Advanced V-based materials for multivalent-ion storage applications

V₂O₅

V₂O₅, with V at the highest oxidation state of +5 which means a relatively larger charge storage capability, usually crystallizes into four polymorphs, i.e., orthorhombic, monoclinic, tetragonal, and orthorhombic. Among them, α-V₂O₅ is the most common polymorph with V-O square pyramid coordination forming a lamellar structure by co-orientation or co-angulation [VO₅] polyhedra^[18]. The structure (Space group: Pmnm, a = 111.150 Å, b = 3.563 Å, and c = 4.370 Å) is usually thermodynamically stable^[19]. It can be easy to regulate the weakly bonded lamellar structure to achieve fast insertion/extraction of different metal ions. However, the poor conductivities of both ions and electrons and intense host-guest interaction readily lead to significant volume change and easy polarization when V₂O₅ is used as a battery cathode. Thus, it is still difficult to achieve reversible and fast ion storage in pure V₂O₅^[20].

VO₂

VO₂ can crystallize into more than a dozen phases, e.g., thermodynamically stable monoclinic VO₂(M), VO₂(B), and metastable tetragonal VO₂(A), VO₂(R)^[21,22]. Among them, VO₂(B) exhibits much better performance for multivalent-ion storage^[12]. VO₂(B) shows a layered structure consisting of [VO₆] octahedral bilayers, resembling a bilayered V₂O₅ structure with the removal of crystal water and interlayer collapse. Corner-shared [VO₆] octahedral bilayers in VO₂(B) contribute to abundant tunneling structures suitable for fast ion diffusion for multivalent-ion storage^[23]. Moreover, the structure also counteracts lattice shear during the charging and discharging processes^[24].

V₂O₃

V₂O₃ usually exhibits a rhombic corundum structure, characteristic of edge-sharing [VO₆] octahedra packed into a 3D tunnel structure, favorable for ion intercalation^[25]. The structure belongs to the $$ \mathrm{R} \overline{3 \mathrm{c}} $$ space group (a = b = 4.9492(2) Å, c = 13.988(1) Å)^[18]. Moreover, the electron conductivity of V₂O₃ is also superior to most transition metal oxides due to available V 3d electron transfer along the V-V chain^[26].

V₆O₁₃

V₆O₁₃ consists of alternating single- and double-twisted [VO₆] octahedral layers in a jagged arrangement containing mixed-valence V⁵⁺/V^4+[23,27]. V₆O₁₃ single- and double-twisted [VO₆] octahedral layers shared corners, showing a 3D open frame structure^[28]. Based on valence bonding and calculations, only some of the vanadium sites in the bilayer show V⁵⁺ properties, while V⁴⁺ occupies the remaining vanadium sites^[23].

V₃O₇

V₃O₇ is of mixed-valence V⁵⁺/V⁴⁺ with an atomic ratio of 2:1, resulting in superior electrochemical properties compared to other vanadium oxides^[12]. The crystal cell of V₃O₇ contains 36 vanadium atoms, 12 in octahedra and 24 in pentacoordination^[18]. Its hydrate V₃O₇·H₂O has a lamellar structure, consisting of V₃O₈ layers stacked along the a-axis with [VO₆] octahedral and [VO₅] square pyramid coordination by sharing corner/edge, and the H₂O molecules are distributed on both sides of the V₃O₈ layer. Adjacent layers are usually interconnected by hydrogen bonds, providing a buffer space due to the vibration of hydrogen bonds^[29]. Therefore, during ion insertion/extraction, large lattice distortion can easily occur without destroying the crystal structure^[30]. Compared to orthorhombic V₂O₅, V₃O₇·H₂O has a much larger layer spacing and is usually crystallized in a one-dimensional nanostructure^[12].

VS₂

VS₂ exhibits a lamellae structure similar to graphite. Each vanadium atom is linked to six sulfur atoms through covalent bonds, forming an S-V-S sandwich layer [Figure 3A] and weakly interlayer van der Waals bonding. The large interlayer spacing of 5.76 Å facilitates rapid ion diffusion and maintains structural integrity during repetitive cycling, making VS₂ a promising candidate for pseudocapacitance^[33]. Compared to vanadium oxides, weakened electrostatic interactions in VS₂ result in a lower diffusion barrier for the cations, enabling reversible ion insertion/desertion. Additionally, VS₂ has good electrical conductivity, which, together with a low ion diffusion barrier, contributes to good performance for metal-ion batteries.

Figure 3. Structures of (A) VS₂, (B) VS₄, (C) NaV₃O₈, (D) MgV₂O₄, (E) NH₄V₃O₈, and (F) VOPO₄. (G) Illustration of selectively etching process to synthesize V₂CT_x MXene. Reproduced with permission^[63]. Copyright 2017, American Chemical Society.

VS₄

VS₄ consists of parallel and one-dimensional atomic chain-like structures with bonding V⁴⁺ and S₂^2- dimer [Figure 3B], and the S₂^2- dimer affords enough ion storage sites. The open channels with a chain spacing of 5.83 Å favor the diffusion and storage of cations. However, pure VS₄ usually exhibits poor cycling performance and severe capacity degradation due to severe polarization, poor electron conductivity, and large volume expansion^[34,35]. Various VS₄ nanostructures, such as nanoribbons, nanorods, nanocones, and nanosheets, have been synthesized and assembled into microspheres or layered structures as electrode materials for batteries to solve the above problems^[36,37].

Alkali metal vanadates

LiV₃O₈ usually demonstrates good Li-storage performance^[40]. Structures of NaV₃O₈ and LiV₃O₈ are similar, consisting of edge- or corner-sharing [VO₆] octahedra layers, and Li⁺ and Na⁺ are located between the layers [Figure 3C]. Due to the large radius of K⁺, it is impossible to form a similar structure. Differently, V₂O₈ units and VO₆ octahedra share edges and corners in KV₃O₈^[41], two VO₅ square pyramids are connected by a VO₆ octahedron to form a (V₃O₁₂)_n chain stacked along the b-axis. Then, another reversed (V₃O₁₂)_n chain connects the above (V₃O₁₂)_n chains to form a KV₃O₈ layer along the b-axis^[42]. Li₃V₆O₁₃ is also a typical alkali metal vanadate. It consists of octahedral [VO₆] and triangular pyramidal [VO₅] units shared on both sides, resembling strings and bands arranged along (110) crystal planes. Layer-structured octahedra and tetrahedra are connected by interstitial Li^+[43], forming a square cone ligand with five O atoms without occupying insertion sites of V₆O₁₃.

Alkali earth metal vanadates

MgV₂O₄, Mg_xV₂O₅, CaV₆O₁₆·2.8H₂O, etc., are common alkaline earth metal vanadates reported for multivalent-ion storage. MgV₂O₄ consists of [VO₆] octahedra and [VO₄] tetrahedra, and Mg²⁺ can be inserted at the tetrahedral sites of the spinel oxide [Figure 3D]. The AV₂O₄-typed spinel (A = magnesium (Mg), calcium (Ca), etc.) is an attractive structure with which electrodes often exhibit abundant 3D channels, good crystal stability, tunable atomic scale structure, and suitable operating voltage^[44]. The structure of Mg_xV₂O₅·nH₂O is similar to bilayer V₂O₅ that consists of [VO₆] octahedra as the basic layer with Mg²⁺ or hydrated Mg²⁺ inserted between the layers^[45]. The biotite talc CaV₆O₁₆·2.8H₂O consists of a reconstructed α-V₂O₅ structure with layers comprising [VO₅] square pyramids and [VO₆] octahedra. The interlayered Ca²⁺ and water molecules raised the layer spacing from 4.37 to 8.10 Å and stabilized the interlayer structure^[46].

Transition metal vanadates

Most ions of transition metals, e.g., Ag⁺, Fe²⁺/Fe ³⁺, Zn²⁺, Co²⁺, Cu²⁺, Mn²⁺/Mn ³⁺, Ni²⁺, etc., readily combine vanadium oxides to form the corresponding vanadates. A_xV₂O₅·nH₂O (A = Zn, manganese (Mn), Ni, Co, etc.) exhibits a layered structure similar to V₂O₅, with alternate [VO₆] octahedral layer and hydrated cations between the layers^[38,47]. ZnV₂O₄, akin to spinel MgV₂O₄, consists of [VO₆] octahedra and [VO₄] tetrahedra. Since the electrochemical performance of low-valent V-based oxides in aqueous solution is limited, researchers tend to oxidize ZnV₂O₄ electrochemically in the first few cycles, which leads to superior electrochemical performance^[48]. Other transition metal vanadates include FeVO₄^[49], Cu₃V₂O₇(OH)₂·H₂O^[50], CuV₂O₆^[51], ZnV₆O₁₆·8H₂O^[52], Fe₂V₄O₁₃^[53], etc., which are also reported efficient for multivalent-ion storage.

Other vanadates

Besides, aluminum (Al)-based and NH₄-based vanadates are relatively less studied. Actually, with a molecular weight relatively lower than that of transition metal vanadates, they generally deliver much larger specific capacities. Besides, insertion of Al³⁺ or NH₄⁺ also makes the interlayer spacings larger and the ion conductivity higher, favoring the diffusion of metal ions^[54]. With edge-shared twisted [VO₆] octahedra, the monoclinic NH₄V₄O₁₀ exhibits a stable bilayer structure. The NH₄⁺ ion tends to act as a backbone cation to stabilize the structure, preventing severe volume changes during insertion of guest ions^[55]. Hollow spheres of H₁₁Al₂V₆O_23.2 with a bilayer structure and low crystallinity showed little lattice distortion and good long-term cycling stability^[56]. Crystallized NH₄V₃O₈ has a twisted zigzag layer structure consisting of VO₅ square cone units and twisted [VO₆] octahedra parallel to the (00l) plane, with NH₄⁺ ions between the layers connecting with oxygen atoms through hydrogen bonding [Figure 3E]^[57].

Mg-metal batteries

With many virtues of Mg, such as rich natural abundance, high capacity, low redox potentials, etc., Mg-metal batteries have been intensively explored. Aurbach et al. reported the first highly reversible rechargeable magnesium battery system using Mo₆S₈ as the cathode in 2000^[69]. However, the low capacity and discharge voltage of Chevel-phase Mo₆S₈ limit further development. So far, many electrode materials have been explored, such as transition metal sulfides^[69], transition metal oxides^[70], polyanionic compounds^[71], and organic materials^[72] for Mg-storage. Among them, V-based materials, with abundant valence states and unique layered structures or open backbones favoring reversible insertion/extraction of considerable Mg²⁺ ions, have been increasingly attractive. However, strong Mg-host interactions due to inherent divalent charge and large radius cause severe ion polarization, which is one of the main reasons hindering the application of V-based compounds.

To alleviate the relevant issues of inherent small layer spacings and strong electrostatic interactions, strategies, such as pre-intercalating large organic ions, adding electrolyte additives or surfactants, doping, etc., have been frequently investigated. Through intercalating large organic cation of C₁₀H₂₂N⁺ in the first discharge, the interlayer spacing of VS₂ was significantly enlarged, raising the diffusion coefficient of Mg²⁺ to 10^-10-10^-12 cm^-2 s^-1[73]. Thus, the corresponding cathode of VS₂ nanosheets achieved a large capacity of 299 mAh g^-1 at 50 mA g^-1. Besides, a capacity of 214 mAh g^-1 was retained even at 2.0 A g^-1. The spontaneous agglomeration of VS₄ was prevented by using surfactants or special self-assembly, leading to unique flower-like or sea urchin-like morphologies that afford abundant surfaces/interfaces and voids for stable Mg²⁺ intercalation/deintercalation^[74]. In addition, S₂^2- dimers in the chain-like crystal structure of VS₄ also provide abundant sites for Mg-storage^[75]. The introduction of dopants such as Mo to replace V would result in the escape of isolated S, then creating abundant S vacancies in a Mo-doped VS₄ cathode. At 50 mA g^-1, a Mg-storage capacity of 120 mAh g^-1 was attained at an optimized Mo content of 3% (atomic ratio)^[76].

In most cases, Mg-storage in vanadium sulfides is based on the Mg²⁺ intercalation mechanism, but intercalation of complexion ions, e.g., MgCl⁺, has also been frequently disclosed. For example, through both theoretical and experimental studies, Pei et al. found that MgCl⁺ reversibly intercalated into/deintercalated out of VS₄@reduced graphene oxide (rGO) rather than Mg²⁺ when 0.25 M [Mg₂Cl₃]⁺[AlPh₂Cl₂]^-/tetrahydrofuran was used as the electrolyte, contributing to 268.3 mAh g^-1 at 50 mA g^-1[77]. Zhu et al. also observed reversible intercalation/deintercalation of MgCl⁺ in VS₄ nanosheets /carbon-coated Ti₃C₂-MXenes hybrid cathodes^[68]. The presence of V-C bonding proved a strong coupling between VS₄ and Ti₃C₂, and this unique layered nano-microstructure improved the accessibility of electrolytes, which reduces resistance of charge transfer. So, the hybrid delivered 492 mAh g^-1 at 50 mA g^-1. After 900 cycles, it also held 80% capacity retention at 0.5 A g^-1. In addition, co-intercalation of Mg²⁺ and MgCl⁺ also happened to some vanadium sulfides. For example, 2-methylhexanamine in situ intercalated VS₂ with a large layer spacing of 9.93 Å reversibly intercalated/deintercalated Mg²⁺ and MgCl⁺, as shown in the corresponding XPS and EDS characterization [Figure 4A-C]^[78]. A polyvinylpyrrolidone/VS₄ composite was also found to simultaneously intercalate Mg²⁺ and MgCl^+[79].

Figure 4. XPS spectra of (A) Mg 2s, (B) Cl 2p, and (C) EDS spectra for expanded VS₂ electrodes. Reproduced with permission^[78]. Copyright 2019, Wiley-VCH GmbH. (D) Charge-discharge curves (A = Li, Na, K) and (E) cycling performance of A-V₃O₈ (A = Li, Na, K) at 100 mA g^-1, and (F) spacing change of NaV₃O₈ at different charge-discharge stages. Reproduced with permission^[84]. Copyright 2019, Elsevier. (G) Formation of CaV₆O₁₆·2.8H₂O and (H) Ca-storage in CaV₆O₁₆·2.8H₂O. Reproduced with permission^[178]. Copyright 2022, Wiley-VCH GmbH. (I) Cycling performance of Zr-NH₄V₄O₁₀ at 0.2 A g^-1. Reproduced with permission^[92]. Copyright 2022, Elsevier.

Compared to vanadium sulfides, the high electronegativity of oxygen leads to higher ionic character of V-O bonding in the oxides, and the strengthened bonding usually raises the electrochemical potential for metal-ion intercalation. Moreover, higher voltage and lower molecular weight will increase the specific energy. For example, monodispersed V₂O₅ hierarchical spheres delivered good performance of 190 mAh g^-1 at 10 mA g^-1[80] because irreversible Mg²⁺ intercalation at the initial charge/discharge process acted as a pillar in the interlayer of V₂O₅.

However, pure V₂O₅ is severely confined for Mg-storage because of poor ion and electron transport processes. It is efficient to improve the conductivity by intercalating electron conductive organics. For example, a 2D organic-inorganic superlattice with alternately arranged monolayered V₂O₅ nanosheets and polyaniline (PANI) monolayers exhibited a Mg-storage capacity of 270 mAh g^-1 at 100 mA g^-1, far superior to only 102 mAh g^-1 of pure V₂O₅ under the same testing conditions. Benefiting from the π-π conjugated chains, monolayer PANI not only served as pillars to enlarge layer spacings but also functioned as electron conducting pathways and active sites for Mg-storage^[81]. Another V₂O₅/PEDOT (3,4-ethylene dioxythiophene) hybrid cathode achieved 348.3 mAh g^-1 at 100 mA g^-1 due to a widened interlayer spacing of 19.2 Å about 4.37 times that of pure V₂O₅^[82]. Besides, it is also effective to improve the kinetics of V₂O₅ by introducing oxygen vacancies. An O-vacancy riched Ti-V₂O_5-x with a honeycomb structure exhibited an impressive electronic conductivity six times higher than that of pure V₂O₅. Due to the improved kinetics, the O-vacancy riched electrode delivered a Mg-storage capacity of 245.4 mAh g^-1 at 100 mA g^-1, except for 79.6% retention after 400 cycles^[83].

Pre-intercalation of metal ions into vanadium oxides also enhances the Mg-storage performance. For example, V₃O₈ with hydrothermally intercalated Li⁺, Na⁺ and K⁺ ions delivered different Mg-storage capacities of 252.2, 204.16 and 37.56 mAh g^-1 at 100 mA g^-1, respectively, [Figure 4D] except for the capacity retention of 42.2%, 85.78%, and 88.6% [Figure 4E], respectively, after 30 cycles^[84]. The V-O layer spacing at different charging and discharging stages does not recover to be the same as the initial [Figure 4F]. Some Mg²⁺ may remain in the interlayers and gradually accumulate during the cycling process, resulting in poor cycling performance. Other metal-ion intercalated vanadium oxides, e.g., Mg_0.3V₂O₅·1.1H₂O,Mn_0.04V₂O₅·1.17H₂O,Mn_0.04V₂O₅·1.17H₂O,NaV₆O₁₅,etc., were also reported with better Mg-storage performance than their parent oxides.

Although interlayer structures of vanadium oxides can be stabilized with intercalated metal ions, the effects of space charge repulsion and occupied active sites by pre-intercalated ions will reduce initial capacities. Differently, water molecules can effectively widen the interlayer spacings and buffer the charge repulsion effect between guest ions and elements from the host structures. There are two common strategies to introduce water molecules. One is in-situ formation of active materials with crystal water in an aqueous solution, and the crystal water acts as both interlayer support and charge shielding layers. The hydrothermally synthesized NaV₃O₈·1.69H₂O nanoribbons exhibited a Mg-storage capacity of 110 mAh g^-1 at 10 mA g^-1, which decays rapidly due to fast Mg consumption from leaching crystal water^[85]. Second, trace amounts of water in organic electrolytes can be more ion-conductive. Meanwhile, it does not lower the electrochemical stabilization windows. For example, NaV₈O₂₀·nH₂O in mixed solvents of tetramethylene glycol dimethyl ether (TEGDME)/water (4:1 by volume) delivered much better performance than that in pure TEGDME (351 mAh g^-1vs. 169 mAh g^-1 at 0.3 A g^-1), except for a wide window voltage of 3.9 V^[86]. Joe et al. improved the diffusion coefficient of 0.3 M Mg (TFSI)₂/AN electrolyte up to 2.45 times by adding 3 M water^[82].

As to V-based phosphates, pre-intercalation of small molecules of H₂O or organics was also explored to improve the Mg-storage performance. For example, water or aniline molecules enhanced the diffusion kinetics of VOPO₄ cathodes due to widened interlayer spacings. Benefiting from a fast MgCl⁺ intercalation mechanism, the cathode delivered 310 mAh g^-1 at 50 mA g^-1 and 192 mAh g^-1 at 0.1 A g^-1 even after 500 cycles^[87]. Similarly, the metal-ion pre-intercalated cathode of Li₃V₂(PO₄)₃ composite delivered 124 mAh g^-1 at 0.1 A g^-1 and 80% capacity retention after 300 cycles at 0.5 A g^-1 in an organic electrolyte with 1.5% water content. Mechanism characterization disclosed that Li⁺ was extracted from the cathode during the first charge and co-intercalated with Mg²⁺ in subsequent cycles, contributing to an enhanced storage capacity^[88]. Table 1 presents a direct comparison of the electrochemical performance of some representatives.

Ca-metal batteries

The lower reduction potential of Ca (-2.87 V vs. SHE) than Mg allows its metal batteries to deliver much higher voltages. Meanwhile, the lower charge density and polarization also contribute to better diffusion kinetics. However, various vanadium oxides suffered severe structural degradation and collapse during ion insertion/extraction. The derivatives with pre-intercalated metal ions, e.g., A_xV₂O₅·nH₂O, where A stands for metal ions, exhibited good structural stability in Ca-storage. At a testing temperature of 50 °C, reversible capacities of 142.4, 109.8, and 86.6 mAh g^-1 were obtained in Mg_0.25V₂O₅·H₂O, Ca_0.26V₂O₅·H₂O, and Sr_0.42V₂O₅·0.7H₂O cathodes after 60 cycles at 100 mA g^-1[89]. The former two cathodes suffered from mono-phase solid-solution reactions during Ca²⁺ insertion/extraction, while the latter performed a two-phase transformation reaction. A similar Ca_0.28V₂O₅·H₂O cathode was reported to suffer from an initial irreversible amorphization before reversible insertion/extraction of Ca ions, which afforded 143 mAh g^-1 at 10 mA g^-1[90]. Moreover, some metal-ion intercalated vanadium oxides also followed a Ca-storage-like ion exchange mechanism. K_0.5V₂O₅ transformed into Ca_0.45V₂O₅, contributing a reversible capacity of 65 mAh g^-1 at 66.6 mA g^-1 and high capacity retention of 92% after 100 cycles^[91].

In addition to oxide derivatives with pre-inserted metal ions, vanadates, such as CaV₆O₁₆·2.8H₂O^[46], are also good candidates for Ca-storage. CaV₆O₁₆·2.8H₂O showed capacities of 175.2 mAh g^-1 at 50 °C and 131.7 mAh g^-1 at room temperature. Ca²⁺ undergoes a solid-solution reaction [Figure 4G] with a diffusion barrier of 0.36 eV along the b-direction [Figure 4H]. A Na-doped NH₄V₄O₁₀ cathode with rod-shaped particles initially discharged 125 mAh g^-1 at 0.1 A g^-1[55]. Another Zr-doped NH₄V₄O₁₀ initially discharged 78 mAh g^-1 at 50 mA g^-1 [Figure 4I], showing a discharge voltage of about 3.0 V vs. Ca²⁺/Ca^[92]. The performance of them is also compared in Table 1.

Al-/Zn-metal batteries

Rechargeable aluminum batteries (RABs) have been intensively focused due to their high safety and rich aluminum abundance^[93]. However, they still face many issues, such as severe corrosion of liquid electrolytes, significant volume change, low discharge voltage, poor reversibility, and so on^[94,95]. Wang et al. reported that a FeVO₄@ PANI nanoribbon composite held 300 mAh g^-1 after 300 cycles at 0.3 A g^-1[93]. Besides, it achieved 268.6 mAh g^-1 after 200 cycles, even at a low temperature of -10 °C. A pre-lithium vanadium oxide derivative of Li₂V₆O₁₃ attained 161.6 mAh g^-1 at 50 mA g^-1 after 300 cycles, far superior to that of pristine V₆O₁₃ whose capacity rapidly decays to 45.4 mAh g^-1 after 50 cycles^[96]. Transition metal sulfides, e.g., VS₄ in nanowires or auricular shapes, were also reported to deliver good Al-storage performance. A channel-rich VS₄ nanowire achieved 252.5 mAh g^-1 at 100 mA g^-1 after five activation cycles and held 138.9 mAh g^-1 after 100 cycles at 0.4 A g^-1[97]. An auricular VS₄ retained 322.2 mAh g^-1 at 200 mA g^-1 after 120 cycles^[98]. However, they displayed quite different ion storage mechanisms. Furthermore, the V₂CT_x MXenes electrode also demonstrated good Al-storage capacity exceeding 300 mAh g^-1 at 100 mA g^-1[63].

Metallic zinc is also a safe anode. However, the research on Zn-metal batteries in organic electrolytes is limited due to lower voltages and capacities. A flower-like NH₄V₄O₁₀ attained a capacity of 486 mAh g^-1 at 0.1 A g^-1 in 1 M Zn (ClO₄)₂/propylidene carbonate (PC) electrolyte [Figure 5A], which displayed little capacity decay at 10 A g^-1 for 3,000 cycles [Figure 5B]^[99]. With the same electrolyte, a paper-like electrode with perfectly aligned Na₂V₆O₁₆·3H₂O nanoribbons exhibited 216 mAh g^-1 at 0.5 A g^-1, and 167 mAh g^-1 was still attained at 5 A g^-1 after 5,000 cycles^[100]. Besides, a composite with V₂O₅·1.6H₂O/Ti₃C₂ MXenes heterostructured nanosheets delivered 205.5 mAh g^-1 at 0.1 A g^-1 in triethyl phosphate electrolyte with a trace of water, and capacity retention of 78.6% was obtained at 0.5 A g^-1 after 4,000 cycles^[101]. The performance is also compared in Table 1.

Figure 5. (A) Rate and (B) long-term cycling performance of Na₂V₆O₁₆·3H₂O. Reproduced with permission under the terms of the Creative Commons Attribution^[99]. Copyright 2020, the Author(s), Springer Nature. (C) Working mechanism and (D) cycling performance of VS₂-GO. Reproduced with permission^[103]. Copyright 2018, Elsevier.

Hybrid-ion batteries

Considering the limitations of single-ion storage, e.g., safety issues and high cost of lithium, high polarization of Mg²⁺, hybrid-ion storage has attracted increasing attention in recent years^[102]. For example, in a Mg/Li hybrid electrolyte, Li⁺ dominates the cathode insertion because the diffusion rate of Li⁺ in the cathode is much larger than that of Mg²⁺. At the discharge, Mg²⁺ ions are dissolved from the Mg anode, while Li⁺ ions are inserted into the cathode [Figure 5C]^[103]. Meanwhile, in the charge process, the situation is just the opposite. A hybrid Mg-Li-ion battery combines the advantages of a Mg anode without dendrite deposition and a fast lithium insertion cathode, making it a better alternative to LIBs for power storage^[104].

Layered V-based compounds are good ion intercalation hosts due to their large specific capacity and multi-electronic reactions. For example, the hybrid batteries of Mg²⁺/Na⁺ and Mg²⁺/K⁺ with VS₂ cathodes were also explored. It was observed that ions of Li⁺, Na⁺, or K⁺ could be co-inserted with Mg²⁺ into VS₂. Differently, co-insertion of Mg²⁺/Li⁺ or Mg²⁺/K⁺ led to the collapse of VS₂, while Mg²⁺/Na⁺ reversibly co-intercalated into VS₂, contributing to a capacity of 170 mAh g^-1 at 0.1 A g^-1 and 96.5% retention after 1,000 cycles^[105]. A Mg²⁺/Li⁺ battery with a graphene-wrapped VS₂ cathode and a Mg anode delivered 235 mAh g^-1 at 90 mA g^-1, and about 146 mAh g^-1 was held at 9.5 A g^-1 after 10,000 cycles [Figure 5d] ^[103]. The mechanism of a Mg²⁺/Li⁺ hybrid-ion battery with a NaV₃O₈·1.69H₂O cathode revealed Li⁺ insertion/extraction at the cathode was accompanied by a small amount of Mg²⁺ adsorption, while the anode is dominated by Mg²⁺ deposition/dissolution^[106]. Differently, both Mg²⁺ and Li⁺ were involved in the cathodic intercalation reaction and accompanied by a change in the valence state of Mo/V in another V₂MoO₈ cathode^[17]. In a Ca²⁺/Zn²⁺ hybrid-ion battery using a Na₃V₂(PO₄)₃ cathode, the open framework in the cathode achieves fast kinetics and good cycling stability for Ca²⁺ storage, and Ca²⁺ preferentially adsorbed on the zinc anode to form an electrostatic shielding layer, which inhibited zinc dendrites and improved the cycling performance^[107].

Mg-/Ca-ion batteries

The available electrode materials for aqueous Mg-ion batteries have faced issues such as limited storage capability due to sluggish Mg-ion diffusion kinetics, easy structure degradation accompanying Mg-ion intercalation resulting from large volume effect and dissolution of active materials, etc. Therefore, relevant references are much less than those about non-aqueous batteries. Zhang et al. used VO₂ as an anode and 1.0 M MgSO₄ as an electrolyte^[109]. A poor cycling performance of only 54.3% retention was achieved after 100 cycles at 0.5 A g^-1. After the first charging process, the VO₂ anode transformed into stabilized MgVO_x, which subsequently served as a host for Mg²⁺ insertion/extraction. In aqueous batteries, the electrochemical performance is severely influenced by temperatures. At low temperatures near the freezing point of electrolytes, lowered interfacial dynamics and ionic conductivity would degrade the performance of the batteries^[110]. In aqueous VO₂/δ-MnO₂ batteries, MgCl₂ [Figure 6A] effectively disrupts the hydrogen bonding network between water molecules and lowers the freezing point [Figure 6B]. This allows the battery to operate from -50 to 25 °C. However, the issue of active material loss due to partial dissolution leads to poor cycling performance. At 100 mA g^-1, 228.5 mAh g^-1 was achieved at room temperature with retention of 35.4% after 30 cycles, while capacities of 97.9 mAh g^-1 at -20 °C with retention of 26% and 37.1 mAh g^-1 at -50 °C with retention of 23% were also attained at the same current rate^[111]. Differently, the Cu₃V₂O₇(OH)₂·2H₂O cathode lasted for 20,000 cycles with retention of 92% at 10 A g^-1 besides a high capacity of 262 mAh g^-1 at 250 mA g^-1. The good performance was attributed to intertwined V₆O₁₃ layers, which avails Mg-ion intercalation and stabilizes the structure of Cu₃V₂O₇(OH)₂·2H₂O^[50]. A mesoporous hierarchical FeVO₄/C [Figure 6C] anode delivered 184.2 mAh g^-1 in 1 M MgSO₄ electrolyte at 50 mA g^-1, and 63.2% capacity was held after 50 cycles [Figure 6D]. The hierarchical pores provide fast pathways for ion diffusion and electrolyte penetration while coating carbon improves the electron conductivity of the anode^[112].

Figure 6. (A) δ-MnO₂//MgCl₂ (aq.)//VO₂ operation from 25 to -50 °C and (B) molecule dynamic simulation of water and MgCl₂ electrolyte. Reproduced with permission^[111]. Copyright 2023, Elsevier. (C) Synthesis of FeVO₄/C and (D) relevant cycling performance. Reproduced with permission^[112]. Copyright 2017, Wiley-VCH GmbH.

Since rechargeable calcium batteries based on organic electrolytes are severely limited in cycling performance and kinetics, Ca-ion batteries with aqueous electrolytes would be an exciting alternative to avoid issues faced by Ca deposition in organic electrolytes and to extend the choice of active materials. An anode material of CaV₆O₁₆·7H₂O synthesized by a molten salt method exhibited an initial discharge capacity of 208 mAh g^-1 at 12.5 mA g^-1, and a high retention of 97% was obtained after 200 cycles. CVs Cyclic voltammetry curves under different pH values of 2.3 and 10 confirmed that Ca²⁺ intercalation rather than H⁺ dominated the energy storage mechanism^[113]. A hydrothermally synthesized K₂V₆O₁₆·2.7H₂O cathode initially discharged 113.9 mAh g^-1 at 20 mA g^-1 in a three-electrode aqueous Ca-ion system and held 78.3% capacity after 100 cycles at 50 mA g^-1[114]. The comparison of these performances is also summarized in Table 1.

Al-ion batteries

With large theoretical capacity, abundant aluminum resources, and high safety, aqueous aluminum ion batteries have been attractive recently. A layered LiV₃O₈ cathode material delivered 205 mAh g^-1 in 2 M Al(CF₃SO₃)₃ aqueous solution at about 500 mA g^-1 [Figure 7A] and held 77.3% capacity after 500 cycles^[115]. Reversible insertion/extraction of 0.94 mol Al³⁺ per mol LiV₃O₈ was disclosed [Figure 7B]. FeVO₄ was converted into Al_xV_yO₄ spinel and amorphous Fe-O-Al after Al³⁺ insertion in 1 M AlCl₃ aqueous solution. It delivered 350 mAh g^-1 at 60 mA g^-1 but decayed rapidly due to vanadium dissolution^[116]. The VOPO₄·2H₂O nanosheets achieved 125.4 mAh g^-1 at 20 mA g^-1. However, the capacity decreased by 40% after 40 cycles due to the loss of crystal water^[117]. A good cycling performance of 2,800 cycles with 86.2% capacity retention was achieved in MoO₃//VOPO₄ aluminum ion battery at 1 A g^-1 when gelatin-polyacrylamide hydrogel electrolyte was used^[118]. A novel ultrathin heterostructured nanocomposite of VOPO₄·nH₂O@MXene exhibited 355.7 mAh g^-1 at 0.5 A g^-1, showing a high discharge potential of 1.8 V^[62]. The bonding of interlayer crystal water and MXenes contributes to extraordinary cycling stability. Table 1 above compares some of the performance.

Figure 7. (A) Al³⁺ storage in LiV₃O₈ cathode and (B) Rate performances. Reproduced with permission^[115]. Copyright 2022, Elsevier. (C) Ex situ XRD characterization of VS₂/VO_x heterostructure and (D) voltage profiles at 1 A g^-1. Reproduced with permission^[128]. Copyright 2020, Wiley-VCH GmbH.

Mn-ion batteries

Unlike Mg and Al metals with high redox potentials, metal Mn with lower redox potentials is a promising candidate material^[119]. Furthermore, Mn has high abundance, good salt solubility, and a small ion radius^[120]. All of these indicate that rechargeable aqueous Mn-ion batteries are feasible. However, there are almost no reports on Mn²⁺ carriers in battery research. A Mn_0.18V₂O₅·nH₂O cathode delivered 83.3 mAh g^-1 at 5.0 A g^-1 in 1 M Mn(CF₃SO₃)₂ aqueous solution and held 86.7% capacity after 200 cycles at 5.0 A g^-1[121]. Yang et al. used V₂O₅ as a cathode, sucrose as a water-splitting inhibitor, and sodium perchlorate (NaClO₄) and glycine as electrolytes; a strong organic-inorganic interface is formed on Mn metal^[119]. The assembled Mn||V₂O₅ battery delivers 180 mAh g^-1 at 0.5 A g^-1 and maintains approximately 100% capacity after 200 cycles at 1.5 A g^-1.

Zn-ion batteries

Zinc has advantages such as high redox potential, high density, large theoretical volumetric energy density, low cost, and high content^[122]. V-based compounds are ideal cathodes for aqueous Zn-ion batteries. The relevant salts used mainly include ZnCl₂, ZnSO₄, Zn(CF₃SO₃)₂, etc. Among them, the use of ZnSO₄ in V-based Zn-ion batteries readily leads to some electrochemically inactive by-products such as ZnSO₄(OH)₆·xH₂O and Zn₂V₂O₇(OH)₂·nH₂O, which led to depletion of the electrolyte and rapid capacity decay^[10,123]. Differently, electrolyte utilizing Zn(CF₃SO₃)₂ allows for fast Zn plating/stripping kinetics due to the weak solvation effect of bulky anions^[124], but its high price means high cost for large-scale application. Currently, one of the main issues faced by aqueous Zn batteries is the short circuit caused by dendrites generated by the zinc anode^[122]. To alleviate this problem, many strategies have been proposed, such as artificial interface layers, 3D structure, alloying, electrolyte engineering, etc.^[125].

(1) Electrochemical performance

The large interlayer spacings of vanadium sulfides facilitate fast Zn²⁺ diffusion and intercalation. For example, VS₂ delivered 159.3 mAh g^-1 at 0.1 A g^-1 in ZnSO₄ electrolyte and held 81% capacity at 0.5 A g^-1 after 200 cycles^[126]. A much better performance of 187 mAh g^-1 at 0.1 A g^-1 and 85% retention after 2,000 cycles at 2 A g^-1 was achieved when VS₂ was used as a cathode^[127]. A VS₂/VO_x [Figure 7C] heterostructure was reported to deliver 310 mAh g^-1 with 75% retention after 3,000 cycles. Moreover, the working potential increased by 0.25 V compared with that of pure VS₂ at 1 A g^-1 [Figure 7D]^[128]. A VS₂@N-C hybrid with enhanced reactivity and interfacial charge transfer by N-doping delivered 203 mAh g^-1 at 50 mA g^-1, and a retention of 97% was retained after 600 cycles at 1 A g^-1[129]. In contrast, VS₄ suffered severe volume changes and dissolution of polysulfides after Zn insertion. Specifically, it was initially converted to Zn_3+x(OH)₂V₂O₇ in the initial cycles, and Zn²⁺ was subsequently inserted into/ extracted out of the open framework structure reversibly. For example, a flower-like VS₄/carbon nanotubes (CNTs) nanocomposite showed a capacity of 182 mAh g^-1 at 0.25 A g^-1 and 93% retention after 1,200 cycles at 5 A g^-1[130]. Another VS₄@rGO electrode delivered 180 mAh g^-1 at 1 A g^-1 with 93.3% retention after 165 cycles^[131].

Vanadium oxides have a wide range of applications in aqueous Zn-ion batteries. V₂O₅ exhibits a theoretical capacity of 589 mAh g^-1 based on V⁵⁺/V³⁺ redox, but the severe deformation accompanying Zn insertion/extraction readily leads to unstable cycling performance^[132]. Proper content of water molecules in interlayers of VO_x polyhedrons avails to shield strong Zn²⁺ host interaction and stabilize the host structure. For example, water molecules in V₂O₅·nH₂O functioned as a buffer layer, weakening the effective charge of intercalated Zn²⁺, leading to good rate performance of 248 mAh g^-1 even at 30 A g^-1 [Figure 8A]^[133]. The cycling performance can also be improved with conductive support. A nano paper electrode comprised of V₂O₅ nanofibers and multiwalled CNTs held 168.5 mAh g^-1 at 10 A g^-1 for 500 cycles^[134]. A nanocomposite with heterostructures of V₂O₅ nanosheets and Ti₃C₂T_x MXenes layer showed enhanced conductivity and robust structure and exhibited stable cycling performance for 5,000 cycles with 99.5% capacity retention at 10 A g^-1[41].

Figure 8. (A) The discharge curves of V₂O₅·nH₂O at 0.3-30 A g^-1. Reproduced with permission^[133]. Copyright 2017, Wiley-VCH GmbH. (B) Diffusion paths in V₂O₅ and Al-V₂O₅. Reproduced with permission^[137]. Copyright 2022, Elsevier. (C) Cycling performances and (D) reaction mechanism of V₂O₃. Reproduced with permission^[139]. Copyright 2021, American Chemical Society. (E) Energy storage mechanism in the Zn|| V₂O₃ cells. Reproduced with permission under the terms of the Creative Commons Attribution^[140]. Copyright 2021, the Author(s), Springer Nature.

Oxygen vacancies usually enhance the conductivity and improve the performance. Dendrites of V₁₀O₂₄·12H₂O, interpreted as V₂O_5-x·nH₂O compound with oxygen vacancies, delivered 164.5 mAh g^-1 at 0.2 A g^-1 and 3,000 cycles with 80.1% retention at 10 A g^-1[135]. After Al doping, structure stability and ion storage capability are highly improved, leading to a high capacity of 534 mAh g^-1 in Al-doped V₂O₅ at 0.1 A g^-1[136]. Relevant theory simulations showed that doping Al significantly reduced the diffusion barrier of Zn²⁺ and increased the conductivity of V₂O₅ [Figure 8B]^[137].

Corundum-type V₂O₃ with unique channels and suitable pore size distribution shows fast insertion/extraction of Zn^2+[138]. Oxygen-deficient carbon-coated V₂O₃ delivered 662 mAh g^-1 at 0.2 A g^-1 [Figure 8C] after turning into Zn_0.4V₂O_5-m·nH₂O during the first charge [Figure 8D]^[139]. Similarly, V-deficient V₂O₃ also delivered enhanced Zn-storage capability because vanadium-defect clusters could afford favorable intercalation sites for Zn ions, as revealed by calculations. In addition, intercalated Zn²⁺ at the V vacancies serves as doped heteroatoms, making the host structure more stable [Figure 8E]^[140].

V₃O₇ with mixed valences (V⁴⁺/V⁵⁺) provides more active sites^[140]. A uniform and ultrafine V₃O₇·H₂O nano-network delivered 481.3 mAh g^-1 at 0.1 A g^-1 [Figure 9A] and 85.4% capacity retention at 5A g^-1 for 1,000 cycles [Figure 9B]^[141]. V₃O₇·nH₂O nanoribbons with rGO exhibited a specific capacity of 410.7 mAh g^-1 at 0.5 A g^-1 and 99.6% retention at 4 A g^-1 after 1,000 cycles^[142]. Core-shell nanowires of V₃O₇·H₂O@V₂O₅·nH₂O showed a capacity of 455 mAh g^-1 at 0.1 A g^-1 and 85% retention at 0.5 A g^-1 for 1,200 cycles^[143].

Figure 9. (A) Voltage profiles and (B) long-term cycling stability of V₃O₇·H₂O cathode. Reproduced with permission^[141]. Copyright 2019, Royal Society of Chemistry. Diffusion paths of Zn ions (C) with and (D) without water and (E) calculated diffusion barriers for paths in (C and D). Reproduced with permission^[28]. Copyright 2019, Wiley-VCH GmbH. (F) Cycling performance of Zn//CO₂-V₆O₁₃ and Zn//P-V₆O₁₃ at 2 A g^-1 and (G) CO₂ molecules modified layer structured material. Reproduced with permission^[145]. Copyright 2021, American Chemical Society.

Similarly, V₆O₁₃ delivered 360 mAh g^-1 at 0.2 A g^-1, benefiting from reduced diffusion barriers of hydrated Zn²⁺ [Figure 9C-E]^[28]. A 3D nested structure of V₆O₁₃ cathodes even exhibited a capacity of 520 mAh g^-1 at 0.5 A g^-1 and 85.3% retention at 2 A g^-1 after 1,000 cycles due to short diffusion depth and large surface^[144]. The electrode of V₆O₁₃ with trapped CO₂ molecules showed a capacity of 471 mAh g^-1 at 0.1 A g^-1 and 80% capacity retention at 2A g^-1 for 4,000 cycles [Figure 9F] due to significantly reduced relative energy of Zn²⁺ diffusion [Figure 9G]^[145].

Unlike other vanadium oxides, tunnel-structured VO₂ showed enhanced structural stability, benefiting from shared corner and edge resistance to lattice shearing accompanying ion insertion/extraction^[146]. However, low conductivity and instability in acidic aqueous solutions limited its ion storage capability and cycle performance. For example, a capacity of 610 mAh g^-1 was achieved at 0.1 A g^-1 by in situ electrochemical oxidation of VO₂ nanorods to V₂O₅·nH₂O^[147]. Similar transitions were also observed for monoclinic VO₂^[148]. Besides V₂O₅·nH₂O, VO₂ could also be in situ converted into ZnV₂O₇, which showed 408.4 mAh g^-1 at 0.1 A g^-1 and 91% retention at 10 A g^-1 for 4,000 cycles [Figure 10A and B]^[149]. Moreover, VO₂ with structure defects, such as Mn-doped VO₂, oxygen vacancy-rich VO, etc., also exhibited improved performance^[150]. The performance of vanadium oxides could be improved by the preintercalation of some ions. The Co_0.247V₂O₅·0.944H₂O nanoribbons delivered a capacity of 432 mAh g^-1 at 0.1 A g^-1 and 90.26% retention at 10 A g^-1 after 7,500 cycles [Figure 10C], much better than those of oxide counterparts^[151]. Similarly, a Cu_0.34V₂O₅ cathode delivered 258 mAh g^-1 at 100 mA g^-1[152].

Figure 10. (A) XRD characterization of the VOP cathode at different charge-discharge stages and (B) phase transition disclosed from the differential capacity curve at various concentrations. Reproduced with permission^[162]. Copyright 2019, Wiley-VCH GmbH. (C) Cycling performance of Co_0.247V₂O₅·0.944H₂O at 4 A g^-1. Reproduced with permission^[151]. Copyright 2019, Wiley-VCH GmbH. (D) Charge-discharge curves of H₁₁Al₂V₆O_23.2 at 0.1-5.0 A g^-1. Reproduced with permission^[56]. Copyright 2020, Elsevier. (E) Cycling performance of (NH₄)_0.38V₂O₅/CNTs paper electrode. Reproduced with permission^[160]_. Copyright 2021, Elsevier. (F) Typical charge-discharge curves of NH₄V₃O₈·0.5H₂O and PANI- NH₄V₃O₈·0.5H₂O electrodes at 1 A g^-1. Reproduced with permission^[57]. Copyright 2022, Elsevier. Typical charge-discharge curves of NH₄V₄O₁₀ at (G) 0.2-1.4 V, (H) 0.2-1.6 V, and (I) 0.2-1.8 V. Reproduced with permission under the terms of the Creative Commons Attribution^[161]. Copyright 2023, the Author(s), Wiley-VCH GmbH.

Vanadates are also good candidates for Zn-storage. The layered LiV₃O₈ discharged 200 mAh g^-1 at 133 mA g^-1 in an aqueous ZnSO₄ electrolyte^[153]. A H₁₁Al₂V₆O_23.2 cathode with an interwoven layer nanosheet structure delivered 288 mAh g^-1 at 0.1A g^-1 [Figure 10D] due to short diffusion length and abundant active sites^[56]. The Ag₂V₄O₁₁ cathode was reported to deliver 213 mAh g^-1 at 0.2 A g^-1 and 93% retention at 5 A g^-1 after 6,000 cycles, benefiting from a pseudo-Zn-air reaction^[154]. K⁺ can act as pillars between the vanadium-oxygen intercalation layers^[138], thus improving the structural stability. A K₂V₈O₂₁ cathode exhibited a high capacity of 247 mAh g^-1 at 0.3 A g^-1, and about 128 mAh g^-1 was retained at 6 A g^-1 after 300 cycles, corresponding to retention of 83%^[87]. A free-standing potassium vanadate/single walled CNTs (KVO/SWCNTs) composite film exhibited a capacity of 379 mAh g^-1, and the capacity only decays from 220 to 200 mAh g^-1 after 10,000 cycles at 5 A g^-1[155]. Different cations can synergistically coexist between layers of vanadium oxides. NaCa_0.6V₆O₁₆·3H₂O nanoribbons with a unique V₃O₈ laminar structure, which energetically favors Zn²⁺ diffusion, delivered 247 mAh g^-1 at 0.1 A g^-1 and retained 83% capacity at 5 A g^-1 after 10,000 cycles^[156]. Layered alkali vanadates have an open-framework structure, thus enabling fast Zn²⁺ diffusion. A self-supported membrane of Zn₃V₂O₇(OH)₂·2H₂O with a porous crystal structure achieved 213 mAh g^-1 at 50 mA g^-1[157]. A layer Fe₅V₁₅O₃₉(OH)₉·9H₂O nanosheet cathode delivered 358 mAh g^-1 at 0.1 A g^-1 and 80% retention at 5 A g^-1 after 300 cycles^[158].

The presence of hydrogen bonding between NH₄⁺ and V-O layers makes a stable structure in ammonia vanadate, resulting in excellent long-term cycling stability^[159]. For example, ultrathin (NH₄)₂V₁₀O₂₅·8H₂O nanoribbons, with large interlayer spacings of 1.045 nm favoring fast Zn²⁺ diffusion, achieved 228.8 mAh g^-1 at 100 mA g^-1 and 90.1% retention at 5 A g^-1 after 5,000 cycles^[54]. A binder-free cathode of (NH₄)_0.38V₂O₅ nanoribbons delivered 465 mAh g^-1 at 100 mA g^-1, and the retention was 89.3% after 500 cycles [Figure 10E]^[160]. An NH₄V₃O₈·0.5H₂O and PANI hybrid initially discharged 397.5 mAh g^-1 at 1A g^-1 [Figure 10F], benefiting from the tailored large interlayer spacings^[57]. The regulation of the larger interlayer spacings was also revealed in NH₄V₄O₁₀ nanoribbons by variation of charged voltages or discharge capacities [Figure 10G-I]. When the cathode was charged to 1.6 V, it displayed 223 mAh g^-1 at 10 A g^-1 and 97.5% retention after 1,000 cycles^[161].

V-based phosphates showed high discharge plateaus due to strong inductive effect of PO₄^3- and represented a type of promising high-energy electrode material for Zn-ion batteries^[58]. However, they also faced various issues. VOPO₄·nH₂O dissolves easily in aqueous solution, leading to poor cycling performance^[61]. Concentrated ZnCl₂ electrolyte was reported to prevent the dissolution of VOPO₄·2H₂O and protect Zn metal from hydrogen evolution reactions and dendrites^[162,163]. Additionally, 29 M ZnCl₂ was adopted to inhibit H⁺ cointercalation and dissolution of LiV₂(PO₄)₃^[162]. The addition of 70% PEG favored reducing free water in the electrolyte and improving the coulomb efficiency^[164]. The presence of high concentration of oxygen vacancies largely improved Zn²⁺ diffusion kinetics in VOPO₄·2H₂O nanosheets [Figure 11A]. Mott-Schottky (impedance potential) measurements also showed that the electronic conductivity was greatly improved due to high concentration of O vacancies, which increases the carrier concentration by about 57 times [Figure 11B]. As a result of these unique characteristics, the specific capacity was 313.6 mAh g^-1 at 0.1 A g^-1, and the retention was 76.8% after 500 cycles at 5.0 A g^-1[61]. Intercalation of aniline significantly increased the hydrophobicity of VOPO₄·2H₂O cathode, thus inhibiting dissolution. Meanwhile, large layer spacing of 16.5 Å and a high diffusion coefficient of 5.7 × 10^-8 cm^-2s^-1 were also achieved^[165]. An open Na superionic conductor with a stable structure facilitates rapid ion diffusion^[166]. Mesoporous graphene oxide-coated Na₃V₂(PO₄)₂F₃ nanoparticles [Figure 11C] delivered 126.9 mAh g^-1 at 0.5 C (1 C = 128 mA g^-1) [Figure 11D], showing a very little capacity decay of only 0.0074% per cycle at 15 C for 5,000 cycles [Figure 11E]^[60].Table 1 also shows the comparison of the performance.

Figure 11. Kinetics in Zn//bulk-VOPO₄ and Zn//bilayer-VOPO₄ batteries. (A) GITT and (B) Mott-Schottky plots. Reproduced with permission^[61]. Copyright 2021, Wiley-VCH GmbH. Material preparation and performance of Na₃V₂(PO₄)₂F₃@rGO. (C) Synthesis procedure, (D) galvanostatic charge-discharge profiles at 0.5 C, and (E) Cycling performance at 15 C. Reproduced with permission^[60]. Copyright 2023, Wiley-VCH GmbH.

(2) Energy storage mechanism

Safe and cost-effective aqueous Zn batteries are well-suited for large-scale applications. However, some reaction mechanisms of the cathodes are currently controversial. The dominant mechanisms include reversible intercalation of Zn²⁺/H⁺ or solvated Zn²⁺/H₂O. Reversible or irreversible phase transitions accompany the ion intercalation process. Various by-products are also generated in the process, such as Zn₃V₂O₇(OH)₂·nH₂O,Zn₄SO₄(OH)₆·nH₂O,Zn_x (OH)_y(CF₃SO₃)·nH₂O. The OH^- in Zn₄SO₄(OH)₆·nH₂O and Zn₃V₂O₇(OH)₂·nH₂O comes from water decomposition in the electrolyte.

Zn₃V₂O₇(OH)₂·nH₂O is a very common by-product of aqueous Zn-ion batteries, and there is considerable controversy about the role of Zn₃V₂O₇(OH)₂·nH₂O in the battery. For example, when Fe₂V₄O₁₃ was used as the cathode, Zn₃V₂O₇(OH)₂·2H₂O could reversibly appear and disappear^[167]. Partial Fe₂V₄O₁₃ converted into Zn₃V₂O₇(OH)₂·2H₂O in the discharge. Meanwhile, the remaining acted as a host to store Zn²⁺. During the subsequent charge process, Zn²⁺ was reversibly extracted, and Zn₃V₂O₇(OH)₂·2H₂O reversibly converted into Fe₂V₄O₁₃. When V₂O₃ was discharged below 0.8 V in the Zn(CF₃SO₃)₂ electrolyte, Zn₃(OH)₂V₂O₇·H₂O was formed, while it disappeared when charging to 1.6 V^[168]. However, the highly crystalline phase of Zn₃V₂O₇(OH)₂·nH₂O electrochemically inactive became dominant when a large amount of it accumulated in aqueous electrolytes^[147]. Therefore, excessive accumulation of Zn₃V₂O₇(OH)₂·nH₂O would result in poor energy storage performance. In contrast, Zn_3+xV₂O₇(OH)₂·2H₂O derived from VS₄ reflected reversible Zn²⁺ insertion/extraction, while Zn₃V₂O₇(OH)₂·nH₂O further transformed to ZnV₃O₈, leading to decay capacity for VS₄@rGO^[130].

Coinsertion of Zn²⁺/H⁺ resulted in variation of pH in the electrolyte, contributing to the formation of those by-products, which indirectly proved the insertion/extraction of H⁺. For example, ζ-V₂O₅ generated from the Cu_0.34V₂O₅ cathode after charging to 1.3 V, suffered cointercalation of Zn²⁺ and H⁺ accompanying the formation of (Zn (OH)₂)₂(ZnSO₄) (H₂O)_n [Figure 12A]^[152]. Reversible H⁺ insertion/extraction happened in various cathodes, such as Cu_0.18V₂O₅·0.72H₂O^[169], Mn-modified V₆O₁₃^[170], Zn_0.36V₂O₅·nH₂O^[171], and etc., implied by appearance and disappearance of Zn₄SO₄(OH)₆·4H₂O, Zn₂V₃O₇(OH)₂·2H₂O, or Zn_x(OTf)_y(OH)_2x-y·nH₂O. Water molecules or hydrogen ions involved in the mechanism were further verified by an organic electrolyte, in which no by-products of Zn₂V₃O₇(OH)₂·2H₂O or Zn_x(OTf)_y(OH)_2x-y·nH₂O were observed.

Figure 12. (A) Zn-storage mechanism in Cu_0.34V₂O₅. Reproduced with permission^[152]. Copyright 2021, American Chemical Society. (B) Zn-storage mechanism in Mg_0.34V₂O₅·0.84H₂O. Reproduced with permission^[45]. Copyright 2018, American Chemical Society. (C) Reaction mechanism of Cu₃(OH)₂V₂O₇·2H₂O at 0.1 A g^-1. Reproduced with permission^[173]. Copyright 2019, Royal Society of Chemistry. (D) Zn-storage in MgV₂O₄. Reproduced with permission^[174]. Copyright 2020, American Chemical Society.

In hydrated ions, solvation water reduces the effective charge density and increases the distance among neighboring cations, leading to decreased coulomb interactions, which is responsible for the high diffusion coefficient. For example, an interlayer spacing of ~13.2 Å was observed when the hydrated Zn ion was intercalated into a porous Mg_0.34V₂O₅·0.84H₂O cathode, which was much larger than the size of Zn²⁺ (~0.7 Å) [Figure 12B]^[45]. CaV₄O₉ exhibited an enhanced charge transfer process due to the cointercalation of Zn²⁺ and H₂O^[172]. Similarly, the content of interlayer water in the Zn_0.25V₂O₅·nH₂O cathode changed with the content of intercalated Zn^[38].

Differently, intercalation of desolvated Zn²⁺ also happened in some circumstances. It was reported that the transformation from Cu²⁺ to metallic Cu⁰ particles occurred when desolvated Zn²⁺ intercalated into copper vanadates, e.g., from Cu₃(OH)₂V₂O₇·2H₂O to Zn_0.25V₂O₅·H₂O; the processes were verified reversible after Zn extraction [Figure 12C]^[173]. Moreover, metallic Cu facilitates good electronic conductivity and superior rate capability. In another example, MgV₂O₄ with intercalated Zn formed in the discharge suffered from the extraction of both Zn²⁺ and Mg²⁺ in the charge process. Compared to Zn²⁺, Mg²⁺ was preferentially extracted. The resultant Zn_xMgV₂O₄ [Figure 12D] served as a stable host for reversible Zn-storage, subsequently^[174].