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.