Unravelling the Mechanism of Rechargeable Aqueous Zn-MnO2 Batteries: Implementation of Charging Process by Electrodeposition of MnO2

Poor cycling stability and mechanistic controversies have hindered the wider application of rechargeable aqueous Zn-MnO2 batteries. Herein, we provide direct evidence of the importance of Mn2+ in this type battery by using a bespoke cell. Without pre-addition of Mn2+, the cell exhibits an abnormal discharge-charge profile, meaning it functions as a primary battery. By adjusting the Mn2+ content in the electrolyte, the cell recovers its charging ability, via electrodeposition of MnO2. Additionally, a dynamic pH variation has been observed during the discharge-charge process, with a precipitation of Zn4(OH)6(SO4)· 5H2O buffering the pH of the electrolyte. Contrary to the conventional Zn2+ intercalation mechanism, MnO2 is first converted into MnOOH, which reverts to MnO2 via disproportionation, resulting in the dissolution of Mn2+. The charging process occurs by the electrodeposition of MnO2, thus improving the reversibility through the availability of Mn2+ ions in the solution.