Unlocking the High Capacity Ammonium‐Ion Storage in Defective Vanadium Dioxide

Abstract Aqueous ammonium‐ion storage has been considered a promising energy storage competitor to meet the requirements of safety, affordability, and sustainability. However, ammonium‐ion storage is still in its infancy in the absence of reliable electrode materials. Here, defective VO 2 (d‐VO) is employed as an anode material for ammonium‐ion batteries with a moderate transport pathway and high reversible capacity of ≈200 mAh g −1 . Notably, an anisotropic or anisotropic behavior of structural change of d‐VO between c ‐axis and ab planes depends on the state of charge (SOC). Compared with potassium‐ion storage, ammonium‐ion storage delivers a higher diffusion coefficient and better electrochemical performance. A full cell is further fabricated by d‐VO anode and MnO 2 cathode, which delivers a high energy density of 96 Wh kg −1 (based on the mass of VO 2 ), and a peak energy density of 3254 W kg −1 . In addition, capacity retention of 70% can be obtained after 10 000 cycles at a current density of 1 A g −1 . What's more, the resultant quasi‐solid‐state MnO 2 //d‐VO full cell based on hydrogel electrolyte also delivers high safety and decent electrochemical performance. This work will broaden the potential applications of the ammonium‐ion battery for sustainable energy storage.