Copper activated near-full two-electron Mn4+/Mn2+ redox for mild aqueous Zn/MnO2 battery

Compared with traditional one-electron Mn4+/Mn3+ redox couple (308 mAh g-1), two-electron Mn4+/Mn2+ redox mechanism doubles the theoretical capacity (616 mAh g- 1) of Zn/MnO2 batteries. However, in most cases, activating the Mn4+/Mn2+ redox couple requires a low pH value, which often leads to corrosion issues. Herein, a binder-free Cu-doped epsilon-MnO2 (CMO) was synthesized through a facile electrodeposition method and the two -electron Mn4+/Mn2+ redox is achieved in near-neutral ZnSO4 electrolyte with a working potential window of 0.8-1.9 V. Detailed characterizations demonstrate that the synergistic effect between Cu and Mn can boost the further reduction of Mn4+ and Mn3+ to dissoluble Mn2+ below 1.1 V. Compared with the capacity of epsilon-MnO2 (MO, 282.4 mAh g(-1)), which mainly involves Mn4+/Mn(3+)redox couple, the CMO cathode demonstrates an improved specific capacity of 397.4 mAh g-1 at 0.2 A g(-1). Besides, the fast reaction kinetics of H+ insertion, Mn dissolution, and Mn deposition reactions endow CMO with excellent rate performance, achieving a high specific capacity of 298.9 mAh g- 1 at 5 A g- 1.