Six-Electron-Redox Iodine Electrodes for High-Energy Aqueous Batteries

Iodine (I₂ ) shows great promising as the active material in aqueous batteries due to its distinctive merits of high abundance in ocean and low cost. However, in conventional aqueous I₂ -based batteries, the energy storage mechanism of I^- /I₂ conversion is only two-electron redox reaction, limiting their energy density. Herein, six-electron redox chemistry of I₂ electrodes is achieved via the synergistic effect of redox-ion charge-carriers and halide ions in electrolytes. The redox-active Cu²⁺ ions in electrolytes induce the conversion between Cu²⁺ ions and I₂ to CuI at low potential. Simultaneously, the Cl^- ions in electrolytes activate the I₂ /ICl redox couple at high potential. As a result, in our case, I₂ -based battery system with six-electron redox is developed. Such energy storage mechanism with six-electron redox leads to high discharge potential and capacity, excellent rate capability, as well as stable cycling behavior of I₂ electrodes. Impressively, six-electron-redox I₂ cathodes can match various aqueous metal (e.g. Zn, Mn and Fe) anodes to construct metal||I₂ hybrid batteries. These hybrid batteries not only deliver enhanced capacities, but also exhibit higher operate voltages, which contributes to superior energy densities. Therefore, this work broadens the horizon for the design of high-energy aqueous I₂ -based batteries.