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

Aqueous zinc battery (AZB) is one of the most promising energy storage systems for large-scale applications due to its high safety, low cost, and environmental friendliness. To match the high capacity of zinc metal (~820 mAh g -1 ), various cathode materials, such as oxides, open structure analogues, chalcogens, and halogens, have been explored for AZBs. Among them, manganese dioxide (MnO 2 ) has been considered to be one of the most attractive candidates due to its high capacity (~308 mAh g -1 for one-electron transfer) and adequate redox potential (~0.70 V vs. SHE). Recently, the MnO 2 dissolution/deposition charge storage mechanism has been demonstrated to further increase the voltage and capacity of Mn-based AZBs. The dissolution/deposition (Mn 2+ /MnO 2 ) is a two-electron transfer process, which doubles the theoretical capacity of MnO 2 cathode to ~616 mAh g -1 and considerably increases the redox potential (~1.229 V vs. SHE). The charge storage via the dissolution/deposition mechanism pushes the aqueous-based energy density even higher, to over 1000 Wh kg -1 . However, this charge storage mechanism involves liquid-solid transition, as well as the thermodynamics and kinetics of this reaction are highly dependent on the acidity and interfacial environment. 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 anion (BF 4 - ) into the sulfate-based electrolyte to regulate the interfacial H + and H 2 O activity. First, BF 4 - hydrolysis increases the electrolyte’s acidity, promoting MnO 2 dissolution. Second, BF 4 - forms an H-bond network with interfacial H 2 O that assists H + diffusion while retaining sufficient H 2 O supply to facilitate MnO 2 deposition. As a result, the cathode-free Zn//MnO 2 electrolytic cell achieves a high plateau of ~1.92 V and energy efficiency of ~84.23 % in the BF 4 - containing electrolyte. 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 for future large-scale energy storage systems.