High-voltage and dendrite-free zinc-iodine flow battery

Abstract Zn-I 2 flow batteries, with a standard voltage of 1.29 V based on the redox potential gap between the Zn 2+ -negolyte (−0.76 vs. SHE) and I 2 -posolyte (0.53 vs. SHE), are gaining attention for their safety, sustainability, and environmental-friendliness. However, the significant growth of Zn dendrites and the formation of dead Zn generally prevent them from being cycled at high current density (>80 mA cm −2 ). In addition, the crossover of Zn 2+ across cation-exchange-membrane also limits their cycle stability. Herein, we propose a chelated Zn(P 2 O 7 ) 2 6- (donated as Zn(PPi) 2 6- ) negolyte, which facilitates dendrite-free Zn plating and effectively prevents Zn 2+ crossover. Remarkably, the utilization of chelated Zn(PPi) 2 6- as a negolyte shifts the Zn 2+ /Zn plating/stripping potential to −1.08 V (vs. SHE), increasing cell voltage to 1.61 V. Such high voltage Zn-I 2 flow battery shows a promising stability over 250 cycles at a high current density of 200 mA cm −2 , and a high power density up to 606.5 mW cm −2 .