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 – /I 0 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 – /I 0, to I 0 /ICl 2 –, and finally ICl 2 – /ICl 4 –, with the prolonged third charging plateau significantly enhancing the specific capacity to 809.2 mAh g –1 of I 2 . During discharge, the cathode undergoes highly reversible but asymmetric conversions, with ICl 3 – 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 – /I 0 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.