Unlocking the capacity of iodide for high-energy-density zinc/polyiodide and lithium/polyiodide redox flow batteries

Highly soluble iodide/triiodide (I-/I-3(-)) couples are one of the most promising redox-active species for high-energy-density electrochemical energy storage applications. However, to ensure high reversibility, only two-thirds of the iodide capacity is accessed and one-third of the iodide ions act as a complexing agent to stabilize the iodine (I-2), forming I-3(-)(I2I(-)). Here, we exploit bromide ions (Br-) as a complexing agent to stabilize the iodine, forming iodine-bromide ions (I2Br-), which frees up iodide ions and increases the capacity. Applying this strategy, we demonstrate a novel zinc/iodine-bromide battery to achieve an energy density of 101 W h Lposolyte+negolyte 1 (or 202 W h L-posolyte 1), which is the highest energy density achieved for aqueous flow batteries to date. This strategy can be further generalized to nonaqueous iodide-based batteries (i.e. lithium/polyiodide battery), offering new opportunities to improve high-energy iodide-based energy storage technologies.