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 VO₂ (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 MnO₂ cathode, which delivers a high energy density of 96 Wh kg⁻¹ (based on the mass of VO₂), 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 MnO₂//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.