Research Progress on Vanadium Sulfide Anode Materials for Sodium and Potassium‐Ion Batteries

Considering environmental changes and the demand for more sustainable energy sources, stricter requirements have been placed on electrode materials for sodium and potassium‐ion batteries, which are expected to provide higher energy and power density while being affordable and sustainable. Vanadium sulfide‐based materials have emerged as intriguing contenders for the next generation of anode materials due to their high theoretical capacity, abundant reserves, and cost‐effectiveness. Despite these advantages, challenges such as limited cycle life and restricted ion diffusion coefficients continue to impede their effective application in sodium and potassium‐ion batteries. To overcome the limitations associated with electrochemical performance and circumvent bottlenecks imposed by the inherent properties of materials at the bulk scale, this review comprehensively summarizes and analyzes the crystal structures, modification strategies, and energy storage processes of vanadium sulfide‐based electrode materials for sodium and potassium‐ion batteries. The objective is to guide the development of high‐performance vanadium‐based sulfide electrode materials with refined morphologies and/or structures, employing environmentally friendly and cost‐efficient methods. Finally, future perspectives and research suggestions for vanadium sulfide‐based materials are presented to propel practical applications forward.


Introduction
[3][4] Lithium only occurs in nature as compounds since it is a typical alkali metal element with high activity and low content when compared to its identical group sodium and potassium. [5,6]As a result, the price of lithium has been steadily increasing due to the difficulties of mining lithium deposits and the high demand.9] In comparison to lithium, sodium has the advantages of higher natural abundance (Na:2.3[12] Potassium not only has a relatively high natural abundance (K:2.09wt.%) but also has a negative reduction potential of −2.93 V (vs standard hydrogen electrode (SHE)), which is close to the reduction potential of lithium (−3.04 V vs SHE), and has a lower tendency to form dendrites, so potassium can provide high energy and is relatively safe (Figure 1a). [13,14]Because of their benefits, sodium and potassium-ion batteries (SIBs and PIBs) have become a new research hotspot for researchers in the past decade, and increase in the number of research papers published on developing materials for both anode and cathode electrodes in SIBs and PIBs (Figure 1b). [15,16][19][20] First, the design and synthesis of many unique crystal structures and varied morphologies, as well as the regulation of chemical composition and proportions, are all made possible by this variable oxidation valence state. [21,22][28][29] As a result, investigations on the use of compounds based on vanadium in SIBs and PIBs have increased during the past ten years (Figure 1c).In particular, due to their adaptable crystal structure and superior theoretical characteristics, vanadium sulfur (VS x ) compounds have also shown good performance as anode materials for SIBs and PIBs.
Vanadium atoms engage in chemical reactions with sulfur atoms, resulting in alterations in the chemical composition and valence state of vanadium. [30]Consequently, a diverse range of materials and structures emerges, contingent upon their ion transport channels.33][34] It is noteworthy that the scientific literature has paid limited attention to vanadium sulfide anode materials thus far, with the majority of articles primarily focusing on the development of vanadium-based cathode materials for SIBs and PIBs. [25,35,36]In this review, we conduct a comprehensive examination of the fundamental crystal structure of vanadium sulfide anodes, with a particular emphasis on the storage mechanism for Na + /K + ions and effective modification strategies aimed at enhancing the electrochemical performance of vanadium sulfide anodes.Ultimately, to provide valuable insights for the advancement of vanadium sulfide materials for SIBs and PIBs, we propose key challenges and their corresponding solutions in the development of vanadium sulfide materials.

Features of VS x
[39][40][41][42] Among these, owing to their exceptionally high theoretical specific capacity, VS 2 and VS 4 have gained significant prominence as anode materials in SIBs and PIBs.The crystal structure of VS 2 is characterized by layering with a layer spacing of 5.76 Å.This structure is composed of hexagonally arranged metal V layers situated between two layers of sulfur atoms.In contrast, the VS 4 crystal exhibits a quasi-one-dimensional chain compound in which V(IV) ions coordinate with S 2 2− dimers.These linear structural units are interconnected through relatively weak van der Waals interactions, resulting in an inter-chain distance of 5.83 Å.It's important to note that the oxidation state of vanadium remains constant in both VS 2 and VS 4 , while the oxidation state of the sulfide differs, with VS 2 containing S 2− monomers and VS 4 featuring S 2 2− dimers.[45] Moreover, it is significant that compounds in the NiAs-type VS x structural class frequently exhibit a predominance of polar and covalent bonding as opposed to ionic interactions.This is because internal instability can result from ionic interactions that are affected by cation-cation repulsion.Consequently, NiAs-type VS x compounds frequently manifest ordered vacancies and may exhibit non-stoichiometric or complex stoichiometry.In addition, multiple crystal forms of VS x as shown in Figure 2 were confirmed, including hexagonal, orthorhombic, monoclinic and tetragonal structures.These characteristics provide many options for Na + /K + storage sites.
In recent years, vanadium sulfide has received widespread attention as electrode material for Na/K-ion batteries.Vanadium sulfide electrodes have high theoretical capacities and multielectron transfer capabilities thanks to their numerous valence states (V 2+ /V 3+ /V 4+ /V 5+ ).The electrochemical characteristics of typical vanadium sulfides are displayed in Table 1.In fact, using DFT simulations, Jing et al. examined the diffusion and V 3 S 4 8 646 5.67 [34]   V 5 S 8 8 419 11.32 [50]   V 2 S 3 6 811 5.7 [32,51]   adsorption of lithium on the VS 2 monolayer. [42]And VS 2 monolayer delivers higher theoretical capacity (466 mA h g − 1 ), lower Li diffusion barrier, and low average open-circuit voltage of 0.93 V (vs Li/Li + ).Multi-layer VS 2 has significant advantages with high theoretical capacity (932 mA h g −1 ).As we all know, the rocking chair mechanism provides the foundation for the operation of both SIB and PIB.Na + /K + -ions use the intercalation and deintercalation mechanisms in the electrolyte medium and separator between the anode and cathode terminals, respectively, to move between them during the discharge and charging.For VS x as the anode electrode, the mechanism of Na + /K + -ions storage changes with the reaction depth and voltage, which can be classified into three types of mechanisms: surface pseudocapacitance mechanism, intercalation reaction mechanism, and conversion reaction mechanism.First, during surface redox reactions, surface adsorption/desorption-induced faradaic pseudocapacitance also makes a significant contribution to charge storage, which occurs on/or near the electrode surface of vanadium sulfides, resulting in ideal capacitive behavior. [46,47]Second, Na + /K + -ions are inserted into the electrode bulk phase, and VS x structure remains stable or undergoes slight deformation during the charging/discharging process, ensuring good cycle stability of the electrode.50] 2.2.Challenges toward VS x VS x have drawn a lot of interest as a developing anode material for SIBs/PIBs and have made encouraging strides in the fields of energy conversion as well as storage.[54][55] Although VS 2 , with a two-dimensional layered crystal structure, is a commonly seen insertion/extraction process, which deliver not bad rate performance.Generally, nonlayered NiAs-type vanadium sulfides conversion reactions at low-voltage can provide high specific capacities, nonetheless, their sluggish kinetics lead to unsatisfactory rate performance.As a result, the electronic migration of the lattice structure, which is mostly governed by the Na + /K + transport kinetics, controls the diffusion of the electrochemical reaction processes in the host crystal.Therefore, it is essential to enhance the diffusion kinetics of Na + /K + ions within electrodes.
[58][59] The simple insertion/extraction of Na + /K + from host VS x materials may be used to charge and discharge SIBs/PIBs.Due to their crystalline character, intercalation-typed anodes may tend to aggregate or collapse nanostructures, which causes a capacity decline throughout the repeat reaction.It will reduce the specific surface area and raise the diffusion resistance.Greater volume change is unavoidable for the structural distortion in the conversion reaction of nonlayered VS x , which results in a number of passive effects.For instance, it is likely that there may be fractures or pulverization, which will separate the active materials from the current collector.The cycling performance will also decline as a result of the repeated creation of new solid electrolyte interface (SEI) layer brought on by fractures.Furthermore, the pulverization or fracture also impedes the diffusion of ions, aggravating the reaction kinetics.
][62] Large volume changes and repeated insertion/extraction of Na + /K + can easily disrupt the integrality of SEI, even though it was first generated in the activation stage.At the same times, the newly exposed surface caused by the volume change would create new SEI film.However, prolonged ion intercalation and extraction, particularly at high current densities, may cause more damage to the SEI layer, which would exhibit poor cycle performance.As a result, there is conflict and competition in the regeneration and dissolution of the SEI layer during the charge and discharge process.

Various Strategies to Boost Na/K-Ions Storage
Many efforts have been made recently to overcome abovementioned challenges and in order to enhance specific capacity, rate performance, and cycle stability of VS x electrode materials in SIBs/PIBs through structural design and optimization.As is well known, the synthesis process, which has a strong correlation with the microstructural and morphological properties, affects the electrochemical performance of the anode materials.Up to now, various strategies have been reported to synthesize the VS x compounds, such as hydrothermal method, solvothermal route, annealing, exfoliation, spray pyrolysis, freeze drying, and template assistance. [19,21,60,63]here is a close relationship between the reaction kinetics and the electrochemical performance as well as the storage mechanism of VS x .From the perspective of material design, all of the countermeasures to improve electrochemical storage capacity and accelerate charge transfer could make sense.These days, morphological design, composite construction (heterostructures, conductive coating), and lattice engineering in particular are often employed methodologies.Figure 2 shows a schematic depiction of the morphology, several crystal structures, and major VS x design techniques.

Lattice Engineering
In recent decades, lattice engineering has also emerged in the field of energy storage and conversion due to advances in precise molecular-level manipulation techniques to obtain the structure and properties of active materials.Utilizing layered lattices allows for the development of novel crystalline structures in two or three dimensions, all while preserving the intrinsic qualities of the two-dimensional lattice.[66] More storage sites and quicker diffusion for the Faraday process are associated with larger layer spacing.As a result, researching novel techniques to reduce the van der Waals forces' strength or increase the interlayer gap has also become popular.For example, by using a molecular pre-intercalation process, the VS 2 nanosheet was interlayer expanded from 5.7 to 9.5 Å. [67] The ion diffusion space is expanded as a result of this increase in layer spacing, which also improves the Na + diffusion kinetics and lowers resistance along the 2D channel, both of which help to raise the ion diffusion rate.This resulted in a 2-3 times increase in capacities even over extended cycles, and considerable gains in capacity at different current densities. [67]Additionally, investigations have revealed that the use of solvothermal synthesis, with octylamine as a key component, plays a crucial role in producing VS 2 nanosheets characterized by remarkably large interlayer spacing.An ultra-expanded interlayer composite of vanadium disulfide with carbon was developed by Chen et al., an expanded crystal plane (001) of 10.2 Å, which effectively reduces ion diffusion distances and offers exceptionally wide channels for the intercalation and extraction of Na + /K + ions. [68]The electrochemical performance of VS x can also be impacted by heteroatom doping, according to researches.For instance, by employing self-assembly hydrothermal approach and subsequent heating treatment procedure, Guan et al. fabricated a unique ordered layered VMoS 2 anode material.It was discovered that by exposing extra defects at the margins of stacked nanosheets to provide an abundance of Na + storage sites, which successfully improved rate performance (452.8 mA h g -1 at 5 A g -1 ). [69]Additionally, vacancy engineering is a useful strategy for enhancing electrochemical performance.Wei's group demonstrated that both ordered and disordered S vacancies can enhance the electrode's Na + diffusion kinetics through experimental and first-principles calculations. [52]Specifically, a simple diffusion channel perpendicular to the VS 2 plane is offered by disordered S vacancies.Excellent rate capability and long-life cycle stability are obtained in this way, and rapid pseudocapacitive sodium ion storage can be facilitated.These cases can effectively illustrate that lattice engineering (e.g., increasing layer spacing, heteroatom doping, and defect) design is a very effective strategy, especially for increasing the diffusion kinetics of Na + /K + ions within VS x electrodes.

Morphological Design
Constructing materials with nanostructures is undoubtedly one of the most widely used techniques for improving battery performance. [11,70,71]When the size is lowered to the nanoscale, the active material's specific area of surface increases dramatically, resulting in an increase in the number of sites of action and intrinsic Faradaic pseudocapacitance near or on the surface.[74] Fick's law states that there is a negative relationship between the ion diffusion time and the diffusion length.This relationship may be represented as ≈L 2 /D, where , L, and D stand for the ion diffusion time, diffusion coefficient, and length in the electrode material lattice, respectively. [75,76]Therefore, nanocrystallization shortens the ion diffusion time, which enhances rate performance.Additionally, the nanoscale building active materials can make it easier for the electrolyte to enter the inner part of the electrode by shortening the diffusion paths for ion and electron transport. [77,78]ased on its crystallographic properties and growth, it may construct various nanostructures, including nanosheets, nanoflakes, nanoflower-like, 3D spheres/hollow spheres, and so on, to enhance the electrochemical performance of VS x materials.
Nanosheets are often the preferred option in most configurations for materials connected with weak Van der Waals force between two stacking layers, such as layer-by-layer VS 2 , hierarchical flower-like VS 2 nanosheets, bowl-like VS 2 arrays on flexible CNFs, and VS 4 nanosheets. [6,11,19,20]In general, these ultra-thin VS x nanosheets or 3D hierarchical made of nanosheets have the ability to enhance the interaction area between electrolytes and electrodes in addition to providing enough room to buffer volume variations brought by frequent intercalation and deintercalation of Na/K-ions.Moreover, the Na/K-ion diffusion channel can be shortened by the very thin layers or nanosheets VS x .With these advantages, the nanosheets VS x showed remarkable electrochemical performance when severed as anode electrodes for SIBs and PIBs about reversible capacity and rate performance.
VS x can also synthesize microspherical structures, hollow microspheres, and hierarchical nanotubes assembled from ultrathin V 3 S 4 @C nanosheets etc., with the use of templates or the addition of surfactants. [49,81,82]With their porous shell and hollow structure, greatly accessible surface area, and strong structural stability, hollow spheres will perform better electrochemically than simple hierarchical structures. [83,84]For example, Yang et al. fabricated microstructure of VS 4 hollow spheres, which exhibit abundant porosity and are surface-coated in 2D ultrathin nanosheets.VS 4 hollow spheres can successfully increase space, which provides more support for the structural stress resulting from the reaction process's volume change, thus increasing the cycle life of active materials. [84]

Composite Construction
In order to optimize the performance of SIBs and PIBs, VS x materials with various morphologies and structures are employed, by modifying the synthesis precursor of the composite or design methods.However, challenges persist due to weak interlayer interactions and the volume changes associated with the repetitive charging/discharging processes.As a consequence, electrode materials tend to deteriorate over time, impeding the attainment of desirable cycling stability in VS x materials.Hence, several strategies have been explored, to enhance reversibility capacity and cycling stability.These approaches include creating micro-nanostructures or heterostructures with confinement or synergy to shorten the diffusion distance of Na + /K + , and VS x rate performance can be enhanced by increasing the rate at which ions/electrons diffuse; [54,59,60,63] As well as improving the conductivity of VS x and enhancing the structural stability of VS x electrodes through element doping and carbonaceous incorporation; [11,33,85] Alternatively, a stable SEI layer can be achieved via surface modification, electrolyte selection, or voltage control to lessen electrolyte side reactions and the crushing of the VS x electrode structure. [61,81,86]For example, multiphase VS 4 /Bi 2 S 3 @C heterostructure was rationally designed, with a self-built-in electric field, leading to boost Na + transport kinetics and displays an superior rate capacity of 410.8 mA h g −1 at 5 A g −1 . [63]Alternatively, designing ultrathin VS x nanosheets or combining them with structurally stable carbon are also common strategies to enhance the structural stability of VS x active materials.For instance, K storage may be accommodated reversibly and structural stability maintained by wrapping multilayer V 5 S 8 nanosheets with hollow carbon spheres made using a simple hollow carbon template induction process. [50]Therefore, V 5 S 8 electrode delivered a high rate performance and extended cycling durability, after 500 and 1000 cycles at 500 and 2000 mA g −1 , capacity of 360 and 190 mA h g −1 were reached, respectively.

VS 2
Vanadium disulfide (VS 2 ) is a typical layered transition metal disulfide compound.VS 2 has an S-V-S sandwich structure connected by covalent bonds within layers and van der Waals forces between layers.The interlayer between S-V-S layers provides a 2D path for K + /Na + diffusion (Figure 3a,b). [87]A monolayer of VS 2 exhibits metallic conductivity, and when utilized as an anode in SIBs, the primary reaction that ensues is the intercalation of Na + ions into VS 2 (Equation (1)), followed by subsequent conversion reactions (Equation (2)).In previous reports, a team has already explored the stable configuration of Na on VS 2 (Figure 3c) through ab Initio Random Structure Searching and calculated that the sodium ion storage of single-layer VS 2 nanosheets is 233 mA h g -1 . [88] Reproduced with permission. [87]Copyright 2017, The Royal Society of Chemistry.c) Lowest energy structure for single Na adsorption on 1H-VS 2 and 1T-VS 2 .Reproduced with permission. [88]Copyright 2016, American Chemical Society.d) Diagrammatic representation of the formation of VS 2 -SNS.e) SEM images of VS 2 -SNS.Reproduced with permission. [91]opyright 2017, Elsevier.f) Principles and techniques for creating sandwich carbon composite materials incorporating ultra large VS 2 .Reproduced with permission. [68]Copyright 2020, Elsevier.g) Cycle performance diagrams for E-VS 2 and A-VS 2 .Reproduced with permission. [67]Copyright 2023, Elsevier.

Layer-by-Layer Stacked VS 2
Furthermore, multilayer VS 2 offers notable advantages, including metallic conductivity, a high theoretical capacity, and the capability to adjust interlayer spacing.Wang et al. prepared VS 2 nanosheets by hydrothermal method and studied the electrochemical properties as anode materials in SIBs through experimental testing and first principles calculations.The results indicate that the material maintains a 2D layered structure at low Na + insertion (x ≤ 2.0 in Na x VS 2 ), and then transforms into V/Na 2 S nanocomposites during deeper sodiation processes. [89]lthough pure phase vanadium sulfide has a considerable theoretical specific capacity, it cannot provide ideal cycling performance in energy storage due to the volume expansion during the process of ions insertion/extraction resulting in low capacity and poor rate performance.The use of interlayer expansion can greatly reduce the ion diffusion energy barrier to improve the cyclic electrochemical kinetics of materials.In 2017, Sun et al. report a novel layer-by-layer stacked VS 2 nanosheets.As shown in Figure 3d, the end of PVP molecules has hydroxyl groups, which can act as anionic surfactants and adsorb on the surface of VS 2 layer through electrostatic interactions. [90]These surfactants will serve as connectors to bridge adjacent VS 2 nanosheets together, resulting in a layered stacking structure (Figure 3e).The layered nanosheet serves as a stable framework to accommodate volume changes during the insertion and extraction of ions, thereby promoting prolonged cyclic stability.Additionally, the abundance of pores situated between these nanosheets facilitates the rapid diffusion of Na ions, ultimately affording a high-rate capability.At 0.2 A g -1 , the VS 2 -SNS anode achieved a reversible discharge capacity of 250 mA h g -1 , and after 600 cycles at a high current density of 5 A g -1 , it still reached a specific capacity of 204 mA h g -1 , demonstrating excellent long-term performance. [91]In addition, some researchers have discovered other methods that can effectively expand the interlayer spacing, which also exhibit excellent electrochemical performance.For instance, Zhang et al. employed a straightforward one-step solvothermal method to synthesize VS 2 nanosheets with rich defects and extended (001) interlayer spacing through a simple one-step solvothermal method.This metal VS 2 nanosheet has abundant vacancy defects and exhibits significantly expanded interlayer spacing, measuring up to 10 Å. [92] In a separate study, VS 2 nanosheets with disordered VS 2-x and ordered VS 2-x sulfur vacancies prepared by Zhao et al, demonstrating that both types of sulfur vacancies may enhance the diffusion kinetics of Na + in the electrode.Especially the disordered S vacancies can provide a simple diffusion pathway perpendicular to the VS 2 plane, significantly reducing both the diffusion length and potential barrier of Na + in the anode, resulting in excellent rate performance and long cycle stability. [52]ltra-enlarged interlayer carbon-containing VS 2 composite material using a simple solvothermal method prepared by Xie et al. and used it as an anode electrode for SIBs and PIBs.The composite material with fully expanded interlayer spacing (10.2 Å) on the crystal plane (001) provides a large channel for rapid insertion/extraction of Na/K-ions (Figure 3f).More importantly, the transverse size at the micrometer level and the large interlayer spacing can provide effective electron paths to shorten the ion diffusion distance.Rich elastic space ensures effective mitigation of volume changes and maintains structural integrity.When utilized as an anode for SIBs, it obtained a high reversible specific capacity of 570.5 mA h g -1 at 0.1 A g -1 , and still maintains a high capacity of 286.0 mA h g -1 even after 800 cycles at 1A g -1 ; while, when served as anode electrode for PIBs, it also presented a specific capacity of 205.2 mA h g -1 after 200 cycles at 0.1 A g -1 . [68]Similarly, in 2023, Wu et al. employed a molecular preintercalation process to expand the interlayer plane (001) from the standard 5.7 to 9.5 Å to obtain E-VS 2 .Benefitting from expanded interlayer distance, weaker van der Waals interaction and adequate space facilitates Na intercalation and diffusion.E-VS 2 exhibits higher reversible capacity at various current densities, while maintaining excellent long-term stability even after long cycles (Figure 3g). [67]

Flower-Like VS 2
In some reported studies, the electrochemical performance of VS 2 is far below its theoretical value.Yu et al. synthesized nanosheet self-assembled hierarchical flower-like VS 2 through a solvothermal pathway and its Na + storage properties are examined.The SEM images (Figure 4a) show that VS 2 is composed of flower shaped nanostructures assembled from nanosheets, exhibiting good dispersion and uniformity.Magnified SEM images (Figure 4b) show that the entire 3D layered structure is composed of dozens of 2D nanosheets with smooth surfaces connected to each other through the center.The flower-like VS 2 delivers a high capacity of above 600 mA h g -1 at 0.1 A g -1 .When the current density is increased to 20 A g -1 , it also shows a high capacity of 277 mA h g -1 , demonstrating a superior rate capability (Figure 4c).Meanwhile, an excellent capacity retention is obtained as 87% after 700 cycles at 5 A g -1 (Figure 4d), indicative of the promising application of flower-like VS 2 as the anode materials for SIBs. [48]Vanadium disulfide spherical nanoflowers (SNF-VS 2 ) with 15 nm thin petals and high purity crystallinity using propylene glycol as the solvent were synthesized by Chen et al., which exhibits rapid ion diffusion, achieving excellent initial Coulombic efficiency (ICE) for SIB and PIB.In SIBs, the prepared SNF-VS 2 electrode provides an initial charging capacity of approximately 392 mA h g -1 , with an excellent ICE of 88.52%, and remains constant at 292 mA h g -1 after 300 cycles of 0.2 A g -1 .In PIBs, SNF-VS 2 showed an initial discharge capacity of approximately 556 mA h g -1 , with an excellent ICE of 74.09%, and a high capacity of 383 mA h g -1 at a current density of 25 mA g -1 . [40]oreover, to gain a deeper insight into the interplay between surfactants, electrochemical performance, and crystal develop-ment patterns.Zang et al. used NMP as a growth promoter and solvent to ensure the growth of VS 2 along the (011) crystal direction, and induced ordered assembly of VS 2 through solvothermal method to form a rose-like structure.Throughout the preparation process, NMP effectively curtailed particle aggregation and preserved the nanostructure of the material.When applied as an anode material for SIBs, the resulting VS 2 (NMP) exhibited outstanding electrochemical performance, especially featuring high ICE and specific capacity.Even at high current density of 5 A g -1 , after 350 cycles, the VS 2 (NMP) electrode can still maintain a high discharge capacity of 565 mA h g -1 , and the electrode also exhibits a high ICE of 92.4%. [93]Based on the aforementioned examples, it is evident that the synthesis of VS 2 nanosheets often involves complex processes and typically requires pre-treatment with surfactants, such as N-Methyl-2-pyrrolidone (NMP), or the use of solvents, like propylene glycol.Wang et al. introduced a one-pot solvothermal method that without surfactants additives and assembled ultra-thin VS 2 nanosheets into microspheres, as illustrated in Figure 4e, which not only provides ample space to accommodate the volume fluctuations resulting from repetitive sodium ion insertion and extraction but also enhances the contact interface between the electrolyte and the electrode.The ultrathin VS 2 layer reduces the diffusion path for electrons and ions.Consequently, when employed as an anode material for SIBs, it maintains a high capacity of 720 mA h g -1 after 100 cycles at a current density of 0.2 A g -1 .Impressively, it attains capacities of 565 mA h g -1 and 479 mA h g -1 at 2 A g -1 and 5 A g -1 , respectively, showcasing remarkable rate performance (see Figure 4f,g). [94]

Coating
Controlling the crystal structures help boost electrochemical kinetics of Na/K-ions, but cyclic stability of VS x still needs to be further improved.[97] Particularly, the conductive poly (styrenesulfonate)-doped poly (3,4ethylenedioxythiophene), layered graphene, TiS 2 layer, crystalline VOOH coated VS 2 nanosheets have been designed, which have also demonstrated the excellent cycling performance. [62,87]As a consequence, coatings can be considered as one of better methods for suppressing the volume expansion owing to its more effective restriction and protection.Exploiting the capacity of coatings to mitigate material volume changes, Li et al. effectively synthesized VS 2 microflowers enveloped in crystalline VOOH via a simple one-pot hydrothermal process.Crystalline VOOH with stable structure can effectively alleviates the volume change of electrode materials, serving as a protective surface layer.Meanwhile, crystalline VOOH, playing as an efficient surface "sensitizer", is beneficial for the transport of Na + and e − due to its pseudocapacitive properties.In addition, crystalline VOOH can also provide more infiltration pathways for the diffusion of Na + , avoiding adverse phase transitions during Na + insertion.The coating of crystalline VOOH reduces the thickness of VS 2 nanosheets and alleviates volume expansion (Figure 4h).The c-VS 2 @VOOH provided a specific capacity of 330 mA h g -1 after 150 cycles at 200 mA g -1 , demonstrating excellent cycling stability.It also exhibits high rate capacities of 356 mA h g -1 and 113 mA h g -1 at Reproduced with permission. [48]Copyright 2017, Elsevier.e) SEM image of VS 2 microspheres.f) Rate and g) cycling performance of VS 2 microspheres.Reproduced with permission. [94]Copyright 2019, The Royal Society of Chemistry.h) Schematic illustration for the discharge process of the c-VS 2 @VOOH with enhanced electrochemical performance in SIBs.Reproduced with permission. [87]Copyright 2017, The Royal Society of Chemistry.i) Cycling performance of N/S-C@VS 2 .Reproduced with permission. [98]Copyright 2022, Elsevier.currents of 0.5 A g -1 and 5.0 A g -1 , respectively. [87]Li et al. used a solvothermal technique to in-situ create N, S co-doped amorphous carbon-coated VS 2 nanosheets that self-assembled into submicroflowers (N/S-C@VS 2 ) with interfacial V-C bonds.This innovative structure incorporates interfacial V-C bonds.Benefitting from the synergistic effects of VS 2 nanosheets, which offer abundant Na + storage sites, N/S-C coatings with high conductivity, and the interfacial V-C bond interactions, the N/S-C@VS 2 anode exhibits remarkable cycling performance in SIBs.After 180 cycles, a capacity of 680 mA h g -1 was obtained at 1.0 A g -1 (Figure 4i). [98]

Composite Construction
Creating heterostructures has shown promise as a recent tactic for improving anode material performance.This approach involves the deliberate and planned combination of two or more material forms.The goal is to maximize the unique benefits of each material while minimizing its drawbacks, aiming for improved electrochemical performance. [99,100]Heterostructures introduce a substantial number of lattice mismatches, distortions, and defects, all of which serve to improve reaction kinetics and introduce long-range disorder. [53,101]Additionally, the heterostructure's internal electric field, formed at the interface, promotes reaction kinetics and facilitates the transport of both electrons and ions. [54,102]raphene serves as a commonly employed conductive substrate for heterostructures, offering an interface that establishes robust contact with particles.This interface effectively mitigates the phenomenon of volume expansion and contraction during particle aggregation.Liu et al. systematically studied the properties of VS 2 /graphene heterostructures and their electrochemical performance as potential anode materials for LIBs and SIBs using first principles calculations.The results indicate that the VS 2 /graphene heterostructure has good structural stability, high adsorption strength, high stiffness, inherent metal properties after Li/Na adsorption, high theoretical specific capacity, shallow average open circuit voltage, and ultra-low ion diffusion barrier.The calculation shows that the total open circuit voltage of the anode of Li/Na ion batteries is as low as 0.65 and 0.46 V, and the maximum theoretical storage capacity is 771 mA h g -1 and 578 mA h g -1 , respectively. [103]Furthermore, Samad et al. found that monolayer VS 2 can grow on monolayer MoS 2 and synthesize MoS 2 @VS 2 .The nanocomposite material has been proven to be a hybrid anode with high electronic conductivity and Na storage capacity using density functional theory.Growth of the unstable monolayer VS 2 over monolayer MoS 2 results in high energy release.Through various computational simulations, they demonstrated the stability of single-layer VS 2 on single-layer MoS 2 , as well as the dynamic stability and metallicity of nanocomposites.The charge density difference shows that there is electron accumulation at the interface between the MoS 2 layer and the VS 2 layer of the MoS 2 @VS 2 nanocomposite.The process of charge redistribution during the growth of a metal monolayer VS 2 atop a monolayer MoS 2 contributes to the enhancement of both conductivity and sodium storage capacity within these nanocomposites.Consequently, the maximum specific capacity of nanocomposites for Na is increased to 584 mA h g -1 . [104]Likewise, Fan et al. produced a distinctive 2D layered, stacked nanosheet heterostructure comprised of VS 2 /MoS 2 .This heterostructure was synthesized by introducing MoS 2 into the VS 2 matrix through a straightforward one-pot hydrothermal method.Due to the heterogeneous interface generated by lattice disorder and the selfbuilt electric field generated by the heterogeneous interface, this material exhibits excellent Na + storage capacity, enhanced Na + reaction kinetics, and good electrode structure stability (Figure 5a).This material exhibits excellent rate performance (capacity of 644.0 mA h g -1 at 10 A g -1 ) and long cycle life performance (capacity of 454.5 mA h g -1 after 1000 cycles at 2 A g -1 ) in SIBs (Figure 5b). [53] VO x layer on the surface of VS 2 by leveraging the spontaneous hydrolysis oxidation coupling reaction of transition metal sulfides in an aqueous medium fabricated by Zhang et al., achieving a close combination of VS 2 and VO x at the nanoscale, and constructing a homologous VS 2 /VO x heterostructure (Figure 5c,d).Thanks to the built-in electric field at the heterogeneous interface, the high chemical stability of VO x , and the high conductivity of VS 2 , the VS 2 /VO x electrode exhibits excellent cycling stability and rate performance.When used as SIBs, after 1000 cycles, it up to a high reversible capacity of 721.6 mA h g -1 at 2 A g -1 and the capacity reached to 654.8 mA h g -1 even at a ultrahigh current density of 10 A g −1 (Figure 5e). [102]Qin et al. successfully prepared nano-roses aggregated by Se doped MoS 1.5 Se 0.5 @VS 2 through a simple hydrothermal reaction and subsequent Se doping process.The heterogeneous interface between MoS 1.5 Se 0.5 and VS 2 phases can effectively promote charge transfer.The heterogeneous interface between MoS 1.5 Se 0.5 and VS 2 phases can effectively promote charge transfer.At the same time, the different redox potentials of MoS 1.5 Se 0.5 and VS 2 alleviate the volume expansion during repeated charging/discharging, improving the electrochemical reaction kinetics and structural stability of the electrode material.In addition, by increasing interlayer spacing and uncovering more active sites, Se doping can enhance active material conductivity and cause charge reconstruction, which will increase the kinetics of diffusion reactions.The heterostructure MoS 1.5 Se 0.5 @VS 2 delivered good rate performance and long-life cycling stability, with a capacity of 533.9 mA h g -1 at 0.5 A g -1 , and rate performance of 424.5 mA h g -1 at a high current density of 5 A g -1 , when utilized as an anode material for SIBs. [105]urthermore, the uniform dispersion of intermediate nanocrystals contributes to robust cycling stability.The distinct redox potentials and electrochemical reactions of various components help reduce strain during the charging/discharging process.Establishing a non-uniform 2D van der Waals structure is an effective method to improve the electrochemical performance of VS 2 nanomaterials.MXene (chemical formula M n+1 X n T x M: transition metal element, X: carbon or nitrogen, T x : surface termination) is a group of 2D layered transition metal carbides, nitrides, or carbonitrides.They have large interlayer spacing, high conductivity, and excellent mechanical flexibility.Zhao et al. designed a graded 2D VS 2 /Ti 3 C 2 T x -MXene hybrid using a simple liquid mixing method, where VS 2 is confined to conductive Ti 3 C 2 T x matrix and chemical connections are established between them (Figure 5f). [106]In situ transmission electron microscopy analysis shows that the hybrid relies on a very fast and reversible insertion/extraction process between Reproduced with permission. [53]Copyright 2022, Royal Society of Chemistry.c)SEM images of VS 2 and d) VS 2 /VO x heterostructure.e) Cycling performance of VS 2 /VO x heterostructure.Reproduced with permission. [102]Copyright 2023, Wiley-VCH.f) Diagram of the preparation and sodium storage of the VS 2 / Ti 3 C 2 T x hybrid.g) Interlayer distances of VS 2 /Ti 3 C 2 T x before and after Na insertion.Reproduced with permission. [106]Copyright 2023, Wiley-VCH.
VS 2 and Na x VS 2 (where x = 1) to store sodium (Figure 5g).Theoretical calculations indicate that the Ti 3 C 2 T x matrix significantly enhances the charge transfer of VS 2 and reduces its volume expansion, especially after the addition of Na + .Therefore, the VS 2 /Ti 3 C 2 T x -MXene hybrid exhibits excellent specific capacity (522 mA h g -1 at 0.2 A g -1 ), rate capacity (342 mA h g -1 at 10 A g -1 ), and cycle life (116% after 3000 cycles).At the same time, Ma et al. successfully synthesized 2D VS 2 /MXene nanostructures through a self-assembly strategy of several layers of MXene nanosheets that were fully dispersed on VS 2 without any surfactants, and through a freeze-drying process. [107]un et al. manufactured flower-shaped VS 2 /N-doped carbon (VS 2 /N-C) with extended (001) planes on rGO via solvent heating and subsequent carbonization processes. [108]In VS 2 /N-C@rGO nanocomposites, ultra-thin VS 2 "petals" are alternately inserted by N-doped porous carbon monolayers to achieve expanded interlayer spacing (10.2 Å), which could effectively reduce ion diffusion barriers, expose rich Na/K embedding active sites, and tolerate large volume changes.N-C and rGO carbon containing materials could significantly improve conductivity and structural stability.Thanks to the synergistic effect, VS 2 /N-C@rGO electrode presented a remarkable reversible capacity for SIBs, it achieved reversible capacities of 407 mA h g -1 and 273 mA h g -1 at 1 A g -1 and 8 A g -1 , respectively, and could still remain 316 mA h g -1 after 1400 cycles at 2 A g -1 .In PIBs, it exhibited reversible capacities of 334 mA h g -1 and 186 mA h g -1 at 0.2 A g -1 and 5 A g -1 , respectively, and could still reach 216 mA h g -1 after 500 cycles at 1 A g -1 . [108][111] The expansion of interlayer spacing facilitates the creation of additional channels for sodiation/desodiation processes between the layers and provides more unsaturated S edges, which are favorable for Na + storage.Li et al. constructed a 3D hierarchical VS 2 microrod assembled by nanosheets comprised of small VS 2 nano-grains using a one-step ethanol insitu etching synthesis route (Figure 6a). [109]Simply adjusting the amount of etching solution in the synthesis system can control the growth direction and thickness of nano-grains, thereby determining the layer edge exposure of nanoparticles.More exposure of layer edges can significantly improve intercalation dynamics, magnification performance, and cycling ability.Specifically, (001) oriented VS 2 nanoparticles with a large thickness to diameter ratio exhibit excellent rate performance, with high capacities of 255 mA h g -1 and 230 mA h g -1 , even at high rates of 1.0 A g -1 and 2.0 A g -1 , respectively.After conducting the following 200 cycles at 0.2 A g -1 , the reversible capacity of 350 mA h g -1 was still maintained. [109]s an anode for SIBs, Liao et al. directly produced Na 2 Ti 2 O 5 (NTO) nanowire arrays covered with VS 2 nanosheets (NTO-VS 2 ) on a collector (Ti foil) using a two-step hydrothermal technique as depicted in Figure 6b.The resulting composite nanowire array electrodes successfully connect the electrochemical functions of individual components, such as the fast electron/ion diffusion and low voltage of VS 2 and the outstanding stability of NTO (Figure 6c). [96]Conversely, Xu et al. prepared CNF@VS 2 by Figure 6.a) Schematic illustration of the in-situ chemical etching procedure for 3D hierarchical VS 2 microrods.Reproduced with permission. [109]Copyright 2018, Elsevier.b) SEM image of NTO-VS 2 .c) Rate performance of NTO-VS 2 .Reproduced with permission. [96]Copyright 2015, Elsevier.d) SEM image of CNF@VS 2 .e) Cycle performance of CNF@VS 2 .Reproduced with permission. [79]Copyright 2020, Elsevier.f) Diagrammatic representation of the OL-VMS manufacturing process.g) Stability measurement of the OL-VMS-based anode at 0.2 A g −1 .Reproduced with permission. [69]Copyright 2021, American Chemical Society.
anchoring an ultra-thin bowl shaped VS 2 nanosheet array onto a highly conductive carbon nanofiber (CNF) substrate through a one-step solvothermal method (Figure 6d).The VS 2 array has the characteristics of large area, high uniformity.In the resultant CNF@VS 2 composite, ultrathin bowl-like arrays could render fast and easy access of Na ions to the surface of VS 2 .The coupling of VS 2 with carbon nanofibers effectively adapts to the volume expansion from the conversion reaction.Therefore, this material exhibits excellent sodium storage capacity when used as an anode for SIBs, with a capacity of up to 659 mA h g -1 at 0.1 A g -1 and capable of cycling up to 6000 times at 2 A g -1 (Figure 6e). [79]Yue et al. employed a straightforward hydrothermal self-assembly process, followed by a heating procedure, to synthesize a novel orderly layered VMoS 2 (OL-VMS) anode material (Figure 6f). [69]hen employed as the anode material in SIBs, the distinctive structure of OL-VMS serves a dual purpose.It not only facilitates the swift migration of sodium ions between the stacked layers but also establishes a robust framework to accommodate volume changes during the intercalation and deintercalation processes.Furthermore, the inclusion of vanadium within the framework introduces additional defects, creating abundant storage sites for sodium ions.Consequently, the OL-VMS-based anode material demonstrates impressive reversible capacities of 602.9 mA h g -1 at 0.2 mA g − 1 and 534 mA h g -1 even after 190 cycles at 2 A g -1 (Figure 6g). [69]

VS 4
Among the vanadium sulfide family, vanadium tetrasulfide (VS 4 ) has 1D atomic-chain structures consisting of V 4+ ions and [S 2 2− ] dimers which are stacked with a large van der Waals (vdWs) gap of 5.83 Å (Figure 7a). [112]This open channel can promote the embedding and diffusion of various metal ions, including Na + (1.02 Å), Mg 2+ (0.72 Å), Al 3+ , K + , and Zn 2+ . [113]Compared with VS 2 , quasi layered VS 4 not only has a wider interlayer spacing, but also has a higher theoretical capacity (1196 mA h g −1 ) due to its high sulfur content. [114]VS 4 undergoes a three-step mechanism when used as an anode discharge for SIB.During the discharge process, Na insertion reaction (Equation ( 3)) first occurs during the initial discharge process, followed by a conversion reaction (Equation ( 4)) where Na x VS 4 decomposes into V and Na 2 S new phases.The subsequent electrochemical reaction is a reversible conversion reaction between Na 2 S and S, while the metal vanadium remains inert, but it improves the conductivity of the electrode during the cycling process. [115]The unique crystal structure and attractive electrical properties of VS 4 make it have broad application prospects in alkali ion batteries and supercapacitors.However, due to significant voltage polarization and electrode fragmentation, the original VS 4 exhibits poor rate ability and severe capacity degradation.Therefore, the mainstream of current research is focused on nanostructure engineering and/or .Reproduced with permission. [112]Copyright 2018, The Royal Society of Chemistry.b) SEM image of VS 4 -20 h.Reproduced with permission. [114]Copyright 2020, Elsevier.c) SEM image of VS 4 .d) Rate performance of the VS 4 electrode.Reproduced with permission. [80]Copyright 2021, Elsevier.e) Diagrammatic representation of the low-temperature solid-state synthesis of IE-VS 4 and IN-VS 4 .f) Long cycling performance of IE-VS 4 and IN-VS 4 .Reproduced with permission. [118]Copyright 2021, Elsevier.
composite material configuration to enhance charge transfer dynamics and alleviate volume expansion during cycling. (3) Various VS 4 nanostructures, such as nanoribbons, nanorods, nanocones, and nanosheets, have been synthesized and assembled into microspheres or layered structures as SIB anodes.

VS 4 Nanostructure
A lot of work has gone into fine-tuning the VS 4 materials' structures to enhance sodium/ potassium storage performance.Numerous nanostructures have been created to exhibit enhanced electrochemical characteristics, such as hollow nanostructures, hierarchical nanostructures, nanorods, nanocones, and nanosheets.Generally speaking, compared to their bulk counterparts, nanostructures show more remarkable size-dependent physicochemical and chemical capabilities.Designing adaptable nanostructures for high-performance anodes has gained more interest due to their notable structural advantages.For example, Li et al. used a simple template free method for hydrothermal synthesis by controlling the pH value of the precursor: nanorods (nanorod-VS 4 ), nanocones, and self-assembled VS 4 microspheres with nanoribbons.When used as an anode material for SIBs, nanorod-VS 4 exhibits better electrochemical performance than the other two dues to its self-assembled structure and synergistic effect of preferred orientation growth along the (110) plane, which provides high reversible capacity of 225 mA h g −1 after 200 cycles at 0.5 A g −1 . [116]Well-distributed VS 4 nanosheets via simple solvothermal methods fabricated by Yang et al.SEM (Figure 7b) and TEM images show that VS 4 with perfect 3D structure can be obtained after 20 hours (VS 4 -20 h) at 180 °C.Thanks to its perfect 3D structure, VS 4 -20 h has excellent sodium storage performance.These nanosheets as anode materials for SIBs presented high reversible capacity (472 mA h g −1 after 100 cycles at 0.2 A g −1 ), excellent cycle stability (410 mA h g −1 at 1 A g −1 after 400 cycles), and excellent rate performance (345 mA h g −1 at 5 A g −1 and 288 mA h g −1 at 10 A g −1 ). [114]he selection of solvents has a certain impact on the structure and morphology of VS 4 , directly affecting its electrochemical performance.According to reports, N-methylpyrrolidone (NMP) has a sufficiently high boiling point (≈200°C) and low viscosity, making it an ideal liquid medium for high-temperature solvothermal reactions.A previous study by Coleman et al. reported that NMP is one of the best solvents for liquid-phase stripping of transition metal disulfide compounds (TMD) materials.As a solvothermal solvent, NMP can effectively preserve nanostructured TMD products and inhibit their further aggregation. [117]Based on this, Zang et al. successfully synthesized VS 4 using NMP as the solvent and solvothermal method.Figure 7c, the SEM images indicate the irregular morphology of VS 4 nanosheets.The large chain spacing of VS 4 is conducive to the diffusion of Na + .In addition, the NMP solvent used here has appropriate surface energy to promote the stability of the VS 4 sample.As the anode of SIB, the VS 4 electrode exhibits excellent cycling stability.After 500 cycles, it provides a reversible specific capacity of 534 mA h g −1 , with an excellent capacity retention rate of 100%.It also exhibits excellent rate performance (743 mA h g −1 and 625 mA h g −1 at 0.2 and 2 A g −1 , respectively), as show in Figure 7d. [80]he ion embedding strategy can expand the interlayer spacing of layered materials, accelerate ion diffusion, and thus improve the rate capability.In addition, some intercalation agents can also reduce the bandgap of electrode materials to improve the intrinsic electron conductivity, thereby further improving the rate performance.Qin et al. developed a low-temperature solid-state method to achieve interlayer engineering on VS 4 .During the synthesis process, the insertion of [NCN] 2-anions into the chains of VS 4 leads to interlayer expansion, providing a wide ion channel for rapid ion diffusion (Figure 7e).In addition, the embedded anions also reduce the band gap of VS 4 and enhance electronic conductivity.Due to these advantages, IE-VS 4 has achieved excellent sodium ion storage performance in terms of ultrafast rate capability (500 mA h g −1 at 20 A g −1 ), high capacity, and excellent cycle stability (550 mA h g −1 at 5 A g −1 after 2500 cycles) (Figure 7f). [118]

3D Spheres VS 4
Synchronizing the improvement of structural stability and Na + storage kinetics is an urgent problem to achieve high performance of pure VS 4 anodes.Valuable insights have been derived from the utilization of 3D self-assembled hollow nanostructures. [49]On the one hand, 3D self-assembly structures can provide buffering for self-expansion and self-contraction, effectively adapting to volume changes during Na-ions insertion/extraction processes.On the other hand, nanoscale building blocks can reduce the transmission length of electrons, shorten the diffusion path of ions, and thus improve the kinetics of electrochemical reactions.For instance, Liu et al. prepared ultrathin VS 4 nanosheets through microwave inorganic production chemistry together with ammonium ion (Figure 8a).Because the nanostructure of hollow VS 4 nanospheres is more resistant to volume changes brought on by Na-ion insertion/extraction, these nanospheres have an ideal long-life cycle stability and a reasonably high specific capacity.When used as an anode electrode material for SIBs, hollow VS 4 nanospheres could display a high capacity of 1226.7 mA h g −1 after 100 cycles at a current density of 0.2 A g −1 , and an average specific capacity (Figure 8b) of 916.8, 852.0, 809.2, 775.7, 748.2 mA h g −1 at a voltage density of 0.2, 0.5, 1.0, 2.0, 5.0 A g −1 . [61]Yang et al. also prepared VS 4 hollow microspheres with ultra-thin 2D nanosheets on the surface through hydrothermal methods, as shown in Figure 8c.The presence of 2D ultra-thin films on VS 4 microspheres not only shortens the diffusion pathway of Na + , but also provides abundant active sites.In addition, the hollow interior allows for full contact between the electrode and electrolyte, and can better adapt to the structural stress caused by volume changes during the reaction process.VS 4 hollow microspheres exhibit a large initial specific capacity (≈1000 mA h g −1 at 0.1 A g −1 ), excellent rate ability (≈360 mA h g −1 at 20 A g −1 ), and excellent stability (capacity retention rate of 73.2% after 1000 cycles at 10 A g −1 ).Even at low temperatures of −40 °C, VS 4 hollow microspheres still exhibit excellent rate performance. [84]A hollow structure that can provide sufficient electrode/electrolyte contact and short Na + and electron diffusion paths, as well as a 3D self-assembled structure that can provide significant physical interception for the expansion/contraction of building blocks, has been proven to have significant structural advantages in advanced energy storage technology.The above advantages can greatly improve the specific capacity, cycling stability, and rate performance of electrode materials. [119]n addition, by controlling the crystal structure of structural blocks and optimizing the connections between structural blocks, the Na + storage dynamics of hollow nanostructures can be further improved.Li et al. first constructed VS 4 microspheres using a simple template free hydrothermal method, and then successfully adjusted the crystallinity of the nanounits by adjusting the micro reaction pressure.A systematic study on the effect of crystallinity on sodium storage performance has shown that the decrease in crystallinity greatly increases surface active sites, thereby promoting pseudocapacitive behavior, ultimately leading to high capacity, long cycle life, and high rate performance.When used as a SIB anode, the VS 4 microspheres with the lowest crystallinity exhibit excellent cycling stability, with a high reversible capacity of 412 mA h g −1 after 230 cycles at 0.2 A g −1 .Even at  [61] Copyright 2022, American Chemical Society.c) SEM image of VS 4 hollow microspheres.Reproduced with permission. [84]Copyright 2022,Wiley-VCH.d) Schematic illustrations of the time-dependent hydrothermal reaction: self-sacrificed evolution mechanism from microsphere to hairball-like structure.e) Long-term cyclic stability of hairball-like VS 4 at high current densities.Reproduced with permission. [120]Copyright 2020, Springer.f) Schematic illustration for the discharge process of (110)-bridged nanoblocks self-assembled VS 4 hollow microsphere.g) Rate and cycling performances.Reproduced with permission. [49]Copyright 2019, Elsevier.
high current densities of 1.0 and 2.0 A g −1 , the reversible capacity can still reach 345 and 293 mA h g −1 , respectively. [112]Ding et al. successfully prepared pure hairball-like VS 4 composed of helical nanowires through a simple template free hydrothermal method, and studied its morphological evolution mechanism by controlling the reaction time (Figure 8d).Specifically, an unlike traditional mechanism of VS 4 for Na + storage was proposed: First, the transformation of VS 4 -Na 2 S and V is partially reversible during the initial ten cycles; second, the increase in capacities can owe to the increment of the reversible conversion reaction between Na 2 S and S to store Na + ; and last, stabilization stage can be described by reversible transformation reaction between Na 2 S and S which was the main reaction mechanism of Na-S batter-ies.The optimized hairball-like VS 4 as the anode of SIBs exhibits high discharge capacity (660 and 589 mA h g −1 at 1 and 3 A g −1 , respectively) and excellent rate performance (maintaining ≈100% at 10 and 20 A g −l after 1000 cycles) at room temperature (Figure 8e). [120]Li et al. synthesized (110) bridged nanoblock self-assembled VS 4 hollow microspheres (PNBH-VS 4 ) using a one-step hydrothermal method. [49]The constructs a Na + conduction channel and e − transfer pathway between nanoparticles and nanoblocks.The self-assembled hollow structure exhibits a dual space physical encapsulation effect on the large volume change of the nanoblocks, improving the electrochemical reversibility of insertion and conversion reactions, and synergistically improving Na + storage kinetics and structural stability (Figure 8f).The Na + storage kinetics and rate ability have significantly increased.Thus, PNBH-VS 4 exhibits synergistic performance, including high capacity, excellent rate performance, and excellent cycling stability.After 250, 200, 350, and 700 cycles at 0.2, 0.5, 1.0, and 2.0 A g −1 , the capacity can reach 629, 564, 428, and 400 mA h g −1 , respectively.Even at 5.0 A g −1 , after 1000 long cycles, the capacity can remain stable at 309 mA h g −1 (Figure 8g). [49]

VS 4 Combined with Various Carbons
Synthesis of VS 4 combined with various carbons is one of the major approaches to enhance mechanical stability and accommodate the volume changes of active materials.A lot of VS 4 combined with carbon-based structures, including VS 4 nanorods onto graphene, VS 4 grown on reduced graphene oxide (rGO), nitrogen-doped carbon nanotube@VS 4 , and carbon modified VS 4 composite.For instance, Chen et al. and Li et al. also anchored VS 4 nanorods onto graphene to obtain composite materials, which exhibited excellent sodium storage performance in SIB. [121,122]For instance, Chen et al. successfully prepared VS 4 nanorods with an average diameter of 30 nm through a fast and simple one pot solvothermal process.In order to further improve the conductivity and electron transfer performance of VS 4 materials, small VS 4 nanorods were uniformly anchored in the conductive network of large-sized RGO nanosheets to obtain VS 4 /RGO nanocomposites.When tested as an anode material in a half cell of SIBs, VS 4 nanorods exhibit outstanding electrochemical performance in terms of high reversible capacity (185 mA h g −1 at a current density of 0.8C), good rate capacity, and long cycle life (182 mA h g −1 after 100 cycles at 0.4C).Similarly, Li et al. successfully exploit dense VS 4 nanorods horizontally anchored on both sides of reduced graphene oxide (rGO) nanosheets and studied their Na + storage performance.The interface lattice interaction and V-C bond are the main coupling modes between VS 4 nanorods and rGO nanoplates.Thanks to VS 4 nanorods with abundant energy storage sites, rGO nanosheets with high conductivity, their interface lattice interactions, and the synergistic effect of V-C bonds, the SIB of a VS 4 @rGO anode showed specific capacities of 264, 176, and 114 mA h g −1 at 1, 5, and 10 A g −1 after 1000 cycles, respectively. [122]In 2015, Sun et al. first reported VS 4 grown on reduced graphene oxide (rGO) as an anode for SIBs.rGO sheets provide a structure for improving conductivity and a substrate for nucleation and growth of VS 4 .The thin folded graphene sheets effectively hinder the aggregation of VS 4 nanoparticles (Figure 9a).The use of VS 4 /rGO as an anode for SIBs exhibits good performance, providing a specific capacity of 362 mA h g −1 at 100 mA g −1 .But after 50 cycles, the capacity significantly decayed to 241 mA h g −1 (Figure 9b). [115]Later, the nanocomposites (labeled VS 4 -G) by stacking vanadium sulfide (VS 4 ) nanosheets onto reduced graphene oxide (RGO) also reported by Li et al. using a simple hydrothermal method.The prepared VS 4 -G nanocomposite material exhibited a stable capacity of ≈463 mA h g −1 after 100 cycles at 100 mA g −1 as a SIB anode. [123]Pang et al. anchored controllable VS 4 nanoparticles onto the surface of graphene sheets (GS) using a simple cationic surfactant (CTAB) assisted hydrothermal method.Small VS 4 nanoparticles are uniformly anchored onto GS and the VS 4 particle size is controlled within 10-40 nm by using different amounts of GS.Thanks to the synergistic effect of high conductivity GS and nanocrystalline VS 4 , when used as an electrode material for sodium ion batteries VS 4 @GS Nanocomposite materials exhibit high specific capacity (349.1 mA h g −1 after 100 cycles at 100 mA g −1 ), excellent long-term stability (84% capacity retention after 1200 cycles), and high rate performance (188.1 mA h g −1 at 4 A g −1 ). [124]Similarly, Wang et al. developed uniform rectangular shaped nanoparticles connected by rGO layers through in-situ rGO template hydrothermal processes.Through the optimization of the rGO contents, we control the ordered microstructure with the uniform nanoparticles wrapped by rGO in porous stack architecture of VS 4 -rGO so as to enhance the pseudocapacitive charge storage ability with the more active surface sites (Figure 9c).The VS 4 -rGO composite materials exhibit excellent electrochemical performance in ester based electrolytes, as displayed in Figure 9d, delivering ultrahigh rates performance of capacity of 123 mA h g −1 at 20 A g −1 . [125]he coating strategy mentioned in the discussion on VS 2 is also applicable to VS 4 .Implementing a uniform and comprehensive carbon coating strategy, combined with a composite structure design that provides fast internal ion conductivity, not only avoids unstable performance caused by uneven local conductivity, but also benefits high power capability and excellent cycling stability from inside to outside.Wang et al. used hydrothermal method combined with rapid cooling strategy to obtain VS 4 /Nb 2 O 5 /GO composite structures with a full range of GO coatings.VS 4 and stable intercalated Nb 2 O 5 are formed through self-assembly during hydrothermal processes.The two monomers are connected through V-O chemical bonds, forming an uninterrupted, fast, and stable electron and ion transport channel from the inside out.In addition, the VS 4 /Nb 2 O 5 precursor and GO were uniformly dispersed through cell fragmentation and further rapidly cooled by liquid nitrogen during the heating process.At the same time, under stress, GO is uniformly wrapped on the surface of VS 4 /Nb 2 O 5 after intense stirring.When used as an anode material for SIBs, it can maintain 268.8 mA h g −1 after 2000 cycles with a current density of 10 A g −1 . [127]oreover, the unique growth pattern of VS 4 can also be combined with various carbon nanoparticles to improve the performance of VS 4 in storing Na + ions.For instance, Yang et al. successfully synthesized a three-dimensional poplar flower-like nitrogen-doped carbon nanotube@VS 4 (NCNt@VS 4 ) composite material using nitrogen-doped carbon nanotubes (NCNt) as a template through solvothermal reaction (Figure 9e). [126]After solvent heat treatment, coral-like VS 4 is uniformly coated on the surface of NCNt, forming a 3D structure resembling a poplar flower.This special structure provides a high surface area and more electron/Na + diffusion channels, and improves the wettability of the electrolyte and the diffusion rate of electrons/Na + in the solid phase.NCNt with a hollow structure can provide a highly conductive network and buffer large volume expansion during electrochemical processes.Thanks to the special structure of composite materials and their clever combination with NCNt, NCNt@VS 4 shown remarkable performance.At the same time, the electrode exhibits excellent rate performance (460 mA h g −1 at 5 A g −1 ) and high stability to current changes (Figure 9f).Interesting, VS 4 /MWCNT composite materials were prepared by Yu et al.In contrast to reduced graphene oxide (rGO) layers, where Reproduced with permission. [115]Copyright 2015, American Chemical Society.c) schematic illustration of the electrons transfers and Na ions charge storage mechanisms of VS 4 -rGO composite electrode.d) Rate performance of VS 4 -rGO.Reproduced with permission. [125]Copyright 2018, Wiley-VCH.e) Schematic illustration of the synthesis process of NCNT@VS 4 .b) Rate and cycling performance of the NCNT@VS 4 .Reproduced with permission. [126]opyright 2020, The Royal Society of Chemistry.
achieving comprehensive wrapping is challenging, MWCNT can establish a robust conductive framework that directs and regulates the nucleation and growth of VS 4 without the concerns of aggregation.This results in an expansion of the specific surface area, the promotion of electrolyte-active material interaction, and the provision of a rapid electronic interconnection pathway for electrochemical reactions.The introduction of MWCNT has led to enhancements in rate performance and Na + storage kinetics for VS 4 .Thus, the VS 4 /MWCNT electrode exhibited excellent performance, with high specific capacity (670 mA h g −1 after 70 cycles at 0.2 A g −1 ) and excellent rate performance (368 mA h g −1 at 5 A g −1 ) for SIBs. [128]Lim et al. also reported N-doped multi-channel carbon nanofibers embedded in amorphous VS nanoparticles (VS 4 @NMCNF) by in-situ thermal vulcanization after electrospinning process.The resulting nanofibers have an ordered pore structure and can contain amorphous vanadium sulfide nanoparticles.This reasonable structure can achieve rapid ion/electron transport, increase the number of active sites used for ion storage, and enhance structural integrity. [129]o far, there are roughly two carbon combination strategies to improve the sodium storage performance of VS 4 .In addition to composites with various carbons, another method is to prepare carbon-modified VS 4 composite materials. [49,115,128]For example, Liu et al. prepared graded VS 4 microspheres using a simple and low-cost solvothermal method, and then modified the VS 4 microspheres using polydopamine through a simple polymerization reaction VS 4 @PDA.Due to its unstable characteristics at high temperatures, VS 4 becomes V 3 S 4 after annealing, and PDA decomposes into N-doped carbon, ultimately resulting in porous structure V 3 S 4 @NC.As an anode material for SIBs, VS 4 @PDA exhibits the best sodium storage performance among.When cycling 150 times at 0.5 A g −1 , a high reversible capacity of 782 mA h g −1 can be obtained.Even at a high current density of 2 A g −1 , the reversible capacity can still be maintained at 289.5 mA h g −1 for 300 cycles.Assembled VS 4 @PDA // Na 3 V 2 (PO 4 ) 3 /rGO full battery also exhibits good cycling stability, with a discharge capacity of 287.6 mA h g −1 after 100 cycles at 0.5 A g −1 . [130]

Build Heterostructures with VS 4
Design strategies for heterostructure electrodes share similarities with those employed for other materials in the energy storage space.Similar to the improvements observed in the electrochemical performance of VS 2 , incorporating VS 4 with carbon materials and creating heterostructures with VS 4 can help overcome issues related to the low conductivity and volume expansion that pure VS 4 during cycling.It is essential to note that there is relatively limited research on this topic.Several proposals for heterostructure design efforts have aimed to construct built-in electric fields to enhance ion reaction kinetics and cyclic stability.To reconstruct the original VS 4 at the structural design level, Wang et al. applied a simple anionic heterogeneous strategy to design a uniformly distributed V 1.11 S 2 /V 1.13 Se 2 heterogeneous interface with a pyrrole derived carbon surface coating (VSSe/C).Unlike traditional methods of constructing heterostructures using disulfides, they used high-capacity VS 4 as the precursor.Thanks to the weaker V-S 2 2− bond, alternating V 1.11 S 2 and V 1.13 Se 2 heterostructures were formed in situ during the subsequent selenization process.Specifically, a stable V-based disulfide/selenide heterojunction interface was constructed, forming a regular built-in electric field and extended interlayer distance to ensure rapid and stable sodium ion transfer.At the same time, a multi defect carbon coating is further formed during the heterogeneous process, which can enhance the rapid ion adsorption and conductivity of the entire structure.Consequently, when applied as SIBs anode materials, a specific capacity of 784.9 mA h g −1 can be achieved at a current density of 2 A g −1 and excellent rate and cycling performance are demonstrated. [131]Theoretical calculations have revealed that MXenes with O-terminated surfaces can effectively bind to polysulfides, thereby stabilizing soluble polysulfides and improving cycling stability.Chen et al. introduced a novel approach involving the electrostatic self-assembly of a few-layer V 2 CT x and vanadium precursors containing VO 3− in solution, and then in situ constructed VS 4 nanosheets anchored on few-layer V 2 CT x by solvothermal synthesis.This sheet-like 3D structure has a large pore structure, allowing for full contact between the electrode and electrolyte.Through the chemical bridging function of S-V-C chemical bonds, a tight heterostructure is formed between V 2 CT x and VS 4 , promoting the flow of electrons from VS 4 to V 2 CT x during the electrochemical reaction process and accelerating the charge transfer ability of VS 4 .In addition, the introduction of V 2 CT x regulated the increase in vanadium chemical composition in the composite material.The VS 4 -V 2 CT x composite material exhibits excellent ultra-long cycle performance for SIBs, with a specific discharge capacity of 322 mA h g −1 after 4000 cycles at a high current density of 10 A g −1 . [132]etal organic frameworks (MOFs) have recently been used as precursors for the synthesis of heterostructure electrode materials due to their stable and diverse structures, which can effectively buffer volume changes during discharge/charging processes.Among them, the cobalt based imidazolium salt skeleton (ZIF-67) can be transformed into a nitrogen doped carbon skeleton after annealing, thereby improving the conductivity of the material.Therefore, people are committed to synthesizing heterogeneous materials based on ZIF-67 templates as anode materials for SIBs and PIBs.In the paper by Li et al., bimetallic sulfides heterostructures with VS 4 nanodots decorated on CoS 2 nanoparticle in nitrogen doped carbon dodecahedron (CoS 2 /NC@VS 4 ) were successfully prepared by sulfidation of ZIF67 template and subsequent hydrothermal process.Firstly, the heterogeneous structure composed of VS 4 and CoS 2 can effectively optimize rate performance.
Meanwhile, this nitrogen doped carbon framework not only improves conductivity, but also prevents volume expansion during cycling.Therefore, when CoS 2 /NC@VS 4 is applied as an anode of SIBs, it exhibits excellent rate capacity and cycle stability with a reversible capacity of 307 mA h g −1 at 1.0 A g −1 after 700 cycles.Moreover, when served as an anode of PIBs, CoS 2 /NC@VS 4 displays superior electrochemical performance with a capacity of 292 mA h g −1 at a current density of 1.0 A g −1 after 430 cycles. [133]ao et al. proposed a homogeneous layered VS 4 /SnS heterostructure with well-defined thin nanosheet morphology, anchoring on the wrinkled graphene scaffold (VS 4 /SnS@C) as an anode of PIBs.The designed heterogeneous VS 4 /SnS phase boundary can limit the coarsening of Sn and the aggregation of intermediates, greatly consolidating the layered nanostructure and maintaining the reversible reaction interface, thus making the reverse conversion reaction between K x S y and metal Sn highly stable.In addition, external graphene scaffolds can enhance the integrity of nanostructures during repeated cycles and improve electrode conductivity by enhancing reaction kinetics.VS 4 /SnS@C heterostructure exhibits excellent rate ability (122.7 mA h g −1 at a super high rate of 10 A g −1 ) and extraordinary ultra-long cycle stability (168.4 mA h g −1 after 6000 cycles of 1 A g −1 ) for PIBs. [54]

V 3 S 4
V 3 S 4 is one of the important members of the vanadium sulfide family.V 3 S 4 , the self-intercalation layer of V, can also be in the form of V 0.5 VS 2 .Due to its unique twisted NiAs structure, it has VS 2 monolayer building blocks and additional V atoms connecting two adjacent layers, which can provide more potential positions and improve charge transfer in rechargeable batteries. [134,135]Liu et al. proposed the sodium storage mechanism of V 3 S 4 : (i) Na + insertion into V 3 S 4 to form Na x V 3 S 4 (Equation (5)); (ii) conversion reaction where Na + reacted with Na x V 3 S 4 to form Na 2 S (Equation ( 6)). [82]3 S 4 + xNa + + xe − → Na x V 3 S 4 (5) Noticeably, the insertion/extraction of sodium ion and conversion processes are reversible, while the irreversible capacity loss is mainly caused by the inactive V residue, S dissolution, and SEI formation in the initial cycle.Like other members of its family, pure V 3 S 4 typically exhibits severe capacity degradation as it rapidly expands when absorbing alkali metal ions.In recent years, many teams have prepared V 3 S 4 based composite materials, such as composite with carbon nanofibers and conductive polymers, which have promoted the improvement of rate ability and long-term cycling stability for SIBs and PIBs.
For instance, Yao et al. designed and synthesized a flexible thin film based on vanadium sulfide using a simple and scalable industrial electrospinning method (Figure 10a,b).The obtained carbon matrix nanofibers were used to construct a flexible membrane, and layered V 3 S 4 was uniformly distributed inside The flexibility of the freestanding electrode.c) Long-term cycling performance of V 3 S 4 @CNF at 5 A g −1 (insert: charge-discharge profiles for special cycles).Reproduced with permission. [136]Copyright 2021, Elsevier.d) Illustration for the typical synthesis of V 3 S 4 @C.e) Comparative long-term cycling behaviors at 1 A g −1 over 4000 cycles.Reproduced with permission. [82]Copyright 2019, Wiley-VCH.f) Rate performance of V 3 S 4 /ppy.Reproduced with permission. [135]Copyright 2023, The Royal Society of Chemistry.
the carbon nanofibers, serving as a flexible independent electrode for SIBs.Flexible electrodes can have astonishing mechanical flexibility, light weight, extremely strong conductivity, and most importantly, excellent electrochemical performance.In addition, compared to traditional electrodes with the addition of inactive materials, flexible independent electrodes without adhesives and secondary conductive agents can effectively express higher energy density.Meanwhile, due to the small size of V 3 S 4 , this special structure provides more reaction sites for Na + .The flexibility V 3 S 4 @CNF electrode exhibits high specific capacity (400 mA h g −1 at 0.1 A g −1 ), high rate performance (185 mA h g −1 at 10 A g −1 ), and excellent long cycle stability (98% capacity retention after 3500 cycles) (Figure 10c). [136]s we all know, the metal ions arranged regularly in MOFs can be easily converted into metal sulfides through simple sulfurization, while organic ligands can form amorphous or graphite nano carbon coatings in situ.Liu et al. employed a straightforward sulfuration process, using nanorod-like V-based MOFs as a template, to synthesize ultrathin V 3 S 4 @C nanosheets assembled into hierarchical nanotubes (V 3 S 4 @C NS-HNTs), as depicted in Figure 10d.The V 3 S 4 @C NS-HNTs have strong structural rigidity, a characteristic attributed to the presence of layered VS 2 sub-units and interlayer-occupied V atoms.Additionally, they offer efficient alkali-ion insertion and extraction, facilitated by the electrically active V 3 S 4 -C interfaces.Benefitting from the synergistic effect of pseudocapacitance and reversible insertion/extraction mechanisms, the V 3 S 4 @C NS-HNTs exhibits excellent long cycling life of 4000 cycles with the capacity of 182 mA h g −1 at 1 A g −1 and excellent rate capacity of 155 mA h g −1 at 10 A g −1 , as shown in Figure 10e.However, it should be noted that the reversible capacity for PIBs remains relatively low in this material. [82]While the V 3 S 4 /C composite material has exhibited enhanced electrochemical performance, the presence of interface resistance between V 3 S 4 and the carbon matrix continues to impede high-speed charge transfer kinetics, leading to elevated charge transfer resistance at the interface.Professor Cao and colleagues have innovatively introduced Mo-C bonds to establish connections between MoSe 2 and graphene, significantly improving the material's conductivity and consequently enhancing sodium storage capacity. [137]Similarly, Zhang et al. assert that the preparation of composite materials with close contact between V 3 S 4 and carbon or the establishment of interface coupling through chemical bonds is imperative to further enhance alkali ion storage performance.They introduced a novel 3D porous material, V 3 S 4 @N-C, which features ultrafine V 3 S 4 crystals integrated with N-doped carbon through V-C bonds.The creation of the 3D porous structure involved employing NaCl as a template and freeze-drying.Differing from prior reports on V 3 S 4 /C, the V-C bonds in V 3 S 4 @N-C promote swift electron transfer at the composite interface while more effectively constraining V 3 S 4 from infiltrating the carbon matrix, thus resulting in accelerated reaction kinetics and heightened structural stability.The optimized V 3 S 4 @N-C electrode delivers a discharge specific capacity of 510 mA h g −1 at a current density of 0.1 A g −1 , along with excellent rate performance, exhibiting a specific capacity of 343 mA h g −1 at 1 A g −1 in SIBs.When serving as an anode for PIBs, the V 3 S 4 @N-C electrode demonstrates a specific capacity of 467 mA h g −1 at 0.1 A g −1 and 302 mA h g −1 at 1.0 A g −1 . [138]otably, the long-term cycling performance of the electrode still requires improvement.Wu et al. reasonably designed a novel necklace shaped V 3 S 4 /carbon composite material composed of V 3 S 4 microspheres wrapped in N-doped carbon nanofibers (V 3 S 4 @NCNFs) as the anode for PIBs.This unique structure can avoid direct contact between V 3 S 4 particles and electrolytes, thereby suppressing side reactions to a certain extent.In addition, carbon nanofibers encapsulate V 3 S 4 microspheres to form a continuous network, which can prevent particle aggregation and V 3 S 4 structure collapse, and provide a conductive pathway for electron transport.V 3 S 4 @NCNFs electrode exhibits a high reversible capacity of 445 mA h g −1 after 300 cycles at 0.2 A g −1 , and an extended cycle stability of 245 mA h g −1 after 1000 cycles at 2 A g −1 . [139]ntegrating vanadium sulfide with conductive polymers is also a feasible method for improving electrochemical performance when compared to vanadium sulfide/carbon composite materials.Conductive polymers are excellent additions for electrode improvement because of their many benefits, including high conductivity, environmental stability, and ease of synthesis. [130,135]or example, Zhang's group chose to build V 3 S 4 /PPy nanocomposites using polypyrrole (PPy), a common conductive poly-mer, as a supporting substrate.Notably, high conductivity of ppy makes it possible for it to both disperse around nanostructures similar to V 3 S 4 and act as a supporting substrate for them, which in turn improves ion transfer and energy storage conductivity.Moreover, when employed as an anode for SIBs, it exhibits noteworthy rate capabilities, with specific capacities of 585 mA h g −1 at 2 A g − ¹ and 535 mA h g −1 at 5 A g − ¹ (Figure 10f). [135]

V 5 S 8
The crystal structure of V 5 S 8 can be characterized as a superstructure composed of ordered V vacancies within NiAs-type structures.Specifically, this structure includes a VS 2 monolayer building block that alternates with an additional V atom that partially occupies a quarter of the available sites between the monolayers, as illustrated in Figure 11a.Therefore, based on this structural feature, the formula for V 5 S 8 can be written as V 0.25 VS 2 . [33]ang et al. summarized the process of using V 5 S 8 for Na + storage in five stages, as depicted in Figure 11b.During the discharge process, it contained three stages, corresponding to intercalation (I), coexistence of intercalation and conversion (II), and conversion (III) reactions, respectively.In contrast, during the charging stage, two processes of conversion (IV) and deintercalation (V) reactions were involved.The (de)intercalation and conversion processes appear to be fairly reversible. [33]5 S 8 + xNa + + xe − → Na x V 5 S 8 (7)   Na x V 5 S 8 + (16 Yang et al. (2016) presented about the creation of ce-V 5 S 8 and V 5 S 8 graphite hybrid nanosheets (ce-V 5 S 8 -C) via a chemical stripping technique.The produced V 5 S 8 has a special crystal structure made up of VS 2 monolayer building blocks, where V atoms occupy 25% of the available crystallographic sites.This special structure facilitates the production and transport of electron/ion charge carriers, thereby promoting rapid electron and Na + diffusion during reversible charge/discharge.As a result, ce-V 5 S 8 -C demonstrated a high reversible discharge capacity of 682 mA h g −1 at 0.1 A g −1 and a respectable cycle life (496 mA h g −1 at 1.0 A g −1 after 500 cycles) for SIBs, as shown in Figure 11c. [33]anadium sulfide/carbon fiber composites (abbreviated as D-V 5 S 8 /CNFs) with sulfur rich defects were prepared by XV et al. through a simple electrospinning technique combined with a one-step sulfurization carbonization process. [140]The unique structure of V 5 S 8 nanoparticles confined within carbon fibers provides short distance channels and abundant adsorption sites for ion storage.In addition, increasing the interlayer spacing not only provides smaller vdW interactions and ion diffusion resistance, but also alleviates the volume change, thereby reversibly storing more Na and K ions simultaneously.The D-V 5 S 8 /CNF anode exhibits excellent storage performance and long-term cycling stability, which exhibits a notable capacity of 462 mA h g −1 at 0.2 A g −1 when employed in SIBs, and maintains a capacity of 190 mA h g −1 even after an extensive 17 000 cycles at 5A g −1 .When served as PIBs, it delivers a capacity of 350 mA h g −1 at 0.1 A g −1 , and maintains a capacity of 165 mA h g −1 after 3000 cycles at 1 A g −1 . [140]Similarly, Liu et al. successfully engineered V 5 S 8 @CNF, wherein V 5 S 8 nanoparticles were  [33] Copyright 2017, The Royal Society of Chemistry.d) SEM image of V 5 S 8 @C. e) Rate performance of V 5 S 8 @C.Reproduced with permission. [50]Copyright 2019, American Chemical Society.
affixed onto carbon nanofibers through the utilization of electrospinning technology, and these composites were subsequently employed as anode electrodes in both SIBs and PIBs.Combine cations and vanadium oxide with organic nanofiber precursors, and sulfurize the resultant nanofibers to create 1D amorphous carbon nanofibers.Particles of metal disulfide compounds were in situ deposited on the surfaces of these nanofibers.The resulting batteries exhibited stable cyclability and excellent rate capability, as well as fast and long-lasting Na and K storage performance.Thus, V 5 S 8 @CNF showed excellent performance with a specific capacity of 351.0 mA h g −1 after 400 cycles at 0.2 A g −1 in SIBs.In addition, it delivered a specific capacity of 289.2 mA h g −1 after 200 cycles at 0.1 A g −1 in PIBs. [141]u et al. reported the fabrication of V 5 S 8 nanoparticles enclosed within amorphous carbon nanorods, forming vanadium sulfide@carbon (VSC) nanorods through a surface MOF-derived vulcanization method.Amorphous carbon can not only alleviate severe volume changes during cycling, but also improves conductivity of active material.Owing to its unique hybrid structure, VSC has demonstrated excellent specific capacity (468 mA h g −1 after 100 cycles at 0.1 A g −1 ) and significant cycling stability (171 mA h g −1 after 4000 cycles at 3 A g −1 ) for PIB anodes. [142]s depicted in Figure 11d, V 5 S 8 nanosheets templated with hollow carbon (V 5 S 8 @C) were fabricated by Li et al. via hollow carbon template-induced technique. [50]The significant internal interlayer spacing of V 5 S 8 provides it with the capacity for reversible K-ions storage.Additionally, the hollow carbon substrate serves as both a structural framework, which mitigates particle aggregation and improvs the overall conductivity of the material, and also induces the formation of VS 4 particles.Then, the VS 4 particles transformed into ultra-thin layers of V 5 S 8 during subsequent annealing treatment.Electrochemical testing shows that V 5 S 8 @C is a promising material for PIBs anode.The as-prepared V 5 S 8 @C electrode exhibited excellent cycling capability of 360 and 190 mA h g −1 after 500 and 1000 cycles even at 500 and 2000 mA g −1 , respectively, as shown in Figure 11e. [50]Similarly, in order to deal with the volume change problem of VS x during the potassiation and depotassiation process.Li et al. employed polystyrene spheres (PS) as soft templates to synthesize honeycomb-like V 5 S 8 @C composite materials, which serve as anodes for PIBs and exhibit excellent K storage performance. [143]he numerous voids within the nanostructures significantly expedite the exchange processes between ion diffusion and charge transfer.Additionally, the exposed V 5 S 8 skeleton provides additional active sites for the storage of K-ions.Furthermore, the multi-scale interactions facilitated by the V-C and V-O-C chemical bonds at the interface between the external graphene matrix and the internal V 5 S 8 skeleton, along with the high pseudocapacitive charge storage mechanism, ultimately enhance the structural integrity of the nanostructure.As a result, the V 5 S 8 @C electrode demonstrates an initial charging capacity of 479.1 mA h g −1 at 0.05 A g −1 and retains a capacity of 121.5 mA h g −1 after 500 cycles at 1 A g −1 .

V 2 S 3
V 2 S 3 is a relatively unfamiliar vanadium sulfide in reported research.Peering into the crystal structure, V 2 S 3 reveals a primitive hexagonal polymorph characterized by abundant and spacious internal tunnels conducive to ion diffusion (with interplanar spacings of d(100) = d(010) = 5.69 Å and d(006) = 3.74 Å). [31,144] Theoretical calculations, based on a complete conversion reaction, indicate that V 2 S 3 could potentially achieve a maximum capacity of ≈811 mA h g −1 . [32]Consequently, V 2 S 3 holds promise as an anode material for SIBs.However, the controllable and stable synthesis of pure phase is difficult, resulting scare attempt on the research of V 2 S 3 recently. [32]The Na + storage mechanism in the V 2 S 3 anode can be characterized by intercalation and conversion reactions as follows Equations: One popular tactic for enhancing electrochemical performance is to modify the particle size of electroactive materials to shorten ion and electron transport lengths.But in this instance, it is challenging to efficiently access every particle via internal electrode circuits as well as ion and electron collectors.To realize the full potential of nanostructures, effective and reliable electrochemical paths with constrained active material volumes must be built. [145,146][149] Thus, building hierarchical structure materials is the ideal solution to shorten charging time and boost rate performance.For instance, Shen et al. developed a general strategy for preparing hierarchical carbon coated metal sulfide spheres. [32]The results indicate that this method is very effective in preparing graded nano/microsphere materials with a conductive matrix, and the conductive matrix is formed in situ without the addition of an external carbon source.By using vanadium glycinate annealing it in an H 2 S environment, they were able to directly produce very uniform carbon-coated V 2 S 3 (V 2 S 3 ⊂ C) spheres.The robust structure and effective electron/ion conduction network of the hierarchical V 2 S 3 ⊂ C sphere allowed it to show an exceptional rate capability of 410 mA h g −1 at 4 A g −1 and a remarkable capacity up to 777 mA h g −1 at 100 mA g −1 for SIB anodes. [32]At the same, A straightforward but effective method to convert V 2 CT x MXene into a sandwich V 2 S 3 @C@V 2 S 3 heterostructure was created by Huang et al. [150] The metal element (vanadium) of MXene could react with CS 2 at high temperatures, V 2 S 3 nanosheets were created, and carbides were converted into carbon in the CS 2 environment.Lastly, it is simple to obtain the sandwich V 2 S 3 @C@V 2 S 3 heterostructure in about one minute of reaction time.This improves poor conductivity and structural degradation of V 2 S 3 .Thus, Na + may be adsorbed and migrated more easily by the ultra-thin V 2 S 3 nanosheets and V 2 S 3 -C heterogeneous interface.As a result, the V 2 S 3 @C anode for SIBs demonstrated remarkable cycle stability (2000 cycles at 20 A g −1 ) and impressive rate performance of 477 mA h g −1 at 10 A g −1 . [150]ationally designing on various phases VS x has come into the spotlight for SIBs and PIBs due to adjustable structure and fascinating electrochemical performances.Nevertheless, there is still a lack of thorough understanding of the synthesis and energy storage mechanisms of VS x , non-stoichiometric compounds VS x , especially as their application in PIBs is still in its infancy.

Summary and Outlook
SIBs and PIBs, as new renewable and sustainable secondary batteries, are being researched extensively for their low production costs, ease of preparation, and environmental friendliness.They are expected to be widespread applications in fields like large-scale energy storage.Due to their varied material combinations, rich crystal structures, valence changes, high specific capacity, and high energy density, vanadium-based sulfides are a very promising anode material for SIBs and PIBs.A lot of vanadium-based sulfides with various morphologies have been prepared, and deliver exceptional high reversible specific capacity.To further enhance electrochemical performance, a growing amount of research interests and attention has been directed into the synthesis of varied morphologies and crystal structures of vanadium-based sulfides in recent years.The several forms of vanadium sulfide and their composites that are found in SIBs and PIBs are included in Table 2 along with information on fabrication methods, morphologies, and electrochemical performance.It should be noted, though, that even though a large number of vanadium-based sulfides have shown improving electrochemical performances, there are still some alluring obstacles in the way of the more developed developments and uses of vanadium-based sulfides, which merit greater interest and effort to overcome.
First, although structuring and micro/nanomaterials, as discussed in the modification design section, can improve rate performance and reduce volume expansion of VS x in SIBs and PIBs, the synthesis routes for these materials are usually complex, time-consuming, and may introduce additional contaminants.Generally, the VS x micro/nanostructures with different morphologies/ composition are first controllably synthesized based on recognizing their inherent nucleation, growth, and reaction behavior.After that, they can be easily reduced or oxidized as other valence-state similar-morphology vanadium sulfides by a simple annealing treatment.The phase purity and valence state of vanadium sulfides are not well regulated in this procedure.Thus, it is a challenging task to identify simpler, faster, and more effective ways to synthesize micro/nanostructures for the whole family of VS x .Second, SEI stability and side reactions are also important research point at present, because they seriously affect the life of active material electrodes.Moreover, the formation and structural changes of the stable SEI layer remain mysterious to researchers because of the limitations of current technology.De-spite the availability of several sophisticated characterization instruments such as ex-situ and/or in situ XRD, Raman, and X-ray photoelectron spectroscopy (XPS) these days, the sensitivity and precision of the detection for the process of evolution remain inadequate.
Third, assessing the potential of these VS x in full cells coupled with a cathode.It is intriguing to observe what level the energy density may reach in such systems.To achieve the practical use of vanadium-based multi-electron reaction electrode materials and to foster future battery technology innovation, efforts in this direction are extremely important.
Another issue that affects the majority of metal sulfides should be brought up as well.Vanadium sulfide has the ability to form polysulfides (NaS x , 4≤x≤8) as a result of the multi-step electrons and ions transfer that occur during the conversion reaction.Sulfur passes past the separator and onto the cathode side as a result of polysulfide decomposition in the electrolyte, which lowers capacity utilization.Therefore, in order to lessen or eliminate undesirable side effects, this is another work that has to be thought about and taken care of.In particular, VS 4 is a particularly promising next-generation anode material due to its enormous reversible capacity and distinctive flat voltage platform.However, the breakdown and distribution of polysulfides, which causes their capacity to fade, remains a severe and urgent issue.The issue must be resolved.Additionally, the voltage platform of VS 4 (≈2.0V) is somewhat higher.It will also result in a reduction in energy density even if it is safe.
Recently, except vanadium-based sulfides, some other vanadium-based compounds, such as vanadium oxides, [18,19,35,153,154] vanadium nitrides, [155][156][157] vanadium carbides, [158][159][160] and so on, have also drawn more interest for the application of SIBs/PIBs own to their chemical and physical properties.For instance, although vanadium oxides are interesting electrode materials with a variety of chemistry compositions, their growth is still constrained by a few intrinsic issues, including poor conductivity, low working voltage, and moderate vanadium dissolution. [153]Vanadium nitride has a high theoretical specific capacity (1238 mA h g -1 ) for SIBs, outstanding electrical conductivity (1.44×10 5 S m −1 ), and strong redox ability.Nevertheless, its production method often requires a lot of steps, high cost, and environmental pollution. [156]As for vanadium carbides, they have not attracted much attention in SIBs/PIBs so far, despite theoretical studies suggest that they may be one of the most promising anode materials. [158]The aforementioned analysis and summary show that while some other compounds based on vanadium also show promising potential for energy storage, there remain some problems and obstacles that have to be resolved.This implies that further efforts should be made in order to achieve these compounds' more promising possible applications for energy storage.

Figure 1 .
Figure 1.a) Comparison of the main properties among Li, Na, and K. b) The number of scientific papers published per year for Na and K-ion batteries.c) The number of papers published per year for vanadium-based materials and vanadium sulfides in Na/K-ion batteries (obtained from Web of Science).

Figure 2 .
Figure 2. Design strategy, morphology, and various crystal structures of VS x .

Figure 3 .
Figure 3. a) Side view of atomic structure of VS 2 crystal and b) Molecular structure of VS 2 crystal.Reproduced with permission.[87]Copyright 2017, The Royal Society of Chemistry.c) Lowest energy structure for single Na adsorption on 1H-VS 2 and 1T-VS 2 .Reproduced with permission.[88]Copyright 2016, American Chemical Society.d) Diagrammatic representation of the formation of VS 2 -SNS.e) SEM images of VS 2 -SNS.Reproduced with permission.[91]Copyright 2017, Elsevier.f) Principles and techniques for creating sandwich carbon composite materials incorporating ultra large VS 2 .Reproduced with permission.[68]Copyright 2020, Elsevier.g) Cycle performance diagrams for E-VS 2 and A-VS 2 .Reproduced with permission.[67]Copyright 2023, Elsevier.

Figure 4 .
Figure 4. a) SEM image and b) Magnified SEM image of flower-like VS 2 .c) Rate performance and d) cycling performance of flower-like VS 2 .Reproduced with permission.[48]Copyright 2017, Elsevier.e) SEM image of VS 2 microspheres.f) Rate and g) cycling performance of VS 2 microspheres.Reproduced with permission.[94]Copyright 2019, The Royal Society of Chemistry.h) Schematic illustration for the discharge process of the c-VS 2 @VOOH with enhanced electrochemical performance in SIBs.Reproduced with permission.[87]Copyright 2017, The Royal Society of Chemistry.i) Cycling performance of N/S-C@VS 2 .Reproduced with permission.[98]Copyright 2022, Elsevier.

Figure 5 .
Figure 5. a) Diagram illustrating the steps involved in creating VS 2 /MoS 2 heterostructure.b) Long cycling performance of VS 2 /MoS 2 .Reproduced with permission.[53]Copyright 2022, Royal Society of Chemistry.c)SEM images of VS 2 and d) VS 2 /VO x heterostructure.e) Cycling performance of VS 2 /VO x heterostructure.Reproduced with permission.[102]Copyright 2023, Wiley-VCH.f) Diagram of the preparation and sodium storage of the VS 2 / Ti 3 C 2 T x hybrid.g) Interlayer distances of VS 2 /Ti 3 C 2 T x before and after Na insertion.Reproduced with permission.[106]Copyright 2023, Wiley-VCH.

Figure 7 .
Figure 7. a) Side view image of monoclinic VS 4 and repeating unit of the 1D chain structure of VS 4 (top).Reproduced with permission.[112]Copyright 2018, The Royal Society of Chemistry.b) SEM image of VS 4 -20 h.Reproduced with permission.[114]Copyright 2020, Elsevier.c) SEM image of VS 4 .d) Rate performance of the VS 4 electrode.Reproduced with permission.[80]Copyright 2021, Elsevier.e) Diagrammatic representation of the low-temperature solid-state synthesis of IE-VS 4 and IN-VS 4 .f) Long cycling performance of IE-VS 4 and IN-VS 4 .Reproduced with permission.[118]Copyright 2021, Elsevier.

Figure 8 .
Figure 8. a) Schematic illustration of the synthesis of VS 4 nanomaterials.b) Rate performance of hollow VS 4 nanospheres.Reproduced with permission.[61]Copyright 2022, American Chemical Society.c) SEM image of VS 4 hollow microspheres.Reproduced with permission.[84]Copyright 2022,Wiley-VCH.d) Schematic illustrations of the time-dependent hydrothermal reaction: self-sacrificed evolution mechanism from microsphere to hairball-like structure.e) Long-term cyclic stability of hairball-like VS 4 at high current densities.Reproduced with permission.[120]Copyright 2020, Springer.f) Schematic illustration for the discharge process of (110)-bridged nanoblocks self-assembled VS 4 hollow microsphere.g) Rate and cycling performances.Reproduced with permission.[49]Copyright 2019, Elsevier.

Figure 9 .
Figure 9. a) Schematic illustration of the formation of VS 4 /rGO composite.b) Cycling performance of VS 4 /rGO composite at a current density of 100 mA g −1 .Reproduced with permission.[115]Copyright 2015, American Chemical Society.c) schematic illustration of the electrons transfers and Na ions charge storage mechanisms of VS 4 -rGO composite electrode.d) Rate performance of VS 4 -rGO.Reproduced with permission.[125]Copyright 2018, Wiley-VCH.e) Schematic illustration of the synthesis process of NCNT@VS 4 .b) Rate and cycling performance of the NCNT@VS 4 .Reproduced with permission.[126]Copyright 2020, The Royal Society of Chemistry.

Figure 10 .
Figure10.a) Schematic illustrating the synthesis of V 3 S 4 @CNF.b) The flexibility of the freestanding electrode.c) Long-term cycling performance of V 3 S 4 @CNF at 5 A g −1 (insert: charge-discharge profiles for special cycles).Reproduced with permission.[136]Copyright 2021, Elsevier.d) Illustration for the typical synthesis of V 3 S 4 @C.e) Comparative long-term cycling behaviors at 1 A g −1 over 4000 cycles.Reproduced with permission.[82]Copyright 2019, Wiley-VCH.f) Rate performance of V 3 S 4 /ppy.Reproduced with permission.[135]Copyright 2023, The Royal Society of Chemistry.

Figure 11 .
Figure 11.a) The top-view and side-view of the atom arrangements in V 5 S 8 .The gray and hollow hatched gray balls represent V atoms, and the yellow balls represent S atoms.b) schematic illustration of the energy storage mechanism of the ce-V 5 S 8 -C hybrid anode at different stages.c)Cycling performance at 1.0 A g −1 for ce-V 5 S 8 and ce-V 5 S 8 -C.Reproduced with permission.[33]Copyright 2017, The Royal Society of Chemistry.d) SEM image of V 5 S 8 @C. e) Rate performance of V 5 S 8 @C.Reproduced with permission.[50]Copyright 2019, American Chemical Society.

Table 1 .
Electrochemical properties of typical vanadium sulfides.

Table 2 .
Fabrication methods, morphology, electrochemical performance of vanadium sulfides and their composites for SIBs and PIBs.Capacity mA h g −1 / Cycles / Current density A g −1