A redox-mediated zinc electrode for ultra-robust deep-cycle redox flow batteries

Zinc-based redox flow batteries are regarded as one of the most promising electricity storage systems for large-scale applications. However, dendrite growth and the formation of "dead zinc" at zinc electrodes particularly at high current density and large areal capacity impede their long-term operation. Here, we report redox-mediated zinc chemistry along with extensive kinetics studies to adequately address these issues under alkaline conditions. A phenazene derivative, 7,8-dihydroxyphenazine-2-sulfonic acid, which is used as the redox mediator in the anolyte, can effectively react with the "dead zinc" and recover the lost capacity, thus leading to drastically enhanced cycling stability. Based on this strategy, alkaline zinc-iron flow batteries using zinc as the anode and ferricyanide as the catholyte active species demonstrated extraordinary cycling performance at high zinc loading of up to 250 mA h cm −2 and near unity utilization. Particularly, a cell with 152 mA h cm −2 zinc areal capacity could operate at near 100% depth of discharge and a current density of 50 mA cm −2 for more than 1500 hours with a capacity fading rate of 0.019% per day (0.0048% per cycle). We believe that this work provides a credible way to ultimately address the "dead zinc" issue for ultra-robust and deep-cycle zinc-based redox flow batteries. Redox-mediated zinc chemistry is proposed to ultimately solve the "dead zinc" problem in zinc-based alkaline flow batteries.