Facing the energy crisis and environmental pollution, aqueous zinc-ion batteries have become an extremely important sustainable energy storage technology, owing to their abundant raw materials, low cost, and high safety. Among various cathode materials, vanadium-based oxides exhibit high specific energy advantages due to their abundant redox pairs, and their unique structures are more favorable for rapid ion transport, making them a current research hotspot. However, the practical application of aqueous zinc batteries has long been severely constrained by the structural instability of cathode materials during electrochemical cycling.

The core challenge in this field lies in structural degradation and active material dissolution during high-voltage charge-discharge processes. During electrochemical cycling, the weakening of vanadium-oxygen orbital coupling leads to a decrease in bond strength, making the oxygen atoms in the structure more susceptible to attack by protons in the aqueous electrolyte, triggering side reactions, local structural distortion, and vanadium dissolution, resulting in rapid capacity decay. Although existing strategies such as defect engineering can improve cycle life, they often come at the cost of sacrificing active sites and capacity. Therefore, how to simultaneously achieve high capacity, high stability, and long lifespan remains a key bottleneck restricting the development of vanadium-based cathodes.

In response to the above issues, a team composed of researchers from Queensland University of Technology and other institutions conducted systematic research using ZEPTOOLS' in-situ TEM measurement system. Through the introduction of fluorine substitution to implement covalency engineering, the team effectively enhanced vanadium-oxygen orbital coupling and chemical bond covalency, successfully alleviating structural distortion and vanadium dissolution at high voltages, and achieving a cathode design for aqueous zinc-ion batteries that combines high specific energy with ultra-long cycle life.

Title:Covalency stabilizes high energy vanadium oxide positive electrode for sustainable aqueous zinc-ion batteries
Journal:Nature Communications
URL:https://doi.org/10.1038/s41467-026-75617-1

To address the long-standing issue of structural distortion and metal dissolution in oxide cathode materials for aqueous zinc-ion batteries during cycling, the research team proposed a novel strategy to stabilize vanadium sesquioxide cathodes through electronic state modulation via covalency engineering. The study shows that while introducing oxygen vacancies into conventional V₂O₃ can improve some electrochemical activity, it weakens the orbital coupling between vanadium and oxygen and reduces chemical bond covalency, leading to increased basicity of axial oxygen atoms and making them more susceptible to proton attack from the electrolyte. To this end, the team introduced fluorine, an element with higher electronegativity, into the V₂O₃ lattice to construct an asymmetric F-V-O local configuration. This strategy promotes more sufficient hybridization between the V 3d and O 2p orbitals, strengthens the covalent interactions of σ and π bonds, and reduces the basicity of axial oxygen atoms, fundamentally inhibiting proton erosion and local structural distortion at high voltages, thus laying the microelectronic foundation for high-stability cathodes.

To visually reveal the structural evolution of materials under real electrochemical environments at the atomic and microscopic scales, the research team conducted real-time in-situ transmission electron microscopy experiments using ZEPTOOLS' in-situ sample holder. By applying electrical signals via the ZEPTOOLS in-situ holder, the researchers captured high-resolution dynamic structural changes of the fluorine-doped V₂O₃ electrode during discharge and zinc-ion intercalation. Experimental observations showed that the electrode material did not undergo significant volume changes or lattice collapse during electrochemical reactions, demonstrating exceptionally high microstructural integrity and mechanical stability. Combined post-cycling strain analysis further confirmed that the unmodified oxygen-vacancy samples exhibited significant lattice distortion and vanadium vacancy defects after cycling, while the fluorine-doped material maintained an intact crystal framework, precisely verifying the decisive role of the covalency enhancement strategy in resisting local structural damage.

On the basis of clarifying the structural stabilization mechanism, the study deeply analyzed the energy storage kinetics and interfacial reaction behavior of the electrode material in aqueous electrolyte. In-situ spectroscopy and electrolyte interface analysis indicated that fluorine doping successfully transformed the vanadium-oxygen redox process, which is prone to material dissolution at high voltages, into a highly reversible vanadium-centered redox reaction. Meanwhile, the strengthened vanadium-oxygen covalent bonds significantly mitigated side reactions and gas evolution at the electrode interface, improved the material's electrical conductivity, and increased the zinc-ion diffusion coefficient by one to two orders of magnitude (reaching 10⁻¹⁰ to 10⁻⁷ cm² s⁻¹). This improvement in kinetic properties allowed the electrode to maintain efficient zinc-ion de/intercalation while minimizing structural damage from proton intercalation, achieving a highly efficient and sustainable energy storage mechanism dominated by zinc-ion insertion.

Ultimately, this covalency engineering strategy demonstrated excellent practical application prospects and high energy density performance at the battery device level. In coin cell tests, the fluorine-doped V₂O₃ cathode achieved an ultra-long cycle life of 60,000 cycles at a high current density of 3 A g⁻¹, with a capacity retention of 80%. In more practically relevant high-mass-loading (approximately 18 mg cm⁻²) ampere-hour-scale pouch cells, the material delivered a specific energy of 95 Wh kg⁻¹ based on the total electrode mass at 50 mA g⁻¹. The research team further assembled four pouch cells in series into a practical battery pack capable of outputting over 3.7 V, successfully powering a remote-controlled drone flight. This fully demonstrates that strengthening vanadium-oxygen covalent bonds not only solves the fundamental scientific challenge of cathode stability in aqueous batteries but also opens a viable industrial pathway for developing high-energy-density, long-life sustainable aqueous energy storage devices.

ZEPTOOLS, as a Chinese high-end precision instrument company, is a first-class supplier of in-situ electron microscopy characterization solutions. Its PicoFemto series of in-situ transmission electron microscopy characterization solutions have successively provided technical support for significant research results of domestic and international users. The following image shows the ZEPTOOLS in-situ TEM sample holder product used in this research: