The Halogen Bond Chemistry in Zn–I Redox Flow Batteries

Zinc–iodine redox flow batteries (ZIRFBs) are promising for safe and sustainable grid storage but are limited by low capacity and voltage due to inefficient iodine utilization. We address this by designing an electrolyte with cost-effective Br–/Cl– redox-mediators that enable a two-electron transfer process (I–/I0/I+). Experimental studies confirm polyhalide intermediates (IX2–) stabilize I+ species, facilitating complete 2e– redox with elevated discharge voltages up to 1.7 V. Crucially, Br– confers higher stability through enhanced I+···Br– halogen bonding, as its diffuse 4p orbital exhibit greater overlap with the vacant 5p orbital of I+ than the 3p orbital of Cl–. Thus, employing 1 M I– with Br–, the ZIRFB delivers a capacity of 53 Ah L–1 at 40 mA cm–2 with an energy density of 65.8 Wh L–1 and a prolonged cyclability over 100 cycles. This halogen-mediated strategy unlocks 100% iodine utilization and enhanced voltage, offering a route toward high-energy aqueous flow batteries.