Hydrophobic Quaternary Ammonium Iodides for Highly Stable Four−Electron Zinc−Iodine Batteries

ABSTRACT Aqueous four−electron zinc−iodine (Zn−I) batteries have attracted extensive attention due to their high capacity, material abundance, and high safety. However, the Zn−I battery suffers from poor reversibility and severe self−discharge due to the polyiodides shuttle effect and high‑valent I + hydrolysis facing by the conventional I 2 cathode. Herein, a series of organic iodides is designed by complexing I 3 − with quaternary ammonium cations with different alkyl chain lengths. Experimental and theoretical analyses reveal that these organic cations form stable complexes with I 3 − , effectively suppressing the shuttle effect of polyiodides and thereby stabilizing Zn metal anode. Meanwhile, their hydrophobic alkyl chains create a physical barrier at the electrode−electrolyte interface, limiting water access and mitigating the I + hydrolysis. Consequently, the as−designed (CH 2 ) 18 N(CH 3 ) 3 I 3 (C 18 I 3 ) cathode for four−electron Zn−I batteries manifest a high reversible capacity of 463.0 mAh g −1 at 0.5 A g −1 and excellent cycling stability with 80.0% capacity retention after 42800 cycles at 10 A g −1 , significantly outperforming the conventional I 2 @active carbon counterpart. Moreover, the reaction mechanism involving the I − /I 3 − /I 0 /I + redox couples in the C 18 I 3 cathode has been well elucidated by in situ characterizations. This work provides new guidelines for designing advanced cathode materials for Zn−I batteries.