Thermodynamic Control of MnO2/Mn2+ Redox Chemistry in Aqueous Batteries.

The MnO2/Mn2+ redox chemistry is attractive for energy-dense aqueous batteries, offering high theoretical capacity and favorable redox potential. However, the practical reversibility of this chemistry is severely limited by competing reaction pathways, including Mn3+ disproportionation, "dead Mn" accumulation, and parasitic reactions. Rather than focusing primarily on material-performance advances in the appealing Zn-MnO2 batteries, this review develops a thermodynamic framework for understanding MnO2/Mn2+ conversion chemistry. It emphasizes the fundamental thermodynamic principles governing MnO2/Mn2+ reaction-pathway selection, interfacial evolution, and failure mechanisms. Within this framework, controlling factors and representative strategies-including local proton and Mn2+ activities, interfacial water structure, and oxide formation/dissolution energetics-are systematically discussed through equilibrium-state, quasi-equilibrium-state, and concatenated thermodynamic regulation perspectives. Future research directions toward operando activity mapping, mediator selectivity design, and full-cell thermodynamic compatibility are also outlined. These insights aim to shift MnO2/Mn2+-based aqueous batteries from empirical optimization toward thermodynamic design rules for stable, efficient, and high-energy redox chemistry.