Regulated Interfacial Proton and Water Activity Enhances Mn2+/MnO2 Platform Voltage and Energy Efficiency

Electrolytic MnO2 batteries store charges via the Mn2+/MnO2 two-electron transfer process with higher capacity and voltage than conventional one-electron (Zn2+ or H+) intercalation reactions. Yet, the opposite effect of interfacial H+ on the dissolution/deposition processes and the role of interfacial H2O are rarely discussed. Here we introduce tetrafluoroborate (BF4–) into the sulfate-based electrolyte to regulate interfacial H+ and H2O activity. First, BF4– hydrolysis increases the electrolyte's acidity, promoting MnO2 dissolution. Second, BF4– forms H-bond networks with interfacial H2O that assist H+ diffusion while retaining a sufficient H2O supply to facilitate MnO2 deposition. As a result, the cathode-free Zn//MnO2 electrolytic cell achieves a high platform of ∼1.92 V and energy efficiency of ∼84.23%. Significantly, the cell delivers 1000 cycles at 1 C with ∼100% Coulombic efficiency and a high energy efficiency retention of 93.65%. Our findings disclose a new strategy to promote Mn2+/MnO2 platform voltage and energy efficiency.