pH-decoupled tin redox chemistry enables high-voltage aqueous Sn-Br batteries

Aqueous bromine batteries hold promise for safe and low-cost large-scale energy storage, but their practical energy density is limited by the restricted voltage of conventional single-electrolyte systems. Herein, we develop a pH-asymmetric aqueous Sn-Br battery by decoupling an acidic bromine catholyte from an alkaline Sn anolyte using a bipolar membrane. The acidic catholyte maintains reversible Br-/Br3- conversion, whereas the alkaline anolyte enables low-potential Sn hydroxo-complex redox chemistry. This acid-alkaline configuration converts the pH-dependent redox asymmetry of Sn into an enlarged full-cell voltage window. A surface-regulated Sn anode is further employed to improve interfacial stability and redox reversibility, while the voltage enhancement primarily originates from pH-decoupled Sn-Br redox chemistry. Potential-alignment analysis shows that switching Sn from acidic Sn/Sn2+ redox to alkaline hydroxo-complex redox increases the theoretical Sn-Br potential gap from approximately 1.2 V to above 2.0 V. In situ Ultraviolet-Visible (UV-vis) Absorption spectroscopy confirms reversible bromine-species evolution during charge/discharge, demonstrating the compatibility of bromine redox chemistry with high-voltage operation. The resulting pH-asymmetric Sn-Br full cell achieves a voltage window of up to 2.3 V, delivers a specific capacity of approximately 172.2 mAh g-1 at 3 A g-1, and maintains 98% capacity retention over 6,000 cycles with Coulombic efficiency above 97%. This study highlights pH decoupling as a powerful strategy for designing high-voltage aqueous conversion batteries based on pH-dependent metal redox chemistry.