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

Zn-I₂ flow batteries, with a standard voltage of 1.29 V based on the redox potential gap between the Zn²⁺-negolyte (-0.76 vs. SHE) and I₂-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 Zn²⁺ across cation-exchange-membrane also limits their cycle stability. Herein, we propose a chelated Zn(P₂O₇)₂^6- (donated as Zn(PPi)₂^6-) negolyte, which facilitates dendrite-free Zn plating and effectively prevents Zn²⁺ crossover. Remarkably, the utilization of chelated Zn(PPi)₂^6- as a negolyte shifts the Zn²⁺/Zn plating/stripping potential to -1.08 V (vs. SHE), increasing cell voltage to 1.61 V. Such high voltage Zn-I₂ 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.