Probing Capacity Decay in Vanadium Oxide Cathodes of Aqueous Zinc-Ion Batteries Using Operando EQCM-D

Zn-ion batteries are promising for their safety and cost-effectiveness. Vanadium-based compounds are notable due to their layered structure and polyvalent nature, enabling high-rate capability and large capacity. The type and properties of charge carriers critically influence the structural and performance stability of vanadium-based electrodes, which, however, remain obscure. Herein, we elucidate the H+-dominated intercalation mechanism of vanadium oxide and its correlation with capacity degradation via an electrochemical quartz crystal microbalance investigation. By tracking real-time mass changes and ion diffusion during the electrochemical process, we demonstrate that H+ is the predominant shuttling cation. H+ intercalation generates OH– at the electrode–electrolyte interface, causing alkaline dissolution of vanadium oxide and the capacity decay. With nanosized modification, the dissolution can be mitigated. The electrode exhibits a capacity retention of 98.2% after 2000 cycles at 5 A g–1. This study deepens the understanding of vanadium oxide’s charge storage, guiding the design of high-performance aqueous batteries.