Zinc–Iodine Batteries with Two-Plus-Two Electron Conversion Chemistry: A Focus Review

Aqueous Zn–Iodine batteries (AZIdBs) with low-valence two-electron (LV2e, I – /I 2 or I 0 ) conversion have recently gained both research and industry attention due to their cost-effectiveness, inherent safety, moderate energy output, and long cycle life. It has also been demonstrated that it is possible to extend the LV2e redox conversion reaction by activating the inherent electrochemistry gained through high-valence two-electron (HV2e, I 2 /I + ) iodine conversion. Despite these advantages, the deployment of AZIdBs has been hindered by their sluggish reaction kinetics due to iodine species interconversion, self-discharge due to I 3 – shuttling during LV2e conversion and I + hydrolysis during HV2e conversion, zinc dendrite formation and corrosion, and the poor selection of cathode hosts. Thus, issues on two-plus-two-electron conversion mechanisms and development limitations are often overlooked. Hence, this Focus Review summarizes the electrochemical mechanisms associated with AZIdBs and their present issues and possible strategies as a step toward their potential development.