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

Abstract

Poor cycling stability and mechanistic controversies have hindered the wider application of rechargeable aqueous Zn-MnO₂ batteries. Herein, direct evidence was provided of the importance of Mn²⁺ in this type of battery by using a bespoke cell. Without pre-addition of Mn²⁺ , the cell exhibited an abnormal discharge-charge profile, meaning it functioned as a primary battery. By adjusting the Mn²⁺ content in the electrolyte, the cell recovered its charging ability through electrodeposition of MnO₂ . Additionally, a dynamic pH variation was observed during the discharge-charge process, with a precipitation of Zn₄ (OH)₆ (SO₄ )⋅5H₂ O buffering the pH of the electrolyte. Contrary to the conventional Zn²⁺ intercalation mechanism, MnO₂ was first converted into MnOOH, which reverted to MnO₂ through disproportionation, resulting in the dissolution of Mn²⁺ . The charging process occurred by the electrodeposition of MnO₂ , thus improving the reversibility through the availability of Mn²⁺ ions in the solution.