Electroactive ferrocene/ferrocenium redox coupling for shuttle-free aqueous zinc-iodine pouch cells

Aqueous zinc-iodine batteries are attractive for grid-scale energy storage because of their high safety and rate capability. However, conventional hosts for iodine cathodes are often electrochemically inactive and show weak interaction with polyiodides, which not only compromises the battery energy density, but also fails to eliminate shuttle effects. Here we report an electroactive redox coupling strategy to enhance energy density and suppress shuttle effects of zinc-iodine batteries by incorporating ferrocene in cathodes. The ferrocene enables a reversible ferrocene/ferrocenium redox conversion that couples with polyiodides to form insoluble ferrocenium-polyiodide complexes, thereby suppressing shuttle effects. Moreover, this redox elevates the overall discharge capacity of cathodes to 160.5 mAh g_cathode^-1 and Coulombic efficiency to over 99.5% at 1 C in coin-cell configurations. A 1.2 ampere-hour pouch cell with high-area-capacity cathodes of 8.4 mAh cm^-2 sustains 600 stable cycles at 0.5 C with an average Coulombic efficiency of 99.8%. This finding provides a strategy to address both shuttle effects and host-derived energy density loss for battery applications.