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DOI: 10.1039/D3CS01043K (Review Article) Chem. Soc. Rev., 2024, 53, 3829-3895

Electrochemical coupling in subnanometer pores/channels for rechargeable batteries

Yao-Jie Lei† a, Lingfei Zhao† b, Wei-Hong Lai† b, Zefu Huang a, Bing Sun a, Pauline Jaumaux a, Kening Sun *c, Yun-Xiao Wang *d and Guoxiu Wang *a
aCentre for Clean Energy Technology, School of Mathematical and Physical Sciences, Faculty of Science, University of Technology Sydney, Sydney, NSW 2007, Australia. E-mail: Guoxiu.Wang@uts.edu.au
bInstitute for Superconducting & Electronic Materials, Australian Institute of Innovative Materials, University of Wollongong, Innovation Campus, Squires Way, North Wollongong, NSW 2500, Australia
cSchool of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 10081, P. R. China. E-mail: sunkn@bit.edu.cn
dInstitute of Energy Materials Science (IEMS), University of Shanghai for Science and Technology, 516 Jungong Road, Shanghai, 200093, P. R. China. E-mail: yunxiaowang@usst.edu.cn

Received 27th November 2023

First published on 4th March 2024

Abstract

Subnanometer pores/channels (SNPCs) play crucial roles in regulating electrochemical redox reactions for rechargeable batteries. The delicately designed and tailored porous structure of SNPCs not only provides ample space for ion storage but also facilitates efficient ion diffusion within the electrodes in batteries, which can greatly improve the electrochemical performance. However, due to current technological limitations, it is challenging to synthesize and control the quality, storage, and transport of nanopores at the subnanometer scale, as well as to understand the relationship between SNPCs and performances. In this review, we systematically classify and summarize materials with SNPCs from a structural perspective, dividing them into one-dimensional (1D) SNPCs, two-dimensional (2D) SNPCs, and three-dimensional (3D) SNPCs. We also unveil the unique physicochemical properties of SNPCs and analyse electrochemical couplings in SNPCs for rechargeable batteries, including cathodes, anodes, electrolytes, and functional materials. Finally, we discuss the challenges that SNPCs may face in electrochemical reactions in batteries and propose future research directions.

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Yao-Jie Lei

Yao-Jie Lei is a Research Associate working under the supervision of Prof. Guoxiu Wang at the Centre for Clean Energy Technology, University of Technology, Sydney. He received his bachelor of science major in chemistry from Lanzhou University, and research master degree from the University of Sydney. He obtained his PhD from the University of Wollongong. His research interests include the synthesis of nanostructured materials for room-temperature Na–S batteries.

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Lingfei Zhao

Lingfei Zhao is currently an associate research fellow at the Institute for Superconducting & Electronic Materials, University of Wollongong, where he earned his PhD in July 2023. His research interest focuses on materials design and electrolyte engineering for high energy density rechargeable batteries, including Li/Na-ion batteries and Li/Na–sulfur batteries.

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Yun-Xiao Wang

Yun-Xiao Wang currently holds the position of Vice Dean at the Institute of Energy Materials Science (IEMS), University of Shanghai for Science and Technology. She received her master's degree from Xiamen University in 2011 and obtained her PhD degree from the University of Wollongong in 2015. She was awarded a Discovery Early Career Researcher Award (DECRA) from the Australian Research Council in 2017. Her research features elaborate material engineering for various emerging battery systems and other energy storage technologies, with a particularly focus on developing room-temperature sodium–sulfur batteries.

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Guoxiu Wang

Guoxiu Wang is the Director of the Centre for Clean Energy Technology and a Distinguished Professor at the University of Technology Sydney (UTS), Australia. Professor Wang is an expert in materials chemistry, electrochemistry, energy storage and conversion, and battery technologies. Currently, he serves as an Associate Editor for Electrochemical Energy Reviews (Springer-Nature), and an Associate Editor for Energy Storage Materials (Elsevier). His research interests include lithium-ion batteries, lithium-air batteries, sodium-ion batteries, lithium–sulfur batteries, and electrocatalysis for hydrogen production. Professor Wang has published more than 700 refereed journal papers. His publications have attracted over 75[thin space (1/6-em)]000 citations with an h-index of 150 (Google Scholar). He has been recognised as a highly cited researcher in both materials science and chemistry by Web of Science/Clarivate Analytics.

1. Introduction

The endeavours to achieve high energy density, high power density, and high cycling stability for various rechargeable battery systems have been the persistent pursuit of the research community for decades.1–4 Among the diverse approaches, reducing the size of electrode materials with essential phase engineering for cathodes and morphology design for anodes is one of the most effective strategies to enhance the electrochemical performance of the batteries.5,6 With the advancement of systematic characterization techniques for optimal electrode materials and in-depth understanding of the electrochemical mechanisms, the fact that the electrode materials with fine structures, especially those exhibiting variations at the subnanometer scale, lead to substantial enhancement in electrochemical redox processes compared to bulk materials7–10 is drawing attention.

Subnanometer pores/channels (SNPCs) are ubiquitous in various rechargeable battery systems, yet the vital roles of SNPCs have long been neglected. Based on the extensive reports on electrode materials with SNPCs, such as metal oxides, metal phosphides, metal nitrides/carbides, poly-anionic compounds, Prussian blue, and carbon materials, the functions of SNPCs can be summarized as (1) size/channel effects on fast ion diffusion. With shortened ion-diffusion length in SNPCs, fast diffusion of ions can be achieved, which is one of the main indicators for the battery performance i.e., high-rate capability.11 In addition, SNPC-based materials can serve as artificial protective shelves on the surface of the metal anodes, which are capable of regulating uniform ion flux and preventing the side reactions of the metal anodes with the electrolyte solvents, thus achieving high Coulombic efficiency and dendrite-free stripping/platting processes.12 (2) Abundant active sites for enhanced ion storage kinetics and capacity. Materials with a larger surface area usually have more active sites, which can provide more storage sites for ions.13–15 By controlling the size, orderliness, and wettability of SNPCs, the ion storage performance in batteries can be significantly improved. For example, modifying the oxygen-containing functional groups in SNPCs can change the surface and bulk structures, conductivity, wettability, and reaction activity of the material, thereby improving the electrochemical reaction kinetics of sodium ion storage.16,17 (3) Pathway regulation of redox reactions. The SNPCs could enable selective transport of various species (ions and/or molecules) through the size effect of the pore/channel or electrostatic effect of the building blocks, so as to avoid undesirable side reactions.18 For instance, the small molecule S species (S2–4) confined in the SNPCs of the carbon matrix could achieve solid-state conversion during the charge/discharge process, prohibiting the formation of soluble polysulfides that lead to parasitic reactions.19 Other than the advantages of SNPCs in performance improvement, some key properties of SNPCs and their underlying relationships with reaction mechanisms are still unclear and have been barely explored to date.20–25

These include the synthesis of electrode materials with uniform SNPCs and maximization of the effective utilization of SNPCs for ion storage. Understanding the contribution of SNPCs to ion storage will help us discover electrode materials with higher capacity and reversibility. In addition, the internal ion diffusion within the SNPCs and the ion diffusion from the interface are the main factors affecting ion kinetics.26,27 When the resistance at the interface of two bulk materials significantly increases, it can lead to a decrease in electrode potential and low ion accessible area, especially at high currents, thereby severely reducing performance. Furthermore, increasing the thickness or mass loading of the electrodes will also severely limit ion diffusion. Ideally, an efficient SNPC should achieve fast electrolyte transport through ion diffusion and have good electrolyte accessibility. Also, ions always exhibit close contact within the SNPCs. In this way, the storage and transport of ions in individual SNPCs are influenced by collective effects. However, these effects cannot be described by traditional channel theories. Importantly, only by studying the electrochemical reactions within SNPCs at the atomic scale can these challenges be fundamentally understood and addressed and their electrochemical performance be further improved.

To the best of our knowledge, no review has been reported yet from the perspective of the electrochemical couplings of SNPCs in rechargeable batteries. Herein, the SNPC families have been systematically classified and summarized from the architecture point of view, i.e., one-dimensional (1D) SNPCs, two-dimensional (2D) SNPCs, and three-dimensional (3D) SNPCs. These SNPCs embrace a wide range of materials including carbonaceous materials, metal organic frameworks (MOFs), covalent organic frameworks (COFs), porous organic cages (POCs), MXenes, layered transition metal dichalcogenides (TMDs), layered transition metal oxides (TMOs), transition metal phosphates (TMPs), and various types of cathode materials. Besides, diverse approaches to alter the structure and building blocks of the SNPCs for anodes, cathodes, electrolytes, and functional interlayers to enhance the electrochemical performance of the rechargeable batteries have been intensively reviewed. Furthermore, the gaps between practical application and fundamental understanding of SNPCs in rechargeable batteries have been carefully illustrated, while future research directions and promising approaches are rationally suggested. We hope that this review could shed light on the forefront of this emerging yet underdeveloped field and stimulate further exploration in this exciting area.

2. Scientific gaps and classifications of SNPCs

2.1. Scientific gaps for understanding SNPCs in batteries

The last century has witnessed great achievements in manifold characterization techniques for materials, enhancing the capability to identify elements, bonds, phase, and morphology of the samples from micrometer to subnanometer scales. These powerful tools are indispensable for the development of various rechargeable batteries, and the progressive understanding of the working mechanisms inside the batteries. The properties of the materials can vary significantly with different sizes from micro or nano to atomic scales, for example, the ratio of surface atoms increases exponentially at subnanometer scales. The angstrom-scale charge carriers exhibit intimate electrochemistry couplings with SNPCs in rechargeable batteries; however, the capabilities of various characterization techniques to understand these interactions in batteries at the subnanometer scale are limited.

Electrochemical studies on SNPCs are expected to overcome the limitations of previous nanopores and achieve higher selectivity and electrochemical reaction efficiency.28,29 Although the application of SNPCs in batteries is still in the early stages, it is undoubtedly a new topic in the development of next-generation materials. At the SNPC-level, the operation of the battery involves ion storage and ion transfer processes. They typically form atomic and molecular-scale circuits. Exploring the atomic-level assembly of cathode and anode electrode materials, electrolytes (solid-state and quasi-solid-state), intermediate layers/isolation layers, and other components from the perspective of SNPCs to form composite electrode structures with efficient ionic carriers and both ion and electron conductivity is of great significance for batteries. This is because the size, shape, and spatial distribution of each component have a decisive impact on the ion storage and diffusion, thereby affecting the charge–discharge performance and rate capability of the battery. As shown in Fig. 1, the research on SNPCs can effectively upgrade various components of batteries, including anodes, cathodes, solid-state electrolyte, as well as interlayers/separators. Optimization of SNPCs can strengthen the interface stability of anodes, especially for the SEI. An excellent SEI layer with appropriate SNPCs can selectively transport ions, avoid excessive SEI formation to reduce electrolyte consumption, and reduce volume changes during the cycling process. In terms of insertion-type cathodes, carefully designed SNPC materials can effectively promote ion diffusion while maintaining material structure stability, enhancing rate performance; for conversion-type cathodes, structural stability and reaction kinetics are crucial performance factors. By improvements in SNPCs, the active material can be better confined within SNPCs and accelerate ion diffusion, which not only suppress the “shuttle effect”, but increase kinetic reactions. Moreover, well-designed SNPCs for solid-state electrolytes can enhance their ionic conductivity, and their strong selectivity advantage can effectively shield unnecessary ions from passing through, avoiding the generation of side reactions. Moreover, by leveraging the advantages of high selectivity and excellent ionic conductivity of SNPCs, ions can rapidly and orderly diffuse through the separators/interlayers, avoiding uneven deposition and side reactions.


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Fig. 1 Schematic illustration of the versatile advantages of SNPCs for various battery components.

However, designing and preparing efficient battery component structures are challenging for the following reasons: (1) there is still insufficient understanding of the dynamic mechanisms of such complex battery systems; (2) the key ion storage properties, as well as electronic and ionic transport properties of many materials have not been determined or there is uncertainty; and (3) involving multiphase and multiscale situations makes the problem more complex. Therefore, studying batteries from the perspective of SNPCs helps to comprehensively understand the electrochemical reaction mechanisms in batteries and achieve high-performance batteries with fast charge–discharge cycles, long life, excellent rate stability, and high energy/power density. Nevertheless, a systematic summary of SNPCs and their electrochemical reactions in batteries has not yet been collected. Therefore, this is the first review to summarize and report on SNPCs and their applications in batteries.

2.2. SNPC families

SNPC families represent a wide range of porous or layered materials with a pore diameter or interlayer spacing of less than 1 nm, including various types of materials such as carbonaceous materials, inorganic transition metal compounds, porous organic materials, and metal–organic materials. The carbonaceous materials include carbon nanotubes and layered graphene analogues such as graphite, graphene oxide, reduced graphene oxide, graphdiyne, and graphitic carbon nitrides (g-C3N4).30 Although phosphorene, silicene, and germanene, are not carbonaceous materials, they exhibit layer structures and deliver similar properties to that of graphene and can be reasonably sorted as GAs.31,32 Inorganic transition metal compounds based SNPCs include transition metal oxides phosphates, dichalcogenides, carbides, and nitrides.33 Porous organic material-based SNPCs include covalent organic frameworks, porous organic cages, and metal–organic frameworks.34 As the shape, diameter, and building blocks of the SNPCs have a crucial influence on the electrochemical performance of the battery materials, logical classification and detailed discussion are essentially required. Therefore, we tentatively classify the SNPCs from the architecture point of view into three categories, i.e., liner shape one-dimensional (1D) SNPCs, channel shape two-dimensional (2D) SNPCs, and interconnected three-dimensional (3D) SNPCs. The typical representatives, distinctive properties, and potential applications of the SNPC families will be comprehensively illustrated in the following sections.
2.2.1. 1D SNPCs.
Carbon nanotubes (CNTs). As presented in Fig. 2, CNTs are cylinders of one or more layers of graphene with open or closed ends, which are usually divided into three types according to the number of the rolling graphitic sheets, i.e., single-walled CNTs (SWNTs), double-walled CNTs (DWCNTs), and multi-walled carbon nanotubes (MWNTs).35,36 Besides, other types of CNTs with non-perfection structures also deserve wide attention, since the doped heteroatoms or defective sites in CNT structures could significantly alter the properties of the CNTs.37,38 CNTs can exhibit either metallic or semiconductor properties, depending on the direction of the graphitic sheets, e.g. zigzag,39 armchair,40 or chiral configurations.41 SWCNTs exhibit excellent chemical stability and provide efficient chemical shielding, which could effectively protect the encapsulated species from external influences.42 For instance, S has been confined in the 1D SNPCs of the SWCNTs with a pore diameter of 0.99 nm, which could facilitate favourable solid-state reactions in glyme-based electrolytes with inhibited S dissolution.43 Other distinctive properties including good mechanical stability, excellent electrical conductivity, and tuneable SNPC size, render them promising as backbones for various composite materials in battery applications.
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Fig. 2 Illustration of representative 1D SNPC materials. (a) Representative materials of CNTs. (b) Representative materials of COFs. (c) Representative materials of MOFs. (d) Representative materials of TMOs/TMPs.

Covalent organic frameworks (COFs). COFs are highly crystalline, covalently linked, periodically extended organic networks consisting of only light elements (i.e., H, B, C, N, and O).44 They exhibit high structural, chemical, and thermal stability, as well as a rich porous structure for a wide variety of applications such as gas storage, separation, catalysis, and sensors.45–48 Notably, there is a growing interest in utilizing COFs in the realm of electrochemical energy storage, including those with 1D SNPCs as presented in Fig. 2, such as COF-1, COF-5, COF-6, ACOF-1, and JUC-505.49 The pore size of COFs ranges from subnanometer to nanometer scales, which can be rationally engineered by altering the building blocks.50–52 The utilization of COFs with 1D SNPCs could facilitate fast ionic transport through SNPCs, while blocking the undesirable species with a diameter larger than the SNPCs. For instance, a redox-active, crystalline COF has been reported as a cathode material for LIBs with excellent rate capability and good cyclability, as a result of enhanced ion transportation derived from the SNPCs of the COF structures.53 COFs possess distinctive qualities, including their inherent insolubility in electrolytes, abundant porosity, ordered open SNPCs that favor ion transportation, and π-conjugated frameworks that enhance charge transfer.54–56 Consequently, the prospects of COF-based electrode materials for rechargeable batteries are undeniably promising.
Metal–organic frameworks (MOFs). MOFs are porous materials consisting of metal ions or clusters coordinated with organic ligands, which build up extended crystalline structures with excellent structural stability upon intercalation/extraction of guest molecules.57–59 There are plenty of MOFs with 1D SNPCs in diverse MOF families, including ZIF-8, ZIF-69, CPO-27, MIL-53, MOF-74, etc., as presented in Fig. 2. In the field of rechargeable batteries, the SNPCs in MOFs can offer a large surface area and adaptable pore size, which facilitates quick diffusion of cations.60–63 These merits make them ideal for intercalation-type electrodes. Additionally, MOFs with SNPCs possess confinement effects to effectively trap S species, enabling their utilization as host materials for conversion-type electrodes in metal–sulfur batteries. Furthermore, the well-ordered channels and unique features of MOFs with SNPCs can enhance the uniformity of cation plating during the charging and discharging process, which could result in the formation of a stable interface layer and enhance cycling stability.64,65 Consequently, MOFs with SNPCs are also attractive for applications in solid-state electrolytes (SSEs) and artificial interfaces. Overall, MOFs with SNPCs have multiple features that give them enormous potential for various kinds of batteries.
Transition metal oxides (TMOs) and transition metal phosphates (TMPs). Ionic channels are indispensable for various cathode materials, and the size of these channels falls within the realm of SNPCs. TMOs and TMPs are among the most representative cathode materials extensively explored for alkali metal ion batteries, of which olivine and tunnel structured cathodes exhibit rich 1D SNPCs.66,67 As presented in Fig. 2, the cations of the cathodes are inserted in the 1D SNPCs build by the M–O (M = transition metals) and/or P–O polyhedrons, which is the case for LiFePO4, NaFePO4, Na0.44MnO2, Li2FeSiO4, and KVOPO4. The shape and size of the SNPCs in the cathodes could be altered using various approaches to modify ion transportation inside the SNPCs, which will be discussed in detail in the following sections.
2.1.2. 2D SNPCs. Since the discovery of graphene by Geim and Novoselov in 2004, 2D materials have surged in various fields of applications, especially in rechargeable batteries.68–71 Diverse 2D materials with 2D SNPCs have been successfully fabricated as promising candidates for rechargeable batteries, including metal sulfides,72 metal oxides,73 metal nitrides,74 metal carbides,75 metallenes,76etc. (Fig. 3) In comparison to other materials, 2D materials exhibit many unique properties, such as excellent conductivity, mechanical flexibility, uniform electronic state and lattice planes, desirable SNPC structures, etc., making them potential electrode materials for various rechargeable batteries.2,77–79
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Fig. 3 Illustration of representative 2D SNPC materials. (a) Representative materials of GAs. (b) Representative materials of MXenes. (c) Representative materials of TMDs. (d) Representative materials of TMOs.

Graphene analogues (GAs). Graphene is the most prestigious 2D material with a single layer of sp2 carbon atoms arranged in a hexagonal lattice structure.80 Graphene possesses many unique properties, including a high surface area, high modulus of elasticity, good mechanical stability, and excellent electrical conductivity.81 It is worth noting that single-layer graphene is relatively scarce and difficult to obtain; most of the research has been carried out on few-layer graphene, which is composed of several parallel graphene layers with weak interlayer van der Waals force.82 The size of the channel between the graphene layers falls into the region of SNPCs, which could allow for the intercalation of various metal ions as well as solvents. For example, graphene oxide laminates were fabricated by Abraham et al., which exhibit distinctive SNPCs with well-defined interlayer spacing ranging from 6.4 to 9.8 Å for ion intercalation.83 On the other hand, generating defects in the graphene layers (holey graphene) could facilitate ion transport across the graphene layers. For instance, holey graphene with SNPCs of 0.5 to 0.8 nm have been produced through high-temperature etching,84 and ion permeable SNPCs can be engineered on the graphene layer through plasma etching.85 Soon after the discovery of graphene, the exploration of 2D materials has been extended to a wide variety of other graphene analogues (GAs) with similar 2D layered structures but different chemical compositions, including graphene oxide (GO), reduced graphene oxide (RGO), graphdiyne, silicene, germanene, g-C3N4, etc. These GAs with distinctive 2D layered structures and rich SNPCs have been widely used in various applications including catalysis, superconductors, sensors, as well as energy storage and conversion devices including various batteries.
MXenes. MXenes stand for a wide variety of 2D metal carbides, nitrides, or carbonitrides with an odd number of layers in which the metal layers (M) are sandwiched between carbon or nitrogen (X) layers.86 Since its first discovery by Yury et al. in 2011, their unique features, including electrical conductivity, mechanical stability, and thermal stability, render them promising for various applications in supercapacitors, batteries, supercapacitors, catalysis, membranes, and sensors.87 Mxenes exhibit the general formula of Mn+1XnTx, where M is the metal, X represents C or N, and T refers to the terminal groups such as –H, –F, and –OH, which are obtained by selectively etching the A layer (usually the Al layer) from pristine MAX precursors. After ten years of development, MXenes have been extended from Ti3C2 to a large family of metal carbides/nitrides with metals such as Ti, Sc, Zr, Hf, V, Nv, Ta, Mo, Cr, Nb, or Ta.6,86 The interlayer space of MXenes ranges from about 3.5–5 Å, making them suitable SNPCs for Li and Na ions storage.6,88,89 Besides, enriched functional groups in the SNPCs of MXenes could efficiently trap polysulfides, thereby inhibiting the shuttle effect in metal–S batteries.90–92 MXenes have shown great potential in various applications when coupled with a diverse range of other materials.
Transition metal dichalcogenides (TMDs). TMDs are layered materials with a general chemical formula of MX2, where M stands for transitional metals and X represents chalcogen elements of S, Se, or Te.93–95 The properties of TMDs can be varied from insulators (HfS2) to semiconductors (MoS2 and WS2), semimetals (WTe2 and TiSe2), and metallic (NbS2 and VSe2), rendering them capable for a variety of applications.96 2D TMDs always feature multi-crystalline structures.97 Taking MoS2 as an example, it has three crystal structures, namely, 2H (hexagonal), 1T (trigonal), and 3R (rhombohedral), each characterized by distinctive coordination models and staking orders. Different from graphene, 2D TMDs can expose more active sites, further improving the electrochemical performance of TMDs.74,98,99 In addition, the interlayer spacing of 2D TMDs at the subnanometer level has strong covalent bonds within the layer and weak van der Waals forces between layers, providing an ideal space for intercalation of ions with small ionic radius, such as Li or Na ions.2,100–102
Layered TMOs. Due to high structural stability, low material cost, large specific capacity, high ionic conductivity, good environmental benignity, and feasible synthesis conditions, layered TMOs have been developed as one of the most popular candidates of cathodes for LIBs and SIBs.103–106 Generally, the structure of layered TMOs consists of alternating MeO2 octahedra layers and alkali metal layers, which can be sorted as O- or P-type depending on the coordination environment of the alkali metal ions.107,108 The interlayer spacings of layered TMOs are in the range of SNPCs, facilitating quick 2D ion transportation through the SNPCs and resulting in excellent rate capabilities.
2.1.3. 3D SNPCs.
MOFs. 1D SNPCs are ubiquitous in various MOFs. For MOFs with 1D SNPCs at intersecting directions, 1D SNPCs would interconnect with each other to generate 3D SNPCs.109 For instance, as illustrated in Fig. 4, Prussian blue analogues (PBAs), one of the earliest discovered MOF materials, exhibit 1D SNPCs at three intersecting directions, generating a 3D SNPC network.110 These PBAs with 3D SNPCs can facilitate quick diffusion of metal ions, together with abundant redox-active sites and good structural stability, making them an ideal choice for alkali metal ion batteries. Similarly, other MOFs with 3D SNPCs, such as MIL-125, UIO-66, MOF-177, and HKUST-1, have also been widely used in various battery systems.111–113 The advantages of utilizing these MOFs with 3D SNPCs for batteries will be discussed in detail in the following sections.
image file: d3cs01043k-f4.tif
Fig. 4 Illustration of representative 2D SNPC materials. (a) Representative structures of MOFs. (b) Representative structures of COFs. (c) Representative structures of POCs. (d) Representative structures of TMOs/TMPs.

COFs. Similar to MOFs with 3D SNPCs, the interconnection of 1D SNPCs in the COFs results in 3D SNPCs. The COFs with 3D SNPCs exhibit not only cross-linked 3D electrically conductive frameworks but also interconnected 3D ion permeable pathways. Therefore, they are promising candidates for active materials or coating layers in various battery systems.114–116 Those frequently reported COFs with 3D SNPCs including COF-102, COF-105, COF-108, COF-300, CTF-1, etc.117
Porous organic cages (POCs). POCs are a series of microporous organic molecules with intrinsic, guest accessible cages.118 They are built through covalent bonds involving non-metal elements, such as carbon–carbon or carbon-heteroatoms, imines, boronic esters, amides, etc. Compared to other porous frameworks, POCs exhibit many merits including large surface areas, pore volumes as well as open and flexible pores, including tuneable SNPCs.28,119,120 In addition, their discrete molecular structure provides excellent solubility in common solvents with high solution dispersion and processability, which is a rare feature that cannot be achieved in insoluble extended porous frameworks such as MOFs and COFs.121,122 A wide variety of POCs including CPOC-102, CPOC-104, CPOC-302, RCC1, and CC-8 have been used in various battery systems, whose functions and advantages would be detailed in the following sections.122,123
TMOs and TMPs. Besides, 1D and 2D SNPCs in olivine and layered type cathodes, 3D SNPCs have also been extensively reported in various cathodes such as Na3V2(PO4)3, which has been widely known as a NASICON (sodium super ionic conductor) type cathode. These NASICON cathodes generally undergo topotactic insertion/extraction Na+ with small volume changes and minimal structural rearrangements, featuring outstanding long-term cycling stability.124,125 Plenty of cathode materials with 3D SNPCs have been widely investigated to exhibit high ionic conductivity in the cathode matrix, which favours high rate performance of the cathodes. Various other cathode materials with 3D SNPCs, such as LiMn2O4 and Na2FeP2O7,126 will also be discussed in the following sections.

It should be noted that simply utilizing one kind of porous material is not the case in most scenarios; instead, heterostructures or composite materials that merge the merits of various components have been frequently reported.127–129 In particular, the built-in electric field at the heterostructure interface can promote rapid transport of ions and electrons.130,131 Heterostructures composed of adsorption components and catalytic components can combine two functions to achieve synergetic effects.132,133 For instance, higher energy density can be achieved in cathode materials by carbon-coating with three-dimensional sodium diffusion channels.134 More rational designs of heterostructures and hybrid materials will be illustrated in detail in the following discussions.

2.3. Unique physicochemical properties of SNPCs

As the size of the pore/channel decreases to the subnanometer level, which is within the scale range for molecules, ions, and atoms, unique physicochemical properties differing from those of the bulk materials would emerge.29,135,136 Some extraordinary properties that are impossible in the view of classic physics would take place, which makes SNPCs a totally new world of exciting wonders. Generally, the ionic conductivity could be defined as:
 
image file: d3cs01043k-t1.tif(1)
where G is the conductance of an ionic channel, S is its cross-sectional area and L is the length. But in fact, the ionic conductivity of nanochannels is quite complex. The diffusion of monovalent ions confined within a pore with a width like the ion diameter can be expressed as:
 
image file: d3cs01043k-t2.tif(2)
where D is the diffusion coefficient of the ion (for simplicity, we will use the same D = D±), ρ± represents the ion density, c = ρ+ − ρ− represents the charge (in units of the elementary charge), and ρΣ represents the total ion density. Additionally, ρmax is the total ion density at close packing. G is a parameter that characterizes the screening of ion–ion electrostatic interactions. To realize G, first, we need to consider the exponential screening of ion–ion electrostatic interactions caused by the pores. In addition, we adopt the Borukhov–Andelman–Orland entropy, S[ρ±].137 The voltage-dependent “external field” includes the electrochemical potential of ions, solvation energy, and van der Waals and image forces between ions and pore walls. The voltage-dependent “external field” is composed of the electrochemical potential of ions, solvation energy, and van der Waals and image forces between ions and pore walls. The external field controls the equilibrium ion density inside the pore, and besides participating in dynamics through initial and boundary conditions, it does not participate in other processes. Dynamics is defined by the equations of continuity: ∂tρ± = −∂χJ±. J± = −Γρ±∂χ(δF/ρ±), where Γ ≡ Γ± = D±/kBT is a phenomenological mobility parameter, D is the diffusion constant, and we assume it is independent of pore width, voltage, and density; kB is the Boltzmann constant. Thus, plugging the free energy F into the continuity equation yields eqn (2),138
 
image file: d3cs01043k-t3.tif(3)

From eqn (3), it can be determined that G is constrained by many factors, including ion density, charge within the channel, electrostatic forces, gradient of ion density along the pore, collective properties, etc. By changing the above parameters, selective transportation of specific ions can be achieved. Additionally, confining restricted species at the molecular scale with distinctly different properties is another appealing feature. For battery couplings, SNPCs also possess unique properties, which can be reasonably classified into six categories size effect, electrostatic effect, confinements effect, suppression effect, buffering effect, and quantum effect (Fig. 5).


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Fig. 5 Schematic illustration of the unique physicochemical properties of SNPCs.
Size effect. The size effect plays an important role in mass transfer. Due to the small size of SNPCs, smaller ions can be allowed to diffuse from one side to the other side while ions with larger size will be inhibited from entering the SNPCs. This function has been widely used in rechargeable batteries, for example, artificial SEIs with abundant ion diffusion channels have been designed, which often only allow smaller ions such as Li+, Na+, K+, etc. to pass through while inhibiting the passage of other larger ions.139–142 This is beneficial for supporting high ion flux for homogeneous metal stripping/plating, and also helps to avoid adverse reactions between larger solvent molecules and ions which are in directly contact with the electrode. Therefore, by adjusting and improving the specific SNPCs of the materials, potential barriers to mass transfer and storage can be reduced.
Electrostatic effect. Mass transport of various species through the SNPCs via diffusion or advection caused by thermal motion or electric force is essential for electrical chemical reactions in the batteries; one of the most important factors impacting the mass transport is the electrostatic effect.143–145 The electrostatic effect of SNPCs can be easily conceived, as the building blocks would attract species with opposite charges while repelling those with the same charges.146–148 It should be noted that the electrostatic effect of SNPCs is generally induced by partial charges caused by the asymmetric distribution of electrons in the chemical bonds, rather than bare charge which tends to be unstable. For instance, Lewis acidic sites in the SNPCs of COFs and MOFs can complex with anions in the electrolyte, which is beneficial for the quick transport of cations.149 The surface of graphene oxide (GO) membranes with SNPCs can be engineered with positive or negative charges, which can repel ions with same charges while allowing the transmission of ions with opposite charges.150 Therefore, the building blocks of the SNPCs can be rationally designed to achieve selective ion transportation through electrostatic effects to screen ions with opposite charges.
Confinements effect. Physical confinement is essential for electrode materials that have poor electric conductivity or exhibit undesirable side reactions with the electrolytes, which is essential for sulfur (S) cathodes since they exhibit both these problems.151,152 Whereas the confinement effect of SNPCs for S cathodes is significantly different from that in the larger pores, where the S molecules are reduced to short chain small molecules (S2–4) instead of long chain S4–8 species. For example, Wan et al. demonstrated that S2–4 molecules can be accommodated in conductive carbon matrix with SNPCs (∼0.5 nm), instead of cyclo-S8 which exhibits a size larger than 0.5 nm in at least two dimensions.153 As such, the formation and dissolution of long chain polysulfides can be avoided, so as to achieve good cycling stability and superior rate capability. Furthermore, it has been evidenced that S encapsulated in the narrow-diameter SWCNTs with SNPCs undergoes solid-state reactions, since the Li ions are de-solvated before they enter the SWCNTs as a result of the confinement effect of SNPCs.43 Similarly, the confinement effect of SNPCs can be extended to other electrode materials to prevent them from side reactions with electrolytes.
Suppression effect. SNPCs often exhibit suppression effects as a result of their good mass transport ability and mechanical stability, which could allow for ions uniformly transmit through the SNPCs while suppressing short circuits caused by dendrite propagation. This property has been widely applied in various battery systems. For example, common SPEs (such as PEO) are difficult to effectively suppress lithium dendrites due to their low mechanical strength, which can easily lead to battery failure due to short circuits. However, adding ceramic fillers (such as LLZAO) with rich SNPC structures and good mechanical properties to polymer electrolytes can not only increase the ion transport pathways and boost the speed of ion migration, but also effectively increase the mechanical strength of the electrolyte which is beneficial for suppressing lithium dendrites.154,155
Buffer effect. When the dimensions of the material (such as the aperture or thickness) are close to the size of a single crystal cell, nanoscale materials exhibit some polymer-like properties due to their similarity in size to polymer chains. When the material dimensions are larger than the nanoscale, they are typically rigid and linear, but when the dimensions are close to or smaller than the nanoscale, they become flexible or curved, like polymer chains.156,157 In the diffusion process of ions, the conformation of SNPCs changes under shear stress, resulting in shear stress being disproportionate to shear rate and exhibiting a good buffering effect. For instance, a block polymer was designed as an interlayer to buffer tbe cathode volume change during discharging and charging processes.158 Therefore, this characteristic can be applied to non-viscous electrodes and solid-state electrolytes in batteries to achieve good mechanical performance, prevent material fracture and resist volume expansion effects.158,159
Quantum effect. The quantum effect refers to a series of distinctive phenomena observed when the size of transported particle is close to the Ångstrom scale range, which follow the rules of quantum mechanics and cannot be explained by classical physics. From the perspective of classical thermodynamics, the chemical selectivity mass transfer through nanoscale channels should be very slow: this limitation is predicted by the Hagen–Poiseuille equation, as the traditional laminar flow has zero flow velocity at the pore wall.160 One of the quantum effects is tunnelling, which refers to the penetration of a particle getting through an energy barrier that is higher than the particle's kinetic energy.161,162 The principle of tunnelling has led to the development of the scanning tunnelling microscope (STM), which has significantly advanced scientific research to atomic scale resolution.163 Another quantum effect for SNPCs is quantum confinement, where the movement of particles will be restricted within the SNPCs, leading to the existence of particles only in the form of quantum states when the size of particles approaches the size of the SNPCs. For instance, quantum confinement occurs between the interlayer space of van der Waals channels.164 In terms of ions diffusion in SNPCs, however, the rapid ion transport is precisely caused by quantum flow, resulting in superfast ion fluid states. A quantum ions fluid can orderly arrange ions, greatly reducing the ion diffusion energy barrier and achieving highly efficient and selective ion diffusion.

3. Electrochemical couplings in SNPCs

As mentioned above, electrochemical coupling of SNPCs in batteries is ubiquitous. To understand the relationship between SNPCs and the electrochemical performance, we have summarized the size of SNPCs and various other electrochemical parameters in batteries (Tables 1–4). We have tentatively sorted the specific capacities (for cathodes and anodes) and the ionic conductivities (for electrolytes and separators/interlayers) according to the size of the SNPCs, to understand the influence of SNPCs on the electrochemical performance of the batteries. As shown in Fig. 6a, capacities of transition metal oxides, polyanions, PBAs, and polymers exhibit not directly relationship with SNPC size. The possible reason is that these materials have enough pores to provide ion diffusion and storage, and the reaction kinetics are all not slow. Instead, their performance is more likely to be influenced by electro-conductivity rather than ionic conductivity. Nevertheless, it is obvious that with the increase in pore size of S/Se cathodes and COFs (Fig. 6b), the performance significantly decreases. This indicates that the reaction kinetics are one of the key factors determining their capacities. As for the anodic materials in Fig. 6c, the pore size of phosphides, sulfides, and polymers, does not directly affect their ion storage. However, for MXenes, graphite-based materials, and hard carbons shown in Fig. 6d, the size of the pores is closely related to ion storage. For MXenes, increasing the size of the pores will lead to a decrease in ion storage performance, while for graphite-based materials and hard carbons, the capacity will increase with the increase in pore size. This indicates that the regulation of the pore size will affect the ion storage capacity of these carbon-based anodic materials. For solid-state electrolytes and quasi-solid-state electrolytes, it is commonly known that ionic conductivity can impact their performance. According to Fig. 3e, however, the ionic conductivity of some materials does not show a linear relationship with SNPC size. It may be because when polymers and MOFs are used as electrolytes, they are usually modified. As described in eqn (3), changing their electrostatic force, ion concentration difference, and other conditions can greatly alter the ion conductivity. Therefore, these materials do not exhibit a linear relationship with size of SNPCs. However, for some materials that are difficult to modify, such as NASICONs and sulfides, their ionic conductivity exhibit a linear relationship with SNPC size. Thus, as the pore size increases, the ion conductivity also increases in Fig. 6f. For interlayer/separators, their research mainly focuses on polymers, oxides, and MOFs. As illustrated in Fig. 6g, there is no clear correlation between the ionic conductivity and SNPC size. Nonetheless, it is obvious that the ionic conductivity of MOFs is affected by the size of SNPC.
Table 1 Summary of the cathode materials with SNPCs (theoretical = T)
Materials Battery types Theoretical capacity (mA h g−1) SNPC size (nm) Average potential (V) Capacity (mA h g−1)@current density (mA g−1)@cycle number Ref.
LiCoO2 LIBs 274 — 4 123@50@10 165
Oxygen vacancy-Li[Ni0.2Li0.2Mn0.6]O2 LIBs 315 — 3.8 121@60@10
Oxygen vacancy-Li2MnO3 LIBs 230 — 2.8 59.5@10@50 166
LiCoO2 LIBs 274 — 4 116@100@20 167, 168
LiNi0.8Mn0.1Co0.1O2 LIBs 190 — — 210@100@50 169
LiNi0.8Co0.15Al0.05O2 LIBs 180 0.47 3.8 170@180@100 170
LiNiO2 LIBs 275 — 3.8–3.9 140@275@100 171
LiNi0.9Co0.1−xTi0.03O2 LIBs 275 — 3.8–3.9 180@275@50
LiNi0.9Co0.08Ti0.02O2 LIBs 275 — 3.8–3.9 120@275@50
LiNi0.9Co0.06Ti0.04O2 LIBs 275 — 3.8–3.9 190@275@50
Li2MnO3 LIBs 458 0.47 4 140@45@20@10 172, 173
LiNi0.8Co0.15Al0.05O2 LIBs 279 0.47 3.8 155@280@100 174
LiNi0.6Co0.2Mn0.2O2 LIBs 279 — 3.6 170@280@100 175
Li1.2Mn0.54Ni0.13Co0.13O2 LIBs 378 — 3.6 177@280@200 176
Li1.13Ni0.30Mn0.57O2 LIBs 343 0.79 3.0 85@340@500 177
Li1.2Ni0.17Mn0.56Co0.07O LIBs 465 — 3.8 172@1400@200 178
LiNi1−yCoyO2 LIBs 276 — 3.7 148.4@27@20 179
LiNi1−xCoxO2 LIBs 276 — 3.8 175@137@50 180
Li12xNi0.85Co0.15O2 LIBs 180 — — — 181
LiNi1/3Co1/3Mn1/3O2 LIBs 279 — 3.8 186@28@50 175
LiNi0.7MnxCo0.3–xO2 LIBs 160 — — — 182
LiNixCoyMnzO2 LIBs 200 0.47 3.8 187@100@100 183
Z-doped LiNi0.6Co0.2Mn0.2O2 LIBs 160 — 3.8 130@53.3@50 184
F-LNCM LIBs 200 0.55 3.75 153.4@200@200 185
5%P@LLO LIBs 250 — 3.5 200@250@100 186
B@LNMO LIBs 200 0.48 3.6 200@100@300 187
LiNi0.6Co0.2Mn0.2O2 LIBs 180 0.47 3.8 141@180@200 188
LiNi0.8Co0.15Al0.05O2 LIBs 180 — 3.8 140@36@1500 189
LiNi0.7Co0.15 Mn0.15O2 LIBs 180 — 3.8 130@36@1500
Mo-doped LiNi0.5Mn0.5O2 LIBs 180 0.47 3.8 105@50@100 190
Nb modified NCM811 LIBs 200 0.47 3.8 182@66@250 191
LiNi0.6Co0.2Mn0.2O2 LIBs 180 0.47 3.9 183.3@180@80 192
Ni-rich LiNi0.8Co0.1Mn0.1O2 LIBs 180 0.47 3.8 178@180@100 193
Ti-doped Ni-rich LiNi0.8Co0.1Mn0.1O2 LIBs 180 0.47 3.8 130@180@150 194
Ni-rich LiNi1−x−yMnxCoyO2 LIBs 190 — 3.8 140@62.7@600 195
LiCoMnO4 LIBs 145 — 4.8 85@14@50 196
LiFe0.5Mn1.5O4 LIBs 147 — 4.2 80@14@50 197
LiFe0.1Mn1.9O4 LIBs 147 — 89@14@50
LiFe0.3Mn1.7O4 LIBs 147 — 70@14@50
LiCu0.5Mn1.5O4 LIBs 145 — 2.6 47@14@14 198
LiNi0.25Cu0.25Mn1.5O4 LIBs 145 — 2.2 57@14@8
LiNi0.5Mn1.5O4 LIBs 145 — 2.7 48@14@20
NaFeO2 SIBs 241.8 0.53 2.8 48@200@100 199
NaNi0.5Mn0.5O2/Super P SIBs 240 0.51 2.8 142@12@100 200
NaNi0.5Mn0.5O2/CNT SIBs 240 0.51 2.8 127@12@100
NaNi0.5Mn0.5O2 SIBs 240 0.54 2.9 74@240@200 201
Na2/3Ni1/3Mn2/3O2 SIBs 173 — 3.2 52@346@500 106
Na3/4(Li1/4Mn3/4)O2 SIBs 291 — 2.6 112@150@100 108
NaNi0.5Mn0.2Ti0.3O2 SIBs 240 0.54 3.0 93@240@200 201
NaLi0.1Ni0.35Mn0.55O2 SIBs 268 — 3.1 110@12@100 202
Na1.2Mn0.4Ir0.4O2 SIBs 203 — 2.5 68@20@50 203
NaFe0.55Mn0.45O2 SIBs — 0.55 3.2 32@240@100 204
Na0.75Ni0.82Co0.12Mn0.06O2 SIBs 200 0.55 2.8 80@200@400 205
Na0.9Ca0.05Ni1/3Fe1/3Mn1/3O2 SIBs 130 — 3.0 102@130@200 206
Na[Li0.05(Ni0.25Fe0.25Mn0.5)0.95]O2 SIBs 170 — 3.4 120@85@40 207
Na0.85Li0.10Ni0.175Mn0.525Fe0.2O2 SIBs 150 — 3.5 115@150@140 208
Na[NiCoMnTi]1/4O2 SIBs 120 0.54 3.0 78@600@400 209
NaMn0.48Ni0.2Fe0.3Mg0.02O2 SIBs 240 — 3.0 100@24@100 210
NaCr1/3Fe1/3Mn1/3O2 SIBs 200 — 2.7 102@5@12 211
Na[Cu0.22Fe0.30Mn0.48]O2 SIBs 240 — 3.3 100@10@100 212
TiS2 LIBs 239 0.57 2.25 200@240@100 213
K0.3MnO2 PIBs 82 0.62 2.8 74@28@50 214
K0.6CoO2 PIBs 80 — 2.5 44@100@120 215
K0.7Fe0.5Mn0.5O2 PIBs 164 0.69 2.0 70@500@200 216
Na0.7Fe0.5Mn0.5O2 SIBs 182 0.56 — 68@500@100 211
Li0.7Fe0.5Mn0.5O2 LIBs 204 0.47 — 20@500@100
Na0.7CoO2 SIBs 175 0.54 2.6 60@30@40 217, 218
100@175@100
NaNiO2 SIBs 237 0.53 2.9 100@23.7@20 219
NaFeO2 SIBs 244 0.56 3.2 68@12@30 220
NaCrO2 SIBs 253 0.53 3.0 121@126@300 221
NaVO2 SIBs 255 0.58 1.6 120@52@15 222
Na0.7VO2 SIBs 190 0.58 1.6 100@38@10
NaNi0.5Mn0.5O2 SIBs 240 0.56 3.5 138@24@12 223
Na0.9Cr0.9Ru0.1O2 SIBs 220 — 2.75 60@1100@500 224
Na2/3[Mg0.28Mn0.72]O2 SIBs 190 — 2.9 83@38@150 225
Na0.6[Cr0.6Ti0.4]O2 SIBs 167 — 3.6 75@16.7@200 226
Na0.80Li0.12Ni0.22Mn0.66O2 SIBs 214 — 3.4 115@21.4@200 227
Na0.70Mn0.60Ni0.30Co0.10O2 SIBs 179 — 3.5 125@90@10 228
K0.41CoO2 PIBs 103 0.56 3.0 56@18@30 229
K0.45MnO2 PIBs 116 0.5 2.7 66@20@100 230
K0.28MnO2·0.15H2O PIBs 77 — 2.5 71.5@20@100
KCrO2 PIBs 218 0.59 2.5 60@10@100 231
K0.5V2O5 PIBs 67 0.6 2.7 70@20@80 232
K0.45Mn0.5Co0.5O2 PIBs 114 — 2.3 47@500@500 233
K0.65Fe0.5Mn0.5O2 PIBs 154 — 2.2 55@100@350 234
K0.44Ni0.22Mn0.78O2 PIBs 112 — 2.4 52@200@500 235
K0.67MnO2 PIBs 237 0.61 2.6 78@50@300 236
K1.39Mn3O6 PIBs 118 — 2.6 80@50@50 237
K0.67MnO2 PIBs 159 0.64 2.5 40@200@400 238
K0.5MnO2 PIBs 125 0.64 2.5 40@200@400
KCrO2 PIBs 218 — 2.5 60@10@100 231
K0.77MnO2 PIBs 176 — — —
K0.32MnO2 PIBs 86 — — —
K0.6CoO2 PIBs 141 — 2.5 65@100@300 232
KCrS2 PIBs 173 — 2.4 60.5@35@300 239
K0.69CrO2 PIBs 167 0.68 2.6 74@100@300 240
K0.8CrO2 PIBs 186 — 2.5 50@186@300 241
KVO PIBs 253 0.52 2.5 87@10 mA g@50 242
K0.5V2O5 PIBs 67 1.0 3.2 79.2@100 mA g@100 243
K2V3O8 PIBs 149 0.53 1.75 60@20 mA g@200 244
K0.83V2O5 PIBs 104 — 2.5 54@100 mA g@200 245
K0.48Mn0.4Co0.6O2 PIBs 119 0.5 2.5 27@12@180 246
K0.45Mn0.9Mg0.1O2 PIBs 119 — 2.4 20@20@100 247
K0.7Mn0.7Mg0.3O2 PIBs 179 — 2.75 83.4@100@400 248
K2/3[Ni1/3Mn2/3]O2 PIBs 155 — 3.0 74@85@200 249
K0.83[Ni0.05Mn0.95]O2 PIBs 187 0.64 3.0 120@52@200 250
K0.67Mn0.83Ni0.17O2 PIBs 158 — 2.1 52@500@200 251
K0.45Mn0.5Co0.5O2 PIBs 113 — 2.3 82@50@50 252
K0.67Ni0.17Co0.17Mn0.66O2 PIBs 157 — 2.9 76@20@100 253
LiFePO4-with Na doping LIBs 170 1.0 3.45 136@1700@3000 254
LiFePO4-with Ni doping LIBs 170 1.0 3.4 150@1700@5000 255
Cl-doped LiFePO4 LIBs 170 1.2 — 105@1700@500 256
Nb-doping LIBs 170 — 3.4 114.3@1700@300 257
LiMnPO4 LIBs 170 0.52 4.1 80@170@500 258
LiMn1−xFexPO4 LIBs 171 0.52 3.8 160@17@50 259
Li0.92Co0.8Fe0.2PO4 LIBs 154.1 0.52 4.8 121@15@500 260
LiCoPO4 LIBs 167.5 0.51 4.8 60@84@140 261
LiNiPO4 LIBs 167.5 0.55 — — 262
LiNi0.5Co0.5PO4 LIBs 160 — — —
Li3V2(PO4)3 LIBs 197 0.51 3.9 57@133@1000 263
Li2MnP2O7 LIBs 110 0.44 4.7 (T) — 264
Li2NiPO4F LIBs 143 0.44 — — 265
Li2CoPO4F LIBs 143 0.44 — — 266
LiCuSO4F LIBs 145 0.50 5.1 (T) — 267
LiCoSO4F LIBs 149 0.56 4.7 (T) — 268
LiNiSO4F LIBs 149 0.54 5.2 (T) —
LiNiOSO4 LIBs 152 — 5.0 (T) — 269
LiCoOSO4 LIBs 152 — 5.1 (T) —
Li2NiOSO4 LIBs 152 — 5.0 (T) —
LiNi0.5Mn1.5O4 LIBs 147 — 4.7 110@140@200 270
LiNiPO4 LIBs 167.5 0.55 5.1 110@167@100 271
LiNi0.5Co0.5PO4 LIBs 167.5 0.55 —
Li2CoPO4F LIBs 287 0.47 4.8 50@2870@50 272
LiNiSO4F LIBs 148 0.54 — —
Li3V2(PO4)3 LIBs 132 0.62 4 90@66@40 273
Na-deficient LIBs 117 — 3.0 65@110@1000 274
Na3.32Fe2.11Ca0.23(P2O7)2
Na deficient LIBs 194 — 2.8 106@550@300 275
Na3.41£0.59FeV(PO4)3
NaFePO4 SIBs 154 — 2.7 110@30@50 276
NaFePO4 SIBs 154 0.38 2.3 140@20@300 277
a-FePO4 SIBs 178 — 2.3 54@50@300 278
VOPO4 SIBs 117 0.6 3.5 100@16.5@250 279
NaVOPO4 SIBs 145 0.51 3.5 75@72@1000 280
Na3V(PO4)2 SIBs 173 0.69 3.25 70@173200 281
Na3V2(PO4)3 SIBs 117 0.52 3.25 48@118@5000 282
Na3V2(PO4)3 SIBs 117 0.54 3.3 30@4680@30000
Na3Fe2(PO4)3 SIBs 115 0.58 2.8 96@115@200 283
Na3Cr2(PO4)3 SIBs 117 0.63 4.5 5@60@20 284
Na2TiV(PO4)3 SIBs 178 — 2.2 73@1250@500 285
Na3FeV(PO4)3 SIBs 111 — 2.6 100@110@1000 286
Na4MnV(PO4)3 SIBs 111 — 3.4 90@110@1000 286
Na3MnZr(PO4)3 SIBs 107 0.64 3.5 92@50@500 287
Na3MnTi(PO4)3 SIBs 177 0.63 2.4 82@3500@3500 288
Na2MnP2O7 SIBs 195 0.62 3.0 90@39@30 289
Na2CoP2O7 SIBs 96.1 0.51 2.6 85@5@10 290
Na3.12Fe2.44(P2O7)2 SIBs 110 — 3.0 92@220@80 291
Na7V3(P2O7)4 SIBs 80 — 4.0 52.5@80@600 292
t-Na2(VO)P2O7 SIBs 93.4 0.5 3.5 66@4.6@10 293
NaVP2O7 SIBs 108 — 3.7 30@5.4@20 294
Na4Co3(PO4)2(P2O7) SIBs 127 — 4.5 70@12.7@100 295
Na4Ni3(PO4)2(P2O7) SIBs 129 — 4.6 100@10@10 296
Na7V4(P2O7)4PO4 SIBs 92.8 — 3.8 81.4@46@300 297
NaVPO4F SIBs 143 0.55 3.5 101@286@1000 298
Na3V2O2(PO4)2F SIBs 130 — 3.7 102@2600@2000 299
Na2FePO4F SIBs 124 3.25 — — 300
Na2CoPO4F SIBs 122 0.59 4.3 40@61@20 301
Na2Fe2(SO4)3 SIBs 118 — 3.5 80@12@50 302
Na2Fe(SO4) ·22H2O SIBs 118 — 3.2 61@12@20 303
Na2Fe2(SO4)3 SIBs 120 — 3.7 97@24@300 304
Na2.5Fe1.75(SO4)3 SIBs 106 — 3.6 90@106@200 305
Na6Fe5(SO4)8 SIBs 120 — 3.7 87.1@240@1000 306
NaFe(SO4)2 SIBs 99 0.64 3.0 78@20@80 307
Na2MnSiO4 SIBs 278 — 3.0 140@278@500 308
Na2FeSiO4 SIBs 276 — 2.4 88@70@200 309
Na2CoSiO4 SIBs 272 0.47 3.3 105@5@25 310
Na4Fe3(PO4)2(P2O7) SIBs 129 — 3.1 65@2600@4400 311
Na2Fe2(SO4)3 SIBs 120 — 3.5 70@60@100 302
KTi2(PO4)3 PIBs 64 0.62 1.7 80@32@100 312
KFeSO4F PIBs 255 0.5 3.6 52@20@100 313
K3V2(PO4)3 PIBs 106 0.47 3.6 50@20@100 314
K3V2(PO4)2F3 PIBs 115 0.58 3.75 59@10@50 315
KVOPO4 PIBs 133 0.55 3.7 104@66@100 316
Fe4[Fe(CN)6]3·5.89H2O LIBs 125 1.03 3.0 68@100@10 317
Fe4[Fe(CN)6]3 LIBs 125 1.02 3.0 70@25@94 318
Fe[Fe(CN)6]0.71 LIBs 125 1.02 3.0 20@25@30 319
Fe[Fe(CN)6]0.87·0.13·3.1H2O LIBs 125 1.02 3.0 96@25@50
LiFeHCF-1 LIBs 170 1.02 3.0 109@190@650 320
LiFeHCF-2 LIBs 170 1.02 3.0 87@190@650
LiFeHCF-3 LIBs 170 1.02 3.0 41@190@650
LiFeHCF-4 LIBs 170 1.02 3.0 61@190@650
LiFeHCF-5 LIBs 170 1.02 3.0 29@190@650
NiFe-PBA SIBs 70 1.03 3.1 100@20@200 321
PB-1 SIBs 112 — 2.9 70@100@100 315
Na2Zn3[FeII(CN)6]2·9H2O SIBs 64.8 — 3.2 96@10@50 322
Ni-based PBAs SIBs 85 — 3.4 72@85@4000 323
PB-S1 SIBs 170 — 3.1 70@100@200 324
PB-S3 SIBs 170 2.9 72@100@500
Fe-based PBAs SIBs 157 — 3.0 80@150@500 325
Mn-based PBAs SIBs 140 1.06 3.3 74@500@2700 326
Na1.7FeFe(CN)6 SIBs 148 1.06 2.9 99@200@100 327
Na1.54FeFe(CN)6 SIBs 170 1.06 3.0 84@200@500 328
Na2FeII[FeII(CN)6] SIBs 170 2.9 71@50@100 329
NaFeIII[FeII(CN)6] SIBs 140 1.0 3.0 50@50@100
FeIII[FeIII(CN)6] SIBs 64.8 1.0 3.2 6@50@100
PB PIBs 87 — 3.6 80@9@500 330
K1.64Fe[FeII(CN)6]0.89·0.15H2O PIBs 155 1.0 3.3 130@30@100 331
K1.75Mn[FeII(CN)6]0.93·0.16H2O PIBs 155 1.0 3.6 120@30@100
K2Mn[Fe(CN)6] PIBs 155 1.02 3.4 150@15@140 332
KFeIIIFeII(CN)6 PIBs 155 1.02 3.3 120@10@100 333
KFe[Fe(CN)6] PIBs 155 1.02 3.3 30@500@1100 334
K0.6Ni1.2[Fe(CN)6] PIBs 155 3.3 62@50@300 335
RGO@PB PIBs 87 3.3 62@50@300 335
K1.75Mn[Fe(CN6)]0.93·0.16H2O PIBs 155 1.02 3.8 128@30@100 331
K1.64Fe[Fe(CN)6]0.89·0.15H2O PIBs 155 1.02 3.3 110@30@100 331
KMHCF PIBs 156 — 3.7 85@156@100 336
NI-KMHCF PIBs 156 — 3.7 102@156@100
K1.6Mn [Fe(CN)6]0.96·0.27H2O PIBs 131 — 3.9 83@50@30 337
K0.220Fe[Fe(CN)6]0.805·4.01H2O PIBs 125 0.51 3.2 117@125@100 338
FeFe-PW PIBs 119.7 — 3.3 90.4@20@100 339
CoFe-PW PIBs 108.2 — 3.4 43@20@15
NiFe-PW PIBs 70.7 — 3.7 64@20@15
CuFe-PW PIBs 59.1 — 3.7 30@20@15
KFeHCF-S PIBs — — — 78@100@300 340
KFeHCF-M PIBs — — — 108@20@15
KFeHCF-L PIBs — — — 8@20@15
PTCDA LIBs 273 0.35 2.4 120@100@280 341
IEP-11-E12 LIBs — 1–2 2.2 46.7@550@9000 342
CMP LIBs — 1.69 101@1000@1500 343
Py-A-CMP LIBs — 2.1 2.2 125@100@400 344
TPE-A-CMP LIBs — 1.56 2.2 170@100@400
PTTPAB LIBs — 0.75∼1 3.8 95@20@50 345
SPTPA LIBs — 1.0 3.75 88@50@1700 346
YPTP LIBs 109.4 1.1, 1.6 3.75 92@50@1700
OPTPA LIBs 109.4 1.7 3.75
SPTPA LIBs 109.4 1, 1.7 3.75
POP SIBs — 1.05 1 180@30@150 347
POP SIBs — 1.4–2 2 200@10@40 348
TAPT-NTCDA SIBs — 1.25 1.5 70@50@50 349
PI PIBs — 4.7 2.2 48.7@50@200 350
PQI PIBs — 4.7 2.2 84.7@50@200
PI-CMP PIBs — 4.7 2.25 93.3@50@200
HAT PIBs 245 0.7–1.4 1.7 169@10000@4600 351
PIBN-G (COF) LIBs 280 1.4 2.3 242.3@280@300 352
BQ1-COF LIBs 773 1.2 2.2 230@1556@1000 353
PIBN LIBs 280 1.45 2.0 206@280@300 352
CT SIBs 454 1.3 2.25 140@500@100 354
N-COF SIBs 515 1.1 1.5 236.5@500@1000 355
DAAQ-COF@CNT PIBs — 2.3 1.5 108@500@500 356
COF PIBs — 1.95 1.5 70@1000@5000 357
MIL-53(Fe) LIBs — 0.65 2.6 75@75@50 358
MIL-132 LIBs 59 — 3.0 50@10@5 359
MOF-1 LIBs — 2 2.3 40@100@50 360
MOF-2 LIBs — 2 2.4 37@100@50
MOF-3 LIBs — 2 2.5 8@100@50
MOF-4 LIBs — 2 2.2 5@100@50
S/MC LSBs 1675 2 1.4 500@167@800 361
AB-S LSBs 1675 2 1.8 400@837@50 362
MICP LSBs 678 1.1 1.8 341@67.8@3000 363
S/OBC LSBs 1675 2.3 2.0 711@167@50 364
S/TBC LSBs 1675 1.4 1.9 401@167@50
S/DBC LSBs 1675 2.5 2.1 291@167@50
FDU/S-40 LSBs 1675 0.46 1.6 780@400@50 365
MPC/Se LSBs 678 1–3 1.7 530@67.8@100 366
ACC SSBs 1675 0.5 1.1 700@1675@2000 367
S/ELSC-40 SSBs 1675 0.7 1.0 518@1675@500 368
APCF-38S SSBs 1675 0.3–0.8 0.9 1000@167@400 369
WBMC SSBs 1675 0.4–1.0 0.9 822@334@100 370
C/S PSBs 1675 0.4–1.0 0.6 868@20@150 371

Table 2 Summary of the anode materials with SNPCs
Materials Battery types Theoretical capacity (mA h g−1) SNPC size (nm) Average potential (V) Capacity (mA h g−1)@current density (mA g−1)@cycle number Ref.
TiS2 LIBs 956 0.57 1.6 213@100@200 372
ZnSe/CoSe2-CN SIBs 433 0.38/0.58 1.0 422@500@300 373
TiO2 SIBs 335 — 0.3 165@6700@2000 374
CoSi3P3 LIBs 2902 0.34 0.3 891@100@200 375
Co3O4 LIBs 890 — 0.8 — 376
TiNb2O7 LIBs 387.6 2 1.6 — 377
BP LIBs 2596 1 0.8 600@1000@100 378
MoO2 LIBs 838 1 0.4 489@100@1050 379
MoS2 SIBs 670 0.64 0.3 172@200@200 380
MoS2 LIBs 670 0.69 1.6 205@200@1400 381
MoSe2 LIBs 422 0.64 0.6 494@422@100 382
e-MoSe2 0.98 0.8 80@422@100
MoSe2@C@MoOx SIBs 422 0.65 0.6 450@200@200 383
MoSe2@C SIBs 422 0.64 0.6 200@200@200
MoSe2 SIBs 422 0.64 0.6 320@200@150
Ti2AlC3 SIBs 351 0.93 0.5 — 384
Ti3C2 SIBs 351 0.99 0.5 —
a-Ti3C2 SIBs 351 1.25 0.5 50@200@500
Ti3C2 PIBs 191 0.99 — —
a-Ti3C2 PIBs 191 1.25 0.25 47@200@500
Mo2TiC2 PIBs 356 0.99 0.5 140@20@94 385
Pillared-Mo2TiC2 PIBs 356 1.12 0.4 250@20@94
Ti3C2Tx SIBs 351 1.2/0.97 0.5 100@20@100 386
Ti3C2Tx SIBs 351 0.99 0.4 76@100@120 387
Ti3C2Tx PIBs 191 0.99 0.3 42@100@120
Ti2C SIBs 331 0.77/1.0 0.6 175@20@100 388
V2C SIBs 335 0.952 0.8 21@1000@300 389
Carbon SIBs 297 0.3/0.5 0.2 280@100@300 390
TiO2 LIBs 335 0.95 — — 391
Hard carbon LIBs — 0.62/0.6 0.01 500@217@200 392
a-C-900 SIBs — 0.55 0.6 280@100@5 393
a-C-1300 SIBs — 0.55 0.7 270@100@5
a-C-1800 SIBs — 0.55 0.7 280@100@5
Hard carbon PIBs — 0.42 0.2 200@100@100 394
Graphite PIBs — 0.34 0.25 16@100@100
SiC-CDC-800 PIBs — 0.52 0.5 284@100@200 395
SiC-CDC-900 PIBs — 1.19 0.4 198@100@200
SiC-CDC-1000 PIBs — 1.53 0.5 164@100@200
L-A-900 PIBs — 0.4 0.3 118@1000@1200 396
B1-A-900 PIBs — 0.5 0.3 95@1000@1200
B2-A-900 PIBs — 0.45 0.4 52@1000@1200
Graphene SIBs — 0.4 0.6 120@50@50 397
C-900 SIBs — 0.6 0.5 150@50@50
C-1300 SIBs — 0.7 0.1 297@50@50
Hard carbon SIBs 0.5 0.1 250@50@200 398
PDCzBT SIBs — 1 0.2 117@100@200 399
LIBs — 1 0.25 300@200@400
DBD-CMP1 LIBs — 0.4–0.6 0.2 600@100@300 400
DBD-CMP2 LIBs — 0.25 400@100@300
DBD-CMP1 SIBs — 0.4–0.6 0.3 241@100@100
DBD-CMP2 SIBs — 0.4 83@100@100
CMP-PyBT PIBs — 0.94 0.75 104@500@500 401
CMP-PyBz PIBs — 1.1 0.5 272@500@500
CTF-0 PIBs — 0.5 0.3 113@100@200 402
CTF-1 PIBs — 0.7 0.8 60@100@200
Si@ZIF-8 LIBs 4200 1.1 0.5 830@200@500 403
ZIF-8-C@PP LIBs 2596 0.8/1.3 0.25 786@100@100 404
P@NMC SIBs 2596 0.6/0.8 0.4 450@1000@1000 405
P@HC SIBs 2596 0.6/1.1 0.3 548@1000@1000 406
P@N-CNT LIBs 2595 0.9/1.1 0.3 523@5000@1500 407
OMC@RP SIBs 2596 0.75 0.4 600@5200@1000 408
Cu-OMC@RP SIBs 2596 1.2/1.4 0.5 750@5200@750
MnS LIBs 616 1–2 0.5 460@1600@1500 409
NiP2 SIBs 1333 — 0.3 244@2000@1000 410
FeP LIBs 926 0.46 0.6 590@2000@1000 411
GeP5 LIBs 2300 — 0.75 2200@200@40 412
ZnP2 LIBs 1477 0.48 0.5 100@1000@200 413
CoNi-LDH LIBs 744 0.81 0.8 440@5000@500 414
CoFe-LDH SIBs — 0.7 0.8 209@1000@200 415
CoFe-LDH LIBs — 0.92 0.8 860@4000@1000 416
Graphene sheets LIBs — 0.57 0.3 718@500@40 417
Graphene sheets SIBs — 0.37 0.25 917@50@100 418
Graphene sheets PIBs — 0.35 0.35 400@500@600 419
Graphite LIBs 372 0.335 0.1 306@1000@800 420
Natural graphite PIBs 278 1.2 0.7/0.75 90@500@100 414
Graphite SIBs 278 0.335 1.1/1.2 125@100@100 421
Graphite SIBs — 0.37 0.3 300@20@30 422
MoP2 LIBs 676 0.56 0.6 525@160@60 423
FeP LIBs — — 0.7 750@200@100 424
SnP0.94 LIBs 488 0.4 0.4 670@120@40 425
NaTi2(PO4)3 SIBs 485 0.366 — — 426
Na2Ti3O7 SIBs 311 0.845 — — 427
Li4Ti5O12 LIBs 175 0.835 0.75 220@125@500 428
MnO2 SIBs 308 0.693 1.25 72.2@500@100 357
F-COF LIBs 314 1.93 0.2 95@1000@5000 357
COF LIBs 421 1.95 0.2 70@1000@4000 357

Table 3 Summary of the electrolytes with SNPCs
Electrolyte SNPC size (nm) Ion types Ionic conductivity (S cm−1) Ref.
PEO-LiTFSI 0.7 Li+ — 429
TEA-TFB 0.6 TEA+ — 430
1.1 BF4−
SiNx film 0.45 H+ — 431
Na+
V-CNF 0.6 Zn2+ 0.61 × 10−3 432
1.3
Zeolite 0.54 Li+ 0.94 × 10−3 433
UiO-66-X 0.6 F−, Cl− ∼10[thin space (1/6-em)] × 10−3 434
CuBTC MOF 0.65 Li+ ∼5.0 × 10−4 435
MOF/polymer 0.8 Li+ 0.54 × 10−3 18
PvDF-LiTFSI-MOF 1.16 Li+ 4.08 × 10−4 436
Ce-UiO-66-Li+ 0.6 Li+ 2.16 × 10−4 437
PEO: LiAsF6 0.5 Li+ — 438
Cellulose nanofibrils 0.9 Li+ 1.5 × 10−3 439
MOF-cellulose nanofibers 1.1 Li+ 7.88 × 10−4 440
PP/UiO-66-NH2 0.4 Li+ 2.45 × 10−4 441
Li9.54Si1.74P1.44S11.7Cl0.3 0.8 Li+ 2.5 × 10−3 442
Li3OCl 0.4 Li+ 6.6 × 10−5 443
Li1+xTi2−xAlx(PO4)3 0.5 Li+ — 444
POSS-PEO/LiTFSI 0.74 Li+ 5.6 × 10−6 445
POSS-IL-LiTFSI — Li+ 4.8 × 10−4 445
EMIM-Cl@UiO-67 0.65 Li+ 0.68 × 10−4 446
F-IL-GEL — Li+ 9.16 × 10−3 447
PIM-EA-TB — FSI− 0.5 × 10−2 448
Li1.5Al0.5Ge1.5P3O12 0.6 Li+ 4 × 10−4 449
GF@ZIF@PEO 0.38/1.31 Li+ 1.78 × 10−4 450
GF@PEO 0.74 Li+ 1.60 × 10−4 450
PEO-E 0.74 Li+ 2.03 × 10−4 450
MOF-LiCl 2 Li+ 2.4 × 10−5 451
MOF-LiBr — Li+ 3.2 × 10−5 451
MOF-LiI — Li+ 1.1 × 10−4 451
PVDF-HFP/LLZO/LiTFSI 3.41 Li+ 4.9 × 10−4 452
Mg2(dobdc) 1.1/1.3 Li+ 3.1 × 10−4 453
MOF 1.0/2.0 Li+ 7.41 × 10−4 454
Li–IL@UIO-67 1.2/1.6 Li+ 4.3 × 10−4 455
EMI-TFSA 1.16 Li+ 0.5 × 10−4 456
Li-IL@MOF 1.2 Li+ 3.0 × 10−4 457
SLE-H 0.6 Li+ 3.3 × 10−4 458
SIL/UIO-66 QSSE 0.6 Li+ 3.7 × 10−4 459
LIM-L 1.2 Li+ 1.0 × 10−4 460
MIL-121/Li 0.87 Li+ 5 × 10−4 461
Li6.75La3Zr1.75Ta0.25O12 — Li+ 1.2 × 10−4 462
LLZO-polymer 1.2 Li+ 10−4–10−3 463
PEO 0.74 Li+ 10−5 464
PAN 1.1 Li+ 1.07 × 10−5 465
PVDF 0.56 Li+ 7.27 × 10−4 466
PAN — Li+ 6.5 × 10−4 460
PEO 0.74 Li+ 1.4 × 10−3 467
PEGDA — Li+ 2.26 × 10−4 462
NaPF6-PEO 0.74 Na+ 5 × 10−6 468
NaTFSI-PEO 0.74 Na+ 4.5 × 10−6 469
NaFNFSI-PEO 0.74 Na+ 2 × 10−6 470
NaTCP-PEO 0.74 Na+ 6.9 × 10−5 471
NaClO4-PVP 1.2/1.3 Na+ 2.5 × 10−6 472
NaBr-PVA 0.34 Na+ 1.36 × 10−6 473
Na3Zr2Si2PO12 0.37 Na+ 1.2 × 10−3 474
Na3.1Zr1.95Mg0.05Si2PO12 0.65 Na+ 3.5 × 10 −3 475
Na3.4Zr1.6Sc0.4(SiO4)2(PO4) 0.54 Na+ 4 × 10−3 476
Na3PS4 0.4/0.5 Na+ 3.9 × 10−4 477
94Na3PS4·6Na4SiS4 0.5 Na+ 7.4 × 10−4 478
Na3P0.62As0.38S4 0.7 Na+ 1.46 × 10−3 479
Na2.9375PS3.9375Cl0.0625 0.4/0.5 Na+ 1.14 × 10−3 480
Na3Pse4 0.73 Na+ 1.16 × 10−3 481
Na2B10H10 0.98 Na+ 1 × 10−2 482
Na2(B12H12)0.5 (B10H10)0.5 0.58 Na+ 9 × 10−4 483
PVDF-HFP 0.3 Na+ 1 × 10−3 477
PEO/Na3.4Zr1.8Mg0.2Si2PO12 0.74/0.4 Na+ 2.4 × 10−3 484
PEO/KbrO3 0.74 K+ 7.74 × 10−8 485
PEO/KBr 0.74 K+ 5.0 × 10−7 486
PEO/CH3COOK 0.74 K+ 2.74 × 10−7 487
PVP/KIO3 — K+ 1 × 10−9 488
PPCB/KFSI 0.74 K+ 1.36 × 10−5 489
PEO/KNO3/KI 0.74 K+ 6.15 × 10−6 490
K2Fe4O7 0.4 K+ 3.5 × 10−2 491
PAF-220 1.3/1.9 Li+ 2 × 10−4 492
PAF-220-Li 1.9 Li+ 5 × 10−4 492
PEO-LiTFSI 0.74 Li+ 2.3[thin space (1/6-em)]× 10−4 493
Halide 0.452 Li+ 1.0 × 10−4 493
Molecular sieve 0.32 Na+ — 494
AO-PIM-1 0.45 K+, Na+, Li+, Cl− 4.4 × 10−4 495
PVDF-HFP-CPT-[PMPyr][TFSI] — Li+ 4.2 × 10−5 496
SSZ-13 + PEO + LiTFSI 0.74 Li+ 5.34 × 10−2 497
PEO + LiTFSI + ZYNa 0.74 Li+ 1.66 × 10−2 498
PHS — Li+ 1.36 × 10−5 499
Mn-PBA 1.0 Na+ 9.1 × 10−5 500
Cubic Na3PS4 0.4 Na+ 2 × 10−4 501
0.5
Na2.4Er0.4Zr0.6Cl6 0.4 Na+ 3.5 × 10−5 502
Na2.9Sb0.9W0.1S4 0.7 Na+ 4.1 × 10−2 503
Na3−xY1−xZrxCl6 0.2 Na+ 6.6 × 10−5 504
K2.2Ba0.4SbSe4 0.5 K+ 4.45 × 10−5 505
Na2MgZnTeO6 0.6 Na+ 1.4 × 10−5 506
Na2Mg2TeO6 0.55 Na+ 2.3 × 10−4 507
CPCSE 0.95 Li+ 5.2 × 10−4 508
COF 0.9 Li+ 1.71 × 10−4 509
MOF 0.34 Li+ 6.7 × 10−4 510
Carbon 0.7 Li+ — 159
PPS@TiO2 0.3/0.4 Li+, Na+ 0.6 to 3 × 10−4 61
MLM 0.56 K+ — 511
MLM-EDTA 0.6 K+ — 511

Table 4 Summary of the functional interlayers/separators with SNPCs
Materials SNPC size (nm) Ion types Ionic conductivity (S cm−1) Ref.
ZIF-7@PCF 0.3 Li+ 2.87 × 10−6/8.09 × 10−7 512
N,S-Mo2C/C-ACF 1.2 Li+ 1.04 × 10−3 513
N-Mo2C/C-ACF 1.4 Li+ 1.31 × 10−3
N-C-AMP 0.5/0.8 Li+ 1.28 × 10−3
Celgard — Li+ 7.6 ± 0.15 × 10−4 514
P(VDF-HFP) — Li+ 2.6 ± 0.04 × 10−4 515
Li-Nafion — Li+ 2.1 ± 0.04 × 10−5
Li-PFSD — Li+ 1.2 ± 0.02 × 10−4
Z-PE 2 Na+ 7.0 × 10−4 516
UiO-66 MMM 0.8 Li+ 0.67 × 10−3 147
LMA 1.1 Li+ 2.3× 10−6 149
MOF-coated LMA 1.1 Li+ 3.3 × 10−3
MOF@GO 0.9 Li+ 3.8 × 10−4 517
MOF@GO 0.9 Li+ 7 × 10−5
S-PE — Li+ 3.5 × 10−4 518
S-GE — — 8 × 10−4
S-CE/S-PE — — 3 × 10−3
S-CE/S-GE — — 8 × 10−3
LLZO 0.35 Li+ 1.03 × 10−3
LLZTO 0.4 Li+ 2 × 10−3
Azo-TbTh 0.36 Na+ 6.9 × 10−3 519
TA/FeIII MOF-PP 1.9 Li+ 1.1 × 10−3 520
Spim-SBF-0.53 0.55 Li+ 5.5 × 10−3 521
Spim-SBF-0.98 0.54 Li+ 1.2 × 10−2
Spim-SBF-1.4 0.54 Li+ 2.6 × 10−2
Spim-SBF-1.67 0.54 Li+ 6.5 × 10−2
Spim-SBF-1.86 0.52 Li+ 7.6 × 10−2
Spim-SBF 0.55 Li+ 2.8 × 10−2
TiO2/PP 0.38 Li+ 0.246 × 10−3 61
MIL-125(Ti)-PP/PE 0.77 Li+ 522
Cpim-1 0.55, 0.9 — 4 × 10−2 523
AO-PIM-1 0.55, 0.9 — 5 × 10−6
MGEs-S0 2 Li+ 9.77[thin space (1/6-em)] × [thin space (1/6-em)]10−5 524
MGEs-S1 2 — 1.80[thin space (1/6-em)] × [thin space (1/6-em)]10−4
MGEs-S2 2 — 5.37[thin space (1/6-em)] × [thin space (1/6-em)]10−4
MGEs-S3 2 — 6.76[thin space (1/6-em)] × [thin space (1/6-em)]10−4
PAN-C — Li+ 1.04 × 10−3 525
oxy-PAN-60 — Li+ 0.461 × 10−3
oxy-PAN-120 — Li+ 0.459 × 10−3
PVDF membrane — Na+ 7.38 × 10− 4 526
PVDF coated PP — Na+ 1.25 × 10−3 527
PEO-KFSI 0.74 K+ 2.7 × 10−4 528
Poly (PC) (PPC)-KFSI — K+ 1.36 × 10−5 529
Al2O3 — Na+ 6.3–6.8 × 10−3 530
NaAlO2 — Na+ 5.5–6.5 × 10−3
MATEPP/MMA/TFMA 0.74, 0.88 Na+ 6.29 × 10−3 531
LLTO/Li3PO4/polymer 1.2 Li+ 5.1 × 10−4 532
2D-SPE — Li+ 7.14 × 10−5 533
SHCPE 0.3 Li+ 8 × 10−5 534
P(MMA-AN-VAc) 0.57, 0.88, Li+ 1.54 × 10−3 535
P(AN-co-MA) 1.1 Li+ 6.7 × 10−4 536
PVN-GPE 0.78 — 2.6 × 10−4 537
PDMS/PVDF 0.56 — 1.17 × 10−3 538
PEO/LiBOB/LLZTO 0,74 Li+ 2.4 × 10−5 539
PEGBCDMA based PEM — Li+ 8 × 10−4 540
HPEI-PGC-PCL — Li+ 5.36 × 10−4 541
PEO — K+ 4.3 × 10−3
ZIF-67@PP 1.1 Li+ 1.64 × 10−3 542


image file: d3cs01043k-f6.tif
Fig. 6 (a) The summary of SPNC size and theoretical capacities for oxides, polyanions, PBAs, polymers, and (b) the summary of SPNC size and capacities for COFs, and S/Se cathodes in rechargeable batteries. (c) The summary of SPNC size and theoretical capacities of oxides, phosphides, sulfides, and polymers, and (d) the summary of SNPC size and capacities of MXenes, graphite-based materials, and hard carbons in rechargeable batteries. (e) The summary of SPNC size and ionic conductivity of polymers and MOF and, (f) the summary of SPNC size and ionic conductivity of NASICONs, sulfides, and hydrides for quasi-solid-state and solid-state electrolytes in rechargeable batteries. (g) The summary of SPNC size and ionic conductivity of polymers and oxides, and the summary of SNPC size and ionic conductivity for interlayers/separators in rechargeable batteries. (Red, green, and blue represent Li, Na, and K, respectively.)

The analysis of Fig. 6 shows that the determining factors of battery performance are not only the reaction kinetics but also the redox reaction activity and electronic conductivity of the materials. Ion diffusion is influenced by various factors, which can alter its kinetic activity and result in differences in battery performance. To help readers better understand the main role of SNPCs in rechargeable batteries, we have compiled and summarized some representative studies.

3.1. Electrochemical couplings of cathode materials with SNPCs

During the electrochemical reactions of rechargeable batteries, the cathode materials coupling with SNPCs serve as the hosts for cations. The redox reactions during charging and discharging processes always cause expansion and contraction of cathode materials, leading to undesired structure distortion, capacity fading, or even battery failure. The design of SNPCs in cathode materials can effectively facilitate ion diffusion and buffer volume changes, ensuring stable performance in rechargeable batteries. In this section, we outline the development and achievement of SNPC design in cathode materials, including intercalation-type and conversion-type materials (Fig. 7).
image file: d3cs01043k-f7.tif
Fig. 7 Illustration of the material structure with SNPC and the improvement strategies for (a) layered oxide cathodes, (b) polyanionic cathodes, (c) metal organic frameworks (MOF), and (d) covalent organic framework (COF) cathode materials.
3.1.1. Intercalation-type cathode materials.
3.1.1.1. Tuning SNPCs in layered oxide cathodes. Layered oxides with SNPCs, which have the chemical formula AMO2 (A normally represents Li, Na, K, etc.; M represents transition metals), are the most popular and well-developed cathode materials under massive production in the current battery industry.543 The first commercial intercalation-type layered oxide cathode material (LiCoO2) with SNPCs was introduced by Goodenough et al. This cathode material attracted global attention due to its high specific capacity (∼140 mA h g−1) and super high working voltage in organic electrolytes, as well as its excellent SNPC structure for excellent reversible Li+ insertion and extraction.544,545 Due to the high cost and geographic distribution issues of cobalt resources, many alternative layered oxides with abundant and low-cost transition metals, such as Ni, Mn, and Fe, were developed to form the layered LiMO2 structure.546–548 Moreover, Li was also replaced with Na or K to form layered NaMO2 and KMO2, which further reduces the cathode cost.549,550 However, with the decrease in the cost, the challenges that affect the structure stabilities, energy density and cycling performances of layered oxide cathode materials have become more prominent.551

The electrochemical energy storage mechanisms of the layered oxides exploit the SNPCs (∼0.4 to 0.7 nm) between the adjacent MO2 layers with edge-shared octahedral structures to allow the ‘A’ ion insertion/extraction during the cycling (Fig. 8a).552–555 This mechanism inevitably subjects the MO2 layers to structural distortion during repeated cation insertion/extraction for long-term cycling, leading to structure degradation and capacity fading. To maintain the performance of layered oxide cathodes, optimizing the size of the SNPC between the adjacent MO2 layers to increase the structure stability and reaction kinetics is essential. Therefore, except for the unary transition metal oxides, binary transition metal oxides (i.e., Mn/Ni, Mn/Fe, and Ni/Fe-based oxides) and ternary transition metal oxides (LiNixCoyMn1−x−yO2) were also intensively investigated to adjust the SNPC for fast ion diffusion kinetics, which increases both high electrochemical performance and low-cost advantages.556–558 Especially, the well-recognized ternary transition metal oxides were successfully applied in industry for large-scale battery packages.559,560


image file: d3cs01043k-f8.tif
Fig. 8 (a) Illustration of Mg doping into CoO2 layers. (b) The electron density differences of NaCoO2 (up) and Na(Co0.92Mg0.08)O2 (down). Reproduced from ref. 552 with permission from The Royal Society of Chemistry, Copyright 2015. (c) HRTEM image of the Na0.8Li0.27Mn0.68Ti0.05O2 Ti-doping cathode. Reproduced from ref. 553 with permission from American Chemical Society, Copyright 2020. (d) Schematic of the migration of Sn during the charge and discharge in the first cycle. Reproduced from ref. 554 with permission from American Chemical Society, Copyright 2022. (e) Illustration of the structure change during the charge of the typical NaMO2 (left) and high entropy NaMO2 (right), where the different colors of balls represent different elements and the different sizes of ball represent the different oxidation states. (f) Schematic of the change between P3 and O3 phase of high entropy NaMO2 during the discharge. Reproduced from ref. 561 with permission from Wiley-VCH, Copyright 2020.

The above SNPC designs alleviate the structure distortion and damage to some degree during ion diffusion. When the Li+ applies as the transport ion ‘A’, the diffusion of the ions inside the MO2 layers slowly distorts the order of the layered structure, which is still affordable for rechargeable batteries. However, if Na+ and K+ ions with much larger ionic radii are used as transport cations, the distortion of MO2 layers becomes severe, damaging the SNPC for the ion transport and significantly shortens the service life of the layered oxide cathodes. Further development of layered oxide materials with appropriate SNPCs to facilitate ion diffusion is required.

Currently, heteroatom doping is one of the most popular strategies to improve the electrochemical performances of layered oxide cathodes with SNPCs. Many elements, such as Mg, Ti, Cu, Sn, Zn, etc., have been investigated to optimize the interlayer distances between adjacent MO2 layers to stabilize the sub-nanochannels for ion diffusion. Su et al. demonstrated that the doping of the Mg element into CoO2 layers to form the Na(Co0.92Mg0.08)O2 cathode material could change the electron density and the valence state of oxygen, and effectively improve the Na+ ion diffusion, making it seven times faster by enlarging and stabilizing the SNPC with stronger Coulombic repulsion between the adjacent oxide layers (Fig. 8b).552 Hu et al. designed the Ti-substituted layered oxide cathode (Na0.8Li0.27Mn0.68Ti0.05O2) and indicated that the doping of the Ti element significantly relieved the structure distortion due to the Jahn–Teller effect, effectively stabilizing the SNPC at around 0.55 nm for ion transport (Fig. 8c).553 Furthermore, Chen et al. discovered that doping Cu element into MO2 layers enlarged the SNPC, preventing the shrinkage of the SNPC during the stripping of ions and increasing the mobility of Na+ ions. As a result, the Cu-substituted layered oxide cathode (Na0.67Mn0.6Ni0.2Co0.1Cu0.1O2) provided a capacity retention of 80.0% after 500 cycles at a high current density of up to 1 A g−1 in SIBs.562 Yun et al. indicated that the doping of Sn into lithium-rich layered oxide (Li2IrO3) effectively enlarged the SNPC and prevented structural collapse at high charging voltage, which benefited from the migration of Sn between the adjacent IrO2 layers during cycling (Fig. 8d).554 Moreover, Zheng et al. reported that Zn-substitution in MO2 layers of materials (K0.02Na0.55Mn0.70Ni0.25Zn0.05O2) reduced the anisotropic coupling between Mn4+ and oxidized O2−. This Zn doping strategy effectively stabilized the SNPC and provided the layered oxides with a superior cycling performance in K-ion batteries (97% capacity retention after 1000 cycles at 100 mA g−1).563

Besides the heteroatom doping strategy, an innovative method to stabilize SNPCs for ion diffusion is to further modify the MO2 layers through high-entropy design. The high-entropy design achieved by doping five or more elements to form a single phase and achieve a high entropy situation in layered oxides aims to provide super stable MO2 layers with homogenous internal stress for fulfilling the requirement of constructing effective SNPCs for fast ion transport. For example, Zhao et al. reported a high entropy sodium layered oxide (NaNi0.12Cu0.12 Mg0.12Fe0.15Co0.15Mn0.1Ti0.1Sn0.1Sb0.04O2) by mixing nine elements (Ni, Cu, Mg, Fe, Co, Mn, Ti, Sn, and Sb) into MO2 layers, which forms very stable SNPCs for fast Na+ ion diffusion.561 The high-entropy MO2 layers effectively delayed the phase transformation from O3 to P3 during cycling and maintained around 60% capacity in the O3 phase region (Fig. 8e and f). Consequently, the high-entropy layered oxide cathode delivered a high-capacity retention of around 83% after 500 cycles at 3C-rate in SIBs.

3.1.2. Tuning SNPCs in polyanionic and spinel-like cathodes. Polyanionic materials are another class of cathode materials coupled with SNPCs to realize redox reactions and cation storage in rechargeable batteries, showing different advantages compared to layered oxides. Applying polyanionic materials as cathodes has attracted great interest since the late 1990s, due to their high operating potential, stable SNPCs (i.e., 3D open framework) with less volume change and no phase transition, high thermal stability, and high safety.564–566 However, the drawbacks of polyanionic materials are their low specific capacities, low electronic conductivities, and poor low-temperature performances, which restrict their wide application under harsh environmental conditions.

Polyanionic materials AxMy(XO4)n consist of tetrahedral polyanionic units (XO4)n− and covalently bonded polyhedra MO6 (where A represents Li, Na, and K, X represents P, S, Si, etc., and M represents transition metals) with SNPCs of around 0.4 to 0.6 nm (Fig. 7b).311,567–571 This special structure of polyanionic materials leads to low conductivity because of the electronic interactions between (XO4)n− units during the charge/discharge process. According to X elements, the polyanionic materials can be classified as phosphate with X = P (orthophosphates, fluorophosphates, pyrophosphates, and mixed pyrophosphates), sulfates with X = S (AM(SO4)2, A2M2(SO4)3), and silicates with X = Si (A2MSiO4), which share similar SNPC structures.566 Similar to the development of layered metal oxides, the heteroatom doping strategy is the most popular way for improving the structure stability and ionic conductivity of the SNPCs inside polyanionic cathodes. However, to further improve the electronic and ionic conductivity of the polyanionic materials, doping modification is commonly accompanied by carbon coating and particle size control strategies.

The heteroatom doping strategy used in polyanionic cathode materials can be categorized as (1) doping in transport ion sites and (2) doping in transition metal sites. The doping in transport ion sites involves incorporating heteroatom ions with large radii and high conductivity to control the size of SNPCs and act as the ‘pillars’ to stabilize the ion diffusion paths. For instance, Lim et al. doped the K element into the Na3V2(PO4)3/C (NVP/C) cathode material at the transport Na+ ion sites to form K0.09-NVP/C.567 Due to the larger ionic radius of K than that of Na, the K elements act as the pillar and enlarge the SNPC for Na diffusion and accommodation (Fig. 9a). With the pillar effect provided by K doping, the doped cathode material showed a significantly smaller volume change than that of the pristine materials (reduced from 9.02% to 6.64%) (Fig. 9b). Consequently, the K0.09-NVP/C cathode achieved a high reversible capacity of 83 mA h g−1 after 200 cycles at the 1C-rate.


image file: d3cs01043k-f9.tif
Fig. 9 (a) Illustration of doping K+ ions into transport ion sites in Na3V2(PO4)3. (b) The volume shrinkage between the charge and discharge of Na3V2(PO4)3/C with different K contents. Reproduced from ref. 567 with permission from The Royal Society of Chemistry, Copyright 2014. (c) Illustration of doping Mn+ ions into transition metal sites in Na3V2(PO4)3. (d) HRTEM image of the Mn doped Na3V2(PO4)3 with 0.373 nm SNPC. Reproduced from ref. 568 with permission from The Royal Society of Chemistry, Copyright 2016. (e) Schematic of the synthesis process for the NVPF@CNT-1 composites. (f) TEM image of the NVPF@CNT-1 composites. Reproduced from ref. 569 with permission from Elsevier B.V., Copyright 2023. (g) Illustration of the Na0.877MnSnO4 structure (h) Schematic showing the bond lengths of (Mn1/Sn1)O6 and (Mn2/Sn2)O6 octahedra inside the Na0.877MnSnO4.by viewing along the a-axial in Figure. (g) Reproduced from ref. 572 with permission from Springer Nature, Copyright 2020. (i) TEM images of the LNMO cathode with 1.0% AlF3-coating. Reproduced from ref. 573 with permission from Elsevier B.V., Copyright 2020.

Doping heteroatom ions in transition metal sites aims to stabilize the framework structure and increase the conductivity of the SNPC. For example, Shen et al. designed a NASICON-type cathode (Na3V2−xMnx(PO4)3/C) by doping Mn2+ ions into the transition metal sites of Na3V2(PO4)3/C (Fig. 9c).568 The doping of Mn2+ effectively enlarged the SNPC (Fig. 9d), stabilized the crystal structure, and promoted the ionic and electronic conductivity due to the large radius of Mn2+ (0.91 Å) compared to that of V3+ (0.64 Å) and self-polarization ability. As a result, the Mn-doped cathode materials achieved a high reversible discharge capacity of 92.5 mA h g−1 after 100 cycles at 5C, which is around 95.9% capacity retention.

Besides heteroatom doping, another pathway to enlarge the ion diffusion channels and increase the ion conductivity of SNPC inside the polyanionic materials is by combining them with high-conductive materials. Li et al. introduced carbon nanotubes into Na3V2(PO4)2F3 (NVPF), which significantly improved the NVPF crystallinity and ion transport (Fig. 9e and f).569 The NVPF@CNT cathode offered a super cycling performance with 77.3% capacity retention after 5000 cycles at 20 C. Guo et al. designed a heterogeneous NASICON-type composite by combining Na3V2(PO4)3 with Na3Fe2(PO4)(P2O7) (NVFPP).570 The introduced NVFPP phase provided enlarged SNPCs. As a result, after 500 cycles, their cathode delivered a 78.6% capacity retention rate at 5C.

Moreover, the conventional polyanionic cathode usually contains the V element, which is expensive and bio-toxic. Substituting the V element with other cheap and eco-friendly transition metals while maintaining the stability and conductivity of SNPCs in polyanionic materials is also worth studying for practical applications. Chen et al. reported a Na3.32Fe2.34(P2O7)2/C composite as the cathode material with obvious eco-friendly advantages. Their V-free cathode also provided sufficient SNPCs for Na+ ion transport.571 Later, they further designed an eco-friendly cathode with a tenable nanosized Na4Fe3(PO4)2(P2O7)/C (NFPP-E) composition, exhibiting enhanced air stability and suitability for all-climate operation conditions. After 4400 cycles at 20C, the SNPC of NFPPE could be maintained at around 0.5 nm.311

The robust 3D frameworks provide the polyanionic cathode with a more stable SNPC than that of the conventional layered oxide materials. In addition, the spinel engineering design can provide a 3D framework for transition metal oxides and form AM[M2]O4 (M = Fe or Mn) spinel-like cathode materials, which is worth mentioning here. In spinel-like materials, the [M2]O4 units form a spinel framework that stabilize the whole structure, allowing the reversible alkali ion insertion and extraction. The spinel-like cathodes cannot offer a high energy density as the traditional layered oxide materials, but they can provide relatively lower costs and high safety like the polyanionic cathode.574 Spinel-like cathodes with 3D SNPCs have been commercialized in LIBs, and their application in sodium-ion batteries is currently attracting researchers.

A popular strategy to improve the electrochemical performance and SNPC stability of spinel-like cathodes is the chemical substitution to form the A[Mn2−xMx]O4, where M is the substituted metal. For example, the well-known LiNi0.5Mn1.5O4 (LNMO) is Ni-substitution modified LiMnO4 spinel-like cathodes. The Ni substitution significantly improves the cathode's capacity (∼130 mA h g−1) and operating voltage (∼4.7 V vs. Li). Furthermore, due to the high voltage and less involvement of the Mn4+ redox reaction during cycling, the Jahn–Teller distortion is effectively reduced in the LNMO cathode, stabilizing the SNPC for ion transport.574,575 Chiring and Senguttuvan reported a spinel-NaMnSnO4 cathode (Na0.877MnSnO4) for SIBs with Sn substitution (Fig. 9g).572 The Sn4+ in the spinel framework effectively reduced the ion diffusion barriers, suppressed the Jahn–Teller effect with no observed change in the axial bonds in Mn3+ centers (Fig. 9h), and thus promoted highly stable SNPCs (∼0.5 nm).

Furthermore, the coating strategy is also commonly used on the surface of spinel-like cathodes against transition metal dissolution, impedance rise, and capacity fade under harsh operating conditions such as high temperatures. For instance, Chu et al. coated AlF3 onto the LNMO.573 They found that the 1 wt% AlF3-coating (∼5.2 nm) can effectively prevent the transition metal dissolution of the spinel-like cathode and maintain the structure integrity of SNPCs (∼0.47 nm) (Fig. 9i). Consequently, the 1 wt% AlF3-modified LNMO cathode delivered enhanced cycling stability with a high-capacity retention of around 81.7% after 100 cycles at 0.2 C and 55 °C, whereas the pristine LNMO cathode only delivered 70.1% capacity retention.

3.1.3. Tuning SNPCs in MOF- and COF-based cathodes. Metal–organic frameworks (MOFs) are another type of popular cathode materials applied in alkali metal ion rechargeable batteries. After decades of investigation, more than twenty thousand types of MOFs have been reported.576 MOFs have structures with metal ion centers (Fe, Co, Ni, Cu, Mn, etc.) and organic ligands (polyamines, carboxylates, hydroxyl, etc.), which give the advantages of tenable porous structures, high surface areas, and homogeneous metal sites to these materials and thus benefit the fast kinetic and ion diffusion (Fig. 7c).577,578 However, this structure also presents significant drawbacks. First, MOFs cathode usually deliver a low specific capacity and limited electric conductivity due to limited redox-active sites that are only based on the metal center (M(n+1)+/Mn+) in the framework. Second, the porous framework not only benefits the ion insertion/extraction by providing sub-nanochannels but also leads to easy structure collapse during the charge/discharge and low volumetric energy density. Third, the existence of crystalline water and lattice vacancies inside the MOFs can lead to low coulombic efficiency and poor cycling stability.

The first MOF-based cathode material for LIBs was MIL-53(Fe) reported by Ferey et al. in 2007. The MIL-53(Fe) cathode provided a low capacity of around 70 mA h g−1 by only exploiting the Fe3+/Fe2+ redox reduction inside the frameworks.358 After that, researchers endeavor to increase the redox-active sites and stabilize the SNPC of MOFs by designing the organic ligands, selecting the metal node, and modifying the nanostructure.

The organic ligand design aims to create redox-active sites on the organic linkers of the MOFs and maintain the structure stability simultaneously. For example, Peng et al. used tricarboxytriphenyl amine (TCA) to replace the conventional quinone-type ligands and form the Cu-TCA cathode for LIBs (Fig. 10a).579 In addition to the redox change of Cu2+/Cu+ in the center of MOFs, the N atoms in TCA also provide redox-active sites by transitioning between neutral and cationic forms in the voltage range from 3.8 to 4.3 V. In the meantime, the TCA ligands also offered good stability to support the frameworks and sub-nanochannels (∼0.5 nm).


image file: d3cs01043k-f10.tif
Fig. 10 (a) Illustration of the Cu-TCA structure. Reproduced from ref. 579 with permission from American Chemical Society, Copyright 2016. (b) Schematic of the crystal structure of the K2FeFe(CN)6 (up) and Ni doping KNi0.05Fe0.95Fe(CN)6 (down). Reproduced from ref. 580 with permission from American Chemical Society, Copyright 2019. (c) Schematic of the high entropy Nax(FeMnNiCuCo)–[Fe(CN)6] crystal structure. Reproduced from ref. 581 with permission from Wiley-VCH, Copyright 2021. (d) Illustration of the energy storage mechanism in the NiDI cathode. Reproduced from ref. 582 with permission from Wiley-VCH, Copyright 2018. (e) Schematic of the 2D Cu-THQ-MOF structure. Reproduced from ref. 583 with permission from Wiley-VCH, Copyright 2019.

The metal node design involves heteroatom doping and high entropy design. The heteroatom doping can effectively change the electronic state of the original metal node, promote the ion transport inside sub-nanochannels, stabilize the frameworks, and enhance the cycling voltage. For instance, Huang et al. successfully doped Ni ions into the Prussian blue analogue (PBA) K2FeFe(CN)6 to form KNi0.05Fe0.95Fe(CN)6 with a sub-nanochannel of around 0.5 nm, which effectively changed the electronic state of the Fe ions and enhanced K ion diffusion inside the channels (Fig. 10b).580 The Ni substitution promoted the redox reaction of Fe2+C6/Fe3+C6 and thus improved the capacity from 40 to 53 mA h g−1 at high charge voltage plateaus. Consequently, their enhanced MOF-based cathode provided a stable cycling performance with 83.1% capacity retention after 300 cycles at 0.1 A g−1. A high entropy design acts the same way as heteroatom doping but with five or more elements sharing the same lattice sites, which further stabilizes the ion diffusion pathways and framework structure. Ma et al. reported a high-entropy design on the PBA (Nax(FeMnNiCuCo)–[Fe(CN)6]) with the sub-nanochannels of around 0.6 nm and employed it as the cathode in SIBs (Fig. 10c).581 The high entropy (HE) structure provided a nearly zero-strain operation during the desertion/insertion of Na+ ions, and thus offered a better structure stability. As a result, the HE–BPA cathode delivered 94% capacity retention after 150 cycles at 0.1 A g−1.

There have been relatively fewer studies on modifiying thestructure morphology of MOF cathodes than the above two strategies, but it can effectively increase the intrinsic electronic conductivity, provide a much higher density of redox-active sites, promote ion diffusion, and offer stable sub-nanochannels for the intercalation of large-size alkali metal ions. For example, Wada et al. reported a 2D-MOF cathode material (NiDI) applied in LIBs.582 Their 2D-MOF cathode has a bis(diimino)nickel framework with a high density of redox-active sites and several redox states provided by both organic ligands and metal ions. Especially, the energy storage mechanism in this 2D-MOF cathode involves the insertion/desertion of both cations (Li+) and anions (PF6−) (Fig. 10d), providing a high specific capacity of around 155 mA h g−1 at a current density of 10 mA g−1. Moreover, Jiang et al. designed a 2D-MOF cathode with a copper–benzoquinoid framework (Cu-THQ MOF) and sub-nanochannels (∼1 nm) using a similar energy storage mechanism with cation and anion insertion/desertion (Fig. 10e).583 Due to the highly porous structure and intrinsic redox characteristics of Cu-THQ, their 2D MOF cathode provides abundant and highly conductive channels for promoting Li+ ion transport. As a result, the 2D Cu-THQ MOF achieved a very high capacity of 387 mA h g−1 during the second cycle and maintained a stable cycling performance with 340 mA h g−1 capacity retention after 100 cycles at 50 mA g−1.

Covalent organic frameworks (COFs) are similar to MOFs, but instead of having metal nodes inside the framework, COFs are formed by covalently bonded organic ligands (Fig. 7d). COFs, as a kind of crystalline porous material, possess abundant well-defined directional meso- and nano-size channels for ion transport, rich redox-active sites, and a stable framework structure, offering them excellent ion diffusion capacity. However, the drawbacks of limited specific capacity and poor electronic and ion conductivity similar to MOFs pose challenges to the application of COFs as cathodes in alkali metal ion batteries. The COF material was first reported by Yaghi's group in 2005 and named COF-1 and COF-5.584 After ten years, Jiang's group first designed the COF-based cathode applied in Li-ion batteries by growing mesoporous COFs (DTP-ANDI-COF with channel size of around 5.06 nm) on carbon nanotubes (CNT).53 However, their cathode only delivered a capacity of 67 mA h g−1 at 200 mA g−1.

In recent years, several strategies, such as SNPC design, pore wall decoration, and morphology modification (exfoliation, crystallinity regulation and orientation, secondary material combination, etc.) have been developed to improve the performance of COF-based cathodes. Generally, the large pores can provide more free space for ion transport, and small pores can be more conductive due to the interaction between the pore wall and transport ions. Combined with the development of enriching redox-active centers on the pore walls, COF-based cathodes can offer significantly enhanced electric and ionic conductivities and excellent electrochemical performance. For example, Shi et al. developed a nitrogen-rich COF with triquinoxalinylene and benzoquinone units (TQBQ-COF).355 The TQBQ-COF has a honeycomb-like and AB stacking structure with well-defined directional SNPCs of around 1 nm (Fig. 11a and b). The SNPC provided the TQBQ-COF with a high electronic conductivity of 1.973 × 10−9 S cm−1, and the abundant N atoms on the pore walls effectively decrease the energy gap between the lowest and highest occupied molecular orbitals, thus promoting the ionic and electronic conductivity. Consequently, the TQBQ-COF cathode in SIBs provided a very high capacity of 327.2 mA h g−1 and around 89% capacity retention after 400 cycles at 0.1 A g−1.


image file: d3cs01043k-f11.tif
Fig. 11 (a) Schematic of the TQBQ-COF chemical structure, electrochemical redox mechanism, and the theoretical capacity of 515 mA h g−1. (b) Illustration of the AB stacking model of TQBQ-COF layers with a packing distance of 3.07 Å. Reproduced from ref. 355 with permission from Springer Nature, Copyright 2020. (c) Schematic of the DAAQ-TFP-COF exfoliation to DAAQ-ECOF as cathodes for LIBs. Reproduced from ref. 585 with permission from American Chemical Society, Copyright 2017. (d) Illustration of the one-layer-conjugated structure of the BQ1-COF and the elements inside the skeleton. (e) TEM images of the BQ1-COF showing the interlayered spacing between the adjacent 2D COF nanosheets. Reproduced from ref. 353 with permission from Elsevier Ltd, Copyright 2020. (f) Schematic of the synthesis route and structure of DAPT-TFP-CPF. Reproduced from ref. 586 with permission from American Chemical Society, Copyright 2018. (g) Schematic synthesis process of the SSWM@Mn3O4 cathode material. (h) SEM image of SSWM@Mn3O4. (i) HRTEM image of the SSWM@Mn3O4 cathode showing the SNPCs. Reproduced from ref. 587 with permission from The Royal Society of Chemistry, Copyright 2018.

The most popular structural morphology modification of COFs is the 2D morphology obtained through direct synthesis or exfoliating from 3D COFs, which can effectively enlarge their surface area, improve the accessibility of redox-active sites, and promote ion diffusion. For example, Wang et al. exfoliated the anthraquinone-based COF (DAAQ-TFP-COF) to redox-active ECOF (DAAQ-ECOF), providing an approximately three times faster Li+ diffusion rate than its non-exfoliated counterpart due to the significantly shortened ion transport paths (Fig. 11c).585 Wu et al. reported a 2D COF cathode (BQ1-COF) through the polycondensation reaction between C6N2O2 and C8N4O2 to deliver C[triple bond, length as m-dash]O and C[triple bond, length as m-dash]N groups (Fig. 11d).353 This design provided SNPCs of ∼1 nm and minimized the inactive group inside the BQ1-COF (Fig. 11e). As a result, the BQ1-COF cathode in LIBs offered an excellent reversible capacity of around 502 mA h g−1 at 0.05C and very stable cycling performance with 81% capacity retention after 1000 cycles at 1.54 A g−1.

It is worth mentioning that conjugated microporous polymers (CMPs), from some perspectives, can be regarded as amorphous COFs. They are another type of microporous material with promising potential as cathode materials in alkali metal ion batteries.588 CMPs have a very similar composition to COFs, but instead of a crystalline structure, CMPs are amorphous. This characteristic not only makes the CMPs more challenging to characterize and process than COFs but also gives CMPs the advantages of high thermal and chemical stability. Moreover, CMPs have a highly conjugated structure and extended π-conjugation system, which benefits the stability of the framework and the ion transport in SNPCs. For instance, Li et al. designed a CMP-based cathode applied in SIBs by cross-linking 1,8-diaminopentacene-5,7,12,14-teraone (DAPT) and 1,3,5triformylphloroglucinol (TFP) to form π-conjugated porous frameworks (DAPT-TFP-CPFs) (Fig. 11f).586 The DAPT-TFP-CPF cathode resulted in fast ionic diffusion thanks to the sub nanometer pores (∼0.8 nm) and the lamellar structure. Furthermore, the extended π-conjugation backbones effectively stabilized the SNPC inside the framework, improving the charge transport stability in cycling. As a result, the DAPT-TFP-CPF cathode exhibited a very stable cycling performance with a high-capacity retention of 145 mA h g−1 after 1000 cycles at 100 mA g−1.

It is worth noting that, in addition to monovalent (Li+, Na+, and K+) ion batteries, tuning of SNPCs in cathodes of multivalent ion (Zn2+, Mg2+, Ca2+, Al3+) batteries can also remarkably enhance their electrochemical performance. Compared with monovalent ion batteries, multivalent ion batteries offer advantages including more affordable cost, higher safety and lower environmental impact. However, challenges such as unstable electrode materials, poor ion diffusion kinetics, and a narrow electrochemical stable potential window hinder the application of multivalent ion batteries. Tuning SNPCs in the cathode materials, through methods such as the abovementioned heteroatom doping, size regulation, pre-intercalation, defect engineering, and freestanding configuration design, could pave the way for breakthroughs and potential applications in these multivalent ion batteries.589 For instance, Zhu et al. designed a freestanding design Mn-based cathode (SSWM@Mn3O4) for Zn-ion batteries using stainless steel welded mesh (SSWM) to grow the flower-like Mn3O4 (Fig. 11g and h). The cathode provided SNPCs of around 0.61 nm (Fig. 11i), allowing Zn2+ intercalation/extraction and effectively improving the cycling stability up to 500 cycles at 500 mA g−1.587 Nam et al. introduced crystal water into the SNPCs (∼0.7 nm) inside the layered manganese oxide to form an enhanced stable cathode for Mg-ion batteries at high voltage (2.8 V vs. Mg/Mg2+).590 The Mg-ion batteries with their cathode offered 62.5% capacity retention after 10[thin space (1/6-em)]000 cycles. Furthermore, Li et al. reported a Co3S4 microsphere cathode material for Al-ion batteries.591 The Co3S4 microsphere provided about 0.2 nm SNPCs for Al3+ intercalation/deintercalation and a porous structure for electrolyte penetration. With this unique structure design, the Al-ion batteries with Co3S4 microsphere cathodes delivered a reversible capacity of 90 mA h g−1 after 150 cycles at 50 mA g−1.

3.1.4. The design of SNPCs in conversion-type cathode materials. Cathode materials based on conversion reactions, such as S and selenium (Se) have been intensively investigated in the past decades due to their high theoretical specific capacities (Fig. 12a and b).592,593 At room temperature, the reactions of S/Se cathodes during charge and discharge processes involve multiple phase-transition steps with the formation of soluble polysulfides (PSs)/polyselenides (PSes) as intermediates. The major challenges faced by these conversion-type cathode materials include poor electronic conductivity of the cathode materials, the “shuttle effect” of PS/PSe intermediates, significant volume variations during cation insertion and extraction, and sluggish reaction kinetics. To solve the abovementioned issues, a strategy which involves introducing multifunctional hosts with appropriate SNPC design to form S/Se-based nanocomposites has made significant achievements so far. The host materials with SNPCs, such as nanostructured porous carbon materials, provide sub-nanoscale conductive networks that significantly increase the electronic conductivity of the S/Se cathodes and buffer the volume change during cation insertion and extraction in different battery systems (e.g., Li/Na/K–S batteries and Li/Na/K–Se batteries). More importantly, the confinement effects of SNPC suppress the “shuttle effect” (i.e., restricting the dissolution of PS/PSe intermediates into the electrolytes), reducing the loss of active materials and the side reactions on the anode counterparts.
image file: d3cs01043k-f12.tif
Fig. 12 (a) Schematic illustration showing the mass/charge transfer at the cathode–electrolyte interface in liquid-electrolyte-based Li–S batteries. S is reduced to generate LiPSs, which could dissolve in the liquid electrolyte and shuttle between two electrodes. Reproduced from ref. 592 with permission from Wiley-VCH, Copyright 2017. (b) Typical charge/discharge profile of Li–S batteries and chemical evolution of LiPS species at different states of (dis)charge. Reproduced from ref. 593 with permission from Wiley-VCH, Copyright 2015. (c) S@SWNT viewed along the axis and side length as computed with dispersion-corrected DFT. Reproduced from ref. 594 with permission from American Chemical Society, Copyright 2018. (d) Schematic illustration of the fabrication processes of the sulfur intercalated reduced graphite oxide by two wet chemical methods. Reproduced from ref. 595 with permission from Wiley-VCH, Copyright 2016. (e) Schematic illustration of Na+ transmission in the internal structures of MMPCS-800@S. Reproduced from ref. 596 with permission from Wiley-VCH, Copyright 2022. (f) Microporous carbon (MPC) with SNPCs as a host for the Se cathode. Reproduced from ref. 366 with permission from Elsevier B.V, Copyright 2019. (g) Atomic model configurations showing the interactions between Na2Fe[Fe(CN)6] and polysulfide Li2Sx (x = 8, 6, 4, and 2). Reproduced from ref. 597 with permission from Wiley-VCH, Copyright 2017.

Carbon-based materials with SNPCs, ranging from 1D CNTs to 2D graphene and 3D porous carbon spheres, are the most intensively investigated hosts for sulfur cathodes.598–604 Compared with the traditional porous structure design, carbon materials with SNPCs have been found to show unusual electrochemical reaction behaviours in the S cathode. For instance, two types of single-walled carbon nanotubes (SWCNTs) with different diameters were prepared as hosts to study the 1D confinement effects of SNPCs on the lithiation/delithiation process of S cathodes in Li–S batteries (Fig. 12c).594 The long-chain S diradicals were formed inside the as-prepared SWCNTs via infusion and polymerization processes. The SWCNTs prepared by the electric arc method (EA-SWCNTs) have an average diameter of ∼1.55 nm with an average inner van der Waals diameter of 12.1 Å for EA-SWNTs. The other SWCNTs obtained from high-pressure carbon monoxide (HiPc0-SWCNTs) have an average diameter of ∼1.0 nm with an average inner van der Waals diameter of 6.6 Å for HiPco-SWNTs. The conventional solid–liquid–solid reactions occurred in the S cathode with EA-SWCNTs in both tetraethylene glycol dimethyl ether (TEGDME)-based and 1,4,7,10,13-pentaoxacyclopentadecane (15-crown-5)-based electrolytes. In contrast, unusual continuous solid-state reactions were observed in S cathodes with HiPc0-SWCNTs. The distinct electrochemical behaviors were explained because of the unique SNPC feature of HiPc0-SWCNTs. The largest van der Waals dimensions of the solvated [Li(TEGDME)]+ and [Li(15-crown-5)]+ ions are 10.87 Å and 11.34 Å, respectively, which are smaller than the diameter of EA-SWCNTs but larger than that of HiPc0-SWCNTs. Therefore, the S cathode with EA-SWCNTs showed similar phase transformation processes to the normal S cathodes as the solvated [Li(TEGDME)]+ and [Li(15-crown-5)]+ ions could still access the inner channels of EA-SWCNTs and react with S. However, the sulfur inside HiPc0-SWCNTs could not directedly react with the solvated cations. Instead, the sulfur was reduced through the wall of SWCNTs via an out-of-plane π-electron interaction.

A 2D graphene material with an SNPC structure (i.e., an interlayer spacing of ∼0.4 nm) was developed as the S host for Li–S batteries. Through a solvothermal method or an interlamellar reaction method, S molecules were intercalated into the graphene interlayers to form a S/reduced graphene oxide (S/RGO) composite (Fig. 12d). The electrochemical performance of the S/RGO composite showed similar phase-transfer behaviors to that of small S molecules in a carbonate-based electrolyte (1 M LiTFSI in DOL/DME), demonstrating that this 2D SNPC design effectively facilitates phase transformation during electrochemical conversion reactions from S to lithium sulfide.595 Another carbon host with 3D SNPC design, micro-mesoporous carbon nanospheres (MMPCS), was reported by Wu et al. for RT Na–S batteries.596 Porous carbon spheres with tailored continuous carbonaceous pores ranging from micrometers to sub-nanometers were synthesized as S hosts (Fig. 12e). Similarly, the design of the 3D SNPC carbon host has been widely applied in the Li/Na–Se battery system. Aboonasr Shiraz et al. synthesized microporous carbon (MPC) with SNPCs of around 1 nm from the PVDF precursor. When employed as a host for Se in Li–Se batteries, the MPC could confine the LiPSes and effectively reduce the side reaction between the Se cathode and the electrolyte (Fig. 12f).366 As a result, the Li–Se batteries exhibited a reversible capacity of 508.8 mA h g−1 after 100 cycles at 0.1C. Furthermore, Zhang et al. designed interconnected conductive polyaniline (PANI) coated hierarchical porous carbon (i-PANI@NSHPC) to accomodate up to 65.7 wt% Se in SNPCs applied as cathodes in Na–Se batteries.595 This cathode offered an excellent reversible capacity of 617 mA h g−1 at 0.2C after 200 cycles due to the enhanced electronic and ionic conductivity even at high Se areal mass loading. The S material confined within those multifunctional channels is highly reactive with full access to Na+ ions. In addition, a highly Na-ion conductive and continuous solid interphase containing sodium sulfide was conformally formed inside the pores, significantly stabilizing the cathode/electrolyte interfaces, and promoting the kinetics of S-involved redox reactions. In addition to the carbon-based materials, MOF and COF 3D frameworks with SNPCs have also been used as hosts for S/Se cathodes. For instance, Su et al. reported a 3D SNPC framework by using the Prussian blue analogues (PBAs) coated with PEDOT (Na2Fe[Fe(CN)6]@poly(3,4-ethylenedioxythiophene)) to capture the sulfur and polysulfides for Li–S batteries (Fig. 12g).597 The MOF-based host significantly enhanced the confinement of soluble Li polysulfides through the Lewis acid–base interaction between the rich open metal sites in the SNPC of the MOF host and polysulfide anions. Finally, their S@Na2Fe[Fe(CN)6]@PEDOT cathode exhibited a stable cycling performance with a capacity retention of 697 mA h g−1 after 100 cycles at 2C. Recently, redox-active polymers have emerged as an attractive option among organic materials for seving as S hosts, offering advantages such as cost-effectiveness, diversity, good processability, unique electrochemical properties, and precise tuning for energy storage applications.605e.g., Park et al. the reported S-rich polymers on the gram scale derived from the vulcanization of trithiocyanuric acid-based materials. The resulting cathode material exhibited stable cycling over 450 cycles, with a retention rate of 83% and excellent rate performance with a current density of C/10 (1210 mA h g−1) to 5C (730 mA h g−1). This performance is attributed to strong bonding energy between redox-reactive polymers and S.606

In conclusion, the SNPCs play an important role in both intercalation-type and conversion-type cathode materials. The size, distribution, and amount of SNPCs inside cathode materials directly affect the accommodation and transportation of metal ions and the confinement of active materials. The advanced material designs to modify the morphology and catalytic functions of the SNPCs in cathodes can further improve the electrochemical performance of rechargeable batteries.

3.2. Electrochemical couplings of anode materials with SNPCs

In addition to their significant role in cathode materials, SNPCs also play critical roles in various anodes. It has been widely reported that the SNPCs can be engineered on the surface of the anodes as artificial solid–electrolyte interphase (SEI) layers, which could facilitate feasible selective ion transport, avoid excessive SEI formation, and alleviate the volume changes upon cycling. The SNPCs can also be engineered inside the nanoparticles of the anode materials, which could generate fast ion transportation to achieve enhanced rate performance. Hereby, the rational design of SNPCs for various types of anode materials has been systematically sorted and discussed.
3.2.1. Intercalation-based anodes. The intercalation process to insert diversified working ions into the interlayers of the crystal planes represents a wide variety of anode materials for versatile battery systems, of which the intercalation of Li+ into the graphite layers has been the fundamental working mechanism for commercial LIBs since 1990s. Carbonaceous materials, conjugated microporous polymers CMPs, titanates, TMDs, and Mxenes generally working as intercalation-based anodes, which exhibit vital importance in practical batteries due to their outstanding cycling performance. These anodes with SNPCs could deliver enhanced electrochemical performance, as a result of the benefits derived from the SNPCs.
3.2.1.1. Carbon-based anodes. Carbon-based materials including graphite, graphene, graphdiyne, hard carbon, soft carbon, carbon nanotubes (CNTs), etc. are all promising anode materials for alkali metal ion batteries. Numerous research studies have incorporated SNPCs into these carbonaceous anodes to achieve outstanding electrochemical performance. For instance, hard carbon has been widely recognised as a versatile anode for alkali metal batteries, the abundant SNPCs in the hard carbon resulting from randomly stacked graphitic layers render it capable of accommodating more ions both between the layers and also in the sub-nano pores, as presented in Fig. 13.607 It is worth noting that, the SNPCs in the hard carbon anodes could contribute a considerable amount of low voltage capacity, which is achieved probably by pore-filling of quasi-metallic Na clusters.608 The size of the SNPCs in the carbon anodes is crucial for the accommodation of alkali metal ions, especially for K+ with a larger diameter. For instance, a nitrogen-doped porous carbon (NPC) has been reported for KIBs, the SNPC size was enlarged from 0.335 to 0.383 nm due to the decomposition of the Na2CO3 during pyrolysis, as presented in Fig. 13b.609 As a result, the NPC delivered a high reversible capacity of 382 mA h g−1 for more than 500 cycles and good rate capability of 185 mA h g−1 at 10.0 A g−1. The improvements in the performance were attributed to the surface driven K+ storage by the enlarged SNPCs, as well as enhanced K+ affinity provided by N-doping sites. Similarly, it has been reported that the interlayer spacing of the graphitized domains in the hard carbon can be expanded to 0.42 nm by incorporating graphene oxide in the precursors; the expanded spacing could serve as fast K+ diffusion channels.394 As a result, high reversible capacity, excellent rate capability, and good cycling stability have been achieved for PIBs.
image file: d3cs01043k-f13.tif
Fig. 13 (a) Schematic illustration of the accommodation of Li+/Na+/K+ in hard carbon anodes through intercalation, adsorption, and pore filling in the SNPCs.603 Reproduced from ref. 603 with permission from Wiley-VCH, Copyright 2019. (b) Schematic illustration of the synthesis of nitrogen doped porous carbon and its working mechanism for K+ storage.594 Reproduced from ref. 594 with permission from Wiley-VCH, Copyright 2018. (c) Illustration of the working mechanism of graphdiyne for Li+ storage.596 Reproduced from ref. 596 with permission from Wiley-VCH, Copyright 2018. (d) Schematic illustration of the intercalation of Li+/Na+/K+ in titania sheets.610 Reproduced from ref. 610 with permission from Wiley-VCH, Copyright 2019. (e) Schematic illustration of the synthesis of MXene and the structural stability of the SNPC upon Na+ intercalation/deintercalation.387 Reproduced from ref. 387 with permission from Nature Publishing Group, Copyright 2015. (f) Schematic illustration of the synthesis of CMP as a universal and ultra robust organic electrode for diverse aqueous battery chemistry.611 Reproduced from ref. 611 with permission from Wiley-VCH, Copyright 2018.

Alternatively, Mai and colleagues reported soft carbon nanosheets with SNPCs of 0.45–0.90 nm, exhibiting a high surface area, large pore volume, and rich defective sites.610 Excellent capacitance-dominated Na+ and K+ storage was achieved, as a result of the extra ion storage sites derived from abundant SNPCs. Significantly, Li and co-workers reported the synthesis of graphdiyne with a channel of 0.546 nm as anode materials for LIBs, which could be expanded to 0.596 nm upon Li+ intercalation, as illustrated in Fig. 13c.611 The self-expanding channel could reduce the energy barrier and electrostatic repulsion for Li+ transport, facilitating fast solid-state Li+ diffusion. Benefiting from self-expanded SNPCs, graphdiyne delivered higher capacity, better cycling stability, enhanced capacity retention, and broaden working temperature range at high current rates. Besides, S-doped hollow carbon nanosheets,612 hard carbon,613 and N,P co-doped hollow microporous carbon,614 have also been reported to deliver enhanced electrochemical performance due to the incorporation of SNPCs, as a result of enhanced ion transportation, improved electrolyte wettability, and decreased SEI thickness. Furthermore, the advantage of the SNPCs in various carbonaceous materials including graphite nanomesh,615 holey graphite,616 and microporous carbon617 have also been revealed via theoretical calculations, further demonstrating the significance of SNPCs for various carbon anodes.


3.2.1.2. Titanates, TMDs, and MXenes. Titanates are widely used intercalation-based anode materials for alkali metal ion batteries, which exhibit good cycling stability and excellent rate performance.618 Various TMDs and MXenes with a well aligned layer structure could also deliver excellent intercalation performance for rechargeable batteries, benefiting from the stable layered structure with SNPCs. Previous research by Geng and co-workers reported stacked titania sheets with pillared interlayer spacing ranging from 0.76 to 1.15 nm, which could facilitate ultrafast intercalation of Li+, Na+ and K+ with high cycling stability, as presented in Fig. 13d.619 The ultrafast ion transportation could be attributed to the open 2D SNPCs, which could even achieve good rate performance for PIBs with a thick electrode of up to 80 μm. Besides, several research groups reported MoS2 and MoO3 based TMDs for LIBs and sodium-ion batteries (SIBs) with SNPCs of ∼1 nm, which delivered excellent cycling stability and rate capability.620,621 The improved electrochemical performance can be attributed to SNPCs between the layered structures, which could accelerate ion transportation and also decrease the ion diffusion pathway. A wide variety of Mxenes with layered structures and abundant SNPCs have also been widely used as anode materials. For instance, Yamada and colleagues reported a TiC2 MXene for Na+ storage in hybrid capacitors and SIBs, which could retain a stable SNPC of 1.01 nm after the first activation process as a result of the Na+ pillars, as illustrated in Fig. 13e.388 The SIB full cells delivered a high specific energy of 260[thin space (1/6-em)]W h kg−1 at a high specific power of 1.4[thin space (1/6-em)]kW kg−1, as a result of the rich and stable SNPC structures. Similarly, a pillared MXene with an expanded interlayer pacing of 1.02 nm has been reported for LIBs, which could enhance the specific capacity, rate capability, and also the cycling stability.622 The improved performance was attributed to SNPCs, which could accelerate ion transportation and provide more ion storage sites. The benefits of the SNPCs for various intercalation based anodes were further verified by numerous previous reports, demonstrating the universality of the SNPCs for layered anodes in delivering enhanced rate performance as a result of the boasted ion transportation in the confined channels between the layers.623–625
3.2.2. COFs. The structure of COFs could enable good electron transport in conductive polymer backbones, as well as effective ion transport through SNPCs. Therefore, COFs with redox active building blocks to accommodate versatile metal ions in SNPCs have been frequently reported as anode materials for various batteries, working with a similar mechanism to that of the Li+ intercalation into the graphite anode. Among which, CMP is a distinctive family of COFs, featuring extended π-conjugation backbones and permanent nanopores.626,627 As presented in Fig. 13f, Chen and colleagues reported a phenazine based CMP with rich SNPCs, which could host various mono- and multi-valent charge carriers (H+, Li+, Na+, K+, Zn2+, and Al3+) for diverse aqueous rechargeable batteries combining rapid kinetics, ultralong lifespan, and chemical rechargeability.543,628 The highly reversible redox activity, facile intramolecular electron transfer, and high ion diffusion coefficient were attributed to the high density of accessible redox sites derived from the cross-linked structure with rich SNPCs. Alternatively, a 2D nitrogen rich CMP with regular SNPCs of 0.7 and 1.1 nm has been rationally designed for LIBs, delivering a high reversible capacity of 701 mA h g−1 at 1 A g−1 and remarkable cycling stability for more than 500 cycles.629 The superior rate performance can be attributed to excellent conductive backbones, as well as the reduced Li+ diffusion pathway derived from the SNPCs. Several excellent reviews have systematically summarized the synthesis, properties, and applications of various COFs,34,44,49,116,626,627 which should be enlightening for follow up research on rational design of high performance CMP anode materials for various rechargeable batteries.
3.2.3. Conversion-based anodes. Conversion based anodes are promising candidates for rechargeable batteries with high specific capacity, whereas suffering from poor cycling stability and rapid capacity degradation.630,631 Recent reports demonstrate that the drawbacks of the conversion based anodes can be significantly relieved by incorporating with SNPCs. As presented in Fig. 14a, ultrathin Co3O4 nanosheets with an interlayer spacing of 0.6 nm have been reported for LIBs, which delivered high specific capacity (1230 mA h g−1 at 0.2 A g−1), excellent rate performance, and excellent cycle capability (1500 cycles).632 The enhanced performance can be attributed to the SNPCs between the layers, which could establish 2D nanofluidic channels offering extra lithium storage sites, accelerate horizontal and vertical Li-ion transport, and alleviate volume changes during the cycling. Alternatively, Park and colleagues reported an exfoliated 2D TMD (MoS2) nanosheets with interlayer channels of ≈1 nm for LIBs, which delivered a high specific capacity of 933.1 mA h g−1 at 0.1 A g−1 and excellent cycling stability with 90% retention after 1000 cycles.633 The SNPCs between the layers with good tolerance to volume expansion and negatively charged channels for fast ion transportation were attributed to the enhancement of the electrochemical performance. It has also been reported that Ni2P anchored in nitrogen doped carbon (Ni2P@N–C) with rich SNPCs could deliver enhanced rate capability (197 mA h g−1 at 2 A g−1) and excellent cycling stability (0.06% capacity decay per cycle), as a result of the high electrical conductivity of the matrix and confinement effect of the SNPCs (Fig. 14b).634 The advantages of SNPCs have also been reported for other conversion-based anodes including silicon oxide,635 SnO2,636 and others high capacity conversion-based anodes with SNPCs can be expected in the following research.
image file: d3cs01043k-f14.tif
Fig. 14 (a) Schematic illustration of ultrathin functionalized Co3O4 nanosheets with massive arrays of 2D nanofluidic channels for Li-ion transport. Reproduced from ref. 632 with permission from Wiley-VCH, Copyright 2017. (b) Schematic illustration of the sodiation/desodiation process for the Ni2P@C-N anode. Reproduced from ref. 634 with permission from Elsevier, Copyright 2019. (c) Schematic illustration of the Si@COF anode with SNPCs for LIBs. Reproduced from ref. 403 with permission from Elsevier, Copyright 2020. (d) Schematic illustration of the sodiation process of the red P confined in SNPCs. Reproduced from ref. 405 with permission from Wiley-VCH, Copyright 2017.
3.2.4. Alloy-based anodes. Alloy based anodes including Si, Ge, Sn, P, Sb, Bi, etc. are well known for their ultrahigh specific capacities resulting from alloying reactions, while their practical applications have been seriously impeded as a result of the huge volume change, unstable SEI layers, and poor cycling stability.637–639 Confining the alloy based anodes in SNPCs has been reported to be an effective approach to address these issues. For instances, Lou and colleagues reported a COF based artificial SEI layer coating on Si nanoparticles, the anode delivered a high specific capacity of 1864 mA h g−1 at a high current density of 2000 mA g−1 and a high capacity retention of more than 60% after 1000 cycles.640 The COF based artificial SEI with SNPCs could facilitate fast ion transportation, reduced electrolyte decomposition, enhanced Coulombic efficiency, as well as improved cycling stability, as illustrated in Fig. 14c. Alternatively, carbon coating layers on Si with abundant SNPCs derived from pyrolyzed metal–organic frameworks (MOF) have been reported for LIBs, resulting in high specific capacity and exceptional cycling stability.403 The increased electrical conductivity, relieved volume change, and enhanced ionic transportation derived from the SNPCs are all responsible for the dramatically enhanced electrochemical performance of the Si anode. Similarly, Yu and coworkers reported a MOF-derived SNPCs (<1 nm) rich carbon to accommodate amorphous Red P as anode material for SIBs (Fig. 14d), the composite anode delivered a high specific capacity of 600 mA h g−1 and excellent cycling performance for 1000 cycles.405 The enhanced performance can be attributed to the buffering effect, enhanced ionic conductivity, and improved electronic conductivity derived from the SNPCs. Various recent research also reported the encapsulation of red P into SNPCs to achieve dramatically enhanced electrochemical performances.404,406–408 With attractive size, buffering, and confinement effects, the enormous potential of SNPCs for various other kinds of conversion-based anodes should be further explored.
3.2.5. Metal anodes. The ultimate choice of anode materials for the rechargeable batteries should be metal anodes, i.e., Li, Na, K, and Zn metal anodes for LIBs, SIBs, PIBs, and aqueous zinc ion batteries (AZBs), respectively.641 For instance, Li metal anodes could deliver the highest theoretical capacity of 3860 mA h g−1 for LIBs as compared to all other kinds of anodes.642,643 Whereas metal anodes exhibit all the problems of other types of anodes, and the extent is much more serious. Besides, the metal anodes are prone to dendrite growth caused by internal short circuits, posing serious safety concerns for the practical application of the metal batteries.
3.2.5.1. Li/Na/K metal anodes. cLi metal anodes have been considered as the “Holy Grail” for LIBs due to their highest specific capacity and lowest redox potential, to Na and K metals for SIBs and PIBs. The alkali metal anodes generally exhibit huge volume change, unstable SEI layers, low Coulombic efficiency, and continuous dendrite growth during repeated cycling.644,645 Various species with SNPCs have been reported as artificial SEI layers to protect the metal anodes. For example, an ACOF coating layer with rich SNPC structures has been used for Li metal anodes (Fig. 15a), exhibiting a size effect that enables selective Li+ transport through the SNPCs while preventing the penetration of electrolyte solvents.646 As a result of effective protection from the ACOF coating layers, significantly enhanced cycling stability for Li metal anodes and Li metal full cells has been achieved. Here, 3D porous frameworks have attracted tremendous interest as metal anode protective layers for rechargeable batteries because of their large active surface areas. Yun et al. demonstrated 3D microporous frameworks grown on Li anodes. Benefiting from the 3D microporous framework induced interfacial activity gradient, Li ions undergoe a uniform deposition, boosting the cycle performance of Li metal batteries.647 As the most representative type of material in the 3D framework, MOFs, COFs and their derived materials have also received a lot of attention. For instance, similarly, a COF film with size effects to block TFSI− while allowing for the selective permeation of Li+ was designed as an artificial SEI layer for Li metal anodes (Fig. 15b), which could generate uniform Li metal plating as compared to the bare Li metal anodes with serious dendrite growth.648 Significantly, Manthiram and co-workers reported the room temperature in situ growth of a COF layer on the Li metal anode, as illustrated in Fig. 15c, which exhibits a suppression effect to prevent dendrite growth, as well as a size effect to achieve uniform Li+ flux.649 Benefiting from the COF protective layer with rich SNPC structures, the Li–S cells delivered exceptional cycling stability for more than 600 cycles with a low-capacity decay of 0.05% per cycle under lean electrolyte conditions. Besides, CMP-based protective layers with SNPCs of 0.5–0.6 nm as ion-selective nanofluidic transport have been reported for Li metal anodes, which delivered extraordinary cycling stability for 2550 h at a high areal current density of 20 mA cm−2.139 The dramatically enhanced cycling performance of the Li metal anode can be attributed to SNPCs with Li+ selective channels, so as to prevent the side reactions of Li metal with anions and electrolytes. Similarly, MOF-based protective layers have been coated on Cu current collectors as artificial SEI layers for Li and Na metal anodes with excellent cycling stability.650,651 Furthermore, expanded graphite with SNPCs of around 0.7 nm as molecular tunnelling has been designed as a host material for Li metal, which could realize dendrite-free Li metal anodes and Li metal full cells with no noticeable capacity degradation after 370 cycles.652 After that, other MOF- and COF-derived materials have also been successfully prepared as protective layer for metal anodes.653–656 The performance enhancement was attributed to the bulk diffusion of superdense Li into SNPCs. It can be anticipated that various SNPC-based artificial SEI layers and host materials could further enhance the cycling stability of alkali metal anodes, paving the way for the practical application of metal anodes for next-generation high energy density rechargeable batteries.
image file: d3cs01043k-f15.tif
Fig. 15 (a) Schematic illustration of ACOF coating with ion selective SNPCs for Li metal anodes. Reproduced from ref. 646 with permission from Wiley-VCH, Copyright 2021. (b) Illustration of the synthesis of COFs with SNPCs for Li anode coating to achieve stable Li metal anodes. Reproduced from ref. 648 with permission from Wiley-VCH, Copyright 2020. (c) Schematic of the synthesis of COFs with SNPCs on Li metal for Li metal anodes. Reproduced from ref. 649 with permission from Wiley-VCH, Copyright 2022. (d) Schematic illustration of the ion transport on the Zn metal anode with a zeolite based protective layer. Reproduced from ref. 657 with permission from Wiley-VCH, Copyright 2023. (e) Schematic illustration of uniform Zn metal plating facilitated by PSPMA coating with SNPCs. Reproduced from ref. 658 with permission from The Royal Society of Chemistry, Copyright 2023. (f) Schematic illustration of VRM coating on Zn metal with ion accelerating SNPCs for stable Zn metal anodes. Reproduced from ref. 635 with permission from Elsevier, Copyright 2022.

3.2.5.2. Zn metal anodes. Aqueous zinc ion batteries (AZBs) have been intensively investigated as alternatives for alkali metal batteries with organic electrolytes, which could deliver high specific capacity and non-flammable properties.659 Low Coulombic efficiency due to hydrogen evolution and short life span as a result of Zn dendrite growth are the prevailing challenges for AZBs. To address these issues, a zeolite based protective layer with SNPCs of 0.349 nm has been designed for Zn metal anodes, which could facilitate selective Zn2+ transportation through channel size restriction and electric field repulsion to the anions, as presented in Fig. 15d.657 As such, the side reactions can be restricted to achieve enhanced Coulombic efficiency. Besides, dendrite-free Zn metal deposition can be achieved due to the homogenized Zn2+ flux derived from uniform SNPCs. Consequently, a long lifespan of 2400 h, a high current tolerance of 100 mA cm−2, and a high capacity retention of 76.4% after 7500 cycles have been achieved. Similarly, an ion regulating interface has been developed for Zn metal anodes, which delivered a high Coulombic efficiency of 99.9% and stable cycling for 2500 h, as a result of the homogeneous Zn2+ flux derived from the coating layer with rich SNPCs (Fig. 15e).658 Interestingly, a vermiculite (VRM) coating with rich negatively charged SNPCs has been designed for Zn metal anodes (Fig. 15f), which could accelerate the transport of Zn2+via electrostatic effects while reducing the water molecules in the Zn2+ solvation shell.635 Therefore, the VRM@Zn anode delivered excellent cycling stability for 5000 cycles at 1A g−1. The feasibility of protective coating layers for Zn metal anodes could be further extended to other SNPC based materials.

Versatile anode materials incorporating SNPCs to achieve enhanced electrochemical performance for various battery systems have been summarized in Table 5. Further exploration of SNPCs for various anode materials is essentially required, which holds promise for enhancing the performance of anodes for next-generation batteries with high energy density, high power density, and desirable cycling stability.

Table 5 Summary of the application of SNPCs for anode materials in various battery systems
Materials Categories SNPCs size (nm) Batteries Ref.
HC Carbon <1 PIBs 394
Graphdiyne Carbon 0.55–0.56 LIBs 611
HPCNS Carbon 0.7–1.3 PIBs 612
SiC-CDC Carbon 0.52–1.0 PIBs 395
HC Carbon 1.0 SIBs 613
SC-NSs Carbon 0.45–0.90 SIBs/PIBs 610
Soft carbon Carbon <1 PIBs 396
C-1300 Carbon 0.8 SIBs 397
PC Carbon 0.43–0.73 PIBs 609
HC Carbon <0.5 SIBs 398
CMP Polymer <1 LIBs/SIBs 660
CMP Polymer 0.4–0.6 LIBs/SIBs 400
CMP Polymers 0.7–1.1 LIBs 629
CMP Polymers 0.5–1.5 SIBs 347
CMP Polymers 0.5 PIBs 402
CMP Polymers <1 ABs 628
Ti1.73O41.04− Intercalation 1.15 LIBs/SIBs/PIBs 619
MoS2/Gr/C Intercalation 0.98 LIBs 620
DMcT-MoO3 Intercalation 1.04 SIBs 621
MXene Intercalation 1.02 LIBs 622
HTO-PANI Intercalation 9.3–9.9 SIBs/PIBs 625
SUCNs-SF Conversion 0.6 LIBs 632
Graphene/MoS2 Conversion <1 LIBs 661
MoS2 Conversion <1 LIBs 633
Ni2P@C-N Conversion 1.0 SIBs 634
Si@ZIF-8 Alloying ∼1.1 LIBs 403
ZIF-8-C@PP Alloying 0.5–1.0 LIBs 404
P@N-MPC Alloying <1 SIBs 405
HPCNS/P Alloying <1 SIBs 406
N-CBCNT@rP Alloying 0.9 LIBs 407
Cu-OMC@RP Alloying <1 LIBs 408
CMP-Li Li metal 0.5–0.6 LMBs 139
ACOF-coated Li Li metal 0.6 LMBs 646
COF-Li Li metal 0.6 LMBs 648
COF-F6@Li Li metal 0.93 LMBs 662
Ploymer-Li Li metal 0.6–1.2 LMBs 12
BDLC Li Metal 0.68 LMBs 652
UiO-66-Li Li metal 1.0–1.2 LMBs 149
ZnA@Zn Zn metal 0.35 AZBs 657
VRM@Zn Zn metal 0.4–1.2 AZBs 663
Zn@MGs Zn metal 0.92 AZBs 664

3.3. Electrolyte

An electrolyte is the medium which carries the ionic charges between the electrodes. Although its role seems trivial, the electrolyte properties drastically influence the electrochemical battery performances. Ideally, the electrolyte should (1) be electrochemically stable at the voltage required for oxidation and reduction reactions at the electrodes; (2) have a high ionic conductivity of >10−3 S cm−1, and a cation transference number close to 1; (3) present high safety; (4) operate over a wide temperature range; and (5) represent a small volume or weight fraction of the battery. Currently, in LIBs, the common electrolytes are based on liquid organic solvent and thermally unstable Li salts. As largely demonstrated in the literature, these types of electrolytes are not satisfactory to further improve the energy density and safety of the next battery generation.665 Quasi-solid and solid-state electrolytes are promising for solving these issues.666 Quasi-solid electrolytes are defined by a liquid electrolyte enclosed in the pores of a solid matrix (polymer matrix, MOFs, etc.), whereas solid-state electrolytes are solely composed of a solid ionic conductor.667 The unique properties of ionic movement in SNPCs have been recently highlighted as a promising way of improving ionic conductivity and resolving electrolyte side reactions in quasi-solid and solid-state electrolytes.668 The following part reviews the latest advance of the application of SNPCs in quasi-solid and solid-state electrolytes.
3.3.1. Quasi-solid-state electrolytes.
3.3.1.1. SNPCs in gel polymer electrolytes. Although gel polymer electrolytes (GPEs) present high safety and relatively good electrochemical properties, the quest to increase the cation transference number (to achieve high-rate performance and suppress dendritic growth) and reduce the side reactions at the electrolyte/electrode interface continues. During the past decades, nanometre and sub-nanometre polymeric matrices have been investigated through intensive experimental and theoretical practices. For instance, in 2011, Li et al. employed multi-axis pulsed-field-gradient nuclear magnetic resonance (NMR), H NMR spectroscopy and synchrotron small-angle X-ray scattering to understand how the PEM structure and morphology influence the ion transport from the sub-nanometre to micrometre scale.669 Theoretical models such as molecular dynamics simulations showed that the ionic transport in SNPC polymer films is ascribed to the negatively charged nanopores (radius of ≈0.3 nm).670 More recently, machine learning was employed to elucidate the energy barrier descriptors from 126 features, this approach could reveal key attributes for the design of ion selectivity sub-nanopores membranes.671 Coupling cutting-edge experimental characterisation studies with theoretical calculations and computing tools could revolutionise the formulation of electrolytes in energy storage systems.

Several research groups demonstrated that Li ions can penetrate SNPC whereas larger anions and solvent molecules remain stranded.392,429 Therefore, employing SNPCs in batteries introduces specific ion selectivity, which leads to a high cation transference number and provides a physical barrier limiting the electrolyte decomposition. Depending on the ion solvation sheath and on the SNPC internal functional groups, various electrostatic interactions dictate the ion selectivity and transport. In organic electrolytes, cations and anions can penetrate the sub-nanochannels with partial solvation sheath.430 As the pore size decreases, the number of solvent molecules removed from the solvation shell increases. Analogically, Lu et al. in situ characterised the dehydration phenomena of alkali metal ions (Li+, Na+ and K+) during ion transport in polymeric SNPCs.431 Interestingly, they observed partial shredding of water molecules from the hydration sheath, which allowed the ions to penetrate pores with smaller size than their solvated size. This fundamental characterization opens exciting routes for aqueous electrolytes to enlarge the electrochemical stability window of the electrolyte and suppress water splitting side reactions. For instance, Li et al. developed a composite membrane based on 2D vermiculite and 1D CNFs with ZnSO4 aqueous electrolyte. Based on dielectric relaxation spectroscopy and molecular dynamics studies, two types of channels form two corresponding types of confined water molecules, with different water relaxation processes and water activities. Due to the fast relaxation process of water in 1D fiber-expanded 2D capillaries, increasing the CNF ratio to 75% will result in the maximum Zn2+ conductivity of 1D/2D membranes, which is far higher compared to the original 2D vermiculite and 1D CNF membranes. Meanwhile, the electrochemical activity of confined water restricted by 1D/2D is limited, including a 78% reduction in corrosion current, and the unique egg-like structure of the elastic modulus of the pebble-like structure is further increased by 155% compared to the vermiculite membrane. Due to the confinement effect of controlled water and enhanced mechanical strength, non-dendritic zinc electrodeposition has been achieved, and the permeation of zinc in the battery is significantly reduced.432

A different quasi-solid electrolyte formulation approach involves introducing SNP particles as fillers in gel electrolytes to promote the cation transference number. Yang et al. engineered a gel polymer electrolyte consisting of a PvDF-HFP micro-structured matrix and two inorganic fillers (SiO2 nanoparticles and Zeolite Socony Mobil-5, ZSM-5).433 As schematically explained in Fig. 16a, the SiO2 nanoparticles adsorb PF6− anions owing to Lewis acid interactions, which favours Li salt dissociation. Meanwhile, the ZSM-5 particles provide many sub-nanopores which enhance the ionic conductivity of Li+, thereby efficiently hindering Li dendrite growth (Fig. 16b). To demonstrate its practical application, the gel polymer electrolyte was applied in Li||NMC811 and Li||LFP batteries. The cells exhibited excellent cycling stability, i.e., 300 cycles with 92% capacity retention for Li||NMC811 batteries and 500 cycles with 96% capacity retention for Li||LFP batteries. In addition, gel polymers are also performed as quasi-solid-state electrolyte for Na batteries. As shown as Fig. 16c, a polyvinylidene fluoride-hexafluoropropylene copolymer (PH) membrane is used as a flexible three-dimensional porous polymer matrix that can incorporate additional microporous polymers.672 By introducing super-crosslinked microporous polymers into the PH host membrane, a continuous polymer network is created that can capture liquid electrolytes and create “liquid SNPCs”. Compared to modern separators used in liquid electrolyte systems, these quasi-solid polymer electrolytes offer higher safety and flexibility without sacrificing the high ion conductivity of traditional liquid electrolytes. Specifically, we synthesized super-crosslinked microporous polymer membranes incorporating polyfuran or polypyrrole into the 3D PH structure. These membranes demonstrate the ability to immobilize liquid electrolytes within the microporous polymer matrix, resulting in quasi-solid electrolytes with high ion conductivity, stability, and cycling life when applied in lithium and sodium metal batteries. Due to a certain affinity for liquids, electrolytes are easily trapped in the microporous polymer. Therefore, GPEs can provide key advantages over SPEs and liquid electrolytes, including acceptable ion conductivity (>104 at room temperature), good wetting of alkali metal anodes, and maintaining close interface contact with the cathode and anode. More importantly, microporous polymer candidate materials with small pore sizes and high porosity can absorb large amounts of liquid electrolyte while providing minimal electrolyte leakage and maintaining the ability to suppress the formation and growth of dendritic substances due to their small pore size. Because of this, the method of trapping liquid electrolytes in microporous polymers has been widely used as gel polymer. Full-cell lithium and sodium batteries containing these microporous polymer electrolyte membranes exhibit comparable rate and interface performance to traditional liquid electrolytes, but with significantly improved cycling performance and coulombic efficiency. Furthermore, the polymer of inherent microporosity (PIMs) has been widely used in functional membranes in recent years because of the stable structure micropores formed by inefficient molecular stacking. Thus, GPE was prepared using bacterial cellulose (BC) as a support membrane and partially acylated PIM-1 (named PIM-CONH2) as a functional layer for use in lithium metal batteries (LMBs), as shown in Fig. 16d. Due to the presence of amide groups, the formed PIM-CONH2/BC GPE exhibits a high Li+ transport number (0.76), good cycling performance (retains 83% capacity after 200 cycles at 0.5C), and good rate capability in LiFePO4|GPE|Li batteries. The layered porous structure in PIM-CONH2/BC GPE can regulate the Li+ transport behavior, and the presence of amide groups promotes the uniform dissolution/deposition of Li+ (Fig. 16e). Acylated PIMs in GPE can play an important role in improving interface affinity and suppressing lithium dendrite growth. Similarity, Zou et al. employed bacterial cellulose (BC) as the support membrane and partially acylated PIM-1 (named PIM-CONH2) as the functional layer, which was performed as the electrolyte membrane of lithium metal batteries (LMBs).673 The PIM-CONH2/BC GPE, designed using the vascular system as a basis, had a multilayer porous structure. This structure enhanced charge transport and reduced volume changes during the cycling process. The amide group in GPE acted as the main transport site. It regulated the nucleation mode of lithium, induced uniform deposition of Li, and inhibited the growth of lithium dendrites. This study provides a new strategy for improving the safety and stability of LMBs to meet the demand for functionalized LMB electrolytes.


image file: d3cs01043k-f16.tif
Fig. 16 (a) Schematic illustration of SZ-GPE. (b) Current–time response of SZ-GPE in Li||Li symmetric cells after applying a constant potential (10 mV). Inset: Nyquist plots before and after polarization and corresponding fitting curves. Reproduced with permission, from ref. 433 Copyright 2022 c. (c) Schematic representation of Li or Na plating/stripping on traditional liquid/separator-based (CG or GF) cells compared to cells based on advanced Li-HCFu-PH or Na-HCPy-PH electrolyte membranes. Reproduced with permission, from ref. 672 Copyright 2020 John Wiley and Sons. (d) Schematic formation of the PIM-CONH2/BC composite GPE. (e) Mechanism of lithium dendrite growth inhibition by PIM-CONH2/BC GPE. Reproduced with permission from ref. 673 Copyright 2023 Elsevier.

3.3.1.2. SNPCs in MOF quasi-solid electrolytes. Solid-state batteries are gaining attention as a potential energy storage system due to their high energy density and improved safety. However, the high resistance between the solid-state electrolyte and electrode leads to slow cation transport. MOF formulation is adaptable to create high specific area SNPC structures, which can accommodate ions and small molecules. Sub-nano confined ions/solvent molecules exhibit chemical and physical properties that are dramatically altered from those in bulk electrolytes. The physical confinement and chemical interactions between the ions/solvent molecules with metal sites inside the MOF nano-channels can facilitate the ion transport, regulate the SEI formation, increase the temperature decomposition of the electrolyte and adjust the electrolyte up-take.18,434,435 Inspired by this, a quasi-solid-state electrolyte (QSSE) based on a solvate ionic liquid (SIL) confined in nanocages of UIO-66 (SIL/UIO-66) is developed. The SIL/UIO-66 QSSE benefits from the spatial confinement of TFSI− by the UIO-66 pores and the strong chemical interactions between SIL and metal atoms. Consequently, the SIL/UIO-66 QSSE exhibits high ionic conductivity and compatibility with electrodes. As a result, Li|QSSE|LFP cells demonstrate excellent rate capability and cycle stability in a wide temperature range of 25–90 °C. This study presents a practical approach for fabricating safe solid electrolytes with excellent compatibility and long cycle life for high-performance QSSE LIBs.

Chang et al. prepared a safe quasi-solid electrolyte by confining a Li-based organic liquid electrolyte inside the SNPCs of a MOF.435 The as-prepared quasi-solid electrolyte exhibits high safety (non-flammability) and a large electrochemical stability window (5.4 V vs. Li/Li+), owing to electrolyte confinement; the Li solvation sheath became more compact and therefore more energy is needed for the solvated PC molecules to be oxidized (Fig. 17a). Meanwhile, the energy density was also improved since only a small amount of liquid electrolyte was necessary to fill the MOF sub-nanochannels. The Li||NMC811 pouch cells cycled for more than 300 cycles at room temperature and at 90 °C with an excellent Coulombic efficiency even though the cathode loading was 20 mg cm−2 and electrolyte up-take was 0.23 μL cm−2. Similarly, ionic liquids, which are considered as a safer alternative to organic solvent, have been infiltrated in the pores of a self-assembled zeolite imidazole framework (ZIF-8) 3D structure (Fig. 17b).436 Recently, a MOF was decorated with halogen atoms inside the channels, leading to a high ionic conductivity of 2.16 × 10−4 S cm−1 at room temperature, a large electrochemical stability window (4.8 V) and dendrite suppression.437 Besides, the MOF nanocrystals were continuously grown on polyimide fibres. The SNPCs of the MOF were infiltrated with a Li-containing ionic liquid electrolyte; meanwhile, the SNPCs between the interconnected fibres were filled in with the PvDF–LiTFSI polymer composite. The SNPCs of the MOF network provided high Li+ conductivity owing to the spatial anion confinement in the sub-nanopores and the Lewis acid interactions between anion-metal sites. Such a quasi-solid electrolyte presented a high ionic conductivity of 4.08 × 10−4 S cm−1 at 30 °C and enhanced the cycling performance of Li||LFP batteries. Furthermore, the MOF structure can be tuned by altering the ligands or the metal sites. In addition, solid-state electrolytes are widely used in solid-state lithium batteries to help improve the slow transmission kinetics caused by high interfacial resistance between the solid electrolyte and the electrode. Thus, a quasi-solid-state electrolyte (QSSE) based on solvent ionic liquid (SIL) confined in UIO-66 nanocages is prepared.459 Due to the effective spatial confinement of TFSI− through UIO-66 SNPCs and the strong chemical interaction between SIL and metal atoms, SIL/UIO-66 QSSE exhibits high ion conductivity and good electrode compatibility (Fig. 17c). Therefore, Li|QSSE|LFP batteries demonstrate excellent rate performance and cycling stability in a wide temperature range of 25–90 °C. This study provides a practical strategy for preparing safe solid electrolytes for high-performance QSSE lithium-ion batteries. Apart from improvement in ionic conductivity, the high interfacial resistance between electrolyte and electrolyte is also a crucial factor to suppress cations transportation. A QSSE based on a solvent-type ionic liquid (SIL) confined in the UIO-66 nanocage was prepared (Fig. 17d).149 Due to the effective space restriction of TFSI- in the UIO-66 pore and the strong chemical interaction between SIL and metal atoms, SIL/UIO-66 QSSE exhibits low interfacial resistance and good electrode compatibility. Therefore, the Li|QSSE|LFP battery shows excellent rate performance and cycle stability over a wide temperature range of 25–90 °C. Additionally, a universal interphase design protocol for durable lithium metal anodes has been proposed by Wu's group. It is recommended to cycle high-capacity Li metal anodes using ion conductors with high lithium transport numbers and high diffusion coefficients. An anion-binding semi-solid interphase constructed with metal–organic frameworks as an ion transport rectifying layer has been used to meet the requirement of suppressing Li dendrite growth on practical Li metal anodes. By using protected thin Li metal, high-loading cathodes, and low electrolyte usage, high energy density Li metal batteries have been demonstrated.


image file: d3cs01043k-f17.tif
Fig. 17 (a) Schematic illustration of the porous PSS polymer with CuBTC MOF decorated inside its channels and its advantages. Reproduced with permission from ref. 435 Copyright 2022 Springer Nature Limited. (b) Schematic illustration of the hierarchically self-assembled MOF network as a 3D ion conductor with continuous Li+ transport. Reproduced with permission from ref. 436 Copyright 2022 John Wiley and Sons. (c) The preparation of SIL/UIO-66 and the assembling of quasi-solid-state batteries. Reproduced with permission from ref. 459 Copyright 2022 John Wiley and Sons. (d) MOF-based SSI formed on LMA with immobilized anions and ionic channels for fast Li+ transport. Reproduced with permission from ref. 149 Copyright 2020 Elsevier. (e) Schematic illustration of MOF–PVDF GPE with anions immobilized for lithium–sulfur batteries. Reproduced with permission from ref. 674 Copyright 2017 American Chemical Society. (f) Illustration of the assembly and design of solid-state batteries based on UN-LiM-IL Sea. Reproduced with permission from ref. 675 Copyright 2023 American Chemical Society.

S/Se cathode batteries are also attracting great attention for promising next-generation rechargeable energy storage systems. They offer advantages like traditional liquid metal oxide batteries and address the drawback of liquid-based metal-chalcogen batteries, which is the “shuttle effect”. However, the poor compatibility of the electrode/electrolyte interface and the low ion conductivity of solid-state electrolytes are key issues hindering their practicality. Thus, a gel polymer electrolyte (GPE) modified with a metal–organic-framework (MOF) is utilized to stabilize the lithium anode in a Li–S battery (Fig. 17e).674 The MOF skeleton contains abundant pores, which immobilize large-size polysulfide anions and cage electrolyte anions, resulting in a uniform flux of Li+ and homogeneous Li deposition. In addition, Wu et al. have designed MOFs by incorporating two types of ionic liquids (ILs) for creating quasi-solid electrolytes.675 The resulting MOF-IL electrolytes provide uninterrupted ion transport channels with functional sulfonic acid groups that serve as lithium ion hopping sites, thus enhancing Li+ transport in both the bulk and at the interfaces (Fig. 17f). These quasi-solid MOF-IL electrolytes exhibit competitive ionic conductivities of over 3.0 × 10−4 S cm−1 at room temperature, wide electrochemical windows over 5.2 V, and good interfacial compatibility. Apart from employing electrolytes, MOFs can also be used as an electrolyte additive to effectively suppress the generation of side reactions. For example, when Li6PS5Cl (LPSCl) is used as an electrolyte, it is easy to release hydrogen sulfide (H2S) in a humid environment, which affects the performance of batteries. To solve this problem, the Jin group used ZIF-8 as a desiccant in LPSCl without changing the Li6PS5Cl electrolyte or electrode structure. By introducing highly ordered porous materials, we have demonstrated the stable cycling performance of the configured LPSCl SSEs battery, due to the effective and lasting desiccating effect.676

Porous organic frameworks (POFs) are porous materials constructed from organic matrices. The inherent pores of POFs meet the demand for constructing rapid Li+ diffusion SNPCs to achieve high ion conductivity in solid polymer electrolytes (SPEs). Although some studies have reported SPEs on POF substrates, most are limited to thin-film frameworks of functional groups (such as boronic esters, β-ketoimines, and triazines), and the transport capacity of Li+ is restricted. In addition, stable covalent bonds are necessary to construct porous materials with rich porous structures to ensure the stability of fast Li+ transport channels in SPEs. Porous aromatic frameworks (PAFs) are a new type of POFs with rigid skeletons, open structures, high specific surface areas, and excellent stability, composed of irreversible carbon–carbon bonds. The abundant SNPCs of PAFs allow for free diffusion of Li+. Therefore, constructing imidazole anion-type PAFs enables easy dissociation and efficient transport of Li+ in PAF-based SPEs. To ensure high Li+ transport in SNPCs, a high Li+ content is necessary according to the classical equation: σ(T) = ∑niqiμi. The Sonogashira–Hagihara coupling reaction was applied to prepare a new imidazole-based PAF (PAF-220), which was then lithiated to obtain a highly lithium-ion conductive lithium-rich anionic PAF (PAF-220-Li) (Fig. 18a).677 The binding energy between imidazole anions and Li+ is low, which facilitates the dissociation of Li+. The conjugated structure of benzimidazole can delocalize negative charges and further reduce the binding energy between imidazole groups and Li+. In addition, the anionic framework can restrict the movement of anions, while the abundant channels in the single-ion PAF-220-Li only allow Li+ diffusion (Fig. 18b). PAF-220-Li was further vacuum-injected with LiTFSI and combined with (polyvinylidene fluoride-co-hexafluoropropylene, PVDF-HFP) to obtain a quasi-solid-state polymer electrolyte (PAF-220-QSPE) with an ion conductivity of 0.206 mS cm−1 at room temperature, up to 0.76. It is worth noting that the rigid structure of PAF-220-Li grants it good stability, providing excellent physicochemical stability with a wide electrochemical window (5.0 V) for PAF-220-ASPE. Therefore, this work provides a valuable strategy for high-performance SPE based on functional PAF and may become a practical means for the design of solid-state LIBs. Similarity, Wang's group reported a strategy to confine low molecular weight polyethylene glycol (PEG, molecular weight = 800) into the channels of COFs, a solvent-free strategy is used to efficiently promote Li+ ion movement within the SNPCs of COFs, enabling rapid and stable conduction of cations over a wide temperature range (Fig. 18c and d).678


image file: d3cs01043k-f18.tif
Fig. 18 (a) Schematic representation of the preparation of PAF-220-SPE and PAF-220-ASPE. (b) Illustrative drawings showing the Li+ transport mechanism in PAF-220-SPE and PAF-220-ASPE. Reproduced with permission from ref. 677 Copyright 2023 John Wiley and Sons. (c) Schematic illustrations of Li+ transport in COFs and (d) structural representations of CD-COF, COF-5, COF-300, and EB-COF. Reproduced with permission from ref. 678 Copyright 2019 American Chemical Society.

SNPCs applied in quasi-solid-state electrolytes provide many benefits. The SNPCs can be tailored to achieve specific properties depending on the application, which can increase the cation transference number by allowing cation conduction while hindering anion movement. This property is critical in metal batteries to mitigate dendrite growth in metal anodes and improve the cycling rate performance for metal-ion batteries. Nonetheless, fully understanding the local properties of ions inside the SNPCs as well as at the interface of bulk electrolyte/SNPCs remains challenging.

3.3.2. Solid-state electrolytes.
3.3.2.1. SNPCs in solid polymer electrolytes. Solid-state polymer electrolytes (SPEs) have garnered significant interest due to their exceptional properties in lithium-ion batteries, including low flammability, resistance to leakage, superior thermal stability, excellent processability, high flexibility, and enhanced safety levels. Polyethylene oxide (PEO) is commonly reported as SPEs since it was discovered that Li ions can hop from one ethylene oxide (EO) group to the next with polymer chain motion, thereby demonstrating some ion conduction.679,680 By definition, in crystalline PEO, the molecular chains are static, thereby hindering ionic conduction. Therefore, PEO-based solid polymer electrolytes suffer from extremely low ionic conductivity at room temperature due to the crystallinity of the PEO chains and can only be applied as electrolyte above their temperature (application above ∼60 °C). Contrarily, Bruce et al. reported Li conduction in a highly ordered crystallized PEO6:LiAsF6 system.681 According to X-ray diffraction (XRD) analysis and ab initio simulations, they demonstrated that in the EO[thin space (1/6-em)]:[thin space (1/6-em)]Li ratio 6[thin space (1/6-em)]:[thin space (1/6-em)]1, the PEO chains adopt a double helical structure forming a cylinder shape.438 The Li ions are located inside the cylinder whereas the anions remain outside. This structure acts as SNPCs with the distance between Li and EO units ranging from 2 to 2.3 Å. In order to increase the ionic conductivity of solid polymer electrolytes, another polymer was explored by Qi and Hu's groups.439 They demonstrated the intercalation of Li ions in elementary cellulose nanofibrils (CNFs). The copper (Cu) coordination with the CNFs allowed the expansion of the interspacing between molecular chains from 0.39 nm to 0.87 nm (Fig. 19a and b). The decoupling of Li ions resulted in excellent ionic conductivity and transference number (Fig. 19c) and cost-efficiency, scalability, and flexibility (Fig. 19d). To prove the practical application of the as-prepared solid polymer electrolyte, an all-solid-state pouch cell based on Li metal anodes and LFP cathodes demonstrated exceptional cycling performance with a capacity retention of 94% after 200 cycles at room temperature (Fig. 19e and f). Furthermore, other ions (Na+ and Zn2+) and polymer (chitosan, CMC, alignate, XG/CNF) combinations are also suitable for creating analogous solid polymer electrolytes.
image file: d3cs01043k-f19.tif
Fig. 19 (a) Schematic illustration of the structure of CNFs. Bottom: Coordination of Cu2+ ions with the hydroxyl groups of cellulose which opens the spacing between the molecular chains, creating cellulose molecular Li+-conducting channels in the CNFs. (b) Scanning electron microscopy (SEM) image of the CNFs. (c) Transference number is plotted against Li+ ionic conductivity for Li–Cu–CNF and for other SPEs PEO-inorganic composites, cross-linked polymers, and a high-Li-concentration electrolyte. (d) Digital photograph of a 1-m-long roll of Li–Cu–CNF membrane. (e) Illustration of a Li|Li-Cu-CNF|LFP all solid-state battery Li-metal as anode, Li–Cu–CNF paper as electrolyte and LFP/CNT/Li–Cu–CNF as a cathode. The Li–Cu–CNF (green fibres) enables transport of Li+ ions (yellow arrows), and CNTs (red fibres) in the cathode enable electron transport (red arrow). (f) Cycle performance of a solid-state LiFePO4 cell made using Li–Cu–CNF ion-conducting binders in the cathode, Li–Cu–CNF electrolyte, and a Li-metal anode. Reproduced with permission from ref. 439 Copyright 2023. Springer Nature. (g) Structure of SSBs with flexible cross-linked MOF chains as SE and the crystal structure of Zr-BPDC-2SO3H. Color code: Zr, purple; O, blue; C, gray; and S, yellow. (h) Bottom–up synthesis of cross-linked MOF chains on the BC skeleton. (i) The ionic conductivities of Zr-BPDC-2SO3H for Li+, Na+, K+, and Zn2+. Reproduced with permission from ref. 440 Copyright 2021 American Chemical Society.

Another approach to improve the ionic conductivity of solid polymer electrolytes through SNC engineering involved utilizing MOFs grown on polymer networks. Zeng et al. in situ grew a MOF functionalized with high-density electronegative groups (Zr-BPDC-2SO3H) on bacterial cellulose (Fig. 19g and h).440 The in situ growth enabled minimization of the interfaces between MOF particles and provided a continuous ion conduction pathway. They demonstrated a Li transference number of 0.88 (Fig. 19i), achieving a high specific capacity of 119 mA h g−1 at a high cycling rate (3C). Recently, Hu et al. employed a layered composite solid polymer electrolyte composed of Uio66-NH2 MOF particles and the PVDF-HFP-LiTFSI-SN-FEC solid polymer.441 The MOF layer acted as an anion sieve owing to the sub-nanopores of the MOF particles,while facilitating high Li conduction. Therefore, the composite solid electrolyte exhibited an ionic conductivity of 0.245 mS cm−1. The Li ions forced through the sub-nanopores of the MOF layer generated a uniform Li flux, which benefit excellent Li plating and stripping cycling performance.


3.3.2.2. SNPCs in inorganic solid electrolytes. Inorganic solid electrolytes can be categorized into three large families according to their chemical structure:682,683 oxides, which regroups perovskite conductors (ABO3 –type, the general formula is LixLa2/3−x/3TiO3, LLTO), anit-perovskite conductors (e.g., Li3OCl and Li3OBr), NASICON conductors (Na1+xZr2P3−xSixO12 and LiM2(PO4)3 (M = Zr, Ti, Hf, Ge or Sn)) and garnet-type conductors (e.g., Li5La3M2O12 (M = Nb, Ta), orthosilicate garnets (A3IIB2III(SiO4)3), A3B5O12 (A = Ca, Mg, Y or Ln = La, or rare-earth elements; B= Al, Fe, Ga, Ge, Mn, Ni, and V), cubic Li7La3Zr2O12 (LLZO)); sulfide-type, which reassembles thio-LISICONs with the formula Li4−xM1−yM′yS4 (M = Si, Ge, and M’ =P, Al, Zn, and Ga), LGPS family (Li10GeP2S12, Li10SnP2S12, and Li9.54Si1.74P1.44S11.7Cl0.3), arygodrites-type with the common formula Li6PS5X (X =Cl, Br, and I); halide-type with the formula—LiaMXb, where X represents I, Br, Cl, and F while M is a metal element (e.g., Li3InCl6, Li3YCl6, Li3ErCl6, and Li2ZrCl6).

Recently, Kang et al. reported the control of superionic conduction in ternary halides (e.g., Li3MCl6 [where the metal (M) is Y or Er]) by manipulating the in-plane lithium diffusion pathways and the stacking layer distances. These two factors are negatively correlated with each other through the partial occupancy of M, which acts as both a diffusion inhibitor and a pillar to maintain interlayer distances. The authors demonstrated the critical range or ordering of M in ternary halides and showed the achievement of high ion conductivity by adjusting the simple M ratio. This work provides a general design standard for superionic ternary halide electrolytes.684 Besides, the inorganic solid electrolytes contain metal cations, which take part in electrochemical reactions. As shown in Fig. 20a, the Li+ conduction occurs through different mechanisms: interstitial hopping, interstitial knock-off and vacancy hopping.666,685 The atomic sites in inorganic solid state electrolytes can be compared to sub-nanopores and the cation conduction analogously occurs through sub-nanochannels. Kato et al. discovered a superionic conductor Li9.54Si1.74P1.44S11.7Cl0.3, which showed the highest Li+ conduction reported for inorganic solid-state electrolytes (Fig. 20d).442 They demonstrated that the double substitution with aliovalent-ion doping (Si and Cl atoms) creates a 3D Li+ conduction pathway in Li9.54Si1.74P1.44S11.7Cl0.3. The 1D pathway is characteristic of the LGPS family, while the 2D pathway could be attributed to the small amount of Cl atoms located in the Cl(1) (8g) sites situated in P(2b)X4 tetrahedra (Fig. 20d and e). Therefore, the ionic conductivity in solid-state electrolytes is closely related with the conduction pathways. Inorganic solid electrolytes are polycrystalline, i.e., contain grains and grain boundaries. The ionic conductivity in the grains, across the grain boundaries and in the grain boundaries varies largely depending on the crystallographic structure of the inorganic solid electrolyte. This leads to two competing conduction pathways: from one grain to the neighbouring grain, i.e., across the grain boundary, named “the granular pathway”, and within the grain boundaries, named “the grain boundary pathway” (Fig. 20b and c). The granular pathway will be preferential when the grain boundaries are more resistive than the grain, for instance, in LATP and Li3OCl. Otherwise, the grain boundary pathway will dominate, which is the case for sulfide-based electrolyte and some oxides such as LLZO.443


image file: d3cs01043k-f20.tif
Fig. 20 (a) Schematic of the Li-ion transport in solid inorganic electrolytes. Reproduced with permission from ref. 666 Copyright 2020. Springer Nature. (b) Schematic of the granular (top) and GB (bottom) conduction in polycrystalline materials. Reproduced with permission from ref. 443 Copyright 2017. American Chemical Society. (c) Arrhenius conductivity plots for the LGPS family and Li9.54Si1.74P1.44S11.7Cl0.3. (d) Crystal structure of Li9.54Si1.74P1.44S11.7Cl0.3. The framework structure consists of 1D polyhedral chains. (e) Nuclear distributions of Li atoms in Li9.54Si1.74P1.44S11.7Cl0.3 at 25 °C. Reproduced with permission from ref. 442 Copyright 2016. Springer Nature.

Several strategies have been explored to increase the ionic conductivity in inorganic solid-state electrolytes. Although not fully understood yet, it was demonstrated that employing a doping and/or substitution strategy can dramatically increase the ionic conductivity. For instance, the introduction of 0.3 Al mol per unit in the LTP crystal enriches the M3 site in Li atoms, thereby opening a new conduction channels M1-M3-M1 site with an activation energy between M1/M3 and M3/M1 much lower than that between M1/M1.444 The new conduction channels enable LATP to exhibit a much higher ionic conductivity than the parent LTP crystal. However, substituting Ge4+ by Si4+ or Sn4+ in LGPS-type solid electrolytes resulted in lower ionic conductivities. It is thought that Si doping narrows the sub-nano channels for Li+ migration; further fundamental investigations on Li+ conduction pathways at a sub-nanoscale are necessary to develop novel superionic conductors.683 The substituted and/or doped element intrinsic characteristics (e.g., atoms size, valence, etc.) greatly influence greatly the bulk ionic conductivity. Besides, the crystal phase also seriously impacts the ionic conductivity due to the different arrangement of atoms. For instance, LLZO solid-state electrolyte has two crystal structures, the tetragonal phase (t-LLZO) and cubic phase (c-LLZO). In t-LLZO, all the Li+ positions in the crystal structure are occupied, whereas in c-LLZO, one site is occupied while the two neighbouring sites are vacant. The c-LLZZO exhibits approximatively twice the ionic conductivity of t-LLZO.686

Another challenge suppressing the development of all-solid-state batteries is the high interface resistance between the electrolyte and the electrode, which has a significant impact on the overall performance of the battery.687 In all-solid-state batteries, electrochemical reactions occur at the solid–solid electrolyte–electrode interface. Ions diffuse from the electrolyte to the electrode through their interconnected region and undergo oxidation–reduction reaction at the electrolyte–electrode interface in contact with the active material and electrons. Therefore, maintaining a good solid–solid electrolyte–electrode interface in the battery is crucial for achieving stable charge transfer reactions. One of the main causes of high interface resistance is the chemical and electrochemical instability between the solid electrolyte and the electrode, such as the space charge layer of local ion vacancies in the electrolyte near the electrode, which severely limits the rate capability of the battery due to the chemical potential difference between the electrode and the electrolyte, causing ions to be extracted from the electrolyte to the electrode side.688 In addition, poor contact between the solid electrolyte (especially inorganic solid electrolyte) and the electrode is also a major issue affecting ion diffusion.689 The volume changes of cathodic and anodic materials during charge and discharge processes may lead to the loss of effective contact between the electrode and the solid electrolyte, thereby limiting the conduction of ions in the interface area. Furthermore, it is worth noting that the diffusion layer of elements formed at the electrolyte–electrode interface is also an important resistance for the deterioration of interface stability during the cycling process.690 Since the solid electrolyte is physically inflexible, the battery manufacturing process usually requires additional heating steps to improve the adhesion between the electrode and the electrolyte (oxides). Therefore, an element mutual diffusion region forms at the solid–solid interface, accompanied by the formation of significant and inevitable interface resistance.

Various methods have been proposed to improve the interface between the solid-state battery electrolyte and electrode. Interface resistance is mainly attributed to the incompatibility between the solid-state electrolyte and electrode, due to effects like space charge, chemical and electrochemical instability, and interdiffusion. Coating ion-conductive and electron-insulating buffer layers on the electrode is an effective way to reduce incompatibility. Ohta et al. found that coating Li0.33La0.557TiO3 (LTO) on LiCoO2 (LCO) particles significantly improved the rate performance of all-solid-state lithium metal batteries based on sulfide electrolytes, due to reduction in the space charge layer effect.691 Besides, introducing an artificial SEI layer with electron insulation and ion conduction is also an effective method to reduce interface instability. Goodenough's team illustrated the formation of an ion-conductive passivation layer composed of Li3P and Li8ZrO6 on LiZr2(PO4)3 solid electrolyte, in contact with Li metal.692 In addition, enhancing the electrode–electrolyte contact area through electrode–electrolyte nanocomposites is a commonly employed method, using techniques such as ball milling, pulsed laser deposition (PLD), and softening glass electrolytes. Hayashi et al. reported that a significant improvement in the solid–solid interface contact area between the electrode and electrolyte was achieved by obtaining electrode–electrolyte nanocomposites through mechanochemical balling. The large volume changes in the electrode during charge and discharge processes would lead to significant interface stress changes, posing a serious problem. Decorating an interface layer that can closely contact the electrode is an interesting approach. Yamamoto et al. reported that introducing an NbO2 layer at the LCO electrode–solid electrolyte interface effectively mitigated interface stress during delithiation.693

Solid-state electrolytes are showing increasingly attractive prospects and potential. Scientists and engineers still need to work hard to develop solid-state electrolytes that are commercially competitive in terms of ion conductivity, interface impedance, mechanical strength, and electrode compatibility, while also being cost-effective. It should be noted that different advantages should be effectively utilized according to different applications, such as high-power density for portable electronic products and electric vehicles, and low maintenance costs for smart grid energy storage, in addition to the required high energy density. Exploring new technologies, such as optimizing battery design structures to improve overall performance, is also essential. Furthermore, modifying existing battery manufacturing processes or reinventing new manufacturing technologies for all-solid-state lithium metal batteries is also important for short-term practical applications.

3.4. SNPCs in separators/interlayers

For rechargeable batteries, the separator plays a crucial role, whose main function is to separate the cathode and anode electrodes to prevent short-circuiting while allowing for the rapid transfer of the ionic charge carrier required by the circuit as the current passes through the cell.694,695 They should be good electronic insulators and could conduct ions through intrinsic ionic conductors or by soaking electrolytes. They should minimize any processes that adversely affect the electrochemical energy efficiency of the battery.

For example, the separator is a very crucial component that influences the mass transport of cations across the electrodes, which affects the anode dendrite growth.435,696 However, the separator currently in use is prone to cause an in-homogenous cation flowing pattern, leading to nonuniform cation transfer that renders dendrite growth.697 To address this issue, to design a separator with appropriate and uniformly distributed SNPCs may aid in the propagation of anode dendrites. For instance, Rajendran et al. discovered a carbon nanomembrane (CNM) with an incredibly thin thickness of 1.2 nm (Fig. 21a).159 This remarkable CNM contained SNPCs measuring 0.7 nm, which acted as an interlayer to control the movement of Li+ during mass transport. The presence of these SNPCs improved the contact and wetting properties at the electrode and electrolyte interface, promoting rapid ion diffusion. Compared to using a Celgard separator alone, the Li-ion transference number increased to 0.67 when the CNM/Celgard separator was employed. A higher Li+ transference number effectively delayed dendrite nucleation, resulting in nearly stable cycling for more than 600 hours. This extended cycling time demonstrated successful dendrite suppression, in contrast to the control experiment where Li dendrites were observed. Additionally, the CNM/Celgard separator exhibited exceptional mechanical strength (Young's modulus ∼10 GPa), preventing dendrite penetration. Besides, the materials with low mechanical strength restrain their application as separator in batteries. The larger and wider pore size distribution of the separator can create steep local concentration gradients at the electrode–electrolyte interface, which further increases the uneven deposition of ions and the formation of fibrous, mossy, and dendritic anode metals.698,699 Recently, in-depth research on separator modification has been implemented. A separator with high mechanical strength can prevent dendrite penetration. A separator with SNPCs can regulate the uniform ion flux near the electrode surface, thereby promoting uniform metal nucleation and growth. Archer and colleagues developed a porous separator with high mechanical moduli and easy ion transport. It comprises a well-ordered porous Al2O3 sheet between microporous poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) polymer layers, forming a sandwich-type composite architecture. The physical properties, including the elastic modulus and ionic conductivity, were influenced by the pore size, which is crucial for electrochemical performance.700 The elastic modulus of the PVDF-HFP/Al2O3 separator is approximately 0.4[thin space (1/6-em)]GPa. The ionic conductivity of the PVDF-HFP/Al2O3 separators was measured at 1 × 10−3 S cm−1. Following optimization, the separators utilizing Al2O3 demonstrated superior performance in terms of low internal resistance, high CE, and long cycling performance. Additionally, MOFs and COFs with a highly uniform SNPC structure make them a promising candidate for constructing separators. Qi et al. prepared a Ti-MOF modified polypropylene/polyethylene separator for rechargeable batteries. Thanks to the better electrolyte wettability, less resistance, highly intrinsic SNPCs and Lewis acid characteristics of Ti-MOFs, this modified separator exhibited the smooth transport of Li+.522 Similarity, a thin film of covalent organic frameworks (COFs) with azobenzene side groups branching from the pore walls was employed as a separator, which acts as ion-hopping sites thus promoting Na+ migration.519


image file: d3cs01043k-f21.tif
Fig. 21 (a) Schematic representation of the synthesis and transfer process of the CNM onto Celgard separators. Reproduced with permission from ref. 159 Copyright 2021 John Wiley and Sons. (b) Energy barrier of polysulfide migration obtained from DFT calculations. (c) The charge density difference of Li2S6 on the SAF-3 skeleton; purple colour represents the electron-rich zone and yellow colour represents the electron-deficient zone, respectively. (d) Charge transfer analysis between Li2S6 and SAF monomers and/or skeletons. Reproduced with permission from ref. 692 Copyright 2021 John Wiley and Sons. (e) Schematic of the fabrication process to produce MOF@GO separators. The MOF nanoparticles and introduced GO laminates synergistically comprise a MOF@GO separator. (f) An illustration of the microporous crystalline structures (HKUST-1). The homogeneous coordinated structures are depicted as sticks, whereas the pores are highlighted in a space-filling representation. Reproduced with permission from ref. 517 Copyright 2016. Springer Nature.

For metal–chalcogen batteries, SNPC modified membrane separators are not only beneficial for cation transport, but also exhibit blocking capability for poly-chalcogen intermediates. On the other side, excessively small pore sizes may hinder the diffusion of cations, resulting in retarded redox kinetics. Therefore, precisely adjusting the size of SNPCs is the key factor for improving the performance of metal-chalcogen batteries. The DFT calculations indicate that the SNPCs with a smaller pore size always exhibit a narrow band gap, which could efficiently block polysulfide shuttling.701 Sun's team synthesized two-dimensional polymer nanosheets, and then deposited the modified layers on a separator to form a composite membrane rich in phenol and triazole units, as well as lithium sites in the ordered channels of the polymer nanosheets.702 Due to its unique SNPCs, Li-COF-based membrane separators can greatly facilitate the transport of lithium ions while limiting the diffusion of polysulfides. As a result, the cells obtained showed higher specific volumes at different rates. Another example involves the synthesis of a porous SNPC aromatic framework (SAF) modified separator, which was designed for all-climate operation of Li–S batteries.703 After rational molecular engineering design, we developed fully conjugated SAF-3 with SNPCs (0.97 nm) and the polysulfide migration barrier up to 33.21 eV (Fig. 21b). In addition, Bader charge calculations (Fig. 21c and d) revealed that SAF-3 transferred more electrons when interacting with Li2S6. In situ X-ray photoelectron spectroscopy (XPS) and in situ XRD tests show that the SA3-modified separators can inhibit the transfer of polysulfide and promote its redox conversion. As a result, SAF-3 modified batteries can work well in a wide temperature range of −40 to 60 °C, showing all-weather battery performance.

Moreover, MOFs with SNPCs can function as a sieve to separate specific ions from an ionic solution based on their sizes and shapes, making them akin to an ionic sieve. MOF-based materials possessing a significant surface area and well-structured pores with adjustable porosity are ideal choices as ionic sieves for reducing the migration of polysulfide ions. Thus, MOF modified separators are tailored for lithium–sulfur batteries. This MOF acts as a battery separator by selectively filtering Li+ ions while preventing polysulfides from passing through. Cu3(BTC)2 (HKUST-1) is selected as the MOF to construct the MOF@GO separator due to its three-dimensional channel structure (Fig. 21e), which contains highly ordered micropores. These micropores have a size window of about 9 Å (Fig. 21f), making them significantly smaller than the diameters of lithium polysulfide (Li2Sn, 4 < n ≤ 8). This property makes it ideal for blocking polysulfides. The MOF@GO separator has demonstrated high efficiency in blocking polysulfides and remarkable stability over long-term cycling in a lithium–sulfur battery.517

Advanced separators are beneficial for battery performance. However, there are still many issues that separators cannot fully solve. Due to the inevitable ion concentration gradient during battery cycling, there can be uneven ion transport efficiency, which can lead to dendrite formation on the anode side. In addition, in metal–chalcogen battery systems, although multifunctional separation membranes can block polysulfide species, the unstable polysulfides species can still result in their migration across the separator, which leads to irreversible loss of active materials and continuous degradation of battery performance. Therefore, embedding an interlayer is crucial for metal-ion batteries with long cycle life, metal anode batteries, and metal–chalcogen batteries. The Manthiram group first proposed the important concept of “interlayer” in 2012 (Fig. 22a). Since then, extensive research has been conducted on interlayer materials resulting in significant improvements. Here, the interlayer of SNPCs plays an important role. In 2012, Yu et al. first prepared the SNPCs hard carbon (HC) to encapsulate Li metal in these SNPCs (Fig. 22b and c), which can prevent the direct contact between electrolyte and anode Li while providing large accommodation space for Li storage, restricting the dendrite growth of Li metal.392 Recently, a MoS2 nanosheet has been used as an interlayer loaded on the Celgard separator, seving as an effective polysulfide barrier, resulting in high Coulombic efficient cycling stability (Fig. 22d).254 It is because the interspace between MoS2 can serve as an ionic sieve to selectively allow Li ion transport while suppressing the shuttle effect. It was also found that chemical reactions could occur between polysulfide and tungsten at the edge of the micron-sized WS2 wafer, effectively improving the performance of Li–S cells.704 When SNPC interlayers are applied on the anode side, the interlayers are commonly anchored to the separator facing the anode side or grown on the surface of the anode. SNPCs can guarantee the smooth ion transfer and avoid the corrosion reaction between the electrolyte and anode side. Moreover, preparing inherent regular SNPCs on the SEI membrane could lead to uniform cation flux at the electrode/electrolyte interface, causing uniform metal deposition. A COF-based ultrafine artificial SEI was fabricated by Guo's group to regulate LiF formation and distribution in the SNPCs in COFs. As illustrated in Fig. 22e, the high Li+ affinity of COFs facilitates the adsorption of lithium salts from the dilute electrolyte, resulting in an increase in the local electrolyte concentration by an electrical double layer (EDL) around the COF skeletons. Upon electrochemical lithiation of the COF film during the first cycle, the reductive decomposition of the strongly bound Li-salts locally generates uniformly distributed LiF domains. Such a homogeneous COF-LiF film serves as a desirable SEI. The discrete LiF grains are confined within the periodic microporous frameworks, endowing the lithium/electrolyte interphase with considerably enhanced passivation and mechanical properties against dendrite growth and electrolyte penetration. Meanwhile, the lithophilic moiety of COFs facilitates Li+ ion transport through the periodic COF skeletons. Consequently, long-cycling stability of symmetric lithium metal batteries has been attained in both ether- and carbonate-based electrolytes.705 Qiu's research group prepared a composite interlayer composed of carbon nanofibers (CNFs) grown from a metal–organic framework (ZIF-67) enriched with Co (Fig. 22f).706 It is worth noting that the ZIF/CNF composite interlayer can synergistically achieve physical blocking and chemical capture capabilities, thereby inhibiting the dissolution of polysulfides and mitigating the shuttle effect. In addition, the three-dimensional fibrous network provides an interconnected conductive framework between each ZIF micro-reactor, facilitating rapid electron transfer during the cycling process, which contributes to excellent rate and cycling performance. The interlayer is also used in quasi-solid-state electrolyte to reduce the loss of liquid electrolyte by adjusting the solvation environment of moving ions in the layer. Thus, Kim et al. introduce a novel approach to enhance the stability and performance of Li metal batteries (LMBs). They present a composite layer consisting of a single-ion-conducting ceramic electrolyte (S-CE) and a single-ion-conducting polymer (S-PE)-based gel electrolyte (S-GE).518 This composite layer, referred to as S-CE/S-GE, acts as a modulator for the reactivity of liquid electrolyte components. By applying the S-CE/S-GE layer onto a Li metal electrode, the loss of liquid electrolyte on the electrode is significantly reduced. This layer also improves the cycling stability of LMBs by altering the solvation environment of Li+ ions within the layer (as shown in Fig. 22g). Interestingly, this effect is not observed when using a composite layer comprising S-CE and a bi-ion-conducting polymer (B-PE)-based gel electrolyte (B-GE). Furthermore, the researchers demonstrate that the S-CE/S-GE layer has an impact on the morphology of Li anodes and the stability of the cathode–electrolyte interface in Li metal batteries. Leveraging the benefits of the S-CE/S-GE composite layer, they successfully developed an energy-dense Li metal pouch (Fig. 22h), which exhibits operation for over 400 cycles with a low mass of electrolyte content of 2.15 g Ah−1. The careful engineering of the solvation structure and the resulting outcomes provide valuable insights into the control of interfacial reactions and offer a practical methodology to enhance the performance of LMBs.


image file: d3cs01043k-f22.tif
Fig. 22 (a) Schematic configuration of the cell with a bifunctional microporous carbon interlayer inserted between the sulphur cathode and the separator. (b) SEM image of the surface of the MCP and (c) transmission electron microscope image of the microporous carbon particles. Reproduced with permission from ref. 392 Copyright 2012 Springer nature. (d) Schematic cell configuration of Li–S batteries using the MoS2/Celgar separator. Reproduced with permission from ref. 254 Copyright 2017 John Wiley and Sons. (e) Schematic illustration of the electrochemical preparation of the COF-LiF hybrid interphase layer. Reproduced with permission from ref. 694 Copyright 2016. Springer Nature. (f) Mechanism of physical barrier and chemical adsorption for polysulfides by the ZIF/CNFs interlayer. Reproduced with permission from ref. 695 Copyright 2021 Elsevier. (g) Schematic illustration of the effects of the S-CE/B-GE and S-CE/S-GE composite layers on the cycling stability of liquid electrolyte-based LMBs and (h) cycling stability with effect of S-CE/S-GE composite layers. Reproduced with permission from ref. 518 Copyright 2023 Springer Nature.

4. Practical challenges

As comprehensively discussed above, electrochemical coupling of SNPCs in a wide variety of electrode materials has been crucial to enhance the performance of the batteries, which is anticipated to be widely recognized and further explored in future research studies. Despite significant advances in the past few years, the employment of SNPCs in rechargeable batteries is still in its infancy. To pave the way for future investigations, it is essential to emphasize the practical challenges for SNPCs at the current stage, as presented in Fig. 23.
image file: d3cs01043k-f23.tif
Fig. 23 The practical challenges associated with SNPCs for rechargeable batteries.

4.1. Angstrom resolutions

The understanding of SNPCs has long been scarce, and one of the major obstacles is the resolution limitations of traditional characterization methods. To acquire accurate structural information of the SNPCs, angstrom scale resolution is required. The surface morphology of the samples is generally obtained using a scanning electron microscope (SEM) which collects scattered electrons for analysis, and the optimal spatial resolution achievable is about 0.5 nm, while transmission electron microscopy (TEM), which collect transmitted electrons for analysis, offers a much better spatial resolution with more structural information such as lattice fringes and crystal structures. TEM can analyze the elemental composition of the samples at high resolution when coupled with energy-dispersive X-ray spectroscopy (EDS) or electron energy loss spectroscopy (EELS); the former is particularly sensitive to heavier elements while the later tends to work best for elements with low atomic numbers. Whereas the specimen for TEM analysis should be very thin (<150 nm) to permit electron transmission, and even below 30 nm is generally required for high resolution images. Scanning transmission electron microscopy (STEM) can be deemed as the combination of SEM and TEM, which can analyze both scattered and transmitted electrons; it requires stable room environments (limited temperature fluctuations, electromagnetic waves, acoustic waves, etc.) and can deliver an optimal spatial resolution of 0.5 Å.707 With the advancement of detectors and computational power, 4D STEM with beam rasters across a 2 dimensional region in real space has been invented.708 It does not necessarily result in higher resolution for 4D STEM; however it can be extremely helpful for specimens with distorted structures due to high strain. It is worth noting that, it generally requires to apply a high dose of electron on a small sampling area to get enough yield of electrons for high resolution analysis, whereas many samples are less tolerable to electrons and can get damaged by the focused electrons. Cryogenic electron microscopy (cryo-EM) is another extension of TEM technology, which can operate at cryogenic temperatures to minimize the sample movement or damage. Cryo-EM has been widely used for structure determination of biomolecules, which has recently been utilized for battery materials, offering significant contributions to the field.709 The invention of cryo-EM enabled the acquisition of atomic structures for a wide variety of sensitive battery materials at the angstrom scale, such as, air and electron sensitive Li/Na metal, air and electron sensitive SEI layers, and volatile S elements.710

In addition to various electron microscopy techniques, other morphology and structural information from diverse characterization techniques are also essentially required. X-ray diffraction (XRD) has been a powerful tool to characterize the lattice patterns of nanocrystals at the angstrom scale, while the lattice distortion may cause broadening and overlap of the diffraction peaks.711 High spatial and angular resolution XRD can be achieved by reducing the beam divergence in high photon fluxes synchrotron sources. For instance, synchrotron XRD with a high resolution of 1.9 Å has been achieved, which could identify light elements such as oxygen and water molecules.712 Besides, X-rays are suited for superficial analysis of shallow depths or thin specimens, while neutron diffraction with its high penetration depth is suited for bulk samples.713 Other angstrom characterization techniques, such as, atomic force microscopy (AFM) and tip-enhanced Raman spectroscopy (TERS) should be further investigated for SNPCs.714 Since the SNPCs cover a wide range of material families with diverse physicochemical properties, it is necessary to choose suitable specimens for characterization of angstrom resolutions to acquire the desired information.

4.2. Operando characterization

For the electrochemical coupling of SNPCs in various batteries, dynamic evolution of various species during the electrochemical reactions is of particular interest. Whereas most of the results were achieved through post-mortem analysis, for which the composition of the electrodes may have changed during the sampling process. Therefore, it is essential to develop various operando or in situ characterization techniques to investigate the dynamic electrochemical evolutions under real time working conditions. Here, the high-resolution in situ transmission electron microscope (TEM) electrochemical experimental platform is very important for observing battery reactions. This technology allows direct visualization and timely discovery of changes in materials during the charging and discharging process, helping researchers to deeply understand the mechanism of SNPCs for ion storage. In addition, high-resolution in situ TEM can also observe the phase change process of the cathode material during battery operation, which helps researchers understand the working mechanism of the battery and make timely adjustments. The development of high-resolution in situ TEM allows researchers to observe the structural changes of materials during charging and discharging at the atomic level in real time. This is crucial for understanding the mechanism of battery reactions and optimizing battery adjustments. Compared with traditional post-reaction characterization, high-resolution in situ TEM can precisely control and capture the dynamic changes of materials, revealing the relationship between the structure and performance. However, the current in situ TEM technology still faces great challenges; first, it is difficult to carry out high-rate (atomic level) characterization; second, the ability to combine other technologies such as EDS and EELS is still weak. The high-resolution in situ TEM would become a powerful tool, not only capturing structural information of nanomaterials in specific environments, but also guiding the design of functional materials. In addition, another feasible option is to use a high-throughput synchrotron light source to penetrate the seal of the battery and generate enough signals for analysis, for instance, synchrotron-based XRD, synchrotron-based X-ray absorption spectroscopy (XAS),715 and synchrotron-based scanning transmission X-ray microscopy (STXM).716In situ synchrotron XRD has been widely used for various battery systems to investigate the lattice parameters, phase transitions, and strain evolution of electrode materials under real time operando conditions, which are extremely useful for an in depth understanding of the working mechanisms. In situ synchrotron XAS could provide the oxidation state and coordination environment of the elements (X-ray absorption near edge structure, XANES), and also the coordination numbers, bonding distances, and the chemical identity of nearest neighbours (X-ray absorption fine structure, EXAFS).717In situ synchrotron STXM could achieve site-specific information on chemical and structural changes of the electrode materials within single particles at high spatial resolution (∼20 nm), therefore, it is powerful for understanding the working mechanisms of the electrode materials under operando conditions. Other synchrotron-based characterization techniques, for instance, synchrotron micro-computed tomography (MCT) while unable to achieve high resolution at the angstrom scale, could also be further explored for SNPCs due to their capability of acquiring various structural information under operando conditions.

4.3. Atomic perceptions

Up to now, most of the reports on SNPCs for battery applications are focused on the microstructures and electrochemical performance of the electrodes, whereas an in depth understanding of the working mechanisms at the atomic level is insufficient. Accompanied by the abovementioned angstrom resolution and operando characterization studies, it is much more desirable to decipher the working mechanism with atomic perceptions. For instance, the encapsulation of S in SNPCs could alter the atomic configuration from cyclic S8 to short chain S2–4 species, which could bypass the generation of soluble long chain polysulfides to minimize S dissolution. This is a good example of the process from quantitative change to qualitative change, in which the impact of the SNPCs should be carefully considered with atomic perceptions. Besides, the atomic configurations of the doping elements in SNPCs, e.g., S doping at different sites in hard carbon, could result in significantly different electrochemical performance due to different working mechanisms, further verifying the significance of angstrom resolution characterization studies and atomic perceptions. For the electrochemical coupling of SNPCs in various batteries, it is highly important to have a closer look at the working mechanisms from an atomic perspective. Especially, the solvation structure of the ions, the composition of the SEI layers, the influence of the doping elements with different atomic configurations, the atomic configuration of the species restricted in the SNPCs, and the interaction of various species with the building blocks of SNPCs should be investigated with atomic perceptions to explore the underlying working mechanisms.

4.4. Theoretical calculations

With the significant advancement of theoretical calculations in the last two decades, many key properties of the batteries can be accurately predicted. The capabilities of theoretical calculations for batteries including theoretical capacity, equilibrium voltage, ion diffusion pathways, electronic conductivity, rate capability, thermal stability, etc.718 These properties are invaluable for battery research community, offering an alternative “prediction and validation” route instead of “trial and error” routine for experimental research. Therefore, the potential of theoretical calculations for electrochemical coupling of SNPCs in carious batteries should be further explored to enhance the fundamental understanding of the working mechanisms. For instance, selective ion transport in SNPCs of MOF based SSEs has been validated through density functional theory (DFT) calculations and molecular dynamics (MD) simulations, which are essential for the atomic understanding of the working mechanism.719 While theoretical calculations for various battery materials have been frequently carried out for crystalized inorganic materials, modelling and calculation of organic and/or amorphous materials have been challenging. The SEI layers on the anode surfaces which are generally composed of organic and inorganic composites are very meaningful yet challenging for DFT calculations. As for the SNPCs, one must consider the size effect of the nano particles, for which the properties may be dramatically changed at the subnanometer scale. For example, nanosized LiFePO4 could deliver ultrafast Li+ extraction, for which the low-energy surfaces of the nanocrystals account for up to 85% of the total surface area.720 Besides, calculations for solid-state materials with known crystal structures have been widely achieved, while those with multiple phases and/or physical fields are difficult, e.g., the interfaces of the electrodes with complex interactions such as electric field, electrostatic field, steric hindrance, solvation structure, ion dissociation, electrochemical voltage window, etc.

Furthermore, with the development of computational power and artificial intelligence, a new branch of computation known as “machine learning” has recently surged. Machine learning is a protocol to feed the program with plenty of known results for the computer and establish an approximate model between input and output, with which artificial intelligence could learn and discover an optimal algorithm to summarize the results with minimum deviation.721 After numerous optimizations, the algorithms can be used to predict the results for unknown species or systems. It is remarkable that we can find the algorithms before understanding the underlying correlations between the properties and the performance, the algorithms in turn could be helpful to discover the decisive parameters for the systems. The capabilities of machine learning should be further explored for SNPCs in batteries, which could certainly lead to a new era for close combination of theories with experiments by data-driven computations.

5. Conclusion and perspectives

To meet the growing demand for batteries, the development of new materials that improve the efficiency of energy conversion and storage systems is crucial. The materials with SNPCs offer opportunities in energy conversion and battery storage applications due to their unique physio-chemical properties. As illustrated in Fig. 24, the most straightforward advantage of the SNPCs is that they could achieve regulated ion transportation, facilitate selective and fast ionic transport through the size effect or electrostatic effect through the SNPCs.435,722–724 Besides, the SNPCs with carbon layers or conductive polymers can serve as an electrically conductive matrix, realizing superior electrical conductivity. The combination of fast ion transfer and high overall electrical conductivity could contribute to high-rate capabilities.392,725 In addition, some SNPCs as artificial SEIs or protecting layers could relieve the volume change of the electrode materials as a result of the suppression effect, to enhance the cycling stability of the electrodes.726,727 The SNPC protecting layers can also deliver good ion and electron conductivity, to enhance the rate performance of the electrodes.728,729 Furthermore, by introducing SNPCs into the materials, higher surfaces area with additional active sites can be achieved; therefore the reversible specific capacity can be increased.730,731 Noticeably, the void formation should be avoided during the SNPCs fabrication process. The generation of voids will greatly affect the inherent electrical conductivity of the matrix. When the void exists, the electrons transformation will be greatly suppressed. The electrons must achieve more energy to overcome the transfer barrier created by the voids. In a word, the SNPCs in the battery materials can enhance the rate capability, enhance the cycling stability, and increase the reversible specific capacity of the anodes. Thereafter, the significance of SNPCs for various anode materials should be further explored in the following research.
image file: d3cs01043k-f24.tif
Fig. 24 Summary of the versatile functions of the SNPCs for rechargeable batteries.

This review comprehensively covers all aspects of SNPCs and their involvement in electrochemical reactions within batteries for the first time. We first define SNPCs and then, we rationally sorted the families of SNPCs materials and summarize the unique physicochemical properties of SNPCs. With respect to the electrochemical applications of SNPCs in rechargeable batteries, we have systematically summarized the electrochemical reactions occurring on various components of the battery in SNPCs. The challenges for the practical application of SNPCs in batteries have also been discussed in detail. Furthermore, to further explore the huge potentials of SNPCs for rechargeable batteries, we have summarized the perspectives for future research as followed.

Synthesis of SNPCs

With the highly sought-after attributes of simplicity, environmental friendliness, and controllability of SNPCs structure synthesis materials, an increase in supply will inevitably lead to a wider range of applications. Compared to the synthesis of materials with larger pores, mesopores, and micropores, the synthesis of SNPCs structures requires carefully designed preparation schemes because their sensitivity to reaction conditions depends on each composition and intermediate phase. In most methods, such as template methods, they may not be applicable to the reconstruction of controllable SNPCs. In addition, the size, structure, and interaction of the components will greatly affect the synthesis of SNPCs structures. Considering these factors, it is crucial to find a template-free, simple, and effective technique to control the synthesis of SNPC materials, like the use of metal–organic frameworks in chemistry. In energy conversion and storage devices, controlling the overall particle size, surface structure, morphology, packing density, orderliness, and high density is also crucial. At the same time, favorable structural parameters are different for different applications. Methods for systematically controlling and optimizing these parameters for specific application systems have not yet been developed.

High specific surface areas and porosity are ideal conditions for improving activity. Nevertheless, these advantages provide more opportunities for adverse reactions. This is particularly evident in batteries, where the combination of a high specific surface area increases reactivity with the electrolyte and leads to uncontrolled solid electrolyte interface reactions. Additionally, in relation to volumetric energy and power density (reflecting how much and how fast energy can be stored in a unit volume of packaging equipment), the low packing density of SNPC materials may be another disadvantage. Furthermore, SNPC materials with nanoscale frameworks and high surface energy often have lower thermal stability, which affects the catalytic performance at high temperatures. Thus, SNPC materials with ultra-high specific surface area will further increase the burden of electrolyte infiltration. SNPCs materials with ultra-high specific surface area typically have lower density. Therefore, the key future research direction is how to use less electrolyte to wet high-load mass electrodes to achieve high utilization of active materials. In addition, the mechanical stability of the materials is also crucial, as excessive porosity may pose a risk of local collapse, and more inconsistent porosity may severely affect the improvement in battery performance. In summary, the synthesis of future sub-nanometer pores should be developed based on applications, and the development of sub-nanometer pores should be beneficial for the reactions involved in those applications. For batteries, the future SNCP designs should prioritize characteristics such as ease of synthesize, high-density, controllable, high toughness and mechanical performance, easy wetting by electrolyte, better compatibility with electrolyte or chemical materials, non-toxic and environmentally friendly, and low-cost, similar to current nanometer pore designs.

Stability of materials with SNPCs

Due to the large surface area of SNPCs, SNPCs can theoretically provide more space to accommodate active materials or cations, which makes the matrix materials more resilient to volume changes and preserve structural stability. However, it is intractable to maintain the SNPC stability for intercalation-type electrodes due to intrinsic working mechanisms. Taking intercalation-type electrodes as examples, the structure will undergo transformation due to intercalation/deintercalation during charge/discharge processes, which make the lattice ions deviate from their original positions, leading to potential risks of structural collapse.732,733 In terms of conversion-type electrodes, especially for metal–sulfur batteries, the structural stability of the SNPCs is relatively hard to maintain. As a S host, the SNPCs have to sustain large volume deformation due to the severe volume expansion of S during redox reactions, causing serious structure damage or even physical disintegration.734 In addition, whether it is an intercalation-or alloy-type anode, unevenly sized and inappropriate SNPCs would result in nonuniform deposition of metal ions during the charging process. As the cycles progress, the structure of the anode electrode will be severely damaged.735,736 Overall, the structural stability of SNPCs is wildly considered one of the main research directions. Therefore, more research attention should be given to enhance stability of materials with SNPCs.

Characterization of SNPCs

This includes the characterization of the sub-nanometer pores themselves as well as the characterization of electrochemical reactions in sub-nanometer pores. Current technological limitations result in material characterization only reaching the nanometer level. Therefore, upgrading and developing new generation characterization devices is crucial and urgent. Three-dimensional atom probe tomography (APT) can provide visual and quantitative analysis of materials. Visual analysis includes observing the internal grain boundaries, phase boundaries, structural interfaces, as well as dislocations and defects at different locations. Quantitative analysis includes determining the composition of materials, calculating the quantity, density, and volume of precipitation and segregation at interfaces or grain boundaries. Moreover, nanoscale electrochemical reactions need to be characterized, such as the confinement of SNPCs, ion diffusion, sub-nanometer solvent effects, etc. How to characterize these effects is a top priority for future research. For this, more advanced characterization techniques need to be combined with advanced electron microscopy for further judgment. For instance, wide-angle X-ray scattering (WAXS) measurements provide a spatial resolution in the range of 0.2 nm–1 nm; By combination of WAXS and SXAS, the capability can be extended to determine the microstructural features for porous materials with SNPCs.

In addition to the characterization of the nanopores themselves, further development is needed for the characterization of electrochemical reactions within nanopores. Due to technical limitations, it is difficult to directly obtain external characterization of the electrochemical reactions of SNPCs, such as transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), XRD, Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), etc. To fully understand the integrated multi-scale regulation mechanism, several challenges related to characterization techniques need to be addressed. The first challenge is the time scale and operating environment. Although offline characterization studies provide valuable information at relatively low cost, their limitations restrict their application: on one hand, there is a lack of direct observation of the movement processes of molecules, atoms, ions, etc., as well as the direct reaction intensity. On the other hand, due to the sensitivity of electrolytes and metal ions to air and moisture, offline characterization cannot accurately represent the reaction processes in batteries. Therefore, in situ/real-time characterization techniques are crucial for a deep understanding of the solvation chemistry in various rechargeable batteries. In addition to using more advanced high-resolution electron microscopy, more instrumental methodologies need to be developed, such as how to further transfer the high-resolution electron microscopy imaging technology to digital intensity, which can help us improve this aspect. Through this means, not only can we observe whether local reactions are intense or not, but we can also observe the movement and reactions of ions, atoms, and molecules. Combined with in situ techniques, this will elevate our understanding of battery reactions from the nano scale to the sub-nano scale.

Advanced theoretical calculation and simulation techniques

Here, we propose the use of material knowledge supported machine learning (MIML) to enhance current material computation methods.737 MIML combines machine learning techniques with explicit prior material knowledge. In fact, many successful cases in materials science already utilize specific forms of MIML, such as deep density functions, chemical synthesis planning, and the use of deep neural networks for ab initio calculations in multi-electron systems. By employing MIML-based algorithms, the computational workflow becomes partially interpretable, and the need for extensive training data is greatly reduced due to the incorporation of prior knowledge or mathematical models. In some cases, it may even achieve zero-data training. Our goal, from the perspective of multiscale simulation in material design, is to develop a general partial differential equation (PDE) solver for subnanometer-scale simulations based on MIML. Additionally, we aim to propose a neural network alternative model for solving ab initio Schrödinger equations with built-in physical constraints in the future. Overall, MIML-based material computation platforms have the potential to extend multiscale simulations to larger systems with higher accuracy.

Furthermore, this is a comprehensive platform based on “artificial intelligence + big data”. Due to its efficiency, artificial intelligence (AI) (whose core is machine learning (ML), high-throughput simulation, and experimentation) and big data technology have attracted more and more attention in molecular design, material development, performance prediction, mechanism analysis, relationship mining, and other fields. Therefore, we propose a comprehensive platform based on “artificial intelligence + big data”. According to the needs, high-throughput computation and simulation based on machine learning are first used to screen and simulate feasible molecules and ions with suitable pore sizes, and then high-throughput synthesis, characterization, and battery testing are carried out by mobile robot technicians. All the results obtained in the above process are stored in the database, providing enough data for machine learning to optimize the design and performance prediction of SNPCs or sub-nanometer structures. Finally, high-performance ideal materials are obtained efficiently and quickly. In addition, this comprehensive platform is an efficient tool for understanding sub-nanometer scale mechanisms or reactions, such as electrochemical reactions in SNPCs that are still difficult due to insufficient experimental techniques.

To better regulate and improve batteries fundamentally and enhance battery performance, a deeper understanding of the structure, electron, surface characteristics, and transfer mechanisms of SNPCs materials at the level of battery and electrochemistry need to be achieved. Given that energy conversion and storage devices will be at the forefront in the coming years, SNPCs materials are bound to become the next material trend. Of course, this not only depends on the development of materials but also relies on the innovation and manufacturing of advanced characterization equipment. Therefore, similar to nanomaterials, it is necessary to conduct fundamental research, manufacturing, and techno-economic analysis on SNPCs materials in order to successfully produce and characterize SNPC materials in a controllable manner and integrate them with cost-effective battery industrial systems. Although there seem to be many difficulties, we still believe that through collaboration and interdisciplinary efforts, there will be more and more exciting results, enabling sub-nanotechnology to be widely applied like current nanotechnology.

Abbreviations

SNPCsSubnanometer pore/channels
MOFsMetal–organic frameworks
COFsCovalent organic frameworks
POCsPorous organic cages
TMDsTransition metal dichalcogenides
TMOsTransition metal oxides
CNTsCarbon nanotubes
GAsGraphene analogues
g-C3N4Graphitic carbon nitrides
MWCNTsMulti-walled carbon nanotubes
SWCNTsSingle-walled carbon nanotues
DWCNTsDouble-walled carbon nanotubes
TMPsTransition metal phosphates
MXenesTransition metal nitrides/carbides
TMDsTransition metal dichalcogenides
POCsPorous organic cages
SSICsodium super ionic conductor
PBAsPrussian blue analogues
POCsPorous organic cages
LIBsLithium-ion batteries
SIBsSodium-ion batteries
PIBsPotassium-ion batteries
LSBsLithium–sulfur batteries
SSBsSodium–sulfur batteries
PSBsPotassium–sulfur batteries
SEISolid–electrolyte interphase
CMPsConjugated microporous polymers
QSSEsQuasi-solid-state electrolytes
GPEsGel polymer electrolytes
PIMsPolymer of inherent microporosity
PAFsPorous aromatic frameworks
POFsPorous organic frameworks
SPEsSolid polymer electrolytes
HR-TEMHigh resolution transmission electron microscopy
HAADF-STEMHigh angle annular dark field scanning transmission electron microscopy
ABF-STEMAnnular bright field scanning transmission electron microscopy
iDPCIntegrated differential phase contrast
XPSX-ray photoelectron spectroscopy
HAXPESHard X-ray photoelectron spectroscopy
XANESX-ray absorption near edge structure
EELSElectron energy loss spectroscopy
XRPDX-ray powder diffraction
NPDNeutron powder diffraction
EDTElectron diffraction tomography
SAEDSelected area electron diffraction
PDFPair distribution function analysis
RIXSResonant inelastic X-ray scattering
NMRNuclear magnetic resonance spectroscopy
WAXSWide angle X-ray/neutron scattering
XCTX-ray computed tomography
XRD-CTX-ray diffraction computed tomography
APTAtom probe tomography
NMRCNuclear magnetic resonance nano pore analysis

Author contributions

Conceptualization, Y. -J. Lei., W. -H. Lai., Y.-X. Wang. and G. Wang.; visualization, Y. -J. Lei., L. Zhao., Z. Huang, and P. Jaumaux.; writing – original draft, Y.-J. Lei., L. Zhao., Z. Huang., and P. Jaumaux.; writing – review and editing, Y.-J. Lei., L. Zhao., W. -H. Lai., B. Sun., Y.-X. Wang., and G. Wang.; funding acquisition, W. -H. Lai., Y. -X. Wang., B. Sun., and G. Wang.; and supervision, G. Wang.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We would like to acknowledge the support from the Australian Research Council (ARC) through the Discovery Projects (DP210101389 and DP220103301), the ARC Future Fellowship (FT180100705), the ARC Discovery Early Career Researcher Award (DE220101113), the ARC Linkage project (LP200200926), and the ARC Research Hub for Integrated Energy Storage Solutions (IH180100020). The authors thank Dr Tania Silver for critical discussion.

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Footnote

† These authors contributed equally to this work.
This journal is © The Royal Society of Chemistry 2024
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