An Fe³⁺-Responsive MnO₂/Mn²⁺ Cathode for Zinc-Ion Batteries: Fe³⁺ Incorporated into MnO₂ during Charging Accelerates its Dissolution during Discharging

We present a novel MnO2/Mn2+ cathode system enabling reversible two-electron charge/discharge cycling in a mildly acidic, acetate-buffered electrolyte. In a three-electrode configuration using carbon cloth as the working electrode, highly reversible MnO2-exclusive dissolution is achieved at low loadings. However, incomplete dissolution at higher MnO2 loadings leads to material accumulation on the electrode surface. The addition of Fe3+ significantly enhances discharge kinetics. During charging, Fe3+ incorporates into the MnO2 matrix, functioning as a solid-phase redox mediator to promote complete MnO2 dissolution, even in the absence of Fe3+ in the bulk electrolyte. Fe3+ remains electrochemically inert above +0.4 V vs Ag/AgCl, minimizing self-discharge. Structural and spectroscopic analyses reveal that MnO2 partially reduces to passivating Mn3+ oxides (e.g., Mn2O3 or Mn3O4) under proton-limited conditions. Residual Mn3+ phases are reduced to Mn2+ via Fe2+ (formed cathodically below +0.4 V), completing dissolution. The resulting Fe2+ negative redox potential chemically reduces the Mn3+ oxides. Reoxidation Fe3+ is electrochemically reduced at the electrode, sustaining the mediation cycle until complete Mn oxide dissolution. In a custom Zn–carbon felt two-electrode cell, the system achieves over 90% Coulombic efficiency (5 mAh cm–2) with 909 Wh kg–1 energy and 364 W kg–1 power densities (based on the cathode active material (MnO2)).