Recent progress of nanostructured metal chalcogenides and their carbon-based hybrids for advanced potassium battery anodes

As can be seen in Fig. 2a and b, N-doped carbon-coated mesoporous α-Fe₂O₃ hollow bowls have been designed and prepared by Qin and coworkers.³⁶ The construction of 3D models and finite element simulations has proved that the hollow bowl inherits the advantages of the hollow sphere, while the void space acts as an effective buffer to combat the volume variation during the successive potassiation–depotassiation process; the enriched pores homogeneously scattered in the shell make a great contribution to the distribution of stress. Benefiting from the N-doped carbon protection and high-rate induced potassiation reactivation in mesoporous hollow structures, this novel anode exhibits desirable cycling durability. Limited by the low electronic conductivity and extreme volume expansion upon potassiation, Fe₃O₄, used as the negative electrode, cannot realize the perfect specific capacity and long cycle life. Qu et al. constructed a three-dimensional N-doped porous graphene framework integrated with Fe₃O₄ nanoparticles (Fe₃O₄/3DNPGF) by a facile chemical blowing method (Fig. 2c).³⁷ The as-prepared hybrid shows a 3D honeycomb-like framework structure with considerable interconnected walls (Fig. 2d), which could effectively elevate the electron transportation efficiency. The carbonaceous nanowalls were a crucial factor for promoting the uniform dispersion of the Fe₃O₄ nanocrystals, which was confirmed by the TEM image. Moreover, poly(vinylidene fluoride) (PVDF) and poly(acrylic acid) (PAA) are used as a binder. The results demonstrated that the unconventional PAA binder does better than the universal PVDF binder over the electrochemical performance. Compared to the integrated PAA-Fe₃O₄/3DNPGF electrode, a spindly crack in the PVDF–Fe₃O₄/3DNPGF electrode appeared after cycling (inset of Fig. 2e). The TEM image (Fig. 2f) shows that the SEI layer was steadily exposed on the surface of the PAA–Fe₃O₄/3DNPGF, and the energy-dispersive X-ray spectroscopy (EDS) elemental mapping revealed the fluoride (F) and sulfur (S) elements derived from the components of the SEI layer. It is an essential procedure to select an appropriate binder to achieve the superior electrochemical performance of PIBs. Iron oxides are capable of delivering a satisfactory reversible K ions storage capacity via the conversion-type mechanism; Li's group proposed an easily scalable chemical bubbling method for the in situ construction of hollow Fe_xO nanospheres fixed on the 3D N-doped few-layer graphene framework (Fe_xO@NFLG) and applied as an anode material for high-performance PIBs.³⁸ The as-synthesized electrode displayed extraordinary rate capability (176 mA h g⁻¹ at 5 A g⁻¹) and cycling recoverability, which retained a high reversible capacity of 206 mA h g⁻¹ over 1000 cycles at 2 A g⁻¹. They carried out the mechanistic research on the K ion storage mechanism of Fe_xO@NFLG; ex situ XPS and selected-area electron diffraction (SAED) were performed to determine the insertion and extraction of K ions at a different state. When discharged to 0.01 V, the reduction of Fe_xO to generate Fe⁰ can be evidently observed. When charged to 3.0 V, the XPS peaks almost reverted to the initial state, which verified the reoxidation of Fe⁰, and the SAED results are in good agreement with the XPS analysis and depict the following steps.

	Fig. 2 Nanostructured metal oxides for potassium batteries: (a and b) N-doped carbon-coated mesoporous α-Fe2O3 hollow bowls for potassium battery. Reproduced with permission.36 Copyright 2019, Wiley-VCH. (c–f) Fe3O4/3DNPGF for potassium battery. Reproduced with permission.37 Copyright 2019, The Royal Society of Chemistry. (g–j) Co3O4@N-doped carbon composite for the potassium battery. Reproduced with permission.45 Copyright 2020, American Chemical Society.

Chemical dealloying technology, a simple and cost-efficient corrosion process, is applied to synthesize 2D nanoporous Co₃O₄ nanosheets and use as the anode for PIBs.⁴³ Taking advantage of the 2D porous nanosheets architecture, faster diffusion of ions, intimate contact of the electrode and the electrolyte, rapid potassiation/depotassiation process can be obtained and further bring about excellent electrochemical performance. Relevant literature survey shows that the mixed systems with different mechanical properties exhibit favorable Li and Na storage capacities, where all of them act as active materials and can accommodate each other. Using this idea, a low-cost and scalable molten salt method combined with ball-milling was adopted and the Co₃O₄–Fe₂O₃/C nanocomposite was successfully prepared by Rahman and coworkers.⁴⁴ The results of this study indicate that the Co₃O₄–Fe₂O₃/C nanocomposite may have potential applications in sustainable and cost-effective PIBs. Noticeably, the hierarchical Co₃O₄@N-doped carbon composite was precisely designed and tested as an anode for PIBs (Fig. 2h);⁴⁵ the resulting electrode exhibited excellent rate capability and sustained the capacity of 213 mA h g⁻¹ at 0.5 A g⁻¹ after 740 cycles (Fig. 2i and j). Impressively, the results of the density functional theory calculation reveal feeble conductivity, high diffusion barrier, and weak potassium interaction, resulting in an unsatisfactory capacity in large-sized alkali-ion storage systems. Specifically, a nitrogen-doped carbon layer and increased Co₃O₄ spacing have been obtained through interface engineering (Fig. 2g), which ameliorated the conductivity and facilitated K ions diffusion, and protected the electrode materials from damage upon potassiation. The results of the DFT calculations revealed that the Co₃O₄@N-doped carbon composite possesses a higher K-adsorption energy compared to Co₃O₄ and the N-doped carbon structure, indicating that the composite can absorb more K ions.

Luo et al. employed a generally used hydrothermal approach to prepare MoO₂ nanoparticles evenly distributed on three-dimensional porous carbon architecture.⁵⁰ A similar topic was further studied by Liu's group, wherein MoO₂ hollow sub-microspheres MoO₂ nanoparticles were uniformly anchored on two-dimensional layered reduced graphene oxide sheets (MoO₂/rGO) (Fig. 3h and i).⁵¹ The details of the electrochemical performance of the MoO₂/rGO composite are amply exhibited in Fig. 3j. The average charge capacities of the MoO₂/rGO composite were found to be 282, 240, 214, and 176 mA h g⁻¹ at current densities of 0.05, 0.1, 0.2, and 0.5 A g⁻¹, respectively. The specific capacity recovers to a highly reversible capacity of 266 mA h g⁻¹ as the current density returns to 0.05 A g⁻¹, which can be retained for more than 100 cycles, suggesting the high-rate capability and good cycling performance. Inspired by the previous reports, the key point that was found was that replacements with favorable diffusion pathways and stable structures are required to accommodate the large-sized K ions during the repeated insertion/extraction processes. Based on this tip, the MoS₂@MoO₂ heterojunction wrapped into the hierarchical Fe and N dual doped carbon framework (MoS₂@MoO₂@Fe@CN) was controllably prepared and used as the anode for PIBs.⁵² The as-synthesized unique heterostructure exhibited a superior rate capability, maintaining 160 mA h g⁻¹ at 10 A g⁻¹ for potassium storage and exceptional long-term cycling stability of 500 cycles without obvious capacity degradation at 0.5 A g⁻¹. Fe, developed as a critical precursor, plays a major part in preparing the MoS₂@MoO₂@Fe@CN heterostructure in this protocol. With the help of Fe, abundant hollow vesicles were generated in the CN matrix and MoO₃ was reduced to MoO₂. In addition, a detailed theoretical calculation of K-adsorption energy was conducted by DFT. The results show that MoS₂@MoO₂@Fe@CN possesses a stronger adsorption capacity toward K atoms; after introducing MoO₂, the transportation capabilities of electrons and ions get enhanced, and more active sites are approached. Moreover, electrons in MoO₂ tend to flow from MoO₂ to MoS₂ for achieving the same Fermi level, which leads to the formation of an internal electrical field between MoO₂ and MoS₂, guiding electron transfer from MoO₂ to MoS₂ at the interface. The calculated DOS for MoO₂ appears to be attenuated at the Fermi level, which further confirms electron transfer from MoO₂ to MoS₂. MoO₂ displays more approximate metallic properties and thus favorable conductivity; the electron transportation capability can be elevated when readily incorporated with MoO₂. In view of the analysis regarding the dynamic relationship between the heterojunction and the K storage performance, Qin and coworkers light up a novel route to construct heterostructures for effective PIB energy storage systems.

	Fig. 3 Nanostructured metal oxides for potassium batteries: (a–c) tremella-like SDVO for K-ion storage. Reproduced with permission.61 Copyright 2019, The Royal Society of Chemistry. (d–g) V2O3@PNCNFs for K-ion storage. Reproduced with permission.60 Copyright 2018, Elsevier. (h–j) MoO2/rGO composite for K-ion storage. Reproduced with permission.51 Copyright 2019, Wiley-VCH.

Vanadium trioxides (V₂O₃), with an open tunnel structure provided by the 3D V-V framework (Fig. 3d), have attracted great attention as fantastic electrode materials in energy storage and conversion systems.⁶⁰ An advanced electrode material, in which V₂O₃ nanocrystals are well embedded in porous N-doped carbon nanofibers (V₂O₃@PNCNFs), has been fabricated through a universal electrospinning strategy and the subsequent calcination process (Fig. 3e). The potassium storage mechanism of V₂O₃ has been investigated by electrochemical kinetic analysis, in situ XRD, ab initio molecular dynamics (AIMD), and DFT calculations. The electrochemical kinetics analysis in Fig. 3g reveals that K storage is mainly dominated by intercalation pseudocapacitance, which is beneficial for high-performance potassium ions storage (Fig. 3f). In situ XRD patterns of V₂O₃@PNCNFs confirmed that K ions intercalate into the tunnels of V₂O₃ accompanied by a faradaic charge-transfer with no crystallographic phase change. The V 2p XPS spectra of V₂O₃@PNCNFs at different electrochemical stages further verified the reversible intercalation pseudocapacitance process (V₂O₃ + xK + xe⁻ → KxV₂O₃). AIMD and DFT calculation revealed that only up to 1 mol K ions can be inserted into the V₂O₃ crystal and occupy the 6e sites of the KV₂O₃ crystal. Unlike the regular transition metal oxides with conversion-type mechanism, this work reveals a unique avenue for the development of a high-rate and ultra-stable PIBs storage system. In addition, a 3D multi-hierarchical tremella-like Sn-doped V₂O₅ (SDVO) nanostructure was prepared through the self-assembly process by Bao and coworkers and served as the negative electrode material in PIBs (Fig. 3a and c).⁶¹ The surface energy calculation results demonstrate the conversion of V₂O₅ from nanoparticles to nanosheets via a strong facet-induced effect. The first-principles calculations and the four-probe method simultaneously confirmed that the incorporation of Sn boosts the electronic conductivity of V₂O₅ remarkably (Fig. 3b). The tremella-like nanostructure with plenty of mesopores and a large specific surface area, which can shorten the transmission path of K ions and increase the permeation of the electrolyte, contributed to a prolonged cyclic performance of 188 mA h g⁻¹ at 0.5 A g⁻¹ over 3000 cycles.

Orthorhombic niobium pentoxide (T-Nb₂O₅) has gained adequate momentum as an ideal anode material for alkali-ion batteries due to the large (001) interplanar lattice spacing, which allows to accommodate massive alkali ions into the layers. Based on this merit, Tang et al. synthesized a T-Nb₂O₅ nanomaterial with a hierarchical urchin-like structure assembled by nanowires via a facile hydrothermal process (Fig. 4a).⁶⁴Ex situ XRD patterns and XPS analyses proved that the potassiation/depotassiation process of the T-Nb₂O₅ electrode follows the intercalation-pseudocapacitive hybrid mechanism. The T-Nb₂O₅ nanomaterial shows enhanced K storage performance with a reversible capacity of 104 mA h g⁻¹ at 0.4 A g⁻¹ after 400 cycles. Furthermore, nanoscale surface-engineering technology is introduced into the field of energy storage for modulating the electrochemical properties of PIBs; Lee et al. examined the electrochemical performance of the partially surface-amorphized and defect-rich black niobium oxide@graphene nanosheets.⁶⁵ The characterization report displays that enormous defects, as well as an amorphous surface layer, are favorable for K ions storage, facilitated electron transport, and enhanced pseudocapacitance energy storage, while seamless contact between the electrode and the electrolyte are responsible for superior-performance PIBs.

	Fig. 4 Other nanostructured metal oxides for potassium batteries: (a) T-Nb2O5 nanowire for K-ion storage. Reproduced with permission.64 Copyright 2018, The Royal Society of Chemistry. (b) HeTiO2eC micro-tubes for K-ion storage. Reproduced with permission.69 Copyright 2019, Elsevier. (c and d) CuO nanoplates for K-ion storage. Reproduced with permission.72 Copyright 2019, Wiley-VCH. (e and f) Sb2O3–RGO composite for K-ion storage. Reproduced with permission.74 Copyright 2019, Wiley-VCH. (g and h) SnO2@C for K-ion storage. Reproduced with permission.77 Copyright 2019, Elsevier. (i and j) SnO2@CF for K-ion storage. Reproduced with permission.78 Copyright 2020, The Royal Society of Chemistry.

Anatase TiO₂, a non-toxic and robust structural stable material, appears to have visible applications in alkali metal-based secondary battery systems.^66–68 The main limitations in the practical application of TiO₂ as an anode of alkali-ion batteries are mostly due to the poor intrinsic electronic conductivity, which is due to the larger bandgap of TiO₂ (3.0 eV), leading to a low specific capacity and a significant capacity loss at a high rate. Nanoengineering and hybridization with conductive carbon materials are commonly applied to enhance the electrochemical performance of TiO₂. To solve these tricky issues, Yang et al. elaborately designed and prepared hierarchical HeTiO₂eC micro-tubes constructed from a myriad of heterostructured TiO₂eC nanosheets by a simple wet-chemical strategy, as displayed in Fig. 4b.⁶⁹ The TiO₂/C heterointerface formed in the TiO₂eC nanosheets optimizes the diffusion of the electrons and K ions, endowing the electrodes with a high reversible capacity and excellent rate capability. In addition, using the less stable Ti₂C as a precursor for the first time to synthesize TiO₂,⁷⁰ Zhu et al. reported that TiO₂ nanoparticles with sizes in between 15 and 25 nm were encapsulated in the RGO nanosheets to form a composite with a sandwich sheet-like structure and tested as anode materials for PIBs. TiO₂ nanoparticles and the incorporation of RGO with high conductivity bring about a remarkable rate ability and a high K ion storage capacity.

Based on the electrochemical conversion reaction with high reversible capacity and appropriate working potential, CuO has aroused significant attention as an anode material for LIBs and SIBs due to its abundance, chemical stability, and environmental benignity. Cu nanoparticles generate and implant in the Li₂O/Na₂O matrix during the discharge process, and then reoxidize to CuO.⁷¹ Lately, Cao and coworkers synthesized copper oxide (CuO) nanoplates and utilized them as high-performance anode materials for PIBs (Fig. 4c and d),⁷² and different K reaction pathways have been elucidated based on various in situ characterization results. The electrochemical reaction mechanism of this CuO nanoplate electrode is determined to be a conversion reaction mechanism that occurs as follows.

Cu nanoparticles are formed during the first potassiation process and then are converted to the Cu₂O nanoparticles in charge. Subsequently, the conversion reaction occurs between the generated Cu₂O and Cu instead of the initial CuO, yielding a theoretical specific capacity of 374 mA h g⁻¹.

It is a strenuous task to look for suitable anode materials to accommodate the large K ions for obtaining high rate capacity and superior stability; painstaking efforts have been made and substantial research progresses have been achieved by several groups.⁷³ Currently, the optimization of electrolyte is an efficient tactic to further ameliorate the K ion storage capacity; Li and coworkers developed a facile solvothermal method to prepare a 2D Sb₂O₃–RGO composite and thoroughly investigated the effects of different electrolytes on the battery performance (Fig. 4e and f).⁷⁴ DFT calculations unveiled the battery using the ether-based electrolyte, which shows a lower energy exchange and migration barrier compared to those using other electrolytes for K ions; the galvanostatic charge–discharge profiles of ether-based electrolytes show no obvious capacity decay. Thus, to get better rate capability and reversibility of the Sb₂O₃ electrodes, optimizing the electrolyte seems to be an indispensable procedure for designing superior-performance PIBs.

SnO₂, a wide band-gap N-type semiconductor material, has intrigued major research areas as an anode material for LIBs and SIBs.⁷⁵ However, due to abundant reserves and relatively low discharge plateau, SnO₂ has also been demonstrated to be electrochemically active in PIBs. Unexpectedly, SnO₂ electrodes usually suffered from low electric conductivity and a non-negligible volume change during potassiation and depotassiation processes, which inevitably resulted in electrode pulverization, rapid capacity decay, and poor rate capability.⁷⁶ An effective method is to combine 3D porous carbon with the active material, which can accommodate the excessive volume expansion as well as prevent the aggregation of nanoparticles. Wang et al. prepared a novel nanocomposite of ultrafine SnO₂ nanoparticles anchored in the 3D porous carbon by freeze-drying, followed by the sintering and dealloying processes (Fig. 4g and h).⁷⁷ After the selective etching of Cu from Cu₆Sn₅ nanoparticles, the final product of SnO₂ nanoparticles was harvested. The distinct microstructure of the 3D carbon networks provides sufficient buffer room for the volume expansion of SnO₂ and abundant channels for the transfer of ions and electrons. Benefitting from these advantages, the 3D SnO₂@C anode delivers a superior rate capability of 145 mA h g⁻¹ at 2 A g⁻¹ and an ultra-long cyclic performance of 110 mA h g⁻¹ over 2000 cycles at 1 A g⁻¹ in PIBs. In addition, Hou et al. reported a novel method for the synthesis of SnO₂@carbon foam (Fig. 4i and j).⁷⁸ SnO₂ nanoparticles were anchored on the 3D carbon foam (SnO₂@CF) through the electrodeposition process; the SnO₂@CF possesses a 3D conductive network, a tight intimate contact between electrode and electrolyte, and accelerated K ion transfer. Encouragingly, the freestanding SnO₂@CF electrode displays an impressive cycle stability of 231.7 mA h g⁻¹ after 400 cycles at 1 A g⁻¹ and an outstanding rate performance of 144 mA h g⁻¹ at 5 A g⁻¹ in the PIBs.

Manganese oxides exist in diverse stoichiometric forms; MnO₂ and Mn₃O₄ have been widely investigated for secondary batteries due to their high theoretical capacity, low oxidation potential, and environment friendliness. However, inferior intrinsic conductivity and drastic volume variation impede the practical application for alkali-ion batteries.^79,80 To overcome these shortcomings, Zhang et al. proposed a simple liquid phase strategy for the synthesis of hierarchical lamellar-structured MnO₂@graphene composite; the distinct hierarchical lamellar structure gives the MnO₂@graphene composite a boost in the rate and cycling performances by providing abundant interfacial interactions for the better transport of K ions.⁸¹ Furthermore, Nithya and coworkers constructed a Mn₃O₄@rGO architecture by anchoring Mn₃O₄ nanospheres on the surface of the rGO sheets; the smart design guaranteed that the expanded Mn₃O₄ nanospheres pose no threat to the electrode materials, thus obtaining a remarkable rate performance of 95 mA h g⁻¹ at a high rate of 10 A g⁻¹ and an ultralong cycling stability of 635 mA h g⁻¹ at 0.5 A g⁻¹ over 500 cycles.⁸²

Prussian blue, consisting of iron ions coordinated to rigid organic cyano groups, has been readily developed to design and synthesize porous nanostructured iron disulfide owing to the strong size-dependent and shape-dependent properties. Xu et al. demonstrated a facile and novel approach for the synthesis of core–shell FeS₂@C nanocubes using Prussian blue as the starting material (Fig. 5a and b).⁸⁵ Harnessing the beneficial structural design wrapped with an amorphous carbon layer is critical for achieving the remarkable performances for PIBs. The inside FeS₂ nanoparticles are favorable for K ion diffusion and the permeation of the electrolyte, while the outside carbon layers can substantively upgrade the electronic conductivity and alleviate the volume variation. The exquisite design of the core–shell structure exhibits fascinating K storage performance with an admirable specific capacity, superior rate capability, and durable cycling stability (Fig. 5c). A novel multi-layer structured FeS₂@C composite composed of considerable 2D nanoflakes was synthesized through a general solvothermal reaction, accompanied by the subsequent sulfidation process.⁸⁶ FeS₂ nanoflakes were tightly wrapped by the coating carbon layer, which served as the anode for the PIBs; the as-synthesized electrode displays an outstanding rate capability of 182 mA h g⁻¹ at a high current density of 10 A g⁻¹ and a long-life cycle performance by retaining a reversible capacity of 295 mA h g⁻¹ after 150 cycles at 1 A g⁻¹. It was claimed that such a superior performance is associated with a multi-layer structure that effectively enhances the electronic conductivity and mechanically constrains the volumetric change, contributing to preserving the structural integrity upon cycling. Zhang's group jointly designed and fabricated graphene-coated FeS₂ nanoparticles embedded in carbon nanofibers (FeS₂@G@CNF) through an electrospinning process.⁸⁷ Dual carbon modification from graphene coating and carbon fibers is advantageous for fast electron and ion diffusion and the structural integrity; the resultant FeS₂@G@CNF electrode was capable of achieving good cycling stability (120 mA h g⁻¹ after 680 cycles at 1 A g⁻¹) and rate capability (171 mA h g⁻¹ at 1 A g⁻¹). Mai et al. reported the metal–organic framework-derived FeS₂ hollow nanocages@reduced graphene oxide composite and used it as the anode for PIBs.⁸⁸ The synergistic effect between the FeS₂ nanocages and the reduced graphene oxide not only curbs the volume variation but also restrains the transformation of the polysulfides upon cycling, hence perfectly enhancing the cycling performance, while the stable specific capacity of 123 mA h g⁻¹ is retained after 420 cycles at 0.5 A g⁻¹.

	Fig. 5 Nanostructured metal sulfides for potassium batteries: (a–c) FeS2@C nanocubes for the potassium battery. Reproduced with permission.85 Copyright 2020, The Royal Society of Chemistry. (d–f) AC@CoS/NCNTs/CoS@CNFs for K-ion storage. Reproduced with permission.91 Copyright 2019, The Royal Society of Chemistry. (g–i) Yolk–shell NiSx@C nanosheets for the potassium battery. Reproduced with permission.99 Copyright 2019, The Royal Society of Chemistry.

Zhang and coworkers accurately fabricated a novel hierarchical structure constructed by nitrogen-doped carbon nanotubes, amorphous carbon wrapped CoS, and CoS-coated carbon nanofibers (AC@CoS/NCNTs/CoS@CNFs) (Fig. 5d and e).⁹¹ Such a 3D structure design enlarges the specific surface area, which decreases the path of K ion diffusion and increases the contact area of the electrode and the electrolyte. When tested as an anode for PIBs, it can exhibit outstanding high-rate cycling stability of 130 mA h g⁻¹ at 3.2 A g⁻¹ after 600 cycles (Fig. 5f). Actually, coupling with conductive carbon materials is a generally used strategy for improving the cycling stability of cobalt sulfides, Lin et al. synthesized porous dual-shell Co₉S₈ hollow spheres coated with protective layers of carbon nanotubes and investigated them as an anode material for PIBs.⁹² The synergistic interaction derived from the hollow structure and carbon nanotubes enhanced the conductivity and delivered an ultra-stable reversible capacity of 164 mA h g⁻¹ at 1 A g⁻¹ after 1000 cycles. To the best of our knowledge, as the diameter is less than 10 nm 0D materials, quantum dots (QDs) have a large surface area and short ion/electron transport distance. The integration of the electrophilic carbon atoms of graphene with the QD materials can prevent its self-aggregation. Along this line, Guo et al. firstly reported a cobalt sulfide and graphene (CoS@G) composite as the negative electrodes for PIBs.⁹³ Quantum dots of the CoS nanoclusters were uniformly anchored on the graphene nanosheets, the hybrid materials with a robust and stable interfacial connection between CoS and graphene. The CoS@G electrode delivers an expected capacity of 310.8 mA h g⁻¹ at 0.5 A g⁻¹ over 100 cycles. After 100 cycles, the CoS nanoclusters with 10–20 nm in size and the existence of the QDs can be observed and further confirmed the superior structural stability of the electrode. Making full use of the protection of the carbon layer on the surface, Yang et al. fabricated carbon-coated mesoporous Co₉S₈ nanoparticles supported on reduced graphene oxide by a simple procedure.⁹⁴ When utilized as the anode for PIBs, the as-prepared sample presented excellent rate performance of 215 mA h g⁻¹ at 5 A g⁻¹ and prolonged cycle life of 210.8 mA h g⁻¹ after 1200 cycles at 1 A g⁻¹. An advanced negative electrode material for PIBs, in which MoS₂ nanosheets grow in situ on the surface of the CoS to generate Co₉S₈/MoS₂ dodecahedral heterogeneous nanocages, was synthesized by the hydrothermal and calcination processes. N–C dual doped bimetallic sulfide heterogeneous nanocages endow PIBs with great electrochemical performance and a reversible capacity of 100 mA h g⁻¹ at 1 A g⁻¹ after 100 cycles.⁹⁵

Different morphologies including solid nanospheres, ultrathin nanosheets, and flower-like structures assembled by the nanoplates show different alkaline ion storage capacities. Feng's group regulated and controlled the Ni₃S₂ morphology by modulating the hydrothermal time.⁹⁷ A highly safe all-solid-state PIB has been systematically investigated; the design of the array structure and the electroconductive matrix enhanced the energy density of PIB, and PEO-based SPE can not only solve the safety problems but also effectively improve the cycling stability caused by the dissolution of polysulfides in a common organic-liquid electrolyte. In addition, a myriad of studies suggest that the incorporation of heteroatoms could modulate the charge distribution and create more reaction sites; recently, Ji's group reported a bifunctional carbon modified hierarchical NiS₂, nitrogen/carbon dual-doped carbon layer, uniform 3D superstructure, and plentiful active sites stably maintain the K ion insertion and extraction.⁹⁸ The as-synthesized anode material exhibited excellent rate capability of 151.2 mA h g⁻¹ at 1.6 A g⁻¹ and outstanding cyclic stability with a reversible specific capacity of 303 mA h g⁻¹ after 100 cycles. The 2D nanosheets can effectively optimize the ion diffusion length and cushion the stress associated with the volume change during the repeated charge and discharge processes. Zhao's group rationally designed and prepared yolk–shell NiS_x@C nanosheets via the effective etching and sulfuration processes for application in PIBs; superhigh K ions diffusion coefficient, and fast potassiation kinetics were achieved for the NiS_x@C electrode (Fig. 5g and h).⁹⁹ Accordingly, the NiS_x@C nanosheets demonstrated a high capacity of 415 mA h g⁻¹ at 0.1 A g⁻¹ and excellent rate performance of 232 mA h g⁻¹ at 2 A g⁻¹ as well as ultra-long cycling life of up to 8000 cycles at 0.5 A g⁻¹ (Fig. 5i).

Similar to other 2D layered materials, monolayered MoS₂ nanosheets tend to restack together because of the high surface energy. To address these key issues, Qin's group recently investigated a novel oleylamine-mediated emulsion templated solvothermal strategy for designing a mesoporous MoS₂-monolayer/carbon composite as an advanced anode for PIBs and successfully prevented the aggregation of adjacent MoS₂ monolayer nanosheets.¹⁰⁴ The MoS₂ monolayer provides a large number of reaction sites, optimized electrons/ions diffusion path, and lesser mechanical strain, resulting in superior K ion storage performance. In the pursuit of high-performance anode materials for PIBs, bamboo-like MoS₂/N-doped-C hollow tubes were prepared and are shown in Fig. 6a–d; the bamboo-like structure offers large gaps along the axial direction in addition to the inner cylindrical hollow space to mitigate the strains in both the radial and vertical directions, which eventually contribute to the good structural integrity for durable cycling stability.¹⁰⁵ Accordingly, a tubular interlayer expanded the MoS₂-N/O doped carbon composite (E-MoS₂/NOC TC), which was accurately synthesized for elevating the K ion diffusion rate and improving the structural stability (Fig. 6e).¹⁰⁶ The tubular structure can effectively stabilize the integral structure by buffering large mechanical strain upon potassiation, When evaluated as the anode for PIBs, the E-MoS₂/NOC TC electrode shows excellent K ion storage performance; it can deliver a high specific capacity of 176 mA h g⁻¹ at 2 A g⁻¹ after 500 cycles with only 0.09% decay per cycle (Fig. 6f). Encouragingly, the novel idea that large-sized ions should accommodate big houses was proposed; a simple induced growth method was adopted to accomplish the self-loading of the MoS₂ clusters inside a hollow tubular carbon skeleton. Step-by-step intercalation and self-loading in the hollow skeleton were performed to construct big houses for K ions. This unique architecture not only mitigates the mechanical strain but also offers wide pathways for fast K ions transition and storage; when used as an anode for PIBs, the resultant electrode exhibits an expected high rate cycling stability of 149 mA h g⁻¹ at 2 A g⁻¹ over 10000 cycles.¹⁰⁷ Previous attempts have shown that expanding the interlayer spacing of 2D materials is favorable for buffering the structural change upon ion insertion/extraction and improving the long-term cycle stability. MoS₂ nanosheets, as an important member of the 2D materials, have proved that expanding the (002) plane can not only accelerate the efficiency of intercalation/deintercalation of the K ions but also shorten the transport length for K ions and electrons. Zhang et al. determined that the synergistic engineering of MoS₂ with expanded (002) planes and a unique yolk–shell architecture can effectively enhance K ion storage performance.¹⁰⁸ Li et al. employed a facile solution method to prepare hierarchical interlayer-expanded MoS₂ assemblies supported on carbon nanotubes, both of which highlight a facile defect and interlayer engineering to accommodate countless K ions.¹⁰⁹ The most attractive investigation of the MoS₂ interlayer enlargement is shown in Qin's work;¹¹⁰ the two-step solvothermal route successfully prepared ultrathin rose-like MoS₂ strongly anchored on reduced graphene oxide sheets (MoS₂@rGO composite) (Fig. 6g and h). Benefiting from the expanded interlayer distance, the strong coupling effect as well as the distinct architecture; the resultant MoS₂@rGO composite displays a high specific capacity of 679 mA h g⁻¹ at 0.02 A g⁻¹ and good cyclic stability of 380 mA h g⁻¹ over 100 cycles at 0.1 A g⁻¹. To further elucidate the electrochemical nature of K ions storage of MoS₂-based PIBs anode materials, in situ Raman and ex situ XRD techniques were employed to observe the composition evolution of MoS₂ during the initial charge/discharge process. As can be seen in Fig. 6i and j, the potassiation process can be described as follows.

	Fig. 6 Nanostructured molybdenum sulfides for potassium batteries: (a–d) bamboo-like MoS2/N-doped-C hollow tubes for K-ion storage. Reproduced with permission.105 Copyright 2018, Wiley-VCH. (e and f) E-MoS2/NOC TC for K-ion storage. Reproduced with permission.106 Copyright 2019, The Royal Society of Chemistry. (g–j) MoS2@rGO composite for potassium battery. Reproduced with permission.110 Copyright 2017, Wiley-VCH.

The naturally narrow interlayer spacing of the layered transition metal sulfide has attracted less attention for the larger K ion insertion, causing sluggish electrochemical kinetics. An expanded interlayer spacing of the 0.610 nm SnS₂ crystals anchored on nitrogen-doped graphene nanosheets has been fabricated by Ou and coworkers;¹¹³ the tight coupling interaction between nitrogen-doped graphene and SnS₂ preserved the nanostructure stability upon repeated charge and discharge process. The synergistic effect of the carbon network and the nanostructured SnS₂ effectively enhanced the electrochemical properties, leading to a superior rate capability of 206.7 mA h g⁻¹ at 1 A g⁻¹ and a desirable cyclic property of 262.5 mA h g⁻¹ over 100 cycles at 0.5 A g⁻¹. Hybridization nanostructured SnS₂ with various carbonaceous materials such as graphene to form a SnS₂/C composite has been extensively developed as the anode for PIBs by numerous groups. Alexey and coworkers prepared nanocrystalline SnS₂ coated onto reduced graphene oxide and utilized it as the anode for PIBs.¹¹⁴ Chen's group did a very good job in PIBs;¹¹⁵ the SnS₂/N-doped reduced graphene oxide composite was prepared through a facile hydrothermal method and electrochemical measurements show that N-doping appears to inhabit the shuttling effect. Accordingly, with the help of a surface-confined approach, Ci et al. reportedly restrained SnS₂ in self-generated hierarchically porous carbon networks;¹¹⁶ the final electrode demonstrated a high reversible capacity of 500 mA h g⁻¹ at 0.05 A g⁻¹ and desirable cycling stability with 298 mA h g⁻¹ after 500 cycles at 0.5 A g⁻¹. By means of collaborative efforts, targeting both the active material and the prepared electrode film, Cao and coworkers controlled the SnS₂ size below 5 nm and further embedded it into the graphene sheets. The highly-resilient electrode film exhibits an unprecedented reversible capacity, extraordinary high-rate capability, and inspiring cycling stability.¹¹⁷ Mai's work caught researchers' attention due to the investigation of the effect of the electrode materials using different electrolytes for PIBs;¹¹⁸ hexagonal SnS₂ nanosheet fixed on reduced graphene oxide surface (SnS₂–RGO) in Fig. 7a–c, when used as the anode for PIBs with ether-based electrolytes, better electrochemical performance could be obtained. The optimization of the electrolytes was performed, as shown in Fig. 7d, suggesting that ether-based electrolytes possess faster electrochemical kinetics and a more stable SEI layer on the surface of the active material. The composites using different concentrations of ether-based electrolytes for K ion storage were also investigated, as shown in Fig. 7e; high concentration ether-based electrolytes bring about better cycling performances. To work out the K ion storage mechanism of the SnS₂–RGO composite with the ether-based electrolyte, phase transitions at various potentials during the initial cycle was recorded by ex situ XRD (Fig. 7f). Based on the ex situ XRD analysis, the potassiation processes are shown in eqn (8) and (9).

	Fig. 7 Nanostructured tin sulfides for potassium batteries: (a–f) SnS2–RGO composites for potassium battery. Reproduced with permission.118 Copyright 2019, The Royal Society of Chemistry. (g–j) SnS2@rGO for K-ion storage. Reproduced with permission.119 Copyright 2019, Wiley-VCH.

When the electrode is fully charged to 3 V, all peaks of K_xSn_y disappear, indicating that the depotassiation process has finished and the formation of the amorphous structure of SnS₂ in the electrode. Xia et al. presented a general solvothermal process for the preparation of few-layered SnS₂ nanosheets supported on reduced graphene oxide (SnS₂@rGO) (Fig. 7g and h),¹¹⁹ the excellent rate performance of the SnS₂@rGO electrode is also demonstrated by the electrochemical measurements at different current densities (Fig. 7i), the as-synthesized SnS₂ nanosheets undergo sequential conversion and alloying reactions upon potassiation and displays in Fig. 7j, which is consistent with Mai's conclusion.

ReS₂, with large interlayer space and weak interlayer coupling, can allow massive alkali ions to diffuse easily between the layers. According to the critical finding, flexible ReS₂ nanosheets/N-doped carbon nanofibers-based paper has been prepared through the feasible electrospinning and hydrothermal process.¹²¹ Electrochemical measurements suggested that ReS₂ can act as a capable anode material for rapid and high potassiation capacity. CuS, a low-priced and less dangerous transition metal sulfide, is a potential anode material with high capacity and excellent rate performance for rechargeable secondary batteries. Abandoning these cumbersome fabrication processes or highly demanding experimental conditions, such as vacuum, controlled high temperature, and high pressure, Yu et al. developed a facile one-step synthetic method to prepare CuS nanosheets that are uniformly anchored on the graphene oxide nanosheets surface (CuS@GO).¹²² Layered GO presence on the material surface meets the demands perfectly for sufficient space to curb volume change during the repeated insertion/extraction processes; the CuS@GO electrode exhibits a satisfactory rate capability and cycling stability. Xu et al. reported a composite consisting of Cu₂S nanoplates and carbon matrix, which was synthesized through a simple method and studied as an anode for PIBs; when used as the anode,¹²³ the Cu₂S@C electrode delivers a capacity of 206.6 mA h g⁻¹ after 400 cycles at a high rate of 2 A g⁻¹. ZnS is undoubtedly a hopeful substitute for PIBs due to its high natural abundance and low cost. To tackle the tough conundrum, mainly including inherent low electronic conductivity and nonignorable volume expansion as well as unstable solid electrolyte interphase (SEI) growth on the active particles interface, Bao et al. proposed an ingenious strategy to fabricate a multilevel hierarchical structure having the advantages of both small particles and reasonable side reaction for enhancing the electrochemical properties.¹²⁴ All carbon-protected uniform zinc sulfide dendrites with the tertiary hierarchical structure were synthesized with zinc sulfide dendrites deeply nested in the tertiary hierarchical structure (ZSC@C@RGO) (Fig. 8h). The distinct building shortens the diffusion path and improves the electronic conductivity from the interior to the exterior for both the K ions and electrons, and generates a stable SEI during the electrochemical reaction, resulting in a stable specific capacity of 208 mA h g⁻¹ at 0.5 A g⁻¹ over 300 cycles. The density functional theory calculation results demonstrated that the interactions between ZnS and the carbon interface can effectively decrease the K ion diffusion barrier and thus promote the reversibility of K ion storage. Vanadium with high valency can exist in different stoichiometric forms, including VS₂, V₃S₄, V₅S₈, and V₂S₃. However, VS₂, V₃S₄, and V₅S₈ have recently been reported in PIBs; hierarchical VS₂ nanosheet assemblies comprised of aligned ultrathin nanosheets (VS₂ NSA) were prepared and applied as the universal host material for K ions (Fig. 8a and b), which displayed remarkable electrochemical performances (Fig. 8c and d), superior to other materials inside or outside the transition metal sulfides family.¹²⁵ Constituting a unique construct of two-dimensional layers grown on one-dimensional nanotubes, ultrathin core–shell V₃S₄@C nanosheets assembled into hierarchical nanotubes were rationally designed and prepared with V-based MOF (MIL-47as) as a precursor (Fig. 8e and f).¹²⁶ The distinctive hierarchical structure was endowed with strong structural rigidness because of the layered VS₂ subunits and interlayer occupied V atoms, and adequate K ion adsorption/diffusion originated from the electroactive V₃S₄-C interfaces. The resultant anode displays unique K-ion-dependent charge storage mechanisms and exceptional long cyclic life in the storage of K ions (Fig. 8g). Moreover, Guo et al. synthesized few-layered V₅S₈ nanosheets coating a hollow carbon sphere through a simple hollow carbon template-induced strategy and was used as the anode for PIBs,¹²⁷ in which hollow carbon is not only applied as a template to prevent particles from aggregating but also induces the formation of VS₄ particles, which is then transformed into ultrathin few-layered V₅S₈ in the subsequent annealing treatment process. The DFT calculations uncovered that the V₅S₈ nanosheets have superior electrical conductivity and low energy barriers for K ion intercalation. The final product shows a high capacity of 645 mA h g⁻¹ at 0.05 A g⁻¹ and an outstanding high-rate cycling stability of 190 mA h g⁻¹ after 1000 cycles at 2 A g⁻¹. Integrating with heteroatom-doped graphene can modulate the reactivity and electric conductivity; Chen et al. presented a general hydrothermal co-assembly approach for the synthesis of Sb₂S₃ nanoparticles uniformly anchored into porous S,N-co-doped graphene backbone and used it as the anode for PIBs.¹²⁸ The interconnected graphene framework buffers the volume swelling and facilitates fast electron and ion transfer, enabling an excellent rate capability of 340 mA h g⁻¹ at 1 A g⁻¹ and exceptional cycling stability. Solution-triggered one-step high-shear exfoliation was developed to fabricate the few-layered antimony sulfide/carbon sheet composite;¹²⁹ Sb₂S₃ with a few-layered structure alleviates the volume expansion upon potassiation and optimizes the ion transport pathways, thus obtaining the excellent rate capability and cycling stability.

	Fig. 8 Nanostructured other metal sulfides for potassium batteries: (a–d) VS2 NSA for K-ion storage. Reproduced with permission.125 Copyright 2017, Wiley-VCH. (e–g) V3S4@C for potassium battery. Reproduced with permission.126 Copyright 2019, Wiley-VCH. (h) ZSC@C@RGO composite for K-ion storage. Reproduced with permission.124 Copyright 2019, American Chemical Society.

Although cobalt selenides with different nanostructures have been reported and have exhibited excellent performance in the PIBs, there are still some questions that need to be solved. Li and coworkers agreed that the integration of Co_0.85Se with carbon materials can curb the volume variation during the repeated discharge/charge processes.¹³³ Improving the dispersion of active materials on the carbon matrix is a critical technique for constructing a robust structure, they proposed a one-step hydrothermal selenization method to synthesize the cobalt selenide quantum dots encapsulated in the mesoporous polyhedral carbon matrix. The QDs, the carbon matrix, and the hierarchical structure endow the Co_0.85Se-QDs/C with ultrahigh reversible capacity, excellent rate capability, and cycling stability. The intrinsically poor conductivity of the cobalt selenides is still the bottleneck for its practical application in PIBs. Zhang et al. presented a synchronous combination of structural engineering and composition of the Co_0.85Se method to considerably optimize the K ions storage performances; the flexible and freestanding electrodes, in which Co_0.85Se@carbon nanoboxes aggregately embedded in carbon nanofibers film (Co_0.85Se@CNFs), have been prepared from the ZIF-67 as a precursor by the electrospinning and selenization reaction (Fig. 9a–e).¹³⁴ Moreover, the robust conductive carbon nanofiber network can maintain the structural stability by alleviating the large mechanical strain upon repeated K ions insertion/extraction, achieving an impressively high rate cycling stability of 299 mA h g⁻¹ at 1 A g⁻¹ over 400 cycles (Fig. 9f). Recently, the high-temperature pyrolysis of metal–organic frameworks (MOFs) to the composite of carbon with selenides as the electrode materials has ushered in a revival. Zeolitic imidazolate framework (ZIF-67), a typical MOF, has been utilized as a starting material to synthesize cobalt selenides; Co_0.85Se nanoparticles encapsulated in N-doped carbon (Co_0.85Se@NC) were prepared by Yang's group,¹³⁵ where the N-doped carbon not only enhances the electron conductivity but also inhibits the aggregation of the Co_0.85Se upon cycling. Meanwhile, the mesoporous nanostructure conquers the volume variation and shortens the diffusion length of K ions, as well as increases the contact between the electrolyte and the electrode. Benefiting from these unique features, the as-prepared anode delivers a high-rate cycling stability of 115 mA h g⁻¹ at 1 A g⁻¹ after 250 cycles. Encouragingly, Gao's group determined the fast capacity decay for CoSe₂ in potassium storage derived from the structure collapse in the initial several cycles.¹³⁶ To settle the aforesaid conundrum, a metallic octahedral CoSe₂ threaded by the N-doped carbon nanotubes as a flexible framework has been designed and prepared by Gao and coworkers; this unique nanostructure elevates the electron transfer and promotes the rate performance of 196 mA h g⁻¹ at 2.0 A g⁻¹, where the zigzag void space can buffer the volume variation during the repetition of long-term cycling, and a desirable cycling stability of 173 mA h g⁻¹ at 2.0 A g⁻¹ over 600 cycles was obtained.

	Fig. 9 Nanostructured metal selenides for potassium batteries: (a–f) Co0.85Se@CNFs for potassium battery. Reproduced with permission.134 Copyright 2019, Elsevier. (g–i) ZnSe/C nanocages for K-ion storage. Reproduced with permission.139 Copyright 2020, The Royal Society of Chemistry.

To date, hollow architectures including yolk–shell, egg-like, and capsule-like nanostructures have been proposed to maintain the large mechanical strain. Enlightened by the previous efforts, Xu et al. demonstrated an innovative and facile approach for the synthesis of ZnSe nanoparticles fixed on nitrogen-doped hollow polyhedron composite through the simultaneous pyrolysis and selenization of the sacrificial templates of ZIF-8.¹³⁸ The TEM image displays that the ZnSe nanoparticles are decorated with a nitrogen-doped carbon layer, which increases the interfacial interaction between carbon and ZnSe when tested as the anode for PIBs, and delivers a superior cyclic stability of 133 mA h g⁻¹ during 1200 cycles at 0.1 A g⁻¹. Moreover, open ZnSe/C nanocages constructed from sub-10 nm nanoparticles with a multi-hierarchy stress-buffering effect were reported by Chu and coworkers (Fig. 9g);¹³⁹ the architecture combined the advantages of the primary ultrafine nanoparticles, the secondary open structure, and the tertiary hollow structure (Fig. 9h), which can dramatically reduce diffusion-induced stresses and maintain the structural integrity, obtaining long-term cycling stability of 189 mA h g⁻¹ at 0.5 A g⁻¹ over 1000 cycles (Fig. 9i). Recently, metal–organic frameworks (MOFs) have been widely used as precursors in the fabrication of porous nanostructures of metal selenides due to their well-defined morphology and large surface areas. Liu and coworkers reported a simple strategy to synthesize highly dispersed ZnSe nanoparticles anchored in a N-doped porous carbon rhombic dodecahedron composite using ZIF-8 as the starting material by a sequential high-temperature decomposition and selenization process.¹⁴⁰ The optimized sample, used as an advanced anode for PIBs, demonstrated excellent electrochemical performance in terms of the good rate capability as well as the long-term cycling stability. Furthermore, a galvanostatic intermittent titration was conducted to elucidate the K ions storage mechanism during the discharge/charge process. During the potassiation process, ZnSe translated into K₂Se; correspondingly, K₂Se transformed into ZnSe during the extraction process.

To get high reversible capacity and desirable cycling performance for PIBs, Zhang et al. reported facile electrospinning and selenization route for the encapsulation of sheet-like MoSe₂ in carbon fibers (MoSe₂/C) (Fig. 10a).¹⁴² Benefiting from the one-dimensional carbon nanofibers with good structural stability and MoSe₂ with an expanded interlayer spacing, the as-prepared electrode sustains high structural integrity and elevates the K ion transfer, resulting in a high discharge capacity of 801 mA h g⁻¹ and a good cyclic performance of 316 mA h g⁻¹ after 100 cycles at 0.1 A g⁻¹. With an expanded MoSe₂ interlayer spacing of 0.85 nm, Guo et al. fabricated a novel pistachio shuck-like MoSe₂/C core–shell (PMC) nanostructure and used it as an advanced anode for PIBs (Fig. 10b).¹⁴³ The PMC electrode displayed a reversible capacity of 224 mA h g⁻¹ at 2 A g⁻¹ (Fig. 10c) and could maintain 226 mA h g⁻¹ at 1.0 A g⁻¹ over a long period of 1000 cycles, which demonstrated that the PMC, comprised of few MoSe₂ nanosheets, was favorable for the fast movement of K ions and electrons, and alleviated the large volume expansion. Surprisingly, integrating MoSe₂ with emerging MXene presented a huge promise for developing more efficient anode materials for PIBs. An innovative anode material, in which 2D layered MoSe₂ anchored on Ti₃C₂ MXene flakes and further coated with a polydopamine-derived carbon layer (MoSe₂/MXene@C), was synthesized by Zhang's group (Fig. 10d).¹⁴⁴ Benefiting from the synergistic effects of MXene and carbon protection, highly stabilized active MoSe₂ nanoparticles, and elevated electron transport in the hierarchical architecture, the as-synthesized electrode demonstrated a remarkable rate capability with 183 mA h g⁻¹ at 10.0 A g⁻¹ and the capacities could be recovered perfectly when switching to the quondam currents (Fig. 10e). Li and coworkers proposed a simple ion complexation induced method coupled with the further selenization within the nanospheres process for the fabrication of nanosized MoSe₂@carbon nanocomposite (N-MoSe₂@C);¹⁴⁵ N-MoSe₂@C was advantageous for K ions and electrons transport while the carbon matrix greatly promoted the conductivity and also acted as a strong support to accommodate the stress upon potassiation. Impressively, considering the practical applications of PIBs, the electrochemical performances under low and high temperatures were also studied, suggesting high-capacity retention at various temperatures. Yu et al. reported a novel MoSe₂/N-doped carbon hybrid and tested it as the anode for PIBs with a new electrolyte.¹⁴⁶ The MoSe₂/N-doped carbon composite composed of carbon-coated MoSe₂ nanosheets displays an excellent rate performance and long-term cycling stability. Furthermore, the reaction mechanism of K ion storage was investigated by ex situ XRD, ex situ Raman, and fully charged HRTEM techniques. Based on the ex situ XRD patterns and ex situ Raman spectra analysis, the electrochemical reactions of the initial discharge process can be described in the following steps.

	Fig. 10 Nanostructured molybdenum selenides for potassium batteries: (a) MoSe2/C composite for K-ion storage. Reproduced with permission.142 Copyright 2019, The Royal Society of Chemistry. (b and c) PMC for potassium battery. Reproduced with permission.143 Copyright 2018, Wiley-VCH. (d and e) MoSe2/MXene@C for K-ion storage. Reproduced with permission.144 Copyright 2019, American Chemical Society.