Recent Progress and Regulation Strategies of Layered Materials as Cathode of Aqueous Zinc‐Ion Batteries

Aqueous zinc‐ion batteries (ZIBs) have shown great potential in the fields of wearable devices, consumer electronics, and electric vehicles due to their high level of safety, low cost, and multiple electron transfer. The layered cathode materials of ZIBs hold a stable structure during charge and discharge reactions owing to the ultrafast and straightforward (de)intercalation‐type storage mechanism of Zn2+ ions in their tunable interlayer spacing and their abilities to accommodate other guest ions or molecules. Nevertheless, the challenges of inadequate energy density, dissolution of active materials, uncontrollable byproducts, increased internal pressure, and a large de‐solvation penalty have been deemed an obstacle to the development of ZIBs. In this review, recent strategies on the structure regulation of layered materials for aqueous zinc‐ion energy storage devices are systematically summarized. Finally, critical science challenges and future outlooks are proposed to guide and promote the development of advanced cathode materials for ZIBs.


Introduction
The demand for sustainable energy is urgent due to the ongoing depletion of traditional fossil fuel sources and the associated environmental problems.The technologies of renewable energy sources (solar, tidal, hydro, wind, and geothermal power) are hampered by their intermittent power generation and long-distance transmission. [1]Owing to their reliability, metal-ion batteries are widely applied in portable electronic devices and electric vehicles. [2,3]Although lithium-ion batteries (LIBs) hold high energy densities and show outstanding cycling performance, the excessive consumption of lithium resources, high cost, and inherent safety issues limit their application in the field of energy storage. [4][11] Zinc metal holds its superiority as the ideal anode for ZIBs, inspiring additional enthusiasm for its use.[14][15] Similar to LIBs, the gravimetric energy density, power density, and cost of ZIBs are largely dependent on the cathode used.Thus, designing and developing cathode materials with a large storage capacity, a high discharge potential, and a robust crystal structure with facile insertion and extraction pathways are great challenges in the development of high-performance ZIBs.
[18][19] As the potential candidates, layered structure materials hold the advantages of commodious-ion (de)intercalated channels, adjustable interlamellar spacing (schematic illustration as shown in Figure 1a), and accommodating various guest ions or molecules, which are illustrated and characterized in Figure 1b,c. [20,21]Layered structured materials have the characteristic of weak van der Waals interactions in the interlayer while possessing strong in-plane chemical bonds, which promote extension toward the in-plane direction. [22]This provides an unimpeded channel for accommodating extrinsic ions, especially polyvalent metal ions or hydrated ions with larger radii. [23]In addition, many studies are devoted to other layered electrode materials, such as vanadium-based oxides, [24] manganese-based oxides, [25,26] and transition metal dichalcogenides for aqueous ZIBs. [27]ayered structure electrode materials possess many significant advantages; however, the perpetual issues of the structure collapsing during the ions' (de)intercalation processes, element dissolution during the electrochemical redox reaction, and the hydrogen evolution and oxygen absorption reaction remain unsolved. [28,29][32][33] The development and application of layered structure materials are hindered by the above problems.Fortunately, these problems have been alleviated through extensive efforts, leading to considerable advancements.Effective strategies have been deduced from the perspective of morphological and structural design, which are widely used in the field of zinc storage. [21]This review will first introduce the different kinds of layered structure cathode materials (α-V 2 O 5 , [34] δ-MnO 2 , [35] VOPO 4 , [36] Mxene, [37] and transition metal dichalcogenides [38] ) for ZIBs, and then systematically introduce the structure design strategies for advanced cathode materials.There are four modifying strategies summarized by the various methodologies as follows: nanostructure design, [39] composite material design, [40] guest species incorporation (metal-ion intercalation, [41] polymer intercalation, [42] water intercalation, [43] and non-metal-ion intercalation [44] ), and defect engineering (cation defect, [45] anion defect, [46] cation doping, [47] and anion doping [48] ).Finally, we look forward to the development of layered structure materials in the field of energy storage in the future.

Types of Cathode Materials
The functional materials could be classified into inorganic, organic, and inorganic-organic hybrid materials in accordance with the difference in elemental composition. [49]Inorganic materials, which could be divided into layered structure and non-layered structure materials based on their crystal characteristics, have been broadly utilized in cathodes for batteries.As with free-standing sheet-like nanomaterials, layered structure materials show a high ratio of lateral size to thickness, with properties including outstanding electrical and thermal conductivities, high specific surface area, the quantum hall effect, a high Young's modulus, and rapid room-temperature carrier mobility. [50]Various synthetic strategies have been proposed to synthesize ultrathin twodimensional (2D) nanomaterials for fulfilling the demand for the development of property modulations.The mature synthetic strategies can be divided into top-down and bottom-up, which include liquid exfoliation (mechanical force assisted, ion intercalation assisted, ion exchange-assisted liquid, and oxidation assisted), micromechanical cleavage, wet-chemical syntheses, and chemical vapor deposition. [50]he comprehensive characterization of physical, electronic, and chemical properties of ultrathin 2D nanomaterials is extremely crucial because various synthetic methods could display different structural characteristics.It is crucial to obtain the compositions, crystal phases, thicknesses, precise sizes, strains, vacancies, defects, doping, surface properties, and electronic states of as-synthesized ultrathin 2D nanomaterials in order to investigate the relevance between the structural features and functionalities. [50]Layered materials, such as transition oxides (V 2 O 5 [34] and δ-MnO 2 [35]   ) and transition metal dichalcogenides (MoS 2 [27] and VSe 2 [51]   ), were assembled layer by layer via the weak van der Waals forces, which could obtain efficient transfer kinetics by controlling the interlayer states.Table 1 summaries the electrochemical performances of common layered materials as cathode in ZIBs, and the following sections will provide detailed discussions about their current developments in the field of ZIBs.

V 2 O 5
V 2 O 5 is a family of common layered materials that have received intensive attention in the fields of energy storage, [52,53] energy conversion, [54,55] and supercapacitors. [56,57]Especially due to the multiple electron transfer (V 5+ to V 3+ ) as well as suitable interlayer spacing, V 2 O 5 compounds are candidates as cathode in ZIBs possessing superior theoretical capacity and ion transfer kinetics.Among three polymorphic phases including α-, β-, and δ-V 2 O 5 , the orthorhombic α-V 2 O 5 , having the space group Pmmn (a = 11.51Å, b = 3.56 Å, and c = 4.37 Å), [21] is the most thermodynamically stable phase which has the weak V-O bond interlinked chains.Highly distorted [VO 6 ] octahedrons with corner and edge sharing formed this chain, which implies that the distorted [VO 5 ] square pyramid with the periodic up-up-down-down sequence bonds with the terminal oxygen atom of the adjacent layers Yuan Yuan is currently a Ph.D. student at Sungkyunkwan University (SKKU), R. Korea.She received her master's degree in 2022 from the School of Chemistry and Chemical Engineering, University of Jinan (UJN), China.Her research mainly focuses on computational chemistry and material, including the design and application of low-dimensional carbon materials in the field of electrocatalysts and batteries.with a long V-O bond as displayed in Figure 2a. [58]α-V 2 O 5 possesses the representative 2D-layered characteristic as well as the intercalation sites for sundry guest ions and polymers owing to the weak and long V-O bond length (2.79 Å). [58] The distorted [VO 5 ] tetrahedron holds three kinds of oxygen atoms, introduced as follows: 1) The double bonds with a length of 1.58 Å consisted of one vanadium atom and one terminal oxygen atom; 2) the bond length of ~1.78 Å formed by two vanadium atoms and one symmetric bridge oxygen; and 3) the bond length varying from 1.88 to 2.02 Å could be attributed to three vanadium atoms bonded to the other asymmetric bridge oxygen atoms. [58]V(O t ) 1/1 (O s ) 1/2 (3O a ) 1/3 (or V 2 O 5 ) could characterize the coordination state of the [VO 5 ] square pyramid unit (O t , terminal oxygen; O s , symmetric bridge oxygen; O a , asymmetric bridge oxygen).Benefiting from the stability of layers and channel architectures for reversible ion (de)intercalation and many valence-state changes (+2 − +5) for rich redox reactions with high capacity as well as promoting partial electronic neutrality, vanadium-based materials hold superior capacity, outstanding rate capacity, and stable cyclic performance.Vanadium-based materials improved the issue of dissolving elements due to more stable architecture and oxidation-state variants compared with manganese-based materials.Moreover, the layered structure compounds hold the character of unique intercalation chemistry, which has tremendous application prospects in the field of energy storage.Layered architecture could easily accommodate the intercalated zinc ions and other alien species compared to other types of architecture, which could maintain structural integrity.Nevertheless, some disadvantages, such as inferior conductivity, structure collapse, and dissolution of elements, still inhibit practical applications. [59]wing to the 2D architecture and excellent theoretical capacity of 588.4 mAh g −1 based on V 3+ /V 5+ redox, anhydrous orthorhombic α-V 2 O 5 has gained attention as the potential cathode for a zinc-based battery. [34]The guest ions are intercalated into the host α-V 2 O 5 via sixfold coordination with oxygen.However, zinc ions are not easily intercalated and transferred into the host structure owing to single V-O layers composed of VO 5 square pyramids being tightly stacked.Different from α-V 2 O 5 , the V 2 O 5 ÁnH 2 O consists of two [VO 6 ] octahedron layers (Figure 2a).The presence of crystal waters expands the interlayer spacing to over 10 Å, which is beneficial to the intercalation and transfer of zinc ions.Zhu et al. synthesized anhydrous α-V 2 O 5 via one-step hydrothermal reaction followed by annealing. [34]The dissolution reaction in water, as well as phase transformation to a hydrated V 2 O 5 Á1.75H 2 O, collectively regulated the stability of anhydrous α-V 2 O 5 .V 2 O 5 Á1.75H 2 O was transformed from α-V 2 O 5 , appropriately dissolving in aqueous ZnSO 4 electrolytes (Figure 2b).The phase transition was discovered to be more rapid after the intercalation of a proton into V 2 O 5 Á1.75H 2 O via in situ and ex situ X-ray diffraction (XRD) and scanning electron microscopy (SEM).Water acting as the driver for dissolution and phase transformation was the separate dissolution research in non-aqueous electrolytes.Both the zinc ion and proton could be intercalated in the α-V 2 O 5 and V 2 O 5 Á1.75H 2 O; the Zn-ion holds lower capacity than the proton, as shown in Figure 2c, due to larger interlayer spacing (∼13.0Å vs ∼4.4 Å).The advantageous phase transformation enlarges the interlayer spacing, showing the capacity of gradual increase for Zn/ α-V 2 O 5 cathode during the initial cycles.The inferior cyclic stability resulting from the dissolution of α-V 2 O 5 could be improved via preimmersion of α-V 2 O 5 in zinc sulfate electrolytes to transform into V 2 O 5 Á1.75H 2 O, while redundant protons and vanadium ions could be removed by washing. [34]

VOPO 4
As the competitive cathode material for zinc batteries, VOPO 4 Á2H 2 O exhibits the (de)intercalation mechanism in interlayer galleries.There are two distinct water molecules in the VOPO 4 Á2H 2 O crystal structure, as shown in Figure 3a, which are contributed by the structural water bonded to the V atom and crystal water that existed in the interlayer between the two PO 4 tetrahedra.From a chemical bonding point of view, the structural water is more strongly bound to the lattice than the crystal water.Two kinds of water molecules could stabilize the structure via hydrogen bonding in the O-O interaction.Hong et al. confirmed that the key to achieving high reversibility was maintaining a certain concentration of water in the electrolyte. [60]VOPO 4 Á2H 2 O exhibits a higher discharge platform around 1.1-1.2V than compared to other vanadium-based materials due to the existence of PO 3À 4 , which brings about enhanced ionicity of V-O bonds. [60]Layered VOPO 4 Á2H 2 O would be a potential candidate with high energy density for zinc batteries because the discharge platform of the cathode determines the energy density.Unfortunately, the electrostatic attraction between zinc ions and the VOPO 4 -layered structure and the dissolution/decomposition behavior of VOPO 4 Á2H 2 O lead to VOPO 4 Á2H 2 O-based electrodes exhibiting unsatisfactory cyclic performance and rate capability.The layered VOPO 4 Á2H 2 O cathode possesses an as-reported capacity of ~170 mAh g −1 , which is much smaller than its theoretical capacity (>300 mAh g −1 ).Thus, the problem demands a prompt solution to offer improved rate capability and long-life cycle performance for aqueous zinc battery applications. [61]hi et al. synthesized layered VOPO 4 ÁxH 2 O electrodes via a reflux reaction between H 3 PO 4 and V 2 O 5 . [62]Nevertheless, VOPO 4 ÁxH 2 O would degrade in the aqueous electrolytes by decomposition into VO x , and the dissolution routes are exhibited in Figure 3b.The voltage platform of VOPO 4 ÁxH 2 O could distinctly decline in Zn(OTf) 2 electrolyte due to the degradation of VOPO 4 ÁxH 2 O into VO x and losing the inductive effect from polyanions, as shown in Figure 3c.Only adding both PO 3À 4 and a high concentration of salt could inhibit the degradation and change the decomposition equilibrium of VOPO 4 ÁxH 2 O, thereby obtaining the appropriate voltage and stable cyclic performance (Figure 3d).This group attempted to utilize the 13 M ZnCl 2 /0.8 M H 3 PO 4 solution as the aqueous electrolyte, which exhibits the desired voltage retention and stable capacity.The H + and Zn 2+ sequentially intercalated into the architecture, exhibiting an outstanding capacity of 170 mAh g −1 at 0.1 A g −1 , as shown in Figure 3e.The fatal problems of decomposition and dissolution of polyanion materials in zinc batteries were demonstrated, and a strategy for effective solution based on fundamental chemical principles was proposed by adding high salt concentration to shift the decomposition equilibrium and prevent dissolution. [62]

δ-MnO2
MnO 2 has attracted attention as the ideal cathode material for zinc storage because of its superior theoretical capacity (308 mAh g −1 ), outstanding energy, and power density. [63]MnO 2 consists of hexagonally close-packed (hcp) MnO 6 octahedral units, where there are six oxygen neighbors per Mn 4+ ion.The crystallographic structures of various polymorph types, as shown in Figure 4a, are formed by basic units connecting with the edges or corners. [64]Different from other MnO 2 with tunnel structure (α-, β-, γ-, and λ-type), birnessite-type δ-MnO 2 holds a typical layered structure with an interlayer spacing of ~7Å to the benefit of fast (de)intercalation of zinc ions without structural rearrangement (schematic illustration as shown in Figure 4b). [65]The low adsorption energy of birnessite could alleviate the co-insertion effect of the proton.Unfortunately, birnessite possesses inferior rate performance and cyclic stability with obvious phase transformation and serious structural collapse, which are attributed to manganese-ion dissolution and the co-intercalation of water molecules.For example,  [20,22] Copyright a) 2020 John Wiley and Sons, b,c) 2021 John Wiley and Sons.Energy Environ.Mater.2024, 7, e12632 Alfaruqi et al. synthesized the layered structure of δ-MnO 2 by the thermal decomposition of KMnO 4 .The as-prepared δ-MnO 2 displayed a capacity of 250 mAh g −1 at 83 mA g −1 and 7 mAh g −1 at 1666 mA g −1 (Figure 4c). [66]am et al. synthesized a layered manganese oxide for zinc battery cathode material with a superhigh crystal water content (∼10 wt.%) via the aqueous electrochemical cycling method. [67]The as-synthesized δ-MnO 2 possesses a large interlayer spacing of 7.25 Å between the neighboring transition metal layers because of the effects of structural water.The charge shielding and hydrated insertion effects of structural water could effectively alleviate the penalty of de-solvation energy as well as promote ion transfer.The strategy for preventing manganese dissolution and intensifying the host structure was controlling the suitable interlayer distance and offering excess structural water (Figure 4d).This δ-MnO 2 showed a superhigh capacity of 350 mA h g −1 at 100 mA g −1 with a discharge platform at 1.53 V.The robust dumbbell architecture of Zn-Mn could obtain the desired stable cyclic performance and superior rate performance. [67]4.MXene "MXene," a brand new member of the 2D material family, was first described by Gogotsi's group, which included transition metal carbides, carbonitrides, and nitrides.[68] MXenes are mainly produced through selectively etching "A" layers from layered ternary MAX counterparts by HF or a mixed solution of HCl and LiF, where M corresponds to the transition metal (such as Sc, Ti, Zr, V, Nb, Cr, or Mo), A corresponds to a group IIIA or IVA element (i.e., Al, Si, and Ga), and X includes carbon and/or nitrogen.The representative structure of a 2D MXene is shown in Figure 5a, and M n + 1 X n T x (n = 1-3) is suggested to illustrate the chemical components of MXenes, where T represents the surface terminal groups (hydroxyl, oxygen, or fluorine) generated during the corrosion procedure.The M-A bond generally possesses metallic characteristics, while the M-X bond possesses mixed ionic/metallic/covalent characteristics.[69] The A in-plane delivers more chemical reactivity because the interlayer M-A bonds as well as the in-plane A-A bonds are weaker compared to the strong M-X bonds.MXene acting as the layered material has shown broad application prospects in many fields, including electrocatalysis, [70] metal-air batteries, [71,72] and photothermal conversion.[73] Importantly, MXene shows high electrical conductivity and excellent hydrophilia in aqueous solutions, which were usually employed to construct advanced electrodes for aqueous metal-ion batteriescontaining ZIBs.[74] Li et al. synthesized a 3D V 2 CTX MXene with a lateral size of 6 μm via the all-in-one in situ electrochemical method utilizing MAX followed by etching in a fluorine-rich electrolyte (Figure 5b), which is a convenient and environmentally friendly procedure in neutral conditions.The exfoliation, electrode oxidation, and redox reaction of MAX were all carried out in the sealed cell.The fabricated MXene cathode maintains the increased capacity during the exfoliation, as shown in Figure 5c, with a capacity of 409.7 mAh g −1 at 0.5 A g −1 with an energy density of 310.3 Wh kg −1 .As shown in Figure 5e, the phase and structure transition where V 2 CTX MXene is oxidized to V 2 O 5 facilitate superior zinc storage performance owing to the higher capacity of V 2 O 5 versus V 2 CTX MXene.Combining the larger interlayer spacing with superior electroconductibility, the transfer kinetics of (de)interlacation are drastically enhanced.This work illustrated evident competitiveness with other V-based counterparts; even at an ultrahigh current density of 64 A g −1 , the battery still maintains a capacity of 95.7 mAh g −1 .It paved the way for designing and fabricating the MXenes devices via a green and facile strategy. [75]

Transition Metal Dichalcogenides
Transition metal dichalcogenides (TMDs) with the formula MX 2 (X = S, Se, or Te) have vital scientific significance in the field of energy storage devices because of their excellent physical and chemical properties.TMD holds a structure similar to that of graphite, consisting of two layers of chalcogen atoms clamped on one layer of metal atoms, as shown in Figure 6a. [76]The weak van der Waals interactions control the two X-M-X layers to adjust the structure.Moreover, the interlamellar spacing of TMD could act as a host structure to accommodate intercalated ions due to its typical 2D characteristic with large interlayer channels.The TMD holds trigonal prismatic (1H phase), octahedral (1T phase), and distorted octahedral (1T' phase) coordination of metal atoms for the monolayer.The 1H layers could be stacked in two ways  [34,58] Copyright a) 2020 John Wiley and Sons, b, c) 2021 American Chemical Society.
Energy Environ.Mater.2024, 7, e12632 to generate 2H and 3R phases based on different stacking orders.Figure 6a shows an illustration of the different structural polytypes.The 2H phase and 1 T phase exhibit three and two degenerate states in the d orbitals of metals, respectively.The d-orbital filling state plays a significant role in the electrical properties of TMDs (metallic conductivity is the partial filling state, and semiconductivity is complete filling).Group 5 TMDs possess the d 1 configuration of a central metal exhibiting metallic conductivity, while the other d metal centers display semiconducting conductivity.However, the application was limited by the inferior performance rate and cycle stability of TMDs due to their sluggish ion diffusion and huge volume expansion.Besides, the side effects during ion (de)intercalation result in the capacity fading quickly. [77]u et al. used the wet-chemical method to synthesize the hexagonal-phase VSe 2 ultrathin nanosheets with a mean lateral size of ~1 μm and an average thickness of ~2.1 nm (corresponding characterization of VSe 2 in Figure 6b,c).The ex situ XRD as well as X-ray photoelectron spectroscopy (XPS) were utilized to observe the two-step ion (de)intercalation mechanism.The van der Waals interaction enables two selenium layers to be alternately clamped between the vanadium layers.The abundance of channels and active sites is attributed to the large interlayer spacing of 6.11 Å.The electron coupling force between the neighboring V 4+ ions induces metallic characteristics.Combined with the reversible ion (de)intercalation mechanism, rapid ion transfer kinetics (D 2þ Zn ≈ 10 −8 cm −2 s −1 ), metallic properties, and stable structure during long-term cycles, the VSe 2 nanosheets exhibit a high energy density of 107.3 Wh kg −1 (at a power density of 81.2 W kg −1 ) and excellent cyclic stability (capacity retention of 80.8% after 500 cycles as shown in Figure 6d).The obvious metallic characteristics (Figure 6e) and optimal ion transfer route (hopping energy barrier: 0.91 eV, Figure 6f) were analyzed by density functional theory (DFT) calculation.Compared with other layered transitional metal dichalcogenides, VSe 2 ultrathin nanosheets have proven to be promising materials with their superior zinc storage performance. [51]

Design Strategies for Cathode Materials
The attenuation of the zinc storage performance of active materials is attributed to electrode dissolution, structure collapse, inferior electroconductibility, and slow diffusion kinetics.To solve these problems, structural design engineering is adapted.This section will systematically introduce structure design strategies for advanced cathode materials.There are four modifying strategies summarized by the various methodologies as follows: nanostructure design, composite material design, guest species incorporation (metal-ion intercalation, polymer intercalation, water intercalation, and non-metal-ion intercalation), and defect engineering (cation defect, anion defect, cation doping, and anion doping). [78]nergy Environ.Mater.2024, 7, e12632

Nanostructure Design
The inferior ion transfer rate of divalent zinc ions in bulk materials is attributed to the strong electrostatic attraction, which results in poor electrochemical performance.Nanomaterials have drawn widespread attention owing to their novel physical and chemical properties in contrast to their bulk counterparts for application in the energy-storage field. [79]The unique advantages of nanomaterials include abundant interlinked channels, a large active surface area, and low thickness, which could accelerate the electron and ion diffusion rate. [78]ang et al. used a hydrothermal method to create novel 2D MnO 2 nanofluidic channels consisting of many monolayer δ-MnO 2 sheets.MnO 2 nanofluidic channels possess rapid zinc-ion diffusion kinetics as well as prominent cyclic performance. [79]This enhanced performance of zinc storage in reasonably constructed MnO 2 nanofluidic channels could be attributed to the following advantageous properties: 1 Multiple interlinked ion diffusion routes of 2D nanochannels allowed the zinc ions to flow uniformly and alleviated the tip effect and dendrite formation during ion (de)intercalation (Figure 7). 2 The implantable multi-walled carbon nanotubes possess an amorphous porous architecture and channels cross-linked with carbon nanotubes in the vertical direction for improving zincion diffusion.
3 The void space inside the nanofluidic channel could effectively relieve the structural stress, buffer the volume change, and prevent the fast capacity decay caused by the structural collapse during the ion (de)intercalation.4 There are no polymers or binders in the membrane electrode, which could drastically enhance the capacity to avoid sacrificing the active substance content of a zinc battery.
This work paved the way to fabricate superior electrodes via 2D nanofluidic channels and apply them to the storage of other metalion batteries.

Composite Material Design
Because of the inherent inferior electroconductibility, irreversible structural change from the phase transformation during the ion (de)intercalation, and electrode dissolution with the redox reaction, storage performance of zinc is unsatisfactory.While the aforementioned methods could effectively enhance electroconductibility as well as alleviate structural change, it is difficult to resolve these problems at the same time.Material A can be combined with material B (both A and B are classical layered materials) by intercalating or coating methods for enhancing the electroconductibility of host materials and preventing the collapse of materials.It is worth noting that this modification strategy plays a multifunctional role in zinc-ion storage performance. [80]i et al. innovatively fabricated the unique sandwich structure of MoS 2 /graphene by embedding the reduced graphene oxide (rGO) into the MoS 2 gallery, which markedly enlarged the interplanar spacing from 0.62 to 1.16 nm, as shown in Figure 8a.The efficaciously enhanced hydrophilicity of MoS 2 /graphene could easily embed zinc ions.The flower-like structure, self-assembled by MoS 2 /graphene hybrid nanosheets, could mitigate stress during the ions (de)intercalation.The MoS 2 /graphene hybrid nanosheets demonstrated excellent rate performance (285.4 mAh g −1 at 0.05 A g −1 and 141.6 mAh g −1 at 5 A g −1 , Figure 8b), as well as stable cyclic performance (88.2% capacity retention after 1800 cycles, Figure 8c).Ultrafast zinc-ion transfer coefficients, superb pseudocapacitive contribution, and low zinc-ion migration barriers were verified by the electrochemical tests and DFT calculation.Ex situ XRD, Raman spectroscopy, and XPS were used to analyze the highly reversible phase transition between 2H-and 1T-MoS 2 .In the end, the MoS 2 /graphene hybrid nanosheet cathode and the PVA/Zn(CF 3 SO 3 ) 2 hydrogel electrolyte were assembled to form the flexible quasi-solid-state zinc batteries, which showed excellent zinc storage performance at various bending angles.It opened a new route for preparing layered materials as high-performance zinc storage cathodes in next-generation energy devices. [81]u et al. successfully deposited a 5-nm-thick poly(3,4ethylenedioxythiophene) (PEDOT) layer on the V 2 O 5 nanosheet array surface as the advanced flexible cathode material for the zinc battery, without utilizing binders or conductive additives.The schematic illustration and TEM image are shown in Figure 8d,e, respectively.As a protective shell, PEDOT could enhance the electroconductibility of host materials and prevent the collapse of materials during the ion (de)intercalation.The tight integration of carbon cloth (CC) with V 2 O 5 nanosheet arrays and the stable interface of conductive PEDOT synergistically promoted the charge transfer kinetics and Zn storage performances, which show the best rate performance (232 mAh g −1 at 20 A g −1 , Figure 8f) and stable cyclic performance (89% retention after 1000 cycles at 5.0 A g −1 , Figure 8g) of the V 2 O 5 @PEDOT/CC electrode.Furthermore, the Zn//V 2 O 5 @PEDOT/CC battery shows a superior energy density of 243 Wh kg −1 at a power density of 90 W kg −1 .The outstanding performance and flexible wearable character of Zn//V 2 O 5 @PEDOT/CC batteries would pave the way for them to be used in future energy storage devices. [82]

Guest Species Incorporation
Charge transfer pathways and coordination states in the lattice could be optimized to promote fast and invertible ion (de)intercalation, which would improve the zinc storage performance in secondary batteries.The interlamination pre-intercalation strategy of guest ions (Li + , Na + , NH 4 + , Zn 2+ , Al 3+ , and conducting polymer) is regarded as an effective Reproduced with permission. [69,75]Copyright a) 2019 Elsevier, and b-e) 2020 John Wiley and Sons.
Energy Environ.Mater.2024, 7, e12632 strategy to adjust the crystal architecture of host materials as well as promote the zinc-ion storage performance.On most occasions, the pillar effect of interlamination pre-intercalated ions could intensify structural stability and enlarge the interplanar spacing to promote ion diffusion and activate abundant active sites.Furthermore, the guest ions could alter the coordination environment, adjust the bandgap, as well as change the route of ion diffusion, thus improving the zinc storage performance. [83]

Metal-Ion Intercalation
The effective method for adjusting the zinc storage performance of layered compounds is metal-ion intercalation.According to the valent state, ion intercalation could be divided into monovalent and multivalent ion intercalation.However, the intercalating effect practically depends on the ionic radius.The column brace effect could optimize structural stability as well as prevent dissolution problems because of the pinning effect.Particularly, the robust bonding between the metal ions and these guest ions maintained in the lattice f) The optimal diffusion pathway of zinc ions and corresponding diffusion barrier curve.Reproduced with permission. [51,76,77]Copyright a) 2020 Royal Society of Chemistry (images of 1H, 1 T, 1 T', 2H, and 3R-MX 2 ) and 2020 John Wiley and Sons (image of MX 2 ), and b-f) 2020 John Wiley and Sons.Reproduced with open-access article. [79]Copyright 2019 Royal Society of Chemistry under CC BY-NC (https://creativecommons.org/licenses/by-nc/3.0/).
Energy Environ.Mater.2024, 7, e12632 9 of 18 could alleviate the phase transformation and the structure change.Moreover, the guest ions could enlarge the interlamellar spacing as well as control electron density near the Fermi level to lower the energy barrier along ion diffusion pathways.The ion transfer kinetics and electroconductibility could be improved simultaneously. [80,84]Xie et al. reported that the different monovalent cations (Li + , Na + , or K + ) were preintercalated into the layered δ-MnO 2 nanosheets via a convenient redox reaction.This work explored the influences of zinc storage behavior and ion transfer mechanisms by guest ions.The cyclic stability and rate performance of δ-MnO 2 were enhanced with the order of K-δ-MnO 2 > Na-δ-MnO 2 > Li-δ-MnO 2 , which were attributed to the large diameter of the potassium ion, promoting the zinc-ion transfer process. [85]he aluminum ions could stabilize the crystal architecture and promote zinc-ion-free access in the lattice.Pang et al. synthesized orthorhombic Al 0.2 V 2 O 5 via pre-intercalated aluminum ions into the host structure (Figure 9a,b).This compound exhibits a superior capacity of 448.4 mAh g −1 at 0.1 A g −1 and an outstanding rate capability of 143.9 mAh g −1 at 10 A g −1 as shown in Figure 9d. [86]u et al. carried out the topochemical synthesis of layered Zn 0.4 VOPO 4 Á0.8H 2 O via intercalation of Zn 2+ into the vanadium phosphate host structure without changing the in-plane atomic arrangement and coordination state (Figure 9e).The electrostatic attraction of the hydrogen bond between the structural water molecules and the phosphate radical receded in the host structure, which reduced the steric hindrance of O-P-O stretching vibration.As a stable cathode material for zinc-ion batteries, this layered material possesses not only a high discharge voltage platform (1.45 V, Figure 9f) but also excellent cycling stability (over 1000 cycles, Figure 9g).The energy density is 219.8Wh kg −1 at a power density of 136.2 W kg −1 , surpassing those of counterparts recently reported.This work utilized the topochemical strategy to pre-intercalate multivalent ions into the host materials without structural change, shedding light on the modification of layered materials for aqueous ZIBs. [87]

Polymer Intercalation
Polypyrrole, polyaniline, and polythiophene, as the representative conductive polymers, possess highly p-conjugated polymer chains, which could enhance the electroconductibility of the composites.Furthermore, conductive polymers offer additional redox pairs for semiconductor materials.Compared with the cations and water molecules, the intercalation of the conductive polymers played the role of a more substantial pillar among the layered structure, which will enable stable charge/discharge processes.The intercalation of the conductive polymers could also enhance the diffusion of electrolyte cations by expanding the interlayer spacing between the layered structure, which brings about superior rate capability.
Hu et al. synthesized the layered VOPO 4 Á2H 2 O with different interlamellar spacing (14.8 Å, 15.6 Å, and 16.5 Å) via the refluxing method followed by one-step phenylamine polymerization, which is the efficient polymeride intercalation strategy.The structural water escaped from vanadium phosphate in the interlayer gallery with the increasing content of the polymeride intercalation.The zinc storage performance is significantly dependent on the interlamellar spacing of phenylamine-intercalated VOPO 4 Á2H 2 O, which exhibits an approximately linear relationship.A maximum capacity of 268.2 mAh g −1 at 0.1 A g −1 , as shown in Figure 10a, is exhibited in this cathode material due to the large interlayer spacing (16.5 Å).The fast zinc-ion transfer kinetics with a high diffusion coefficient (~5.7 × 10 −8 cm −2 s −1 , Figure 10b) are attributed to the vast interlamellar spacing via the analysis of the experimental data and theoretical calculation, as in Figure 10c,d.The hydrophobicity of VOPO 4 Á2H 2 O after intercalating the phenylamine molecules was significantly increased, which shows remarkable cyclic stability after 2000 cycles at 5.0 A g −1 with a capacity of ~200 mAh g −1 as well as inhibited decomposition/dissolution of VOPO 4 Á2H 2 O (Figure 10e).This work delivers a promising  and c) long-term cycling stability at 1 A g −1 of MoS 2 /graphene.d) Schematic illustration of the fabrication of V 2 O 5 @PEDOT/CC.e) TEM image of the as-prepared V 2 O 5 @PEDOT nanosheet arrays.f) Rate capabilities of V 2 O 5 @PEDOT/CC.g) Cyclic performances of V 2 O 5 @PEDOT/CC at 5 A g −1 for 1000 cycles.Reproduced with permission. [81,82]Copyright a-c) 2021 John Wiley and Sons, and d-g) 2018 John Wiley and Sons.
Energy Environ.Mater.2024, 7, e12632 solution to the slow zinc-ion transfer kinetics and inferior stability of layered vanadium phosphate.Furthermore, the interlayer chemistry principle was clearly analyzed to pave the way toward nextgeneration ZIBs. [88]

Water Intercalation
Structural water is a widespread strategy for intercalation modification.The structural water could not only intensify the host architecture but also alleviate the electrostatic attraction between guest ions and the host architecture due to the charge shield effect.As a "lubricant," this structural water as intercalated species could promote the zinc-ion diffusion kinetics and enhance the architecture stability.The structural water intercalation strategy is widely used to regulate the zinc storage performances of layered host materials, which benefit from the advantages mentioned above. [89] et al. synthesized a novel layered Na 5 V 12 O 32 Á11.9H 2 O with a 14.8 wt.% maximized crystal water content and a supramaximal lattice space of 12.75 Å using the molten salt method in 5 min (Figure 11a).As for superior aqueous cathode materials, such a large lattice space offered spacious channels for fast zinc-ion (de)intercalation.The abundant content of structural water could moderate the electrostatic interactions between zinc ions and the host materials due to the chargeshielding effect.The electrical conductivity of Na 5 V 12 O 32 Á11.9H 2 O has a positive correlation with the high structural water content as determined by the DFT calculation.Both the large lattice space and high crystal water content promote the zinc-ion diffusion kinetics and cycling stability.Particularly, layered Na 5 V 12 O 32 Á11.9H 2 O demonstrated 103.7% capacity retention after 3862 cycles at 1 A g −1 (Figure 11b).The reaction mechanism of Na 5 V 12 O 32 ÁnH 2 O transfers from one-electron to two-electron redox reactions with an increase in the structural water content, as shown in Figure 11c.Na 5 V 12 O 32 Á11.9H 2 O displayed a superior reversible capacity of 430.5 Reproduced with permission. [86,87]Copyright a-d) 2020 Elsevier, and e-g) 2020 American Chemical Society.

Non-Metal-Ion Intercalation
Ammonium ions and protons, as diminutive non-metal ions, were utilized to pre-intercalate the precursor.To assure adequate interlayer spacing for zinc-ion embedding, the diameter of pre-intercalation ions should be larger than that of zinc ions if the solvation sheath was totally dissolved at the interface.Moreover, the column brace effect of ammonium ions is more effective than that of the other guest ions due to the effect of the hydrogen bond between the ammonium ions and host materials, which could promote a more stable architecture and alleviate volume expansion/extraction during cycling.The ammonium-pillared host materials exhibit superior zinc storage performance, indicating that ammonium ions are the optimal preintercalation strategy.Moreover, compared with metal ions, ammonium ions with a lower molecular weight may possess superior gravimetric capacity. [91]ang et al. fabricated thin, flexible (NH 4 ) 0.38 V 2 O 5 nanoribbons that were only 6.7 nm in thickness via the one-step hydrothermal method to obtain a binder-free paper cathode that depicted spontaneous knitting behavior.Edge-sharing VO 6 octahedron composed the monoclinic (NH 4 ) 0.38 V 2 O 5 unit cell to build a robust bi-layered architecture; the atomic structure is shown in Figure 12a.NH þ 4 acted as a "pillar" to intensify the architecture, which prevented volume changes during the charge/discharge processes.The (001) interlayer space of (NH 4 ) 0.38 V 2 O 5 is noticeably achieved to be 9.67 Å.The conductive multiwalled carbon nanotubes were mixed up with the paper cathode, and the hybrid membrane offered a capacity of 465 mAh g −1 at 100 mA g −1 , with the capacity retention exceeding 89.3% after 500 cycles at 100 mA g −1 (Figure 12b).The energy density of this paperlike membrane battery was as high as 40.94Wh kg −1 at a power density of 733.8 W kg −1 . [92]ang et al. designed long H 0.08 MnO 2 Á 0.7H 2 O nanobelts with a high aspect ratio (∼30) by the ion-exchange method from a K 0.33 MnO 2 Á0.5H 2 O precursor, which was initially utilized in the microporous and freestanding H 0.08 MnO 2 Á0.7H 2 O/MWCNT membrane (Figure 12c).H 0.08 MnO 2 Á0.7H 2 O cathode materials hold the primitive hexagonal cell with a layer spacing of 7.3 Å; the structure is shown in Figure 12d.The protons were retained between the lattice oxygen and [MnO 6 ] octahedron by electrostatic attraction.The preintercalation of protons could co-intercalate in the host materials with the zinc ions to command the various voltage stages, which is the intercalation competition for H + and Zn 2+ .This phenomenon shows a capacity of 277.6 mAh g −1 at 0.2 A g −1 as well as outstanding cyclic performance after 1000 cycles at 3.0 A g −1 (Figure 12e,f).The energy density of this membrane battery reaches up to 368.3 Wh kg −1 at a power density of 300 W kg −1 .This work illustrated that the proton-exchange material H 0.08 MnO 2 Á0.7H 2 O has obvious advantages as the potential and unique cathode material for zinc batteries.Employing a macroporeassisted binder-free method paves the door for layered nanostructures to be used as next-generation energy storage devices. [93]

Defect Engineering
According to the defect formation mechanism, defects in crystalline materials could be divided into interstitial defects, higher-dimensional defects, atomic vacancies, and substitutional defects. [94]Vacancy and doping manifest themselves in various types of defects owing to the significant and unique functionalities in the fields of electronic, magnetic, and optical properties of the materials. [95]At present, the application of defect engineering in rechargeable batteries, including lithiumion batteries, sodium-ion batteries, and metal-air batteries, has achieved Reproduced with permission. [88]Copyright 2021 Royal Society of Chemistry.
Energy Environ.Mater.[97] Defect strategies can drastically alleviate the inherent problems of the layered structure by controlling the surface characteristic as well as electronic structures of materials. [98]Defect engineering technology has achieved excellent advances in the modification of layered structures, resulting in enhanced electrochemical performance of materials.This section focuses on promoting the reported strategies of defect design (oxygen vacancy, cation vacancy, anionic doping, and cationic doping). [99]

Cation Defect
Cation defect is a desirable option to promote the migration of zinc ions by offering additional diffusion paths.Copyright a,b) 2021 Elsevier, and c-f) 2021 American Chemical Society.

Anion Defect
Anion defects have attracted widespread attention for strategies in the design of electrode materials.Anion defects could change the stable state to the metastable state of materials, resulting in the evolution of electrochemical activity.Anion defects rearrange the electronic architecture topically to multiply the concentration of charge carriers.Anion defects could shorten ion transport paths and enhance zinc storage performances through defects of the energy band. [101,102]Wu et al. prepared bilayer VOPO 4 nanosheets with high-concentration oxygen vacancies [Vo ÁÁ ] by a liquid exfoliation strategy in isopropanol (Figure 14a).Compared to the bulk counterpart, zinc-ion coefficient of the [Vo ÁÁ ]-rich bilayer VOPO 4 nanosheets (4.6 × 10 −7 cm −2 s −1 , Figure 14b) is of six orders of magnitude higher by galvanostatic intermittent titration technique study.This 2D nanosheet with optimized Zn 2+ transport possesses a particularly low diffusion barrier (0.08 eV, Figure 14c) detected by DFT simulation, which results in rapid diffusion kinetics and excellent zinc storage performance.The high-concentration [Vo ÁÁ ] significantly boosts electronic conductivity with ~57 times increased carrier concentration as determined by Mott-Schottky measurement.This cathode holds a superior capacity (313.6 mAh g −1 at 0.1 A g −1 ), a high discharge plateau of 1.1 V, as shown in Figure 14d, and excellent rate capability (168.7 mAh g −1 at 10 A g −1 , Figure 14e).This strategy may pave the way for prospective energy storage devices because of its prominent zinc storage performance, safety, and flexibility. [46]

Cation Doping
The enhancement of zinc storage performance by cation doping could be attributed to the following reasons: 1) Excellent structural stability derived from lower crystallinity after cation doping; 2) cation doping prevented the accumulation of zinc ions in the cathode; and 3) cation doping improved the diffusion rate of Zn 2+ and then promoted the rate and cycle performance. [103]He et al. prepared the Ti-doped NH 4 V 4 O 10 nanobelts (Figure 15b) with the advantage of interlinking pores in the NH 4 V 4 O 10 bilayer architecture (Figure 15a) as well as the improved stability performance and faster diffusion mechanism caused by titanium doping.The effect of microstructure and zinc storage performance of NH 4 V 4 O 10 nanobelts after titanium doping was investigated in this study.The Ti-doped NH 4 V 4 O 10 nanobelts possess capacities as high as 146 mAh g −1 with 89.02% capacity retention at 2 A g −1 after 2000 cycles (Figure 15c).The in-depth analysis of the effects of titanium doping influenced by zinc storage performances provides guidance for exploring suitable methods to optimize cathode materials. [47]

Anion Doping
Anion doping could stimulate the zinc-ion storage sites caused by anion defects in amorphization.Moreover, anion doping could alleviate the electrostatic attraction between the multivalent charge of Zn 2+ and the cathode structure, which improved the electrochemical reaction kinetics as well as effectively enhanced the electronic conductivity. [104]As a robust cathode for rechargeable aqueous ZIBs, sulfur-doped MnO 2 nanosheets were synthesized via a low-temperature sulfurization method by Zhao et al.The fabrication process and morphology of sulfur-doped MnO 2 nanosheets are shown in Figure 16a,b, respectively.This sulfur-doped cathode possesses an excellent discharge capacity of 324 mAh g −1 at 200 mA g −1 and superior cyclic performance of 150 mAh g −1 with capacity retention of 95% at 3000 mA g −1 over Reproduced with permission. [45]Copyright 2021 Elsevier.
Energy Environ.Mater.2024, 7, e12632 1000 cycles, as shown in Figure 16c,d.With the method of structural measurement and theoretical calculation, the author confirmed that sulfur doping could both lower the bandgap as well as enhance the intrinsic electronic conductivity, as shown in Figure 16e, to adjust the electronic structure of MnO 2 and alleviate the electrostatic interaction between zinc ions and the host structure.It is particularly significant for increasing Zn 2+ diffusion kinetics by oxygen defects with anionic doping and the extrinsic pseudo-capacitance associated with the amorphous surface (Figure 16f).This work throws new light on the sulfur-doping effect in MnO 2 nanosheets via a facile and effective strategy as a highefficiency cathode material for aqueous zincion batteries, which could be expanded to other compounds for structural optimization and practical application. [48] Summary and Perspectives Aqueous ZIBs have been deemed alternative candidates for large-scale applications because of their low cost, inherent safety, and environmental friendliness.Layered structure materials with tunable interlayer spacing, convenient ion (de)insertion channels, and the capacity to accept a variety of guest ions or molecules are favorable qualities that have drawn a lot of attention.However, further development and application of cathodes are notably obstructed by many severe problems, such as their poor electronic  [46] Copyright 2021 John Wiley and Sons.[47] Copyright 2020 Elsevier.conductivity, sluggish kinetics, structural collapse, active material dissolution, and the irreversible phase transition of the cathode.From the above analysis, plentiful design engineering strategies open a feasible and efficient avenue to handle the abovementioned problems.In this review, various structural and morphological engineering strategies for the cathode materials of aqueous ZIBs are systematically summarized.There are four modifying strategies summarized by the various methodologies as follows: nanostructure design, composite material design, guest species incorporation (metal-ion intercalation, polymer intercalation, water intercalation, and non-metal-ion intercalation), and defect engineering (cation defect, anion defect, cation doping, and anion doping).Nonetheless, the development of aqueous ZIBs is still in its infancy, and many fundamental issues need to be urgently settled to satisfy the requirements for future industrial applications.
The inferior ion transfer rate of divalent zinc ions in bulk materials is attributed to the strong electrostatic attraction, which results in poor electrochemical performance.Nanomaterials have drawn widespread attention due to their novel physical and chemical properties that distinguish them from their bulk counterparts for use in the energy storage field.The unique advantages of nanomaterials are their abundance of interlinked channels, large active surface area, and reduced thickness, which could accelerate the electron and ion diffusion rates.
It would result in an adverse impact on zinc storage performance because of the inherent inferior electroconductibility, irreversible structural change from the phase transformation during the ion (de)intercalation, and electrode dissolution with the redox reaction.While the aforementioned methods could effectively enhance electroconductibility as well as alleviate structural change, it is difficult to resolve these problems at the same time.Material A can be combined with material B (both A and B are classical layered materials) by intercalating or coating methods for enhancing the electroconductibility of host materials and preventing the collapse of materials.
Charge transfer pathways and coordination states in lattices could be optimized to promote fast and invertible ion (de)intercalation, enhancing the zinc storage performance in secondary batteries.The interlamination preintercalation strategy of guest ions (Li + , Na + , NH 4 + , Zn 2+ , Al 3+ , and conducting polymer) is regarded as an effective strategy to adjust the crystal architecture of host materials as well as promote the zinc-ion storage performance.The pillar effect of interlamination pre-interlacated ions could intensify structural stability and enlarge the interplanar spacing to promote ion diffusion and activate abundant active sites on most occasions.Furthermore, the guest ions could alter the coordination environment, adjust the bandgap, and change the route of ion diffusion, thus improving the zinc storage performance.Defects of crystalline materials could be divided into interstitial defects, higher-dimensional defects, atomic vacancies, and substitutional defects according to the defect formation mechanisms.Vacancy and doping manifest themselves in various types of defects in order to endow materials with significant and unique functionalities in the fields of electronics, magnetism, and optics.Defect strategies, as previously mentioned, can significantly improve the inherent problems of the layered structure by controlling the surface characteristics as well as electronic structures of materials.Defect engineering technology has achieved excellent advances in the modification of layered structures, resulting in enhanced electrochemical performance of materials.
Wu is currently an engineer at the Shandong Hi-Speed Engineering Test Co., Ltd., China.He received his bachelor's degree in 2017 from the Inner Mongolia University of Technology (IMUT), China, and his master's degree in 2020 from the School of Chemistry and Chemical Engineering, University of Jinan (UJN), China.His research mainly focused on the understanding mechanism of electrocatalysts and batteries, and functionalization of low-dimensional carbon materials.Xiaoxue Song obtained her BS degree from Shandong University, China, in 2008 and MS degree in Chemical Engineering from Sungkyunkwan University (SKKU), R. Korea, in 2012, and joined Chemistry Department at SKKU as a Ph.D. candidate in 2020.Her current research interest mainly focuses on computational design of novel functional nanomaterials and their applications in energy storage and conversion.Energy Environ.Mater.2024, 7, e12632

Jin
Yong Lee graduated and obtained Ph.D. (Chemistry) from POSTECH supervised by Prof. Kwang S. Kim in 1997.He worked with Prof. David Chandler in Berkeley as a postdoc and with Prof. Shaul Mukamel as a Research Associate.He moved to Sungkyunkwan University in 2005, where he was promoted to Professor in 2012.His research interest includes understanding molecular mechanism of photocatalysts, electrocatalysts, batteries, OLEDs, spin/electron transport, and excited-state dynamics.He received the Pople medal from APATCC in 2012 and was elected as an associate member of the Korean Academy of Science and Technology in 2015.Baotao Kang (born in 1986) is currently an associate professor in the School of Chemistry and chemical engineering at the University of Jinan (UJN), China.He received his BS degree (2008) from Shandong University, China, and Ph.D. degree (2013) in Chemistry in 2011 from Sungkyunkwan University (SKKU), R. Korea.After 2 years of postdoctoral research at SKKU, he started his academic career at UJN in 2015.His research field is computational chemistry and material, including fluorescence probe, fuel cell, battery, etc.He has published 60+ papers in international journals regarding functional molecules and materials.Energy Environ.Mater.2024, 7, e12632

Figure 1 .
Figure 1.a) A schematic illustration for extending the interlaminar spacing of V 2 O 5 .b) Schematic illustration of zerovalent metal intercalation into 2Dlayered crystals.c) Selected area electron diffraction (SAED) patterns of different metal-intercalated Bi 2 Se 3 crystals.Reproduced with permission.[20,22]Copyright a) 2020 John Wiley and Sons, b,c) 2021 John Wiley and Sons.

Figure 2 .
Figure 2. a) Crystal structures of orthorhombic α-V 2 O 5 and V 2 O 5 ÁnH 2 O. b) TEM image of the α-V 2 O 5 cathode after soaking in a 2 M ZnSO 4 aqueous electrolyte.c) Cycling performances of carbon, asprepared V 2 O 5 , and the soaked V 2 O 5 electrode.Reproduced with permission.[34,58]Copyright a) 2020 John Wiley and Sons, b, c) 2021 American Chemical Society.

Figure 3 .
Figure 3. a) Crystal structure of VOPO 4 Á2H 2 O (hydrogen atoms are not shown for clarity).b) VOPO 4 ÁxH 2 O degradation pathways in aqueous solutions.Voltage profiles and the associated differential capacity curves of the VOPO 4 ÁxH 2 O cathode cycled at 0.5 A g −1 in zinc cells with c) 3 M Zn(OTf) 2 and d) 13 M ZnCl 2 /0.8 M H 3 PO 4 aqueous electrolytes.e) Voltage profiles of the VOPO 4 ÁxH 2 O cathode in the 13 M ZnCl 2 /0.8 M H 3 PO 4 electrolyte.Reproduced with permission. [60,62]Copyright a) 2019 John Wiley and Sons, and b-e) 2019 John Wiley and Sons.

Figure 5 .
Figure 5. a) Side view of 2D MXene.b) SEM images of the cathode after 400 cycles in the coin-type battery at 10 A g −1 .c) Comparison of the discharge capacity for the three consecutive rounds.d) Rate performance of the ZIB for three consecutive rounds.e) Illustration of phase and structure transition of V 2 AlC cathode during the whole cycle at 5 A g −1 .Reproduced with permission.[69,75]Copyright a) 2019 Elsevier, and b-e) 2020 John Wiley and Sons.

Figure 8 .
Figure 8. a) Crystal structures of bulk MoS 2 and MoS 2 /graphene.b) Rate capability at 0.05-5 A g −1and c) long-term cycling stability at 1 A g −1 of MoS 2 /graphene.d) Schematic illustration of the fabrication of V 2 O 5 @PEDOT/CC.e) TEM image of the as-prepared V 2 O 5 @PEDOT nanosheet arrays.f) Rate capabilities of V 2 O 5 @PEDOT/CC.g) Cyclic performances of V 2 O 5 @PEDOT/CC at 5 A g −1 for 1000 cycles.Reproduced with permission.[81,82]Copyright a-c) 2021 John Wiley and Sons, and d-g) 2018 John Wiley and Sons.

Figure 9 .
Figure 9. a) TEM image and b) SEM image of the Al 0.2 V 2 O 5 sample.c) Schematic illustration of the working mechanism of the Al 0.2 V 2 O 5 /Zn battery.d) Rate performance of Al 0.2 V 2 O 5 at different current densities.e) SXRD pattern of the Zn 0.4 VOPO 4 Á0.8H 2 O single crystal along the b axis.f) Charge-discharge curves at a current density of 100 mAh g −1 .g) Long-term cycle stability of VOPO 4 Á2H 2 O and Zn 0.4 VOPO 4 Á0.8H 2 O cathodes at a current density of 100 mA g −1 .Reproduced with permission.[86,87]Copyright a-d) 2020 Elsevier, and e-g) 2020 American Chemical Society.
He et al. synthesized Zn 0.3 (NH 4 ) 0.3 V 4 O 10 Á0.91H 2 O with abundant cation vacancies by a two-step hydrothermal reaction; the structural illustration is given in Figure 13a.The pre-intercalated zinc ions, as well as structural water and residual NH 4 + , synergistically strengthened the stability Zn 0.3 (NH 4 ) 0.3 V 4 O 10 Á0.91H 2 O.The interlayer spacing of Zn 0.3 (NH 4 ) 0.3 V 4 O 10 Á0.91H 2 O was utilized by ion transport at high current density and low temperature, which could be

Table 1 .
The electrochemical properties of common layered materials for ZIBs.