Thermodynamic Control of MnO 2 /Mn 2+ Redox Chemistry in Aqueous Batteries

ABSTRACT The MnO 2 /Mn 2+ 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 Mn 3+ disproportionation, “dead Mn” accumulation, and parasitic reactions. Rather than focusing primarily on material‐performance advances in the appealing Zn‐MnO 2 batteries, this review develops a thermodynamic framework for understanding MnO 2 /Mn 2+ conversion chemistry. It emphasizes the fundamental thermodynamic principles governing MnO 2 /Mn 2+ reaction–pathway selection, interfacial evolution, and failure mechanisms. Within this framework, controlling factors and representative strategies—including local proton and Mn 2+ 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 MnO 2 /Mn 2+ ‐based aqueous batteries from empirical optimization toward thermodynamic design rules for stable, efficient, and high‐energy redox chemistry.