Tailoring electronic structure to enhance the ammonium-ion storage properties of VO2 by molybdenum doping toward highly efficient aqueous ammonium-ion batteries

Recently, research on ammonium-ion storage has gained widespread interest, and it is still a major problem and a popular research area to produce high-performance electrode materials for aqueous ammonium ion batteries (AAIBs). Herein, the electronic structure of tunnel-like vanadium dioxide (VO2) is tailored by molybdenum doping (denoted as VO2-Mo) to enhance ammonium-ion storage properties toward highly efficient AAIBs. VO2-Mo with a unique nanobelt structure is designed and synthesized by adjusting the content of Mo via a facile hydrothermal method. Density functional theory (DFT) simulations and experimental data both demonstrate that molybdenum atoms in the VO2 structure can improve mass transfer, speed up ion transport, and accelerate kinetics, showing boosted NH4+-storage properties. With 2% Mo doping, at 0.1 A g-1, VO2-Mo exhibits a specific discharge capacity of around 370 mA h g-1, surpassing VO2 (232 mA h g-1) and the vanadium oxide-based materials that have been reported for NH4+-storage. After approximately 6000 successive charging and discharging cycles at 2 A g-1, it essentially maintains the specific capacity of 140 mA h g-1. Using VO2-Mo, polyaniline (PANI) and 1 M (NH4)2SO4 as the anode, cathode, and electrolyte, respectively, a VO2-Mo//PANI full battery was further built, and at 0.2 A g-1, it reached a specific discharge capacity of up to 232 mA h g-1, surpassing the performances of the most state-of-the-art AAIBs. At 89 W kg-1, the VO2-Mo//PANI battery can achieve an energy density (E) up to 133 W h kg-1. This study provides new ideas for tailoring electrode materials with enhanced NH4+-storage for AAIBs.