Regulating the electrolyte network to accelerate reversible I-/I2Br- conversion and suppress zinc dendrite formation in advanced zinc-iodine flow batteries

Zinc-iodine flow batteries are promising candidates for large-scale electrochemical energy storage owing to their high energy density, safety, and low-cost features. However, the limited utilization of iodine species by liberating I- to stabilize I-2 and severe anodic dendrite growth are still seriously challenging the real battery performance and limiting its commercialization applications. Herein, we report a unique ternary hydrated eutectic electrolyte based on zinc bromide and acetamide to manipulate the solvent structure and realize favorable interface engineering at both electrodes simultaneously. Specifically, solvating Br- in the sheath with Zn2+ facilitates the formation of a strong halogen bond that could stabilize the elemental iodine effectively and then enhance the I-/I2Br- conversion kinetics and reversibility with the iodine reutilization efficiency reaching almost 100%. Simultaneously, such bifunctional hydrated eutectic electrolytes could also greatly minimize hydrogen evolution side reactions and direct uniform electrodeposition to suppress the anodic dendrite growth. As expected, the electrochemical performance of the designed flow cell is remarkably enhanced, delivering a specific capacity and energy density of 200 A h L-catholyte(-1) and 283 W h L-catholyte(-1), respectively, at 20 mA cm(-2) and maintaining a high energy efficiency of 83% even after 1200 hours of extended operation.