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

ABSTRACT

VO2 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 V5 O12 ∙6H2 O, 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 (V2 O3 , VO2 , and V2 O5 ) 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.