Engineering Oxygen Vacancies and Ion Diffusion Channels via Ni2+/K+ Co-Doping for Aqueous Ammonium-Ion Batteries

In ammonium-ion batteries, manganese dioxide (MnO2) exhibits promising ammonium storage capabilities but suffers from inherent limitations such as poor electrical conductivity and structural instability. To address these challenges, this study proposes a cation doping strategy involving individual (Ni2+ or K+) and dual doping strategies. The incorporation of these dopants significantly enhances the ammonium storage performance of MnO2. Single doping (Ni2+ or K+) and dual doping strategies significantly improve the ammonium storage performance of MnO2. Ni2+ doping induces oxygen vacancies and modulates the electronic structure, extending the cycling lifespan to 727 cycles (60% capacity retention) and reducing charge transfer resistance to 9.42 Ω. K+ doping forms a KMn8O16 phase with 2 × 2 tunnel structures, elevating the NH4+ diffusion coefficient to 10−10~10−9 cm2 s−1 and improving rate capability (73.75 mAh g−1 at 1 A g−1). The dual-doped system (NiK-MnO2) achieves a specific capacity of 165.94 mAh g−1 at 0.1 A g−1 through synergistic effects. It maintains 60% capacity retention after 917 cycles at 1 A g−1 with an impedance of 9.08 Ω and a resistivity of 3.02 mΩ cm. Dual doping improves the electrical conductivity of the electrode material and enhances the ammonium storage performance of MnO2 through a synergistic mechanism of oxygen vacancies and tunneling structure.