Construction of MnO₂ Polymorphs via Trace Potassium Ion Doping for High-Performance Zinc-Ion Batteries

Manganese dioxide (MnO2) emerges as a promising cathode active material for aqueous zinc-ion batteries (AZIBs) due to its inherent safety and environmental friendliness. However, various crystalline forms of MnO2 (e.g., α- or β-MnO2) exhibit low structural stability or limited ion diffusion, resulting in poor cycling stability, particularly under high current density. Herein, a MnO2 polymorph (PM-MnO2) is constructed via the in situ doping of trace potassium ions (K+) into β-MnO2, remarkably addressing these challenges. By leveraging the high structural stability of the β-MnO2 phase and the facilitated ion diffusion in α-MnO2 phases, PM-MnO2 with a low atomic ratio of K to Mn of only 0.02 demonstrates excellent electronic and ionic conductivity, thereby achieving enhanced charge storage kinetics as a cathode material. The assembled AZIBs exhibit a high specific capacity of 295.3 mA h g–1 at a charge–discharge rate of 10 C (1 C = 308 mA h g–1), with an average capacity decay rate of only 0.017% per cycle over 3000 cycles, indicating significant potential for practical applications. This research provides experimental insight into how K+ doping affects the crystal structure of MnO2 and highlights the crucial role of polymorphic materials in enhancing the performance of MnO2-based AZIBs.