An in-depth mechanistic insight into the redox reaction and degradation of aqueous Zn-MnO2 batteries

Rechargeable aqueous Zn/MnO2 batteries raise massive research activities in recent years. However, both the working principle and the degradation mechanism of this battery chemistry are still under debate. Herein, we provide an in-depth electrochemical and structural investigation on this controversial issue based on α-MnO2 crystalline nanowires. Mechanistic analysis substantiates a two-electron reaction pathway of Mn2+/Mn4+ redox couple from part of MnO2 accompanying with a reversible precipitation/dissolution of flaky zinc sulfate hydroxide (ZSH) during the discharge/charge processes. The formation of the ZSH layer is double-edged, which passivates the deep dissolution of MnO2 upon discharging, but promotes the electrochemical deposition kinetics of active MnO2 upon charging. The cell degradation originates primarily from the corrosion failure of metallic zinc anode and the accumulation of irreversible ZnMn2O4 phases on the cathode. The addition of MnSO4 to the electrolyte could afford supplementary capacity contribution via electro-oxidation of Mn2+. However, a high MnSO4 concentration will expedite the cell failure by corroding the metallic zinc anodes. The present study will shed a fundamental insight on developing new strategies toward practically viable Zn/MnO2 batteries.