High valence MnO₂ as an aqueous zinc ion battery cathode prepared using a secondary hydrothermal method

Aqueous zinc-ion batteries (AZIBs) have emerged as promising energy storage systems due to their inherent safety and high capacity, with manganese oxides attracting attention for their cost-effectiveness and environmental compatibility. However, the poor cycling stability of manganese-based oxides, primarily due to Jahn-Teller distortions caused by Mn³⁺, limits their practical applications. Herein, a high valence MnO₂ (H-MnO₂) material was prepared via a simple secondary hydrothermal method, yielding an increased average manganese valence from 3.31 to 3.89. A Zn/H-MnO₂ aqueous battery that utilized H-MnO₂ as a cathode achieves an exceptional capacity of 420 mA h g^-1 at 0.1 A g^-1 and retains a capacity of 92.6% after 900 cycles at 2.0 A g^-1. The structural transformation of the electrode material and changes in the elemental content during charging and discharging reveal that the H-MnO₂ electrode undergoes a chemical transformation mechanism during these processes. This work demonstrates that increasing the average manganese valence state is a critical strategy for improving both capacity and cycling stability in manganese-based AZIBs.