Unravelling the Mechanism of Rechargeable Aqueous Zn–MnO ₂ Batteries: Implementation of Charging Process by Electrodeposition of MnO ₂

Abstract

Poor cycling stability and mechanistic controversies have hindered the wider application of rechargeable aqueous Zn–MnO ₂ batteries. Herein, direct evidence was provided of the importance of Mn ²⁺ in this type of battery by using a bespoke cell. Without pre‐addition of Mn ²⁺, the cell exhibited an abnormal discharge–charge profile, meaning it functioned as a primary battery. By adjusting the Mn ²⁺ content in the electrolyte, the cell recovered its charging ability through electrodeposition of MnO ₂. Additionally, a dynamic pH variation was observed during the discharge–charge process, with a precipitation of Zn ₄(OH) ₆(SO ₄)⋅5H ₂O buffering the pH of the electrolyte. Contrary to the conventional Zn ²⁺ intercalation mechanism, MnO ₂ was first converted into MnOOH, which reverted to MnO ₂ through disproportionation, resulting in the dissolution of Mn ²⁺. The charging process occurred by the electrodeposition of MnO ₂, thus improving the reversibility through the availability of Mn ²⁺ ions in the solution.

Abstract

Taking charge: This work provides direct evidence of the importance of Mn ²⁺ in rechargeable aqueous Zn–MnO ₂ batteries. The charging process is implemented by the electrodeposition of MnO ₂, thus improving the reversibility by the availability of Mn ²⁺ in the solution. A conversion reaction between MnO ₂ and MnOOH occurs, rather than Zn ²⁺ intercalation into MnO ₂.