Elucidating and tackling capacity fading of zinc-iodine redox flow batteries

Abstract

As novel and rapidly growing battery technologies, zinc-iodine redox flow batteries (ZIFB) with high energy density exhibit great potential for large-scale energy storage. However, their capacity fade and elusive operational instability over charge-discharge cycling severely hinder their commercialization. Herein, the capacity fade in ZIFBs is investigated by systematically evaluating electrochemical performance and electrolyte properties. It is found that the differential hydraulic pressure at both sides of the porous separator leads to colossal electrolyte transport from catholyte to anolyte via convection. Consequently, an accumulation of (poly)iodide at the negative side is established as cycling proceeds, leading to substantial capacity fade of the flow cells. To remediate the capacity fade, an effective strategy is proposed by adjusting electrolyte flow rate ratios to regulate the induced convection by balancing the hydraulic pressure. Theoretical calculations and experimental analysis confirm that an asymmetric flow rate condition drastically inhibits catholyte transport and (poly)iodide crossover. Therefore, a strategically designed ZIFB with an optimal catholyte to anolyte flow rate ratio of 1 to 7 is able to achieve an energy efficiency (EE) of 82% and cycle life of 1,100 cycles at high current density of 80 mA cm−2, which is the highest performance of all the reported ZIFBs. The deep insight gained into the capacity fade mechanism and the proposed methodology to sustain capacity substantially benefit the commercialization of flow batteries.