A review on recent developments of vanadium‑based cathode for rechargeable zinc‑ion batteries Tungsten

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Tungsten (2021) 3:289–304

https://doi.org/10.1007/s42864-021-00091-9 REVIEW PAPER

A review on recent developments of vanadium‑based cathode for rechargeable zinc‑ion batteries

Yan Wu^1,2 · Tian‑Yi Song² · Li‑Na Chen¹

Received: 29 December 2020 / Revised: 2 February 2021 / Accepted: 2 February 2021 / Published online: 1 July 2021

Abstract

Benefiting from their high safety, low cost, and excellent performance, aqueous zinc-ion batteries are regarded as a promising candidate for next-generation commercial energy storage devices. High-performance cathodes are urgently needed to accelerate practical application of zinc-ion batteries (ZIBs). Among various cathodes reported previously, vanadium-based materials attract a great deal of attention since they hold high capacity and good cycling stability. Though fruitful achieve- ments have been made, there are amounts of crystal structures and energy storage mechanisms are still unclear, which will significantly affect performance of full batteries. This review presents a comprehensive overview of structure characteristics, relevant electrochemical behavior, and proposed energy storage mechanisms of reported vanadium-based materials which provide effective Zn-storage performance. Meanwhile,recent developments of vanadium-based materials for ZIBs are sys- tematically summarized. Last but not least, the future perspectives are also discussed. We hope that this review can provide suggestions to design high-performance electrode materials and promote the development of ZIBs.

Keywords Aqueous zinc-ion batteries · Vanadium-based materials · Cathode · Structure · Energy storage mechanism

1 Introduction

Considering the limit of traditional energy resources stocks, serious environmental pollution problems, and continuous developments of electric vehicles and portable electronics, it is urgent to explore a reversible energy storage device for renew- able energy with low cost, good safety, as well as remarkable energy density. Currently, the most commonly commercial choice is lithium-ion batteries (LIBs) [1] with superior energy densities and cycle stability, but their large-scale application is strongly impeded by the high cost, low safety, and the toxic substance produced in cycling. This motivates people to explore alternative rechargeable batteries. Zn-ion batteries (ZIBs) [2]

have attracted a lot of attention, because Zn²⁺ ions have similar ionic radius with Li⁺ ions (0.075 nm vs. 0.076 nm) [3], indicating the similar energy barriers. Divalent zinc ions have

higher electrostatic interaction when inserting into the electrode materials than that of lithium ions [4]. In addition, zinc metal shows high stability under atmospheric environment and can be used directly as anode materials for ZIBs, enabling low cost [5], low redox potential (− 0.76 V vs standard hydrogen electrode) [6], and high theoretical capacity (≈ 820 mAh·g⁻¹) [7]. Fur- thermore, the mild aqueous electrolytes with high safety, low price, and easy-to-use [8] have an advantage over their organic counterparts [9]. Therefore, there are a lot of works focused on investigating appropriate positive materials for aqueous ZIBs (AZIBs), such as the Mn-based materials [10], V-based material [11], and Prussian blue analogues (PBAs) [12]. Among them, Mn-based materials and PBAs show high operating voltage, but their capacity and reversibility during cycling are still not satisfactory [13]. These disadvantages hugely limit their further application of next-generation AZIBs. In this case, V-based materials are regarded as the most promising candidate, which provide high capacity and excellent cycle stability [14–16]. Crystal structure of the active materials and Zn-ion storage mechanisms have significant effect on the electrochemical behaviors. Various crystal structures of V-based materials have been explored and their energy storage mechanisms are still controversial. Thus, in terms of crystal structure and storage mechanism, this review aims to provide a comprehensive

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www.springer.com/42864

* Li-Na Chen linachen@hit.edu.cn

1 School of Materials Science and Engineering, Harbin Institute of Technology, Shenzhen 518055, China

2 Department of Chemistry, Center of Super‐Diamond and Advanced Films (COSDAF), City University of Hong Kong, Hong Kong 999077, China

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summary of the developments of V-based materials for AZIBs.

We believe that this review could provide a suggestion in designing high-performance cathode materials and promoting the development of AZIBs.

2 Structure of vanadium‑based materials

Vanadium-based materials mainly contain vanadium oxides and metal vanadate, varying from layered structure, tunnel based structure, other structures, etc. (Fig. 1) [17–19].

2.1 Layered structure 2.1.1 Normal layer

Thanks to their abundant zinc-ion storage sites and favorable ions transfer channels, layered vanadium‐based oxides have been identified as promising candidates for AZIBs. V₂O₅ consisting of VO₆ square pyramids [20] is the most common layered V-based electrode material for AZIBs, displaying

a remarkable theoretical capacity of 589 mAh·g⁻¹ [21].

Layered V₂O₅ cathode prepared by atomic layer deposition showed a high capacity of 513 mAh·g⁻¹ at 0.5 A·g⁻¹ and reversible capacity of 439 mAh·g⁻¹ at 5 A·g⁻¹ even after 1000 cycles. Interestingly, the electrolyte’s pH value could affect phase transformation during cycling. For instance, no new phase produced but only partial dissolution took place if pH value is below 3.2, but the phase transition occurred when pH values are above 3.8 [10]. Zhang et al. [22] discovered that the morphology of V₂O₅ cathode changed during the electrochemical process. In detail, the commercial V₂O₅ powder changed into porous nanosheets with more active sites for Zn²⁺ storage (Fig. 2a).

As shown in Fig. 2b, VS₂ is a typical layered structure material and active for insertion/extraction of Zn²⁺. There is a vanadium layer located in two sulfur layers, providing a large interlayer spacing (0.576 nm) and delivering a high capacity of 190.3 mAh·g⁻¹ at 0.05 A·g⁻¹ [23]. As shown in Fig. 2c, VOPO₄ is another typical layer structural cathode, in which VO₆ octahedra corner-shared with each other and simultaneously connected with PO₄ tetrahedra to form layer

Fig. 1 Crystal structures of typical vanadium‐based electrodes for AZIBs. Crystal structure on the left is reproduced with permission from Ref. [17], Copy- right 2019 American Chemical Society (ACS). Crystal structure on the upper right is reproduced with permission from Ref. [18], Copyright 2018 Wiley. Crystal structure on the lower right is reproduced with permission from Ref. [19], Copyright 2020 Royal Society of Chemis- try (RSC)

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structure. The VOPO₄/Zn battery extended the voltage win- dow to 2.1 V with the activated oxygen redox because of the water‐in‐salt electrolyte, resulting in a higher average operating potential of 1.56 V [24].

Above all, even normal layer V-based cathodes show many advantages. For example, V₂O₅ delivers high specific capacity and VOPO₄ shows high operating voltage. They still face some problems, such as low ion diffusion, low conductivity, dissolution of materials, self-aggregation for most V₂O₅ cathodes, relatively low specific capacity and poor cycle stability for VS₂ as well as VOPO₄.

2.1.2 Intercalated layer

Inspired by the excellent property of intercalated layered materials in other types of rechargeable ion batteries, pre- intercalating different ions and molecules into layered vanadium-based materials were used to obtain lager space for ions transfer.

For instance, as shown in Fig. 3a, by pre-inserting Na⁺ ions into the V₂O₅ layer, the interlayer space can be expanded to 0.501 nm, resulting in an excellent capacity of 367.1 mAh·g⁻¹ at 0.1 A·g⁻¹ and 93% retention for 1000 cycles [25]. Ming et al. [17] synthesized Mg²⁺-intercalated V₂O₅

Fig. 2 a Schematic illustration of a rechargeable aqueous Zn-V₂O₅ battery (left) and galvanostatic cycling performance at 0.2, 0.5, and 1.0 A·g⁻¹ and the corresponding coulombic efficiency at 0.2 A·g⁻¹ (right). Reproduced with permission from Ref. [22]. Copyright 2018 ACS. b Schematic illustration of operation mechanism of Zn-VS₂ batteries (left) and charge/discharge curves of Zn-VS₂ batteries at the

current densityfrom 0.05 to 2.0 A·g⁻¹ (right). Reproduced with permission from Ref. [23]. Copyright 2017 Wiley. c Crystal structure of VOPO₄ (left) and charge/discharge curves of Zn/VOPO₄ batteries at 0.05 A·g⁻¹ (right). Reproduced with permission from Ref. [24]. Cop- yright 2019 Wiley

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with large layer space (1.34 nm) as the cathode material for AZIBs, providing a capacity of 353 mAh·g^–1 at 0.1 A·g^–1 (as shown in Fig. 3b). Cathode materials composed of anionic [V₃O₈] with the large interlayer space usually exhibit high specific capacity. Therefore, Cai et al. [26] designed the Na⁺ pre-inserted V₃O₈ layers with enhanced interlayer distance, which can store more Zn²⁺ as host materials in AZIBs. When further mixed with nanoribbons/graphene to form nanocom- posites, they achieved excellent reversibility of 223 mAh·g⁻¹ at 0.3 A·g⁻¹. Cations such as NH₄⁺ also show good results for modifying V₂O₅ cathode. The obtained (NH₄)₂V₁₀O₂₅·8H₂O exhibits a robust bi-layered structure, which makes the assem- bled cell display high energy density of 341 Wh·kg⁻¹ [27].

Pre-intercalation of a great deal of metal ions may decrease the reversible capacity of V₂O₅-based materials.

Yang et al. [28] pre-intercalated a bit of transition metal ions, such as Fe²⁺, Co²⁺, Ni²⁺, Mn²⁺, Zn²⁺, and Cu²⁺, into the V₂O₅ interlayer, and confirmed that this universal strategy can significantly improve ions transfer kinetic, structure stability, and temperature adaptability of V₂O₅. Hydrated layer materials such as V₂O₅·nH₂O demonstrated an energy density of ≈144 Wh·kg⁻¹ at 0.3 A·g⁻¹, thanks to the “lubri- cating” effect from H₂O [29].

Ions and water co-insertion is also demonstrated to be a positive way to enlarge the layer space and enabled higher electrochemical performance of layered V₂O₅. For example, Yang et al. [30] reported a series of LixV₂O₅·nH₂O with a spacing of 1.377 nm in the (001) face as cathodes for AZIBs, delivering high capacities, excellent cycling performance, and excellent temperature adaptability. Kundu et al. [31] prepared the Zn_0.25V₂O₅⋅nH₂O, in which V₂O₅ interlayer was supported by the Zn ions and the interlayer distance of the (200) planes is 0.537 nm. The presence of pillars leads to reversible Zn²⁺ (de)insertion at high rates, excellent capacities (~ 300 mAh·g⁻¹), and long-term performance (> 1000 cycles). Compared with Zn_0.25V₂O₅⋅nH₂O, Ca₀.₂₅V₂O₅⋅nH₂O achieved an expanded layer space of 1.06 nm and delivered higher gravimetric capacity of 340 mAh·g⁻¹ at 0.2 C [32].

Hydrated layer V₃O₈ also exhibits excellent performance in AZIBs. For example, V₃O₈ layers were investigated to form V₃O₇·H₂O, in which hydrogen-bond vibration could provide the elastic buffer space to make Zn²⁺ insertion/extraction easier and maintain the stable structure [33]. Zhang et al. [34] reported a high‐performance V₅O₁₂·6H₂O nanobelt cathode, which delivered a capacity of 354.8 mAh·g⁻¹ at 0.5 A·g⁻¹, a high energy

Fig. 3 a Schematic illustration of zinc‐storage mechanism in the Na_0.33V₂O₅ electrode (left) and galvanostatic charge/discharge curves of it at 0.2 A·g⁻¹ (right). Reproduced with permission from Ref. [25]. Copyright 2018 Wiley. b Structural characterization of MgxV₂O₅·nH₂O cathodes (left), and scanning electron microscope

(SEM), transmission electron microscope (TEM), high resolution TEM (HRTEM) images, and TEM–energy dispersive X-ray spec- trometry (EDS) elemental mappings of prepared MgxV₂O₅·nH₂O (right). Reproduced with permission from Ref. [17]. Copyright 2018 ACS

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density of 194 Wh·kg⁻¹, and good cycle stability. Layered barium vanadate with different amount of Ba ions and water atoms were also investigated, in which water co-intercalation in Ba_1.2V₆O₁₆·3H₂O contributed to robust structure, a high capacity of 108.8 mAh·g^–1 at 10 A·g^–1, and capacity retention of 95.6% over 2000 cycles [35]. The co-introduction of hydrated, monovalent- and divalent-cations into the galleries of V₃O₈ layers can contribute to pseudocapacitive behavior and improved layer structure stability [36, 37]. For example, the prepared NaCa_0.6V₆O₁₆·3H₂O cathode shows excellent performance [38].

Layered VOPO₄ [39] also can be modified by pre-intercalating H₂O molecules to weaken the electrostatic repulsion between the intercalating Zn²⁺ and the host. Simultaneously, using electrolyte with controlled water amounts could further improve the ion transfer and reversibility. However, in common Zn(CF₃SO₃)₂(Zn(OTF)₂) electrolyte, it still under- goes significant voltage decay. After introducing high con- centration ZnCl₂ salt in the electrolyte, Zn/VOPO₄ battery delivered a high capacity (170 mAh·g⁻¹) and stable retention of both capacity and voltage over 500 cycles [40].

To sum up, chemical pre-intercalation of ions (such as monovalent ions, Li⁺, Na⁺, K⁺, NH₄⁺, divalent ions, Mg²⁺, Cu²⁺, Ba²⁺.) and molecules (H₂O, small organic molecules,

etc.) was proved to be an effective strategy to increase the interlayer space, enhance the stability of normal layered structure, and suppress vanadium dissolution upon the electrochemical process. However, the intercalation of metal ions may reduce the specific capacity of layer materials.

2.2 Tunnel‑based structure

Tunnel-based vanadium oxides also exhibit excellent performance in AZIBs because of their unique tunnel pathways for Zn²⁺ migration. Taking VO₂(B) as an example, as shown in Fig. 4a, it displayed ultrafast kinetics of Zn²⁺ transportation with little volume change during cycling. After 50 cycles, the VO₂(B) cathode still delivered a reversible capacity of 357 mAh·g⁻¹ at 0.25 C [41]. Reduced graphene oxide (rGO)/

VO₂ contributed an excellent power density, energy density, and cycling stability, demonstrating an energy density of 65 Wh·kg⁻¹ even at a power density of 7.8 kW·kg⁻¹ as well as a capacity retention of 99% after 1000 cycles at 4 A·g⁻¹ [42].

As depicted in Fig. 4b, α-Zn₂V₂O₇ with distorted trigonal bipyramid ZnO₅ polyhedral layers connected via VO₄ tetrahedra could provide the tunnel network for Zn-ion transfer.

Then, a high capacity of 138 mAh·g⁻¹ over 1000 cycles was

Fig. 4 a Schematic illustration of a rechargeable aqueous Zn-VO₂(B) battery (left) and its cycling performance at 0.25 C (inset: typical galvanostatic charge-discharge curve) (right). Reproduced with permission from Ref. [41]. Copyright 2018 Wiley. b Galvanostatic cycling

performance of a rechargeable aqueous Zn-Zn₂V₂O₇ battery at a current density of 4.0 A·g⁻¹ (insert: schematic of electrochemical regula- tion in α-Zn₂V₂O₇ nanowires for AZIBs). Reproduced with permission from Ref. [43]. Copyright 2018 RSC

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achieved [43]. Analogously, CuV₂O₆ has also been considered as the active materials for AZIBs. It demonstrated reversible phase transformation between CuV₂O₆ and ZnV₂O₆ during cycling, delivering a high capacity of 427 mAh·g^–1 and capacity retention of 99.3% after 3000 cycles [18].

In addition, Guo et al. [44] compared the electrochemical performances between different structural silver vanadate cathodes, and found that tunnel structural Ag_0.33V₂O₅ show reversible Zn²⁺ (de)insertion during cycling and good cycle stability of 83% capacity retention after 100 cycles at 1 A·g⁻¹. Similarly, Guo et al. [45] also compared the electrochemical performances between the NaV₃O₈-type layered structure (Na₅V₁₂O₃₂) and β-Na_0.33V₂O₅-type tunneled structure (Na_0.76V₆O₁₅). They discovered that although the tunneled structure has lower ion diffusion coefficients than layered structure, it displays superior cyclic stability. The channel provided by irreversible exchanging between Ca²⁺

in Ca_0.67V₈O₂₀·3.5H₂O and Zn²⁺ enabled a capacity of 466 mAh·g^–1 and a balanced energy power-density behavior [46].

Moreover, tunneled sodium super ionic conductor (NASICON)-typed materials with very stable framework and rapid ionic diffusion capability, have been recognized as a promising Zn²⁺ storage as well as intercalation hosts [47]. After the extraction of Na⁺ ions, it shows a fast diffusion-controlled Zn-ion storage kinetic in Zn(CH₃COO)₂ electrolyte, delivering a capacity of 97 mAh·g⁻¹ as well as only 26% decay after 100 cycles (as shown in Fig. 5a) [48]. Hu et al. [49] revealed that its high structural stability mainly depends on mixed occupation of Na⁺/Zn²⁺ at both two inequivalent Wyckoff sites including 6b sites and 18e sites. The excellent electronic conductivity and mechanical properties were then achieved. In a more recent study, Hu et al. [50] realized a simultaneous Zn²⁺/Na⁺ de/insertion of Na₃V₂(PO)₃/rGO microspheres in 2 mol·L⁻¹ Zn(OTF)₂

Fig. 5 a Schematic illustration of Zn ions’ insertion and extraction in Na₃V₂(PO₄)₃ (up), galvanostatic charge-discharge curves (lower left), and cycling performance at a current density of 0.5 C for the Zn//0.5 mol·L⁻¹ Zn(CH₃COO)₂//Na₃V₂(PO₄)₃ battery. Reproduced with permission from Ref. [48]. Copyright 2016 Elsevier. b Sche-

matic illustration of a rechargeable aqueous zinc‐ion battery with Na₃V₂(PO₄)₂F₃ as cathode and Zn foil as anode (left) and cycling performance of a carbon film functionalizing (CFF)-Zn//N₃VPF@C battery at a current density of 1 A·g⁻¹ (right). Reproduced with permission from Ref. [52]. Copyright 2018 Elsevier

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electrolyte and delivered a specific capacity of 114 mAh·g⁻¹ with an operating voltage of 1.23 V (vs. Zn²⁺/Zn).

Moreover, inspired by that F atoms having a strong affinity with surroundings [51], Na₃V₂(PO₄)₂F₃ was investigated as positive materials for AZIBs, achieving 0.5 V higher work voltage than that of Na₃V₂(PO₄)₃. The operating voltage was further enhanced to 1.62 V after it was combined with carbon nanotube (CNT) and good long cycle performance of only 5% capacity decay over 4000 cycles was obtained (Fig. 5b) [52].

To sum up, V-based materials with tunnel structure show excellent performance. For example, VO₂ with a shear-type structure which results in larger resistance to lattice shear- ing against the insertion/extraction of ions, and some NASI- CON-typed materials achieved high working voltage (~ 1.6

V), which is higher than most of V-base materials with other structures. However, they also have some disadvantages, such as low ion and electrical conductivity, which need to be further improved to meet the demand of next-generation AZIBs.

2.3 Other structures

There are also some vanadium-based cathode materials with other different structures, such as the VS₄ (as shown in Fig. 6a) with chain‐like structure. Its loosely stacked structure had open channels for transfer and storage of Zn ions, achieving excellent capacity of 310 mAh·g⁻¹ [19].

Zhang et al. [53] proved that Mn cation defective ZnMn₂O₄ with spinel structure (Fig. 6b) has abundant Mn vacancies to enable fast Zn²⁺ ion diffusion and good

Fig. 6 a Lateral and vertical views of the crystalline structure of VS₄ (left), galvanostatic charge/discharge curves of the Zn-VS₄ battery at different current densities (middle), and cycling performance of the Zn-VS₄ battery at a current density of 2.5 A·g⁻¹ (right). Repro- duced with permission from Ref. [19]. Copyright 2020 RSC. b Sche- matic illustration of Zn-ion storage of ZnMn₂O₄ (ZMO) cathode in Zn(CF₃SO₃)₂ Electrolyte (left), schematic illustration of Zn²⁺ insertion/extraction in an extended three-dimensional ZMO spinel frame-

work (middle) and proposed Zn²⁺ diffusion pathway in ZMO spinel without and with Mn vacancies (right). Reproduced with permission from Ref. [53]. Copyright 2016 ACS. c Probable Zn-ion-binding sites for coordination with the V-MOF-48@carbon nanotube fibers (CNTF) (left), specific capacities of V-MOF-12//Zn, V-MOF-24//Zn, V-MOF- 48//Zn, and V-MOF-60//Zn at various current densities (middle) and Ragone plot of fiber-shaped V-MOF-48//Zn batteries (right). Repro- duced with permission from Ref. [54]. Copyright 2019 Elsevier

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structural stability. 3-dimensional (3D) conductive V-based metal–organic frameworks (MOFs) (Fig. 6c) with abundant active sites, good conductivity, and hierarchical porosity were also considered as electrode materials for AZIBs. After being arrayed on carbon nanotube fibers, the combination delivered a volumetric capacity of 101.8 mAh·cm⁻³ at 0.1 mA·cm⁻³ and an excellent rate capability. More importantly, its high energy density of 17.4 mWh·cm⁻³ at a power density

of 1.46 W·cm⁻³ offered a prospect of conductive MOFs nanowires-based energy storage devices [54].

In addition, vanadium nitride with surface-oxide (VN_xO_y) used as electrode materials for AZIBs is face-centered structure, which possesses high electrical conductivity, ionic conductivity, and improved supercapacitor‐like surface redox reactions. There are Zn²⁺ and H⁺ de‐/intercalation accompanied by multiple redox reactions including V³⁺↔V²⁺ and N³⁻↔ N²⁻,

Fig. 7 a Zinc ion diffusion in layered V₂O₅: diffusion pathways viewed along the [100], [010], and [001] directions (upside) and corresponding calculated minimum energy paths of Zn²⁺ diffusion.

Blue, red, and grey balls represent V, O, and Zn atoms, respectively.

Black balls indicate the most energetically favorable location for Zn²⁺

intercalation. Reproduced with permission from Ref. [58]. Copyright

2019 Elsevier. b Schematic illustration of the Zn-V₂O₅ battery with 2 mol·L⁻¹ ZnSO₄ aqueous electrolyte (left), capacitive contribution current at 0.1 mV·s⁻¹ (upper right), and the capacitive contribution ratio at different scan rates (lower right). Reproduced with permission from Ref. [59]. Copyright 2019 Elsevier

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and release/uptake of anions (OH⁻) on the VNxOy surface, delivering a high capability of 200 mAh·g⁻¹ at 30 A·g⁻¹ [55].

To sum up, the structure of V-based materials used as the active materials for AZIBs is mainly the layered structure, ion or/and water atoms pre-inserted layered materials, tunneled materials, and so on. Current works mainly focus on enlarging the interlayer space and exploring materials with different structures which could be an excellent host and transmission medium for Zn²⁺ ions.

3 Storage mechanisms

For the vanadium-based cathode, its energy storage mechanism involves traditional reversible Zn-ion insertion/de-insertion mechanism, H⁺-Zn²⁺ co-insertion mechanism, and H₂O-Zn²⁺

co-insertion mechanism. Detailed works are discussed below.

3.1 Zn‑ion insertion/de‑insertion mechanism A lot of vanadium-based cathodes in AZIBs adopt the traditional reversible Zn-ion insertion/extraction mechanism, such as polyaniline (PANI)-intercalated V₂O₅ [56], double- layered calcium vanadium oxide bronze (Ca_0.25V₂O₅·nH₂O, CVO), and two-dimensional (2D) amorphous V₂O₅/graphene heterostructures (A-V₂O₅/G) [57]. As shown in Fig. 7a, the density functional theory (DFT) calculation found that the preferred coordination environment of Zn²⁺ ions is similar to that of Li⁺ ions and intercalated into the interlayer region with the most favorable pathway along the [100] channel [58]. Figure 7b demonstrated the capacitive contribution ratio at different scan rates of the V₂O₅ nanopaper and confirmed that Zn-ion storage behavior is usually manipulated by both capacitive reaction and ion diffusion-controlled process [59].

In addition, He et al. [25] reported the Na_0.33V₂O₅ (NVO) electrode with the improvement conductivity benefiting from the intercalation of Na-ions among [V₄O₁₂]n layers, in which a new phase Zn_xNa_0.33V₂O₅ (0.42 < x < 0.96) formed/disap- peared in a voltage range between 0.7 V and 0.2 V, and confirmed that Na-ions are unchanged during the charge/

discharge process and only play a “binders/pillars” role to maintain structural stability. Similarly, two new phases of Zn_0.25V₂O₅·nH₂O and Zn_0.29V₂O₅ also occurred for the LixV₂O₅·nH₂O cathode upon charging/discharging [30].

3.2 H₂O–Zn²⁺ Co‑insertion mechanism

Besides the traditional Zn²⁺ carrier mechanism, H₂O and Zn²⁺ co-intercalation also commonly exists in vanadium- based cathode [15, 28, 35, 55, 60–62]. For example, Zn²⁺

accompanied H₂O in the form of complex intercalated in the layer gap was also observed in layered Mg_xV₂O₅·nH₂O

cathod e via X-Ray Diffraction (XRD) analysis and the schematic illustration is shown in Fig. 8a [17]. In layered V₂O₅ with the chemical pre-intercalation of Cu²⁺, Zn₃(OH)₂V₂O₇·2H₂O and Zn_0.25V₂O₅·H₂O appeared during the first cycling (Fig. 8b [28]). H₂O molecules de-intercalated with Zn²⁺ ions into the Zn_0.25V₂O₅·nH₂O could interact with the oxygen layers, buffer the high charge density, and make charge transfer easier at the electrode interface [31].

Generally, the multivalent ions usually distort the material’s original crystal structure, because its strong charge density hinders the diffusion of charge carriers through the host matrix. However, co-intercalation water can shield the electrostatic interactions and facilitate the insertion of multivalent ions at the interface and the host framework [2].

Additionally, it is worth noting that when the interlayer space is enlarged upon cycling, the Zn²⁺ accompanied by water molecules inserted in the host may be observed [28].

In summary, water molecules have a great positive effect on the process of Zn²⁺ transport and structure stability.

3.3 H⁺– Zn²⁺ Co‑insertion mechanism

H⁺– Zn²⁺ co-insertion mechanisms are widely studied in vanadium-based cathode in AZIBs, such as NaV₃O₈·1.5H₂O (NVO) nanobelts cathode. During the first discharging, the water from the aqueous ZnSO₄/Na₂SO₄ electrolyte decom- posed as H⁺ and OH⁻. Then, OH⁻ ions coupled with Zn²⁺

from ZnSO₄ to form Zn₄SO₄(OH)₆·4H₂O. Simultane- ously, the H⁺ coupled with Zn²⁺ inserted into NVO cathode and H_3.9NaZn_0.5V₃O₈·1.5H₂O was formed. After being fully charged, the H⁺ and partial Zn²⁺ are simultaneously extracted and NaZn_0.1V₃O₈·1.5H₂O is formed (as shown in Fig. 9a) [63]. Moreover, Li et al. [64] reported that there is a competition between proton and Zn²⁺ insertion in VO₂ cathode. They also found that structural distortion resulting from proton insertion is minimal (as shown in Fig. 9b).

The similar H⁺ and Zn²⁺ co-insertion mechanism was also reported in Zn_0.3V₂O₅·1.5H₂O cathode [65], carbon nanotube delaminated V₂C novel 2D nanostructured materials (MXene) cathode [66], as well as KV₃O₈·0.75H₂O (KVO)/

single wall carbon nanotube (SWCNT) cathode [62].

3.4 Cationic and anionic oxidation co‑exist mechanism

To our knowledge, most vanadium-based cathodes per- form an insertion/extraction mechanism with the charge compensation from the V⁵⁺/V⁴⁺/V³⁺ redox reaction.

Nonetheless, reversible oxygen redox chemistry also exists, especially for VOPO₄ cathode. In these materials, charge loss generated from the process of Zn²⁺ insertion/

extraction in the low-voltage range usually compensates

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by the V⁴⁺/V⁵⁺ redox reaction. Lattice oxygen oxidation and reduction reaction proceeded without Zn²⁺ insertion/

extraction during the voltage range of 1.8– 2.1 V. The oxygen oxidation process increases the energy density and enhances the rate capability and cycling performance [24].

Additionally, Fang et al. [55] reported a novel simultaneous cationic and anionic redox reaction in the VNxOy cathode. The Zn²⁺ insertion/extraction reaction depends on a redox reaction including V³⁺ ↔ V²⁺ and N³⁻ ↔ N²⁻ (as shown in Fig. 10a). In conclusion, cationic and anionic

Fig. 8 a Schematic illustrations of storage mechanism of MgxV₂O₅·nH₂O cathode upon charging/discharging. Reproduced with permission from Ref. [17]. Copyright 2018 ACS. b Ex-situ XRD patterns and corresponding galvanostatic charge/discharge (GCD) curves

at a current density of 0.1 A·g⁻¹ (up), and schematic illustration of storage mechanism in the first cycle of Cu_0.1V₂O₅·0.08H₂O electrode (lower part). Reproduced with permission from Ref. [28]. Copyright 2019 Elsevier

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oxidation co-exist mechanism has a profound significance for high energy density cathode and still needs further investigation.

3.5 Insertion and conversion co‑exist mechanism Apart from the traditional intercalation mechanism, conversion reaction also exists in some vanadium-based cathodes. For example, VS₄@rGO cathode adopted an intercalation and conversion coexistence mechanism. There was a Zn₃(OH)₂V₂O₇·2H₂O phase generated in the full discharge state of 0.35 V and diminished after charging to 0.8 V.

Until charged to 1.8 V, the Zn₃(OH)₂V₂O₇·2H₂O phase was reformed again with orthorhombic sulfur formation. There- fore, during the charging process, the conversion can be expressed as Eq. (1):

However, upon the discharge process, other complicated conversion reactions also occurred (as shown in Fig. 10b) [67]. The conversion reaction with the formation of Zn₃(OH)₂V₂O₇·2H₂O can also observe in the Mg_0.19V₂O₅·0.99H₂O cathode [68]. In addition, Ding et al. [69] observed the product of NH₄⁺ in electrolyte for VN_0.9O_0.15 in initial charge state, which suggested that VN(O_poor) was converted to VN_1−x(O_rich) with large V and O vacancies/defects, accor ding to the following Eq. (2):

(1) 2VS₄+ 11H₂O + 3Zn²⁺+

→ Zn₃(OH)2V₂O₇⋅2H₂O + 8S + 16H⁺+ 10e⁻.

(2) VN(

O_poor)

+ 2xH₂O→VN_1−x( O_rich)

+ xNH⁺₄ + xe⁻.

Fig. 9 a Second charge/discharge curve of NaV₃O₈·1.5H₂O nanobelts at 0.1 A·g⁻¹ (left), corresponding ex situ XRD patterns (right). Repro- duced with permission from Ref. [63]. Open Access, Copyright 2018 Fang Wan. b Schematic illustration of the electrochemical process in

a Zn-VO₂ battery during discharging in a ZnSO₄ aqueous electrolyte.

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300 Y. Wu et al.

The extra X-ray photoelectron spectroscopy (XPS) testing process demonstrated a complete conversion reaction upon initial charging. Additionally, the conversion reaction also happened in some vanadium oxide with polyaniline intercalation [70, 71].

In summary, even the energy storage mechanisms of AZIBs are very complex and affected by various factors.

Structure of vanadium-based materials plays very important role. For example, as depicted in Table 1, layered V-based cathode always demonstrate insertion mechanism. Conver- sion mechanism, and conversion and insertion co-mechanism are more common in V-based materials with other structures. Therefore, clearly understanding the mechanism has great significance in designing a high-performance cathode. It is still a challenge and needs further investigation through various advanced characterization technology.

4 Conclusions and perspectives

Vanadium-based materials have been regarded as a kind of promising cathode materials for AZIBs. This review sys- tematically summaries main structures of V-based cathode, including layered, tunnel and other structure. Layered structure attracts much attention due to their large number of active sites and intrinsic fast ions diffusion channels. By pre-intercalating guest ions, atoms, or even molecules into the structure, the interlayer space can be enlarged, and the structure can be stabilized. Thus, enhanced performance can be achieved. Furthermore, electrochemical reactions in aqueous systems are much more complicated than those in their organic counterpart. Various energy storage mechanisms have been revealed, including Zn-ion insertion/extraction mechanism, H₂O-Zn²⁺ co-insertion, H⁺-Zn²⁺ co-insertion,

Fig. 10 a High angle annular dark field (HAADF) and elemental mapping images of VNxOyna noflake (left) and ex situ XRD patterns of VNxOy electrodes during initial cycle (right). Reproduced with permission from Ref. [55]. Copyright 2019 Wiley. b Ex-situ XRD

patterns in the first charge/discharge cycling at 0.1 A·g⁻¹. The inset shows the enlarged image of the XRD pattern of the D_0.35 state in the 2 theta range from 11.4° to 12.6°. Reproduced with permission from Ref. [67]. Copyright 2018 RSC

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Table 1 Typical vanadium-based cathode materials in AZIBs Cathode materialsRef.ElectrolyteCapacityCyclabilityMechanism Normal layer structure Atomic layer deposition (ALD)-derived V2O5[21]3 mol·L−1 Zn(CF3SO3)2513 mAh·g−1 (0.5 A·g−1)439 mAh·g−1 1000 (5 A·g−1)Zn2+/H+ co-intercalation Ball-milled commercial V2O5[22]3 mol·L−1 Zn(CF3SO3)2470 mAh·g−1 (0.2 A·g−1) 91.1% 4000 (5 A·g

−1)Zn2+/H2O co-intercalation VS2 nanosheets [23]1 mol·L−1 ZnSO4190.3 mAh·g−1 (0.05 A·g−1)

98.0% (0.5 A·g

−1)Zn2+/H2O co-intercalation VOPO4[24]21 mol·L−1 LiTFSI /1 mol·L−1 Zn(Tr)2139 mAh·g−1 (0.05 A·g−1)

93% 1000 (1 A·g

−1)Oxygen redox Zn2+ (de)intercalation Intercalated layer structure Na0.33V2O5 (NVO) nanowire [25]3 mol·L−1 Zn(CF3SO3)2367.1 mAh·g−1 (0.1 A·g−1)

93% 1000 (1 A·g

−1)Zn2+ (de)intercalation (NH4)2V10O25·8H2O [27]1 mol·L−1 ZnSO4376 mAh·g−1 (0.5 A·g−1)

93% 1000 (10 A·g

−1)Zn2+ (de)intercalation Zn0.25V2O5·nH2O [31]1 mol·L−1 ZnSO4220 mAh·g−1 (4.5 A·g−1)

80% 1000 (1.2 A·g

−1)Zn2+/H2O co-intercalation Ca0.24V2O5·0.83H2O nanobelt [32]1 mol·L−1 ZnSO4340 mAh·g−1 (0.2 C)

96% (80 C) 3000

Capacitive behavior Zn2+ (de)intercalation Mg0.34V2O5· 0.84H2O nanobelt [17]3 mol·L−1 Zn(CF3SO3)2264 mAh·g−1 (1 A·g−1)97% (5 A·g−1) 2000Capacitive behavior Zn2+ / Mg2+ intercalation CuxV2O5·nH2O [28]2 mol·L−1 ZnSO4410 mAh·g−1 (0.5 A·g−1)180 mAh·g−1 10,000 (10 A·g−1)Zn2+/H2O co-intercalation NaCa0.6V6O16·3H2O nanobelt [38]3 mol·L−1 Zn(CF3SO3)2347 mAh·g−1 (0.1 A·g−1)83% (5 A·g−1) 10,000Pseudocapacitive Zn2+/H2O co-intercalation H2V3O8 nanowire [36]3 mol·L−1 Zn(CF3SO3)2423.8 mAh·g−1 (0.1 A·g−1)94.3% (5 A·g−1) 1000Capacitive behavior Zn2+ (de)intercalation V3O7·H2O nanogrid [33]3 mol·L−1 Zn(CF3SO3)2481.3 mAh·g−1 (0.1 A·g−1)85.4% (5 A·g−1) 1000Pseudocapacitance Zn2+ (de)intercalation Polypyrrole (PPy)-VOPO4[39]1 mol·L−1 Zn(CF3SO3)2 in acetonitrile 10% water86 mAh·g−1 (25 mA·g−1)86 mAh·g−1 350 (0.1 A·g−1)Zn2+ (de)intercalation VOPO4·H2O [40]13 mol·L−1 ZnCl2/ 0.8 mol·L−1 H3PO4 mixed electrolyte

170 mAh·g−198 mAh·g−1 500 (2A·g−1)Capacitive behavior Zn2+/H+ (de)intercalation Tunnel structure VO2 (B) nanofibers [41]3 mol·L−1 Zn(CF3SO3)2357 mAh·g−1 (75 mA·g−1)171 mAh·g−1 (90 A·g−1)Zn2+ (de)intercalation VO2 nanorods [65]1 mol·L−1 ZnSO4272 mAh·g−1 (3.0 A·g−1)75.5% (1 A·g−1) 945Zn2+/H+ (de)intercalation α-Zn2V2O7 nanowire [43]1 mol·L−1 ZnSO4170 mAh·g−1 (4.4 A·g−1)85% (4 A·g−1) 1000Zn2+ (de)intercalation

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302 Y. Wu et al.

cationic and anionic oxidation co-exist, and insertion and conversion co-exist mechanisms. However, they are still in debate and need to be further investigated. Therefore, future efforts about vanadium-based cathode are suggested to be made in the following directions.

First, to obtain high-performance cathode materials for ZIBs, intrinsic structure should be properly controlled, vanadium-based materials with large number of active sites, fast and favorable ion diffusion channels, and stable structure must be designed and fabricated. Such a s introducing metal cations act as "pillars" to enhance the structural stability, expanding the interlayer space to buffer the Zn²⁺ electrostatic force through pre-intercal ating cations or water molecules, combining the conductive substrate or matrix to form composites to improve its electronic conductivity, downsiz- ing the materials to nanoscale to shorten the transfer path thereby increasing the ion transfer kinetic, and designing the compatible electrolyte with the cathode or adding additives to electrolyte to activate the anionic redox of cathode thereby further increasing its capacity.

Second, to further explore the electrochemical reactions during the charging/discharging process, high-resolution in- situ characterization technologies and simulation are also needed, which will contribute a lot to design V-based cathodes with ideal properties for AZIBs.

Acknowledgements This work was financially supported by the School Research Startup Expenses of Harbin Institute of Technology (Shenz- hen) (Grant No. DD29100027).

Compliance with Ethical Statement

Conflict of interest The authors declare no conflict of interest.

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6. Wu Z, Yuan X, Jiang M, Wang L, Huang Q, Fu L, Wu Y. Zinc- carbon paper composites as anodes for Zn ion batteries: keys Table 1 (continued) Cathode materialsRef.ElectrolyteCapacityCyclabilityMechanism Ag0.33V2O5[45]2 mol·L−1 ZnSO4350 mAh·g−1 (50 mA·g−1)83% (1 A·g−1) 100displacement mechanism Zn2+ (de)intercalation Na3V2(PO4)3[49]0.5 mol·L−1 Zn(CH3COO)297 mAh·g−1 (0.5 C)

74% (0.5 C) 100

Zn2+ (de)intercalation Na3V2(PO4)3/rGO microspheres [51]2 mol·L−1 Zn(CF3SO3)2107 mAh·g−1 (50 mA·g−1 )75% (0.5 A·g−1 ) 200Zn2+ /Na+ co-intercalation

Carbon coated Na

3V2(PO4)2F3[53]2 mol·L−1 Zn(CF3SO3)261.7 mAh·g−1 (0.2 A·g−1)95% (1 A·g−1) 4000Zn2+ (de)intercalation Other structure VNxOy nanoflake [56]2 mol·L−1 ZnSO4200 mAh·g−1 (30 A·g−1)150 mAh·g−1 2000 (20 A·g−1)Zn2+/H+ intercalation Surface anionic redox VS4[19]1 mol·L−1 ZnSO4310 mAh·g−1 (0.1 A·g−1)85% (2.5 A·g−1) 500Capacitive Zn2+ (de)intercalation VS4@rGO [68]1 mol·L−1 Zn(CF3SO3)2180 mAh·g−1 (1 A·g−1)93.3% (1 A·g−1) 165Conversion Zn2+ (de)intercalation VN0.9O0.15[70]3 mol·L−1 Zn(CF3SO3)2124 mAh·g−1 (102.4 A·g−1)

100% 1500 (4.26 A·g

−1)Conversion reaction