Ultrafast Zn2+ Intercalation and Deintercalation in Vanadium Dioxide

One-step hydrothermal synthesis of vanadium dioxide/carbon core-shell composite with improved ammonium ion storage for aqueous ammonium-ion battery

Aqueous nonmetallic ion batteries have garnered significant interest due to their cost-effectiveness, environmental sustainability, and inherent safety features. Specifically, ammonium ion (NH4+) as a charge carrier has garnered more and more attention recently. However, one of the persistent challenges is enhancing the electrochemical properties of vanadium dioxide (VO2) with a tunnel structure, which serves as a highly efficient NH4+ (de)intercalation host material. Herein, a novel architecture, wherein carbon-coated VO2 nanobelts (VO2@C) with a core-shell structure are engineered to augment NH4+ storage capabilities of VO2. In detail, VO2@C is synthesized via the glucose reduction of vanadium pentoxide under hydrothermal conditions. Experimental results manifest that the introduction of the carbon layer on VO2 nanobelts can enhance mass transfer, ion transport and electrochemical kinetics, thereby culminating in the improved NH4+ storage efficiency. VO2@C core-shell composite exhibits a remarkable specific capacity of ∼300 mAh/g at 0.1 A/g, which is superior to that of VO2 (∼238 mAh/g) and various other electrode materials used for NH4+ storage. The NH4+ storage mechanism can be elucidated by the reversible NH4+ (de)intercalation within the tunnel of VO2, facilitated by the dynamic formation and dissociation of hydrogen bonds. Furthermore, when integrated into a full battery with polyaniline (PANI) cathode, the VO2@C//PANI full battery demonstrates robust electrochemical performances, including a specific capacity of ∼185 mAh·g-1 at 0.2 A·g-1, remarkable durability of 93 % retention after 1500 cycles, as well as high energy density of 58 Wh·kg-1 at 5354 W·kg-1. This work provides a pioneering approach to design and explore composite materials for efficient NH4+ storage, offering significant implications for future battery technology enhancements.

Seed-Assisted Reversible Dissolution/Deposition of MnO2 for Long-Cyclic and Green Aqueous Zinc-Ion Batteries

Manganese oxide (MnO2) based aqueous zinc-ion batteries (AZIBs) are considered to be a promising battery for grid-scale energy storage. However, they usually suffer from the great challenge of capacity attenuation due to Mn dissolution and irreversible structural transformation. Herein, full use of the shortcomings is made to design high-performance cathode-free AZIBs. Manganese-based Prussian blue analog (Mn-PBA) is selected as a seed layer to provide a stable MnO2 electrodeposition surface. Thanks to the large specific surface area and manganophilic nature of Mn-PBA, the deposition/dissolution kinetics between Mn2+ and MnO2 are significantly enhanced. Systematic studies revealed the mechanism of MnO2 deposition-dissolution related to the reversible transformation of manganese oxide hydroxide and zinc hydroxide sulfate hydrate. Based on this, the developed cathode-free AZIBs exhibit outstanding rate performance (with a specific capacity of 273.7 mAh g-1 at 1 A g-1) and extraordinary cycle stability (maintaining a specific capacity of 52.3 mAh g-1 after 50 000 cycles at 20 A g-1). Furthermore, the AZIBs with non-toxic, biocompatible materials can be directly discarded after use, without causing pollution to the environment, which is expected to help achieve the sustainable development goals.

Bilayered Vanadium Oxides Pillared by Strontium Ions and Water Molecules as Stable Cathodes for Rechargeable Zn-Metal Batteries

Vanadium-based compounds have attracted significant attention as cathodes for aqueous zinc metal batteries (AZMBs) because of their remarkable advantages in specific capacities. However, their low diffusion coefficient for zinc ions and structural collapse problems lead to poor rate capability and cycle stability. In this work, bilayered Sr0.25V2O5·0.8H2O (SVOH) nanowires are first reported as a highly stable cathode material for rechargeable AZMBs. The synergistic pillaring effect of strontium ions and water molecules improves the structural stability and ion transport dynamics of vanadium-based compounds. Consequently, the SVOH cathode exhibits a high capacity of 325.6 mAh g-1 at 50 mA g-1, with a capacity retention rate of 72.6% relative to the maximum specific capacity at 3.0 A g-1 after 3000 cycles. Significantly, a unique single-nanowire device is utilized to demonstrate the excellent conductivity of the SVOH cathode directly. Additionally, the energy storage mechanism of zinc insertion and extraction is investigated using a variety of advanced in situ and ex situ analysis techniques. This method of ion intercalation to improve electrochemical performance will further promote the development of AZMBs in large-scale applications.

Hydrogen Peroxide Tuned Morphology and Crystal Structure of Barium Vanadate-Based Nanostructures for Aqueous Zinc-Ion Storage Properties

Improving the layered-structure stability and suppressing vanadium (V) dissolution during repeated Zn2+ insertion/extraction processes are key to promoting the electrochemical stability of V-based cathodes for aqueous zinc (Zn)-ion batteries (AZIBs). In this study, barium vanadate (Ba2V2O7, BVO) nanostructures (NSs) are synthesized using a facile hydrothermal method. The formation process of the BVO NSs is controlled by adjusting the concentration of hydrogen peroxide (H2O2), and these NSs are employed as potential cathode materials for AZIBs. As the H2O2 content increases, the corresponding electrochemical properties demonstrate a discernible parabolic trend, with an initial increase, followed by a subsequent decrease. Benefiting from the effect of H2O2 concentration, the optimized BVO electrode with 20 mL H2O2 delivers a specific capacity of 180.15 mA h g-1 at 1 A g-1 with good rate capability and a long-term cyclability of 158.34 mA h g-1 at 3 A g-1 over 2000 cycles. Thus, this study provides a method for designing cathode materials with robust structures to boost the electrochemical performance of AZIBs.

Rational Regulation of Optimal Oxygen Vacancy Concentrations on VO2 for Superior Aqueous Zinc-Ion Battery Cathodes

VO2 with its special tunnel structure and high theoretical capacity is an ideal candidate for cathode materials for aqueous zinc-ion batteries (ZIBs). However, the slow kinetics and structural instability due to the strong electrostatic interactions between the host structure of VO2 and Zn2+ hinder its application. Defect engineering is a well-recognized strategy for improving the intrinsic ion-electron dynamics and structural stability of this material. However, the preparation of oxygen vacancies poses significant difficulties, and it is challenging to control their concentration effectively. Excessive or insufficient vacancy concentration can have a negative effect on the cathode material. Herein, we propose electrode materials with controlled oxygen vacancies prepared in situ on carbon nanofibers (CNF) by a simple, one-step hydrothermal process (Ov-VO2@CNF). This method can balance the adsorption energy and migration energy barrier easily, and we maximized the adsorption energy of Zn2+ while minimizing the adsorption energy barrier. Notably, the Ov2-VO2@CNF electrode delivered a high specific capacity (over 450 mAh g-1 at 0.1 A g-1) and excellent cycle stability (318 mAh g-1 at 5 A g-1 capacity after 2000 cycles with a capacity retention of 85%). This rational design of precisely regulated defect engineering provides a way to obtain advanced electrode materials with excellent comprehensive properties.

Mo modulating the structure of monoclinic vanadium dioxide boosting the aqueous ammonium-ion storage for high-performance supercapacitor

Supercapacitors (SCs) using ammonium-ion (NH4+) as the charge carrier (NH4+-SCs) have attracted continuous attention and vanadium-based materials are proved to have high-efficient NH4+-storage properties. Monoclinic vanadium dioxide, VO2(B), as an anode material applied to SCs has been rarely reported and modulating its electronic structure for boosted NH4+-storage is full of challenge. In this work, molybdenum-doped VO2(B) (Mo-doped VO2(B)) is designed and synthesize to enhance its NH4+-storage. The introduction of Mo atom into the crystal structure of VO2(B) can modulate its crystal structure and bring in some defects. Experimental results manifest that Mo-doped VO2(B) with 2 % Mo-doping shows the best electrochemical properties. Mo-doped VO2(B) achieves the specific capacitance of 1403 F g-1 (390 mAh g-1) at 0.1 A g-1 and the capacitance retention of about 98 % after 5000 cycle, superior to that of VO2(B) (893 F g-1, 248 mAh g-1 at 0.1 A g-1 and 60 % capacitance retention. The hybrid supercapacitor (HSC) assembled by Mo-doped VO2(B) and active carbon delivers good electrochemical performance with the energy density of 38.6 Wh kg-1 at power density of 208.3 W kg-1. This work proves that the Mo-doping is an efficient strategy for boosted NH4+-storage of VO2(B) and this strategy is like a Chinese idiom "like adding wings to a tiger" to guide the design of electrode materials for high-efficient NH4+-storage.

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.

On the Dendrite-Suppressing Effect of Laser-Processed Polylactic Acid-Derived Carbon Coated Zinc Anode in Aqueous Zinc Ion Batteries

A major bottleneck limiting the commercialization of aqueous zinc ion batteries (AZIBs) is dendrite formation on the zinc (Zn) anode during the plating/stripping process, which leads to rapid deterioration in performance and, consequently to the device failure. In this regard, researchers are trying to design more stable anodes toward suppressing dendrite formation. One possible solution to tackle this problem and to extend the cycling life of AZIBs is to modify the zinc anode surface by coating carbonaceous materials, enabling more controlled charge flux and uniform ion distribution. This work reports sustainable and bio-derived polylactic acid (PLA) as a coating layer on the zinc anode. Carbonizing this polymer under ambient conditions using a high-power nanosecond laser forms a carbon-coated zinc foil, which was directly utilized as the anode in aqueous zinc ion batteries. The fabricated laser-processed PLA-derived carbon-coated zinc anode demonstrated an extended cycling life of almost 1600 hours, significantly outperforming the bare zinc anode. A full aqueous zinc ion battery assembled from as-modified anode and as-prepared V2O5 nanofibers as cathode was able to deliver a specific capacity of 238 mAh g-1 at 1.0 A g-1 with a capacity retention of 70 % after 1000 cycles.

Enhancing ammonium-ion storage in Mo-doped VO2 (B) nanobelt-bundles anode for aqueous ammonium-ion batteries

The recent surge in interest in aqueous ammonium ion rechargeable batteries (AAIBs) has been fueled by their eco-friendliness, efficiency, safety, and sustainability. However, finding the optimal anode material for effective ammonium ion (NH4+) storage remains a nascent and significant challenge. The research presented here focuses on the enhancement of aqueous ammonium rechargeable batteries by incorporating Mo atoms into VO2 (B) (denote as MVO), a material that has shown promise as an anode for NH4+ storage. The introduction of Mo ions was found to optimize the electronic structure and morphology of pristine VO2 (B) (label as PVO), resulting in the transformation of its nanobelts into thin nanobelt-bundles. This alteration exposes more active sites and increases oxygen vacancies, which in turn improve the conductivity and diffusion rate of NH4+ ions, thereby enhancing the overall electrochemical performance of the material. The MVO material demonstrates a high initial capacity of 283.5 mA h g-1 at 0.3 A g-1, and maintained 86.7% of its capacity after 4500 cycles, indicating excellent long-term stability. To further validate the practical application, a full cell was garnered utilizing MVO as the anode and Cu3[Fe(CN)6]2 (CuHCF) as the cathode. The resulting AAIB displays remarkable cycling stability, with 81.5% capacity preservation after 1000 cycles and large energy density of 57.9 W h kg-1. The study reveals that the doping of Mo ions can significantly improve both the stability and NH4+ storage capacity of PVO, offering a promising new direction for the exploitation of efficacious and sustainable NH4+ host materials for rechargeable batteries.

Self-Charging Aqueous Zn//COF Battery with UltraHigh Self-Charging Efficiency and Rate

Self-charging zinc batteries that combine energy harvesting technology with batteries are candidates for reliable self-charging power systems. However, the lack of rational materials design results in unsatisfactory self-charging performance. Here, a covalent organic framework containing pyrene-4,5,9,10-tetraone groups (COF-PTO) is reported as a cathode material for aqueous self-charging zinc batteries. The ordered channel structure of the COF-PTO provides excellent capacity retention of 98% after 18 000 cycles at 10 A g-1 and ultra-fast ion transfer. To visually assess the self-charging performance, two parameters, namely self-charging efficiency (self-charging discharge capacity/galvanostatic discharge capacity, η) and average self-charging rate (total discharge capacity after cyclic self-charging/total cyclic self-charging time, ν), are proposed for performance evaluation. COF-PTO achieves an impressive η of 96.9% and an ν of 30 mAh g-1 self-charge capacity per hour in 100 self-charging cycles, surpassing the previous reports. Mechanism studies reveal the co-insertion of Zn2+ and H+ double ions in COF-PTO of self-charging zinc batteries. In addition, the C═N and C═O (on the benzene) in COF-PTO are ortho structures to each other, which can easily form metal heterocycles with Zn ions, thereby driving the forward progress of the self-charging reaction and enhancing the self-charging performance.

Heterojunction tunnelled vanadium-based cathode materials for high-performance aqueous zinc ion batteries

Rechargeable aqueous zinc ion batteries (ZIBs) have emerged as a promising alternative to lithium-ion batteries due to their inherent safety, abundant availability, environmental friendliness and cost-effectiveness. However, the cathodes in ZIBs encounter challenges such as structural instability, low capacity, and sluggish kinetics. In this study, we constructed BiVO4@VO2 (BVO@VO) heterojunction cathode material with bismuth vanadate and vanadium dioxide phases for ZIBs, which demonstrate significant advancements in both aqueous and quasi-solid-state ZIBs. Benefitting from the heterojunction structure, the materials present a high capacity of 262 mAh g-1 at 0.1 A g-1, superb cyclic stability with 96% capacity retention after 1000 cycles at 2 A g-1, and outstanding rate property with a specific capacity of 218 mAh g-1 even at a high rate of 5.0 A g-1. Furthermore, the flexible quasi-solid-state ZIBs incorporating the BVO@VO cathode demonstrate prolonged cyclic life performance with a remarkable specific capacity of 234 mAh g-1 over 100 cycles at a current density of 0.1 A g-1. This study potentially paves the way for the utilization of heterointerface-enhanced zinc ion diffusion for vanadium-based materials in ZIBs, thereby providing a new approach for the design and investigation of high-performance zinc-ion systems.

Boosting the reversibility of Zn anodes via synergistic cation-anion interface adsorption with addition of multifunctional potassium polyacrylate

Rechargeable aqueous Zn batteries have the edge in resource reserve, cost, energy and conversion efficiency due to the inherent features of metal Zn anodes. However, the application of Zn-based batteries is being seriously hindered by Zn dendrites and water-induced side-reactions. Here, potassium polyacrylate (K-PAM) is proposed as the electrolyte additive to form a synergistic cation-anion interface on Zn surface. The carboxyl anions and K+ cations are preferentially adsorbed on the Zn surface due to the intrinsic surfactant characteristics, which could homogenize Zn plating and suppress parasitic reactions. The synergistic regulation of K-PAM additive endows the ZnZn symmetric cells with excellent cyclic durability of 1250 h at 1 mA cm-2, which is significantly better than the polyacrylic acid additive only with carboxyl anions. Moreover, trace K-PAM addition into traditional ZnSO4 electrolyte endows the ZnCu batteries with a considerable average Coulombic efficiency of 99.2 %. Additionally, higher capacity retention and excellent cycling stability of ZnVO2 cells further mark K-PAM as a potentially impressive aqueous electrolyte additive for high-performance Zn-based batteries. This work will provide a promising method for the synergistic regulation with cations and anions of electrolyte additives to improve the stability and reversibility of Zn anodes.

Preparation of Expanded Graphite-VO2 Composite Cathode Material and Performance in Aqueous Zinc-Ion Batteries

Due to safety problems caused by the use of organic electrolytes in lithium-ion batteries and the high production cost brought by the limited lithium resources, water-based zinc-ion batteries have become a new research focus in the field of energy storage due to their low production cost, safety, efficiency, and environmental friendliness. This paper focused on vanadium dioxide and expanded graphite (EG) composite cathode materials. Given the cycling problem caused by the structural fragility of vanadium dioxide in zinc-ion batteries, the feasibility of preparing a new composite material is explored. The EG/VO2 composites were prepared by a simple hydrothermal method, and compared with the aqueous zinc-ion batteries assembled with a single type of VO2 under the same conditions, the electrode materials composited with high-purity sulfur-free expanded graphite showed more excellent capacity, cycling performance, and multiplicity performance, and the EG/VO2 composites possessed a high discharge ratio of 345 mAh g-1 at 0.1 A g-1, and the Coulombic efficiency was close to 100%. The EG/VO2 composite has a high specific discharge capacity of 345 mAh g-1 at 0.1 A g-1 with a Coulombic efficiency close to 100%, a capacity retention of 77% after 100 cycles, and 277.8 mAh g-1 with a capacity retention of 78% at a 20-fold increase in current density. The long cycle test data demonstrated that the composite with expanded graphite effectively improved the cycling performance of vanadium-based materials, and the composite maintained a stable Coulombic efficiency of 100% at a high current density of 2 A/g and still maintained a specific capacity of 108.9 mAh/g after 2000 cycles.

Tunnel-Oriented VO2 (B) Cathode for High-Rate Aqueous Zinc-Ion Batteries

Tunnel-type vanadium oxides are promising cathodes for aqueous zinc ion batteries. However, unlike layer-type cathodes with adjustable layer distances, enhancing ion-transport kinetics in tunnels characterized by fixed sizes poses a considerable challenge. This study highlights that the macroscopic arrangement of the electrode crucially determines tunnel orientation, thereby influencing ion transport. By changing the material morphology, the tunnel orientation can be optimized to facilitate rapid ion diffusion. In a proof-of-concept demonstration, it is revealed that (00l) facets-dominated VO2 (B) nanobelts with dispersive morphology (VO2-D) tend to adopt a stacking pattern with directional ion transport along the c-axis on the electrode and guarantee fast ion diffusion. Compared with the aggregated sample (VO2-A) that tends to random arrangement on the electrode with isotropic and slow ion transfer behavior, the electrode featuring dispersive (00l) facets-dominated VO2 (B) nanobelts displays directional and fast ion diffusion behavior, thus exhibits an ultrahigh-rate performance (420.8 and 344.8 mAh g-1 at 0.1 and 10 A g-1, respectively) and long cycling stability (84.3% capacity retention under 5000 cycles at 10 A g-1). The results suggest that simultaneous manipulation of exposed crystal facet and morphology-related electrode arrangement should be promising for boosting the ion-transport kinetics in tunnel-type vanadium oxide cathodes.

Structure regulation and synchrotron radiation investigation of cathode materials for aqueous Zn-ion batteries

In view of the advantages of low cost, environmental sustainability, and high safety, aqueous Zn-ion batteries (AZIBs) are widely expected to hold significant promise and increasingly infiltrate various applications in the near future. The development of AZIBs closely relates to the properties of cathode materials, which depend on their structures and corresponding dynamic evolution processes. Synchrotron radiation light sources, with their rich advanced experimental methods, serve as a comprehensive characterization platform capable of elucidating the intricate microstructure of cathode materials for AZIBs. In this review, we initially examine available cathode materials and discuss effective strategies for structural regulation to boost the storage capability of Zn2+. We then explore the synchrotron radiation techniques for investigating the microstructure of the designed materials, particularly through in situ synchrotron radiation techniques that can track the dynamic evolution process of the structures. Finally, the summary and future prospects for the further development of cathode materials of AZIBs and advanced synchrotron radiation techniques are discussed.

Porous layered ZnV2O4@C synthesized based on a bimetallic MOF as a stable cathode material for aqueous zinc ion batteries

Vanadium-based oxides are considered potential cathode materials for aqueous zinc ion batteries (AZIBs) due to their distinctive layered (or tunnel) structure suitable for zinc ion storage. However, the structural instability and sluggish kinetics of vanadium-based oxides have limited their capacity and cycling stability for large-scale applications. To overcome these shortcomings, here a porous vanadium-based oxide doped with zinc ions and carbon with the molecular formula ZnV2O4@C (ZVO@C) as the cathode material is synthesized by the pyrolysis of a bimetallic MOF precursor containing Zn/V. This electrode demonstrates a remarkable specific capacity of 425 mA h g-1 at 0.5 A g-1 and excellent cycling stability with about 97% capacity retention after 1000 cycles at 10 A g-1. The excellent electrochemical performance of ZVO@C can be attributed to more reaction active sites and the faster reaction kinetics for zinc ion diffusion and storage brought about by the porous layered spinel-type tunnel structure with high surface area and massive mesoporosity, as well as the enhanced electron transport efficiency and more stable channel structure achieved from the doped conductive carbon. The reaction mechanism and structural evolution of the ZVO@C electrode are analyzed using X-ray diffraction and X-ray photoelectron spectroscopy, revealing the formation of a new phase of ZnxV2O5·nH2O during the first charge, which participates in reversible cycling together with ZVO@C during the charging and discharging processes. Moreover, the energy storage mechanism is revealed, in which zinc ions and hydrogen ions jointly participate in intercalation and extraction. The present study demonstrates that constructing composite vanadium-based oxides based on bimetallic organic frameworks as precursor templates is an effective strategy for the development of high-performance cathode materials for AZIBs.

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.

A High-Energy Tellurium Redox-Amphoteric Conversion Cathode Chemistry for Aqueous Zinc Batteries

Rechargeable aqueous zinc batteries are potential candidates for sustainable energy storage systems at a grid scale, owing to their high safety and low cost. However, the existing cathode chemistries exhibit restricted energy density, which hinders their extensive applications. Here, a tellurium redox-amphoteric conversion cathode chemistry is presented for aqueous zinc batteries, which delivers a specific capacity of 1223.9 mAh gTe -1 and a high energy density of 1028.0 Wh kgTe -1. A highly concentrated electrolyte (30 mol kg-1 ZnCl2) is revealed crucial for initiating the Te redox-amphoteric conversion as it suppresses the H2O reactivity and inhibits undesirable hydrolysis of the Te4+ product. By carrying out multiple operando/ex situ characterizations, the reversible six-electron Te2-/Te0/Te4+ conversion with TeCl4 is identified as the fully charged product and ZnTe as the fully discharged product. This finding not only enriches the conversion-type battery chemistries but also establishes a critical step in exploring redox-amphoteric materials for aqueous zinc batteries and beyond.

One-Step Hydrothermal Synthesis of NVO Cathodes with Varied Lattice NH4 + Content: Effect on Structural Evolution and Electrochemical Performance

Aqueous zinc ion batteries have been extensively researched due to their distinctive advantages such as low cost and high safety. Vanadium oxides are important cathode materials, however, poor cycle life caused by vanadium dissolution limits their application. Recent studies show that the lattice NH4 + in vanadium oxides can act as a pillar to enhance structural stability and play a crucial role in improving its cycling stability. Nevertheless, there is still a lack of research on the effect of the lattice NH4 + content on structural evolution and electrochemical performance. Herein, we synthesize vanadium oxides with different contents of lattice NH4 + by a one-step hydrothermal reaction. The vanadium oxides with lattice NH4 + exhibit high initial capacity, as well as good cycling stability and rate performance compared to bare vanadium oxide. Combined with electrochemical analyses, ex-situ structural characterizations, and in-situ X-ray diffraction tests, we reveal that the lattice NH4 + content plays a positive role in vanadium oxides' structural stability and cation diffusion kinetics. This work presents a direction for designing high-performance vanadium cathodes for aqueous zinc ion batteries.

Design Strategies toward High-Utilization Zinc Anodes for Practical Zinc-Metal Batteries

Aqueous Zn-metal batteries (AZMBs) hold a promise as the next-generation energy storage devices due to their low cost and high specific energy. However, the actual energy density falls far below the requirements of commercial AZMBs due to the use of excessive Zn as anode and the associated issues including dendritic growth and side reactions. Reducing the N/P ratio (negative capacity/positive capacity) is an effective approach to achieve high energy density. A significant amount of research has been devoted to increasing the cathode loading and specific capacity or tuning the Zn anode utilization to achieve low N/P ratio batteries. Nevertheless, there is currently a lack of comprehensive overview regarding how to enhance the utilization of the Zn anode to balance the cycle life and energy density of AZMBs. In this review, we summarize the challenges faced in achieving high-utilization Zn anodes and elaborate on the modifying strategies for the Zn anode to lower the N/P ratio. The current research status and future prospects for the practical application of high-performance AZMBs are proposed at the end of the review.

Inhibition of Vanadium Cathodes Dissolution in Aqueous Zn-Ion Batteries

Aqueous zinc-ion batteries (AZIBs) have experienced a rapid surge in popularity, as evident from the extensive research with over 30 000 articles published in the past 5 years. Previous studies on AZIBs have showcased impressive long-cycle stability at high current densities, achieving thousands or tens of thousands of cycles. However, the practical stability of AZIBs at low current densities (<1C) is restricted to merely 50-100 cycles due to intensified cathode dissolution. This genuine limitation poses a considerable challenge to their transition from the laboratory to the industry. In this study, leveraging density functional theory (DFT) calculations, an artificial interphase that achieves both hydrophobicity and restriction of the outward penetration of dissolved vanadium cations, thereby shifting the reaction equilibrium and suppressing the vanadium dissolution following Le Chatelier's principle, is described. The approach has resulted in one of the best cycling stabilities to date, with no noticeable capacity fading after more than 200 cycles (≈720 h) at 200 mA g-1 (0.47C). These findings represent a significant advance in the design of ultrastable cathodes for aqueous batteries and accelerate the industrialization of aqueous zinc-ion batteries.

Recent Advances in Graphene-Based Materials for Zinc-Ion Batteries

Zinc-ion batteries (ZIBs) are a promising alternative for large-scale energy storage due to their advantages of environmental protection, low cost, and intrinsic safety. However, the utilization of their full potential is still hindered by the sluggish electrode reaction kinetics, poor structural stability, severe Zn dendrite growth, and narrow electrochemical stability window of the whole battery. Graphene-based materials with excellent physicochemical properties hold great promise for addressing the above challenges foe ZIBs. In this review, the energy storage mechanisms and challenges faced by ZIBs are first discussed. Key issues and recent progress in design strategies for graphene-based materials in optimizing the electrochemical performance of ZIBs (anode, cathode, electrolyte, separator and current collector) are then discussed. Finally, some potential challenges and future research directions of graphene-based materials in high-performance ZIBs are proposed for practical applications.

Low-current-density stability of vanadium-based cathodes for aqueous zinc-ion batteries

Vanadium-based cathodes have received widespread attention in the field of aqueous zinc-ion batteries, presenting a promising prospect for stationary energy storage applications. However, the rapid capacity decay at low current densities has hampered their development. In particular, capacity stability at low current densities is a requisite in numerous practical applications, typically encompassing peak load regulation of the electricity grid, household energy storage systems, and uninterrupted power supplies. Despite possessing notably high specific capacities, vanadium-based materials exhibit severe instability at low current densities. Moreover, the issue of stabilizing electrode reactions at these densities for vanadium-based materials has been explored insufficiently in existing research. This review aims to investigate the matter of stability in vanadium-based materials at low current densities by concentrating on the mechanisms of capacity fading and optimization strategies. It proposes a comprehensive approach that includes electrolyte optimization, electrode modulation, and electrochemical operational conditions. Finally, we presented several crucial prospects for advancing the practical development of vanadium-based aqueous zinc-ion batteries.

Regulating Crystal Orientation in VO2 for Aqueous Zinc Batteries with Enhanced Pseudocapacitance

Although aqueous zinc batteries have attracted extensive interest, they are limited by relatively low rate capabilities and poor cyclic stability of cathodes. The crystal orientation of the cathode is one important factor influencing electrochemical properties. However, it has rarely been investigated. Herein, VO2 cathodes with different crystal orientations are developed via tuning the number of hydroxyl groups in polyol, such as using glycerol, erythritol, xylitol, or mannitol. The polyols serve as a reductant as well as a structure-directing agent through a hydrothermal reaction. Xylitol-derived VO2 shows a (110)-orientated crystalline structure and ultrathin nanosheet morphology. Such features greatly enhance the pseudocapacitance to 76.1% at a scan rate of 1.0 mV s-1, which is significantly larger than that (61.6%) of the (001)-oriented VO2 derived from glycerol. The corresponding aqueous zinc batteries exhibit a high energy storage performance with a reversible specific capacity of 317 mAh g-1 at 0.5 A g-1, rate ability of 220 mAh g-1 at 10 A g-1, and capacity retention of 81.0% at 10 A g-1 over 2000 cycles. This work demonstrates a facile method for tailoring VO2 crystal orientations, offers an understanding of the Zn2+ storage mechanism upon different VO2 facets, and provides a novel method to develop cathode materials toward advanced aqueous zinc batteries.

Thioacetamide Additive Homogenizing Zn Deposition Revealed by In Situ Digital Holography for Advanced Zn Ion Batteries

Zinc ion batteries are considered as potential energy storage devices due to their advantages of low-cost, high-safety, and high theoretical capacity. However, dendrite growth and chemical corrosion occurring on Zn anode limit their commercialization. These problems can be tackled through the optimization of the electrolyte. However, the screening of electrolyte additives using normal electrochemical methods is time-consuming and labor-intensive. Herein, a fast and simple method based on the digital holography is developed. It can realize the in situ monitoring of electrode/electrolyte interface and provide direct information concerning ion concentration evolution of the diffusion layer. It is effective and time-saving in estimating the homogeneity of the deposition layer and predicting the tendency of dendrite growth, thus able to value the applicability of electrolyte additives. The feasibility of this method is further validated by the forecast and evaluation of thioacetamide additive. Based on systematic characterization, it is proved that the introduction of thioacetamide can not only regulate the interficial ion flux to induce dendrite-free Zn deposition, but also construct adsorption molecule layers to inhibit side reactions of Zn anode. Being easy to operate, capable of in situ observation, and able to endure harsh conditions, digital holography method will be a promising approach for the interfacial investigation of other battery systems.

Organic Dye Molecule Intercalated Vanadium Oxygen Hydrate Enables High-Performance Aqueous Zinc-Ion Storage

Although the layered vanadium oxide-based materials have been considered to be one of the candidates for aqueous Zn-ion batteries (AZIBs), it still faces inevitable challenges of unsatisfactory capacities and sluggish kinetics because of strong electrostatic interactions between Zn-ions and structure lattice. This work addresses the strategy of pre-inserting guest materials to vanadium oxide cathode using different intercalants. To achieve this goal, the small organic dye molecules, methyl orange (MO), and methylene blue (MB) are proposed as the intercalants for vanadium oxygen hydrate (VOH). It has been demonstrated that use of these intercalants can facilitate reaction kinetics between Zn2+ and VOH, leading to an improvement of specific capacity (293 mAh g-1 at 0.3 A g-1 for MO-VOH and 311 mAh g-1 for MB-VOH) compared to VOH, a large enhancement of excellent energy density (237.1 Wh kg-1 for MO-VOH, 232.3 Wh kg-1 for MB-VOH), and a prolong lifespan operation at 3 A g-1 . The mechanism studies suggest that the weakened electrostatic interactions between the Zn-ions and V-O lattice after intercalating organic molecules contribute to boosting the electrochemical performance of AZIBs unveiled by charge density difference and binding energy.

Operando crystal-amorphous transformation cathode for enhanced zinc storage

Aqueous zinc-ion batteries have obtained broad attention due to their high safety, eco-friendliness, and low cost. However, they are still in the developing stage considering the limited candidate of high-performance cathode materials. Furthermore, the intrinsic storage zinc mechanism is also needed to uncover. Here, we propose an operando crystal-amorphous transformation of tunnel-type VOOH and the obtained amorphous V2O5 serves as a high-performance zinc-ion battery cathode. In-situ X-ray diffraction corroborates the unique operando crystal-amorphous transformation of VOOH during the initial charging process. X-ray photoelectron spectroscopy and transmission electron microscopy further demonstrate the element valence evolution and the lattice structure change, respectively. The operando electrochemical crystal-amorphous transformation allows the obtained amorphous V2O5 to achieve the high capacity of ∼ 350.7 mAh g-1, rate performance (151.2 mAh g-1 at 6.4 A g-1), energy and power densities (137 Wh kg-1 at 6831 W kg-1), unveiling a promising approach of cathode materials via operando crystal-amorphous transformation to achieve the enhanced zinc storage.

Interface engineering of heterostructured vanadium oxides for enhanced energy storage in Zinc-Ion batteries

Rechargeable aqueous Zn-ion batteries (RAZIBs) with the merits of cost effectiveness and high safety have been rejuvenated as tantalizing energy storage systems to meet the demand for grid-scale applications. Currently, the energy storage capability of the positive electrode (cathode) holds the key for the overall performance of RAZIBs. In this work, we reveal VO2, V10O24·12H2O (HVO), and VO2/HVO can be prepared via hydrothermal reaction by using different reducing agents. VO2 exhibits high capacity of 237 mAh/g at 4 A/g, while it suffers from quick capacity decay with 48 % retention after 2000 charge/discharge cycles. On the contrary, HVO demonstrates moderate capacity but meritorious cycle stability (i.e., 173 mAh/g at 4 A/g and 82 % after 2000 cycles). By integrating the merits of high-capacity VO2 and high-stability HVO, the biphasic VO2/HVO sample exhibits promising electrochemical performance with high capacity (317 and 239 mAh/g at 1 and 4 A/g, respectively) and good cycle stability (80 % after 2000 cycles). As examined by band structure analysis, the superior electrochemical performance of VO2/HVO is attributed to the presence of a heterojunction between VO2 and HVO enabling a built-in electric field to boost electron transport kinetics, leading to high attainable capacity and reliable cycle performance in RAZIBs.

Multi-Ion Engineering Strategies toward High Performance Aqueous Zinc-Based Batteries

As alternatives to batteries with organic electrolytes, aqueous zinc-based batteries (AZBs) have been intensively studied. However, the sluggish kinetics, side reactions, structural collapse, and dissolution of the cathode severely compromise the commercialization of AZBs. Among various strategies to accelerate their practical applications, multi-ion engineering shows great feasibility to maintain the original structure of the cathode and provide sufficient energy density for high-performance AZBs. Though multi-ion engineering strategies could solve most of the problems encountered by AZBs and show great potential in achieving practical AZBs, the comprehensive summaries of the batteries undergo electrochemical reactions involving more than one charge carrier is still in deficiency. The ambiguous nomenclature and classification are becoming the fountainhead of confusion and chaos. In this circumstance, this review overviews all the battery configurations and the corresponding reaction mechanisms are investigated in the multi-ion engineering of aqueous zinc-based batteries. By combing through all the reported works, this is the first to nomenclate the different configurations according to the reaction mechanisms of the additional ions, laying the foundation for future unified discussions. The performance enhancement, fundamental challenges, and future developing direction of multi-ion strategies are accordingly proposed, aiming to further accelerate the pace to achieve the commercialization of AZBs with high performance.

Synergistically Stabilizing Zinc Anodes by Molybdenum Dioxide Coating and Tween 80 Electrolyte Additive for High-Performance Aqueous Zinc-Ion Batteries

Recently, aqueous zinc-ion batteries (ZIBs) have become increasingly attractive as grid-scale energy storage solutions due to their safety, low cost, and environmental friendliness. However, severe dendrite growth, self-corrosion, hydrogen evolution, and irreversible side reactions occurring at Zn anodes often cause poor cyclability of ZIBs. This work develops a synergistic strategy to stabilize the Zn anode by introducing a molybdenum dioxide coating layer on Zn (MoO2@Zn) and Tween 80 as an electrolyte additive. Due to the redox capability and high electrical conductivity of MoO2, the coating layer can not only homogenize the surface electric field but also accommodate the Zn2+ concentration field in the vicinity of the Zn anode, thereby regulating Zn2+ ion distribution and inhibiting side reactions. MoO2 coating can also significantly enhance surface hydrophilicity to improve the wetting of electrolyte on the Zn electrode. Meanwhile, Tween 80, a surfactant additive, acts as a corrosion inhibitor, preventing Zn corrosion and regulating Zn2+ ion migration. Their combination can synergistically work to reduce the desolvation energy of hydrated Zn ions and stabilize the Zn anodes. Therefore, the symmetric cells of MoO2@Zn∥MoO2@Zn with optimal 1 mM Tween 80 additive in 1 M ZnSO4 achieve exceptional cyclability over 6000 h at 1 mA cm-2 and stability (>700 h) even at a high current density (5 mA cm-2). When coupling with the VO2 cathode, the full cell of MoO2@Zn∥VO2 shows a higher capacity retention (82.4%) compared to Zn∥VO2 (57.3%) after 1000 cycles at 5 A g-1. This study suggests a synergistic strategy of combining surface modification and electrolyte engineering to design high-performance ZIBs.

Regulating Zinc Storage Behaviors of Tunnel Structure Cathodes Via Tungsten Induction

Aqueous zinc-ion batteries have received continuous interests because of applying low-cost and eco-friendly aqueous electrolytes and having high safety. Beyond energetically to explore new-type cathode materials, it is of great significance to regulate the zinc storage behavior of the existing cathodes in order to understand the underlying working mechanism. Therefore, as a proof of concept, this work achieves the regulation of zinc storage behaviors of the tunnel structure tunnel structure B-phase vanadium dioxide (VO2 (B)) and vanadium oxide (V6 O13 ) cathodes via a simple chemical tungsten-doping induction approach. Under low-concentration tungsten-doping induction of 1, 2 and 3 at.%, the tunnel sizes of VO2 (B) can be controlled readily. Moreover, the V6 O13 with large size tunnels can be achieved by medium-concentration tungsten induction of 6 and 9 at.%. It is demonstrated that tungsten induced VO2 (B) can achieve zinc storage without lattice structure change via operando X-ray diffraction analyses. Remarkably, via operando and non-operando analyses, tungsten induced V6 O13 with lager size tunnels can realize the oriented 1D zinc ion intercalation/deintercalation. The further kinetics analysis shows that the zinc storage is mainly diffusion control, which is different from most of vanadium-based cathodes with capacitance control. This viable tungsten-doping induction strategy provides a new insight into achieving the controllable regulation of zinc storage behaviors.

Atomic scale analysis of Zn2+ storage in robust tunnel frameworks

Realizing rapid and reversible Zn2+ storage at the cathode is imperative for the advancement of aqueous Zn-ion batteries (ZIBs), which offer an excellent option for large-scale electrochemical energy storage. However, owing to limitations of the structural stability of previously investigated frameworks, the Zn2+ storage processes remain unclear, thus hindering progress towards the above goal. Herein, we present the novel application of MoVTe oxide with an M1 phase (MVT-M1) as a potential cathode material for ZIBs. MVT-M1 features broad and robust tunnels that facilitate reversible Zn2+ insertion/extraction during cycling, as well as rich redox centers (Mo, V, and Te) to aid in charge redistribution, resulting in good performances in ZIBs. The exceptional resilience of MVT-M1 to high-energy electron beams allows for direct observation of Zn2+ insertion/extraction at the atomic scale within the tunnels for the first time using high-angle annular dark field scanning transmission electron microscopy; the storage location of zinc ions within the cathode is accurately determined layer by layer from the surface to the bulk phase by employing time-of-flight secondary ion mass spectrometry. Additionally, solvent molecules (H2O and methanol) are also found inside the tunnels along with Zn2+. Due to the broader heptagonal tunnels and Te ions in the hexagonal tunnels, MVT-M1 exhibits good cycling stability, outperforming MoVTe oxide with the M2 phase (no heptagonal tunnels) and MoV oxide with the M1 phase (no Te). These findings hold significant importance in advancing our understanding of the Zn2+ storage mechanism and enable the design of novel materials specifically optimized for efficient Zn2+ storage.

In-situ preparation of amorphous VO2@rGO cathode for ultra-high capacity and ultra-long cycle life aqueous zinc ion batteries

Aqueous zinc-ion batteries (AZIBs) are considered as a promising alternative to lithium-ion batteries for stationary energy storage due to their environmental benignity and cost-effectiveness. However, the development of AZIBs continues to be plagued by a lack of cathode materials with high specific capacity and superior lifetime. Herein, we in-situ synthesize amorphous VO₂@rGO assisted by controlling the charging cut-off voltage. Experimental results and theoretical calculations confirm that the amorphous VO₂(A)@rGO can effectively reduce the migration energy barrier of Zn²⁺, improve the conductivity of the electrode, and promote the insertion/extraction of Zn²⁺. Consequently, the Zn//VO₂(A)@rGO battery exhibits an ultra-high specific capacity of 527.0 mAh·g^-1 at 1 A·g^-1 after 100 cycles, an ultra-long cycle stability of 183.4 mAh·g^-1 at 20 A·g^-1 after 30,000 cycles, and an energy of 316.1 Wh·Kg^-1 at a power density of 6082.9 W·Kg^-1 power density. Meanwhile, we reveal that the amorphous VO₂@rGO electrode follows a hybrid mechanism of classical Zn²⁺ insertion/de-insertion and the reversible phase transition from amorphous VO₂ to V₂O₃. This study highlights that in-situ preparation of amorphous VO₂@rGO cathode materials by controlling the charging voltage interval, opening up further possibilities for the development of high-performance AZIB cathodes.

Self-Transformation Strategy Toward Vanadium Dioxide Cathode For Advanced Aqueous Zinc Batteries

In the lithium-dominated era, rechargeable Zn batteries are emerging as a competitive alternative. However, the sluggish kinetics of ion diffusion and structural destruction of cathode materials have thus far hampered the realization of future large-scale energy storage. Herein, an in situ self-transformation approach is reported to electrochemically boost the activity of a high-temperature, argon-treated VO₂ (AVO) microsphere for effective Zn ion storage. The presynthesized AVO with hierarchical structure and high crystallinity allows efficient electrochemical oxidation and water insertion to induce self-phase transformation into V₂ O₅ ·nH₂ O within the first charging process, which leads to rich active sites and fast electrochemical kinetics. Using AVO cathode, an outstanding discharge capacity of 446 mAh g^-1 at 0.1 A g^-1 , high rate capability of 323 mAh g^-1 at 10 A g^-1 and excellent cycling stability for 4000 cycles at 20 A g^-1 with high capacity retention are demonstrated. Importantly, such zinc-ion batteries with phase self-transition can also perform well at high-loading, sub-zero temperature, or pouch cell conditions for practical application. This work not only paves a new route to design in situ self-transformation in energy storage devices, but also broadens the horizons of aqueous zinc-supplied cathodes.

Green Environmentally Friendly "Zn(CH3SO3)2" Electrolyte for Aqueous Zinc-Ion Batteries

Aqueous zinc-ion batteries are considered as an ideal substitute for lithium-ion batteries due to their abundant resource storage, high safety, and low price. However, zinc anodes exhibit poor reversibility and cyclic stability in most conventional aqueous electrolytes. Herein, an environmentally friendly Zn(CH₃SO₃)₂ electrolyte is proposed to solve the problems of common aqueous electrolytes. The bulky CH₃SO₃^- anions can regulate the solvation structure of Zn²⁺ by replacing some water molecules in the primary solvation sheath of Zn²⁺, thus slowing the hydrogen evolution side reactions and formation of zinc dendrite. Additionally, the changing solvation structure weakens the bonding between Zn²⁺ and the surrounding water molecules, which is conducive to the transport and charge transfer of Zn²⁺, thus improving the battery capacity. In the Zn(CH₃SO₃)₂ electrolyte, Zn plating/stripping exhibits a high Coulombic efficiency of >98% and long-term cyclic stability over 800 h. The specific capacity of the assembled Zn//V₂O₅ cell in 3 mol L^-1 Zn(CH₃SO₃)₂ reaches 350 mA h g^-1 at 0.1 A g^-1, much higher than that in the ZnSO₄ electrolyte (213 mA h g^-1). In conclusion, this work offers insights into the exploration of advanced green electrolyte systems for zinc-ion batteries.

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.

How About Vanadium-Based Compounds as Cathode Materials for Aqueous Zinc Ion Batteries?

Aqueous zinc-ion batteries (AZIBs) stand out among many monovalent/multivalent metal-ion batteries as promising new energy storage devices because of their good safety, low cost, and environmental friendliness. Nevertheless, there are still many great challenges to exploring new-type cathode materials that are suitable for Zn²⁺ intercalation. Vanadium-based compounds with various structures, large layer spacing, and different oxidation states are considered suitable cathode candidates for AZIBs. Herein, the research advances in vanadium-based compounds in recent years are systematically reviewed. The preparation methods, crystal structures, electrochemical performances, and energy storage mechanisms of vanadium-based compounds (e.g., vanadium phosphates, vanadium oxides, vanadates, vanadium sulfides, and vanadium nitrides) are mainly introduced. Finally, the limitations and development prospects of vanadium-based compounds are pointed out. Vanadium-based compounds as cathode materials for AZIBs are hoped to flourish in the coming years and attract more and more researchers' attention.

Fluoride-Based Stable Quasi-Solid-State Zinc Metal Battery with Superior Rate Capability

Aqueous zinc metal batteries are limited in practical applications due to their short lifespans. Herein, a LaF₃-coated Zn anode (LF@Zn) is investigated to induce the uniform Zn deposition and successfully build a separator-free quasi-solid-state zinc metal battery. The LF@Zn enables smooth and dendrite-free Zn deposition, owing to the homogeneous Zn²⁺ flux regulated by the LaF₃-based quasi-solid-state electrolyte. It can also suppress the corrosion side reactions by modulating the [Zn(H₂O)₆]²⁺ solvation sheath. The polarization of plating and stripping is relatively modest due to the reduced diffuse energy of desolvated Zn²⁺ in the quasi-solid-state electrolyte. In a separator-free symmetric cell, the LF@Zn anode shows a significantly prolonged lifespan of over 1300 h at 2 mA cm^-2 and a superior rate performance with only 156 mV at an ultrahigh current density of 50 mA cm^-2. A LF@Zn//VO₂ quasi-solid-state full cell exhibits outperforming rate capability and a long cyclic performance for up to 3000 cycles at 6.0 A g^-1. A stable Zn anode is established in this work with a fluoride-based quasi-solid-state electrolyte, opening up a new avenue for protecting metal anodes.

Vanadium Oxide: Phase Diagrams, Structures, Synthesis, and Applications

Vanadium oxides with multioxidation states and various crystalline structures offer unique electrical, optical, optoelectronic and magnetic properties, which could be manipulated for various applications. For the past 30 years, significant efforts have been made to study the fundamental science and explore the potential for vanadium oxide materials in ion batteries, water splitting, smart windows, supercapacitors, sensors, and so on. This review focuses on the most recent progress in synthesis methods and applications of some thermodynamically stable and metastable vanadium oxides, including but not limited to V₂O₃, V₃O₅, VO₂, V₃O₇, V₂O₅, V₂O₂, V₆O₁₃, and V₄O₉. We begin with a tutorial on the phase diagram of the V-O system. The second part is a detailed review covering the crystal structure, the synthesis protocols, and the applications of each vanadium oxide, especially in batteries, catalysts, smart windows, and supercapacitors. We conclude with a brief perspective on how material and device improvements can address current deficiencies. This comprehensive review could accelerate the development of novel vanadium oxide structures in related applications.

Synthesis of V2O5·nH2O nanobelts@polyaniline core-shell structures with highly efficient Zn2+ storage

Aqueous zinc-ion batteries (AZIBs) are regarded as attractive candidates for next-generation energy storage devices. Among various cathode materials, V₂O₅·nH₂O (VOH) possesses a high theoretical capacity but poor cycle stability due to the susceptibility of its open structure to damage by the quick shuttling of Zn²⁺. Herein, the structural stability of VOH is directly improved by wrapping polyaniline (PANI) on the VOH nanobelts (VOH@PANI). As a cathode material for AZIBs, the VOH nanobelts@PANI core-shell structures exhibit an outstanding cycle stability of 98% after 2000 cycles at 2 A g^-1. The improved conductivity and additional energy storage contribution of the PANI endow VOH@PANI with a specific capacity as high as 440 mAh g^-1 at 0.1 A g^-1, substantially higher than pure VOH (291 mAh g^-1). At the same time, high energy and power densities of 349 Wh kg^-1 and 3347 W kg^-1 are achieved. This work not only demonstrates that p-type doped PANI coatings on VOH can boost the Zn²⁺ storage of VOH, but also provides a novel method to enhance cathode materials for high electrochemical performance.

Oxygenated copper vanadium selenide composite nanostructures as a cathode material for zinc-ion batteries with high stability up to 10 000 cycles

The development of aqueous zinc-ion batteries (AZiBs) towards practical implementations is hampered by unsuitable host cathode materials. Herein, we reported a high-capacity, stable, and long-cycle-life (10 000 cycles) oxygenated copper vanadium selenide composite material (Cu_0.59V₂O₅/Cu_0.828V₂O₅@Cu_1.8Se₁/Cu₃Se₂, denoted as O-CuVSe) as a cathode for AZiBs. The newly constructed O-CuVSe composite cathode can be operated in the wide potential window of 0.4-2.0 V, exhibiting a high specific capacity of 154 mA h g^-1 at 0.2 A g^-1 over 100 cycles. Interestingly, the O-CuVSe composite cathode delivered excellent specific capacities of 117 and 101.4 mA h g^-1 over 1000 cycles at 1 and 2 A g^-1, respectively. Even at a high current density of 5 A g^-1, the cathode delivered a high reversible capacity of 74.5 mA h g^-1 over an ultra-long cycling life of 10 000 cycles with no obvious capacity fading. Apart from this, the cathode exhibited excellent rate capability at different current densities. The superior electrochemical properties originate from the synergistic effects between the oxygen vacancy engineering and interlayer doping of Cu ions to increase the structural stability during the cycling, enhancing the electron/ion transport kinetics. Moreover, the Zn²⁺ storage mechanism in the Zn/O-CuVSe aqueous rechargeable battery was explored. This study provides a new opportunity for the fabrication of different kinds of a new class of cathode materials for high-voltage and high-capacity AZiBs and other energy storage devices.

A Foldable Aqueous Zn-Ion Battery with Gear-Structured Composite as Freestanding Cathode

A foldable battery with high flexibility provides great potential in various wearable electronic devices for health and fitness tracking, chronic disease management, performance monitoring, navigation tracking, and portable gears for soldiers. We report a highly flexible, self-healing Zn-ion battery with a free-standing cathode that is composed of a 3D gear-like NH₄ V₄ O₁₀ @C composite on carbon paper. The battery retained a capacity of up to 102.4 mAh g^-1 even after being folded 60 times with a high angle of 180°. An aqueous hydrogel consisting polyvinyl alcohol, glycerin and Zn(CF₃ SO₃ )₂ was used as electrolyte, which showed as high as 580 % tensile strain under a loading weight of 78 N. The battery exhibited a better capacity retention of over 100 mAh g^-1 and Coulombic efficiency of over 99.8 % after cutting and twisting to 90°, thereby indicating a great self-healing performance. The gear-like geometry greatly improved the volume accommodation due to the increased interval space between the blades and the outward configuration. Meanwhile the Zn²⁺ ionic conductivity was improved by rapid re-binding of many existing hydroxy groups from the electrolyte and the enhanced contact surface area and diffusion route from the cathode material. The highly flexible, safe aqueous Zn-ion battery opens a practical way to power various carry-on electronics under mechanical agitation.

Electrolyte for High-Energy- and Power-Density Zinc Batteries and Ion Capacitors

Growth of dendrites, limited coulombic efficiency (CE), and the lack of high-voltage electrolytes restrict the commercialization of zinc batteries and capacitors. These issues are resolved by a new electrolyte, based on the zinc(II)-betaine complex [Zn(bet)₂ ][NTf₂ ]₂ . Solutions in acetonitrile (AN) avoid dendrite formation. A Zn||Zn cell operates stably over 10 110 h (5055 cycles) at 0.2 mA cm^-2 or 110 h at 50 mA cm^-2 , and has an area capacity of 113 mAh cm^-2 at 80% depth of discharge. A zinc-graphite battery performs at 2.6 V with a midpoint discharge-voltage of 2.4 V. The capacity-retention at 3 A g^-1 (150 C) is 97% after 1000 cycles and 68% after 10 000 cycles. The charge/discharge time is about 24 s at 3.0 A g^-1 with an energy density of 49 Wh kg^-1 at a power density of 6864 W kg^-1 based on the cathode. A zinc||activated-carbon ion-capacitor (coin cell) exhibits an operating-voltage window of 2.5 V, an energy density of 96 Wh kg^-1 with a power density of 610 W kg^-1 at 0.5 A g^-1 . At 12 A g^-1 , 36 Wh kg^-1 , and 13 600 W kg^-1 are achieved with 90% capacity-retention and an average CE of 96% over 10 000 cycles. Quantum-chemical methods and vibrational spectroscopy reveal [Zn(bet)₂ (AN)₂ ]²⁺ as the dominant complex in the electrolyte.

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.

In-situ electrochemical conversion of Na5V12O32@graphene for enhanced cycle stability in aqueous zinc ion batteries

HYPOTHESIS

Na5V12O32 (NVO) is a potential cathode for aqueous zinc ion batteries (AZIBs). However, it suffers severe capacity decay due to the dissolution of the active material. The structural design may be an effective solution to the problem.

EXPERIMENTS

Herein, we construct a typical two-dimensional hierarchical structure of Na5V12O32@graphene (NVO@G) via a facile molten salt method.

FINDINGS

The capacity fading problem is solved by the in-situ conversion of NVO@G to a more stable hierarchical system during cycling. The in-situ formed zinc pyrovanadate (Zn3V2O7(OH)2·2H2O, ZVO) nanosheets on the surface of graphene exhibits excellent zinc-ion storage stability. The presence of graphene induces the growth of NVO nanobelts to construct the typical two-dimensional hierarchical structure. Additionally, the in-situ conversion makes the formed ZVO nanosheets contact with graphene better. Benefitting from the hierarchical nanostructure and in-situ phase conversion, the NVO@G electrode shows excellent long-term stability (96.4% retention after 340 cycles at 0.3 A g-1, 85.7% retention after 4400 cycles at 5 A g-1) and high zinc ion storage capacity (220 mAh g-1 at 0.3 A g-1), which is superior to those of most electrode materials previously reported.

High-Performance Aqueous Zinc-Ion Batteries Enabled by Binder-Free and Ultrathin V2O5-x@Graphene Aerogels with Intercalation Pseudocapacitance

As a result of the absence of solid-state diffusion limitation, intercalation pseudocapacitance behavior is emerging as an attractive charge-storage mechanism that can greatly facilitate the ion kinetics to boost the rate capability and cycle stability of batteries; however, related research in the field of zinc-ion batteries (ZIBs) is still in the initial stage and only found in limited cathode materials. In this study, a novel V₂O_5-x@rGO hybrid aerogel consisting of ultrathin V₂O₅ nanosheets (∼1.26 nm) with abundant oxygen vacancies (Vö) and a three-dimensional (3D) graphene conductive network was specifically designed and used as a freestanding and binder-free electrode for ZIBs. As expected, the ideal microstructure of both the material and the electrode enable fast electron/ion diffusion kinetics of the electrode, which realize a typical intercalation pseudocapacitance behavior as demonstrated by the simulation calculation of cyclic voltammetry (CV), ex situ X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and first-principles density functional theory (DFT) calculation. Thanks to the elimination of solid-state diffusion limitation, the V₂O_5-x@rGO electrode delivers a high reversible rate capacity of 153.9 mAh g^-1 at 15 A g^-1 and 90.6% initial capacity retention at 0.5 A g^-1 after 1050 cycles in ZIBs. The intercalation pseudocapacitance behavior is also realized in the assembled soft-pack battery, showing promising practical application prospects.

Synergistic Chaotropic Effect and Cathode Interface Thermal Release Effect Enabling Ultralow Temperature Aqueous Zinc Battery

Although rechargeable zinc-ion batteries are promising candidates for next-generation energy storage devices, their inferior performance at subzero temperatures limits their practical application. Here, a strategy to destroy the H-bond network by adding synergistic chaotropic regents is reported, thus reducing the freezing point of the aqueous electrolyte below -90 °C. Owing to the synergistic chaotropic effect between urea and Zn(ClO₄ )₂ and the thermal release effect on the cathode interface during charging, Zn//VO₂ batteries feature a specific capacity of 111.4 mAh g^-1 and stability after ≈1000 cycles with 81.9% capacity retention at -40 °C. This work demonstrates that the synergistic chaotropic effect and the thermal effect on the interface can effectively widen the operation range of temperature of aqueous electrolytes and maintain fast kinetics, which provides a new design strategy for all-weather aqueous zinc batteries.