Transition Metal Dichalcogenides for High−Performance Aqueous Zinc Ion Batteries

Abstract

Aqueous zinc ion batteries (ZIBs) with cost—effectiveness, air stability, and remarkable energy density have attracted increasing attention for potential energy storage system applications. The unique electrical properties and competitive layer spacing of transition metal dichalcogenides (TMDs) provide dramatical freedom for facilitating ion diffusion and intercalation, making TMDs suitable for ZIB cathode materials. The recently updated advance of TMDs for high−performance ZIB cathode materials have been summarized in this review. In particular, the key modification strategies of TMDs for realizing the full potential in ZIBs are highlighted. Finally, the insights for further development of TMDs as ZIB cathodes are proposed, to guide the research directions related to the design of aqueous ZIBs while approaching the theoretical performance metrics.

1. Introduction

In order to overcome the energy crisis and environmental degradation in today’s society, developing reliable, renewable, and clean energy resources have been considered as a crucial strategy [1]. Lithium ion batteries (LIBs) are the most deeply researched energy storage systems in the commercialized market [2]. However, the widely used organic solvent electrolytes of LIBs may lead to safety concerns and increased costs. Compared with LIBs, aqueous zinc ion batteries (ZIBs) have some advantages, such as cost−effectiveness, air stability, environmental friendliness, and high theoretical energy density, and they have been considered an ideal candidate for next−generation energy devices [3,4,5]. Thus far, vanadium−based oxides [6], manganese−based oxides [7], and Prussian blue analogs [8] have exhibited remarkable Zn²⁺ storage capabilities, making them suitable for ZIB cathode materials [9]. However, the reversible Zn²⁺ storage behavior exhibited by these materials is not superior enough. For instance, Mn−based or V−based oxides as ZIB cathode materials suffer from poor cycling performance and unsatisfactory conductivity [7]. For Prussian blue analogues, the unstable crystal structure induced phase transformation and limited the capacity [3]. Therefore, exploiting a cathode with a large specific capacity and long−time cycling stability to further increase the Zn²⁺ storage capabilities of ZIBs is highly desired.

Transition metal dichalcogenides (TMDs), such as MoS₂, WSe₂, and VS₂, have been considered as another promising cathode material in energy storage systems, benefitting from unique electrical properties and competitive layer spacing [10]. Compared with the above−mentioned cathode materials, TMDs process the unique hexagonal crystal structure and chemical stability, which may make TMDs exhibit better cycling performance. In particular, the appealing property of TMDs, the wide interlayer distance, is feasible for ion diffusion and intercalation, which is ideal for high−capacity aqueous ZIBs [11]. However, the inserted Zn²⁺−induced electrostatic interactions and the poor electrical conductivity hinder the capacity of bulk TMDs. As a result, bulk MoS₂ and WS₂ exhibit unsatisfactory capacity of 18 and 22 mAh·g⁻¹ as the aqueous ZIB cathode materials, respectively [12]. Hence, efficient strategies for improving the Zn²⁺ storage ability of TMD cathodes are highly desired. In this review, recent advanced approaches to optimize the Zn²⁺ storage ability of TMDs, including interlayer engineering, phase engineering, defect engineering, metallic 1T TMDs, oxidation, and hybridization, have been comprehensively summarized (Scheme 1), which would provide clear guidance to design high−performance TMD−based aqueous ZIBs.

2. Basic Structural Characteristics of TMDs

These TMDs are a class of materials with the formula MX₂, where M is a transition metal element of group IV, group V or group VI, while X is a brass element. X−M−X is the form of layered TMD materials, in which the brass atoms are located in two hexagonal planes separated by transition metal atoms [19], as shown in Figure 1a,b. In energy−related applications, the most representative TMD is MoS₂.

The bulk MoS₂ usually consists of monolayer MoS₂ van der Waals stacked together. MoS₂ is the extensively researched electronic material for various energy−related devices. In general, MoS₂ has three different crystalline phases: 2H (hexagonal symmetry), 3R (rhombohedral symmetry) and 1T (tetragonal symmetry) [20]. For different phases, the arrangement of MoS₂ monolayers is different, which may lead to different physical and chemical properties and which may provide flexibility for material design. More importantly, due to the large interlayer spacing (0.62 nm), TMDs can ensure the diffusion of a larger radius of cations favorable as ZIB cathode materials. The year−on−year increase in the number of published articles is evidence of the growing interest in TMD materials for ZIBs (Figure 1c) [10].

3. Proposed Modifying Strategies for the TMDs Cathode of Aqueous ZIBs

In general, the specific capacity of pristine MoS₂ as the ZIB cathode is about 1–40 mAh·g⁻¹ [12]. In order to accommodate hydrated Zn²⁺, the host material must embrace a layer spacing larger than its radius (0.40–0.43 nm). Although the layer spacing of bulk MoS₂ (≈0.62 nm) is larger than the radius of hydrated Zn ions, the poor electrochemical activity and low electrical conductivity of intrinsic MoS₂ are not feasible for reversible intercalation of Zn²⁺. When pristine MoS₂ and WS₂ obtained in nature were tested in aqueous ZnSO₄ solutions, no observable redox peaks were observed, suggesting that the intrinsic MoS₂ and WS₂ cannot store reasonable amounts of Zn²⁺ to show acceptable capacity [12].

Based on the above discussion, modification strategies become important in order to obtain outstanding performance from TMDs for functional ZIB cathodes. In this section, six effective methods to overcome the high Zn²⁺ intercalation energy barriers of TMDs and to ensure the structural stability of TMDs are proposed: (1) interlayer engineering, (2) phase engineering, (3) defect engineering, (4) metallic 1T TMDs, (5) oxidation, and (6) hybridization.

3.1. Interlayer Engineering

The intercalation of small guest layers, such as ions and molecules, can tune the interlayer spacing of TMDs due to the unique van der Waals stacking. Therefore, interlayer engineering is a widely used means to modulate the electrochemical properties of TMDs. Via interlayer engineering, TMDs can not only achieve larger interlayer spacing but can also obtain more stable structures. Meanwhile, the energy barrier of Zn²⁺ intercalation can be lowered, and the storage of Zn²⁺ can also be enhanced in the TMDs framework with larger interlayer spacing. Furthermore, the interlayer engineering becomes more critical if the diffused hydrated Zn²⁺ is to be separated through a narrow gap in MoS₂. As the layer spacing of MoS₂ increases, the effective energies can be lowered at similar hydration levels [21]. Based on these benefits, there are many researchers exploiting the various intercalators to modify the MoS₂ interlayer spacing to functionalize the ZIBs.

Zhang et al. synthesized MoS₂−nH₂O nanoflowers for storing Zn²⁺ via hydrothermal synthesis [22], and the layer spacing of MoS₂ can be enlarged by water (Figure 2a). In addition, the electrostatic interaction between Zn²⁺ and the host material can be shielded, which further increases the Zn²⁺ diffusion channel. MoS₂−nH₂O exhibited a high reversible capacity of 165 mAh·g⁻¹ at 0.1 A·g⁻¹ (Figure 2b,c). The capacity retention was 88% after 800 cycles at 2 A·g⁻¹. Moreover, the insertion of highly conductive carbon material between MoS₂ layers is another approach often used in interlayer engineering [13,15,23]. The synergistic effect of expanding the layer spacing and enhancing the conductivity can effectively improve the performance of MoS₂−based ZIBs. Liu et al. [13] used an electrostatic self−assembly strategy to successfully insert graphene between molybdenum disulfide layers, leading to the enlarged layer spacing from 0.62 to 1.16 nm (Figure 2d), and the reduced graphene oxide and 1T−rich phase MoS₂ gave the material better ionic/electronic conductivity. The “sandwich” structure of MoS₂/graphene composite nanosheets self−assembled into a nanoflower structure can effectively suppress the stacking of molybdenum disulfide and graphene layers, which is conducive to sufficient electrolyte infiltration and rapid diffusion of zinc ions to maintain structural stability. Compared with bulk molybdenum disulfide (21.9 mAh·g⁻¹ at 0.05 A·g⁻¹), the material has more than 10 times higher zinc storage capacity (285.4 and 141.6 mAh·g⁻¹ at 0.05 and 5 A·g⁻¹, respectively) and remarkable cycling stability, as displayed in Figure 2e. Except for the interlayer engineering, the highly flexible and reversible transition of MoS₂ between the 2H−phase and 1T−phase during Zn²⁺ insertion/extraction also contributed to the remarkable storage capacity. The cycling stability is remarkable (about 88.2% capacity retention after 1800 cycles), which benefits from the large spacing−induced low Zn ion migration barriers that are also verified by density function theory calculation (Figure 2f). By combining interlayer polymerization with template−assisted insertion of N−doped carbon between MoS₂ layers, Li et al. produced MoS₂ nanocages with multilevel structures with expanded interlayer spacing, as displayed in Figure 2g [23]. The cage structure suppresses the accumulation of nanosheets. Meanwhile, the ion migration induces the volume expansion of alleviates during (non)charging. As a result, the layer spacing−enhanced MoS₂ exhibits high zinc storage capacity (247.8 mAh·g⁻¹ at 0.1 A·g⁻¹) and remarkable capacity retention (85.6% after 3200 cycles at 1.0 A g⁻¹) (Figure 2h).

3.2. Phase Engineering

Phase engineering has been verified as a potential technique to tune the electrochemical properties of materials [24]. Monolayer TMDs can exist in three different types according to the atomic configuration, namely 1T, 2H, and 3R. Monolayer TMDs can be composed of either a trigonal phase (2H) or an octahedral phase (1T). Phase control may be key to the electronic properties modification of TMDs. This is because the TMD phase is strongly related to the electron d−orbital density of the transition metal. When the Zn diffusion pathway was simulated, 2H MoS₂ exhibits a higher Zn energy barrier than 1T MoS₂, regardless of the interlayer spacing [14], indicating that the reversible Zn²⁺ storage can be effectively activated via phase control. Interleaving alkaline metals is the most common method for controlling TMDs phase transitions. Taking Li ion batteries as an example, electrons are transferred to the TMD via the insertion of an alkaline metal, leading to an increase in the d−orbital electron density. As a result, the phase transition of TMDs from 2H to 1T can be accomplished through increasing the d−orbital electron density [25]. Lithium−containing compounds, such as n−butyllithium diluted in hexane, were used to achieve the phase transition of TMDs [26]. Furthermore, 1T MoS₂ with increased interlayer spacing can be obtained by intercalation of Na⁺ into 2H MoS₂ as demonstrated by Zeng et al. Therefore, alkali metal intercalation−induced phase transition of TMDs has been verified as a synergistic method to activate MoS₂ and TMDs for Zn²⁺ storage [27].

Figure 2. Interlayer engineering for optimized MoS₂−based ZIBs. (a) HRTEM images of MoS₂·nH₂O. (b) Rate performance of Zn/MoS₂·nH₂O and Zn/MoS₂−400 battery at 2 A·g⁻¹. Reprinted with permission from Ref. [22]. Copyright 2021, Elsevier. (c) Cycling performance of Zn/MoS₂·nH₂O and Zn/MoS₂−400 battery. (d) Crystal structures of bulk MoS₂ and graphene−enlarged MoS₂. (e) Rate capability of the graphene−enlarged MoS₂ at 0.05−5 A·g⁻¹. (f) Migration barriers associated with MoS₂ and graphene distances. Reprinted with permission from Ref. [13]. Reprinted with permission from Ref.Copyright 2021, Wiley-VCH GmbH, Weinheim. (g) Illustration of the synthetic process of C−MoS₂−NC. (h) The capacity retention performance comparison for the various MoS₂ cathodes. Reprinted with permission from Ref. [23]. Copyright 2022, Elsevier.

Figure 2. Interlayer engineering for optimized MoS₂−based ZIBs. (a) HRTEM images of MoS₂·nH₂O. (b) Rate performance of Zn/MoS₂·nH₂O and Zn/MoS₂−400 battery at 2 A·g⁻¹. Reprinted with permission from Ref. [22]. Copyright 2021, Elsevier. (c) Cycling performance of Zn/MoS₂·nH₂O and Zn/MoS₂−400 battery. (d) Crystal structures of bulk MoS₂ and graphene−enlarged MoS₂. (e) Rate capability of the graphene−enlarged MoS₂ at 0.05−5 A·g⁻¹. (f) Migration barriers associated with MoS₂ and graphene distances. Reprinted with permission from Ref. [13]. Reprinted with permission from Ref.Copyright 2021, Wiley-VCH GmbH, Weinheim. (g) Illustration of the synthetic process of C−MoS₂−NC. (h) The capacity retention performance comparison for the various MoS₂ cathodes. Reprinted with permission from Ref. [23]. Copyright 2022, Elsevier.

For the first time, Liu et al. prepared defective MoS₂ with different phases as ZIB cathode materials (Figure 3a). 1T−MoS₂ performed more remarkable specific capacity and more remarkable capacity retention than 2H−MoS₂ in aqueous ZIBs. The combination of experiments and DFT calculations reveals that 1T−MoS₂ can effectively lower the Zn²⁺ diffusion energy barrier (Figure 3b). Tang et al. demonstrated that 1T−WS₂ nanosheets obtained via phase engineering can be used as a ZIB cathode candidate. Benefiting from the interlayer expanding (Figure 3c) and conductivity increasing of the 1T phase, the reversible discharge capacities of the synthesized WS₂−200 at current densities of 50, 100 and 200 mA·g⁻¹ were 233.26, 206.25 and 179.99 mAh·g⁻¹, respectively, with 179.99 mAh·g⁻¹ far exceeding the discharge capacity of the commercial 2H−WS₂ (22 mAh·g⁻¹ at 200 mA·g⁻¹), as displayed in Figure 3d.

To further enhance the transfer kinetics of zinc ions, the vertically aligned 1T phase MoS₂ was fabricated by Liu et al., as displayed in Figure 3e [28]. The vertically aligned freestanding structure of the 1T−MoS₂ contributed to a remarkable specific capacity of 198 mA h·g⁻¹ at 0.1 A·g⁻¹ (Figure 3f). More importantly, it also exhibits remarkable cycling stability with a specific capacity retention of 97.2% after 1000 cycles at a current density of 2 A·g⁻¹ (Figure 3g).

Figure 3. Phase−engineered TMDs for ZIB cathode. (a) Illustration of the rechargeable Zn/MoS₂ battery system. (b) Relative Zn²⁺ diffusion energies associated with the interlayer spacing in 2H and 1T MoS₂. Reprinted with permission from Ref. [14]. Copyright 2020, Elsevier. (c) High−resolution XPS spectra of S_2p in the commercial WS₂ and WS₂−200 (1T−WS₂) samples. (d) Galvanostatic charge/discharge curves of WS₂−200 with various current densities. Reprinted with permission from Ref. [29]. Copyright 2022, Elsevier. (e) SEM images of vertically aligned 1T−MoS₂. Inset: Section TEM images of 1T−MoS₂. (f) Specific capacity of 1T−MoS₂ from 0.1 to 2 A·g⁻¹. (g) Capacity retention of 1T−MoS₂ at 2A·g⁻¹. Reprinted with permission from Ref. [28]. Copyright 2022, Elsevier.

Figure 3. Phase−engineered TMDs for ZIB cathode. (a) Illustration of the rechargeable Zn/MoS₂ battery system. (b) Relative Zn²⁺ diffusion energies associated with the interlayer spacing in 2H and 1T MoS₂. Reprinted with permission from Ref. [14]. Copyright 2020, Elsevier. (c) High−resolution XPS spectra of S_2p in the commercial WS₂ and WS₂−200 (1T−WS₂) samples. (d) Galvanostatic charge/discharge curves of WS₂−200 with various current densities. Reprinted with permission from Ref. [29]. Copyright 2022, Elsevier. (e) SEM images of vertically aligned 1T−MoS₂. Inset: Section TEM images of 1T−MoS₂. (f) Specific capacity of 1T−MoS₂ from 0.1 to 2 A·g⁻¹. (g) Capacity retention of 1T−MoS₂ at 2A·g⁻¹. Reprinted with permission from Ref. [28]. Copyright 2022, Elsevier.

3.3. Defect Engineering

Defect engineering is another widely studied method to change the physical and chemical properties of TMDs [15,30,31,32,33]. Defect engineering causes structural changes in TMDs, which facilitate the optimization of Zn²⁺ ion diffusion and charge transfer processes [34]. In addition, defect engineering can increase the number of ion storage sites and can achieve higher charge capacity. Defect engineering has been verified by many studies that can help improve the kinetics of reactions and electrochemical properties due to the active sites increasing [35]. Therefore, the implementation of defect engineering in TMDs is a feasible approach to tuning electrochemical properties for functional cathodes.

Vacancy is one of the most common structural defect of materials, such as sulfur vacancies, transition metal vacancies, edges, and holes in TMDs lattice, as shown in Figure 4a [34,36]. These vacancy defects are particularly attractive in two−dimensional monolayer TMDs because the transport of charge carriers in the two−dimensional plane is significantly enhanced. Moreover, the creation of vacancies exposes sites in the TMDs that can then be used for chemical functionalization [37,38]. It has been reported by Liu et al. that the formation energy of S vacancies in MoS₂ (2.35 eV) is lower than that of Mo vacancies (8.02 eV) [39], indicating that it is easier to generate S vacancies than Mo vacancies, which is also verified by Koh et al. [40]. In particular, S vacancy is critical for the storage mechanism due to the ability to facilitate ion transit through the MoS₂ layer [41]. Xu et al. investigated defect−engineered MoS_2−x nanosheets as cathodes for zinc ion batteries [42]. Compared with defect−free MoS₂, the defect−rich MoS_2−x nanosheets provided more intercalation sites for zinc ions, resulting in higher capacity (Figure 4b). More importantly, it also exhibits remarkable cycling stability with a specific capacity retention of 87.8% after 1000 cycles at a current density of 1 A·g⁻¹ (Figure 4c).

Recently, Liu et al. successfully unlocked the molybdenum disulfide substrate by implanting structural defects and lattice oxygen (D−MoS₂−O) in molybdenum disulfide through in situ molecular engineering (Figure 4d) [15], expanding the layer spacing from 6.2 to 9.6 Å. The enriched 1T phase gives the material better electrical conductivity and hydrophilicity and achieves efficient three−dimensional transport of zinc ions along the ab−plane and c−axis of molybdenum disulfide. The prepared molybdenum disulfide nanosheets containing structural defects and lattice oxygen are grown vertically on a carbon cloth substrate, which effectively suppresses the stacking of molybdenum disulfide layers and facilitates sufficient electrolyte infiltration and rapid diffusion of zinc ions. Benefiting from the synergistic engineering (structural defects and 1T−phase transition) that induces interlayer expanding, the material exhibits remarkable multiplicative performance (reversible capacities of 261 and 102.4 mAh·g⁻¹ at 0.1 and 10 A·g⁻¹, respectively) and outstanding cycling stability (90.5% capacity retention after 1000 cycles). Electrochemical tests of the system indicate that the material has a high zinc ion diffusion coefficient, while DFT theoretical calculations demonstrate a low Zn²⁺ migration energy barrier and a feasible three−dimensional zinc ion transport mechanism in the material (Figure 4e).

3.4. Metallic 1T TMDs

Beyond the 1T phase transition of 2H MoS₂ and WS₂, the direct selection of metallic 1T phase TMDs is another effective strategy to ensure large layer spacing and high electric conductivity, while the structure is also more easily stabilized compared to that of 1T−MoS₂/WS₂ during the phase transition process.

Li et al. developed metallic 1T− VS₂ directly grown on stainless steel (VS₂@SS) mesh cathode for an aqueous ZIB [43]. The cell exhibited a remarkable Zn ion storage capacity (198 mAh·g⁻¹) and stable cycling performance (more than 80% capacity retention over 2000 cycles at 2 A·g⁻¹), which suggests that the electrode is promising for practical applications. Wu et al. observed the remarkable zinc ion storage properties in ultrathin VSe₂ nanosheets (Figure 5a) [44]. The X−ray photoelectron spectrum (XPS) unveiled the two−step intercalation/de−intercalation reaction mechanism of VSe₂ (Figure 5b). VSe₂ nanosheets exhibited a specific capacity of 131.8 mAh·g⁻¹ at 0.1 A·g⁻¹ and a high energy density of 107.3 Wh·kg⁻¹ (power density of 81.2 W·kg⁻¹). Moreover, the ZIBs with this material exhibit remarkable cycling stability (80.8% capacity retention after 500 cycles) (Figure 5c).

Recently, TiSe₂ was verified to be an ideal candidate for aqueous ZIBs by Wen et al., as shown in Figure 5d [16]. The TiSe₂ with 0.601 nm layer spacing and good electron conduction properties make it suitable for efficient (de)intercalation of zinc ions. As a result, the TiSe₂ electrode showed a capacity of 128.0 mAh·g⁻¹ at 0.20 A·g⁻¹ (Figure 5e) and recoverability of 70.0% after 300 cycles. Especially, the high concentration distribution of zinc ions in the fully discharged state demonstrates the accomplished intercalation of zinc ions (Figure 5f).

3.5. Oxidation

The use of in situ electrochemistry to oxidize the inserted ions to the corresponding oxides without changing the ion insertion position of the TMD, not only increases the active site but also enhances the hydrophilicity of the ions, leading to an effective diffusion of Zn²⁺ [45,46].

To verify the effectiveness of in situ oxidation, Yu et al. constructed VS₂/VOx composite structures with porous VO_x nanosheets uniformly distributed on VS₂ using electrochemical pretreatment [47]. In particular, the lattice fringes of 0.295 and 0.345 nm correspond to the (400) and (100) planes of V₆O₁₃ (Figure 6a). Benefiting from the synergy of the high conductivity of VS₂ and the remarkable stability of VO_x, the VS₂/VO_x heterostructure facilitates the regulation of guest ion intercalation and improves the chemical stability of the VS₂ backbone. The buffer volume was changed by the gradual insertion of Zn²⁺ while accelerating the reversible reaction kinetics. The VS₂/VO_x electrode maintained a high capacity of 75% after 3000 cycles at 1 A∙g⁻¹. (Figure 6b). In addition, MoS₂−O was prepared as a cathode for ZIBs by a simple solvothermal method by Jia et al. [17]. EDS and XPS characterization verifies the existence of O insertion in MoS₂ (Figure 6c,d). MoS₂−O reduced the ion diffusion resistance and maintained the stability of the MoS₂ layer. The optimized MoS₂−O electrode exhibited a high specific capacity of 206.7 mAh·g⁻¹ at a current density of 0.1 A·g⁻¹ (Figure 6e) and remarkable cycling stability after 300 cycles, further confirming the effectiveness of the oxidation strategy.

3.6. Hybridization

Hybridization is another well−proven method for altering the properties of a material or for giving it new properties [48]. Two−dimensional TMDs can be compounded with other materials to obtain new properties. In general, DMTs have low electrical conductivity, which can considerably limit the specific capacity that they can achieve. To overcome the challenge, the researchers demonstrated that the electrochemical properties of the entire composite can be improved via hybridizing the host TMDs with other materials, such as other TMDs, carbon materials, and so on [49].

The hybridization strategy is particularly applicable between TMD materials because the host and guest TMDs usually have similar structures and the lattice mismatch can be neglected. Generally, the host and guest TMDs can be hybridized by epitaxial growth, and the composites are often able to exhibit stronger intrinsic properties, multifunctionality, or even new properties. For example, a MoS₂−MoSe₂ heterostructure assembled by liquid−phase ultrasound−assisted exfoliation was reported by Yang et al. [50]. Benefiting from the unique van der Waals interlayer coupling, the MoS₂−MoSe₂ heterostructure is able to provide more electrochemical active sites. Moreover, the fluffy construction of the MoS₂−MoSe₂ heterostructure enhances ion transport. Pu et al. fabricated VS₂@VOOH via hydrothermal synthesis (Figure 7a) [51]. The presence of V−O−V and O−H vibrational was demonstrated by FT−IR spectroscopic characterization at typical 480 and 1610 cm⁻¹ adsorption peaks, respectively (Figure 7b), which further solidifies the existence of VOOH in VS₂. The O−H in VOOH not only improved the wettability of the electrode and electrolyte, but also reduced vanadium dissolution and improved the electrochemical performance. This composite displayed a remarkable capacity of 107.5 mAh·g⁻¹ after 350 cycles at 1.5 A·g⁻¹. In particular, after 400 cycles at a high current density of 2.5 A·g⁻¹, it maintains a capacity of 91.4 mAh·g⁻¹ (Figure 7c). In the hybridization of cathode materials for energy storage systems, both carbon nanotubes and graphene are widely used for their remarkable electrical and mechanical properties [52,53], and by hybridizing TMDs and CNT/graphene, the conductivity of the whole composite can be effectively improved while maintaining a large layer spacing colleague. A novel two−dimensional layered composite composed of graphene sheets were grown vertically on metallic 1T−VS₂ nanosheets by a one−step solvothermal strategy by Chen et al. [54], as shown in Figure 7d. The rGO−VS₂ was confirmed by Raman spectroscopy (Figure 7e). Due to the advantageous synergy between the layered VS₂ and graphene nanosheets, the rGO−VS₂ composites demonstrated a high capacity of 238 mAh·g⁻¹ at 0.1 A·g⁻¹ and capacity retention of more than 93% after 1000 cycles at 5 A·g⁻¹ as ZIB cathodes (Figure 7f).

Yan et al. synthesized marble−like rGO−VSe₂ nanobridges [18]. Benefiting from the inhibition of electrostatic buildup of VSe₂ nanosheets by the strongly conducting rGO, the electrolyte diffusion path is shortened, while the highly conductive rGO network provided a three−dimensional highway for electron transport and improved reaction kinetics. The rGO−VSe₂ nanohybrids were used as the ZIB cathode candidate, exhibiting high capacity 221.5 mAh·g⁻¹ at a current density of 0.5 A·g⁻¹, and performed 91.6% capacity retention after 150 cycles. Wang et al. combined metal 1T phase MoS₂ with multi−walled carbon nanotubes (MWCNT) to construct ZIB cathode materials by the hydrothermal method [55]. The synergistic effect of phase engineering and hybridization was utilized to accelerate the zinc ion transfer kinetics and to cost−effectively improve electrochemical properties at room temperature and low temperature. The hybrid electrode exhibited a high reversible capacity of 161.5 mAh·g⁻¹ with good cycling stability after 100 cycles at 0.1 A·g⁻¹ and capacity retention of 84.6% after 500 cycles at 1 A·g⁻¹.

4. Conclusions and Perspective

In this review, potential ZIB cathode materials, TMDs, are systematically discussed. The timeline and electrochemical performance of TMD materials as ZIB cathode is summarized in Figure 8 and

Table 1. Summary of the electrochemical performance of TMDs as ZIB cathode.

Strategies	Cathode Material	Electrolyte	Voltage	Capacity [mAh·g−1]	Cycle Stability	Ref
Interlayer engineering	MoS2·nH2O	3M Zn (CF3SO3)2	0.2–1.25 V	165 at 0.1 A g−1	88% after 800 cycles at 2.0 A g −1	[22]
	MoS2−graphene	3M Zn (CF3SO3)2	0.2–1.5 V	283.9 at 0.1 A g−1	88.6% after 1800 cycles at 1.0 A g−1	[12]
	C−MoS2−NC	2 M ZnCl2	0.2 to 1.4 V	247 at 0.1 A g−1	85.6% after 3200 cycles at 1.0 A g−1	[23]
Phase engineering	1T−MoS2	3M Zn (CF3SO3)2	0.25–1.25 V	168 at 0.1 A g−1	98.1% after 400 cycles at 1.0 A g−1	[14]
	1T−WS2	1M ZnSO4	0.1–1.5 V	206 at 0.1 A g−1	/	[29]
	Vertical 1T−MoS2	3M Zn (CF3SO3)2	0.25–1.25 V	198 at 0.1 A g−1	87.8% after 2000 cycles at 1.0 A g−1	[28]
Defect engineering	MoS2−x	3M Zn (CF3SO3)2	0.25–1.25 V	138 at 0.1 A g−1	87.8% after 1000 cycles at 1.0 A g−1	[42]
	D−MoS2−O	3M Zn (CF3SO3)2	0.2–1.25 V	261 at 0.1 A g−1	90.5% after 1000 cycles at 1.0 A g−1	[15]
Metallic 1T	VS2	1M ZnSO4	0.4–1.0 V	159 at 0.1 A g−1	98% after 200 cycles at 1.0 A g−1	[43]
	VSe2	2M ZnSO4	0.1–1.6 V	132 at 0.1 A g−1	80.8% after 500 cycles at 1.0 A g−1	[44]
	TiSe2	2M ZnSO4	0.05–0.6 V	128 at 0.2 A g−1	70% after 300 cycles at 1.0 A g−1	[16]
Oxidation	VS2/VOx	25M ZnCl2	0.1–1.8 V	260 at 0.1 A g−1	75% after 3000 cycles at 1.0 A g−1	[47]
	MoS2−O	3M Zn (CF3SO3)2	0.2–1.3 V	206 at 0.1 A g−1	/	[17]
Hybridization	VS2@VOOH	3M Zn (CF3SO3)2	0.4–1.0 V	165 at 0.1 A g−1	86% after 200 cycles at 1.0 A g−1	[51]
	rGO−VS2	3M Zn (CF3SO3)2	0.4–1.7 V	238 at 0.1 A g−1	93% after 1000 cycles at 1.0 A g−1	[54]
	rGO−VSe2	2 M ZnSO4	0.4–1.7 V	221at 0.1 A g−1	91.6% after 150 cycles at 1.0 A g−1	[18]
	1T−MoS2−CNT	2 M ZnSO4	0.2–1.3 V	161.5at 0.1 A g−1	84.6% after 500 cycles at 1.0 A g−1	[55]

. In order to effectively reduce the diffusion potential of Zn²⁺ in TMDs and maintain the stability of the material over multiple cycles, six possible strategies for eliciting satisfactory electrochemical performance of TMDs, including (1) interlayer engineering, (2) phase engineering, (3) defect engineering, (4) metallic 1T TMDs, (5) oxidation, and (6) hybridization are comprehensively summarized in this review. Although these modification strategies have only been initially validated in some TMD materials, more members of the TMDs family will definitely become excellent ZIB cathode materials as the research continues, driving the flourishing development of aqueous ZIBs. To further the continuous upgrade of these modification strategies, we also conclude by listing a few key breakthrough directions:

(1)

Focusing on mechanism exploration behind the ZIBs technology:

Although previous studies have analyzed the electrochemical properties of TMD−based ZIBs and their energy storage mechanisms, the link between the lattice structure, crystal defects and properties of cathode materials needs to be deeply resolved. Fully revealing the effects of various strategies, engineered TMDs on Zn²⁺ storage behavior is crucial to achieving higher performance ZIB batteries.

(2)

Increasing synergistic engineering between different strategies:

Each of the strategies mentioned above addresses a specific performance enhancement of TMD−based ZIBs. To achieve the overall enhancement, the interplay between different modified strategies is required, such as the synergy of defect engineering and phase engineering increasing the layer spacing [15], reducing the Zn²⁺ transport barriers, and enhancing the cycling characteristics. The hybridization of metallic 1T−TMD materials and graphene enhances the stability of the material while guaranteeing conductivity. More strategy combinations provide new freedom to design high−performance ZIBs.

(3)

Solving cyclic stability from a system perspective:

In ZIBs, the storage capacity can approach theoretical values by material selection and modifications. However, the storage capacity suffers from rapid fading upon long−term cycling, which is more crucial for the commercialization of ZIBs. For TMD−based ZIBs, improving material structure and chemical stability are the basic guarantees. In addition, in−depth exploration of the underlying and overall correlation between TMD cathode materials, electrolytes, and Zn anodes is the further direction.