Discovery of High-Capacity Asymmetric Three-Stage Redox Reactions of Iodine for Aqueous Batteries

Iodine-based batteries have emerged prominently in grid energy storage due to their cost-effectiveness and versatility. However, traditional iodine cathodes featuring I^-/I⁰ mechanisms struggle to meet the current demands for high-energy-density batteries, considering their limited specific capacity and voltage. Here, we discover a unique eight-electron-transfer asymmetric three-stage conversion of iodine facilitated by the formation of interhalogens. This mechanism involves a three-stage sequential charging from I^-/I⁰, to I⁰/ICl₂^-, and finally ICl₂^-/ICl₄^-, with the prolonged third charging plateau significantly enhancing the specific capacity to 809.2 mAh g^-1 of I₂. During discharge, the cathode undergoes highly reversible but asymmetric conversions, with ICl₃^- as the intermediate. The mechanism is achieved by a regulated "chloride-in-acid" electrolyte with interlocking H-bond structures, which effectively reduces the free water content and stabilizes the interhalogen species. The iodine-hydrogen gas battery demonstrates stable cycling performance with an average Coulombic efficiency exceeding 98.2% for over 1000 cycles and an increased voltage from 0.47 to 0.75 V compared with the I^-/I⁰ mechanism, which can be further enhanced to 1.43 V by utilizing zinc anode. This study broadens the application of interhalogen chemistry into conversion reactions, presenting great prospects for high-energy-density aqueous batteries.