Unlocking the High Capacity Ammonium-Ion Storage in Defective Vanadium Dioxide.

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.