Emerging Amorphous to Crystalline Conversion Chemistry in Ca-Doped VO₂ Cathodes for High-Capacity and Long-Term Wearable Aqueous Zinc-Ion Batteries

Amorphous transition metal oxides have attracted significant attention in energy storage devices owing to their potentially desirable electrochemical properties caused by abundant unsaturated dangling bonds. However, the amorphization further amplifies the shortcoming of the poor intrinsic electronic conductivity of the metal oxides, resulting in unsatisfying rate capability and power density. Herein, freestanding amorphous Ca-doped V2O5 (a-Ca-V2O5) cathodes are successfully prepared via in situ electrochemical oxidation of Ca-doped VO2 nanoarrays for wearable aqueous zinc-ion batteries. The doping of Ca and construction of freestanding structure effectively uncover the potential of amorphous V2O5, which can make full use of the abundant active sites for high volumetric capacity and simultaneously achieve fast reaction kinetics for excellent rate performance. More importantly, the introduction of Ca can notably reduce the formation energy of VO2 according to theoretical calculation results and realizes amorphous to crystalline reversible conversion chemistry in the charge/discharge procedure, thereby facilitating the reversible capacity of the newly developed a-Ca-V2O5. This work provides an innovative design strategy to construct high-rate capacity amorphous metal oxides as freestanding electrodes for low-cost and high-safe wearable energy-storage technology.