Halogenated Organic Acid Additive Rendered Highly Reversible Four‐Electron Transfer Zinc‐Iodine Batteries by Strengthened Interhalogen Interaction and Protons Supply

Abstract The pursuit of high‐energy‐density aqueous zinc (Zn) iodine (I 2 ) batteries has driven the focus on four‐electron I − /I 0 /I + electrochemistry, which provides high theoretical capacity and voltage plateaus. However, the practical implementation of four‐electron Zn‐I 2 batteries is significantly impeded by the inherent instability of electrophilic I + species. Herein, bromoacetic acid (BrAc) is introduced as a functional electrolyte additive to activate and stabilize high‐valence iodine via strengthened interhalogen interaction and protons supply, which guarantees the reversibility of four‐electron transfer iodine conversion. In situ Raman spectroscopy, integrated with DFT calculations, confirms that the reversibility of I 0 /I + redox couple originates from the formation and stabilization of I‐BrAc complex. In addition, BrAc can also regulate the Zn 2+ deposition to mitigate the anode corrosion and dendritic growth. Thanks to these merits, the assembled Zn−I 2 batteries achieve exceptional cycling stability, in terms of the maintenance of 303 mA h g −1 (96% retention) after 6000 cycles at 4 A g −1 . This strategy provides a viable route for exploring high‐capacity and reversible zinc iodine batteries.