English

• 1. Introduction

  The rapid development of electric vehicles and various intelligent devices lead to the huge demand for energy storage devices with high energy density, long-cycle-life, safety and cheap price [1-5]. Among various batteries, alkaline ion batteries (Li⁺, Na⁺, K⁺) are easily standing out because of their ability to provide stable output voltage, which increase the reliability of the equipment operation. Lithium-ion-batteries are most commonly used as commercial rechargeable batteries due to their high energy density and low self-discharge [6-10]. However, potential dangers of electrode materials and the limited lithium resources in the earth crust have made it a challenging task for large-scale applications [11-13]. For sodium-ion batteries, the low operation voltage and the relatively large ionic size of Na⁺ lead to sluggish reaction kinetics and poor electrode stability [14]. As for potassium ion batteries, there are severe issues in transport kinetics and cycling stability [15].

  Rechargeable aqueous zinc-ion batteries (ZIBs) have been recognized as a promising alternative owing to its merits and advantages: (1) high operational safety [16]; (2) low redox potential of Zn²⁺/Zn of anode (−0.76 V vs. SHE) compared to other anodes in aqueous batteries [17]; (3) high capacity of Zn (820 mAh/g, 5855 mAh/cm³) [18]; (4) stability in water due to high overpotential for hydrogen evolution [19]. Thus the ZIBs have great potential in the future energy storage field. Due to the low redox potential of Zn and the formation of Zn dendrite, the near-neutral or slightly acidic aqueous electrolyte is more suitable for ZIBs, which may successfully eliminate dendritic Zn deposition prevalent in alkaline electrolytes and further promote the development of ZIBs [20]. To date, various cathode materials in aqueous ZIBs, such as manganese (Mn)-based compound [21], Prussian blue [22], organic material [23], Chevrel phase compound [24] and vanadium-based compound have been developed and gained impressive progress. In short, manganese-based compound electrode materials have been reported in aqueous ZIBs due to the diverse structures and multivalent phases of Mn: +2, +3 and +4 [25-27]. The structure of Mn-based materials usually includes tunnel structure, layered structure and 3D structure, which are built by MnO₆ octahedra sharing corners or edges into chains or tunnels [28-30]. The large interspacing structures allow reversible (de)intercalation of Zn ions [31]. α-MnO₂ is built by the corner-sharing MnO₆ octahedra. During the cycling process, the dissolution of Mn ions in the electrolyte changes the equilibrium of Mn dissolution from the MnO₂ electrode. It means that the irreversible distorted framework reduces the capacity. Prussian blue and its analogues possess a face-centered cubic structure, and the open framework has large channels to allow the rapid diffusion of metal ions [32]. However, the aqueous ZIBs with Prussian blue show poor electrochemical performance [17, 20, 33]. The structure of organic cathode materials is pinned by weak van der Waals forces. The soft lattice allows molecular reorientation for facile and reversible intercalation of Zn ions with the smallest volume change [34-36]. The Chevrel phase compounds' crystal structure composed of three-dimensional arrays of Mo₆T₈ units can form tridirectional channels consisting of metal ions [24]. This structure can host various cations. But the theoretical capacity of that is very low (129 mAh/g) [37]. Besides, vanadium-based compounds usually present a layered structure consisting of square pyramidal VO₅ units that link each other by sharing edges [38]. Because of the large interlayer spacing, the vanadium-based cathode in aqueous ZIBs has a high Zn ions diffusion rate and valuable specific capacities. However, the problems of sluggish kinetics and capacity degradation of vanadium-based cathode materials must urgently be promoted.

  With a focus on the vanadium-based materials for aqueous ZIBs, this article offers a comprehensive review on the development of vanadium-based cathodes with controlled structure for energy storage. We begin with a brief introduction to the reaction mechanism of vanadium-based aqueous ZIBs. We then discuss various strategies involved in interlayer cationic and anions ions, lattice water, as well as devisable oxygen vacancy to increase the lattice spacing to enhance the diffusion rate of Zn ions with desirable capacity and stability, followed by an extensive presentation of diverse morphologies, and combination of other carbon materials to improve electrochemical performance. At the end, we give a brief summary about vanadium-based materials in aqueous ZIBs, together with perspectives on the challenges, opportunities, and new directions for future development. It is hoped this article provides not only a comprehensive review of the vanadium-based cathodes in aqueous ZIBs but also the necessary knowledge to push their commercial application.

  2. Reaction mechanism of vanadium-based aqueous ZIBs

  The charge and discharge process of vanadium-based aqueous ZIBs is the insertion/extraction of hydrous Zn ions into/from their hosts. For many kinds of cathode materials, interlayer ions (M_xV_nO_m, M = inserted ions or structural water) were acted as pillars to improve structural stability during the insertion/extraction of Zn ions into the layered oxide. As shown in Fig. 1, for the discharge process, the negative electrode undergoes an oxidation reaction to generate Zn ions, which diffuses to the electrolyte and intercalate into the vanadium oxide layer. Usually, the hydrous Zn ions combine with six water molecules and co-insert into the vanadium oxide layer, which can easily destroy the host structure [39]. For instance, the discharge process in the first cycling reaction of V₂O₅ is as follows:

  (1)

  Figure 1

  

  Figure 1.  Reaction mechanism of vanadium-based aqueous ZIBs.

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  For the subsequent charge process, the Zn ions between V₂O₅ layers extract and move to the anodewith the electrolyte. The reaction is as follows:

  (2)

  Various layered frameworks of vanadium-based compounds are linked by sharing corners/edges polyhedron, with structural water acted as pillars between the layers. For instance, the V₂O₅ layers are built by VO₆, VO₅ and VO₄ polyhedron [40]. The water-based shield formed by structural water reduces the "effective charge" of Zn ions, and increases the distance between Zn ions and the neighboring oxygen-ions in VO_x polyhedron, resulting in the lower diffusion coefficient [41]. During the charge/discharge process, the phase transformation companied by solid-solution behavior can be observed during the (de)intercalation of Zn ions. During discharging, a new phase occurs when the Zn ions intercalate into vanadium oxide, followed by the transformation to the original state during charging [42]. The more completely the cathode transforms, the more reversible the host structure is, which means the cathode has excellent electrochemical performance. Sometimes the formation of the irreversible byproduct, such as ZnSO₄Zn₃(OH)₆•5H₂O, can prevent the (de)intercalation of Zn ions, leading to the failure of battery at last [43].

  3. Vanadium-based compound with interlayer cationic ions

  3.1 Vanadium-based compound with interlayer monovalent cations

  Vanadium-based compounds with interlayer monovalent cations as cathodes have been studied for years. Yang et al. introduced an effective strategy by the chemical intercalation of Li ions into the interlayer of V₂O₅•nH₂O, with enlarged layered spacing (Fig. 2a) [44]. The Ganlvanostatic Intermittent Titration Technique (GITT) in Fig. 2b reflected a high diffusion coefficient, which benefited from the chemical intercalation of Li ions, enlarging the interlayer spacing. The obtained materials presented a discharge capacity of 232 mAh/g after 500 cycles at 5 A/g. He's group prepared Na_0.33V₂O₅ nanowires with Na⁺ intercalating between the layers of [V₄O₁₂]_n (Figs. 2c and d) [45]. The Na ions not only improved the stability of the tunnel structure as pillars during the insertion/extraction of Zn ions, but also enhanced the conductivity of V₂O₅. The obtained Na_0.33V₂O₅ electrode delivered reversible intercalation reaction mechanism (Fig. 2e) and high capacity as well as long-term cyclic stability with a capacity retention over 93% for 1000 cycles. Hu et al. [46] prepared Na₂V₆O₁₆•1.63H₂O nanowires and studied the influence of crystalline water on the properties of materials. They certified that crystalline water played an important role in the electrochemical properties of Na₂V₆O₁₆•1.63H₂O cathode materials, which might increase the capacity and improve the stability of the materials. In addition, the other research work about Na₂V₆O₁₆•2.14H₂O nanobelts [47] also showed a high capacity of 438 mAh/g at the rate of 0.1 A/g. Du et al. [48] synthesized Mn-doped NaV₈O₂₀•nH₂O nanobelts via one-step hydrothermal method. They found the co-insertion of Na ions and Mn ions enlarged the interlayer spacing, resulting in the stable structure and excellent electrochemical performance.

  Figure 2

  

  Figure 2.  (a) The HRTEM image with SAED pattern of Li_xV₂O₅•nH₂O at 0.1 A/g. (b) The discharge/charge curves in GITT measurement of Li_xV₂O₅–250 and the corresponding diffusivity coefficient (D) of Zn ion in the discharge and charge processes of the Li_xV₂O₅–250 and V₂O₅–250. Copied with permission [44]. Copyright 2018, the Royal Society of Chemistry. (c) The crystal structure, (d) SEM, HRTEM and (e) ex situ XRD patterns of Na_0.33V₂O₅. Copied with permission [45]. Copyright 2018, Wiley-VCH. (f) The crystal structure, (g) XPS spectra, (h) HRTEM and (i) TEM images of Ag_1.2V₃O₈. Copied with permission [54]. Copyright 2019, Elsevier.

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  In recent work, K_0.5V₂O₅ cathode was prepared by intercalating of K ions into V₂O₅, constructing a stable crystal structure by forming chemical bonds between V₂O₅ layers. The obtained K_0.5V₂O₅ electrode exhibited excellent durability [49]. The K-O bonds stabilized the structure which liked the pillars and slowed down the dissolution process of V element from V₂O₅. The insertion of K ions led to the reduced lattice space due to the enhancement of attraction between K and O ions. The smaller lattice space might inhibit the intercalation of K ions into V₂O₅ layers. In another research work, K_0.23V₂O₅ with tunnel structure had been prepared by hydrothermal method and employed as cathode material for aqueous ZIBs, in which, K ions also acted as pillars between the V₂O₅ layers [50]. This material delivered excellent structural stability and high capacity retention rate of 92.8%. It could remain 92.8% of original capacity after 500 cycles at the rate of 2 A/g. K₂V₈O₂₁ and K_0.25V₂O₅ with tunnel structure also has stable structures [51]. The pre-intercalation of potassium ions still enlarged the interspacing of V₂O₅. K_0.486V₂O₅ as cathodes in aqueous ZIBs showed a large interlayer spacing of 0.95 nm, which was much larger than that of V₂O₅ (0.436 nm) [52]. The advantage of structures ensured reversible Zn ions insertion/extraction. Another work evidenced that K ions intercalated V₂O₅ nanorods with exposed layered structure provided facile access for guest ions to intercalation/deintercalation into/from structure, which led to better performance: discharge capacities of 439 and 286 mAh/g at current densities of 50 and 3000 mA/g [53].

  Owing to unique electrical properties and high specific capacity, silver vanadates, in which two types of redox metal centers existed, showed great potential as cathode materials in energy storage devices. A series of Ag ions doped vanadium-based oxides with different structure and Ag content was used in aqueous ZIBs, for example, Ag_0.33V₂O₅ with a rigid three dimensional tunneled structure, Ag_1.2V₃O₈, Ag₂V₄O₁₁ with a typical layered structure, β-AgVO₃ with chain-like structure and Ag₄V₂O₇ with island-like structure [54]. The ratio of Ag/V played an important role in structure and electrochemical behaviors. For Ag_1.2V₃O₈, Ag⁰ nanoparticles would be generated upon Zn ions intercalation process and were largely reversible oxidized to Ag ions upon Zn ions de-intercalation process, showing highly structural and cycle stability. The V(V) was reduced to V(IV) and turned back to the original state during the reaction progress, which demonstrated the highly reversibility (Figs. 2f and g). Since the in-situ generation of highly conductive Ag⁰ and their grid structures, Ag_1.2V₃O₈ and Ag₂V₄O₁₁ with moderate Ag/V ratio presented good rate performance. Some Ag⁰ particles could be observed on Ag_1.2V₃O₈ nanorod during discharging (Fig. 2h), and still existed in the fully charged state (Fig. 2i), which could contribute the significant increase of electronic conductivity and rate capacity. Shan's group revealed the combination displacement/intercalation (CDI) reaction in Zn/Ag_0.4V₂O₅ system [55]. They found that the characteristic peaks of Zn₂(V₃O₈)₂ in XRD spectra became stronger during discharging process and disappeared during the charging process, demonstrating the Zn²⁺/Ag⁺ displacement reaction occurred during Zn ions insertion/extraction in the Ag_0.4V₂O₅ structure. The Zn ions intercalated into Ag_0.4V₂O₅ and replaced the Ag ions sites, while Ag ions were reduced to metallic Ag⁰. The reaction mechanism offered Ag_0.4V₂O₅ a rigid structure and good electrochemical properties.

  Ammonium vanadate presented a similar structure with other vanadium-based compounds. The existence of hydrogen bonds between ammonium ions and V₂O₅ layer offered a more stable layer structure [56]. (NH₄)₂V₆O₁₆ nanobelts showed a high capacity of 323.5 mAh/g at 2.4 A/g and a long cycle performance of 78.3% retention after 2000 cycles at 5 A/g [57]. (NH₄)₂V₆O₁₆•1.5H₂O nanobelts showed better cycling performance because the hydrated ammonium ions acted as pillars to stabilize the structure. At the current density of 3 A/g, the capacity of 209.6 mAh/g after 1000 cycles could be obtained. Both T. Wei et al. [58] and B. Tang et al. [59] demonstrated the ammonium ions and crystal water molecules stabilized the structure. Zhao et al. [39] synthesized (NH₄)_0.58V₂O₅•0.98H₂O and revealed the Zn ions storage mechanism. The large interlayer distance of 10.9 Å in (NH₄)_0.58V₂O₅•0.98H₂O meant more Zn ions could be de/intercalated into V₂O₅ layers and this structure allowed more [Zn(H₂O)₆]²⁺ to insert into V₂O₅ layers. When the interlayer could not accommodate more [Zn(H₂O)₆]²⁺, [Zn(H₂O)₆]²⁺started to dehydrate as Zn ions and water molecules, and the formed Zn ions intercalated into V₂O₅ layers. With the deeply discharging, the formation of ZnSO₄Zn₃(OH)₆•5H₂O byproduct accelerated the desolvation of [Zn(H₂O)₆]²⁺ into Zn ions. The Zn ions with small radius were more easily inserted into V₂O₅ layers. The charging progress was fully reversible. The inserted cations not only enlarge the interlayer spacing to accommodate more Zn ions for high capacity, but also stabilize the host structure by bond force.

  3.2 Vanadium-based compound with interlayer divalent cations

  Vanadium oxide bronze pillared by interlayer Zn ions can lead to more than one Zn ion per formula unit during reversible Zn ions (de)intercalation storage process, thus showed high capacity. L. F. Nazar et al. [60] prepared Zn_0.25V₂O₅•H₂O with excellent layered structure by microwave hydrothermal method. As shown in Fig. 3a, the ZnO₆ octahedron was arranged in order in the layered V₂O₅, which supported the V₂O₅ layers and expanded the channel for the insertion/extraction of Zn ions as pillars. In addition, this excellent layered structure enabled water molecules in the electrolyte to be intercalated into V₂O₅ layers, further increasing the layer distance of V₂O₅ (Fig. 3b). During charging and discharging, V₂O₅ layer space could reach 12.9 Å. The obtained materials presented a high capacity of 282 mA/g at 0.3 A/g (Fig. 3c). Compared to Zn_0.25V₂O₅•nH₂O, the double-layered calcium vanadium oxide bronze (Ca_0.25V₂O₅•nH₂O) was more suitable for aqueous ZIB intercalation cathode because the larger CaO₇ polyhedra in Ca_0.25V₂O₅•nH₂O may further expand the cavity size between V₄O₁₀ layers. The structure of Ca_0.25V₂O₅•xH₂O was shown in Fig. 3d, with a length of Ca-O bond of 2.38–2.57 Å [61], which was longer than the length of Zn-O bond (2.03–2.13 Å) in δ-Zn_0.25V₂O₅•xH₂O. The interlayer space of Ca_0.25V₂O₅•nH₂O increased from 10.6 Å to 14.1 Å after the insertion of water molecules from the electrolyte into its interlayer space (Fig. 3e). The Ca_0.25V₂O₅•nH₂O offered 4-folds higher electrical conductivity than Zn_0.25V₂O₅•nH₂O nanobelt (Fig. 3f), which could boost the electron transfer during Zn ion (de)intercalation, thereby enhancing battery performance.

  Figure 3

  

  Figure 3.  (a) The structure analysis, (b) HRTEM image and (c) galvanostatic discharge and charge profiles of Zn_0.25V₂O₅•H₂O. Copied with permission [60]. Copyright 2016, Springer Nature. (d) The crystal structure, (e) XRD pattern and (f) electricity conductivities of Ca_0.25V₂O₅•xH₂O. Copied with permission [61]. Copyright 2017, Wiley-VCH. (g) The XRD pattern, (h) XPS spectra and (i) charge/discharge curves of Mg_0.26V₂O₅•0.73H₂O. Copied with permission [65]. Copyright 2020, Wiley-VCH.

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  In addition, the recent research showed that Ca_0.23V₂O₅•0.95H₂O nanobelt with a large interlayer spacing of 13.0 Å also had high capacity (355.2 mAh/g at 0.2 A/g) and excellent cycling performance (97.7% capacity retention over 2000 cycles at 5 A/g) [62]. Patridge et al. [63] studied the changes of the local structure of Ca_xV₂O₅•xH₂O during the intercalation of Zn ions and the impacts on cycling performance. They proved that Ca_xV₂O₅•xH₂O was a promising cathode for the future storage device. Ming's group [64] prepared the Mg_0.34V₂O₅•nH₂O nanobelts. After magnesium ions and crystalline water were inserted into the V₂O₅ layer, the distance of the V₂O₅ layer was achieved to 13.4 Å, which resulted in a fast diffusion coefficient of Zn ions and good rate performance. This material also presented excellent cycling performance. At the current density of 5000 mA/g, 97% of the initial capacity was maintained after 2000 cycles. Wang et al. [65] studied the advantages of the intercalation of hydrated Zn ions into Mg_0.26V₂O₅•0.73H₂O. The XRD pattern of Mg_0.26V₂O₅•0.73H₂O and the Rietveld refinement result are shown in Fig. 3g. Mg ions improved the interlayer spacing by acting as pillars, and the larger interlayer space allowed more incorporated Zn cations with six water molecules to insert, which required lower intercalation energy. The reversible electrochemical reduction can be discovered in the V 2p XPS region (Fig. 3h). The co-inserted water molecules could shield the effective charge of Zn ions, which increased the diffusion coefficient of Zn ions, leading to a high capacity of 212 mAh/cm² at 0.05 A/g (Fig. 3i).

  Liu et al. [66] prepared manganese expanded hydrated vanadate with high stability, in which, the Mn(II) cations acted as structural pillars, expanded the interplanar spacing, connected the adjacent layers, and partially translated pentavalent vanadium cations to tetravalent. They found that the expanded interplanar space of 12.9 Å reduced electrostatic interactions. And transition metal cations collectively catalyzed more Zn ions intercalation at higher discharge current densities with enhanced reversibility and cycling stability. Li et al. prepared δ-Ni_0.25V₂O₅•nH₂O that Ni ions coordinated two oxygen ions, which led to more apical oxygen ions in V₂O₅ layers [67]. The oxygen ions made Zn ions easier to intercalate. And the structure with double layers was stable. In addition, other transition metal ions (Fe, Co, Ni, Mn, Zn, Cu) doped vanadate system could also improve the capacity by enlarging interlayer spacing and stabilized the structure [68]. These cathodes showed good cycling performance even in a range of testing temperatures (0–50 ℃).

  Intercalating copper-ions produced another electrochemical mechanism that reduction displacement reaction occurred during the discharge process [69]. After Zn ions being intercalated into the cathode material, Cu ions were reduced to Cu⁰ nanoparticles, then parts of Cu⁰ nanoparticles were oxidized to Cu ions charging. This process promoted multiple electron transfer and increased battery capacity. CuV₂O₆ nanowires prepared by Yu et al. [70] proved that the multistep reduction reactions of Cu ions and V ions with the intercalation of Zn ions provided high capacity (338 mA/g at 0.1 A/g). In another research, Liu et al. [71] prepared CuV₂O₆ nanobelts and found that the displacement between Zn and Cu was reversible and would not damage the VO₆-octahedron chains.

  3.3 Vanadium-based compound with both interlayer cations and anions

  In typical hollandite-type vanadium oxide, the hydrogen atoms attached to the edge-sharing oxygen atoms of oxide ions in the channels to form OH groups, resulting in highly stable structure. Jo et al. [72] synthesized V_1-xAl_xO_1.52(OH)_0.77 by a solvothermal synthesis method. As shown in Fig. 4a, the distorted VO₆ octahedra shared an edge to form VO₆ double chains, and there was an oxygen ion in the tunnel. This tunnel-type structure had a larger space to accommodate more ions (Fig. 4b). The X-ray absorption near edge structure (XANES) spectroscopy data indicated successful doping of Al into the hollandite VO_1.52(OH)_0.77 tunnel structure (Fig. 4c). The Al-O bond strongly stabilized the structure, resulting in better electrochemical properties compared with Al-free VO_1.52(OH)_0.77 (Fig. 4d). Zn₃V₂O₇(OH)₂•2H₂O also had a tunnel-type structure that was composed of zinc oxide layers separated by V₂O₇⁴⁻ groups (Fig. 4e) [73]. The open-framework structure allowed fast Zn ions migration and better performance (Fig. 4f). The reversible interlayer expansion could be observed due to the stable structure (Figs. 4g and h). Cu₃V₂O₇(OH)₂•2H₂O [74] had a structure that was built up of Cu₃O₆(OH)₂ layers separated by V₂O₇ pillars and water molecules. During the discharging process, Cu⁰ formed and the metallic Cu⁰ had not been oxidized at the initial stage of charging, which resulted in high electrical conductivity, fast electron transfer, and rapid extraction of Zn ions. Peng and coworkers [75] prepared Fe₅V₁₅O₃₉(OH)₉•9H₂O nanosheets with a high capacity of 385 mAh/g at 0.1 A/g. During discharging and the insertion of Zn ions, V⁵⁺ was reduced to V⁴⁺ and V³⁺, and the concomitant oxidation of Fe³⁺ to Fe²⁺. Both Fe and V were electrochemical active for Zn storage mechanism.

  Figure 4

  

  Figure 4.  (a) The model structure, (b) HRTEM image and (c) K-edge XANES spectra of V_1-xAl_xO_1.52(OH)_0.77. (d) First discharge–charge curves of VO_1.52(OH)_0.77, V_0.95Al_0.05O_1.52(OH)_0.77 and V_0.95Al_0.05O_1.52(OH)_0.77. Copied with permission [72]. Copyright 2017, the Royal Society of Chemistry. (e) HRTEM image, (f) the corresponding Zn ions coefficients, (g) ex situ XRD patterns of Zn₃V₂O₇(OH)₂•2H₂O. (h) HRTEM of the Zn₃V₂O₇(OH)₂•2H₂O at initial, fully discharged (0.2 V) and charged (1.8 V) state. Copied with permission [73]. Copyright 2017, Wiley-VCH. (i) Zn storage mechanism illustrations, (j) CV curves and (k) initial three charge/discharge profiles of Na₃V₂(PO₄)₂F₃. Copied with permission [80]. Copyright 2018, Elsevier.

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  Additionally, the vanadium-based compound with polyanionic has been reported as cathode for aqueous ZIBs. Many groups showed interest on NASICON A_xMM'Z(XO₄)₃ (A = Li, Na, Mg, etc.; M, M' = Ti, V, Cr, Fe, etc.; X = P, Si, S, etc.) compounds [76]. (PO₄)³⁻ polyanion could moderate the energy of the transition metal redox couple to generate relatively high operating potentials and the strong covalent units could provide structural stability even at high charge states and address thermal safety concerns [77]. Na₃V₂(PO₄)₃ also had a layer framework [78]. And there were two Na sites with different coordination environments, one of which could be extracted from Na₃V₂(PO₄)₃ to electrolyte with the transformation of Na₃V₂(PO₄)₃ into NaV₂(PO₄)₃ during the charging process. The other Na site provided a large open site that could accommodate the other four Na ions. During discharging, Zn ions achieved reduction reaction, and Na ions were inserted into NaV₂(PO₄)₃ to form back Na₃V₂(PO₄)₃. In the monoclinic Li₃V₂(PO₄)₃ cathodes, the (PO₄)³⁻ polyanions facilitated to form three dimensional pathway for Li ions insertion/extraction with a very high Li-ion diffusion coefficient (from 10⁻⁹ cm/s to 10⁻¹⁰ cm/s) [79]. The three pairs of redox peaks revealed the two-lithium-ion de/intercalation behaviors. Li₃V₂(PO₄)₃ showed stable cycle performance that the capacity retention was 85.4% after 200 cycles at 0.2 C. Due to the strong affinity of F atoms toward the surroundings, Na₃V₂(PO₄)₂F₃ showed a more stable structure than Na₃V₂(PO₄)₃ [80]. As shown in Fig. 4i, the framework of Na₃V₂(PO₄)₂F₃ provided more convenient pathways for the insertion/extraction of Zn ions. The initially cathodic scan corresponds to the extraction of Na ions from Na(2) sites and the anodic scan corresponds to the insertion of Zn ions (Fig. 4j). And Na₃V₂(PO₄)₂F₃ showed good cycling performance due to the facile diffusion of H ions into the host structure (Fig. 4k). Na₃V₂(PO₄)₂O_1.6F_1.4 [81] exhibited a high operation potential of ~1.46 V and good electrochemical performance. The polyanions can help generate a three-dimensional framework and abundant large vacancies, which leads to promising electrochemical performance.

  In general, the inserted cations and anions not only strengthen atomics force and change the valence states to enhance the conductivity of vanadium-based cathodes, but also promote water molecular to intercalate into the interlayer, leading to the enlarged lattice spacing and increased capacity.

  4. Oxygen vacancy in vanadium compound

  V₃O₇ is a typical low-valent vanadium compound with oxygen vacancy. Kundu et al. [82] prepared layered V₃O₇•H₂O that connected by hydrogen bonds, which was composed of VO₆ octahedra and VO₅ trigonal bipyramids. The hydrogen bonds strongly stabilized the structure compared with other vanadium compounds that were combined by Van der Waals force between layers. V₃O₇•H₂O showed a high capacity of 400 mAh/g at 375 mA/g and small charge transfer resistance of 20 Ω, manifesting fast interface kinetics. Crystalline water is also crucial to the property of V₃O₇. Wang et al. [83] compared the properties between V₃O₇•H₂O and V₃O₇. V₃O₇•H₂O presented higher capacity, meaning that crystalline water maintained the layer structure for the (de)intercalation of Zn ions. Pang et al. [84] synthesized H₂V₃O₈/graphene via a one step hydrothermal method (Fig. 5a). The vacancy sites between V–O octahedrons could accommodate the intercalation of Zn ions (Fig. 5b), leading to a high capacity of 394 mAh/g at 1 C rate (Fig. 5c). V₆O₁₃ with alternative single/double layers showed high capacity and good long-term cyclability [85]. Different from other previously reported Zn ions storage mechanisms, a highly reversible phase of Zn_0.25V₂O₅•H₂O appeared during discharging and disappeared during charging. The phase of Zn_0.25V₂O₅•H₂O not only stabilized the structure, but also produced more phase boundaries, resulting in more active sites, which was conducive to numerous Zn ions storage. Shin et al. [86] further studied the electrochemical mechanism of V₆O₁₃ nanosheeets/Zn system, the morphology was shown in Fig. 5d. They found that co-intercalated water could shield electrostatic interactions with the host matrix and enlarge the channel dimensions (Figs. 5e and f). The materials of V₁₀O₂₄•12H₂O with bilayer structure showed high specific energy of 88.5 Wh/kg with a high power density of 12247.8 W/kg [87].

  Figure 5

  

  Figure 5.  (a) Schematic illustration, (b) crystal structure and (c) cycling performance of H₂V₃O₈/ graphene. Copied with permission [84]. Copyright 2018, Wiley-VCH. (d) SEM image, (e) ex situ FT-IR spectra and (f) kinetic behavior of Zn of V₆O₁₃. Copied with permission [86]. Copyright 2019, Wiley-VCH.

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  Recently, lots of low-valent vanadium compounds with cations, such as K₂V₆O₁₆•2.7H₂O [88], Na₂V₆O₁₆•3H₂O [89], are promising cathodes for aqueous ZIBs. The (NH₄)₂V₆O₁₆•1.5H₂O synthesized by Lai et al. delivered a capacity of 268 mAh/g with 87% retained capacity after 1000 cycles at 5 A/g [90]. Alfaruqi's group [91] revealed the electrochemical mechanism of Zn ions insertion in LiV₃O₈. The inserted Zn ions occupied the sites of Li and the stoichiometric ZnLiV₃O₈ phase formed during the discharge process. With the progress of discharging process, continuous insertion of Zn ions led to the formation of Zn_yLiV₃O₈. During the charging process, the extraction of Zn ions promoted the recovery of the LiV₃O₈. The high reversibility contributed to a high capacity of 267 mAh/g at 16 mA/g. Wang and coworkers [92] synthesized Ba_xV₃O₈-type nanobelts with robust structures. Compared to Ba_xV₂O₅•nH₂O, Ba_xV₃O₈ suppressed the cathode dissolution and eliminated the irreversible byproduct Zn₄SO₄(OH)₆•xH₂O, leading to superior rate capability and long-term cyclic stability. Liu et al. [93] synthesized CaV₆O₁₆•3H₂O based on microwave reaction, and it was a typical vanadium-bronze mineral. The Ca ions were located in the interlayer space and coordinated by oxygens from the two vanadium slabs. Ca ions improved the interplanar spacing and the electrode material reversibly intercalated up to 4.5 equiv. of Zn ions per mole of CaV₆O₁₆•3H₂O, delivering high specific capacity with excellent capacity retention. The preinsertion of dual cations into the V₃O₈ structure could benefit from the advantageous role of both monovalent and divalent cations. Zhu et al. [94] selected Na ions and Ca ions to pre-insert into V₃O₈ layer structure. The NaCa_0.6V₆O₁₆•3H₂O nanobelts showed excellent electrochemical performances: the original capacity reached 347 mAh/g at 0.1 A/g, and the capacity retention rate was 94% after 2000 cycles at 2 A/g.

  It can be concluded that bring oxygen vacancy into Vanadium oxide layers can improve the battery performance in different ways. Intercalating cations can significantly enlarge the interlayer spacing and stabilize the structure, which can improve the specific capacity and electrochemical performance.

  5. Vanadium-based composites with different morphologies

  The past decade has witnessed the successful synthesis of cathode materials in a rich variety of morphologies used for ZIBs, which includes lamellar, linear, ribbon, spherical, etc. The shape not only controls its physicochemical properties but also determines its merit for electronic transmission. For example, the nanowires with ultra-long structure (Fig. 6a) and nanoplates with ultra-thin morphology (Fig. 6b) are favorable to provide a large contact area and shorten the ion diffusion path [45, 85]. Liang et al. prepared hollow nanospheres consisted of nanoflakes (Fig. 6c), which provided more active sites and increased the ion diffusion ability (Fig. 6d) [95]. Some morphologies with micropores provided more ion diffusion ways [96, 97]. As shown in Figs. 6e and f, the scroll shells were composed of 10~20 ultrathin layers (thickness of 2~3 nm) with the spaces of 1~3 nm between adjacent layers, which facilitated easy electrolyte penetration and enhanced the active sites [98]. Microspheres (Fig. 6g) and nanoflowers (Figs. 6h and i) create more pathways, resulting in high capacity [96, 99-101]. These kinds of structures with a large specific surface area can supply more active sites, shorten the transmission channel of electrolyte ions, and have strong structural stability, which leads to superior electrochemical performance.

  Figure 6

  

  Figure 6.  (a) SEM of Na_0.33V₂O₅. Copied with permission [45]. Copyright 2018, Wiley-VCH. (b) SEM of V₆O₁₃. Copied with permission [85]. Copyright 2019, Wiley-VCH. (c) TEM and (d) GITT curves of V₂O₅ nanospheres. Copied with permission [95]. Copyright 2019, Springer. (e) Schematic image and (f) SEM of V₆O₁₃-δ@C nanoscrolls. Copied with permission [98]. Copyright 2020, Wiley-VCH. (g) SEM of V₂O₅ microspheres. Copied with permission [96]. Copyright 2019, the Royal Society of Chemistry. (h) SEM of MgV₂O₄ microsphere. Copied with permission [100]. Copyright 2020, ACS. (i) SEM of NaV₆O₁₅. Copied with permission [101]. Copyright 2020, the Royal Society of Chemistry. (j) TEM and (k) cycle performance of Na_1.1V₃O_7.9. Copied with permission [108]. Copyright 2018, Elsevier.

  DownLoad: Full-Size Img PowerPoint

  Many vanadium based materials suffers from poor electrical conductivity, which results in poor cycling performance and ionic transport kinetics. Graphene [102-105] and CNTs [106, 107] modified cathode materials to stabilize the structure and improve conductivity by providing more efficient pathways for ion/electron transportation. Cai [108] prepared pilotaxitic Na_1.1V₃O_7.9 nanoribbons wrapped by graphene film through facial hydrothermal reaction. The existence of rGO provided modified electronic conductivity and the ability to buffer the variation of stress and volume during discharging/charging process (Fig. 6j). When it was used in aqueous ZIBs, the Na_1.1V₃O_7.9@rGO cathode demonstrated the satisfying electrochemical properties (Fig. 6k). In another literature, carbon and rGO were used together to improve the conductivity and enhance the electrochemical performance of Na₃V₂(PO₄)₃ [78]. The Na₃V₂(PO₄)₃ particles were distributed uniformly on the surface of rGO nanosheets. The rGO sheets enhanced the charge transfer via the interconnected conducting scaffolds and acted as the heterogeneous nucleation sites to facilitate the growth of Na₃V₂(PO₄)₃ particles. CNTs significantly boosted the electrochemical stability of V₂O₅, which has a capacity retention of 72% after 6000 cycles at 5 A/g [106]. CNTs could serve as the electron connecting network, which was beneficial to the Zn ions diffusion [109]. Compared to the pure V₂O₅, the composite film composed of carbon nanotube film and V₂O₅ (CNTF@V₂O₅) showed a super high and stable capacity. [110]. Z. Niu et al. prepared KV₃O₈•0.75H₂O and further incorporated them into single-walled carbon nanotubes (SWCNTs) network, achieving freestanding KVO/SWCNTs composite films. The KVO/SWCNTs cathodes exhibited a Zn²⁺/H⁺ insertion/extraction mechanism, resulting in fast kinetics of ion transfer [111].

  The architecture of nanostructure and carbon-based composite materials can provide more and shorter ion diffusion ways, reduce structural stress during cycles, improve the conductivity, and also reinforce the structure.

  6. Summary and prospect

  Recent years have witnessed great progress in exploiting vanadium-based aqueous ZIBs with regard to the synthesis, characterization, electronic structures and exceptional performances. They have shown excellent electrochemical properties due to the multivalence and layered structure. This review has outlined the various tactics to improve the properties of vanadium-based cathodes, including ions insertion, morphology adjustment, and carbon materials modification. The corresponding synthesis method, electrochemical performance and energy storage mechanism were also discussed.

  Despite the tremendous achievements arising from the vanadium-based aqueous ZIBs in recent years, there are still several issues should be carefully considered for future design of more effective devices. In this section, the associated challenges are summarized and perspectives are also proposed, which was shown in Fig. 7.

  Figure 7

  

  Figure 7.  Drawbacks and prospective strategies of aqueous ZIBs with vanadium-based cathodes.

  DownLoad: Full-Size Img PowerPoint

  (1) The energy storage mechanism of aqueous ZIBs is complex. The interlayer ions, crystal water, or oxygen vacancy may powerfully increase the lattice spacing and provide more ion diffusion ways with reduced kinetic potential energy for the diffusion of hydrous Zn ions. A quantitative understanding for the contribution of different factors in vanadium-based compounds to energy storage mechanism is still lacking for the operation process. Deep understanding for the microstructure evolution of the active material and the reaction mechanisms with the assistance of in-situ technology will beneficial for theoretical guidance for the rational design and application of aqueous ZIBs in the future.

  The unfortunate shortcoming of vanadium-based compounds is their solubility in water system electrolytes. A variety of vanadium oxide or vanadate always undergo detrimental dissolution, exfoliation, or shape degeneration, leading to the structure collapse and drastically change their cycling stability, which makes it difficult to realize large-scale commercial application. In order to successfully use vanadium-based compounds into future renewable energy systems, it is necessary to find new guest species to pre-intercalate into vanadium oxide layers as pillars and the suitable electrolytes.

  (3) The discharge voltage platform of vanadium-based cathodes is relatively low, with usually poor energy density, which makes it difficult to realize efficient application. In order to successfully use vanadium-based cathodes into future renewable energy systems, it is necessary to utilize functional groups, such as PO₄³⁻, F⁻ to increase the operating voltage and the energy density.

  The dendrite growth of Zn anode is still a challenge in aqueous ZIBs, which leads to the capacity fading and battery life degradation. Thus, it is urgently necessary to find new strategies such as designing the structure of Zn anode, changing suitable electrolytes, coating the vanadium-based compounds with the carbon or building artificial inorganic layers to inhibit the growth of Zn dendrite.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was financially supported by the State Key Lab of Advanced Metals and Materials (No. 2020-Z14), the Startup Funds from the Henan University of Science and Technology (Nos. 13480095 and 13480096), the National Natural Science Foundation of China (No. 52002119).

  
  
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  Figure 1  Reaction mechanism of vanadium-based aqueous ZIBs.

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  Figure 2  (a) The HRTEM image with SAED pattern of Li_xV₂O₅•nH₂O at 0.1 A/g. (b) The discharge/charge curves in GITT measurement of Li_xV₂O₅–250 and the corresponding diffusivity coefficient (D) of Zn ion in the discharge and charge processes of the Li_xV₂O₅–250 and V₂O₅–250. Copied with permission [44]. Copyright 2018, the Royal Society of Chemistry. (c) The crystal structure, (d) SEM, HRTEM and (e) ex situ XRD patterns of Na_0.33V₂O₅. Copied with permission [45]. Copyright 2018, Wiley-VCH. (f) The crystal structure, (g) XPS spectra, (h) HRTEM and (i) TEM images of Ag_1.2V₃O₈. Copied with permission [54]. Copyright 2019, Elsevier.

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  Figure 3  (a) The structure analysis, (b) HRTEM image and (c) galvanostatic discharge and charge profiles of Zn_0.25V₂O₅•H₂O. Copied with permission [60]. Copyright 2016, Springer Nature. (d) The crystal structure, (e) XRD pattern and (f) electricity conductivities of Ca_0.25V₂O₅•xH₂O. Copied with permission [61]. Copyright 2017, Wiley-VCH. (g) The XRD pattern, (h) XPS spectra and (i) charge/discharge curves of Mg_0.26V₂O₅•0.73H₂O. Copied with permission [65]. Copyright 2020, Wiley-VCH.

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  Figure 4  (a) The model structure, (b) HRTEM image and (c) K-edge XANES spectra of V_1-xAl_xO_1.52(OH)_0.77. (d) First discharge–charge curves of VO_1.52(OH)_0.77, V_0.95Al_0.05O_1.52(OH)_0.77 and V_0.95Al_0.05O_1.52(OH)_0.77. Copied with permission [72]. Copyright 2017, the Royal Society of Chemistry. (e) HRTEM image, (f) the corresponding Zn ions coefficients, (g) ex situ XRD patterns of Zn₃V₂O₇(OH)₂•2H₂O. (h) HRTEM of the Zn₃V₂O₇(OH)₂•2H₂O at initial, fully discharged (0.2 V) and charged (1.8 V) state. Copied with permission [73]. Copyright 2017, Wiley-VCH. (i) Zn storage mechanism illustrations, (j) CV curves and (k) initial three charge/discharge profiles of Na₃V₂(PO₄)₂F₃. Copied with permission [80]. Copyright 2018, Elsevier.

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  Figure 5  (a) Schematic illustration, (b) crystal structure and (c) cycling performance of H₂V₃O₈/ graphene. Copied with permission [84]. Copyright 2018, Wiley-VCH. (d) SEM image, (e) ex situ FT-IR spectra and (f) kinetic behavior of Zn of V₆O₁₃. Copied with permission [86]. Copyright 2019, Wiley-VCH.

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  Figure 6  (a) SEM of Na_0.33V₂O₅. Copied with permission [45]. Copyright 2018, Wiley-VCH. (b) SEM of V₆O₁₃. Copied with permission [85]. Copyright 2019, Wiley-VCH. (c) TEM and (d) GITT curves of V₂O₅ nanospheres. Copied with permission [95]. Copyright 2019, Springer. (e) Schematic image and (f) SEM of V₆O₁₃-δ@C nanoscrolls. Copied with permission [98]. Copyright 2020, Wiley-VCH. (g) SEM of V₂O₅ microspheres. Copied with permission [96]. Copyright 2019, the Royal Society of Chemistry. (h) SEM of MgV₂O₄ microsphere. Copied with permission [100]. Copyright 2020, ACS. (i) SEM of NaV₆O₁₅. Copied with permission [101]. Copyright 2020, the Royal Society of Chemistry. (j) TEM and (k) cycle performance of Na_1.1V₃O_7.9. Copied with permission [108]. Copyright 2018, Elsevier.

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  Figure 7  Drawbacks and prospective strategies of aqueous ZIBs with vanadium-based cathodes.

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