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 ε-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 ε-MnO₂ (MO, 282.4 mAh g^-1), which mainly involves Mn⁴⁺/Mn³⁺ 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.