V2O4·2H2O nanocrystal as a novel cathode material for aqueous zinc ion batteries

Aqueous Zn ²⁺ batteries (AZIBs) have become a popular research topic due to their impressive performance, low cost, and non-flammable properties. Developing promising cathode materials is still crucial for making satisfactory AZIBs. Herein, V ₂O ₄·2H ₂O, a layer-type hydrated VO ₂ phase, is employed as a novel cathode material in AZIBs. It is produced via a hydrothermal process, has a ribbon-like nanocrystal morphology, and exhibits an appreciable capacity (412.7 mAh/g at 0.1 A/g), considerable capacity preservation at high rates, and good cyclability. Ex-situ structural and elemental valence state analysis discloses the structural evolution of the cathode material. V ₂O ₄·2H ₂O is electrochemically oxidized to low-crystalline V ₂O ₅·nH ₂O in the first charging process. In the subsequent charging/discharging processes, the emerged V ₂O ₅·nH ₂O is converted to crystalline Zn ₃(OH) ₂V ₂O ₇·2H ₂O upon discharging, which is transformed back to V ₂O ₅·nH ₂O when charging. This work presents that V ₂O ₄·2H ₂O is a potential candidate for AZIB cathode materials.