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.