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

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 iodinebased 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 L_posolyte^-1 (6.0MI^- in catholyte). Therefore, the zinc/iodine RFB delivers a high energy density of 221.34Wh L_posolyte^-1, and the polysulfide/iodine RFB achieves a highenergy density of 165.62Wh L_posolyte^-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. [ABSTRACT FROM AUTHOR]