Reversible multielectron transfer I⁻/IO₃⁻ cathode enabled by a hetero-halogen electrolyte for high-energy-density aqueous batteries

The ever-increasing need for energy-dense batteries with high safety is fuelling global research and innovations in new redox chemistry and device design. Here we show an aqueous battery employing highly concentrated hetero-halogen electrolytes that contain I⁻ and Br^-, resulting in a multielectron transfer process of I⁻/IO₃⁻. The intermediate bromide species IBr and Br₂, generated during the electrochemical process, enhances the reaction kinetics and alleviates the potential gap between oxidation and reduction. When using a 6 M I⁻ electrolyte to achieve over 30 M electron transfers, the I⁻/IO₃⁻ cathode displayed a high specific capacity of over 840 Ah l_catholyte⁻¹. A battery with Cd/Cd²⁺ as the anode demonstrated a high energy density of over 1,200 Wh l_catholyte⁻¹. Even at an exceptionally high current density of 120 mA cm⁻², an energy efficiency of 72% was obtained. Our work demonstrates that safe aqueous batteries with high energy density are possible, offering a development option for grid-scale energy storage and even electric vehicles.