1. Introduction

In recent years, the demands for electric energy are growing day by day with the development of economic society and industrial civilization. The declining of traditional fossil energy sources forces people to turn their attention to renewable clean energy resources [1]. However, the energy output of renewable clean energy resources is intermittent, which is difficult to meet the customer requirements. So the reliable and efficient electrical energy storage systems (EESS) are considered as a key support to solve the problem of unstable new energy power generation [2]. The ideal EESS possess the following characteristics: pollution-free, high reliability, low cost, high safety, long cycle life, high gravimetric energy, high energy efficiency, and high power density [3]. Lithium-ion batteries with higher energy density (150–265 Wh/kg) have become the star performer of EESS [4, 5], which change the communication ways and the power supply for portable equipment [6-8]. But "Lithium dendrites" formed on the surface of negative electrodes can penetrate the diaphragm, causing an electrical short circuit. And the use of highly-flammable organic electrolytes has a huge negative impact on the safety of lithium-ion batteries. Beside, the cost of manufacturing lithium-ion batteries has been rising. Therefore, rechargeable aqueous metal-ion batteries are brought into focus due to their high safety and low cost.

Aqueous zinc ion batteries (AZIBs) stand out from numerous rechargeable aqueous metal-ion batteries due to its various advantages, such as high theoretical capacity (820 mAh/g or 5855 mAh/cm³), low redox potential (–0.762 V vs. standard hydrogen electrode), high stability and abundant zinc resources [9-14]. However, many cathode materials paired with zinc anode face more problems including low specific capacity, low lifespan and high solubility [15-17], which can greatly affect the overall electrochemical performances of AZIBs. The commercial success of alkaline zinc-manganese dioxide batteries has led to the rapid development of rechargeable Zn-MnO₂ batteries [18-21]. However, the volume expansion and poor conductivity of MnO₂, and the formation of irreversible electrochemical-inert products during the charge/discharge process reduce the capacity of Zn-MnO₂ batteries and shorten its lifespan [22-25]. Whereafter a variety of cathode materials for AZIBs including vanadium-based derivatives [26-28], Prussian blue analogs (PBAs) [29], and organic compounds [30] are gradually developed. There are few studies on PBAs materials due to their limited capacity (< 150 mAh/g), the oxygen evolution reaction under high operating voltage [31-33] and organic compounds owing to the dissolution in the aqueous electrolyte. Vanadium-based compounds have the characteristics of multiple oxidation states, variable crystal structures, high theoretical capacity (> 300 mAh/g), and highly abundant element in the Earth's crust, which has become a research hotspot [34-39].

In this review, the latest investigations of Zn-ion storage mechanism based on the crystal structures of vanadium-based cathodes was summarized. The flexible VO_x polyhedron unit in vanadium-based compounds can be assembled into various open structures such as 1D tunnel, 2D layer and 3D framework by means of different connections to allow for the reversible de/insertion of zinc ions. Additionally, the cations in vanadates as "pillars" can optimize the layer distance to improve the electrochemical performances of AZIBs. The inductive effect of [PO₄] tetrahedron in alkali vanadium phosphates can also provide a higher operating voltage for Zn²⁺ storage. Such variable crystal structures endow vanadium-based compounds excellent electrochemical performance. However, most V-compounds face some problems in inherently poor electrical conductivity, poor structure stability and vanadium dissolution. Finally, the modification strategies for vanadium dissolution and structural instability are proposed for high-performance AZIBs.

2. Vanadium-based materials

Vanadium-based materials are very competitive candidates for AZIBs cathodes. Variable ion diffusion channels of vanadium-based materials affect the electrochemical activity of active materials. At present, there are various types of energy storage mechanisms for AZIBs including Zn²⁺ insertion, H⁺ insertion, Zn²⁺/H⁺ co-insertion, Zn²⁺/H₂O co-insertion and chemical conversion reactions. In this section, the vanadium-based materials are classified based on the material types and their storage mechanisms are discussed based on crystal structures.

2.1. Introduction of vanadium oxides in AZIBs

Vanadium has 5 valence electrons that can be lost, so it has the ability to adopt multi-oxidation states (from +2 to +5) and to form numerous complexes. The vanadium-oxygen coordination polyhedron in vanadium oxides varies with their oxidation states from tetrahedron through trigonal bipyramid and square pyramid to distorted octahedra and rectilinear octahedra (Fig. 1a), in which the oxidation state of vanadium atom decreases from +5 through +5/+4 to +3 valence [40]. These polyhedrons link to form the chain structure by sharing corner, which link in turn into layers and then three-dimensional (3D) frameworks (Fig. 1b). These open structures can provide a variety of channels for Zn²⁺ insertion/extraction [41, 42].

2.1.1. Vanadium sesquioxides (V₂O₃)

The trigonal phase V₂O₃ (R3c) has a 3D framework structure, which allows 3d electrons to move back and forth through the V-V chain followed by displaying metallic behavior. V atoms occupy two-thirds of octahedral interstices of O atoms, meanwhile two [VO₆] octahedrons link by sharing corner, face and edge with the adjacent one to form a tunnel-like 3D structure (0.288 × 0.291 × 0.291 × 0.410 nm⁴ along the (111) lattice plane). These tunnel structures are conducive to the insertion/extraction of metal ions, so V₂O₃ materials are often used as electrode materials in electrochemical energy storage. Ding et al. [43] firstly applied porous V₂O₃ pyrolyzed from vanadium-based MOFs precursors as cathode of AZIBs. The cyclic voltammetry (CV) plot in Fig. 2a shows two coupled redox peaks at 0.92/1.04 and 0.56/0.72 V, which involved the two-step de/insertion reaction of Zn²⁺. This electrode displays a specific capacity of 300 mAh/g at 100 mA/g calculated based on Fig. 2b and also exhibits outstanding cycling stability with a capacity retention of 90% after 4000 cycles at 5 A/g (Fig. 2c). Ex-situ X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Raman tests were adopted to analyze the Zn²⁺-storage mechanism. In the discharge cycle, H₂O molecules along with Zn²⁺ were migrated into the tunnel structures of V₂O₃ and then were leftover in the charge process, which would help to expand the ion diffusion channel and reduce the diffusion resistance of Zn²⁺ (Fig. 2d). The first-principle calculation further confirmed that the intercalated-Zn²⁺ occupied the octahedral interstices of O atoms to form a stable structure (Figs. 2e and f). The differential charge state density in Fig. 2g showed a chemical bond formation between Zn²⁺ and O²⁻ from V₂O₃. The inserted Zn²⁺ reduced the distance and strengthened the interaction between the two atoms, resulting in the improved cycle stability of AZIBs. However, the octahedral interstice was not large enough for more than one zinc ion storage, which was confirmed by the corresponding calculation results. Subsequently, Ding et al. [44] found that the vanadium vacancies could also facilitate the Zn²⁺ intercalation/extraction tested via operando XRD and Raman analysis. Defect-rich V₂O₃ obtained from V₂O₅ nanosheets calcinated in NH₃/Ar atmosphere by in-situ electrochemical lattice conversion reactions. Defect-rich V₂O₃ as cathode achieved a high capacity of 382.5 mAh/g and a remarkable rate performance. In the first charge process, the lattice structure of V₂O₃ was distorted to obtain vanadium vacancies, which were favorable for the Zn²⁺ de/intercalation in the subsequent cycles. The activated V₂O₃ by fabricating the vanadium vacancies could fasten the reaction kinetics of AZIBs, but the detailed reaction mechanism was still unclear. Luo et al. [45] investigated the reaction mechanism of fastening the electrochemical kinetics by preparing hollow-carved carbon-coated V₂O₃ microcuboids with abundant channels, short ion diffusion distance and good electrical conductivity (Fig. 2h). The electrolyte concentration played an important role in the electrochemical performance of V₂O₃ electrodes. The capacity and cycle stability in 2 mol/L Zn(CF₃SO₃)₂ were superior to those of 1, 3 and 4 mol/L. The maximum capacity, high rate performance and capacity retention in 2 mol/L electrolyte was 625 mAh/g at 0.1 A/g, 486 mAh/g at 20 A/g, and 100% after 10,000 cycles, respectively (Fig. 2i). H₂O as reductant provided the electron to V₂O₃ to form V₁₀O₂₄·12H₂O (Fig. 2j) in the initial charge process, and then layer Zn_3+x(OH)_2+3xV_2−xO_7−3x·2H₂O appeared in the subsequent discharge process. V₁₀O₂₄·12H₂O and Zn_3+x(OH)_2+3xV_2−xO_7−3x·2H₂O gradually were converted into V₂O₅·nH₂O and Zn_xV₂O₅·nH₂O, respectively after 20^th charging/discharging.

2.1.2. Vanadium dioxide (VO₂)

VO₂ phase with different space groups (B (C2/m), M (P2), A (P4), D (P2/c)) has been widely studied as the cathode material for AZIBs. The [VO₆] octahedron as the basic structural unit of the above-mentioned four phases links by edge or vertice to form a tunnel structure. The two octahedrons of VO₂(B) connect along the b-axis direction and the second layer moves (1/2, 1/2, 0) (fractional coordinates in unit cell) relative to the first layer, shaping into a tunnel structure. The V-V atoms in monoclinic VO₂(M) form a zigzag-chain structure along the c axis, causing a staggered transverse displacement of vanadium atoms and an oxygen octahedron distortion. Hence, VO₂(M) has denser tunnels than VO₂(B). Similar to the VO₂(M) structure, VO₂(D) is built based on one chain composed of alternating the edge-sharing distorted [V(1)O₆] octahedron linking to another individual chain composed of [V(2)O₆] octahedron through corner-shared oxygen atoms. For VO₂(A), four sets of two-sided octahedrons on the c plane form a 2 × 2 square. These squares stack along the c axis by edge-sharing to form long Z-shaped V-atom chains.

VO₂(B) is suitable for use as the electrode materials of AZIBs due to higher theoretical capacity and the tunnel structure with plus sizes (0.34, 0.82 and 0.5 nm³ along a-, b- and c-axes, respectively) for the Zn²⁺ migration. Ding et al. [46] investigated the interaction behavior of Zn²⁺ with VO₂(B) in AZIBs assembled with VO₂(B) nanofibers as cathode, Zn foil as anode, and Zn(CF₃SO₃)₂ as electrolyte (Fig. 3a). VO₂(B) cathode displayed a specific capacity of 357 mAh/g (Fig. 3b) and superior rate capability (171 mAh/g at 51.2 A/g in Fig. 3c), ascribing to the intercalation pseudocapacitance behavior and the ultrafast kinetics of Zn²⁺ into the tunnel structures of VO₂(B). The lattice parameter variation with the Zn²⁺ intercalation into the tunnels (paralleled to the b- and c-axes of VO₂(B)) tested by in-situ XRD diffraction was less, suggesting that these tunnels were stable and large enough to ensure the fast Zn²⁺ intercalation/de-intercalation (Fig. 3d). Therefore, VO₂(B) cathode with tunnel-structure was proved feasible to permit the insertion/extraction of Zn²⁺ in laboratory-scale research. Park et al. [47] further revealed the electrochemical reaction mechanism of AZIBs using a bond-valence sum energy map and a first-principle calculation. Among the four sites (Zn_c, Zn_A1, Zn_A2, Zn_c') of inserting Zn²⁺ into VO₂(B) structure shown in Fig. 3e, Zn_A2 site was the optimal location of Zn²⁺ into VO₂(B) and the predicted redox potential of Zn_xVO₂(B) was ~0.61 V (Fig. 3f), close to the experimental value of ~0.7 V (VO₂(B) + 0.57Zn²⁺ + 1.14e⁻ ↔ Zn_0.57VO₂(B), Fig. 3g). The pseudocapacitance contribution of Zn²⁺ intercalating into VO₂(B) structure, evaluated using potentiodynamic electrochemical impedance spectroscopy (PDEIS) technique was negligible. But the strong Coulombic ion-lattice interaction between Zn²⁺ and VO₂(B) structure slowed down the kinetics and further limited the improvement of electrochemical properties. Oxygen-deficient VO₂ nanostructure could address the above-mentioned problem through providing the jumping charge transport sites via enlarging the b tunnels for fast zinc-ion diffusion. VO₂ with high-concentration oxygen vacancies prepared by Li et al. [48] displayed a discharge specific capacity of 375 mAh/g at 100 mA/g and long-term cyclic stability with a retained capacity of 175 mAh/g at 5 A/g over 2000 cycles in 3 mol/L Zn(CF₃SO₃)₂. However, in aqueous electrolytes, Zn²⁺ often exists in the form of Zn(H₂O)₆²⁺, indicating that larger desolvation energy and insertion energy need to be overcome in the process of Zn²⁺ insertion. And the strong electrostatic repulsion between Zn²⁺ and host structure also causes poor diffusion kinetics and mild deformation in the host structure. So Li et al. [49] announced a H⁺ (de)insertion mechanism with less structure distortion of VO₂(B) than the Zn²⁺ insertion mechanism. The VO₂(B) cathode exhibited excellent electrochemical performances of the first discharge capacity (353 mAh/g at 1.0 A/g), a 75.5% capacity retention after 945 cycles at 3.0 A/g in 1 mol/L ZnSO₄ (Fig. 3h). The whole reaction steps were shown in Fig. 3i. The generated H⁺ in the deposition process of Zn₄(OH)₆SO₄·5H₂O could insert into VO₂(B) to contribute most of capacity by the first-principle analysis (Fig. 3j) and keep long-term cycle stability. Zhang et al. [50] further verified the proton insertion mechanism by the bond valence method using VO₂(M) as cathode and ZnSO₄ as electrolyte. While storing Zn²⁺ in VO₂(D) phase, H⁺ and Zn²⁺ could simultaneously insert into and extract from VO₂(D) during the cycling process, resulting in a phase transition from VO₂(D) to V₂O₅·xH₂O [51]. H₂O molecules with Zn²⁺ co-intercalation into host structure facilitated the intercalation of Zn²⁺ due to shielding the electrostatic interactions between charge carrier and VO₂(D).

2.1.3. Vanadium (IV, V) oxide (V₆O₁₃)

The monoclinic V₆O₁₃ (C2/m) with mixed-valence vanadium (V⁴⁺/V⁵⁺) consists of alternating single- and double-layer distorted [VO₆] octahedrons connected by sharing corner to form chains zigzag along the b-axis [52], which can provide more active sites for ion storage (such as Li⁺ capacity: 417 mAh/g) [53, 54]. As shown in Fig. 4a, the valence states of V(1) atoms in single-layer and V(3) atoms in double-layer are +4, while the valence state of V(2) atoms in the double layer is +5. So the ratio of V⁴⁺/V⁵⁺ is 2:1 and an average mixed vanadium valence is V^4.33+ [55]. Moreover, V₆O₁₃ at room temperature has a metallic character.

Shan et al. [56] prepared V₆O₁₃ cathode and investigated the Zn²⁺ storage mechanism in AZIBs. The V₆O₁₃ electrode displayed a specific capacity of 206 mAh/g at 10 A/g after 3000 cycles and an excellent rate performance owning to higher Zn²⁺ ion diffusion coefficient, better electronic conductivity than V₂O₅ and VO₂ (Figs. 4b and c). Interestingly, the highly-reversible Zn_0.25V₂O₅·H₂O phase promoted the storage of Zn²⁺ verified by ex-situ XRD, high-resolution transmission electron microscope (HRTEM) and XPS (Fig. 4d). Fig. 4e illustrates the Zn²⁺ insertion/extraction mechanism. During the discharge process of aqueous Zn/V₆O₁₃ cell, Zn²⁺ is inserted into the empty sites of V₆O₁₃ phase to form a new Zn_0.25V₂O₅·H₂O phase; while in the subsequent charge process, Zn²⁺ is extracted from V₆O₁₃ and Zn_0.25V₂O₅·H₂O phase simultaneously disappears. A structural engineering strategy of oxygen-deficient is usually adopted to fasten the kinetics and boost the Zn²⁺ diffusion pathways for the reversibility and capacity of V₆O₁₃ cathode. As an example, the oxygen-deficient V₆O₁₃ structure was fabricated by a two-step method from solution-redox-based self-assembly method to thermal treatment under a reducing N₂/H₂ atmosphere, which was explored to create more Zn²⁺ intercalating sites [57]. The resulting V₆O₁₃ cathode retained a high specific capacity of 400 mAh/g after 200 cycles, reaching a utilization rate of 95% of the theoretical specific capacity. Its capacity retention still remained 86% after 2000 cycles because the vacancies of V₆O₁₃ provided more pathways of Zn²⁺. The surface electron density difference analysis proved that the charge distributions of oxygen-deficient V₆O₁₃ structure in Figs. 4h and i were more concentrated than those of perfect V₆O₁₃ in Figs. 4f and g, resulting in the dual-direction Zn²⁺ insertion formation and the capacity increase. As we all know, the strong electrostatic attraction between Zn²⁺ and host structure could hinder the Zn²⁺ diffusion, so CO₂-trapped in V₆O₁₃ was designed to modify the electrochemical performance [58]. The as-prepared CO₂-V₆O₁₃ electrode showed a maximum capacity of 471 mAh/g (Fig. 4j) and an excellent cyclic stability of 80% capacity retention after 4000 cycles at 2 A/g (Fig. 4k).

2.1.4. Vanadium pentoxide (V₂O₅)

The layered orthorhombic V₂O₅ structure (Pmmn) is build-up of zigzag double chains of edge and corner sharing distorted [VO₅] square pyramids. The layers are connected along the c-axis by weak electrostatic interactions, which are readily available for the intercalation of Zn²⁺. The interlayer spacing of 0.577 nm in V₂O₅ is much larger than the radius of Zn²⁺ (0.074 nm), so it can deliver a high theoretical capacity of 589 mAh/g.

Zhang et al. [36] assembled AZIBs based on ball-milled commercial V₂O₅ cathode, Zn anode and 3 mol/L Zn(CF₃SO₃)₂ electrolyte, whose reversible capacity reached 470 mAh/g at 0.2 A/g (Fig. 5a) and capacity retention was 91.1% over 4000 cycles at 5 A/g (Fig. 5b). Chen et al. [59] prepared porous V₂O₅ nanofibers (Fig. 5c) by electrospinning technology, which displayed the capacity of 319 mAh/g at 20 mA/g and capacity retention of 81% over 500 cycles at 2 C (Fig. 5d) in Zn(CF₃SO₃)₂ electrolyte. A two-step conversion mechanism was adopted to explain the Zn²⁺ insertion/extraction (Fig. 5e): the open-structured zinc pyrovanadate (Zn_3+x(OH)₂V₂O₇·2H₂O) formation in the first discharge and then the reversible insertion/extraction of Zn²⁺ between Zn_3+x(OH)₂V₂O₇·2H₂O and Zn_3+y(OH)₂V₂O₇·2H₂O. Optimizing electrode manufacture is an effective strategy to enhance the specific capacity of active materials. Javed et al. [60] directly grew 2D V₂O₅ nanosheets on Ti substrate to increase the contact between V₂O₅ and collector to avoid agglomeration, which as cathode exhibited a discharge capacity of 503.1 mAh/g at 100 mA/g and long-term stability with 86% retention after over 700 cycles at 500 mA/g.

The pre-intercalation of light alkali metal ions into V₂O₅ structure was also an effective strategy to improve the electrochemical properties through enlarging the interlayer distance. For instance, Na_0.3V₂O₅ nanowire cathode had a high capacity of 367.1 mAh/g at 0.1 A/g and its capacity retention reached 93% after 1000 cycles in Zn//Na_0.3V₂O₅ batteries [61]. The Na⁺ as "pillars" between V₂O₅ layers not only enhanced the conductivity of V₂O₅ but also improved the structural stability of V₂O₅ structure during the Zn²⁺ insertion/extraction. The incorporation of K⁺ into V₂O₅ layers adopted the Zn²⁺/H₂O co-insertion storage mechanism. The K_0.5V₂O₅ cathode showed excellent zinc storage performances with a reversible capacity of 397 mAh/g after 100 cycles at 1 A/g and 251 mAh/g after 1000 cycles at 5 A/g [62]. Even when the temperature was as low as –20 ℃, the capacity still reached to 241 and 115 mAh/g after 1000 cycles at 1 and 5 A/g, respectively. Density functional theory calculations further demonstrated that the introduction of K⁺ promoted the electron migration in the layer and thus boosted the (de)intercalation kinetics of Zn²⁺. In addition, there are other metal ions such as Li⁺ [63], Al³⁺ [64], Ag⁺ [65], Mg²⁺ [66], Zn²⁺ [67] and Ca²⁺ [68] to be investigated.

2.2. Introduction of vanadates in AZIBs

Vanadates as the derivatives of vanadium oxides are widely used in AZIBs owing to various cations. Cations in vanadates as pillars are inserted into the layers of different vanadium oxides to form a variety of vanadates (M_xV₃O₈, M_xV₂O₇, M_xV₆O₁₆, and M_xV_yO_z) with highly stable structure.

2.2.1. Main group trivanadates (M_xV₃O₈)

The monoclinic M_xV₃O₈ structure is formed by the stacked [V₃O₈] layers and these layers are connected by M ions. Each [V₃O₈] layer consists of [VO₆] octahedrons and [VO₅] trigonal bipyramids. [VO₆] octahedron links with [VO₅] trigonal bipyramids by sharing vertice, resulting in an octahedral vacancy formation in the middle. M ions occupy the vacancy of [VO₆] octahedron, in which ions are escaped uneasily [69-71]. Among them, H₂V₃O₈ is an important member. H atoms between the layers link with O atoms in [VO₆] polyhedron by a strong hydrogen bond, which can buffer the expansion/contraction of H₂V₃O₈ structure during the ion insertion/extraction [70]. The ratio of V⁵⁺/V⁴⁺ is 2:1 in H₂V₃O₈, which contributes to higher electrical conductivity than other vanadium oxides. For example, H₂V₃O₈ nanowires (Fig. 6a) as cathode delivered a high initial capacity of 423.8 mAh/g at 0.1 A/g (Fig. 6b) and a capacity retention rate of 94.3% after 1000 cycles at 5 A/g (Fig. 6c) [72]. The large interlayer spacing of H₂V₃O₈ contributed to an excellent Zn²⁺ storage performance. The similar phenomenon occurs in other monovalent ions (Li⁺, Na⁺, K⁺). LiV₃O₈, similar to the crystal structure of H₂V₃O₈, also showed good electrochemical performance due to expanding the interlayer spacing [71]. ZnLiV₃O₈ transition phase formed at the early stage of discharge when Zn²⁺ were inserted into the Li-empty sites of LiV₃O₈ (Fig. 6d), and then ZnLiV₃O₈ transformed to Zn_yLiV₃O₈ (y > 1) in the continuous intercalation of Zn²⁺. However, in the charging process, Zn_yLiV₃O₈ was directly reduced to LiV₃O₈, suggesting that the intercalation/extraction of Zn²⁺ was reversible and Li⁺ was irreversible.

The ion radius of Na⁺ (0.102 nm) is larger than those of H⁺ and Li⁺ (0.076 nm), so NaV₃O₈ with larger layer spacing (~0.71 nm) can promote ion transfer [73]. For instance, disordered NaV₃O₈ cathode exhibited a capacity of 265 mAh/g at 0.05 A/g in 3 mol/L ZnSO₄ and 117 mAh/g at 0.4 A/g over 2000 cycles in 3 mol/L ZnSO₄/0.5 mol/L Na₂SO₄ mixed electrolyte [74]. The storage behavior of Zn²⁺ was sequential two-stage intercalation of Zn²⁺ and H⁺ during discharging. If 1 mol/L Na₂SO₄ was added into 1 mol/L ZnSO₄ solution, NaV₃O₈·1.5H₂O nanoribbons could co-insert/extract of H⁺ and Zn²⁺, and then maintain a reversible capacity of 380 mAh/g at 0.05 A/g and a capacity retention of 82% over 1000 cycles at 4 A/g [75]. If 0.3 mol/L VOSO₄ was added in 3 mol/L ZnSO₄ electrolyte, fiber-like NaV₃O₈ cathode underwent a reversible three-ions insertion of VO²⁺, Zn²⁺ and H⁺ at the discharged state and displayed a high specific capacity of 450 mAh/g at 0.05 A/g and a capacity retention rate of 82% at 0.4 A/g after 500 cycles [76]. VO²⁺ carrier with large ion radius (0.16 nm) can still intercalate the layer of host structure and contribute the storage capacity. KV₃O₈ electrodes with an interlayer distance of ~0.77 nm delivered a discharge capacity of 182 mAh/g at 1.75 A/g with ~82.8% retention of initial capacity for 500 cycles at 1.25 A/g in 1 mol/L Zn(CF₃SO₃)₂ electrolyte [77]. The pillar of K⁺ in host structure promoted an excellent capacity retention even at high rates due to its larger ion size.

2.2.2. Main group polyvanadates (M_xV₂O₇)

The monoclinic α-Zn₂V₂O₇ (C2/c) and trigonal Zn₃V₂O₇(OH)₂·2H₂O (P3m1) phases are the typical representatives of M_xV₂O₇. [V₂O₇] groups consist of a pair of [VO₄] tetrahedrons both in α-Zn₂V₂O₇ and Zn₃V₂O₇(OH)₂·2H₂O structure, which align along c-axis to form the layer structure. [ZnO₅] polyhedron in α-Zn₂V₂O₇ is a distorted trigonal bipyramid and [ZnO₆] polyhedron in Zn₃V₂O₇(OH)₂·2H₂O is octahedron. The V-O-V pillars (tetrahedra) separate each Zn-O layer and H₂O molecules can be randomly accommodated to ensure the facile diffusion of Zn ions [78-84]. The α-Zn₂V₂O₇ nanowire displayed a high capacity retention of 85% after 1000 cycles at 4 A/g [80]. Its unique 1D nanowire morphology shortened the diffusion pathway of Zn²⁺ and electron transport distance because the growth direction of nanowires was parallel to the (002) plane (c-axis). Similarly, Zn₃V₂O₇(OH)₂·2H₂O nanowires with porous framework prepared by a simple microwave method improved the intercalation of Zn²⁺ [81] and delivered the capacities of 213 and 76 mAh/g at 50 and 3 A/g, respectively, as well as good cycling stability. Cao et al. compared the electrochemical performances of 1D nanowires (ZnVW) and 2D nanoflakes (ZnVF) for Zn₃V₂O₇(OH)₂·2H₂O (Figs. 7a–f) [78]. The nanowire cathode achieved more excellent discharge capacity of 108 mAh/g after 700 cycles at 2 A/g than that of the nanoflake cathode due to lower polarization, higher electrochemical activity, and faster ion diffusion capability of 1D materials. Hence, the electrochemical performances of M_xV₂O₇ are highly dependent on preparation methods that influenced its morphology, particle size, and crystallinity beside inherent crystal structure.

2.2.3. Hewettite (M_xV₆O₁₆·nH₂O)

The monoclinic M_xV₆O₁₆·nH₂O comprises [V₃O₈] layers, which are constructed by edge-sharing [V₂O₈] square pyramids and [VO₆] octahedrons, and interlayer hydrated M ions connect them together [85]. Much interest focuses on Na₂V₆O₁₆·nH₂O and K₂V₆O₁₆·nH₂O. Hu et al. [86] constructed 1D Na₂V₆O₁₆·1.63H₂O nanowire (Fig. 8a) by hydrothermal method with a high specific capacity of 352 mAh/g at 50 mA/g, a good rate performance and a high retention rate of 90% after 6000 cycles at 5 A/g (Fig. 8b). Zn²⁺ and H₂O molecules were simultaneously inserted into the host structure in the first discharge (shown in Fig. 8c), and then only Zn²⁺ were reversibly inserted/extracted in the subsequent cycles. 1D Na₂V₆O₁₆·1.63H₂O nanobelt electrode adopted a similar Zn-storage mechanism and also exhibited superior electrochemical performance including a large discharge capacity of 466 mAh/g at 100 mA/g and excellent cycle stability with capacity retention of 90% over 2000 cycles at 20 A/g [87]. Sambandam et al. synthesized K₂V₆O₁₆·2.7H₂O nanorods, which as cathode achieved a reversible capacity of nearly 180 mAh/g at 6 A/g [88]. H₂O molecules co-intercalated with Zn²⁺ as lubricant efficiently reduce the "effective charge" of Zn²⁺, thereby resulting in a higher diffusion coefficient of solvated Zn²⁺ and good electrochemical performance. 1D nanomaterials can offer more electrochemically active sites to promote the significant interfacial redox reaction. So the nanostructured morphologies of vanadium-based materials as desirable zinc ion hosts can contribute to improving their electrochemical properties.

Compared to common intercalation metal ions, the existence of hydrogen bonds between NH₄⁺ and [V₃O₈] layer offers a more stable layer structure, contributing to long-term cycling performance. Additionally, the lower molecular weight would provide higher gravimetric specific capacities theoretically. Zhang et al. [89] reported that there was the intensive hydrogen bond interaction between H atoms from H₂O and NH₄⁺ and O atoms from [V₃O₈] layer, which value was calculated to be 208.01 meV/Ų by first-principles, higher than 49.58 meV/Ų of V₂O₅, suppressing the dissolution of (NH₄)₂V₆O₁₆·1.5H₂O. In the static immersion experiments of V₂O₅, V₂O₅·1.5H₂O, NH₄V₃O₈ and (NH₄)₂V₆O₁₆·1.5H₂O cathodes, only (NH₄)₂V₆O₁₆·1.5H₂O materials remained colorless after 200 days, suggesting the strong stability in the electrolyte. Furthermore, Wang et al. [90] presented (NH₄)₂V₆O₁₆·1.5H₂O nanoribbons, which not only contributed to a high reversible capacity of 479 mAh/g at 0.1 A/g but also remained the specific capacity of 152 mAh/g at 5 A/g even after 3000 cycles. The results of ex-situ XRD, FTIR, XPS and HRTEM verified that the electrochemical reaction mechanism of (NH₄)₂V₆O₁₆·1.5H₂O electrode was as follows: Zinc ions were intercalated into (NH₄)₂V₆O₁₆·1.5H₂O along with a new phase of Zn₃(OH)₂V₂O₇·2H₂O formation during the discharge process and were extracted from (NH₄)₂V₆O₁₆·1.5H₂O accompanied by the new phase disappearing during the charging process.

2.3. Introduction of alkali vanadium phosphates (M_xV₂(PO₄)₃, M = Li, Na) in AZIBs

M_xV₂(PO₄)₃ (M = Li, Na) structure is composed of [VO₆] octahedrons and [PO₄] tetrahedrons by sharing all vertices to form the "lantern" [V₂(PO₄)₃] unit framework, where M atoms occupy the voids of tetrahedrons and octahedrons (Fig. 9a). The open frame structure can endure the significant volume change (radius of Na⁺ (0.102 nm) and Li⁺ (0.076 nm) vs. Zn²⁺ (0.074 nm)) in the ion insertion or extraction process [91-97]. Monoclinic Li₃V₂(PO₄)₃ (P2, LVP) and rhombohedral Na₃V₂(PO₄)₃ (R3c, NVP) cathodes not only provide a robust structure but also a high working voltage (~1.8 V vs. Zn²⁺/Zn). Zhao et al. [98] compared the electrochemical mechanisms of Li₃V₂(PO₄)₃ (LVP) and Na₃V₂(PO₄)₃ (NVP) cathodes in mixed aqueous electrolyte (1 mol/L Li₂SO₄ and 2 mol/L ZnSO₄). NVP cathode experienced Na⁺ and Li⁺ co-insertion while only two Li⁺ insertion for LVP cathode. Zn²⁺ precipitated on the surface of zinc metal, leading to initial discharge capacities of 128 and 96 mAh/g at 0.2 C for LVP and NVP cathodes, respectively. Compared with the low charge of Li⁺/Na⁺, Zn²⁺ with strong electrostatic interaction force was difficult to be inserted into the LVP/NVP lattice. However, Zn²⁺ could intercalate Na-deficit NVP structure (NaV₂(PO₄)₃) through NVP to form Zn_xNaV₂(PO₄)₃ in the first discharge process and de-intercalate after charging shown in Fig. 9b [29], resulting in a reversible capacity of 97 mAh/g at 0.5 C and a capacity retention rate of 74% after 100 cycles in 0.5 mol/L Zn(CH₃COO)₂ electrolyte. If AZIBs assembled with NVP@reduced graphene oxide (NVP@rGO) microsphere cathode and 2 mol/L Zn(CF₃SO₃)₂ electrolyte [99], Zn²⁺ and Na⁺ were simultaneously inserted into NVP structure tested by ex-situ XPS and XRD. Galvanostatic intermittent titration technique (GITT) test showed that the divalent Zn²⁺ had higher diffusion coefficient than monovalent Na⁺. The complete electrochemical reaction equation was as follows: Na_xV₂(PO₄)₃ + zZn²⁺ + yNa⁺ + (2z+y)e⁻ ↔ Zn_zNa_(x+y)V₂(PO₄)₃. The specific capacity of NVP@rGO cathode reached up to 74 mAh/g at 500 mA/g after 200 cycles and its rate performance was also excellent (107 mAh/g at 50 mA/g and 82 mAh/g at 2 A/g) (Fig. 9c). Ko et al. [100] proposed a quasi-two-stage Na⁺ and Zn²⁺ insertion mechanism based on the analysis results of synchrotron XRD and Rietveld refinement using porous NVP as cathode and 0.5 mol/L Zn(CH₃COO)₂ as electrolyte. The formation of Na-deficit NVP structure (NaV₂(PO₄)₃) at the initial charging stage resulted in the predominant intercalation of Na⁺ followed by the Zn²⁺ intercalation into Na2 (18e site) place of NVP, which was more active than Na1(6b site) place. When discharged to 0.6 V, Zn_0.25NaV₂(PO₄)₃ phase with good electrochemical reversibility formed. However, the reason for Zn²⁺ occupying only Na2 site was still unknown. Hu et al. [101] revealed the Na⁺ and Zn²⁺ mixed occupation at both Na1 and Na2 sites of Zn_xNaV₂(PO₄)₃ based on experimental and theoretical methods, which improved the conductivity, crystal structure and structural stability of Zn_xNaV₂(PO₄)₃ after Zn²⁺ insertion. Na1 sites were activated due to the concerted migration of Na⁺/Zn²⁺. Recently, Li et al. [102] demonstrated that the NASICON-type compounds were not suitable for AZIB because of their structure decomposition during the long cycles (Fig. 9d) after comparing the structure stability of Na₃V₂(PO₄)₃ and Na₃V₂(PO₄)₂F₃ cathodes in 1 mol/L Zn(CF₃SO₃)₂. During cycling, Na₃V₂(PO₄)₃ decomposed into Zn₃V₂O₈, V₂O₅, and V₂O, while Na₃V₂(PO₄)₂F₃ decomposed into V₂O₅, VPO₅, and Zn₃(OH)₂V₂O₇·2H₂O, greatly reducing the stability of NASICON-type compounds.

2.4. Introduction of other vanadium-based materials in AZIBs

Vanadium disulfide (VS₂) is a representative member of transition metal dichalcogenide, which possesses the unique chain layered structure analogous to graphite. V atoms and S atoms in VS₂ are covalently bonded, and neighboring layers (S-V-S) with a large interlayer distance of 0.58 nm are connected by weak van der Waals interaction, which can accommodate more the guest ion storage of Zn²⁺, Al³⁺ and Mg²⁺ [103-107]. Even if these guest ions are intercalated in the host structure, the structure of VS₂ is not significantly broken [108, 109]. For instance, He et al. [26] firstly investigated the insertion mechanism of Zn²⁺ for VS₂ nanosheet by means of in-situ Raman, ex-situ XRD and ex-situ XPS tests. The phase transitions from VS₂ to Zn_0.09VS₂ and then to Zn_0.23VS₂ occurred within a voltage range of 0.65–0.82 and 0.45–0.65 V during the discharge process, respectively. The interlayer expansion of VS₂ along (002) after Zn²⁺ inserting is only 1.73%. Hence VS₂ nanosheet cathode for AZIBs provides a reversible specific capacity of 190.3 mAh/g at 0.05 A/g under the voltage window of 0.4–1.0 V. Even at 0.5 A/g, the first capacity retention rate reached 98% after 200 cycles. But in fact, the narrow voltage window (0.4–1.0 V vs. Zn²⁺/Zn) limited the energy density of VS₂. Yu et al. [110] increased the working potential by in-situ fabricating heterostructural VS₂/VO_x materials. The average discharge voltage of VS₂/VO_x cathode was 0.9 V (vs. Zn/Zn²⁺), which is 0.25 V higher than that of the pristine VS₂ cathode. So the composite electrode provided a reversible capacity of 150 mAh/g at 0.1 A/g. Even at 1 A/g, the capacity retention rate reached 75% after 3000 cycles. Liu et al. [111] adopted N-doped carbon as the conductive supports instead of VO_x to improve the chemical stability and long-cycle performance with the help of strong interfacial interaction between VS₂ and N-doped carbon. The resulting cathode displayed a high specific capacity of 203 mAh/g at 50 mA/g and long-term cycling stability with a capacity retention of 97% after 600 cycles at 1 A/g.

Monoclinic VS₄ (I2/c) as an analog of VS₂ is a unique 1D atomic-chain structure bonded by weak van der Waals forces, which is consisted of V⁴⁺ coordinated to sulfur dimers (S₂²⁻). VS₄ with more S atoms shows larger interlayer space (0.583 nm) than that of VS₂ [112-115]. Qin et al. [116] prepare VS₄@rGO composite by the hydrothermal method, which displayed a capacity as high as 180 mAh/g at 1 A/g after 165 cycles and a capacity retention of 93.3%. During the battery charge/discharge, a conversion reaction occurred between VS₄ and Zn₃(OH)₂V₂O₇·2H₂O.

Hexagonal vanadium diselenide (VSe₂, P3m1) is also a typical layered structure, where V atoms and Se atoms combine with covalent bonds to form the Se-V-Se sandwich structure. And the layers connect each other by van der Waals forces along c-axis direction. The electronegativity of Se is lower than those of O and S, which reduced the electrostatic interaction between the intercalation ions and the host structure and lowered the electron migration barrier [117]. Wang et al. [118] firstly presented VSe₂ cathode with a large interlayer spacing of 0.61 nm and showed a discharge storage capacity of 250.6 mAh/g at 200 mA/g and a reversible specific capacity of 132.6 mAh/g even at a high current density of 5 A/g. Bai et al. [119] adopt defect engineering to fabricate the stainless steel (SS)-supported VSe₂ nanosheets with defects to improve the conductivity and fasten the ion transmission speed (D_Zn²⁺ ≈ 10⁻⁸ cm²/s). Therefore, the synergistic effects of selenium defects and self-supporting structure improved the VSe_2-x-SS structure stability and increased the specific capacity (265 mAh/g).

3. Conclusions and outlooks

Generally, the zinc-ion storage mechanisms of vanadium-based cathodes are the crystal structure-related theory. Table 1 lists the electrochemical performance and energy storage mechanism of vanadium-based cathodes. The storing Zn²⁺ ways of the monoclinic VO₂, V₆O₁₃ and VS₄ phases with 1D-tunnel structure heavily depend on different lattice types. The VO₂(D) and VO₂(M) cathodes with monoclinic-P lattice structure adopt the Zn²⁺/H⁺ co-insertion mechanism. The VO₂(B) and V₆O₁₃ phases with monoclinic-C lattice structures are inclined to the Zn²⁺ insertion mechanism. However, in other monoclinic-C lattice structure (I2/c), VS₄ cathode prefers to the conversion reactions and Zn²⁺ insertion mechanism.

Most layered vanadium-based materials such as V₂O₅, VS₂, VSe₂, Zn₃V₂O₇(OH)₂·2H₂O and Na₂V₆O₁₆·2.14H₂O provide 2D diffusion channels of facilitating Zn²⁺ insertion. The guest ions intercalated into layered V₂O₅-type compounds act as pillars to stabilize the layered structure, contributing more superior cycling performance than that of the tunnel structure in Table 1. Their expanding layer space can accommodate Zn²⁺ and H₂O co-insertion. Moreover, H₂O molecules not only act as a charge shielding medium to stabilize structure, but also reduce the "effective charge" of Zn²⁺ and promote the intercalation rate of Zn²⁺.

The framework-structured V₂O₃ and NASICON-type compounds have various interpenetrating 3D diffusion channels, which can accommodate more Zn²⁺. Hence the framework-structured cathodes with 3D tunnel can adopt all of the above-mentioned Zn-storage mechanisms. In addition, NASICON-type compounds can also permit Zn²⁺/Na⁺ co-insertion mechanism due to involving the Na⁺ de/intercalating.

The goal of this review is to summarize the recent progress of the vanadium-based compound family, including vanadium oxide, vanadates, alkali vanadium phosphates and other vanadium-based materials, and to present the relationship between the structural features and Zn²⁺ storage mechanisms, which are crucial for designing and preparing high-energetic materials. Although vanadium-based materials have made impressive progress in AZIBs cathodes, suitable anodes, electrolytes, current collectors and binders are equally important for the development of AZIBs. Hence, accelerating the development of high-performance AZIBs needs to pay enough attention to the following aspects.

(1) The dissolution of vanadium-based compounds in electrolytes during cycling causes the capacity fading and cycling stability reducing. Modifying electrolytes, constructing the integrated electrode and designing the cathodes with stable structure can address the above-mentioned shortcomings.

(2) The inherent low operating voltage of vanadium-based compounds hinders their practical applications. Introducing the electron-withdrawing groups on vanadium-based compounds and higher electronegative atom substitution or exploring more advanced materials could increase the voltage platform.

(3) The mechanism for energy storage plays a crucial role in guiding the design of the cathode materials. Many energy storage mechanisms have been put forward, but they do not reach an agreement so far. Therefore, more and more high-resolution in-situ characterization technologies need to be applied to provide further proof to elucidate the real storage mechanisms, which is of great significance to the application of vanadium-based materials in AZIBs cathodes.

(4) The inherently-poor electrical conductivity of vanadium-based materials causes their distinct volume changes during cycling. The highly-conductive additives such as carbon-based materials are an effective strategy to address this problem. Beside, metal element doping modification and defect engineering can improve electrochemical performance by boosting the chemical environment of vanadium-based compounds.

(5) Vanadium-based compounds with layered structures are gradually damaged due to the repeated insertion/extraction of Zn²⁺. The design and synthesis of core-shell and hollow structures will help vanadium-based compounds to maintain the structural stability.

Declaration of competing interest

We have no conflicts of interest to declare.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos. U1910210, U1810204 and 22004122), Research Foundation for the Returned Overseas in Shanxi Provence (No. 2020-048) and the Central Guidance on Local Science and Technology Development Fund of Shanxi Province (No. YDZJSX2021A021).