Catholyte engineering to release the capacity of iodide for high‐energy‐density iodine‐based redox flow batteries

Abstract Due to the high solubility, high reversibility, and low cost of iodide, iodine‐based redox flow batteries (RFBs) are considered to have great potential for upscaling energy storage. However, their further development has been limited by the low capacity of I− as one‐third of the I− is used to form I3− (I2I−) during the charging process. Herein, we have demonstrated that the pseudohalide ion, thiocyanate (SCN−), is a promising complexing agent for catholyte of iodine‐based RFBs to free up the I− by forming iodine‐thiocyanate ions ([I2SCN]−) instead of I3−, unlocking the capacity of iodide. Applying this strategy, we have demonstrated iodine‐based RFBs with full utilization of iodide to achieve high capacity and high energy density. Both the zinc/iodine RFB and polysulfide/iodine RFB with SCN− complex agent achieve their theoretical capacity of around 160 A h Lposolyte−1 (6.0 M I− in catholyte). Therefore, the zinc/iodine RFB delivers a high energy density of 221.34 W h Lposolyte−1, and the polysulfide/iodine RFB achieves a highenergy density of 165.62 W h Lposolyte−1. It is believed that this effective catholyte engineering can be further generalized to other iodine‐based RFBs, offering new opportunities to unlock the capacity of iodide and achieve high energy density for energy storage.