Recent Advance and Modification Strategies of Transition Metal Dichalcogenides (TMDs) in Aqueous Zinc Ion Batteries

In recent years, aqueous zinc ion batteries (ZIBs) have attracted much attention due to their high safety, low cost, and environmental friendliness. Owing to the unique layered structure and more desirable layer spacing, transition metal dichalcogenide (TMD) materials are considered as the comparatively ideal cathode material of ZIBs which facilitate the intercalation/ deintercalation of hydrated Zn2+ between layers. However, some disadvantages limit their widespread application, such as low conductivity, low reversible capacity, and rapid capacity decline. In order to improve the electrochemical properties of TMDs, the corresponding modification methods for each TMDs material can be designed from the following modification strategies: defect engineering, intercalation engineering, hybrid engineering, phase engineering, and in-situ electrochemical oxidation. This paper summarizes the research progress of TMDs as cathode materials for ZIBs in recent years, discusses and compares the electrochemical properties of TMD materials, and classifies and introduces the modification methods of MoS2 and VS2. Meanwhile, the corresponding modification scheme is proposed to solve the problem of rapid capacity fading of WS2. Finally, the research prospect of other TMDs as cathodes for ZIBs is put forward.


Introduction
Due to the deterioration of the environment and the deficit of fossil energy, it is increasingly important to develop environmentally friendly, sustainable, and renewable energy [1][2][3]. At present, some renewable energy power generation can meet the requirements of environmental protection and sustainable development, such as solar power, wind power, and tidal power generation, but these energy sources suffer from regional limitations and instability, limiting their wider application. As an important part of energy for sustainable development, electrochemical energy storage has become an active research field in recent decades [4][5][6]. In the current energy market, lithium-ion batteries (LIBs) are dominant in automobile, medical equipment, portable wearable equipment, and other industries because of their excellent energy density and good environmental performance [7,8]. However, most lithium-ion batteries use organic solvents as electrolytes, which may cause safety problems and increase costs [9][10][11][12]. Compared with non-aqueous batteries, aqueous rechargeable batteries have the characteristics of low cost, non-toxicity, and non-flammability, which makes them safer, more environmentally friendly, and more economical [13][14][15].
In addition to LIBs, many other rechargeable aqueous metal ion batteries have been developed, including sodium ion batteries [16], potassium ion batteries [17], aluminum ion batteries [18], calcium ion batteries [19], and zinc ion batteries. Compared with other rechargeable aqueous metal ion batteries, zinc ion batteries have many advantages [20][21][22]: (1) Zinc ion batteries can be directly assembled in the air without inert environment, which can reduce the battery assembly cost. (2) Zinc ions can be electrodeposited reversibly in aqueous solution, so the zinc sheet can be directly used as the anode of the battery.
(3) Zinc as anode has a higher theoretical capacity and lower oxidation/reduction potential (-0.76 V) than the standard hydrogen electrode, which indicates that there is a higher open circuit voltage when coupled with cathode. Therefore, in many rechargeable water-based metal ion batteries, the research on aqueous zinc ion batteries is increasingly concerned.
As shown in Figure 1a, the aqueous zinc ion battery (ZIB) is mainly composed of the battery shell, cathode material, anode material, electrolyte, and diaphragm. There are many kinds of cathode materials, including manganese-based materials [23], vanadium-based materials [24], and Prussian blue analogues [25], all of which possess a certain capacity of Zn 2+ storage. Since the theoretical interlayer spacing of transition metal disulfide compounds is larger than the diameter of Zn 2+ , Zn 2+ can be intercalated and deintercalated in TMD material, which also indicates that TMD material is feasible as the cathode of ZIB [26]. At present, few studies have been conducted on TMD as a cathode material of ZIB, and there is not a systematic exposition, which also shows from the aspect that TMD as cathode of ZIB is quite novel. Figure 1b displays the trend of the increasing number of publications, evidencing the increasing attention paid to TMD materials in ZIB. The latest research progress of TMD series materials as cathodes of ZIB in recent years is reviewed in this paper, and several modification methods which can improve the Zn 2+ storage capacity and structural stability of TMD materials are expounded. rechargeable aqueous metal ion batteries, zinc ion batteries have many advantages [20][21][22]: (1) Zinc ion batteries can be directly assembled in the air without inert environment, which can reduce the battery assembly cost. (2) Zinc ions can be electrodeposited reversibly in aqueous solution, so the zinc sheet can be directly used as the anode of the battery.
(3) Zinc as anode has a higher theoretical capacity and lower oxidation/reduction potential (-0.76 V) than the standard hydrogen electrode, which indicates that there is a higher open circuit voltage when coupled with cathode. Therefore, in many rechargeable water-based metal ion batteries, the research on aqueous zinc ion batteries is increasingly concerned. As shown in Figure 1a, the aqueous zinc ion battery (ZIB) is mainly composed of the battery shell, cathode material, anode material, electrolyte, and diaphragm. There are many kinds of cathode materials, including manganese-based materials [23], vanadiumbased materials [24], and Prussian blue analogues [25], all of which possess a certain capacity of Zn 2+ storage. Since the theoretical interlayer spacing of transition metal disulfide compounds is larger than the diameter of Zn 2+ , Zn 2+ can be intercalated and deintercalated in TMD material, which also indicates that TMD material is feasible as the cathode of ZIB [26]. At present, few studies have been conducted on TMD as a cathode material of ZIB, and there is not a systematic exposition, which also shows from the aspect that TMD as cathode of ZIB is quite novel. Figure 1b displays the trend of the increasing number of publications, evidencing the increasing attention paid to TMD materials in ZIB. The latest research progress of TMD series materials as cathodes of ZIB in recent years is reviewed in this paper, and several modification methods which can improve the Zn 2+ storage capacity and structural stability of TMD materials are expounded.

Characterization and Synthetic Methods of TMD
Two-dimensional layered transition metal disulfides (TMDs) generally have X-M-X structure, where M is the transition metal, such as molybdenum, vanadium, tungsten, bismuth, and other metal elements, while X is generally sulfur, selenium, and other elements, as shown in Figure 2a. These layered structures facilitate the transport of various carriers and can also adapt to the volume change during ion insertion. Because of their different chemical composition and unique crystal structure, as well as the fact that the dorbitals can be filled with different elements, TMDs can be used as functional materials for electronic insulation, semiconductors, and superconductivity [27].
Among all TMDs materials, molybdenum disulfide (MoS2) has received extensive attention as a typical representative. Molybdenum disulfide is composed of two layers of sulfur atoms and one layer of molybdenum atoms, with the metal molybdenum layer interposed between the two sulfur layers, alternately stacked to form a sandwich-like structure. The sulfur atoms are bound together by van der Waals force, while the S-Mo-S atoms are linked by strong covalent bonds [28,29]. MoS2 not only has a layered structure, but also has different phases (1T, 2H, and 3R), and each phase also has different physical properties and chemical characteristics [30]. As shown in Figure 2b, MoS2 in 2H and 3R phases both demonstrate the triangular prismatic coordination of Mo atoms, and 2H-MoS2 is very

Characterization and Synthetic Methods of TMD
Two-dimensional layered transition metal disulfides (TMDs) generally have X-M-X structure, where M is the transition metal, such as molybdenum, vanadium, tungsten, bismuth, and other metal elements, while X is generally sulfur, selenium, and other elements, as shown in Figure 2a. These layered structures facilitate the transport of various carriers and can also adapt to the volume change during ion insertion. Because of their different chemical composition and unique crystal structure, as well as the fact that the d-orbitals can be filled with different elements, TMDs can be used as functional materials for electronic insulation, semiconductors, and superconductivity [27].
Among all TMDs materials, molybdenum disulfide (MoS 2 ) has received extensive attention as a typical representative. Molybdenum disulfide is composed of two layers of sulfur atoms and one layer of molybdenum atoms, with the metal molybdenum layer interposed between the two sulfur layers, alternately stacked to form a sandwich-like structure. The sulfur atoms are bound together by van der Waals force, while the S-Mo-S atoms are linked by strong covalent bonds [28,29]. MoS 2 not only has a layered structure, but also has different phases (1T, 2H, and 3R), and each phase also has different physical properties and chemical characteristics [30]. As shown in Figure 2b, MoS 2 in 2H and 3R phases both demonstrate the triangular prismatic coordination of Mo atoms, and 2H-MoS 2 is very stable because of the two layers of units stacked in hexagon symmetry. Meanwhile, 3R-MoS 2 has rhombus symmetry, and each unit has three layers. On the contrary, the Mo atoms of 1T-MoS 2 (metal phase) are octahedral coordinated and most unstable [31,32]. VS 2 is also a common material in TMDs, where the sulfur and vanadium layers are stacked together in a sandwich-like structure by van der Waals force interactions [33,34]. Due to the large interlayer spacing, VS 2 has great potential in the intercalation/deintercalation of ions, such as Li + , Na + , Zn 2+ , Mg 2+ , and Al 3+ [35][36][37]. stable because of the two layers of units stacked in hexagon symmetry. Meanwhile, 3R-MoS2 has rhombus symmetry, and each unit has three layers. On the contrary, the Mo atoms of 1T-MoS2 (metal phase) are octahedral coordinated and most unstable [31,32]. VS2 is also a common material in TMDs, where the sulfur and vanadium layers are stacked together in a sandwich-like structure by van der Waals force interactions [33,34]. Due to the large interlayer spacing, VS2 has great potential in the intercalation/deintercalation of ions, such as Li + , Na + , Zn 2+ , Mg 2+ , and Al 3+ [35][36][37]. In order to obtain layered and nanoflower-like sulfide materials, TMD materials are generally prepared by hydrothermal and solvothermal methods. The synthetic methods of TMD materials as ZIB cathode are summarized in Table 1.

TMDs as ZIBs Cathode
Layered MoS2, VS2, WS2, and VSe2 have been proven to be feasible cathode materials for ZIB in recent years. MoS2 is the earliest and most extensively material studied in ZIB owing to its unique layered structure and different phases. However, compared with manganese-based materials and vanadium-based materials, TMDs have lower conductivity and larger capacity changes, which will lead to the lower rate performance and poor cycle performance of electrode materials during charge and discharge, which is also the biggest challenge for TMDs materials. In order to improve the electrochemical performance of TMDs, different modification strategies of TMDs were designed. In this section, the research progress of MoS2, VS2, WS2, and VSe2 materials as cathode materials for ZIB In order to obtain layered and nanoflower-like sulfide materials, TMD materials are generally prepared by hydrothermal and solvothermal methods. The synthetic methods of TMD materials as ZIB cathode are summarized in Table 1.

TMDs as ZIBs Cathode
Layered MoS 2 , VS 2 , WS 2 , and VSe 2 have been proven to be feasible cathode materials for ZIB in recent years. MoS 2 is the earliest and most extensively material studied in ZIB owing to its unique layered structure and different phases. However, compared with manganese-based materials and vanadium-based materials, TMDs have lower conductivity and larger capacity changes, which will lead to the lower rate performance and poor cycle performance of electrode materials during charge and discharge, which is also the biggest challenge for TMDs materials. In order to improve the electrochemical performance of TMDs, different modification strategies of TMDs were designed. In this section, the research progress of MoS 2 , VS 2 , WS 2 , and VSe 2 materials as cathode materials for ZIB is described, and various modification methods of MoS 2 and VS 2 materials are discussed and summarized.

MoS 2 and Modification
Liu et al. [53] did not obtain obvious redox peaks in CV curves of MoS 2 and WS 2 , indicating that it is difficult for MoS 2 and WS 2 to store zinc ions. The measured cyclic discharge capacities provided by MoS 2 and WS 2 were 18 mAh·g −1 and 22 mAh·g −1 , respectively, and the EDS images of MoS 2 and WS 2 in the fully discharged state also had low zinc contents, so their conclusions showed that MoS 2 and WS 2 materials were not suitable as cathode materials for ZIB. The electrochemical performance of unmodified MoS 2 in ZIB is undesirable, such that more and more scholars have paid attention to the modified MoS 2 as the cathode material of ZIB.

Defect Engineering
Defect engineering is used to change the surface properties and structure of TMDs to improve the electrochemical performance, which is undoubtedly a preferable modification strategy. The possible vacancy defects in TMDs consist of sulfur vacancies, transition metal vacancies, edge vacancies, and holes in the crystal lattice [54][55][56]. The main effects of defects on electrode materials are divided into three types: (1) More storage and adsorption sites of foreign ions are provided to improve the battery capacity. The introduction of defects increases the number of active sites significantly, which improves the electrochemical performance. (2) It changes the local structure and charge distribution on the surface of electrode materials, which can improve the conductivity, cycle stability, and rate performance of electrode materials. (3) Defects can make the structure more flexible and stable when external ions are intercalated and deintercalated. Therefore, the introduction of defects in the crystal structure of TMDs and the selection of appropriate chemical modification molecules or groups using the defect sites constructed on the surface of TMDs can improve the electrical conductivity, reversible capacity, and initial Coulombic efficiency (ICE) and enhance the reversibility of Zn 2+ intercalation/de intercalation, thus improving the electrochemical performance of TMD electrode materials in ZIB.
For example, Xu et al. [38] designed the first MoS 2−x /Zn cell and confirmed that defect engineering could be used to enhance the Zn storage capacity of MoS 2 . They predicted that the edge vacancies and sulfur vacancies were the main adsorption sites of Zn 2+ by DFT, while the original surface was inert in storing Zn 2+ due to the lack of these vacancies. As shown in Figure 3, owing to the absence of some chalcogen atoms, there is a large number of vacancies existing in the neatly arranged sulfur layer. These vacancies include edge vacancies and sulfur vacancies, which can easily accommodate more Zn 2+ for intercalation, resulting in a significant increase in the reversible capacity of ZIBs. Such a layered nanostructure with rich defects can also reduce the diffusion energy barrier and significantly increase the diffusion rate of Zn ions [57]. Figure 4a shows that the capacity of defect-free MoS 2 nanosheets is very low at a current density of 100 mA·g −1 , while the defect-rich MoS 2 nanosheets provide a high reversible capacity of 135 mAh·g −1 . After 1000 cycles at a current density of 1000 mA·g −1 , the defect-rich MoS 2 electrode still delivers a high reversible capacity of 88.6 mAh·g −1 with a capacity retention of 87.8%. The electrochemical reaction mechanism of the MoS 2−x /Zn battery at cathode and anode can be expressed as: Anode:

Interlayer Engineering
The non-bonded characteristics of 2D layered materials with unique vdW gaps between adjacent atomic layers allows the insertion of atoms, ions, and molecules without breaking covalent bonds through a chemical intercalation process, thus changing the physical and chemical properties of the material [58]. TMDs are generally composed of 2D single-molecule layers combined with van der Waals forces and weak chemical bonds stacked along the direction perpendicular to the layers, a structure that has a larger specific surface area and is also conducive to the insertion of small intercalators, such as ions and molecules. Figure 5 shows that the insertion of external ions can expand the interlayer spacing of TMDs.

Interlayer Engineering
The non-bonded characteristics of 2D layered materials with unique vdW gaps between adjacent atomic layers allows the insertion of atoms, ions, and molecules without breaking covalent bonds through a chemical intercalation process, thus changing the physical and chemical properties of the material [58]. TMDs are generally composed of 2D single-molecule layers combined with van der Waals forces and weak chemical bonds stacked along the direction perpendicular to the layers, a structure that has a larger specific surface area and is also conducive to the insertion of small intercalators, such as ions and molecules. Figure 5 shows that the insertion of external ions can expand the interlayer spacing of TMDs.

Interlayer Engineering
The non-bonded characteristics of 2D layered materials with unique vdW gaps between adjacent atomic layers allows the insertion of atoms, ions, and molecules without breaking covalent bonds through a chemical intercalation process, thus changing the physical and chemical properties of the material [58]. TMDs are generally composed of 2D single-molecule layers combined with van der Waals forces and weak chemical bonds stacked along the direction perpendicular to the layers, a structure that has a larger specific surface area and is also conducive to the insertion of small intercalators, such as ions and molecules. Figure 5 shows that the insertion of external ions can expand the interlayer spacing of TMDs.
as well as a capacity retention rate of 98.6% after more than 600 cycles with good cycling stability. The electrochemical reaction mechanism of the cathode and anode of E-MoS2/Zn battery is expressed as: Anode: xZn 2+ + 2xe -→ xZn (4) Liang et al. [40] made MoS2-O to store Zn ions more efficiently by adjusting the interlayer spacing and designing hydrophilicity tuning engineering. A small amount of oxygen (5%) was added to MoS2, and the incorporation of oxygen improved the hydrophilicity of MoS2-O and made it possible for water to intercalate in MoS2-O, which also increased the interlayer spacing of MoS2-O from 0.62 to 0.95 nm, as shown in Figure 4b. Furthermore, if a diffusing hydrated Zn 2+ were to diffuse through a narrow void separation in MoS2 (S to S layer separation), i.e., 0.31 nm, instead of the 0.62 nm, i.e., Mo to Mo layer separation, interlayer engineering becomes even more crucial. As a result, the incorporation of oxygen increased the diffusion rate of Zn 2+ by three orders of magnitude and the storage capacity of Zn 2+ was increased by10 times. When the current density is 100 mA•g −1 , the capacity reaches 232 mAh•g −1 , which is higher than that of MoS2-based electrode materials in other ZIB.
Unusually, Zhang et al. [41] designed a material with crystalline water introduced into MoS2 nanoflowers as cathode materials for ZIB. The MoS2•nH2O nanoflowers obtained by the one-step hydrothermal method are composed of layered nanosheets with low stacking height and uniform distribution, as shown in Figure 4c. The insertion of crystalline water not only increases the interlayer distance of MoS2 to 0.65 nm, but also shields the electrostatic interaction between zinc ions and matrix materials, and lubricates and increases the diffusion ion channels as a way to promote the migration of Zn 2+ . Under the synergistic effect of crystal water lubrication and nanoflower structure, the reversible capacity of MoS2•nH2O was 165 mAh•g −1 at 0.1 A•g −1 . At the high current density of 2 A•g −1 , the capacity retention rate after 800 cycles was 88%.
Huang et al. [42] first designed a molybdenum disulfide/polyaniline (MoS2/PANI) hybrid material with overlapping heterostructures as an excellent cathode for ZIB. The Li et al. [39] prepared expanded interlayer distance E-MoS 2 nanosheets vertically aligned on carbon fiber cloth by a one-step glucose-assisted hydrothermal method as cathode materials for ZIB. The layer spacing of commercial MoS 2 is 0.62 nm, while the interlayer spacing of E-MoS 2 nanosheets is 0.70 nm. The increased interlayer spacing is due to the incorporation of hydrated Na + and NH 4 + into the MoS 2 crystal framework during the hydrothermal reaction. The E-MoS 2 electrode also showed good performance with a specific capacity of 202.6 mAh·g −1 and a specific energy density of 148.2 Wh·kg −1 at 0.1 A·g −1 as well as a capacity retention rate of 98.6% after more than 600 cycles with good cycling stability. The electrochemical reaction mechanism of the cathode and anode of E-MoS 2 /Zn battery is expressed as: Anode: Liang et al. [40] made MoS 2 -O to store Zn ions more efficiently by adjusting the interlayer spacing and designing hydrophilicity tuning engineering. A small amount of oxygen (5%) was added to MoS 2 , and the incorporation of oxygen improved the hydrophilicity of MoS 2 -O and made it possible for water to intercalate in MoS 2 -O, which also increased the interlayer spacing of MoS 2 -O from 0.62 to 0.95 nm, as shown in Figure 4b. Furthermore, if a diffusing hydrated Zn 2+ were to diffuse through a narrow void separation in MoS 2 (S to S layer separation), i.e., 0.31 nm, instead of the 0.62 nm, i.e., Mo to Mo layer separation, interlayer engineering becomes even more crucial. As a result, the incorporation of oxygen increased the diffusion rate of Zn 2+ by three orders of magnitude and the storage capacity of Zn 2+ was increased by10 times. When the current density is 100 mA·g −1 , the capacity reaches 232 mAh·g −1 , which is higher than that of MoS 2 -based electrode materials in other ZIB.
Unusually, Zhang et al. [41] designed a material with crystalline water introduced into MoS 2 nanoflowers as cathode materials for ZIB. The MoS 2 ·nH 2 O nanoflowers obtained by the one-step hydrothermal method are composed of layered nanosheets with low stacking height and uniform distribution, as shown in Figure 4c. The insertion of crystalline water not only increases the interlayer distance of MoS 2 to 0.65 nm, but also shields the electrostatic interaction between zinc ions and matrix materials, and lubricates and increases the diffusion ion channels as a way to promote the migration of Zn 2+ . Under the synergistic effect of crystal water lubrication and nanoflower structure, the reversible capacity of MoS 2 ·nH 2 O was 165 mAh·g −1 at 0.1 A·g −1 . At the high current density of 2 A·g −1 , the capacity retention rate after 800 cycles was 88%.
Huang et al. [42] first designed a molybdenum disulfide/polyaniline (MoS 2 /PANI) hybrid material with overlapping heterostructures as an excellent cathode for ZIB. The MoS 2 /PANI prepared by the hydrothermal method is a micro-nanoflower assembled by some ultra-thin wrinkled nanosheets with a layer spacing of 1.03 nm, as shown in Figure 4d.
An appropriate amount of PANI was inserted into the MoS 2 matrix, thus expanding the interlayer spacing and shielding the electrostatic interaction between some Zn 2+ and MoS 2 substrates. The intercalation of polyaniline not only expands the Zn 2+ diffusion channel, but also reduces the band gap between the conduction band and the valence band of MoS 2 , which improves the charge transfer efficiency. Compared with the original MoS 2 , the zinc storage performance of the MoS 2 /PANI hybrid cathode was significantly improved. At a current density of 1.0 A·g −1 , it provides a high reversible capacity of 106.5 mAh·g −1 and maintains 86% capacity after 1000 cycles. This also provides a modification strategy for developing other TMD materials as good cathode materials for ZIB.

Hybridization
The conductivity of TMDs is poor, which limits the diffusion power of ions. In electrochemical energy storage, carbon nanotubes, carbon fiber cloth, and graphene are the most commonly used hybrid materials. Carbon nanotubes and graphene materials have unique structures, high electrical conductivity, good mechanical properties, and large specific surface areas [59,60]. As shown in Figure 6, coating with a certain material or hybridizing with carbon nanotubes, carbon cloth, and graphene on the surface of the TMD can increase the surface area and charge transfer ability of the material to improve the electrochemical performance of the composite. MoS2/PANI prepared by the hydrothermal method is a micro-nanoflower assembled by some ultra-thin wrinkled nanosheets with a layer spacing of 1.03 nm, as shown in Figure  4d. An appropriate amount of PANI was inserted into the MoS2 matrix, thus expanding the interlayer spacing and shielding the electrostatic interaction between some Zn 2+ and MoS2 substrates. The intercalation of polyaniline not only expands the Zn 2+ diffusion channel, but also reduces the band gap between the conduction band and the valence band of MoS2, which improves the charge transfer efficiency. Compared with the original MoS2, the zinc storage performance of the MoS2/PANI hybrid cathode was significantly improved. At a current density of 1.0 A•g −1 , it provides a high reversible capacity of 106.5 mAh•g −1 and maintains 86% capacity after 1000 cycles. This also provides a modification strategy for developing other TMD materials as good cathode materials for ZIB.

Hybridization
The conductivity of TMDs is poor, which limits the diffusion power of ions. In electrochemical energy storage, carbon nanotubes, carbon fiber cloth, and graphene are the most commonly used hybrid materials. Carbon nanotubes and graphene materials have unique structures, high electrical conductivity, good mechanical properties, and large specific surface areas [59,60]. As shown in Figure 6, coating with a certain material or hybridizing with carbon nanotubes, carbon cloth, and graphene on the surface of the TMD can increase the surface area and charge transfer ability of the material to improve the electrochemical performance of the composite. Huang [43] dispersed multi-walled carbon nanotubes in aqueous solution mixed with glucose and deionized water, and prepared ultra-thin MoS2 nanosheets with expanded layer spacing directly grown on the surface of carbon nanotubes (CNTs) by a onestep hydrothermal method (as shown in Figure 4e) as the ZIB cathode material. When the interlayer spacing of MoS2 increases to 1.0 nm, the increase of interlayer spacing reduces the diffusion energy barrier of Zn 2+ between layers, and the main chain of CNTs promotes the electron transfer of Zn 2+ between layers. More importantly, the layered structure hinders the aggregation of MoS2 nanosheets, increases the surface area, and provides a large number of active sites for electrochemical reactions. At the current density of 0.1 A•g −1 , the reversible capacity of the MoS2@CNTs electrode was about 180.0 mAh•g −1 . When the current density increases to 3 A•g −1 , it can maintain 51.7% capacity, and after 500 cycles (the current density is 1.0 A•g −1 ) it can maintain 80.1% capacity. Therefore, MoS2@CNTs as cathode can provide high reversible capacity and good cycle stability for ZIB.

Phase Engineering
Phase engineering has been proven to be an effective method to regulate the electrochemical performance of TMDs [61]. According to different stacking methods, the materials can be divided into three bulk phases: 1T, 2H and 3R. The cell of 1T phase is octahedral, and the cell of 2H phase is prismatic. Due to different atomic configurations, singlelayer TMD materials can also exist in three different forms of 1T, 2H, and 3R, respectively. MoS2 is a typical TMD which has different phase structures, different metal coordination Huang [43] dispersed multi-walled carbon nanotubes in aqueous solution mixed with glucose and deionized water, and prepared ultra-thin MoS 2 nanosheets with expanded layer spacing directly grown on the surface of carbon nanotubes (CNTs) by a one-step hydrothermal method (as shown in Figure 4e) as the ZIB cathode material. When the interlayer spacing of MoS 2 increases to 1.0 nm, the increase of interlayer spacing reduces the diffusion energy barrier of Zn 2+ between layers, and the main chain of CNTs promotes the electron transfer of Zn 2+ between layers. More importantly, the layered structure hinders the aggregation of MoS 2 nanosheets, increases the surface area, and provides a large number of active sites for electrochemical reactions. At the current density of 0.1 A·g −1 , the reversible capacity of the MoS 2 @CNTs electrode was about 180.0 mAh·g −1 . When the current density increases to 3 A·g −1 , it can maintain 51.7% capacity, and after 500 cycles (the current density is 1.0 A·g −1 ) it can maintain 80.1% capacity. Therefore, MoS 2 @CNTs as cathode can provide high reversible capacity and good cycle stability for ZIB.

Phase Engineering
Phase engineering has been proven to be an effective method to regulate the electrochemical performance of TMDs [61]. According to different stacking methods, the materials can be divided into three bulk phases: 1T, 2H and 3R. The cell of 1T phase is octahedral, and the cell of 2H phase is prismatic. Due to different atomic configurations, single-layer TMD materials can also exist in three different forms of 1T, 2H, and 3R, respectively. MoS 2 is a typical TMD which has different phase structures, different metal coordination geometries, different stacking sequences, and different properties. For example, the natural bulk MoS 2 crystal has a 2H phase with triangular prism coordination geometry, but the coordination of molybdenum and sulfur atoms is octahedral in 1T phase which shows metallicity [62,63]. In fact, the conductivity of 1T phase MoS 2 is about 105 times higher than that of the 2H phase MoS 2 corresponding to semiconductor [64,65]. It can be reasonably speculated that MoS 2 with different phases exhibits different properties in application. Phase engineering has also been proven to be an effective strategy to regulate the catalytic activity of MoS 2 [66].
For example, Liu et al. [44] prepared MoS 2 with different phases by regulating the reaction temperature of the hydrothermal reaction, and most samples showed a flowerlike structure of self-assembled nanosheets. As the synthesis temperature decreases, the interlayer spacing of MoS 2 increases from 0.62 nm to 0.68 nm, as shown in Figure 4f. Different from MoS 2 nanosheets corresponding to 2H, MoS 2 nanosheets with high 1T phase content (about 70%) have excellent specific capacity, and the reason for this is that the Zn 2+ diffusion barrier of 1T-MoS 2 is much lower than that of 2H-MoS 2 . At a current density of 1.0 A·g −1 , the reversible capacity of 1T-MoS 2 is about 140 mAh·g −1 , but the capacity decreases rapidly after 100 cycles. However, the capacity attenuation of 2H-MoS 2 can be ignored after 400 cycles, showing good cycle performance. How to make MoS 2 show excellent specific capacity and maintain stable cycle performance is still worth exploring.

VS 2 and Modification
In recent years, vanadium disulfide (VS 2 ) based nanomaterials have become potential electrode materials for various energy storage batteries due to their highly controllable structure and chemical composition. The VS 2 layer shows metallicity and has a considerable electronic state at the Fermi level, which is conducive to promoting electron and electrochemical activity [67]. At present, using VS 2 as the cathode of ZIB is still considered to be a relatively new method. So far, there are only a few reports on VS 2 as the cathode of ZIB. For example, He et al. [45] firstly synthesized rose-like VS 2 nanoflowers assembled by nanosheets with diameter of 5-8 µm and thickness of 50-100 nm, whose interlayer spacing is 0.576 nm, as shown in Figure 7a. These nanosheets can realize the intercalation/deintercalation of Zn 2+ in VS 2 nanosheets. At a current density of 0.05 A·g −1 , VS 2 nanosheets have a high capacity of 190.3 mAh·g −1 and an energy density of 123 Wh·kg −1 . The capacity retention rate reached 98.0% after 200 cycles at a current density of 0.5 A·g −1 , which confirmed the feasibility of VS 2 as the cathode of ZIB. They summarize the electrochemical reactions occurring in the cathode of Zn/VS 2 batteries, as shown in Equation (5): VS 2 + 0.09Zn 2+ + 0.18e − → Zn 0.09 VS 2 ; Zn 0.09 VS 2 + 0.14Zn 2+ + 0.28e − → Zn 0.23 VS 2 (5) In other reports, VS 2 was modified to obtain electrode materials with better electrochemical performance. Consistently, we reviewed the research progress of modified VS 2 as electrode material for ZIB and summarized two corresponding modification strategies in this section.

Hybridization
Jiao et al. [46] synthesized hierarchical 1T-VS 2 directly on stainless steel mesh (VS 2 @SS) by the hydrothermal method. The prepared VS 2 was composed of bending nanosheets with a transverse size of 5-8 µm, forming a highly layered network flower structure from Figure 7b. There is a large number of interlayer channels for electrolyte penetration in VS 2 nanoflowers, which enhances the contact area between electrolyte and VS 2 and promotes the transmission of electrons and ions. Therefore, the VS 2 @SS electrode with VS 2 loading of 4-5 mg·cm −2 has a high capacity of 198 mAh·g −1 at a current density of 50 mA·g −1 .
After 2000 cycles at a current density of 2 A·g −1 , the capacity retention rate was about 80%, showing a superior cycle life. When the mass load of VS 2 is about 11 mg·cm −2 , the electrode can maintain 90% capacity in 600 cycles (only 0.017% loss per cycle), which indicates that the stability of the composite structure is enhanced after the hybrid of TMD material and SS. crystals (0.575 nm). They also calculated that the surface area of rGO-VS2 was 34.2 m 2 •g −1 , which was larger than that of commercial VS2 (26.5 m 2 •g −1 ), indicating that the introduction of graphene increased the specific surface area of the composites. VS2 nanosheets are closely connected with graphene, which can effectively prevent the dissolution of VS2 and improve the cycle stability of the battery. Therefore, when the current density is 0.1 A•g −1 , it has a high capacity of 238 mAh•g −1 . At a high current density of 5 A g −1 , the capacity is 190 mAh•g −1 , and the capacity retention is over 93% after 1000 cycles. Figure 7. (a) HR−TEM image and rate performance of layered VS2 [45]; (b) SEM image of VS2 grown on SS mesh and rate performance of VS2@SS electrode [46]; (c) HRTEM image and rate performance of the rGO−VS2 composites [47]; (d) SEM image and rate performance of the VS2@VOOH−18h [48]; (e) HRTEM image and rate performance of VS2•NH3 electrode [49]; (f) TEM image and rate capability of in−situ electrochemical oxidation formed VS2/VOx [50].
Metal oxides, polymers, C3N4, and MXene can also be hybridized with TMD materials [71][72][73]. Pu et al. [48] prepared a rose-like VS2@VOOH material with hydrophilic VOOH coating for the first time. The size of VS2@VOOH is about 10 μm and a large number of nanosheets are assembled into uniform rose-like morphology, as shown in Figure   Figure 7. (a) HR−TEM image and rate performance of layered VS 2 [45]; (b) SEM image of VS 2 grown on SS mesh and rate performance of VS 2 @SS electrode [46]; (c) HRTEM image and rate performance of the rGO−VS 2 composites [47]; (d) SEM image and rate performance of the VS 2 @VOOH−18h [48]; (e) HRTEM image and rate performance of VS 2 ·NH 3 electrode [49]; (f) TEM image and rate capability of in−situ electrochemical oxidation formed VS 2 /VO x [50].
Graphene and reduced graphene oxide (rGO) are idealized cathode materials with large specific surface area, excellent electrical conductivity, and excellent mechanical elasticity [68]. In addition, the end-functional group surface of rGO further provides the possibility of strong interface coupling between the active material and the conductive skeleton, and it also realizes the rapid charge transfer and high structural stability against repeated electrochemical cycles [69,70]. Due to the unique layered structure and large specific surface area of graphene and VS 2 , the electrochemical performance of the electrode can be further improved by hybridizing graphene and VS 2 . For example, Chen et al. [65] synthesized vertically grown ultra-thin VS 2 nanosheets (rGO-VS 2 ) on graphene sheets by the solvothermal method. The interlayer distance of these ultrathin VS 2 nanosheets is about 0.97 nm, as shown in Figure 7c, which is much larger than that of commercial VS 2 crystals (0.575 nm). They also calculated that the surface area of rGO-VS 2 was 34.2 m 2 ·g −1 , which was larger than that of commercial VS 2 (26.5 m 2 ·g −1 ), indicating that the introduction of graphene increased the specific surface area of the composites. VS 2 nanosheets are closely connected with graphene, which can effectively prevent the dissolution of VS 2 and improve the cycle stability of the battery. Therefore, when the current density is 0.1 A·g −1 , it has a high capacity of 238 mAh·g −1 . At a high current density of 5 A g −1 , the capacity is 190 mAh·g −1 , and the capacity retention is over 93% after 1000 cycles.
Metal oxides, polymers, C 3 N 4 , and MXene can also be hybridized with TMD materials [71][72][73]. Pu et al. [48] prepared a rose-like VS2@VOOH material with hydrophilic VOOH coating for the first time. The size of VS 2 @VOOH is about 10 µm and a large number of nanosheets are assembled into uniform rose-like morphology, as shown in Figure 7d. Hydrophilic VOOH coating is conducive to the penetration of electrolyte in ZIB, and the structure is still intact even after O-H is exchanged with water in VOOH. Their research also showed that the O-H bond in VOOH could not only improve the permeation of electrolyte to the electrode, but also reduce the possibility of vanadium dissolution. Therefore, after 350 cycles at a current density of 1.5 A·g −1 , the specific capacity of VS 2 @VOOH composite can reach 107.5 mAh·g −1 . Even after 400 cycles at a current density of 2.5 A·g −1 , the VS 2 @VOOH material still maintained a specific capacity of 91.4 mAh·g −1 , which further verified the potential of the modified VS 2 as a ZIB cathode.

In Suit Electrochemical Oxidation
Electrochemical oxidation, as a feasible and multi-purpose strategy, has been proven to enhance the electrochemical behavior of low-valent active substances relative to evolutionary forms and newly formed high-active substances [74][75][76][77]. As shown in Figure 8, electrochemical oxidation of the inserted ions into the corresponding oxides without changing the position of the ion insertion into the TMD can provide more active sites or enhance the hydrophilicity of the ions, which is beneficial to the diffusion of Zn 2+ and the expansion of the TMD layer spacing. Yang et al. [49] prepared a hollow spherical VS 2 ·NH 3 material with layer spacing expansion as the cathode of ZIB. The VS 2 ·NH 3 hollow flower ball assembled by nanosheets has a porous structure, and the expanded layer spacing is 0.98 nm, as shown in Figure 7e. They found that during the first charge process, the expanded VS 2 ·NH 3 transformed into V 2 O 5 ·nH 2 O with a layer spacing of 1.21 nm after electrochemical oxidation. Then, Zn 2+ was intercalated/deintercalated in V 2 O 5 ·nH 2 O with large interlayer spacing, which provided high capacity and good stability for subsequent charge-discharge cycles. When the current density is 0.1, 0.2, 0.5, 1.0, 2.0, and 5.0 A·g −1 , the capacity of the Zn/VS 2 ·NH 3 battery is 390, 372, 320, 260, 189, and 107 mAh·g −1 , respectively, which is much higher than the VS 2 -based cathode of other ZIB previously reported. It is worth noting that the electrode formed by in-situ electrochemical oxidation into V 2 O 5 ·nH 2 O reached an amazing 110% capacity retention compared with the initial capacity after 2000 cycles at a high current density of 3 A·g −1 .
7d. Hydrophilic VOOH coating is conducive to the penetration of electrolyte in ZIB, and the structure is still intact even after O-H is exchanged with water in VOOH. Their research also showed that the O-H bond in VOOH could not only improve the permeation of electrolyte to the electrode, but also reduce the possibility of vanadium dissolution. Therefore, after 350 cycles at a current density of 1.5 A•g −1 , the specific capacity of VS2@VOOH composite can reach 107.5 mAh•g −1 . Even after 400 cycles at a current density of 2.5 A•g −1 , the VS2@VOOH material still maintained a specific capacity of 91.4 mAh•g −1 , which further verified the potential of the modified VS2 as a ZIB cathode.

In Suit Electrochemical Oxidation
Electrochemical oxidation, as a feasible and multi-purpose strategy, has been proven to enhance the electrochemical behavior of low-valent active substances relative to evolutionary forms and newly formed high-active substances [74][75][76][77]. As shown in Figure 8, electrochemical oxidation of the inserted ions into the corresponding oxides without changing the position of the ion insertion into the TMD can provide more active sites or enhance the hydrophilicity of the ions, which is beneficial to the diffusion of Zn 2+ and the expansion of the TMD layer spacing. Yang et al. [49] prepared a hollow spherical VS2•NH3 material with layer spacing expansion as the cathode of ZIB. The VS2•NH3 hollow flower ball assembled by nanosheets has a porous structure, and the expanded layer spacing is 0.98 nm, as shown in Figure 7e. They found that during the first charge process, the expanded VS2•NH3 transformed into V2O5•nH2O with a layer spacing of 1.21 nm after electrochemical oxidation. Then, Zn 2+ was intercalated/deintercalated in V2O5•nH2O with large interlayer spacing, which provided high capacity and good stability for subsequent charge-discharge cycles. When the current density is 0.1, 0.2, 0.5, 1.0, 2.0, and 5.0 A•g −1 , the capacity of the Zn/VS2•NH3 battery is 390, 372, 320, 260, 189, and 107 mAh•g −1 , respectively, which is much higher than the VS2-based cathode of other ZIB previously reported. It is worth noting that the electrode formed by in-situ electrochemical oxidation into V2O5•nH2O reached an amazing 110% capacity retention compared with the initial capacity after 2000 cycles at a high current density of 3 A•g −1 . Yu et al. [50] first used the in situ electrochemical pretreatment of VS2 in aqueous medium and obtained VS2/VOx heterostructures as cathode materials for ZIB. The fluffy VOx nanosheets are uniformly grown on VS2, forming an interwoven porous electrode rich in VS2 and VOx heterostructures. The VS2/VOx heterostructure combines the high conductivity of VS2 and the high chemical stability of VOx, which is conducive to regulating the intercalation of guest ions and improving the chemical stability of the VS2 skeleton. The step-by-step insertion of Zn 2+ changed the buffer volume, making the reaction kinetics faster and more reversible. As shown in Figure 7f, the VS2/VOx electrode maintains 75% high capacity in 3000 cycles at a current density of 1 A•g −1 . After 1000 cycles, VS2/VOx still Yu et al. [50] first used the in situ electrochemical pretreatment of VS 2 in aqueous medium and obtained VS 2 /VO x heterostructures as cathode materials for ZIB. The fluffy VO x nanosheets are uniformly grown on VS 2 , forming an interwoven porous electrode rich in VS 2 and VO x heterostructures. The VS 2 /VO x heterostructure combines the high conductivity of VS 2 and the high chemical stability of VO x , which is conducive to regulating the intercalation of guest ions and improving the chemical stability of the VS 2 skeleton. The step-by-step insertion of Zn 2+ changed the buffer volume, making the reaction kinetics faster and more reversible. As shown in Figure 7f, the VS 2 /VO x electrode maintains 75% high capacity in 3000 cycles at a current density of 1 A·g −1 . After 1000 cycles, VS 2 /VO x still maintained the original crystal structure and nanosheet morphology, which again proved that the VS 2 /VO x heterostructure had high reversibility. As a consequence, the design of TMD heterostructures by in situ electrochemical oxidation provides a new strategy for the efficient energy storage of ZIB.

WS 2 and Modification
WS 2 is a natural metal sulfide, where the S atom is located in the lattice position of the closely packed hexagonal structure. The plane where the W atom lies is sandwiched between two S layers and each W atom is coordinated with the S atom. The W layer and the S layer are stacked together through a weak van der Waals effect to form a prismatic structure [78,79]. The theoretical layer spacing of WS 2 is 0.618 nm, while the diameter of Zn 2+ is only 0.404~0.43 nm. Theoretically, it can also be used as the electrode material of the energy storage battery, but there are few reports on it as the ZIB cathode material. A report [53] on WS 2 pointed out that the interlayer spacing of commercial WS 2 was insufficient, and the inherent conductivity of Zn 2+ for reversible intercalation between layers was low, which indicates that the storage capacity of commercial WS 2 for Zn 2+ was poor. Another reason for this is that 2H-WS 2 has a large Zn 2+ intercalation barrier, which seriously hinders the diffusion of Zn 2+ to electrode materials.
Tang et al. [51] proved for the first time that 1T-WS 2 nanosheets are a promising candidate cathode material for ZIB. Thus, 1T phase WS-200 is stacked by ultrathin nanosheets, and the interlayer spacing is expanded from 0.618 nm to about 0.90 nm. After the introduction of 1T-WS 2 , the capacity of an almost inactive Zn/WS 2 battery increased by eight times, and the reversible capacity was 179.99 mAh·g −1 at a current density of 200 mA·g −1 . However, WS 2 has the problem of rapid capacity degradation in the long cycle at different current densities from Figure 9a. Their explanation for this phenomenon is that the layered WS 2 structure collapsed and dissolved, and some 1T-WS 2 dissolved in the electrolyte during the intercalation/deintercalation of Zn 2+ , which provides a research direction for improving the reversible capacity and cycle stability of WS 2 .  In order to improve the electrochemical performance of WS 2 , a modification strategy similar to MoS 2 and VS 2 can be designed to modify WS 2 , the most important of which is hybridization and intercalation. For example, Debela et al. [80] synthesized WS 2 @NGr material by hybridizing WS 2 with nitrogen-doped graphite. The reversible capacity of WS 2 @NGr electrode reached 963 mAh·g −1 . Ratha et al. [81] synthesized layered WS 2 and reduced graphene oxide (RGO) hybrid materials by a simple hydrothermal method. WS 2 /RGO composites showed excellent cycle stability in supercapacitors. Liu et al. [82] successfully synthesized a highly stable 1T-WS 2 nanobelt with a special zigzag chain superlattice structure. The NH 4 + insertion not only extends the interlayer spacing of WS 2 to 0.95 nm, but also makes the structure highly stable. Although the above three WS 2 modified materials are not applied to ZIB, they also provide many strategies. Starting from the microstructure of WS 2 and the storage mechanism of Zn 2+ in WS 2 , the carbon-based materials and WS 2 can be hybridized, or external ions, such as NH 4 + and crystal water, can be used for intercalation, so as to overcome the challenge of rapid capacity decline faced by WS 2 .

Other Materials
VSe 2 is also a typical layered structure, and the vanadium layer is alternately located between two selenium layers, forming a sandwich structure formed by van der Waals interaction [83,84]. Larger interlayer spacing (0.611 nm) provides sufficient transport channels and active sites for active cations. Moreover, the electronic coupling force between adjacent V 4+ -V 4+ induces metal properties and excellent electronic conductivity, making it an attractive candidate electrode material [85,86]. Wu et al. [52] prepared ultra-thin VSe 2 nanosheets and used them as cathode materials for ZIB. As shown in Figure 9b, the specific capacities of VSe 2 are 250.6 and 132.6 mAh·g −1 at 200 and 5000 mA·g −1 , respectively. After 800 cycles at 2 A·g −1 , the capacity retention was 83%. The factors influencing the excellent electrochemical performance of VSe 2 nanosheets cathode are attributed as follows: (1) Highly reversible Zn 2+ intercalation/deintercalation storage mechanism; (2) Thanks to its ultra-thin two-dimensional morphology, Zn 2+ has rapid diffusion kinetics (DZn 2+ ≈ 10 −8 cm −2 s −1 ); (3) The metal characteristics of VSe 2 are conducive to the thermodynamics and kinetics of Zn 2+ storage process; (4) Stability of crystal structure of VSe 2 electrode material during long cycle. They summarized the storage mechanism of VSe 2 /Zn battery as 6 and 7: Cathode: VSe 2 + 0.23Zn 2+ + 0.46e − → Zn 0.23 VSe 2 Zn 0.23 VSe 2 + 0.17Zn 2+ + 0.34e − → Zn 0.4 VSe 2 (6) Anode: 0.4Zn 2+ + 0.8e − → 0.4Zn (7) A few other TMD materials can also be used in the water-based zinc ion battery. For example, Zhang et al. [87] proved that CoS 2 could be used as anode material for a sulfur redox type non-aqueous zinc battery. Li et al. [88] synthesized a novel pre-cured titanium disulfide (Na 0.14 TiS 2 ) as anode material for ZIB. However, these materials are all anode materials for ZIB, which are not described too much here.

Conclusions and Future Work
Since ZIB offer good safety and environmental protection, and with the deepening research on ZIB, they are expected to become a substitute for LIB. The special layered structure of TMDs is conducive to the transport of carriers such as Zn 2+ . The nanoflowerlike structure synthesized by the ordered stacking of monolayers confers TMD materials with a larger specific surface area. Some TMDs also have interlayer spacing larger than the diameter of hydrated zinc ion. Therefore, TMD materials are considered to represent a candidate cathode of ZIB with significant potential. Although TMDs are conductive, they also face the challenges of low reversible capacity and rapid capacity decay.
In recent years, some studies have reported that the modified TMD materials have excellent reversible capacity and cycle stability as cathodes of ZIB. Based on these research reports, the research progress of typical TMDs materials, such as MoS 2 , VS 2 , WS 2 , and VSe 2 , as cathodes of ZIB was described in this paper. The reversible capacity and long-period cycle performance are summarized in Table 2 and compared. At the same time, the modification ideas of TMD cathode materials for ZIB were combed, and the corresponding modification methods were summarized and discussed: (1) defect engineering, providing more ion storage sites and active sites by constructing chemical defects; (2) interlayer engineering, namely through the intercalation of guest ions to expand the layer spacing; (3) hybridization with carbon-based materials or other materials, making the microstructure of the composites more stable or maintained in a stable phase structure; (4) phase engineering, controlling the mutual transformation between 1H and 2R phases to reduce the diffusion energy barrier of Zn 2+ and make the structure more stable; (5) in-situ electrochemical oxidation, without changing the position of ions in the interlayer, where the low-valent active substances are oxidized into kinetic high-valent active substances by electrochemistry.  [52] Although these modification strategies are currently only applied to MoS 2 and VS 2 , they should also be applicable to other TMD materials because the TMDs have similar special layered structures. For example, in view of the rapid decline of WS 2 capacity, we may venture to assume that the hybridization of WS 2 and carbon-based materials improves the structural stability of the composites, thus improving the poor cycle stability of WS 2 electrodes. With the modification strategies put forward, TMD materials are bound to receive more attention, and more series members of TMDs will become excellent cathode materials for ZIB.