Activation and degradation mechanisms of α-V2O5 cathode materials in Zn-ion battery

Vanadium oxides with high specific capacity have been widely used as cathode materials for aqueous Zn-ion batteries. However, the interactions between the aqueous electrolytes and Vanadium oxides are still unclear. In particular, the intercalation of proton and water which can play a critical role in the degradation process is less well understood. Moreover, the mechanism for the capacity increase during the electrochemical cycling, known as the activation process, is still under debate. In this study, we systematically investigated the α-V2O5||Zn battery system with state-of-the-art characterization tools and electrochemical testing techniques to reveal the activation and degradation processes. It is discovered that the activation can be mainly attributed to the intercalation of protons and water in the first 100 cycles, which destabilizes the overall structural integrity, leading to capacity degradation afterward. It is also revealed that two main cathode electrolyte interface (CEI) components can be formed on the cathode surface, namely Zn3(V2O7)(OH)2·2H2O and Znm(CF3SO3)n(OH)2m-n·nH2O. The formation of Zn3(V2O7)(OH)2·2H2O-based CEI component is irreversible, while Znm(CF3SO3)n(OH)2m-n·nH2O-based CEI component can be reversibly cycled. Different structure-stabilizing charge/discharge protocols are proposed accordingly, which could leverage the structural integrity with thorough phase transition in the activation stage to enhance the cycling performance of the V-based cathodes. We hope to spur further interest in the fundamental understanding of the cathode materials in Zn-ion batteries.