Unlocking the performance degradation of vanadium-based cathodes in aqueous zinc-ion batteries

Vanadium-based materials stand out as promising cathode options for rechargeable aqueous zinc-ion batteries (ZIBs), primarily because of their high capacity and superior rate capability. Nevertheless, the utilization of vanadium-based cathodes in advancing ZIBs toward commercial viability is hindered by their insufficient stability and a notable lack of comprehension of the mechanisms driving capacity degradation. Herein, we identified the formation of Zn3(OH)2V2O7·2H2O (ZOV) as a key contributor to the capacity decay of vanadium-based cathodes. Through a series of compelling experiments, we revealed that dissolved vanadium ions react with zinc salts or layered zinc hydroxide (formed from H+ insertion) during the immersion or cycling of vanadium-based cathodes in aqueous electrolytes, ultimately leading to the formation of detrimental ZOV. Strong evidence shows that ZOV is inactive for Zn2+ storage owing to the presence of the pure tetrahedral frameworks of vanadium. To suppress the formation of ZOV, adjustments were made to the electrolyte composition, including the solvents and solutes. Consequently, the absence of ZOV enables an impressive capacity retention of 85 % in the ZnSO4 electrolyte after 150 cycles at 0.2 A g−1. Overall, this study directly unlocks the capacity decay mechanism in vanadium-based cathodes and offers valuable insights for the design of innovative electrolytes and novel vanadium-based cathode materials for ZIBs.