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DOI: 10.1039/C9NR08217D (Minireview) Nanoscale, 2019, 11, 23092-23104

Modulation engineering of 2D MXene-based compounds for metal-ion batteries

Yadong Yu ab, Jian Zhou *ab and Zhimei Sun *ab
aSchool of Materials Science and Engineering, Beihang University, Beijing 100191, China. E-mail: jzhou@buaa.edu.cn
bCenter for Integrated Computational Materials Science, International Research Institute for Multidisciplinary Science, Beihang University, Beijing 100191, China. E-mail: zmsun@buaa.edu.cn

Received 24th September 2019 , Accepted 1st November 2019

First published on 4th November 2019

Abstract

The increasing demand for next generation rechargeable metal-ion batteries (MIBs) has boosted the exploration of high-performance electrode materials. Two-dimensional (2D) transition metal carbides/nitrides (MXenes), the largest family of 2D materials, show extremely competitive potential applications in electrodes due to their excellent electrical conductivity, chemical diversity, and large specific surface area. However, the problems of uncontrollable surface functionalization, interlayer restack and collapse significantly hinder their practical applications. To this end, effective strategies to modify traditional MXenes for targeted electrochemical performance are highly desirable. In this mini review, we briefly summarize the most recent and constructive development in the modulation engineering of 2D MXene-based transition-metal compounds. Firstly, to modify traditional MXenes by intercalating, surface decorating and constructing heterostructures. Secondly, to design novel transition-metal compounds beyond MXenes by precisely controlling the atomic structures, proportions and compositions of constituent elements. Moreover, the critical challenges and perspectives for future research on MXene-based materials are also presented.

image file: c9nr08217d-p1.tif

Yadong Yu

Yadong Yu is currently a Ph.D. student supervised by Prof. Zhimei Sun at the School of Materials Science and Engineering, and Center for Integrated Computational Materials Science, International Research Institute for Multidisciplinary Science, Beihang University. Her research interests focus on the first-principles calculations of two-dimensional materials for energy storage and conversion.

image file: c9nr08217d-p2.tif

Jian Zhou

Jian Zhou received his Ph.D. degree from the Institute of Metal Research, Chinese Academy of Sciences in 2003. Then, he did his postdoctoral research at RWTH (Aachen University, Germany) and KTH (Royal Institute of Technology, Sweden) from 2003 to 2007. He worked as an associate professor at Xiamen University and Beihang University. His current research interests include intermetallics, superalloys, thermoelectric materials and computational materials.

image file: c9nr08217d-p3.tif

Zhimei Sun

Zhimei Sun is a Cheung Kong Scholar Chair Professor in the School of Materials Science and Engineering at Beihang University, China. Professor Sun received her Ph.D. in Materials Science from the Institute of Metal Research, Chinese Academy of Sciences in 2002. Before joining Beihang University, she worked at RWTH Aachen University (Germany) and Uppsala University (Sweden) from 2002 to 2007, and at Xiamen University (China) from 2007 to 2013. Her research interests include structure and properties of phase-change materials for random access memory, mechanical properties of high performance structural materials and optoelectronic properties of two-dimensional materials by both experiments and computational simulations.

1. Introduction

Energy crisis and environmental pollution are driving the further development of energy storage devices for renewable energies.1 Lithium-ion batteries (LIBs), as the state-of-the-art energy storage technology, have become indispensable in our modern day electronics. Meanwhile, under the soaring demand for energy storage devices with greater emphasis on high energy density and safety as well as low cost with sufficient reserves, conventional LIBs are reaching their ceilings.2 Therefore, rechargeable metal-ion batteries (MIBs) utilizing other abundant metal ions such as Na+, K+, Mg2+, Ca2+, and Al3+, which share similar structures and working principles with LIBs, have come into being and show promising potential in addressing energy storage issues.3

As one of the most critical parts of rechargeable MIBs, the performance of electrodes plays key roles. Two-dimensional (2D) materials have become the predominant choice of electrode materials in the pursuit of high energy and power densities owing to their extraordinary physical and chemical properties, such as large surface-area-to-volume ratio and rapid ion-diffusion channel.4,5 Particularly, the family of 2D transition metal carbides and nitrides, as the emerging and fast-growing materials, also referred to as MXenes, is usually obtained by selectively etching the reactive “A” layers in their laminated ternary carbide or nitride precursors (also known as “MAX” phases), as shown in Fig. 1(a).6,7 It has been reported that MXenes show great prospects for their use as electrode materials and receive increasing attention in the application of MIBs due to their excellent electrical conductivity, chemical diversity, and hydrophilicity.8–11 In 2012, Naguib et al. realized the application of MXenes in LIBs for the first time.12 Then, Zhou et al. theoretically predicted the feasibility of using MXenes as electrode materials for LIBs through DFT calculations.13 Following these two pioneering studies, an explosive study has been conducted on the MXene electrodes for rechargeable MIBs both experimentally and computationally.


image file: c9nr08217d-f1.tif
Fig. 1 (a) Structure of MAX phases and the corresponding MXenes. Reproduced with permission.6 Copyright 2013, John Wiley and Sons. (b) Schematic illustration of the synthesis of Ti2CTx by the LiF–HCl treated MAX phase. Reproduced with permission.28 Copyright 2017, WILEY-VCH. (c) Long-term cycling performance of Nb4C3Tx, and schematic illustration of structural changes during charge/discharge cycle. (d) XRD patterns of Nb4C3Tx before and after cycling. (e) Inter-planar spacing as a function of the number of cycles. HRTEM images of Nb4C3Tx after (f) 0, (g) 160, and (h) 1000 cycles. Reproduced with permission.30 Copyright 2017, Elsevier.

However, in practice, due to the electrostatic forces between the layers, MXenes tend to restack and collapse, leading to a severe decrease in electrical conductivity, specific surface area and active site. On the other hand, there are a large amount of dangling bonds on the surface of MXenes, which are readily saturated by some T groups (T = OH, F and O), thus further influencing their electrochemical performance.11,14–17 The surface groups of MXenes, such as OH and O groups, are not easy to precisely control, although fluorine-free high-purity MXenes have been synthesized.18 All of these greatly affect the intrinsic properties of MXenes, especially for electrochemical performance. Therefore, it is of great significance and urgency to modify MXenes when they are actually used as electrode materials for rechargeable MIBs, yet there is still no systematic induction for this intriguing aspect. In this brief review, we will summarize the latest and instructive research progress of modulation engineering in 2D MXene-based transition-metal compounds for MIBs with enhanced performance. Two major modulation directions can be taken into account: one is to effectively modify the traditional MXenes by intercalating, surface decorating and constructing heterostructures, and the other is to design novel transition-metal carbides/nitrides/borides beyond MXenes by precisely controlling the atomic structures, proportions and compositions of constituent elements. Besides, we also propose the future opportunities for this promising class of materials in energy storage and conversion.

2. MXenes in rechargeable batteries

MXenes have a wide chemical and structural variety, and most bare and/or T-group terminated MXenes possess electronic conductivity, which makes them more competitive than other 2D materials when used as electrodes for secondary batteries.13 Besides, as the performance of one single material is limited by its inherent properties, combining MXenes with other nanomaterials is an efficient strategy to deliver the desired electrochemical performance, which should be attributed to the synergistic effects of the composites and the purposefully designed architectures. The performances of MXenes and MXene-based materials for MIBs are summarized in Table 1. It is noteworthy that for modified MXene-based materials, the capacity of metal ions and the cycling stability during charging and discharging are significantly better than those of traditional bare MXenes. The details will be discussed in the following sections.
Table 1 Summary of electrochemical performances of MXenes and MXene-based materials for MIBs
Materials Applications Current rate (mA g−1) Reversible capacity (mA h g−1) Cycle number Capacity after cycle (mA h g−1) Ref.
LIBs, SIBs, MgIBs and KIBs refer to lithium ion batteries, sodium ion batteries, magnesium ion batteries and potassium ion batteries, respectively. Among them, MgIBs use volumetric specific capacity to characterize the battery performance. For current rate, the unit C is sometimes employed, where 1C is equal to the current rate with a value of the theoretical capacity.
Ti2CTx LIBs 0.04C 225 80 110 12
Ti3C2Tx LIBs 1C 123.6 75 118.7 29
Nb4C3 LIBs 100 310 100 380 30
CTAB@Ti3C2Tx MgIBS 50 300 mA h cm−3 250 135 mA h cm−3 31
CTAB-Sb@Ti3C2Tx KIBs 50 516.8 15 475.09 34
Sn@V2C LIBs 100 1284.6 90 1262.9 36
Ag/Ti3C2 LIBs 1C 330 5000 310 37
CNTs@Ti3C2 LIBs 1000 479 250 445 39
CNF/Ti3C2 LIBs 1C 320 2900 320 40
TiO2 (nanorod)/Ti3C2Tx LIBs 500 204 200 209 43
SnO2 (nanowire)/Ti3C2Tx LIBs 1000 560 500 530 43
Fe3O4@Ti3C2 LIBs 1C 336.8 1000 747.4 44
VO2/Ti3C2 SIBs 100 280.9 200 280.9 45
Si@SiOx@C/Ti3C2Tx LIBs 0.2C 1674 200 1547 38
Na0.55Mn1.4Ti0.6O4/Ti3C2Tx LIBs 5000 440 5000 350.68 46
MoS2/Mo2TiC2Tx LIBs 100 646 100 509 49
SnS2/Ti3C2Tx SIBs 100 882 125 322 50
PDDA-BP/Ti3C2 SIBs 100 1635 500 1112 52
Graphene/Ti3C2Tx SIBs 0.25C 600 100 230 57
rGO/Ti3C2 LIBs 50 335.5 1000 212.5 41
δ-MoN LIBs 0.05C 336 200 320 64

2.1. Intercalations

During the synthesis of MXenes, some surface T groups are initially adsorbed. It has been demonstrated that tailoring the surface chemistry of MXenes can further regulate their electronic conductivity.19 Therefore, numerous studies have been carried out to reveal the effects of different functional groups on both voltage and capacity of MXenes as electrode materials, and their significant influence on changing the intercalating behavior of metal ions in MXenes. Compared to OH and F, bare MXenes exhibit complete advantages in terms of capacity and diffusion barrier, while they show lower capacity compared to O terminated MXenes.13,20–23 Besides, MXenes with low weights, such as Ti2C, V2C, Nb2C and Sc2C, have higher gravimetric capacities compared to heavy MXenes.21 Wu et al. investigated the electrode properties of a series of M2C-type MXenes and explored the effect of vacancies on their electrochemical properties for simulating the real experimental conditions.24 However, the surface energy of the thinner M2CTx is higher than that of the thicker M3C2Tx and M4C3Tx, thus M2CTx layers tend to attract each other to decrease energy and establish stable structures, which leads to smaller c-lattice parameters (interlayer spacing) for the storage of metal ions or even restacking.9 In order to avoid MXenes from collapsing and restacking, the most simple and effective way is ultrasonication, which can not only effectively delaminate the MXene flakes and hinder stable interlayer bonds to form again but also accelerate the etching process.25 Mashtalir et al. have synthesized the MXene “paper” with extraordinary electrical properties by ultrasonication for applications in LIBs even at extremely high charging rates.26 Therefore, after weakening the interlayer interactions of MXene layers by intercalating agents such as ions and molecules, ultrasonication-assisted delamination is often required to properly separate the materials into single or few-layer sheets. After that, Sun et al. prepared the intercalated Ti3C2Tx with DMSO under the assistance of ultrasonication.27 Besides, some inorganic compounds have also been introduced into MXenes to increase their interlayer spacing. For example, Kajiyama et al. prepared the steric chloride terminated Ti2CTx with increased interlayer spacing between the individual 2D MXene sheets, as shown in Fig. 1(b).28 Similarly, the high-valence cation Al3+ pre-intercalated Ti3C2Tx was constructed, offering an effective interspace of MXenes to accommodate more metal-ions, and accelerate the ion transport.29

Intriguingly, for Nb-based MXenes, owing to the increased interlayer spacing during cycling, their capacity for metal ions increases gradually. Zhao et al. carried out the long-term cycling test of Nb4C3Tx to reveal its electrochemical performance, as shown in Fig. 1(c–h).30 We can see that with cycling, the capacity of Li+ increased from 116 mA h g−1 to 320 mA h g−1 after 1000 cycles at a current density of 1 A g−1. XRD patterns indicate that the structure of Nb4C3Tx is highly stable and the spacing is reversible during lithium insertion and extraction, since the main peaks were well maintained after cycling. However, the main (002) diffraction peak shifted continuously towards lower angles, which indicates a significant expansion in the interlayer distance. The high-resolution TEM (HRTEM) images further confirm that the d-spacing increased after cycling, consistent with the XRD results. More importantly, the increased interlayer spacing can not only efficiently improve the accessibility of metal ions and afford more active sites, but also accommodate the larger radius and multivalent metal-ions such as Na+, K+ and Mg2+ to provide a fast ion channel, thereby improving the electrochemical performance of MIBs. Xu et al. endowed the freestanding Ti3C2Tx electrode with Mg2+ storage capability by pre-intercalating cationic surfactant cetyltrimethylammonium bromide (CTAB) among delaminated Ti3C2Tx layers.31 Both structural analyses and DFT calculations verified that the intercalated CTA+ can improve the transport kinetics of Mg2+ on the surface of Ti3C2Tx, resulting in significant improvement in the storage capability. The function of the CTAB additive is like a connecting switch that increases the Mg2+ capacity of the Ti3C2Tx electrode from almost zero to an impressive value of 300 mA h cm−3 at 0.05 A g−1.

Very recently, Xia et al. prepared vertically aligned MXene sheets by mechanical shearing of the discotic lamellar liquid-crystal phase of MXenes. As shown in Fig. 2, on the one hand, the vertical alignment of MXene flakes enables directional ion transport, thus reducing the diffusion resistance of metal ions and improving their electrochemical performance.32,33 On the other hand, this can also lead to the thickness-independent electrochemical performances in thick films. Therefore, as the precise control of directional ion transport is of fundamental importance to the fields of electrochemical energy storage, it is highly desirable to extend the application of materials with this morphology to MIBs.


image file: c9nr08217d-f2.tif
Fig. 2 Schematic illustration of ion transport in (a) horizontally stacked and (b) vertically aligned Ti3C2Tx MXene films. Reproduced with permission.33 Copyright 2018, Springer Nature.

2.2. Composites

The strategies for searching MXene-based composites with electrode applications include surface decorations by introducing foreign atoms or molecules, and constructing heterostructures by hybridizing MXenes with other nanostructures.11,17
2.2.1. Surface decorations. The strategy of combining MXenes with other active heterogeneous particles can not only increase the active sites on the surface of MXenes, but also play the role of a “pillar effect” to further prevent the restacking between MXenes layers, thereby greatly improving their electrochemical performance. Hence, some active nanoparticles with high electronic conductivity and energy density such as Sn, Si, Bi, etc. have been introduced to decorate the surface of MXenes.34–36 The as-prepared composites provide an electronic highway to facilitate electron transport and accommodate the volume change in the cycling process. For example, Sn nanoparticles were introduced to decorate Ti3C2Tx through a facile liquid-phase CTAB pre-pillaring and Sn4+ pillaring method, as shown in Fig. 3(a).35 Owing to the pillared structure and synergistic optimization, the energy storage space increased and the resistance of ionic diffusion or charge transfer reduced, endowing the nanocomposites with excellent capacity and electrochemical kinetics of metal ions. Similarly, Sn4+ decorated V2C MXenes in the form of Sn@V2C composites not only exhibit a highly enhanced Li+ capacity of 1284.6 mA h g−1 at 0.1 A g−1, but also possess an outstanding rate and cycling stability of 1262.9 mA h g−1 at 0.1 A g−1 after 90 cycles (Fig. 3(b–d)).36 Besides, Zou et al. introduced metallic Ag nanoparticles into the surface of Ti3C2Tx by directly reducing AgNO3 in Ti3C2Tx aqueous solution.37 On the one hand, Ag spacers can prevent the restacking of MXene layers, permitting more stable chemical properties. On the other hand, Ag metal particles on the surface of MXenes can further improve the electrical conductivity of the composites.
image file: c9nr08217d-f3.tif
Fig. 3 (a) Schematic illustration of the preparation of CTAB–Sn(IV)@Ti3C2. Reproduced with permission.35 Copyright 2017, American Chemical Society. (b) Electrochemical performance of Sn@V2C for (i) rate performance, (ii) galvanostatic charge and discharge curves, and (iii) long-term cycling performance and coulombic efficiency at different current densities. Reproduced with permission.36 Copyright 2019, John Wiley and Sons. (c) MXene/Si@SiOx@C and (d) bare Si electrodes (inset: corresponding digital photographs) after 1000 cycles at 10C. (e) Corresponding schematic preparation procedure of the MXene/Si@SiOx@C nanohybrid. Reproduced with permission.38 Copyright 2019, American Chemical Society.

Another blending protocol is to incorporate carbon-based materials with MXenes. Zheng et al. successfully prepared the carbon nanotube (CNT) and MXene (CNTs@Ti3C2) hybrid structures by a facile microwave irradiation method under ambient conditions.39 Benefiting from the synergetic effect of the connecting CNT bridges and the fine conductive MXene matrices, these composites exhibit excellent electrochemical properties as anode materials in LIBs (430 mA h g−1 and 408 mA h g−1 at 0.1 A g−1 before and after 200 cycles). Other carbon-based materials such as carbon nanofibers (CNFs)40 and reduced graphene oxide (rGO)41 have also been combined with MXenes and demonstrated superior electrochemical performance in MIBs. Additionally, a novel MXene/MOF derivative 2D hybrid (N-Ti3C2/C) was fabricated by in situ nucleation and conversion of ZIF-67 on MXenes.15,42 The uniformly decorated composites can not only isolate MXene nanosheets from restacking, but also leave sufficient Lewis acidic adsorption surfaces exposed on MXenes, thus giving them better electrode performance.

Moreover, the combination of transition metal oxides (TMOs) with high capacity and MXenes with good electronic conductivity is promising for batteries because of the possible complementary optimization between these two components. Liu et al. reported a general route to simply self-assemble TMO nanowires and nanorods on MXene nanosheets through van der Waals interactions.43 By minimizing the surface energy, TMOs can be automatically assembled on MXene flakes. As the structural collapse caused by volume expansion and aggregation of TMOs during charge/discharge cycles and the reduction of active sites caused by restacking of MXenes have been relieved or even avoided, the micelle-like heterostructures exhibit excellent electrochemical properties. Besides, Fe3O4@Ti3C2 hierarchical composites44 and VO2/MXene hybrid architecture have also been synthesized.45 These composites all deliver outstanding capacity and excellent cycle performance for MIBs due to the synergistic effect between the two building blocks. In addition, in order to further improve the electrochemical performance and long-term cycling stability of TMO/MXene composites, Zhang et al. rationally designed and fabricated the MXene/Si@SiOx@C superstructure in a layer-by-layer manner, as shown in Fig. 3(c–e).38 The porous 3D-like architecture synergistically releases the strain, prevents the fracture or pulverization, and keeps the flexible interspace of the electrode materials during the charge and discharge process, showing almost 100% capacity retention after ultra-high cycle times. Besides, based on the bipolar material MXene/Na0.55Mn1.4Ti0.6O4, a stable symmetric MIB was developed.46 Benefitting from its high conductivity and electrochemically active redox couples, this symmetric full cell exhibits the highest energy density among all the symmetric full cells reported so far.

2.2.2. Constructing heterostructures. Integrating different materials to construct van der Waals heterostructures can combine their respective advantages and eliminate the associated shortcomings of individual building blocks to achieve synergy, which can make full use of both the characteristics of components in various aspects such as physicochemical properties.47 As emerging potential candidates for metal-ion storage, various transition metal dichalcogenides (TMDs) have been combined with MXenes to achieve high performance MIBs, especially VS2,48 MoS2[thin space (1/6-em)]49 and SnS2.50 These heterostructures not only enhance their individual conductivity by stacking dissimilar materials together to achieve the multilayer adsorption of metal ions, but also relieve the premature failure of electrode caused by volume expansion upon cycling. Typically, Chen et al. synthesized the intimately contacted 2D MoS2-on-MXene heterostructure with metallic properties and open structure through in situ sulfidation of Mo2TiC2Tx, as shown in Fig. 4(a and b). It takes advantage of the surface Mo–O motifs in Mo2Ti2Tx that can be converted into Mo–S bonds under certain conditions to form MoS2, just like the transformation from MoO2 or MoO3 to MoS2. The synergistic effect renders the MoS2/Mo2Ti2Tx electrode with the superior reversible capacity of Li+, and long-term cycling stability even at high current density (about 509 mA h g−1 at 100 mA g−1 after 100 cycles). Besides, a unique heterostructured graphitic carbon nitride (g-C3N4) and MXene were prepared (Fig. 4(c)).51 Driven by the hydrogen-bonding interaction (e.g., –NH2⋯O–Ti), dicyandiamide molecules were preferentially adsorbed on the surface of Ti3C2Tx, and then followed by thermal treatment in argon to facilitate the in situ growth of g-C3N4. These can regulate the electronic state of heterostructures to enhance the overall kinetic process (ionic and electronic transport) and stabilize the active material upon repeated cation (de)intercalation, leading to improved electrochemical performance.
image file: c9nr08217d-f4.tif
Fig. 4 (a) Schematic illustration of the preparation of the MoS2/MXene hybrid, and (b) its XRD patterns. Reproduced with permission.49 Copyright 2018, John Wiley and Sons. (c) Schematic illustration of the preparation of the MXene@CN heterostructure. The magnified part represents the H bonding that drives the assembly of dicyandiamide molecules on the surface of MXene. Reproduced with permission.51 Copyright 2019, Elsevier. (d) Schematic illustration of the preparation of PDDA-BP/Ti3C2 nanosheets. The corresponding (e) cross-sectional SEM image (inset shows the flocculation), (f) TEM image (inset shows that the film is flexible and freestanding), and (g) HAADF-STEM and elemental mapping images. (h) The charge/discharge and (i) rate performance of different electrodes. (j) Long cycling performance for the PDDA-BP/Ti3C2 heterostructure electrode. (k) Powered LED by two SIBs based on the PDDA-BP/Ti3C2 electrode. Reproduced with permission.52 Copyright 2019, Elsevier.

By performing DFT calculations, it has been found out that the adhesion energy, charge transfer, and band structure of these heterostructures are all sensitive to the surface functional group of MXenes and their stacking order.53 In addition, the weak interaction between the interfaces of different materials can be a promising way to adjust the interlayer space, which provides a good guarantee for the rapid diffusion of metal ions during charge and discharge, especially for Na+ and K+ with larger ionic radius, and multivalent Mg2+ with larger polarization. Demiroglu et al. examined the most stable stacking configurations of different MXene/graphene heterostructures and the possible adsorption sites of metal ions on their surface.54 The calculations disclose that the presence of heterostructures not only avoids the restacking of MXene layers but also enhances the electrical conductivity and metal-ion adsorption strength (while maintaining a high metal-ion mobility). These favorable attributes collectively lead to the excellent performance of MXene/graphene electrodes.41,55

Furthermore, with the increasing demand for wearable electronics and soft robots, the role of flexible batteries is becoming more and more important. 2D heterostructures can combine different mechanical properties of various materials to obtain good and isotropic mechanical properties, which not only ensures the structural stability, but also provides new ideas and paradigms for the design of flexible electrodes.56 Zhao et al. prepared the molecular-level PDDA-BP/Ti3C2 heterostructure through a flocculation process of exfoliated Ti3C2 nanosheet and PDDA-BP (poly diallyl dimethyl ammoniumchloride modified exfoliated black phosphorene) nanosheet with oppositely charged states (Fig. 4(d–k)). As observed from SEM, TEM, and HAADF-STEM with elemental mapping images, a uniformly dispersed and flexible film was obtained. Due to the face-to-face contact of both components, the parallel 2D interlayer spacing provides more synergistic adsorption sites and effective charge transfer and diffusion channels. Consequently, it exhibits extreme structural stability, an ultrahigh reversible Na+ capacity of 1112 mA h g−1 at 500th cycle at 0.1 A g−1 and an ultra-long cycling stability of 658 mA h g−1 with only 0.05% degradation per cycle within 2000 cycles at 1.0 A g−1. Particularly, a red light-emitting diode (LED) (Fig. 4(k)) can be easily lit by two PDDA-BP/Ti3C2 heterostructure electrode devices assembled in series, and efficiently discharges for more than 6 h after charging for only 60 s to reach 3.0 V, confirming the feasible and potential applications of the device.52 A 2D flexible MXene/graphene heterostructure was also manufactured using spray-assisted layer-by-layer assembly.57 It can be directly used as an anode for metal-ion storage without current collectors or binders and exhibits improved electrochemical performance compared to pure MXene and rGO films in terms of capacity, rate performance, and cycling stability. In addition, Li et al. systematically investigated the mechanical and electrochemical properties of the heterostructure composed of MoS2 and Ti2CTx through first principles calculations.56 This further demonstrates that the formation of the heterostructure provides excellent large ultimate strains (>20%) and Young's modulus, which is beneficial for their applications in flexible batteries.

3. 2D transition metal compounds beyond MXenes in rechargeable batteries

In the synthesis process of MXenes, the species and proportions of terminated T groups are arbitrary and inevitable. Thus, the performance of MXenes is usually uncontrollable, which will seriously affect their practical applications. Therefore, it is essential to design novel 2D transition metal-compounds beyond MXenes that with avoided surface functionalization to obtain materials with controlled properties, thereby realizing their practical and broader applications in MIBs.

3.1. MXene-like transition metal carbide/nitride structures

The diversity of the 2D structure of transition-metal compounds (carbides/nitrides) can be achieved depending on the change of the atomic position.16,58 In order to avoid the surface functionalization of MXenes and the harsh fluoride-based etching conditions, Xu et al. first reported the growth of large-area and high-quality 2D ultrathin α-Mo2C crystals without defects, disorders or impurities being observed by the chemical vapor deposition (CVD) method, as shown in Fig. 5.59,60 The crystals are over 100 μm in size and very stable under ambient conditions. Meanwhile, the electrochemical properties of the 2D α-Mo2C structure used as the electrode for MIBs have been studied theoretically by Sun and coauthors.61 Thanks to its large capacity, high mobility, and cycling ability for metal ions, the results highlight that the Mo2C monolayer can act as an appealing candidate anode material for MIBs. Additionally, this CVD method can also be used for preparing other 2D high quality ultrathin structures, such as hexagonal WC and cubic TaC crystals.59,62 For example, the stable and uniform 2D in-plane WC–graphene heterostructure (i-WC–G) was fabricated by CVD in one step,62 and its electrochemical performance and potential application in MIBs is expected to be explored in the near future.
image file: c9nr08217d-f5.tif
Fig. 5 (a) Schematic illustration of 2D carbide crystals produced by CVD. Inset shows the growth of perfect α-Mo2C crystals. (b) Single-layer Mo2C (top) and 3 nm-thick crystals of α-Mo2C (bottom). Reproduced with permission.60 Copyright 2015, Springer Nature. (c) Optical image of ultrathin α-Mo2C crystals. Scale bar, 20 μm; (d) HRTEM shows that Mo atoms (white dots) are arranged in a hcp structure, and C atoms (black dots indicated by red) are located at the center of six Mo atoms (black dots indicated by green circle). Reproduced with permission.59 Copyright 2015, Springer Nature.

Thereafter, more and more transition metal carbide/nitride derived MXenes with different structures from conventional MXenes have been proposed constantly. Recently, a 2D ScCxOH structure has been synthesized through the selective etching of the alternative ScAl3C3 precursor in aqueous organic base solutions. The OH groups are located at the top of the centers with three neighboring Sc atoms, and the Sc atomic layer is sandwiched by the OH groups and C atomic layer.63 In combination with experimental studies and DFT calculations, the underlying mechanisms of the selective etching process, electrochemical properties, and potential applications of this structure were investigated systematically. Besides, Joshi et al. synthesized an MoN nanosheet with tunable composition and grain size by a much higher yield template method, in which MoO3 nanosheets were vertically grown on a substrate by using a hot Mo filament reacting with O2 as a template, and then transformed into δ-MoN.64 When used as an electrode in LIBs, it behaves with an excellent Li+ insertion/extraction mechanism that does not involve chemical transformation. Electrochemical studies have shown that the 2D layered MoN exhibits a stable Li+ capacity of 336 mA h g−1 at C/20 and a long-term cycling stability of 320 mA h g−1 at C/20 after 200 cycles without degradation or structural transformation, indicating its promising potential applications in MIBs.

3.2. Transition metal compounds with increased carbon/nitrogen compositions

Based on the structure of MXenes, an increase in their carbon stoichiometry and exposure of the carbon atoms to the surface play a critical role in protecting metal atoms from exposing on the surface, thereby preventing the surface functionalization by T groups. On the other hand, this strategy can reduce their relative molecular weight and modulate their lattice constants to accommodate more metal ions, thus greatly improve the specific capacity and the safety of rechargeable MIBs.65 For most commonly studied 2D Ti-based compounds with increased carbon content, a large number of studies have been conducted to investigate the structural changes, intrinsic properties, and their potential applications in MIBs.66–70 Sang et al. successfully synthesized a hexagonal TiC single layer with the surface of the Ti3C2 MXene substrate as a source material by thermal exposure and electron-beam irradiation.67 The schematic illustrations of the homoepitaxial growth process and the corresponding h-TiC are presented in Fig. 6(a–e). Based on ab initio calculations, Fan et al. reported the tetragonal TiC and TiN phase, which possesses the intrinsic metallic nature, high thermal stability, and satisfactory metal-ion storage and rate capability.66 Then, Meng et al. proposed a 2D MXene-like structure of the Ti3C4 monolayer through a carbonized Ti3C2 structure.68 By means of increasing the carbon content and exposing half of the carbon atoms on the surface in the form of C2 dimers, not only the stability of the structure can be improved, but also metal ions can be adsorbed directly to enlarge the capacity. After that, Zhao et al. continued to increase the carbon stoichiometry in the TiC structure and resulted in a rectangular lattice, exposing all the C2 dimers to the surface completely.69 This strategy further improves the structural stability and decreases the relative molecular weight of 2D TixCy monolayers, all of which are desirable as electrode materials for MIBs. Besides, Yu et al. designed a metallic graphene-like TiC3 monolayer in the presence of unusual n-biphenyl units, as shown in Fig. 6(g), providing remarkable high storage capacity, low diffusion barrier and open-circuit voltage (OCV) of metal ions. Even in the bulk-stacked limits, TiC3 exhibits better electrode performance compared to Ti3C2 MXene.70
image file: c9nr08217d-f6.tif
Fig. 6 (a) Schematic illustration of the homoepitaxial growth process of h-TiC through electron-beam irradiation and heating. (b) Crystal structure of monolayer Ti3C2Tx and atomic resolution STEM image acquired from monolayer Ti3C2Tx along the [001] zone axis at (c) room temperature, (d) 500 °C and (e) 1000 °C. Reproduced with permission.67 Copyright 2018, Springer Nature. (f) Top and side view of (i) Mo2C, (ii) MoC and (iii) MoC2 and ELF of the section. Reproduced with permission.65 Copyright 2019, Royal Society of Chemistry. (g) Structure of the TiC3 monolayer, zigzag Ti atom chain and n-biphenyl structural unit and ELF map. Reproduced with permission.70 Copyright 2018, American Chemical Society.

All the aforementioned structures have proved to be stable and exhibit excellent electrochemical properties, indicating that they have great potential application prospects in MIBs. Particularly for the structure of TiC2, the transition-metal layer is sandwiched between the top and bottom C2 dimers without being exposed on the surface. Thus, the problem that the surface of MXenes is composed of transition-metal atoms and thus requires surface functionalization has been avoided or solved. Therefore, scientists focused their attention mainly on the study of MC2 style 2D materials hereafter. Recently, Yu et al. reported a novel Mo-based monolayer through first principles swarm structural search, which shows an iso-structure with the previously reported TiC2 monolayer. They also revealed the bonding nature and stabilizing mechanism of the MoC2 structure.65 As shown in Fig. 6(f), as the carbon chemical composition increased and exposed on the surface, dangling bonds like in MXenes were observed, indicating that the surface functionalization has been reduced or even disappeared. Besides, they further demonstrated that the formation of the C2 dimer plays an important role in stabilizing the metallic 2D structures, and modulating their lattice constants to accommodate more metal ions, thus greatly improving the specific capacity and safety of MIBs. In addition, 2D TaC2, VC2, and bi-transition-metal carbides of V1/2Mn1/2C2 similar to those of the TiC2 monolayer were consecutively presented.71–73 The theoretical storage capacity of metal ions is much higher than that of MXenes and commercial graphite, and the rate performance and stability can be well maintained. Furthermore, for the newly synthesized 2D material system of 1H-MoN2 with an increased nitrogen content, Zhang et al. have studied the adsorption and diffusion behavior of various metal ions on its surface to show its promising prospects in electrochemical applications.74

3.3. Transition metal borides (MBenes)

In order to expand the extensive compositional varieties of MXenes and improve their stability and electrode performance simultaneously, MBenes (M = transition metal, B = boron), which are considered as the analogs to graphene-like MXenes, have been proposed and prepared by replacing carbon and/or nitrogen atoms with boron atoms. Initially, Guo et al. first proposed the structure of MBenes, such as Mo2B2, and Fe2B2, and predicted their possible synthetic method from MAB (M = transition metal, A = aluminum, and B = boron) precursors in the experiment, as shown in Fig. 7(a).75 Meanwhile, Jiang et al. identified about twelve stable MBene sheets that may exist in reality by using ab initio high-throughput calculations.76 Khazaei et al. further extended the potential family members of the precursor MAB phase theoretically, and revealed the mechanism of the successful exfoliation of MBenes by the difference of bonding energy among B–B, M–B, B–B and M–B.77 Further research has found that the emerged MBenes are metallic with excellent electronic conductivity, omnidirectional small diffusion barrier and high storage capacity for metal ions, which are ideal for applications in MIBs. Therefore, the increasing interest of scientists in exploring the applications of 2D MBene-based materials in rechargeable batteries has been sparked. Based on the several considered factors for electrode materials, including high stability, high electronic/ionic conductivity, small molecular weight, and sufficient metal-ion accommodating site, Bo et al. reported a series of 2D transition metal borides.78–80 Among all those predicted materials, tetragonal and trigonal Mo2B2, hexagonal Ti2B2 and Zr2B2 structures are the most prominent, which possess sufficient capacity, stability, safety and excellent charge/discharge capability under operating conditions.
image file: c9nr08217d-f7.tif
Fig. 7 (a) Atomic configuration of Mo2B2 during the step-by-step interaction of HF via the Mo2AlB2 edge (the red, green, black, cyan, and pink balls represent Mo, B, Al, F, and H atoms, respectively). Reproduced with permission.75 Copyright 2017, Royal Society of Chemistry. (b) ADF-STEM image of isolated, delaminated MoB sheets inside an etched cavity (left), higher-magnification and contrast-enhanced image of the Mbene sheets (middle), and structure of the delaminated region of MBene sheets (right). Reproduced with permission.81 Copyright 2018, American Chemical Society. Adsorption and diffusion behavior of Li/Na on 2D TiB (c) ELF maps of the TiB monolayer with one layer of Li/Na atoms. (d) Considered diffusion paths and (e) corresponding diffusion energy. (The purple and yellow spheres represent Li and Na atoms, respectively.) Reproduced with permission.82 Copyright 2019, Springer Nature.

More importantly, previous research suggests that MBenes may be accessible by using alternative etchants that are safer compared to the fluoride-based etchants required for MXene synthesis. To demonstrate such feasibility, a 2D CrB nanosheet was first prepared by the selective etching of Al from Cr2AlB2 in diluted hydrochloric acid (HCl) at room temperature.83 Hereafter, Alameda et al. synthesized the isolated MoB MBene monolayer by room-temperature reaction with NaOH in a stepwise manner.81 They further revealed the mechanism of Al deintercalated in the MoAlB precursor by high-resolution microscopic (ADF-STEM) investigation, which we can see from Fig. 7(b). It has been demonstrated that the topochemical deintercalation of Al from the MoAlB phase occurs in a stepwise manner, involving the formation of stacking fault bands that grow over time until a high density of stacking faults is reached, and then Al is removed from stacking faults to form 2D MoB (MBene). Very recently, by combining both theoretical and experimental studies, Wang and co-workers obtained the layered TiB MBene by the removal of the In atom layer through dealloying of the nascent parent Ti2InB2 MAB phase at high temperature under high vacuum.82 The obtained TiB exhibits superior stability over the conventional MXenes. Besides, DFT calculations also predicted that the conductive nanosheet would be a promising anode material for MIBs with respect to low OCV, low diffusion barrier, and high theoretical capacity for metal ions, as shown in Fig. 7(c–e). Therefore, it is expected that more newly developed 2D MBenes will be further manufactured and applied in rechargeable batteries, which is regarded as the extension of the fascinating MXene-like materials.

4. Conclusions and perspectives

In conclusion, we have briefly reviewed the most recent and instructive developments in modulating the MXene-based electrodes with enhanced performance for rechargeable MIBs. The modulation engineering is highlighted from the following two aspects: effective modification of traditional MXenes by intercalating, surface decorating or constructing heterostructures, and design of novel transition-metal carbides/nitrides/borides beyond MXenes by precisely controlling the atomic structures, proportions and compositions of constituent elements. Recent studies have shown that the above-mentioned strategies are not only useful for improving the structural stability and obtaining the surface-controllable 2D MXene-based materials, but also for achieving high-performance electrodes with sufficient metal-ion capacity, safety, and excellent charge/discharge capability. It is of great significance for guiding the development of 2D materials and renewable energy technology.

Beyond the exciting achievement, there are still many opportunities and challenges that we should take into consideration and overcome immediately. Firstly, the research of MXene-based materials for MIBs is still in a preliminary stage. To date, more than 70% of all MXenes research has focused on the first discovered MXene, Ti3C2Tx. As there are more than 30 MXene composites being synthesized, attention should also be extended to various MXenes to explore their application potential in energy storage. Besides, the difficulty in the large-scale preparation of single/few-layer MXenes and the realization of controllable surface remains the gap between laboratory research and practical applications. Thus, it is promising to develop new methods for synthesizing high-quality MXenes with large lateral dimensions, no defects and controlled surface terminations. Furthermore, MXene flakes with a vertically aligned architecture can remarkably improve ion/electron transport. Advanced technologies such as electrospinning, electrophoretic deposition and electrochemical deposition could be used to fabricate vertically aligned MXenes.

Secondly, the study of novel 2D transition-metal compounds beyond MXenes remains mostly by theoretical methods. Particularly for 2D MxCy monolayers with increased carbon/nitrogen stoichiometry, computations have predicted that they have excellent electrical conductivity, more surface active sites for the adsorption of metal ions, and high feasibility for experimental synthesis. Therefore, extensive investigation by experiments is urgently required to realize the preparation or even commercialization of such promising materials. Additionally, it is highly desirable to discover more and more novel 2D MXene-based materials, to dig into their exciting properties in depth and to expand their intriguing applications in many other energy-related fields. Specifically, under the consideration of their structural and compositional diversity, energy storage applications such as supercapacitors, hybrid capacitors, lithium–sulfur batteries, and lithium–air batteries could be designed purposefully. Furthermore, MXene-based materials can be used as catalysts/co-catalysts in energy conversion applications due to their more open architectures. They have shown promising prospects in the electro/photocatalytic water splitting, oxygen reduction reaction (ORR), reduction of carbon dioxide, and ammonia fuel production, exhibiting great potential to replace noble metal catalysts.

Finally, computations would further guide the study of the preparation and application of these MXene-based materials in the future to avoid the cost and time-consuming trial-and-error experiments. Overall, we look forward that both theoretical study and experimental research could be conducted together to provide new and deeper insights into the design and synthesis of 2D materials as well as their applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Key Research and Development Program of China (No. 2017YFB0701700) and the National Natural Science Foundation of China (No. 51871009).

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