Cyclable Manganese Inventory as a Mechanistic Descriptor for Electrolyte Design in Rechargeable Aqueous Zn–MnO2 Batteries

Abstract

The cycling stability of rechargeable aqueous Zn–MnO2 batteries is constrained by the irreversible loss of active manganese species during charge–discharge cycling. Here, the “cyclable manganese inventory” is defined as the total amount of manganese species that can repeatedly undergo Mn2+ oxidation/deposition to MnO2 during charging and MnO2 reduction/dissolution back to Mn2+ during discharging. Manganese acetate(Mn(CH3COO)2) is demonstrated as an effective electrolyte modifier that enhances reversible manganese utilization through Mn2+ supply and acetate-assisted MnO2 deposition. Consequently, the Mn(CH3COO)2-containing electrolyte delivers a peak specific discharge capacity of 320.7 mAh g-1, substantially exceeding that of the conventional MnSO4-containing electrolyte. To clarify how electrolyte chemistry regulates the cyclable manganese inventory, the Mn(CH3COO)2 system is compared with an isolated acetate(CH3COO-)-containing system and a citric acid (C6H8O7)-containing system. CH3COO-consumes protons and suppresses MnO₂ reduction, thereby decreasing the amount of manganese available for reversible cycling. In contrast, C6H8O7 regulates proton availability and suppresses basic zinc sulfate hydroxide (ZSH)-related side reactions, helping to maintain the cyclable manganese inventory during extended cycling. These results support the cyclable manganese inventory as a mechanistic framework for understanding electrolyte-dependent capacity evolution in rechargeable aqueous Zn–MnO2 batteries.