Better engineering layered vanadium oxides for aqueous zinc‐ion batteries: Going beyond widening the interlayer spacing

Aqueous zinc‐ion batteries (ZIBs) are regarded as among the most promising candidates for large‐scale grid energy storage, owing to their high safety, low costs, and environmental friendliness. Over the past decade, vanadium oxides, which are exemplified by V2O5, have been widely developed as a class of cathode materials for ZIBs, where the relatively high theoretical capacity and structural stability are among the main considerations. However, there are considerable challenges in the construction of vanadium‐based ZIBs with high capacity, long lifespan, and excellent rate performance. Simple widenings of the interlayer spacing in the layered vanadium oxides by pre‐intercalations appear to have reached their limitations in improving the energy density and other key performance parameters of ZIBs, although various metal ions (Na+, Ca2+, and Al3+) and even organic cations/groups have been explored. Herein, we discuss the advances made more recently, and also the challenges faced by the high‐performance vanadium oxides (V2O5‐based) cathodes, where there are several strategies to improve their electrochemical performance ranging from the new structural designs down to sub‐nano‐scopic/molecular/atomic levels, including cation pre‐intercalation, structural water optimization, and defect engineering, to macroscopic structural modifications. The key principles for an optimal structural design of the V2O5‐based cathode materials for high energy density and fast‐charging aqueous ZIBs are examined, aiming at paving the way for developing energy storage designed for those large scales, high safety, and low‐cost systems.


| INTRODUCTION
2][3][4][5] As a key type of energy storage system, electrochemical batteries play an essential role in the huge field of energy storage and conversion.Among the existing and extensively developed battery systems, [6][7][8] lithium-ion batteries (LIBs) have attained overwhelming success in commercialization and large-scale production over the past more than two decades, owing to their high open-circuit voltage, generally good reliability in cycling, and relatively high energy density. 9,10However, the application of LIBs in large-scale energy storage is handicapped by several apparent drawbacks, for example, the poor safety and long-term nonsustainability of the key metals for both anode and cathode.On the one hand, the everrising demand-driven mining and utilization of limited lithium metal and transition metals reservoirs make the technology unsustainable in the long term. 11On the other hand, the employment of environmentally unfriendly liquid organic electrolytes and their disposals present high risks of hazards both during the in-use and post-use, thus raising strong concerns at all levels of fabrication, service, and recycling.4][25][26][27][28] Compared with organic electrolytes, aqueous electrolytes possess the superior characteristics of low cost, environmentally benign, nonflammability, and high ionic conductivity. 29Among the aqueous metal-ion batteries mentioned above, ZIBs have received considerable attention as one of the most promising candidates, especially for their potential in large-scale grid energy storage, where high safety and huge cost competitiveness are the key considerations.1][32] In addition, zinc element is earthabundant, the reservoir of which is about 300 times higher than lithium, thus possessing the apparent benefit of low-cost and long-term sustainability.Furthermore, Zn metal can be easily recycled with a low degree of toxicity, and it also meets the general requirement of high safety and environmental benignity.
As a key component in determining the final output energy density at the cell level, among the most widely investigated cathode materials for aqueous ZIBs are the Mn-based oxides and nonoxide materials, [33][34][35] V-based oxides and compounds, 2,36 Prussian blue analogs (PBAs), 37 and certain organic compounds.Although the Zn 2+ ions storage mechanisms in these cathode materials have been widely explored, it remains a big challenge if one is to look for an ideal cathode material with an appropriate balance among all the key electrochemical metrics, including high capacity, desired rate performance, good cycling stability, and so forth.][40][41][42][43][44][45][46] Among the notable studies, Xu et al. first employed α-MnO 2 as the cathode material for ZIBs, which was shown to possess an open-circuit voltage of about 1.5 V and a high capacity of 210 mA•h/g at 0.5 C. 47 However, all Mn-based oxides can easily undergo structure transformation, large volume change, structural degradation, and eventual collapse due to the partial dissolution into the electrolyte, and phase transition upon repeated charge/discharge processes, 48,49 thus resulting in poor cycle stability and rapid fading of capability.It is rather challenging to clarify the exact complex multiple storage mechanisms involved during the transformations along with further Zn 2+ ions intercalation. 50Following on, PBAs and organic compounds with open-framework structures were explored and were shown with much better cyclability due to the largely uniform ion diffusion tunnels and robust covalent bonds, but they show poor Zn 2+ ions storage ability with the overall capacity is much lower than 100 mA•h/g. 51In contrast, V-based materials, such as vanadium nitride (VN), vanadium oxides, and so forth, can offer a relatively good balance between capacity and cycling stability.While VN exhibits a structural instability and the poor conductivity may hinder its further development, V-based oxide cathodes, exemplified by those V 2 O 5 -based, show higher reversible capacities, outstanding rate performance, longer cyclic lifespans, and better reactivity than those of Mn-based oxides, PBAs, and other V-based materials, especially benefiting from higher structural stability and higher conductivity.For instance, the multivalent redox reactions of vanadium in V 2 O 5 enable a high theoretical capacity of 585 mA•h/g.In addition, these V-based oxides provide more opportunities for structural selfarrangement owing to the diversity of the V─O polyhedrons to accommodate inserted hydrated Zn 2+ ions in the process, and they will suffer a small volume change during the charging/discharging process, benefiting from the unoccupied nonbonding orbitals of vanadium, thus offering better cyclability. 31In several of the recent studies on V-based cathode materials, vanadium oxides, exemplified by V 2 O 5 , are among the most popular choices, which will be focused on in this review.Although the crystal structures of vanadium oxides vary considerably according to the different ratios of V and O, with or without other molecular components, they have considerable similarities in the open frameworks and Ocoordination.One of the typical open frameworks of orthorhombic V 2 O 5 with layered structure can be found in Figure 1B.
Although these V-based oxide compounds are regarded as a promising type of cathodes for aqueous ZIBs, they still face several challenges for large-scale practical applications, due to the unpredicted structural degradation and then eventual collapse after long cycles and the sluggish kinetics of Zn 2+ ions traveling between the restricted VO layers.Given the electrochemical performance of the electrode materials is largely dependent on their lattice/layer structures, surface states, and redox reactions, 52,53 several strategies have been explored, in attempts to improve the structural stability and optimize the local electron/charge distribution and electrochemical redox couples, to overcome the sluggish transport kinetics of Zn 2+ ions.In addition, the exact charge storage mechanism for some of these vanadium oxides in aqueous ZIBs is still under strong debate.For those guest-ion pre-intercalations in vanadium oxides, among the questions are exactly where these guest ions are going, and how the guest ions affect the VO layers, and what is the actual role(s) of lattice water.All these puzzlements require further and clear classification and explanation by further investigations.
To properly address the above remaining issues associated with vanadium oxides, when used as the cathodes in ZIBs, in this review, we will examine several approaches that have been taken more recently, including the manipulation of lattice defects at atomic levels, structure engineering of the interlayer spacing by guestion pre-intercalation, use of structural water, and structural designs at macroscopic levels.In particular, we will also provide new insights into the fundamental understandings of the pre-intercalated guest ions leading to the enhancement of electrochemical properties, and the key effects of structural water molecules in maintaining the layer-structural stability and effective Zn 2+ ions diffusion.Going beyond, new perspectives on the structural construction of vanadium oxides at lattice levels are presented for high-performance aqueous ZIBs.

| ISSUES REMAINING FOR VANADIUM OXIDE CATHODE
When used as the cathode material in ZIBs, V-based oxides show several advantages in terms of abundant resources, low cost, accessible redox couples, and high theoretical capacities.They also possess great capabilities for metal-ions storage, owing to their tunable layered structures and/or tunneled configurations.There are a large number of vanadium oxides varying in the V to O ratio, structure, and amounts of other ions and crystal water.Among them, V 2 O 5 is the best known and has been regarded as the basic representative of the layered structure for vanadium oxides.The detailed lattice structure of the basic vanadium oxide, V 2 O 5 (Pmmm space group) consists of the square bipyramids sharing corners and edges in the orthorhombic phase, as shown in Figure 1B, where the square pyramids are formed by coordinated O atoms around V atoms, forming vanadium pentoxide layers with symmetry. 54Among the VO 2 polymorphs with the same valence, there are phase transformations between the B-phase, A-phase, and Rphase, which have been observed, where for example the B-phase can transfer into A-phase when the VO 6 octahedra change from edge shared to face-shared, and the A -phase can further transfer into R-phase with half of VO 6 octahedra being reoriented.While for the mixedvalance vanadium oxides (those containing both V 5+ and V 4+ ), such as V 3 O 7 , V 4 O 9 , and V 6 O 13 , the structure tends to form a network in 3D with different coordination polyhedra (Figure 3A). 55,56n the past decade, the application of vanadium oxides as the cathode materials in ZIBs has been widely investigated, due to their excellent and tunable characteristics and promising electrochemical performance toward the high theoretical capacity.In aqueous ZIBs, Zn 2+ ions storage mechanisms are varied with different structures in vanadium oxides, which can be classified into four types: Zn 2+ ions insertion/deintercalation, 45 H + /Zn 2+ ions co-insertion, 57,58 H + insertion/extraction, 59 and the combination displacement/intercalation. 60In detail, a steady Zn 2+ ions insertion/extraction can be observed in the layered V 2 O 5 and also in metal-ion pillared hydrated V 2 O 5 with a reversible transformation.For H + /Zn 2+ ions co-insertion, a simultaneous intercalation/deintercalation of H + /Zn 2+ takes place in some metal vanadate with ZnSO 4 electrolyte and hydrated VO 2 with Zn(CF 3 SO 3 ) 2 electrolyte.In addition, the H + insertion mechanism in VO 2 is complicated with the formation of HVO 2 and the deposition of Zn 4 (OH) 6 -SO 4 ⋅5H 2 O.As for the combination displacement/intercalation mechanism, it occurs in some metal-ions preintercalated V 2 O 5 with the most pre-intercalated metal ions being replaced by Zn 2+ ions and reduced into metal deposited onto the cathode. 61It is noted that the mechanisms taking place in different host structures are also influenced by different electrolyte choices. 62lthough extensive works have demonstrated that vanadium oxides can deliver an enhanced performance of aqueous ion batteries, they still face several intrinsic drawbacks (Figure 1C), which handicap their further development toward large-scale applications.In association with the inherent chemical nature of vanadium oxides in an aqueous environment (exemplified by V 2 O 5 ), there is the tendency of dissolution of vanadium in the aqueous electrolyte, which is one of the main reasons causing the poor cycle stability of cathode for ZIBs.For example, Yang's group has claimed that the dissolution of vanadium-based oxide materials appears to be inescapable once the electrode is exposed to the aqueous electrolyte, because of the thermodynamic favorability toward the low lattice energy. 63A feasible strategy to suppress the undesired dissolution is to qualify the cathode with a chemically stable structure.Considering that the dissolution process takes place at the electrode/electrolyte and solid/liquid interface, manipulating electrolytes has been considered as a potential approach to control electrode dissolution.Indeed, Zhi's group has proposed applying a hydrogel as the electrolyte in ZIBs, 64 where the stability of the V 2 O 5 cathode could be remarkably improved, upon altering the solid/liquid to solid/semi-solid interface.For instance, there has been a steady development of electrolytes focusing on the concepts of the "water-in-salt" electrolytes and solid-state electrolytes, which shall lead to an improvement in the electrochemical stability of vanadium oxides over those aqueous electrolytes.Nevertheless, it is rather challenging to develop a class of truly satisfactory solidstate electrolytes for Zn 2+ ion batteries. 65,66he dissolution of vanadium-based oxide materials can be further accelerated by a slightly acidic situation.Aside from the original H + concentration in the electrolytes, there is the released H + during the formation of byproducts, which can also present a further serious issue for the cathode dissolution, resulting in a fast capacity reduction.Although some strategies have been attempted, such as the use of a concentrated solution or adding vanadium salt to the electrolyte, the rise in manufacturing cost is among the other concerns.
More recently, there have been several alternative strategies aiming to enhance the structural stability of vanadium oxides, including structural water introduction, metal ions pre-intercalation, and defects engineering, some of which have been reported very recently in the past couple of years.They will be discussed in detail in the following section.
A further challenge with vanadium oxides, when used as the cathode in ZIBs, is the relatively low electrical conductivity in association with the small polaron transport process, which leads to the slow kinetics of Zn 2+ ions during the insertion/extraction process.Taking V 2 O 5 , the most representative vanadium oxide as an example, the small polarons are created by the V 4+ valence state, where the concentration can be increased by charge injection.A promising pathway to achieve a high charge injection is through metal-ion pre-intercalation, which can effectively raise the number of polarons, resulting in higher conductivity.For instance, Wei et al. have observed that the electrical conductivity of V 2 O 5 can be enhanced by up to 100 folds by intercalating with an appropriate level of Cu ions, demonstrating the feasibility of improving conductivity through certain ion pre-intercalations. 67Several other approaches have also been explored to improve the conductivity of vanadium oxides, for example, by combining vanadium oxides with conductive polymers, carbon-based materials, or even MXenes by the construction of heterostructures.
An additional issue with vanadium oxides being used as the cathodes in ZIBs is the low operating voltage, which makes the electrochemical performance largely not so satisfactory, especially with the limited energy density.Typically, the capacity above 1.0 V is less than 70 mA•h/g; thus, over 80% of the capacity is delivered below 1.0 V, leading to an overall low energy density of <250 W•h/kg.Several approaches have been taken to address the issue of low operating voltage.For example, Ma et al. demonstrated ZIBs of high-voltage output and long-lifespan by using the Co 0.247 V 2 O 5 ⋅0.944H 2 O cathode, implying that an appropriate ion-pre-intercalation can not only improve the conductivity but also raise the operating voltage. 57Besides, in the past decade, considerable efforts were made with both understanding and manipulation of the interlayer spacing between VO layers in vanadium oxides, as the limited interlayer spacing appears to be a key parameter in restricting the diffusion kinetics and storage of Zn 2+ ions.Compared with Li + ions, Zn 2+ ions possess a much larger ionic radius, which raises the difficulty for the large ions to efficiently diffuse between the VO layers.More difficult is the multicoordinated hydrated Zn 2+ ions, which are even larger in size (~5.50Å), and thus with much lower kinetics.Due to the strong electrostatic interactions between the hydrated Zn 2+ ions and O in the VO layers, the diffusion of hydrated Zn 2+ ions in the cathode lattice is further impeded, and the hydrated Zn 2+ ions tend to be trapped in the lattice structure, leading to low reversibility.As a result, the layered structure of vanadium oxides will gradually degrade and then collapse, with a steady increase in the trapped hydrated Zn 2+ ions during the repeated cycling.

| STRATEGIES TO IMPROVE PERFORMANCE
As has been mentioned above, the functioning of vanadium oxide as cathodes in ZIBs faces several challenges of obstacles in relation to their inherent chemical characteristics, lattice and layer structures, as well as the electrostatic interactions with Zn 2+ ions.Herein, several approaches have been designed to manipulate the vanadium oxides at varying structural levels, going beyond the simple widening of the interlayer spacing, which will be summarized into four aspects in this review, including pre-intercalation by guest ions, optimization in structural water, defect engineering, and macroscopic structure construction (Figure 2).An appropriate regulation in structure design at either atomic, micro/macro levels, or both can lead to a significant improvement in the electrochemical performance of ZIBs, which also inspires further tuning of vanadium oxides as a class of cathodes in other metal-ion batteries.

| Interlayer manipulation by guestion pre-intercalation
In general, vanadium oxides, such as V 2 O 5 , exhibit an open-framework structure giving rise to the ability for metal ions to transfer into/out and be stored, while their structural stability is generally poor together with a restricted interlayer or tunnel spacing.The relatively weak Van der Waals forces between the VO layers can be easily broken, making it much easier to be damaged during the intercalation/deintercalation process of the hydrated Zn 2+ ions.The introduction of guest metal-ions pillars, such as Na + , 68 Cu 2+ , 69 and Ca 2+ , 70 into the interlayers of vanadium oxides, offers an effective pathway to enhance the overall structural stability. 71However, upon the introduction of these metal ions between the V 2 O 5 layers, the crystal structure can show different conversions, depending on the type(s) and amounts of intercalated metal ions. 51Metal guest ions tend to locate at particular sites in the host structures, as shown in Figure 3B, where the sites with symmetrical distances to F I G U R E 2 Summary of structural engineering strategies for vanadium oxides, where V 2 O 5 is exemplified, ranging from the atomic scales to the macroscopic level, and their effects on the structures and performance.
F I G U R E 3 (A) Different vanadium coordination polyhedra.Reproduced with permission. 42Copyright 2021, Royal Society of Chemistry.(B) Crystal structures and insertion sites in the most stable V 2 O 5 and VO 2 phases: α-V 2 O 5 , β-V 2 O 5 , VO 2 (R), and VO 2 (B).The V, O, and M (M = Li, Na, Mg, Al) atoms are shown in cyan, red, and green colors, respectively.Reproduced with permission: Copyright 2017, Royal Society of Chemistry. 72(C) Schematic illustration of the reaction mechanism of Ca 0.23 V 2 O 5 ⋅0.95H 2 O with the (de)intercalation of major Zn 2+ ions and minor H + .Reproduced with permission: Copyright 2019, American Chemical Society. 73ridge oxygens and vanadyl oxygens in V 2 O 5 are preferred, while those octahedral sites are energetically favored in VO 2 (R) phase. 72

| Single-metal-cation pre-intercalation
Several monovalent alkali metal ions have been applied as the pre-intercalation ions, which can offer an improvement in electrical conductivity and can also help stabilize the layered structure-indigenous ions acting as the pillars between the V─O layers.For example, previous research has indicated that Li + and Na + insertion generally results in a small distortion to the occupied sites between layers in α-V 2 O 5 . 72Monovalent ions with lower electronic density would not result in strong electrostatic interactions with the host structure, and they also enable a higher Zn 2+ ions storage ability by partially replacing monovalent guest ions with Zn 2+ ions.For example, He et al. have reported that preintercalation of Na + ions could change the layered structure into a more stable 3D tunnel structure, where the intercalated Na + ions acted as pillars to buffer the structure expansion upon hydrated Zn 2+ ions insertion, 68 giving rise to a high degree of reversibility upon cycling.However, too much pre-intercalation of Na + ions could also limit the number of inserted Zn 2+ ions, because they would occupy some of the active sites or tunnels that are intended for the incoming Zn 2+ ions. 51ifferent from the monovalent cations that possess lower molecular weights benefiting higher gravimetric capacity, multivalent cations have stronger interactions with the host layer structure than monovalent cations, resulting in more stable structures for vanadium oxides.In such cases, transition metal ions are popular as the guest ions, where the reported examples are Mn 2+ , Cu 2+ , and Co 2+ .Among these transitional metal ions studied, Cu 2+ was reported as an excellent choice for the V 2 O 5 cathode.The Cu 2+ -intercalated cathode could deliver an ultrastable cyclic performance with 88% capacity retention after 10,000 cycles at 10 A/g.The admirable electrochemical performance benefits from the enlarged and stabilized lattice spacing (11.45 Å in Cu 2+intercalated cathode, compared with 10.32 Å in the initial cathode), and the enhanced electrical conductivity. 69Ca 2+ with a larger ionic radius of 0.99 Å, when inserted into V 2 O 5 , has also been demonstrated to expand the interlayer spacing in V 2 O 5 (Figure 3C), accelerating the Zn 2+ ions diffusion between the V-oxide layers. 74For example, it was reported that Ca 2+ ions were successfully inserted into layered V 2 O 5 forming Ca 0.23 V 2 O 5 ⋅0.95H 2 O nanobelts, which could effectively expand the interlayer spacing to 13.0 Å and enabling a high capacity retention of 97.7% upon 2000 cycles at 5 A/g. 75Coincidentally, Ca 0.67 V 8 O 20 ⋅3.5H 2 O nanobelts were prepared with different quantities of guest Ca 2+ cations being inserted, showing that a lower capacity retention rate was exhibited with a lower quantity of guest Ca 2+ , and the Ca 2+ had benefited the stabilization of lattice structure by acting as ion pillars and accelerating the reversible fast Zn 2+ ions insertion/extraction. 73 In addition, the doublelayered V 2 O 5 pillared by Ni 2+ ions could also stabilize the structure and give rise to faster Zn 2+ ions intercalation/ deintercalation with more apical oxygen ions. 69he pre-intercalation by higher valance state ions, such as Al 3+ (0.53 Å) and Sn 4+ (0.69 Å), has also been reported to work as the pillars to enhance structural stability.For instance, Luo et al. first applied the Al 3+ pre-intercalation strategy in V 2 O 5 as the cathode for ZIBs.Al 3+ ions were shown to exhibit larger binding energy than those of other common metal cations, and the more positive charges induced much stronger interactions with negative-charge oxygen ions in the host structure. 76herefore, the thus-prepared Al x V 2 O 5 cathode shows an enlarged degree of reversibility by preventing "dead Zn 2+ sites," and delivers a cycling stability of 61.4% capacity retention over 5000 cycles at 5 A/g.It has also been suggested that the intercalated Al 3+ ions could occupy the interstitial sites of V 2 O 5 , and decrease its crystallinity, so it would be essential to adjust the population of guest ions to get overall better properties and performances.To the best of our knowledge, Sn 4+ has not been properly studied as the guest cation in vanadium oxide cathodes for ZIBs.Nonetheless, there have been reports of attempts in the preparation of pyrovanadate and MnO 2 for ZIBs. 77,78In particular, the intercalation of Sn 4+ ions into pyrovanadate introduces Sn─O─V bonding, benefiting better mechanical stability and higher Zn 2+ ions migration kinetics as a result of the charge screening effect, which has inspired the modification of vanadium oxides and other types of cathode materials.By a preliminary summary, cathodes with varied guest ions intercalated and electrolytes will undergo different storage mechanisms, where guest ions can well show different behaviors in terms of being stable in the host structures or coming out/in during the repeated cycling.In some cases, for example, with a combination displacement/deposition mechanism, such as in Agintercalated vanadium oxides, guest ions can be replaced by Zn 2+ ions during the discharging, and then be reduced to metal deposited on the cathode surface.In some cathodes with reversible Zn 2+ ions insertion/extraction, it has been observed that the guest ions are able to stay in the host structures stably.Furthermore, metal ions with varying molecular weights and numbers of charge tend to perform differently in affecting electronic structures, mechanical properties, and thus the electrochemical performance.It should be noted that the use of suitable metal cations can increase the number of active sites, while there is still a general lack of systematic studies on the comprehensive understanding as to how to verify the best choice of metal cations because each of them exhibits one set of its characteristics in terms of the interactions with the layered vanadium oxides.It would be of considerable value to clarify the electrochemical behaviors of guest ions with different electrolytes, which will help clarify the still foggy issues.Recently, the dual/ multications intercalation approach has been taken as a newly emerging pathway to better satisfy the broad needs of improving the overall performance of ZIBs.Therefore, manipulating the interlayer spacing of V 2 O 5 with stable dual/multiguest ions architecture would be explored further, although the simple widening of the interlayer spacing by one particular type of metal ions appears to have reached its threshold of limitations.Nevertheless, the rather limited number of reports conducted so far have demonstrated that the multi-ion co-intercalation could be practicably achieved by balancing the coordination and impacts raised by the different ions, which will be discussed in the following section.

| Multi-ion co-intercalation
As discussed above, there have been extensive research works focusing on the introduction of a single type cation to tune and stabilize the vanadium oxide lattice(s), such as V 2 O 5 .For instance, the different metal ions inserted into the vanadium oxide affects the VO frameworks differently, thus attributing differing effects on the overall electrochemical properties.Therefore, performing a strategy of multi-ions co-insertion is expected to generate coordinative/synergistic benefits.In 2019, for example, Huang et al. first demonstrated an ultrastable cathode V 3 O 8 co-intercalated by both Na + and Ca 2+ , where these two types of cations functioned as the alternating structural stabilizing "pillars" and "binder," and together lead to excellent cyclic stability. 79This coinsertion strategy was also applied to V 2 O 5 in the year that followed.Du et al. 41 developed both monovalent and divalent cations co-preinserted V 8 O 20 nanobelts as the cathode in ZIBs.They found that Mn ions could change the electronic structure of NaV 8 O 20 •nH 2 O (NVO) by strengthening the chemical bonds, accounting for the outstanding performance of (Na, Mn)V 8 O 20 •nH 2 O thus observed.The structure with intercalated Mn 2+ ions results in an increased electron density near the Fermi level as shown in density functional theory (DFT) results in Figure 4A, and offers some well-organized tunnels for Zn 2+ ions transport, suggesting a higher electrical conductivity to boost the electrochemical performance of Mn-doped NVO.Undisputedly, Na + ions are also an integral part to enhance the performance, where the energy barrier for Zn 2+ ions migration is lowered (Figure 4B).Benefiting from both Mn 2+ and Na + , (Na, Mn)V 8 O 20 •nH 2 O gave rise to a high capacity of 377 mA•h/g at 0.1 A/g, and a capacity retention of 88% after 1000 cycles at 4 A/g, while NVO displayed a worse capacity retention with some Zn 2+ ions trapped into the host lattice after the first cycle, which was confirmed by the existence of characteristic peaks of intercalated and absorbed Zn 2+ ions at the fully charged state in highresolution X-ray photoelectron spectroscopy results (Figure 4C).However, the different roles of these two metal cations in improving the electrochemical performance of vanadium-based materials still need to be further studied.For example, there is a possibility that one type of metal ions is performing to widen the interlayer spacing to enhance capability, while the other type tends to bring higher cyclic stability with strengthened spacing.Inspired by the new strategy of co-insertion using two types of metal ions to improve the performance, Zhang et al. studied an aqueous ZIB employing a bimetallic ion intercalation hydrated vanadium oxide (VOH) by onevalent metal K + and divalent alkaline earth metal Mg 2+ (KMgVOH) fabricated by a one-step hydrothermal method, which is shown in Figure 4D. 80The different effects of K + and Mg 2+ ions in improving the electrochemical performances were investigated in detail.They observed that the interlayer spacing of VOH was increased by the presence of Mg 2+ ions, resulting in an expanded ion transport channel and thus improved specific capacity.With the intercalation of K + ions, the Zn 2+ ions diffusion process was accelerated to a great extent.In Figure 4E, KMgVOH electrode displayed the smallest slope, which indicated that it would offer a faster Zn 2+ ions diffusion rate than the other three electrodes, leading to better electrochemical performance.Meanwhile, K─O bonds were formed between the V─O layers, which were made closer, leading to a degree of stabilization in the structure and a high reversibility observed.In addition, as shown in Figure 4F, cyclic voltammetry (CV) curves of KMgVOH maintain similar shapes under different scan rates, with only slightly shifted redox peaks being observed.Apart from good cycle stability, the Zn//KMgVOH cell appears to possess good kinetics contributed by both ionic diffusion and surface capacitance, which would largely determine the electrochemical performance, as suggested by the relationship between the current and voltage (the slope b between log(i) and log(v) of peaks a, b, c, and d) obtained through CV test.A similar piece of research work on δ-(Ni, Ca)V 2 O 5 @C also demonstrated that the electrochemical performance of VOH was greatly improved due to the synergistic effects of these two metal ions. 81Moreover, a metal ion with a large ionic radius has proven to form strong M─O bonds, which is favorable to the electrochemical performance of the intercalated vanadium oxide materials.Ni 2+ , Ca 2+ , and Mn 2+ exhibit the ionic radius of 0.072, 0.100, and 0.082 nm, respectively, while Cs + possesses an even much larger ionic radius (0.167 nm) and lower electronegativity. 82Considering these factors, Na 0.33 Cs 0.03 V 2 O 5 has been synthesized and shown excellent long-term cycling stability.The interlayer spacing between VO layers was expanded by Cs + ions of large ionic radius, allowing for fast Zn 2+ ions diffusion.The insertion energy of Zn 2+ ions was accordingly reduced, accompanied by an increase in the population of surface oxygen defects in Na 0.33 Cs 0.03 V 2 O 5 .The accommodation of Na + ions as pillars is in favor of forming the two-dimensional tunnel structure of vanadium oxide nanowires, composed of VO layers connected by the corner-sharing oxygen atoms of ladder chains.As a positive indication of the electrochemical performance, Na 0.33 Cs 0.03 V 2 O 5 demonstrated a reversible phase change during the cycling process after a conversion reaction in the initial discharge process. 83o date, several pieces of research work have suggested that it is feasible to intercalate two different types of metal ions at the same time through a one-step process, for example, intercalation by hydrothermal method.Nevertheless, one of the key unresolved debates is exactly how the charge storage of the guest-ion reconstructed vanadium oxides will change with an increase in the interplanar spacing, which is indeed not well understood.Furthermore, it is still of challenge to precisely control the proportion of the co-intercalated ions to aim for the best performance.It is common to adjust the ratio of different guest ions through the concentration regulation in a mix salt solution, with a set of concentration gradients, which can lead to relatively better performance in a certain ratio range.There is feasibility to employ computational methods to aim for the best-performance points by giving theoretical guidance to some degree to incorporate multiple guest ions, although the ratio may not be accurately controllable in reality because it may be also influenced by the reaction time and other parameters.Moreover, to better realize the intercalation behavior and the effects of incorporation ions, it would be necessary to conduct thorough studies on the exact preferred sites for one or multiple types of the intercalated ions in the different host structures with the combination of experiments, simulation, and advanced characterization techniques.As intercalation is a site-selective process based on charge density and energy, computational simulation is an effective avenue to predict the preferred interaction sites with the local minimal energy states among the competitions of multiple guest ions.With the development of advanced characterization techniques, it would be possible to confirm the precise positions of multiple incorporated cations through atomic-resolution mapping. 85While in the present stage, the deep study of intercalation chemistry is hindered by the lack of advanced techniques with automated detecting and real-time tracking of precise ionic position at the atomic levels.In addition to adding two or more together, an alternative would be to introduce the dual metal ions in sequence at the cathode processing stage and to further manipulate the electrochemical behavior.Very recently, for example, Wang's group 81 has pioneered a completely new "dual-ion-in-sequence" intercalation approach, based on rapid quenching from high temperature, where the two different types of metal ions are introduced in a purposely designed sequence.The novel research work enables a continuous manipulation of the d-spacing of the V 2 O 5 layered structure.With a steady expansion in interlayer spacing of the V 2 O 5 layered structure, the Zn 2+ ions storage mechanism showed an interesting transformation from the solid-state diffusion to the intercalation pseudocapacitance behavior.Due to the much-raised diffusion kinetics from the intercalation pseudocapacitance and the enhanced electrical conductivity arising from the structural defects, the dual-ion reconstructed Li@MnVO cathode by the in-sequence approach shows a superior reversible capacity and remarkable cycling stability, together with high rate capability.
In addition to metal ions, organic cations also have been widely intercalated into various layered materials to regulate the structure and performance of host materials.Indeed, some of these organic cations have the potential to effectively manipulate the interlayer spacing of vanadium oxides, [86][87][88] where the interlayer spacing can well be controlled by changing the length/dimensions of the organic cationic groups (Figure 4G).They generally possess larger ionic radii/molecular dimensions than those of metal ions, which can effectively reduce the electrostatic interactions between Zn 2+ ions and the VO frameworks, thus efficiently promoting the Zn 2+ ions diffusion kinetics and providing more active sites for charge accommodation.For example, Liu and coauthors reported a co-preinserted vanadium oxide ([N(CH 3 ) 4 ] 0.77 Zn 0.23 )V 8 O 20 •3.8H 2 O cathode, by both organic cations and Zn 2+ ions.The [N(CH 3 ) 4 ] + preinsertion enlarged the interlayer spacing and served as pillars. 89Both Zn─O and Zn─C bonds have formed upon Zn 2+ ions intercalation during the discharge process.The interactions between Zn 2+ ions and C are actually weaker than that between Zn 2+ ions and O, promoting an enhanced reversibility of the Zn 2+ ions intercalation/ (de)intercalation and improving the structural stability.It delivered an impressive discharge capacity of 181 mA•h/g at 8 A/g with 99.5% capacity retention after 2000 cycles.With similar considerations, there was a study involving both iron ions and alkylammonium cations in the cointercalated strategy. 84Such co-intercalated vanadium oxide cathode was shown to exhibit a high reversible specific capacity of 408 mA•h/g at 0.1 A/g and excellent cycling stability.Interestingly, as shown in Figure 4H, alkylammonium cations were found to help the FeVO-12 electrode to become more hydrophobic with a contact angle of 156.7°larger than that of FeVO (54.4°), which would effectively inhibit the dissolution of vanadium.The co-intercalated strategy of iron ions and alkylammonium cations helps clarify the functions of different types of guest ions.Indeed, the metal ions and organic cations appear to act concurrently, for instance, resulting in an improvement in capacity through partially reducing V 5+ into V 4+ and/or introducing some oxygen vacancies to keep electroneutrality.However, they also show differences in functionality when acting as the intercalated ions.By the pre-intercalation of metal cations, the electronic conductivity would be enhanced intrinsically, and the layered crystal structure is also stabilized, and the vanadium dissolution is suppressed inherently with metal ions contributing to strong metal-oxygen interaction.Organic cations would help reduce the interaction between Zn 2+ ions and VO frameworks not only through the expanding of the interlayer spacing but also supplying C atoms to replace some Zn─O interactions with Zn─C interactions.Moreover, the organic cations will enhance the chemical stability of cathodes with the reduction of vanadium dissolution by raising the hydrophobicity of active materials. 84Although there has been less number of studies reported on the dualcation pre-intercalations, compared with those with single-cation intercalations, 81 there is no doubt that they have led to impressive electrochemical performances of ZIBs, by properly tuning the dual-cation insertion.Of course, some in-depth understanding is still needed, to clarify the exact mechanisms involved in the process.For example, what would be the optimal combination of the two different ion types, if one is to choose two different types of cations, and what would be the general principles to choose them, as well as the mechanism behind the much-improved performance?
While there have been considerable efforts to widen the interlayer spacing in the V 2 O 5 -based cathodes, by various pre-intercalation approaches, it is of considerable interest to examine the relationships between the interlayer spacing and specific capacity.As shown in Figure 5 and Table 1, the enlarged interlayer spacing can increase the attainable specific capacity, but there is an indication of approaching the saturation, which means that there is a limit in the achievable energy density by simply widening the interlayer spacing V 2 O 5 -based layer structures.More interestingly, it is noted that the interlayer spacing versus capacity relation possesses a relatively concentrated distribution by pre-intercalations with similar types of guest ions, which exhibit similar electronic structures, electronegativities, and ionic radii.The interlayer spacing of the vanadium oxides can be enlarged to the next high level with the intercalation of large organic cations (~1.2 nm), while there is no noticeable boost in the capacity of polymer-intercalated V 2 O 5 cathodes, where the large polymer molecules may occupy the transport tunnels and block some of the storage sites of Zn 2+ ions.A further interesting set of statistical results show that there is an almost monotonic increase by pre-intercalation with selected transition metal ions, within a certain range.Although the intrinsic mechanism for such an interesting phenomenon is not fully understood, the pre-intercalation with transition metal ions appears to give a degree of controllability in the structure design of vanadium oxides.However, an excess level of pre-intercalated ions does not lead to any further increase in the energy density, which can well be F I G U R E 5 Summary of the specific capacity based on the interlayer spacing based on different ions/molecules pre-intercalation, the data were collected at the current density of 100 mA/g.due to the other parameters involved.Therefore, a suitable amount of intercalated metal ions can keep the relatively large interlayer spacing, high specific capacity, as well as long-term cycle stability.
In general, an expanded ionic transport channel in the layered structures of vanadium oxides by introducing guest cations is an effective pathway to improve the overall electrochemical performance of ZIBs.Furthermore, the deeper reason for guest ions to optimize electrochemical performance is the reconstruction of the electronic structure of cathode materials resulting in the intrinsically enhanced conductivity to boost the electron/ charge transfers and reach high rate capability.Moreover, the existence of some guest ions may cause more newly exposed active O sites in VO layer due to electroneutrality, which will improve the Zn 2+ storage capacity.With an appropriate level of cation pre-intercalation, the interlayer spacing can be broadened and Zn 2+ ions diffusion kinetics can thus be accelerated.Another impact is to stabilize the layer structures of vanadium oxides with the formation of strong metal-oxygen bonds, by the intercalating metal cations or the presence of organic protective layers as a result of incorporating the organic cations, which are significant for effectively restraining vanadium dissolution and endowing the cathode with excellent cyclic stability.However, there are still some unsettled issues and challenges in association with the guest-ion preintercalation, for example, (i) there can be two possible locations for the guest ions in the layered structures: those inserted between the interlayers of VO and those doped within VO frameworks by substituting V atoms.Although several reports have claimed that the guest ions largely exist between VO layers, they still need to be confirmed by giving more intuitive experimental evidence through more advanced characterizations.(ii) Whether the guest ions can exist in the host materials leads to a high stability, depending on the type and amount of the guest ions.
Although several pieces of research works have claimed that the guest ions could stabilize the VO layered structures, for example, by acting as a "pillar" or "binders."However, it is not clear whether these guest ions would be able to deintercalate during the repeated Zn 2+ ions insertion/deinsertion or even interact with the host framework.(iii) It is important to balance the dynamic process of ionic transportation/storage and the structural stability of the cathode by adding a suitable type and an amount of guest ions, where they can vary considerably as has been shown by a number of previous studies.(iv) The introduction of guest ions will apparently make the structure of materials more complex, which will in turn affect the Zn 2+ ions transport and storage mechanism.Consequently, it would be crucial to elucidate the exact roles of each type of the guest ions and their effects on the insertion/deinsertion or reaction mechanism, where there can be synergistic and/or complementary effects, which are not well documented.(v) More efforts are expected to comprehensively understand the different electrochemical behaviors brought by inserting multiple guest ions at the same time or in sequence.Indeed, for the same two/multi-ion combination, there are differences in the overall performance, between the "adding-in at the same time" and the "adding-insequence."In this connection, the materials preparation methods need to be further developed, on top of the very few that have been reported, such as by hydrothermal synthesis, that can differentiate the different inserting sequences.

| Controls of structural water
In addition to guest-ion pre-intercalations discussed above, structural H 2 O molecules are also among the key species involved in structural changes and tuning the performance of vanadium oxides, where the involvement of water molecules is normally inescapable in the hydrothermal synthesis process, and when the aqueous electrolytes are used.Structural water molecules deliver an intuitionistic effect on the regulation of host structures mainly by expanding the interlayer spacing and changing the overall morphology.Intercalated water molecules can effectively increase the interlayer spacing of layered vanadium oxides, which is an essential pathway to reduce the interactions between Zn water molecules, while the d-spacing of VOG-350 then shrank obviously.Having conducted the structural studies, they concluded that the interlayer spacing had increased upon Zn 2+ ions intercalation, which was different from what was indicated by the XRD phase analysis results of Zn 0.25 V 2 O 5 under the operando conditions.The structural water molecules can largely accelerate Zn 2+ ions diffusion, by the admirable "lubricating" effect of structural water.The existence of structural water molecules could also effectively reduce the electrostatic interactions between Zn ions and the host oxide framework.As a result, a diffusion coefficient of three times higher for Zn 2+ ions was achieved together with a high degree of reversibility.Specifically, the VOG provided a 71% capacity retention after 900 cycles, and a much superior rate performance compared with that of VOG-350.It could give a discharge capacity of 248 mA•h/g even at the high rate of 30 A/g.The hydration strategy by water molecules is also applicable to other vanadium oxides and vanadates.For example, Zhang et al. 40  As has been demonstrated by a couple of previous studies, [117][118][119] thermal treatment strongly impacts the structure and electrochemical properties of V 2 O 5 •nH 2 O, when applied in LIBs.Similarly, an optimum water content is a key parameter leading to an improvement in the electrochemical performance of hydrated V 2 O 5 , while the introduction of water molecules does not work in "the more the better" fashion.For example, too much structural water can well cause a set of unsatisfyingly weak interlayer forces and result in the structural collapse of active materials and thus poor behavior in Zn 2+ ions storage.Recently, for example, Sun et al. 115 purposely controlled different heating temperatures to obtain a series of hydrated VOH xerogels with varying water contents (n) for ZIBs.The interlayer spacing of V 2 O 5 •nH 2 O could well be modulated by changing the content of structural water.Moreover, the amount of the intercalated solvent water and the degree of V-dissolution were also largely determined by the initial value of "n" in V 2 O 5 •nH 2 O materials.Their studies show that, when n equals 0.26, the V 2 O 5 •nH 2 O electrode exhibits the best electrochemical performances with a specific capacity of 456.5 mA•h/g at the current density of 0.1 A/g and a cycling retention of more than 94% retention over 2000 cycles at 3 A/g.It was also found that the structural water molecules could promote the desolvation process of [Zn(H 2 O) 6 ] 2+ at the electrolyte/ electrode interface, which tends to lower the energy barrier and boost the insertion and deinsertion kinetics of multihydrated Zn 2+ ions. 120On the other hand, the structural water also serves as a "modulator" to interact with the intercalated Zn 2+ ions and weaken the interactions with the VO bilayers in vanadium oxides, allowing for fast Zn 2+ ions diffusion and promising admirable structural stability, which can be proved by the reversible peak shifts in ex situ XRD result during a discharge/charge cycle (Figure 6F,G).Besides, the Zn 2+ ions would bond with skeleton O and structural water O upon forming [Zn(H 2 O) 6 ] 2+ , which could lower the electron/charge accumulation and avoid the repulsion caused by charge concentration.Therefore, an optimal content of structural water in the layer structure of vanadium oxides can provide a smooth diffusion channel for Zn 2+ ions, leading to much enhanced electrochemical performance.
In general, the effects of structural water molecules can be concluded into these main aspects: (i) enlarge the interlayer spacing, where the crystal water molecules act as pillars to widen the lattice spacing providing a larger diffusion tunnel leading to fastened Zn 2+ ions transport kinetics; (ii) shield the electrostatic process, where the structural water molecules effectively reduce the electrostatic interactions between Zn 2+ ions and the VO framework, further accelerating Zn 2+ ions diffusion; (iii) promote the compatibility of the electrode and aqueous electrolyte, where water molecules can reduce the energy barrier for Zn 2+ ions transport across the interface between the electrode and electrolyte.Nevertheless, too higher a crystal water content would expand the interlayer spacing of vanadium oxides to an unsatisfactory degree, which may result in the structural collapse of active materials, thus degrading the structural stability and leading to poor Zn 2+ ions storage.Moreover, although the existing studies have shown structural water is a good way to guarantee smooth Zn 2+ intercalation/deintercalation and to improve cycling stability, there is no evidence indicating water molecules can protect vanadium pieces from dissolution.Besides, concerning the Zn 2+ ions storage capability, whether the water molecules will occupy active sites of Zn 2+ storage is still a question to be answered.Furthermore, although some previous reports have adapted controlledthermal treatment to adjust the content of structural water molecules, there is still a general lack of understanding on what would be the most suitable content of crystal water that can deliver admirable performance to manifest a precise and effective control, where the amount of lattice water molecules introduced shall not be a blurry value obtained by few controlled experiments.Hence, it would be necessary to clarify where is the exact balance between the structure stability and ion kinetics, and also the reliable methods to obtain the preferred amount of water molecules.In addition, the stability of structural water also requires to be determined preciously.For example, on the basis of TG results, some studies have claimed that structural water is lost at about 340 °C, while some others claimed water loss at just over 100 °C, as shown in Figure 6H.The overall electrochemical performance is determined by several factors, where the presence of crystal water is just a factor in stabilizing the structure and an important contributor to enhancing Zn 2+ ions diffusion and thus the overall electrochemical process of ZIBs.

| Defect engineering
In connection with the poor conductivity and limited understanding of the active sites, both the types and numbers, one of the key issues is the poor cycling stability of V 2 O 5 (which also applies to other layered vanadium oxides), when used as the cathode in ZIBs.As has been shown by previous studies on several electrode materials explored for LIBs, introducing appropriate defects into the cathode materials can be an effective approach to boost the electrical conductivity and electrochemical activity by manipulating the localized electrons distribution and active sites, as shown in Figure 7A,C.Similar effects can be achieved in vanadium oxides by inducing defects at atomic scales, such as oxygen anion defects and vanadium cation defects.Indeed, several previous studies have shown that an oxygen-deficient structure can introduce tunnel type (c-axis) instead of the regular Zn 2+ ions diffusion in the ab-plane (Figure 7B,D), 121,122 which effectively reduces the interactions between the Zn 2+ ions and the host structure frameworks, and promotes fast insertion/ extraction kinetics and thus high reversibility of ZIBs.Meanwhile, the introduction of oxygen vacancies can also be beneficial to raising the population of active sites and suppressing the undesired phase transitions, allowing for high utilization of capacity for Zn 2+ ions storage and improved Zn 2+ ions diffusion/extraction kinetics.As has been demonstrated by Liao et al., 121 oxygen-deficient vanadium oxides show an enhanced specific capacity and improved stability.In their studies, oxygen defects were introduced by the solution-redox-based self-assembly process in a reducing atmosphere.The oxygen defects are expected to form delocalized electron clouds and active sites with spare electrons, which would be more attractive intercalation sites for Zn 2+ ions, thus achieving a higher capacity.For example, a V 6 O 13 cathode with oxygen defects was successfully synthesized, showing a high capacity and excellent rate capability. 121The oxygen-deficient cathode could avoid the formation of strong Zn─O bonds upon Zn 2+ ions intercalation and enable high reversibility, where the strong interactions between Zn 2+ ions and oxygen ions would restrain Zn 2+ ions from being released during the charging process.According to theoretical calculations, the introduction of oxygen defects results in a lower free energy for Zn 2+ ions intercalation and the formation of delocalized electron clouds, promoting a larger capacity of ZIBs.For example, Zhang et al. 42 prepared an oxygen-deficient hydrated vanadium dioxide cathode with polypyrrole coating (O d -HVO@PPy) through a simple hydrothermal process for aqueous ZIBs with 3 M Zn(CH 3 F 3 SO 3 ) 2 as the electrolyte and reduced graphene oxide (rGO)/carbon cloth as the current collector, as shown in Figure 7E.Oxygen vacancies were then introduced during the in situ polymerization process, and they helped generate more active sites for Zn 2+ ions to increase the capacity (Figure 7F).This admirable electrode delivered a high reversible capacity of 337 mA•h/g at 200 mA/g and an amazing energy density of 228 W•h/kg.The excellent electrochemical performance is attributed to the extraction of oxygen atoms and the formation of delocalized electron cloud with spare electrons.Aside from the reducing atmosphere, a strong reducing agent can also be a feasible method to prepare the oxygen defects.For example, VO 2 (B) could be reduced to VO 1.88 with oxygen vacancy defects being generated by the use of a strong NaBH 4 -reducing agent. 123With the presence of oxygen defects, the cathode shows much improved electrochemical performance with a higher specific capacity, better structural stability, and better reversibility.The defect engineering approach can also be taken together with other strategies to further promote electrochemical performance.For example, a new strategy was developed by purposely reconstructing the CaV 4 O 9 cathode into oxygen-deficient V 5 O 12-x •6H 2 O coated by gypsum layers (GP-HVO d ) through in situ electrochemical conversion. 124This novel cathode offers a high reversible Zn 2+ ions capacity owing to the existence of oxygen defects, and it also delivers fast interfacial kinetics with reduced absorption energy for hydrated Zn 2+ ions benefiting from the hydrophilicity of gypsum coats.In general, the introduction of oxygen defects is an effective and feasible pathway to improve the electrochemical performance through delocalizing electron clouds, lowering the Gibbs free energy for Zn 2+ ions insertion, and avoiding the strong interactions between intercalated Zn 2+ ions and the oxide frameworks. 125On the one hand, the existence of oxygen vacancies and delocalization of electrons may enhance the V─O interactions and lower the formation energy of defective cathodes, promoting higher structural stability to confront the dissolution of vanadium.As a result, higher specific capacity and much-prolonged stability can be achieved by the vanadium oxide cathode with appropriate oxygen defects.Nevertheless, both the sites and amount of oxygen defects are strong impacting parameters, and therefore it would be essential to develop an optimum quantity and preferred sites of defects to boost the electrochemical performance when the oxide cathode is applied to Zn 2+ ions storage, and further investigation would be needed before the final conclusion.
In addition, the introduction of cation defects in the layer-structured oxides has also been demonstrated to be an effective pathway to improve the electrochemical performance of ZIBs with more active sites being provided and the reduced interactions between Zn 2+ ions and cathodes.On the one hand, compared with anion defects, cation defects can act a little differently, which is to generate higher hole concentration near the Fermi surface improving the electronic conductivity. 126n the other hand, it is more difficult to generate metallic cation vacancies because of the high formation energies, and this is the reason why there are fewer applications of cation defects in defect engineering.A previously reported successful attempt was conducted in the Mn-deficient MnO cathode. 35With the presence of Mn ion vacancies, the defective MnO cathode becomes more electrochemically active, benefiting from more defective sites for Zn 2+ ions storage and being more structurally stable owing to the presence of larger channels for Zn 2+ ions transportation.Theoretical calculations were conducted to clarify the reaction kinetics involved, which suggested that the defective MnO cathode exhibited a higher conductivity due to the increased electron density around the Fermi level.As a result, the defective oxide cathode with more available active sites and enhanced electrical conductivity shows a high reversible capacity.Similar advantages can also be verified in the vanadium oxide cathode.For instance, vanadium vacancies were introduced into V 2 O 3 via an in situ electrochemical lattice conversion process, and they were observed in V 2 O 3 by the electron paramagnetic resonance signature at about 3670 G.These vanadium vacancies facilitate the formation of larger Zn 2+ ions diffusion tunnels and promoted ultrafast Zn 2+ ions storage. 127Even though the vanadium vacancies can partially break the long-range order of the VO layer, the lattice structure of vacancy-rich V 2 O 3 can also be well maintained even after 800 cycles of ultrafast Zn 2+ ions insertion/extraction, showing an excellent structural stability.In addition, the diffusion coefficient of Zn 2+ ions in defective V 2 O 3 is proved about two orders higher than that of V 2 O 3 with regular lattice structures, demonstrated by the galvanostatic intermittent titration technique test, facilitating the ionic transferring.
Generally, heteroatom doping is a popular strategy to introduce defects and guest ions into the host materials.Similar to the pre-intercalation, heteroatom doping, especially those anion dopings (N, P, B, S, etc.), has been proven to be an effective pathway to gain the desired properties and performance of cathodes.In particular, the presence of hetero dopants and oxygen vacancies can tailor the electronic structures reducing the bandgap for more metallic properties with higher conductivity. 128,129At the same time, lower diffusion energy and higher binding energy of Zn 2+ ions would facilitate fast Zn 2+ diffusion kinetics and reduce the "Dead Zn 2+ " sites.Defective structures offer additional active storage sites for Zn 2+ to achieve higher capacity.Additionally, a metallic heteroatom can also be doped into V-based oxide crystal frameworks and generate abundant defects to enhance the conductivity and provide more absorption sites for Zn 2+ ions. 130In a preliminary summary, anion dopants can change the charge distribution and improve the capacity raised from surface reaction. 131Metal dopants tend to enhance the conductivity inherently and stabilize the host structure.Some active metal ions may also participate in redox reactions for more Zn 2+ ions storage.
In conclusion, the presence of both anion and cation defects is demonstrated to be able to effectively enhance the electrochemical performance of vanadium-oxide cathode materials by generating more active sites, accelerating ion and electron transferring, and improving structural stability.Nevertheless, the effects of the exact quantity and sites of some of these vacancies on the electrochemical performance have not yet been fully investigated.Similar to and also different from the structural water molecules, both the concentration and location sites of defects in vanadium oxide lattices have an important effect on the electrochemical performance, when used as the electrode materials in ZIBs, for which essential and in-depth characterization shall be conducted accordingly.Furthermore, it would be imperative to explore new avenues to synthesize vanadium oxide cathodes with proper controls in defects and/or crystal water, both the type and amount, where the respective concentration shall also be optimized.

| Micro/macroscopic structure engineering
Low conductivity is one of the typical issues of vanadium oxides when used as the cathodes in ZIBs, which limits the transportation of electrons/charges, thus handicapping their overall electrochemical performance.Apart from modifications at atomic scales, including the guest-ions intercalation, structural water optimization, and defects introduction as has been discussed above, compositing vanadium oxides with other conductive materials is an important pathway at micro/macroscopic scales.According to several previous reports, carbon-based conductive materials, conductive polymers, as well as some metal nanowires or films, have been the potential candidates to boost the ionic and electrical conductivities of vanadium oxides, including V 2 O 5 . 132,133Among these candidate materials, carbon-based materials, including rGO, carbon cloth, carbon nanotubes (CNTs), and so on, are among the most commonly used categories.These carbon-based materials not only can act as a conductive network to accelerate the electron transfer but also can buffer the volume variation during the cycling process.For instance, a binder-free V 2 O 5 nanofibers/CNTs hybrid film cathode was synthesized via a hydrothermal process. 134The nanohybrid cathode could deliver a higher specific capacity of 390 mA•h/g, while the pure V 2 O 5 electrode only gave 263 mA•h/g at the current density of 1 A/g.Further, it exhibited a prominent capacity of 250 mA•h/g at a higher current density of 5 A/g, as well as an excellent rate performance.This indicates the introduction of CNTs with high conductivity enables a higher electrical conductivity of the nanohybrid cathode, thus boosting the electrochemical performance.In addition, a binder-free cathode with centimeter-long V 2 O 5 nanofibers on carbon cloth was designed, showing excellent stability in Zn(OTf) 2 electrolyte. 135In this free-standing structure, the carbon cloth was pretreated by soaking in sulfuric acid, and V 2 O 5 nanofibers were deposited on the carbon cloth through a simple one-step hydrothermal process.The capacity retention was found to be greater than 90%, showing that a stable cathode structure was obtained without the presence of a polymeric binder.Jiang et al. made a successful attempt in a free-standing cathode with hydrated ammonium vanadate in situ grown on activated carbon cloth (NV Nss@ACC). 136With the three strategies of pre-intercalation, nanostructuring, and combination without binder, the resulting freestanding cathode exhibits several superiorities in one body, including a remarkable rate performance due to the high conductivity of ACC, high Coulombic efficiency achieved by preinserting NH 4+ , and fast kinetics with shortened Zn 2+ ions transfer pathway at nanoscales.There are other examples of constructing freestanding cathodes with various vanadium oxides and morphologies together with different substrates.For instance, Juggernauth et al. 137 prepared a 3D V 2 O 5 /CNT hybrid architecture by the chemical vapor deposition technique, where the vanadium oxide nanoarchitectures are morphologically tunable and possess translatable functionality.Their work confirmed that the surface areas and densities of hybrid structures could be tailored by controlling the operating pressure to tune the Gibbs free energy of crystal growth and expressed the significance of morphology design based on the hybridization with conductive materials.For instance, Cui et al. 138 synthesized VO 2 (B) nanobelts and rGO nanocomposites for Zn 2+ ions storage.With the direct deposition of VO 2 (B) nanobelts on rGO by a hydrothermal process, the nanocomposite cathode showed a large capacity of 456 mA•h/g at 100 mA/g and excellent rate performance.The remarkable rate capability is attributed to the shortened electron and ionic transport pathway, which is achieved by the connection of VO 2 (B) nanobelts with rGO.Moreover, the morphology of electrodes would also affect the capacitive process, because a larger surface area of active materials can offer more react sites for Zn 2+ ions storage.
Even though compositing with conductive materials can alter the conductivity to a large degree, the electrode preparation process often involves certain organic binders, such as PVA, PVDF, and so forth.These organic binders usually possess a rather insulative nature, which will be against the original aim to improve the ionic and electrical conductivity.To this end, a binder-free freestanding structure with vanadium oxides tightly interconnected has been proposed, which can not only boost the ion and electrons transfer but also tolerate the volume change of vanadium oxides during cycling.For example, Javed et al. 132 demonstrated the fabrication of a flexible ZIB employing a binder-free structure combining V 2 O 5 nanosheets and titanium substrate, which delivered an admirable discharge capacity and cycle stability.The superior performance is attributed to the enhancement of electrical conductivity and exposure of active sites.Zhang et al. 139 reported edge-rich vertical graphene nanosheets to bridge between V 2 O 5 and substrate, which could further reduce the interfacial resistance of the oxide cathode.As expected, the graphene nanosheets can well anchor V 2 O 5 and show good tolerance for the volume change during the cycling process, thus improving the stability of V 2 O 5 for a prolonged cycle lifespan.Xu et al. 140 successfully constructed conducting polymer shells (poly(3,4-ethylenedioxythiophene)) on V 2 O 5 nanosheets and deposited the coated arrays on carbon cloth.Benefiting from the exposed active sites by V 2 O 5 nanosheets and the well-maintained structure by the polymer protective layers, the optimized electrode shows a high rate capacity and excellent capacity retention after a long-time charge/discharge process.Instead of mixing with organic insulate binders, both the in suit growth of vanadium oxides on substrates and the use of the bridgelike or coat-like agent with superior characteristics are positive, in terms of being able to maintain the structure and morphology of active materials with more exposed active sites and shortened ionic transfer pathway, thus boosting the overall electrochemical performance.Therefore, constructing binder-free free-standing structures can effectively address the issue of poor rate performance through largely improved electronic conductivity.Moreover, a proper design of morphology together with an appropriate loading of active materials is also critical to harvest high Zn 2+ ions storage capacity and rate capability, where there can be a synergistic role with the free-standing structure to reach high electrochemical performance.Developing free-standing structures with well-designed active materials, such as hierarchical structures, is an attractive research direction for flexible aqueous ZIBs.
However, it shall be noted that the composite electrodes often possess randomly dispersed microstructures, which can be detrimental to the electrochemical performance due to the labyrinthian-like Zn 2+ ions diffusion paths, especially for the fast-charging process.Thus, precisely constructing a novel microstructure with uniform ion diffusion paths would be of interest.Besides, an excellent Zn 2+ ions storage capacitance can also be developed by exposing rich active sites.For instance, a number of studies were conducted to focus on the structure construction of vanadium oxides at the nanoscale level.On the basis of the morphology design, a hollow V 2 O 5 nanosphere cathode was prepared for aqueous ZIBs (Figure 8A), which exhibits a large specific surface area for electrode-electrolyte contact and helps buffer the volume change during Zn 2+ ions intercalation/ deintercalation. 25Tamilselvan et al. 141 first grew V 6 O 13 nanobelts on carbon cloth through a hydrothermal process, and then applied it as the cathode in aqueous ZIBs.Due to the better contact between the active materials and the current collector, the impedance for electron transfer can be significantly lowered, and the results shown in Figure 8B,C demonstrated the improved stability of the composite structure.This successful attempt demonstrated a pathway for assembling flexible metal-ion batteries by fabricating a free-standing structure on bendable conductive substrates.Liu et al. 134 proposed a cathode with V 2 O 5 nanofibers grown on CNTs.Owing to the shortened ion transport distance by the nanofiber structure and the exposed active sites, the transfers of electrons and ions are significantly accelerated, promoting excellent electrochemical performance.In addition, a nanopaper cathode of V 2 O 5 was synthesized with V 2 O 5 nanofibers and CNTs to supply shortened ionic diffusion distance. 142Nanostructures in 2D morphologies have also been designed on different conductive substrates.For example, Xu et al. 140 designed V 2 O 5 nanosheet arrays grown on carbon cloth with conducting polymer coating on the surface.The 2D V 2 O 5 nanosheets were directly grown on the titanium substrate, successfully forming a flexible cathode with high performance. 132Aside from these, the formation of hierarchical structures is getting popular owing to their extremely porous morphologies with large specific surface areas and rich active sites, 143 which would boost the electrochemical performance of cathodes, as shown in Figure 8D. 116Wang et al. 24 designed a hierarchically porous Zn 0.3 V 2 O 5 ⋅1.5H 2 O cathode, which exhibited a high capacity (426 mA•h/g at 200 mA/g) and ultralongterm cyclic stability (capacity retention of 96% after 20,000 cycles at 10 A/g).The hierarchical nanospheres consisted of flaky precipitates, which provided abundant active sites and guaranteed a larger contact area of electrode and electrolyte.Moreover, the hierarchical structure could accelerate the diffusion kinetics by shortening the ion transport pathway and promise long-term stability via providing buffering for lattice strain during cycling.The micro/macroscopic design of cathode materials is of great importance to enhance electrochemical performance.Composited with highly conductive materials can effectively improve the electrical conductivity and stability of the cathode materials.Morphology design is a key direction to modifying the cathode by offering improvements in ion diffusion kinetics and structural stability.However, too high a ratio of conductive additives can hinder the capacitive performance and too large a surface area may be detrimental to preventing the dissolution of active materials.Studies about the effects of composite ratio and surface area on electrochemical performance are still in general lack, thus the investigation into determining the suitable composite ratio and design of suitable morphology needs to be further conducted.It is also expected to combine the microscopic modification and macroscopic optimization to improve electrochemical performance furthest.
On the basis of the fundamental comprehension of design strategies for cathode materials, it would be necessary to fully understand the structural design principles of cathode materials for ultrafast Zn 2+ ions storage to meet the ever-rising energy demand.In one of the recently reported designs of electrode materials for fast-charging ZIBs, the focus is mainly on the manipulation of size scales and well-organized ionic diffusion tunnels of active materials.Extensive studies have confirmed that the size scaling of active materials impacts electrochemical performance in kinetics, thermodynamics, and also mechanical properties.Nanostructuring leads to better performance when several structural parameters are considered. 145For example, nanometer-scale architectures can possess a network of nanosized tunnels with few defective block areas, which will shorten the ionic diffusion length, thus achieving high Zn 2+ ions diffusion kinetics and thus high rate performance of the cathode materials.According to thermodynamics, nanosizing will reduce the miscibility gap to deliver a higher reversible capacity and generate excess surface energy for reaction voltage.In addition, the mechanical stability of active materials can be enhanced by controlling into a scale smaller than the typical crack size to avoid stresses arising from the concentration gradient and suppress the degradation/ collapse of the electrode structure under an ultrafast electrochemical process.However, an excessive nanoscaling will raise the manufacturing cost and make the industrial process less attractive.It is thus suggested to develop hierarchical structures with porous architectures consisting of both micro-and nanoscale substructures, which can lead to fast charging with high energy density for ZIBs, benefiting from the combination of both microstructures and nanoarchitectures.Besides, wellorganized ion diffusion tunnels/pathways in electrodes are expected to accelerate the mass transport kinetics, which is a key limiting factor of the electrochemical process.The design of more channels at suitable scales would also allow the cathode to maintain structural stability and provide more active sites for ultrafast Zn 2+ ions insertion and extraction.Moreover, fast intercalation/deintercalation of Zn 2+ ions generally requires the host structure to provide a cushion for lattice stress caused by the volume change and concentration gradient. 146As discussed before, certain free-standing structures with conductive additives can exhibit a stronger tolerance of volume change and fast electron transfer kinetics, preventing morphological degradation/ collapsing and promising high cyclic stability for fast charging.Therefore, the overall structural design of electrodes at an appropriate set of size scales and with tunable ionic diffusion tunnels, as well as the combination of engineering with conductive additives, are of great significance for the development of advanced fastcharging energy storage with the high power density and excellent rate performance for ZIBs.

| REMARKS AND PERSPECTIVES
Vanadium oxides, exemplified by V 2 O 5 , show great potential in Zn 2+ ions storage benefiting from their layered open-framework or tunnel structure for fast Zn 2+ ions migration and available multivalence for high theoretical capacity.However, these vanadium oxides often show poor cyclic stabilities due to the steady destruction of the layered structure and irreversible transformation caused by hydrated Zn 2+ ions insertion/ extraction and the dissolution of active materials into the aqueous electrolytes.Widening of the interlayer spacing in the layered vanadium oxides by pre-intercalations has been extensively explored over the past several years.However, there is a saturation point in terms of improving the energy density of ZIBs, by simply expanding the interlayer spacing in V 2 O 5 -based cathode materials, although various metal ions and organic cations/groups have been investigated.Several new strategies have been designed more recently aiming to further improve the stability of cathode materials and also the overall electrochemical performance, among which the guest-ions pre-intercalation strategy is the most widely used.With the guest ions located in the host lattices acting as the proper pillars, the layered structures of vanadium oxides would be stabilized with proper interlayer spacing or even form a tunneled structure for ionic migration and storage.Moreover, these guest cations can also enhance the intrinsic electrical conductivity of V 2 O 5 -based cathode by several folds, to boost the electron/charge transfer.As a result, the overall electrochemical performance of the vanadium oxide cathodes is improved with much enhanced cyclic stability, fast kinetics of ionic transport, rate performance, and long lifespan.However, as discussed above, there are further understandings that are needed with the guest-ion preintercalation approach, rather than just staying in the stage of studying enlarged interlayer spacing.Although several reports have claimed that the guest ions are present in the space between VO layers, there is still a possibility for some of these ions going into the VO framework and even substituting V atoms to form new octahedrons.Therefore, more intuitive experimental evidence by advanced characterization technologies would be imperative.In the repeated charging/discharging cycles, there are also questions about whether the guest ions can stay in the host materials during the long multicycling process.If they behave in the same/similar way as the inserted Zn 2+ ions, it would be hard to imagine why these guest ions cannot come in/go out together with Zn 2+ ions, as has been suggested in some of the previous reports.Consequently, it would be crucial to elucidate the trajectory of the guest ions and their effects on the reaction mechanism during the repeated battery cycles.Therefore, more efforts are expected to comprehensively understand the dynamic behaviors of these guest ions and their impact on electrochemical performance of ZIBs.An optimization would also be needed with the different preparation methods reported, and also fine-tuning in the processing parameters, going much beyond the simple hydrothermal synthesis.
Controls of both structural/crystal water molecules and the introduction of certain structural defects can both improve the stability of vanadium oxides during the repeated cycling of Zn ion batteries.In general, the effects of structural water in V 2 O 5 can be considered into three main aspects: (i) adjusting the interlayer spacing of vanadium oxides, which has been the most common structural feature considered; (ii) shielding the electrostatic effects by the presence of water molecules in the interlayer space; (iii) facilitating the compatibility of the electrode structure and aqueous electrolytes, where water molecules improve the hydrophilicity of the layered oxides, such as V 2 O 5 .The presence of structural/crystal water molecules creates a different solvation environment for the incoming Zn 2+ ions from the liquid electrolyte, thus reducing the energy barrier for Zn 2+ ions transport both within the layered structure and at the interface between the electrode and electrolyte.However, there is still a debate as to whether the water molecules will occupy active sites competing for Zn 2+ ions storage, which would need further confirmation by high-resolution in situ or operando studies, together with computational studies.And there is still a general lack of understanding on the optimum amount of crystal water being present if one aims for further improved electrochemical performance.Besides, too many structural water molecules will expand the interlayer spacing of vanadium oxides to an unsatisfactorily high degree, resulting in the dissolution of active materials and the eventual collapse of the entire layered structure.Thus, it would be necessary to clarify exactly where is the actual and optimal balance between the stability and fast ion kinetics.Furthermore, concerning the defect engineering approach, given the presence of both anion and cation defects can effectively improve the electrochemical performance, it would be desirable to explore new avenues to synthesize the purposely designed vanadium oxide cathodes with precise controls in the types and amount of defects, including both/all types and optimum concentrations.Besides, similar to the structural water, it would also be necessary to clarify the site locations and the movement of these defects in the vanadium oxide lattice, during the repeated cycling process.Apart from the structural modifications at atomic scales, micro/ macroscopic structure optimization of the layered vanadium oxides is also an important pathway, where there is a need for a comprehensive understanding of the structural design principles of cathode materials aimed for ultrafast and efficient energy storage in ZIBs.Scale miniaturization of the vanadium oxides together with purposely designed ion diffusion channels/avenues can well boost the Zn 2+ ions diffusion and storage kinetics.
Combing with an appropriate free-standing/straintolerating structure, the volume changes during the fast Zn 2+ ions intercalation/deintercalation can well be accommodated for the long-term device cyclability.
Aqueous ZIBs have been regarded as a class of great candidate batteries for safety-centric applications and large-scale energy storage and conversion, benefiting from the low cost, high safety, and relatively high energy density.Although extensive explorations and investigations have been conducted over the past few years, in effects to promoting the maturation of vanadium oxides, where V 2 O 5 is the best-known example, as the cathodes in ZIBs, the overall evolution still seems to be shrouded in a fog and the progress appears to be slow.They are overall still in the infancy, because there is still a general lack of deep understanding on the exact type of structure with efficient charge transfer and storage as well as longterm stability, although great progress has been made so far with some of the approaches that have been taken, including (pre-)intercalating cations, optimizing structural water, introducing atomic defects, as well as the macrostructural design.There is doubt that the efforts will continue, and they will lead to further and much clearer clarification of the Zn 2+ ions transfer and storage mechanisms in the purposely engineered structure of vanadium oxides, such as those V 2 O 5 -based, to achieve high capacity, desirable lifespan, and fast charging.The on-going approaches and mechanism studies, going much beyond simple widening of the interlayer spacing will overcome most of the outstanding issues, both scientifically and technically, and pave the way for the large-scale applications of aqueous ZIBs in the near term.
history and main roadmap of cathode materials for aqueous ZIBs.(B) The lattice structure of orthorhombic V 2 O 5 crystalline viewed in different facets.(C) Summary of the main issues of vanadium oxides as the ZIBs cathodes.PBA, Prussian blue analog; ZIB, zinc-ion battery.

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I G U R E 4 (A) Density of states of the NVO, Mn1-NVO, and MnVO.(B) Energy barriers along Zn 2+ -migration pathways for Mn1-NVO.(C) High-resolution XPS spectra of zinc in the fully charged state.Reproduced with permission: Copyright 2020, Wiley-VCH. 41(D) Schematic configuring of metallic zinc//KMgVOH aqueous ZIBs.(E) The corresponding relationship between frequency and real resistance in the low-frequency region.(F) CV curves with diverse scan rates.Reproduced with permission: Copyright 2021, Elsevier. 80(G) Schematic illustration of iron ion and alkylammonium cation co-intercalated vanadium oxide (FeVO-X) by a hydrothermal method followed by ionexchange and electrostatic interaction.(H) The contact angle tests of the FeVO and FeVO-12.Reproduced with permission: Copyright 2022, Elsevier. 84CV, cyclic voltammetry; DOS, density of state; NVO, NaV 8 O 20 •nH 2 O; VOH, hydrated vanadium oxide; XPS, X-ray photoelectron spectroscopy.
2+  ions and host framework to enhance conductivity and facilitate the fast Zn 2+ transport achieving high rate performance.For example, Wang et al. have prepared a layered hydrated vanadium oxide (V 3 O 7 •H 2 O) cathode with high Zn 2+ ions capacity and good stability.111It is confirmed that water molecules are critical to maintaining the layered structure of cathodes through the comparison between layered V 3 O 7 •H 2 O and nonlayered V 3 O 7 with nanorods morphology.Obviously, the presence of water molecules will effectively improve electrochemical performance due to their pillar and shield role.Hydrated layered cathode offers higher capacity owing to the enlarged open ionic transport and storage channels between layers, remarkably enhanced stability with the support of crystal water, and better rate performance due to smaller charge transfer resistance than nonlayered V 3 O 7 .This pillar effect has also been examined in V 10 O 24 •12H 2 O with an interlayer spacing of up to 14 Å.112In this work, Wei et al. demonstrated that the enlarged interlayer spacing by hydration would facilitate the diffusion of large-radius hydrated Zn 2+ cations with weaker interactions between Zn and O in the cathode, which helped avoid structural damage otherwise caused by the repeat insertion/extraction of Zn 2+ ions.On top of expanding the interlayer spacing, as has been verified through X-ray diffraction (XRD) phase identification, high-resolution transmission electron microscopy, and other analytical techniques,113 H 2 O molecules can also effectively reduce the energy barrier for the incoming Zn 2+ ions through solvation to decrease the electrostatic interactions of Zn 2+ ions with the VO layers,101 where the structural H 2 O molecules work like a lubricant to fasten the Zn 2+ ions diffusion kinetics.For example, Yan et al. prepared V 2 O 5 •nH 2 O/graphene (VOG) and VOG-350 (annealing VOG at 350 °C for 2 h in a vacuum) and investigated the important role of structural water in the electrochemical reaction processes.39As shown in the solid-state magic-angle-spinning nuclear magnetic resonance results in Figure6A-C, the presence of structural water between VO layers was confirmed by the resonance of 1 H at 5.6 ppm, which would experience an upward shift and increased intensity upon charging in aqueous electrolyte.In addition,13 C and19 F signals indicate the intercalation of electrolyte ions together with water molecules during the charging process.Their thermogravimetric/differential scanning calorimetermass spectroscopy (TG/DSC-MS) results showed that structural water was lost at temperatures of around 340 °C.The interlayer spacing of VOG was nevertheless maintained at 12.6 Å with the presence of structural F I G U R E 6 (A-C) MAS NMR spectra were collected for pristine VOG (black line), VOG after charging to 1.3 V (blue line), and discharging to 0.2 V (red line).Deconvolution of the 1 H spectrum of VOG at 0.2 V (pink dot lines) shows that the sharp peak at 5.1 ppm contributes to 20% of the total signal.The spinning speed is 30 kHz for 1 H and13 C, and 20 kHz for19 F with spinning side bands marked with *.Reproduced with permission: Copyright 2017, Wiley-VCH.39(D) Schematic diagram of the flexible quasi-solid-state Zn-VOH battery chemistry using a VOH cathode, gelatin/Zn(CF 3 SO 3 ) 2 water-based gel electrolyte, and Zn foil anode.(E) Typical discharge/charge curves under the flat state, as well as the 90°and 180°bending states.Reproduced with permission: Copyright 2019, Wiley-VCH. 40(F) Ex situ XRD patterns at selected states, and (G) Corresponding discharge/charge curves at the first cycle.Reproduced with permission: Copyright 2022, Wiley-VCH. 115(H) TG curves of A-VOH sample.Reproduced with permission: Copyright 2021, American Chemical Society. 116MAS, magicangle spinning; NMR, nuclear magnetic resonance; TG, thermogravimetric; VOG, V 2 O 5 •nH 2 O/graphene; VOH, hydrated vanadium oxide; XRD, X-ray diffraction.
have demonstrated the formation of a layered structure of hydrated V 5 O 12 •6H 2 O (VOH) cathode on a stainless-steel substrate, which displayed excellent flexibility without capacity change under different bending states (Figure6D,E).This cathode showed an excellent rate performance with high capacities under different current densities, attributed to the stabilized crystal structure by water molecules.There is no doubt that the structural water molecules in V 2 O 5 play a key promotional role for Zn 2+ ions movement and storage, although there is still a generally poor understanding of the detailed fundamental mechanisms behind.To complement the experimental investigations, there have been several DFT studies, where one was conducted for a model bilayer V 2 O 5 to study the effect of structural water molecules.114The structural water molecules could modify the crystal structure, intercalation voltage, migration energy barrier, electronic structures, and capacity in the bilayer dry-V 2 O 5 and hydrated V 2 O 5 .On top of acting as a "charge shield," the structural H 2 O molecules can provide extra host sites through O to accept electrons from Zn, increasing the open-circuit voltage.Furthermore, benefiting from a smoother electrostatic environment created by the presence of structural water molecules, Zn 2+ ions diffuse more steadily due to the reduced diffusion resistance.

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I G U R E 7 (A, B) Charge distribution and corresponding structure of p-VO.(C, D) The charge distribution and corresponding structure of O d -VO.(E) Configuration of the Zn/O d -VO battery and its behavior during discharge; the oxygen-deficient sites (represented by red spheres) are incorporated into the vanadium oxide framework.(F) GITT profiles of p-VO and O d -VO electrodes.Reproduced with permission: Copyright 2019, Wiley-VCH. 121GITT, galvanostatic intermittent titration technique; rGO, reduced graphene oxide.

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I G U R E 8 (A) Configuration of the aqueous ZIBs with hollow V 2 O 5 as the cathode materials.Reproduced with permission: Copyright 2019, Elsevier.25 (B) Galvanostatic cyclic performances and corresponding Coulombic efficiencies of V 6 O 13 @gCC and V 6 O 13 @bCC measured at 4.5 A/g.(C) Discharge/charge profile of V 6 O 13 @gCC cathode at 4.5 A/g.Reproduced with permission.141Copyright 2020, Elsevier.(D) Schematic illustration of the preparation process for 3D CNT-stitched Zn 0.3 V 2 O 5 ⋅1.5H 2 O NSs@OCNT fiber.Reproduced with permission: Copyright 2019, American Chemical Society.144CNT, carbon nanotube; ZIB, zinc-ion battery.
Interlayer spacing and electrochemical performance of V-based oxide cathode materials, data were collected for the current density of 100 mA/g.
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