Bi Doping-Enhanced Reversible-Phase Transition of α-MnO₂ Raising the Cycle Capability of Aqueous Zn-Mn Batteries

Rechargeable aqueous zinc-manganese oxide batteries have attracted extensive attention in energy-storage systems owing to their high safety and low cost but still suffer from the lack of advanced cathode materials with both high capacity and a long cycle life. Here, the bismuth-doped α-MnO₂ was synthesized by a hydrothermal method. The preintercalation of Bi³⁺ effectively enlarges the lattice spacing and boosts the electrochemical performance of Zn/MnO₂ batteries. The systematical studies suggest that Bi doping significantly optimized the electrochemical behavior and especially enhanced the reversibility of dissolution-deposition and phase transition processes. As a result, the Bi-doped α-MnO₂ cathode achieves a superior performance: high reversible specific capacity (325 mA h g^-1 at 300 mA g^-1) and long cycling stability (90.9% capacity retention after 2000 cycles at 1000 mA g^-1). By comparison with the α-MnO₂ electrode, the Bi-doped α-MnO₂ electrode exhibits a longer and stabler discharge plateau. It is different from most anionic doping methods, which attribute the performance improvement to superior ion diffusion kinetics and enhanced structural stability. Therefore, this work offers a new viewpoint and approach to improve the electrochemical property of Zn/MnO₂ batteries.