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

Electrolytic MnO 2 batteries store charges via the Mn 2+ /MnO 2 two-electron transfer process with higher capacity and voltage than conventional one-electron (Zn 2+ or H + ) intercalation reactions. Yet, the opposite effect of interfacial H + on the dissolution/deposition processes and the role of interfacial H 2 O are rarely discussed. Here we introduce tetrafluoroborate (BF 4 – ) into the sulfate-based electrolyte to regulate interfacial H + and H 2 O activity. First, BF 4 – hydrolysis increases the electrolyte’s acidity, promoting MnO 2 dissolution. Second, BF 4 – forms H-bond networks with interfacial H 2 O that assist H + diffusion while retaining a sufficient H 2 O supply to facilitate MnO 2 deposition. As a result, the cathode-free Zn//MnO 2 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 Mn 2+ /MnO 2 platform voltage and energy efficiency.