Boosting Zn-Ion Storage Performance of Bronze-Type VO2 via Ni-Mediated Electronic Structure Engineering

Trigger a multi-electron reaction by tailoring electronic structure of VO2 toward more efficient aqueous zinc metal batteries

VO2 (B) is recognized as a promising cathode material for aqueous zinc metal batteries (AZMBs) owing to its remarkable specific capacity and its unique, expansive tunnel structure, which facilitates the reversible insertion and extraction of Zn2+. Nonetheless, challenges such as the inherent instability of the VO2 structure, poor ion/electron transport and a limited capacity due to the low redox potential of the V3+/V4+ couple have hindered its wider application. In this study, we present a strategy to replace vanadium ions by doping Al3+ in VO2. This approach activates the multi-electron reaction (V4+/V5+), to increase the specific capacity and improve the structural stability by forming robust V5+O and Al3+O bonds. It also induces a local electric field by altering the local electron arrangement, which significantly accelerates the ion/electron transport process. As a result, Al-doped VO2 exhibits superior specific capacity, improved cycling stability, and accelerated electronic transport kinetics compared to undoped VO2. The beneficial effects of heterogeneous atomic doping observed here may provide valuable insights into the improvement electrode materials in metal-ion battery systems other than those based on Zn.

Reversible Deposition/Dissolution of Double Hydroxides to Modulate Electrolyte pH Enabling High-Performance Aqueous Zinc-Ion Batteries

Vanadium oxide has been extensively studied as a host of zinc ion intercalation but still suffers from low conductivity, dissolution, and byproduct accumulation during cycling. Here, we hydrothermally synthesize the VO2@MXene Ti3C2 (MV) composite and find that in the MV//3 M Zn(CF3SO3)2//Zn system, the double hydroxide Zn12(CF3SO3)9(OH)15·nH2O (ZCOH) uniformly covers VO2 during the charging process and dissolves reversibly during the discharge process. In situ X-ray diffraction of the MV combined with in situ pH measurements reveals that ZCOH acts as a pH buffer during cycling, which is beneficial to the cycling stability of batteries. And the theoretical calculation indicates that the decomposition energy required by ZCOH on the MV surface is lower than that on pure VO2, which is more conducive to ZCOH dissolution. The coin battery exhibits high-rate performance of 65.1% capacity retention at a current density of 15 A g-1 (compared to 0.6 A g-1) and a long cycling life of 20,000 cycles with a capacity retention of 80.7%. For a 22.4 mA h soft-packaged battery, its capacity remains at 72.1% after 2000 cycles. This work demonstrates the active role of ZCOH in the electrochemical process of VO2 and provides a new perspective for exploiting this mechanism to develop high-performance aqueous zinc-ion battery vanadium oxide cathode materials.

Understanding the Super-Theoretical Capacity Behavior of VO2 in Aqueous Zn Batteries

VO2 material, as a promising intercalation host, is widely investigated not only in aqueous lithium-ion batteries, but also in aqueous zinc-ion batteries (AZIBs) owing to its stable tunnel-like framework and multivalence of vanadium. Different from lithium-ion storage, VO2 can provide higher capacity when storing zinc ions, even exceeding its theoretical capacity (323 mAh g-1), but the specific reason for this unconventional performance in AZIBs is still unclear. The present study proposes a catalytic oxygen evolution reaction (OER) coupled with an interface oxidation mechanism of VO2 during the initial charging to a high voltage. This coupling induces a phase transformation of VO2 into a high oxidation state of V5O12∙6H2O, enabling a nearly two-electron reaction and providing additional zinc storage sites to achieve super-theoretical capacity. Furthermore, it is demonstrated that these vanadium oxide cathodes (V2O3, VO2, and V2O5) will all undergo phase change after the first charge or short cycle. Notably, water molecules participate in the final formation of layered vanadium-based hydrate, highlighting their crucial role as "pillars" for stabilizing the structure. This work significantly enhances the understanding of vanadium-based oxide cathodes.

Additives for Aqueous Zinc-Ion Batteries: Recent Progress, Mechanism Analysis, and Future Perspectives

Aqueous zinc-ion batteries (ZIBs) stand out as a promising next-generation electrochemical energy storage technology, offering notable advantages such as high specific capacity, enhanced safety, and cost-effectiveness. However, the application of aqueous electrolytes introduces challenges: Zn dendrite formation and parasitic reactions at the anode, as well as dissolution, electrostatic interaction, and by-product formation at the cathode. In addressing these electrode-centric problems, additive engineering has emerged as an effective strategy. This review delves into the latest advancements in electrolyte additives for ZIBs, emphasizing their role in resolving the existing issues. Key focus areas include improving morphology and reducing side reactions during battery cycling using synergistic effects of modulating anode interface regulation, zinc facet control, and restructuring of hydrogen bonds and solvation sheaths. Special attention is given to the efficacy of amino acids and zwitterions due to their multifunction to improve the cycling performance of batteries concerning cycle stability and lifespan. Additionally, the recent additive advancements are studied for low-temperature and extreme weather applications meticulously. This review concludes with a holistic look at the future of additive engineering, underscoring its critical role in advancing ZIB performance amidst the complexities and challenges of electrolyte additives.

V2O3@C Microspheres as the High-Performance Cathode Materials for Advanced Aqueous Zinc-Ion Storage

Vanadium oxides attract increasing research interests for constructing the cathode of aqueous zinc-ion batteries (ZIBs) because of high theoretical capacity, but the low intrinsic conductivity and unstable phase changes during the charge/discharge process pose great challenges for their adoption. In this work, V₂O₃@C microspheres were developed to achieve enhanced conductivity and improved stability of phase changes. Compounding vanadium oxides and conductive carbon through the in-situ carbonization led to significant improvement of the cathode materials. ZIBs prepared with V₂O₃@C cathodes produce a specific capacity of 420 mA h g^-1 at 0.2 A g^-1. A reversible capacity of 132 mA h g^-1 was achieved at 21.0 A g^-1. After 2000 cycles, the electrode could still deliver a capacity of 202 mA h g^-1 at the current of 5.0 A g^-1. Besides, the energy density of batteries constructed with the thus-prepared electrodes was about 294 W h kg^-1 at 148 W kg^-1 power. The in-situ compounding of V₂O₃ and carbon resulted in a microstructure that facilitated the stable phase transformation of Zn_xV₂O_5-a·nH₂O (ZnVOH), which provided more Zn²⁺ storage activity than the original phase before electrochemical activation. Moreover, the in-situ compositing strategy presents a simple route to the development of ZIB cathodes with promising performance.

Polysaccharide hydrogel electrolytes with robust interfacial contact to electrodes for quasi-solid state flexible aqueous zinc ion batteries with efficient suppressing of dendrite growth

Flexible aqueous zinc-ion batteries (AZIBs) require high conductive and adhesive hydrogel electrolytes. However, high adhesion tends to hinder ion conduction rate. Herein, we designed a water/glycerol binary solvent coordinating the hydrophilic polymers to reconstruct the water molecules' environment in the hydrogel. As a consequence, the interface adhesion strength between Zn and the hydrogel reached 3.0 kPa and the ionic conductivity was up to 16.8 mS cm^-1. In addition, inspired by the slurry electrode preparation method, we developed a simple blade coating technique using a non-Newtonian polysaccharide liquid solution to construct an ultra-thin hydrogel electrolyte in situ on the cathode. The thickness of the obtained hydrogel reached 70 μm, and the ultrathin flexible AZIBs were easily constructed by pasting a Zn anode directly on the adhesive hydrogel, showing the potential of flexible AZIBs scalable assembly. In addition, the Zn//Zn symmetrical cells with the hydrogel electrolyte provided stable cycling performance for over 400 h at 0.1 mA cm^-2 with suppressed dendrite growth. The assembled Zn//Polyaniline battery and Zn//V₂O₅ battery also exhibited excellent capacity retention after cycles. This work has realized the hydrogel electrolyte with high adhesion and conductivity, which has good adaptability to metal electrodes and opened up a new practical way for large-scale assembly of flexible energy storage devices.

Tungsten-oxygen bond pre-introduced VO2(B) nanoribbons enable fast and stable zinc ion storage ability

The tunnel structure of the bronze phase vanadium dioxide (VO2(B)) can be used as the zinc ion storage active sites. However, the intense charge repulsion of divalent Zn2+ causes a sluggish reaction kinetics in the tunnel VO2(B). Here, a tungsten-oxygen bond pre-introduced (TOBI) approach is proposed to modulate the tunnel structure of VO2(B). The VO2(B) cathodes with TOBI of 0.5 at% to 3.0 at% have been controllably synthesized by a simple hydrothermal method. The results from structural analysis uncover that the pre-introduced W6+ replaces the V4+ in VO2(B) to form WO6 octahedra. Benefiting from the rapid diffusion kinetics, enhanced structural stability and improved conductivity enabled by the TOBI, the optimal VO2(B) nanoribbons with 1.5 at% shows a high reversible capacity of 265 mAh g-1, a high rate-performance of up-to 10 A g-1 and a long cycling stability of 2000 cycles. Moreover, a pseudo-capacitive dominated Zn2+ intercalation/de-intercalation behavior is solidly determined by the electrochemical kinetics testing and structural characterizations. This TOBI method is referential for developing other multivalent ion battery cathodes with outstanding performances.

VO2·0.26H2O nanobelts@reduced graphene oxides as cathode materials for high-performance aqueous zinc ion batteries

A simple microwave-assisted hydrothermal approach is adopted to synthesize VO₂·0.26H₂O nanobelts@reduced graphene oxide (VO₂·0.26H₂O@rGO), which is regarded as a promising cathode material for ZIBs. Compared to VO₂·0.26H₂O, VO₂·0.26H₂O@rGO has a large specific surface area and low charge transfer resistance, improving its electrochemical properties. The large lattice spacing, high capacitive Zn²⁺ storage behavior, fast Zn²⁺ transfer rate and stable structure are also responsible for the performance.

Element doping: a marvelous strategy for pioneering the smart applications of VO2

Smart materials are leading the future of materials by virtue of their autonomous response behavior to external stimuli; it is widely believed their development and application will bring a new revolution. Among them, vanadium dioxide (VO₂) is a special one showing a unique multi-stimulus responsive metal-insulator transition (MIT) accompanied by a structural phase transition (SPT) with striking changes of physical properties including optical, electrical and thermal properties, etc., making it ideal for smart windows, micro-bolometers, actuators, etc. Since the attractive performances of VO₂ are rooted in MIT behavior (coupled with SPT), element doping becomes a powerful tool in tailoring VO₂ performance. Oriented on the practical requirements, element-doped VO₂ is more promising and competitive in terms of performance, prospect, and cost. Here we focus specifically on element-doped VO₂, the recent progress and potential challenges of which are discussed. We devote attention to the crucial roles of element doping in modulating the properties and driving the practicality of VO₂, aiming to inspire current research to pioneer new applications of VO₂.

Recent Developments and Challenges of Vanadium Oxides (Vx Oy ) Cathodes for Aqueous Zinc-Ion Batteries

The rapid depletion of lithium resources and the increasing demand for electrical energy storage have stimulated the pursuit of emerging electrochemical energy storage. Aqueous zinc ion batteries (ZIBs) are highly sought after for their low cost, high safety, and increased environmental compatibility. However, the search for suitable cathode materials is still tricky for a wide range of researchers. Vanadium oxides (V_x O_y ), with their abundant vanadium valence, easily deformable V-O polyhedrons, and tunable chemical compositions, are of significant advantage in developing emerging materials. This work provides a detailed review of different V_x O_y for the application in aqueous ZIBs. The current problems and optimization strategies of V_x O_y cathode materials are systematically discussed. Finally, the current challenges and possible directions for future research of V_x O_y cathode materials in aqueous ZIBs are presented.

Oxygen Defect Hydrated Vanadium Dioxide/Graphene as a Superior Cathode for Aqueous Zn Batteries

Zinc ion batteries have become a new type of energy storage device because of the low cost and high safety. Among the various cathode materials, vanadium-oxygen compounds stand out due to their high theoretical capacity and variable chemistry valence state. Here, we construct a 3D spongy hydrated vanadium dioxide composite (O_d-HVO/rG) with abundant oxygen vacancy defects and graphene modifications. Thanks to the stable structure and abundant active sites, O_d-HVO/rG exhibits superior electrochemical properties. In aqueous electrolyte, the O_d-HVO/rG cathode provides high initial charging capacity (428.6 mAh/g at 0.1 A/g), impressive rate performance (186 mAh/g even at 20 A/g), and cycling stability, which can still maintain 197.5 mAh/g after 2000 cycles at 10 A/g. Also, the superior specific energy of 245.3 Wh/kg and specific power of 14142.7 W/kg are achieved. In addition, MXene/O_d-HVO/rG cathode materials are prepared and PAM/ZnSO₄ hydrogel electrolytes are applied to assemble flexible soft pack quasi-solid-state zinc ion batteries, which also exhibit excellent flexibility and cycling stability (206.6 mAh/g after 2000 cycles). This work lays the foundation for advances in rechargeable aqueous zinc ion batteries, while revealing the potential for practical applications of flexible energy storage devices.

Structure evolution and energy storage mechanism of Zn3V3O8 spinel in aqueous zinc batteries

Spinel-type materials are promising for the cathodes in rechargeable aqueous zinc batteries. Herein, Zn₃V₃O₈ is synthesized via a simple solid-state reaction method. By tuning the Zn(CF₃SO₃)₂ concentration in electrolytes and the cell voltage ranges, improved electrochemical performance of Zn₃V₃O₈ can be achieved. The optimized test conditions give rise to progressive structure evolution from bulk to nano-crystalline spinel, which leads to capacity activation in the first few cycles and stable cycling performance afterward. Furthermore, the energy storage mechanism in this nano-crystalline spinel is interpreted as the co-intercalation of zinc ions and protons with some water. This work provides a new viewpoint of the structure evolution and correlated energy storage mechanism in spinel-type host materials, which would benefit the design and development of next-generation batteries.

Direct Detection and Visualization of the H+ Reaction Process in a VO2 Cathode for Aqueous Zinc-Ion Batteries

Because they are safer and less costly than state-of-the-art Li-ion batteries, aqueous zinc-ion batteries (AZIBs) have been attracting more attention in stationary energy storage and industrial energy storage. However, the electrochemical reaction of H⁺ in all of the cathode materials of AZIBs has been puzzling until now. Herein, highly oriented VO₂ monocrystals grown on a Ti current collector (VO₂-Ti) were rationally designed as the research model, and such a well-aligned VO₂ cathode also displayed excellent zinc-ion storage capability (e.g., a reversible capacity of 148.4 mAh/g at a current density of 2 A/g). To visualize the H⁺ reaction process, we used time-of-flight secondary-ion mass spectrometry. With the benefit of such a binder-free and conductor-free electrode design, a clear and intuitive reaction of H⁺ in a VO₂ cathode is obtained, which is quite significant for unraveling the accurate reaction mechanism of VO₂ in AZIBs.

Chemically assembling chromium vanadate into an urchin-like porous rich matrix with ultrathin nanosheets for rapid Zn2+ storage

Aqueous zinc ion battery (AZIB) is a promising battery system developed in recent years, which has the advantages of safety, environmental protection and low price. However, it is still a puzzle to develop and improve cathode materials with satisfactory performance. In this paper, the chromium vanadate (CrVO₃) electrode material was reported for the first time. The obtained CrVO₃ have mesoporous structure (the mesopore sizes: 2-50 nm), excellent conductivity, high surface area (129.3 m² g^-1) and uniform thickness of 2 nm, which provides a short path for rapid transfer of zinc ions, a large surface area for high pseudocapacitance, and sufficient voids to mitigate volume expansion. Given these structural advantages, the CrVO₃ cathode delivers high capacities of 188.8 and 112.8 mAh g^-1 at 0.5 and 4 A g^-1 and excellent long cycle stability, respectively. More importantly, the Zn//CrVO₃ battery provided an energy density of 231.9 Wh kg^-1 at a power density of 100.4 W kg^-1. Meanwhile, insight into the formation mechanism and Zn²⁺ storage mechanism by ex situ methods. The results show that the porous CrVO₃ is a promising cathode material for AZIBs, which provide a valuable idea for the design of porous vanadate with significantly enhanced performances in electrochemical energy storage.

VO2(B)@carbon fiber sheet as a binder-free flexible cathode for aqueous Zn-ion batteries

A flexible VO2(B)@CFS electrode exhibits a high capacity and a long cycle life for zinc-ion batteries.