Review on Oxygen-Free Vanadium-Based Cathodes for Aqueous Zinc-Ion Batteries

Aqueous zinc-ion batteries (AZIBs) are considered as one of the most promising next-generation electrochemical energy storage systems owing to their high-power density, environmental benign, intrinsic safety, and the low cost of the abundant zinc resources. However, their further development is still plagued by the inferior electrochemical performance of cathode materials. Though extensive research has been conducted to investigate various cathode materials (including manganese oxides, vanadium oxides, Prussian blues analogy, and organic materials), design of high-performance cathodes with satisfying capacity and long-term cycling stability still faces great challenges. Oxygen-free vanadium-based compounds, owing to their better conductivity, larger interlayer spacing, lower ion diffusion barrier and higher theoretical specific capacity than those of vanadium oxides, have gained increasing attention recently. In this review, we summarize the recent development about the emerging oxygen-free vanadium-based compounds in AZIBs, emphasizing the methods to design electrode materials with desired structures, effective strategies to improve their electrochemical performance, and the fundamental electrochemical mechanisms. Finally, the current challenges and outlooks of oxygen-free vanadium-based compounds are proposed, providing a novel perspective and useful guidance for the design of high-performance vanadium-based cathode materials for AZIBs.