Dual-ion pre-intercalation in intralayer and interlayer sites for enhanced ammonium‑ion storage performance of MnO2

Abstract

MnO₂ has emerged as a promising electrode material for aqueous ammonium ion supercapacitors due to its low cost and high theoretical capacitance. However, its practical application is hindered by inherently low electrical conductivity, insufficient practical capacitance, and poor structural stability during cycling. Herein, a synergistic optimization strategy is developed via intralayer and interlayer Mo/NH₄⁺ dual-ion pre-intercalation. Intralayer Mo-doping modulates the microstructure and crystal structure of MnO₂, increases active sites, and enhances conductivity through the introduction of oxygen vacancies. Meanwhile, in-situ electrochemical activation is employed to pre-intercalate NH₄⁺ ions, which further elevates the oxygen vacancy concentration and significantly improves the structural stability of the material. The optimized Mo-MnO₂/AC electrode delivers a high specific capacitance of 668.5 F g^-1 at 2 mA cm^-2 and retains 97.92% of its initial capacity after 10,000 cycles at 25 mA cm^-2. An asymmetric supercapacitor assembled with Mo-MnO₂/AC as the positive electrode and activated carbon cloth as the negative electrode achieves an energy density of 1.52 mWh cm^-2 at a power density of 2 mW cm^-2. This work provides a synergistic pre-intercalation strategy for developing high-performance electrode materials for ammonium-ion energy storage.