High-Voltage Manganese Oxide Cathode with Two-Electron Transfer Enabled by a Phosphate Proton Reservoir for Aqueous Zinc Batteries

MnO2 cathodes typically undergo one-electron transfer in aqueous zinc batteries. The two-electron MnO2/Mn2+ reaction provides double capacity and higher voltage. However, this requires a highly acidic environment, which challenges the Zn metal anode. Herein, we present a proton reservoir for the MnO2/Mn2+ reaction. Zinc hydrophosphate is codeposited with MnO2 at the cathode. The former deprotonates to release protons and enhances the reduction of MnO2 to Mn2+. The resulting zinc phosphate further interacts with MnO2 and realizes spontaneous water desorption from the MnO2 surface as revealed by theoretical calculations, which facilitates the dissolution process. The hydrophosphate species is reversibly generated upon charge. Based on this reaction mechanism, the cathode achieves a high discharge voltage of 1.75 V. It also delivers 0.99 mAh cm–2 capacity with 99% Coulombic efficiency. Stable capacity retention is realized for over 3000 cycles. This work demonstrates an effective strategy to access the two-electron process of MnO2 cathode materials in aqueous zinc batteries.