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Review  |  Open Access  |  25 Mar 2024

Advanced V-based materials for multivalent-ion storage applications

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Energy Mater 2024;4:400026.
10.20517/energymater.2023.82 |  © The Author(s) 2024.
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Abstract

Multivalent-ion batteries, as promising alternative or supplementary technologies to lithium-ion batteries, have increasingly attracted attention recently. Various advanced materials have been presented to pursue potential breakthroughs in energy and power. Among them, vanadium (V)-based materials benefiting from abundant resources, various polymorphs and valences, especially most with large interlayer spacings, are good candidates for multivalent-ion storage. However, limited by multiple inherent issues, e.g., strong electrostatic interactions, poor electronic conductivity, structure collapse or materials dissolution under battery operation, etc., various strategies have sprung many advanced materials and applications and also brought about new challenges that are in urgent need to clarify and summarize. Hence, advanced V-based compounds developed for multivalent-ion storage in the past few years are selectively summarized and systematically analyzed, including vanadium oxides and sulfides, vanadates, and V-based MXenes and phosphates. Not only crystal structures and electrochemical properties but also mainstream ion storage mechanisms are critically reviewed. Through analyzing the challenges accompanying multivalent-ion storage, potential opportunities are anticipated.

Keywords

Multivalent-ion, charge storage, reaction mechanism, efficient energy storage

INTRODUCTION

With ever-growing need of energy storage and electric vehicles, various rechargeable batteries have gained much attention. Among them, lithium-ion batteries (LIBs) are currently dominating the global market for mobile power sources, but issues from safety, cost, and limited resources have hindered their further development[1-3]. Multivalent-ion batteries, such as Mg2+, Ca2+, Zn2+, Al3+, etc., are attracting more and more interest due to their relatively high safety, considerable resource reserves, and good environmental friendliness[4-8]. However, multivalent ions carry more charges per ion than monovalent ions, which means a much stronger Coulomb interaction resulting in sluggish kinetics for ion diffusion[9]. Therefore, compared with monovalent-ion storage, it is much more crucial to search for suitable insertion hosts for efficient multivalent-ion storage.

There are many candidate electrodes, e.g., some layered oxides, Prussian blue analogs, organic compounds, etc., appropriate for multivalent-ion storage, but they are generally challenged by issues such as poor cycling stability, inherent low capacity, severe dissolution, and so forth[10]. Among them, vanadium (V)-based compounds with abundant valences and rich resource reserves guarantee high theoretical capacity and cost-effective scale-application prospects[11,12]. Meanwhile, changeable V-O coordination chemistry of VO4 tetrahedra, VO5 triangular bipyramidal/square pyramidal, and VO6 aberrant/ortho-octahedral and types of polymorphs and microstructures such as laminar, three-dimensional (3D) tunneling, chain, rock-salt structures, etc., also offer a wide structure regulation freedom[12]. Crucially, most V-based compounds exhibit large interlayer spacings conducive to fast insertion/extraction of various multivalent ions. However, issues such as active material loss due to dissolution also occurred in some V-based compounds similarly[13]. Complex reaction mechanisms and low average operating voltage are also occasional challenges. Overall, V-based materials exhibited obvious advantages in emerging multivalent-ion storage. After years of rapid development, it is necessary to comprehensively review relevant achievements and discuss the challenges to present reasonable anticipation for future prospects and development trends. With this consideration, the manuscript systematically introduces the applications of different V-based compounds in various multivalent-ion batteries, as illustrated in Figure 1, including their crystal structure, electrochemical properties, and energy storage mechanism. Firstly, the structural characteristics and potential electrochemical applications of various V-based compounds will be discussed in Section "OVERVIEW AND CATEGORIES". Subsequently (Section "MULTIVALENT-ION STORAGE APPLICATION"), the electrochemical performance of Mg2+, Ca2+, Al3+, and Zn2+ storage in non-aqueous or aqueous batteries, supercapacitors are separately reviewed, and especially, reaction mechanisms of Zn2+ are discussed to comb the emerging complex zinc (Zn) storage processes. Before the conclusion (Section "CONCLUSIONS AND FUTURE PROSPECTS"), various challenges of multivalent-ion storage for V-based materials are summarized, and their possible future trends and directions are predicated.

Advanced V-based materials for multivalent-ion storage applications

Figure 1. Illustration of V-based materials for multivalent-ion storage.

OVERVIEW AND CATEGORIES

Before introducing the Mg2+, Ca2+, Al3+ and Zn2+ multivalent-ion storage performance, various categories of V-based materials and their structures are summarized below. For simplification, they are generally divided as vanadium oxides (VOx), vanadium sulfides (VSx), vanadates (MVOx), V-based phosphates (MVPO4), and V-based MXenes (VXenes), respectively.

Vanadium oxides

Vanadium oxides show different valences of V from 0.4 to 5, forming a variety of symmetries such as triclinic V4O7, V8O15, V7O13, and V6O11, monoclinic VO2 and V2O4, orthorhombic V4O9, tetragonal VO0.2, VO1.27 and V2O5, and cubic VO0.9 and VO. They can typically be treated as corner-, edge-, or face-sharing V-O polyhedra with different oxygen coordination. Some common vanadium oxides [Figure 2] in energy storage include octahedra, pentagonal bipyramid, square pyramid, and tetrahedra coordinated V2O5, VO2, V6O13, V2O3, and V3O7[14]. Various valences and structures afford diverse multi-electron transfer chemistries and abundant vacancies needed for superior ion storage capability and fast kinetics for ion transport, especially for Mg2+, Ca2+, Zn2+, and Al3+ multivalent ions[15,16]. However, unlike monovalent ions, multivalent ions generally exhibit strong Coulomb interaction with the host lattices, leading to considerable polarization and sluggish kinetics in mass and charge transfer processes. Meanwhile, poor electron conductivity and metastable structure of V-O polyhedral layers due to weak van der Waals bonding easily lead to serious capacity decay after repeatedly inserting/extracting multivalent ions. Various vanadium oxides still suffer from the problems of poor rate performance and serious capacity decay[17]. Hence, it is necessary to summarize and clarify their structure characteristics related to energy storage.

Advanced V-based materials for multivalent-ion storage applications

Figure 2. Structures of some typical vanadium oxides, O in red and V in cyan.

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+/V4+[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].

Vanadium sulfides

Vanadium sulfide is available in many forms, e.g., VS2, VS4, V2S3, VS, V2S5, V3S, and V6S. Among them, VS2 and VS4 are usually used in batteries[31]. Their graphene-like structures afford large layer spacings, facilitating cation intercalation and diffusion. Vanadium atoms in VS2 and VS4 exhibit the same oxidation state, while sulfur atoms appear as S2- monomers in VS2 and S22- dimers in VS4, leading to entirely different physicochemical and electrochemical properties[32]. However, sulfides of vanadium are apt to be oxidized, especially when staying for a long duration in air atmosphere. This general phenomenon suggests that stringent conditions and atmosphere are essential for preparing or storing vanadium sulfide.

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.

Advanced V-based materials for multivalent-ion storage applications

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 V2CTx 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 S22- dimer [Figure 3B], and the S22- 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].

Vanadates

Vanadates usually show better electrochemical performance than vanadium oxides due to structure optimization effects of extra metal ions. Similar to Zn0.25V2O5[38], many vanadates are made by inserting different cations into vanadium oxides, a strategy that preserves the rich chemical valence of the original vanadium oxide. Insertion of cations also increases the internal spacing of vanadium oxide, thus effectively mitigating the capacity loss. Furthermore, cations serve as pillars between layers, which prevents relative slip between adjacent V-O and supports both layers, stabilizing the V-O structure and avoiding vertical collapse[39]. According to the type of cations, vanadates can be divided into alkali metal and alkaline earth metal vanadate, transition metal vanadate, and other vanadates. Most vanadates were prepared using the hydrothermal method, so insertion of metal ions is usually accompanied by water molecules, forming vanadates with some crystallized water.

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, MgxV2O5, 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 MgxV2O5·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. AxV2O5·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 H11Al2V6O23.2 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].

V-based phosphates

A prominent feature of V-based phosphates is the output voltage higher than that of vanadates or vanadium oxides due to the inductive effect of the (PO)43- group[58]. Moreover, V-based phosphates also exhibit good structure stability and fast ion diffusion[58,59]. VOPO4, Na3V2(PO4)2F3, Li3V2(PO4)3, Na3V2(PO4)3, etc., are common explored V-based phosphates. VOPO4 displays a typical layered crystal structure with vertex-sharing VO6 octahedra and PO4 tetrahedra in a 1:1 ratio [Figure 3F][60]. Hydrothermal synthesized VOPO4 tends to form the hydrate VOPO4·nH2O, which easily decomposes into VOx during cycling, so the cycling performance is poor[61]. The 3D framework of Na3V2(PO4)3 cathode consists of strongly bonded tetrahedral [PO4] and octahedral [VO6] units with large gaps, favoring fast ion transport. However, due to the poor electron conductivity, Na3V2(PO4)3 is often doped with metal ions or coated with conductive carbon to improve the conductivity. The introduction of highly electronegative F- in Na3V2(PO4)3 can further increase the operating voltage[62]. Stable V-F bonding in the skeleton facilitates the formation of a new polyanion system, further enhancing its inductive effect. Na3V2(PO4)2F3 consists of [V2O8F3] double octahedra with [VO4F2] octahedra sharing corners with [PO4] tetrahedra through an F atom, while [VO4F2] octahedra are connected to [PO4] tetrahedra through O atoms, and sodium (Na) is located in the a-axis and b-axis in the position of open tunnels. Alignment and stacking in the Na3V2(PO4)2F3 framework provide many channels that offer convenient ion diffusion paths. However, similar to most phosphates, they also exhibit very poor electronic conductivity and low capacity[60].

V-based MXenes

MXenes (M and X stand for an early transition metal and C/N elements, respectively) represent a series of two-dimensional (2D) inorganic compounds comprising several atomic layers of transition metal carbides, nitrides, or carbon-nitrides. They are typically produced by selectively removing metal ions in the A layer in the MAX phase with a combination of HF or LiF/HCl aqueous solution as etchants [Figure 3G][63]. Meanwhile, they also exhibit extraordinary physical, chemical, and electrochemical properties, such as hydrophilic surfaces, ultra-high electrical conductivity, and accordion-like laminate structure, making MXenes a good candidate to form a series of functional composites[64]. The primary four types of MXenes are titanium, niobium, vanadium, or molybdenum-based. Among them, most studies are Ti-based MXenes with large interlayer spacings, and abundant active sites allow for fast insertion/extraction of various guest ions[65]. However, the non-environmentally friendly preparation mostly involves harmful and hazardous HF, limiting the application of MXenes. Currently, V2CTx (Tx representing different surface functional groups) is the widely used V-based MXenes in multivalent-ion batteries[63], which undergoes multi-electron redox reactions. Due to its low valence of V (+2 and +3), it usually exhibits a relatively low capacity. To enhance the ion storage capability, in situ electrochemical activation was used to increase the valence state of V in V2CTx while preserving its V-C-V 2D layered structure[66]. In addition, V-based MXenes were also ideal supports to form composite electrode materials with Mg0.2V2O5·nH2O[67], VS4[68], and other V-based compounds for multivalent-ion storage.

MULTIVALENT-ION STORAGE APPLICATION

Non-aqueous batteries

Batteries with non-aqueous electrolytes usually exhibit a much wider electrochemical window and higher energy density than aqueous ones. However, active metal anodes in organic electrolytes tend to form insulative and passivated interphases, resulting in sluggish ion transport. It is a significant obstacle to reversible plating/stripping processes for Mg-/Ca-metal batteries. Differently, the ion diffusion rate in aqueous electrolytes is faster than in organic ones due to the fact that water has a charge shielding effect which can reduce the polarization of ions to the host lattice[69]. Therefore, small amounts of water are sometimes introduced into the organic electrolytes to reduce polarization and improve the reaction kinetics by obtaining a charge-shielding effect or converting metal ions into low-polarized solvated ions.

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, S22- 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].

Advanced V-based materials for multivalent-ion storage applications

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-V2O5-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., Mg0.3V2O5·1.1H2O,Mn0.04V2O5·1.17H2O,Mn0.04V2O5·1.17H2O,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.

Table 1

Performance comparison of V-based materials for multivalent-ion batteries

MaterialsApplicationElectrolyteCapacity (mAh g-1)Cycle performance
NaV6O15[179]MIB0.5 M Mg(ClO4)2/AN137 (0.05 A g-1)80%, 10 mA g-1 (100 cycles)
Mg0.3V2O5·1.1H2O[16]MIB0.3 M Mg (ClO4)2/AN164 (0.1A g-1)80%,2 A g-1 (10,000 cycles)
FeVO4[49]MIB0.3 M Mg(ClO4)2/AN270 (0.5 A g-1)85%,1 A g-1 (10,000 cycles)
H11Al2V6O23.2[180]MIB0.3 M Mg(ClO4)2/AN165 (0.1 A g-1)87%,1 A g-1 (3,000 cycles)
Li3V2(PO4)3[88]MIB0.3 M Mg(ClO4)2/PC124 (0.1 A g-1)80%,0.5 A g-1 (300 cycles)
VS2[73]MIB0.4 M APC-PP14Cl/THF214 (2 A g-1)78%,1 A g-1 (300 cycles)
Mn0.04V2O5·1.17H2O[181]MIB0.3 M Mg(ClO4)2/AN145 (0.05 A g-1)82%,1 A g-1 (10,000 cycles)
CaV6O16·2.8H2O[46]CIBCa(TFSI)2/G2134.7 (0.1 A g-1)75%,0.1 A g-1 (50 cycles)
Mg0.25V2O5·H2O[182]CIB0.8 M Ca(TFSI)2 in carbonate120 (0.02 A g-1)86.9%,0.5 A g-1 (500 cycles)
Ca0.28V2O5·H2O[90]CIB0.5 M Ca(ClO4)2/PC142 (0.01 A g-1)74%,0.03 A g-1 (50 cycles)
K2V6O16·2.7H2O[114]CIB5 M Ca(NO3)2114 (0.02 A g-1)78.5%,0.05 A g-1 (100 cycles)
NH4V4O10[92]CIB0.25 M Ca(TFSI)2/PC77 (0.05 A g-1)89%,0.2 A g-1 (500 cycles)
Ca0.26V2O5·H2O[89]CIB0.8 M Ca(TFSI)2 in carbonate196 (0.02 A g-1)93.6%,1 A g-1 (2,500 cycles)
FeV3O9·1.2H2O[183]CIB0.5 M Ca(ClO4)2/AN96 (0.2 A g-1)79%,0.2 A g-1 (400 cycles)
Li2V6O13[96]AIB[EMIm]Cl:AlCl3 = 1:1.3159 (0.1 A g-1)73%,0.05 A g-1 (300 cycles)
VS4[98]AIB[EMIm]Cl:AlCl3 = 1:1.3408 (0.1 A g-1)39%,0.5 A g-1 (500 cycles)
V2O5[184]ZIB2 M ZnSO4425 (0.3 A g-1)78.5%,3 A g-1 (200 cycles)
VO2[185]ZIB3 M Zn(CF3SO3)2280 (0.1 A g-1)86%,3 A g-1 (5,000 cycles)
V3O7[186]ZIB2.5 M Zn(CF3SO3)2233 (0.2 A g-1)96.2%,2 A g-1 (1,120 cycles)
V6O13[28]ZIB3 M Zn(CF3SO3)2360 (0.2 A g-1)92%,4 A g-1 (2,000 cycles)
V2O3[140]ZIB3 M Zn(CF3SO3)2196 (0.1 A g-1)81%,5 A g-1 (30,000 cycles)
VS2[127]ZIB1 M ZnSO4187 (0.1 A g-1)80%,2 A g-1 (2,000 cycles)
VS4[130]ZIB2 M Zn(CF3SO3)2265 (0.25 A g-1)93%,5 A g-1 (1,200 cycles)
NH4V4O10[187]ZIB3 M Zn(CF3SO3)2298 (0.1 A g-1)89%,2 A g-1 (2,000 cycles)
LiV3O8[188]ZIB3 M Zn(CF3SO3)2298 (1 A g-1)85%,5 A g-1 (4,000 cycles)
Na0.33V2O5[189]ZIB3 M Zn(CF3SO3)2367 (0.1 A g-1)93%,1 A g-1 (1,000 cycles)
Mg0.2V2O5[67]ZIB3 M Zn(CF3SO3)2346 (0.1 A g-1)83.7%,5 A g-1 (10,000 cycles)
ZnV3O8[190]ZIB3 M Zn(CF3SO3)2294 (0.1 A g-1)74.6%,2 A g-1 (1,200 cycles)
Fe2V4O13[53]ZIB2 M Zn(CF3SO3)2380 (0.2 A g-1)83%,10 A g-1 (1,000 cycles)
Na3V2(PO4)2F3[191]ZIB2 M Zn(CF3SO3)260 (0.2 A g-1)95%,1 A g-1 (4,000 cycles)

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., AxV2O5·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 Mg0.25V2O5·H2O, Ca0.26V2O5·H2O, and Sr0.42V2O5·0.7H2O 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 Ca0.28V2O5·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. K0.5V2O5 transformed into Ca0.45V2O5, 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 V2CTx 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.

Advanced V-based materials for multivalent-ion storage applications

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].

Aqueous batteries

Compared with organic electrolytes, aqueous electrolytes, benefiting from good conductivity, low cost, high safety, etc., have attracted intensive attention in energy storage[108]. The multivalent metal-ion storage of V-based compounds in aqueous electrolytes is discussed in the following.

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 MgVOx, 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].

Advanced V-based materials for multivalent-ion storage applications

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 AlxVyO4 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.

Advanced V-based materials for multivalent-ion storage applications

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/VOx 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 Mn0.18V2O5·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/VOx [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 Zn3+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 VOx 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 Ti3C2Tx 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].

Advanced V-based materials for multivalent-ion storage applications

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 V2O5-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 Zn2+[138]. Oxygen-deficient carbon-coated V2O3 delivered 662 mAh g-1 at 0.2 A g-1 [Figure 8C] after turning into Zn0.4V2O5-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].

Advanced V-based materials for multivalent-ion storage applications

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 Co0.247V2O5·0.944H2O 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 Cu0.34V2O5 cathode delivered 258 mAh g-1 at 100 mA g-1[152].

Advanced V-based materials for multivalent-ion storage applications

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 Co0.247V2O5·0.944H2O at 4 A g-1. Reproduced with permission[151]. Copyright 2019, Wiley-VCH GmbH. (D) Charge-discharge curves of H11Al2V6O23.2 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 H11Al2V6O23.2 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. NaCa0.6V6O16·3H2O 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 PO43- 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.

Advanced V-based materials for multivalent-ion storage applications

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,Znx (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, Zn3+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 Cu0.34V2O5 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 Cu0.18V2O5·0.72H2O[169], Mn-modified V6O13[170], Zn0.36V2O5·nH2O[171], and etc., implied by appearance and disappearance of Zn4SO4(OH)6·4H2O, Zn2V3O7(OH)2·2H2O, or Znx(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 Znx(OTf)y(OH)2x-y·nH2O were observed.

Advanced V-based materials for multivalent-ion storage applications

Figure 12. (A) Zn-storage mechanism in Cu0.34V2O5. Reproduced with permission[152]. Copyright 2021, American Chemical Society. (B) Zn-storage mechanism in Mg0.34V2O5·0.84H2O. 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 Mg0.34V2O5·0.84H2O 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 Zn0.25V2O5·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 Zn0.25V2O5·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 ZnxMgV2O4 [Figure 12D] served as a stable host for reversible Zn-storage, subsequently[174].

Supercapacitors

Compared with batteries, supercapacitors have lower sensitivity to temperature, better tolerance to charge/discharge cycles, superior power performance, and good cycling stability[175]. V-based materials are also considered as promising high-energy electrodes for electrochemical capacitors due to their excellent specific capacitance, long cycling stability, and good electrochemical reversibility[176], but poor electrical conductivity has hindered their further use in supercapacitors. A VOSO4 additive was reported to dramatically improve the cycling stability of V2O5-based supercapacitors, leading to 91.23% retention at 10 A g-1 after 10,000 cycles[177].

CONCLUSIONS AND FUTURE PROSPECTS

This review combed recent advances of multivalent-ion storage applications for a variety of advanced V-based materials, including vanadium oxides, vanadates, vanadium sulfides, and V-based MXenes and phosphates. The features for typical structures were analyzed with representative materials. The relevant electrochemical properties and energy storage mechanisms for different advanced V-based electrodes were systemically discussed. The discussion covered devices of not only non-aqueous batteries and aqueous batteries but also supercapacitors. For different devices, challenges from poor conductivity, slow ion diffusion, dissolution and structural collapse, low operating voltage, etc. were discussed with the corresponding representative electrodes.

Based on the review, we disclosed that issues for V-based materials could be alleviated, to some extent, by common material engineering strategies such as nanosizing, doping, encapsulating, constructing vacancies and heterostructures, etc. Further, electrolyte design, e.g., highly concentrated electrolytes, organic/aqueous hybrid electrolytes, hybrid ions electrolytes, etc., are also beneficial to improve main factors of structure stability, ion storage capability and diffusion kinetics due to optimized surface/interface, weakened coulomb interactions, and enhanced storage pathways. Overall, to obtain better multivalent-ion storage applications for V-based materials, cooperation from material engineering and electrolyte design is possibly a promising avenue. Meanwhile, various advanced in-situ characterization techniques are also needed to clarify the relevant complex interactions between materials and electrolytes.

DECLARATIONS

Authors’ contributions

Proposed the topic of this review: Song H, Wang C

Prepared the manuscript: Guo W

Writing - review & editing: Guo W, Fu D, Song H, Wang C

Availability of data and materials

Not applicable.

Financial support and sponsorship

The work is financially supported by the National Natural Science Foundation of China (grant nos. 91963210, 52322107) and the Natural Science Foundation of Guangdong Province (grant nos. 2020B0101690001, 2022A1515010723).

Conflict of interest

All authors declared that there are no conflicts of interest.

Ethical approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Copyright

© The Author(s) 2024.

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Guo W, Fu D, Song H, Wang C. Advanced V-based materials for multivalent-ion storage applications. Energy Mater 2024;4:400026. http://dx.doi.org/10.20517/energymater.2023.82

AMA Style

Guo W, Fu D, Song H, Wang C. Advanced V-based materials for multivalent-ion storage applications. Energy Materials. 2024; 4(2): 400026. http://dx.doi.org/10.20517/energymater.2023.82

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Guo, Weihua, Danchen Fu, Huawei Song, Chengxin Wang. 2024. "Advanced V-based materials for multivalent-ion storage applications" Energy Materials. 4, no.2: 400026. http://dx.doi.org/10.20517/energymater.2023.82

ACS Style

Guo, W.; Fu D.; Song H.; Wang C. Advanced V-based materials for multivalent-ion storage applications. Energy Mater. 2024, 4, 400026. http://dx.doi.org/10.20517/energymater.2023.82

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