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

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 I₂ cathode. Herein, a series of organic iodides is designed by complexing I₃ ^- with quaternary ammonium cations with different alkyl chain lengths. Experimental and theoretical analyses reveal that these organic cations form stable complexes with I₃ ^-, 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 (CH₂)₁₈N(CH₃)₃I₃ (C₁₈I₃) 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 I₂@active carbon counterpart. Moreover, the reaction mechanism involving the I^-/I₃ ^-/I⁰/I⁺ redox couples in the C₁₈I₃ cathode has been well elucidated by in situ characterizations. This work provides new guidelines for designing advanced cathode materials for Zn-I batteries.