Cyclable manganese inventory as a mechanistic descriptor for electrolyte design in rechargeable aqueous Zn-MnO₂...

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.