Uncovering and Retrieving the Internal Vanadium Migration Caused Voltage Fade in Vanadyl Phosphate Cathode for Aqueous Zinc Batteries

Aqueous zinc batteries are expected to be applicable for large-scale energy storage. The polyanion cathode materials provide high voltage, but they experience fast voltage fade in the aqueous zinc system. Herein, we uncover the electrolyte-depending degradation mechanisms and propose the corresponding solutions with a representative layered hydrated VOPO4 (VOP) polyanion cathode material. The voltage fade of VOP in the 10 m ZnCl2 electrolyte is caused by the transition to VOx, which loses the inductive effect from phosphate. With further suppressed water activity with a "water in bi-salt" electrolyte, on the other hand, the voltage drops as a result of internal V migration from the VOP layer to the space between layers, the latter of which experiences a weakened inductive effect. Accordingly, a deep charging method is applied to retrieve the migrated V and recover the decayed voltage. As a result, the VOP cathode presents stable retention of both the voltage and capacity.