Reversible Multi-electron Transfer I-/IO3- Cathode enabled by Hetero-halogen for Ultra-high Energy Density Aqueous Batteries

The ever-increasing need for more energy-dense batteries is fuelling global research and innovations in new redox chemistry and device design. Lithium-ion batteries adopting organic electrolytes have the potential to deliver high energy densities, however, they usually have to compromise on safety‒a metric of equal importance. Here, we show an aqueous battery that exhibits a volumetric energy density far beyond the state of the art, together with long cycling life and excellent power performance. The high energy density is realized by employing highly concentrated hetero-halogen electrolytes that contain I- and Br-, which can realize a multi-electron transfer process of I-/IO3- with high reversibility and excellent kinetics. The intermediate bromide species can enhance the reaction kinetics and alleviate overpotential, leading to reversible and fast multi-electron transfer. Using a 6 M halogen electrolyte to achieve more than 30 M electron transfers per litre (the highest reported so far is 9 M), the I-/IO3- cathode displayed super high specific capacity of 830 Ah L-1. A battery with Cd/Cd2+ as anode demonstrated an energy density of >1200 Wh L-1catholyte, which is by far the highest value for aqueous batteries. Even at a current density of 120 mA cm-2 an energy efficiency of 72% can be obtained. Our work demonstrates that safe aqueous batteries with high energy density are possible, offering a new and evolutional option for grid scale energy storage and electric vehicles.