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

Abstract

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.