Interface-Controlled Redox Chemistry in Aqueous Mn²⁺/MnO₂ Batteries

Manganese dioxide (MnO₂) deposition/dissolution (Mn²⁺/MnO₂) chemistry, involving a two-electron-transfer process, holds promise for safe and eco-friendly large-scale energy storage. However, challenges like electrode/electrolyte interface environment fluctuations (H⁺ and H₂O activity), irreversible Mn degradation, and limited understanding of degradation mechanisms hinder the reversibility of the Mn²⁺/MnO₂ conversion. This study demonstrates a vanadyl/pervanadyl (VO²⁺/VO₂ ⁺) redox-mediated interface designed for high-energy Mn²⁺/MnO₂ batteries. Unlike flow systems, this work uncovers, for the first time, the mechanism of a static redox-mediated interface in regulating interfacial H⁺ and H₂O activities. Significantly, the VO²⁺/VO₂ ⁺ chemical redox mediation targets Mn³⁺ intermediates, suppressing their hydrolysis and enabling 100% Mn²⁺/MnO₂ conversion. The redox-mediated interface enhances the Mn redox electron transfer process, achieving a stable ≈95% coulombic efficiency and ultrahigh capacity of 100 mAh cm^{- 2} with an areal energy density of 111 mWh cm^{- 2}, outperforming flow systems. The electrode also exhibits an average specific capacity of 593 mAh g^-1, approaching the theoretical limit of 616 mAh g^-1, and a specific energy density of 721 Wh kg^-1 at high MnO₂ loadings (50-150 mg cm^-2). The findings highlight the critical role of interfacial redox mediation in regulating H⁺ and H₂O activities and underscore the significance of interface dynamics.