Rethinking the Four-Electron Iodine Redox Mechanism in Zn–I ₂ Batteries

Four-electron Zn–I 2 batteries (FEZIBs) have garnered increasing research interest due to their potential in grid-scale energy storage. Compared with the energy density (<300 Wh kg –1 ) of conventional two-electron iodine cathodes, the I – /I 0 /I + redox pair endows four-electron iodine cathodes with an energy density of >600 Wh kg –1 . However, the formation of I + species is hindered by the high energy barrier of I 0 /I + oxidation and their intrinsic chemical instability. In this Perspective, we elucidate the S N 2-type nucleophilic mechanism as a fundamental principle for activating the I 0 /I + conversion and propose that the instability of I + species originates from shuttling effects and hydrolysis reactions. Based on these insights, feasible strategies for I 0 /I + activation and I + species stabilization are proposed. Moreover, we outline the key targets, opportunities, and challenges for the future development of FEZIBs. This Perspective may provide insights into further advancement of Zn–I 2 batteries.