Understanding the Super‐Theoretical Capacity Behavior of VO2 in Aqueous Zn Batteries

Abstract

AbstractVO2 material, as a promising intercalation host, is widely investigated not only in aqueous lithium‐ion batteries, but also in aqueous zinc‐ion batteries (AZIBs) owing to its stable tunnel‐like framework and multivalence of vanadium. Different from lithium‐ion storage, VO2 can provide higher capacity when storing zinc ions, even exceeding its theoretical capacity (323 mAh g−1), but the specific reason for this unconventional performance in AZIBs is still unclear. The present study proposes a catalytic oxygen evolution reaction (OER) coupled with an interface oxidation mechanism of VO2 during the initial charging to a high voltage. This coupling induces a phase transformation of VO2 into a high oxidation state of V5O12∙6H2O, enabling a nearly two‐electron reaction and providing additional zinc storage sites to achieve super‐theoretical capacity. Furthermore, it is demonstrated that these vanadium oxide cathodes (V2O3, VO2, and V2O5) will all undergo phase change after the first charge or short cycle. Notably, water molecules participate in the final formation of layered vanadium‐based hydrate, highlighting their crucial role as “pillars” for stabilizing the structure. This work significantly enhances the understanding of vanadium‐based oxide cathodes.