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

Four-electron Zn–I2 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–/I0/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 I0/I+ oxidation and their intrinsic chemical instability. In this Perspective, we elucidate the SN2-type nucleophilic mechanism as a fundamental principle for activating the I0/I+ conversion and propose that the instability of I+ species originates from shuttling effects and hydrolysis reactions. Based on these insights, feasible strategies for I0/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–I2 batteries.