Reversible Insertion of I–Cl Interhalogen in a Graphite Cathode for Aqueous Dual-Ion Batteries

Anion storage in cathode of dual-ion batteries provides leeway for new battery chemistries. For high energy density and better safety, it is desirable but challenging to reversibly intercalate chloride in a graphite cathode because either the oxygen or chlorine evolution reaction can prevail over chloride insertion. The primary barrier is the lack of suitable aqueous electrolytes that suppress these parasitic reactions. Herein, we report an aqueous deep eutectic solvent gel electrolyte that allows reversible chloride storage for graphite based on a chloride-based electrolyte via the formation of iodine–chloride interhalogens. The results suggest three reversible steps: iodine plating on the host surface, oxidation to form I-Cl interhalides, and then intercalation into graphite. As a result, the graphite cathode delivers a high reversible capacity of 291 mAh g–1 with stable cycling performance. Facilitated by the same mechanism, a porous graphenic carbon delivered a record-high capacity of over 1100 mAh g–1.