Polyhalide Ionic Liquid Phase-Separation Strategy Enables High-Performance Four-Electron Transfer Zinc-Iodine...

Aqueous zinc-iodine batteries are promising for grid-scale energy storage but suffer from irreversible capacity loss when pursuing the high-energy four-electron redox chemistry, primarily due to the hydrolysis of high-valent iodine species (I⁺) and severe corrosion of the zinc anode. Herein, we propose a polyhalide ionic-liquid phase-separation strategy enabled by the dual-functional additive 1-ethyl-3-methylimidazolium ([EMIm]⁺). We find that [EMIm]⁺ preferentially coordinates with the electrogenerated polyhalide [IBr₂]^- to form a hydrophobic ionic liquid (EMImIBr₂), which spontaneously separates from the aqueous electrolyte. This phase separation physically isolates I⁺ from water, effectively suppressing hydrolysis and enabling highly reversible I⁰/I⁺ conversion. Meanwhile, [EMIm]⁺ mitigates Br^--induced corrosion, guides Zn deposition along the dendrite-suppressing (002) plane, and improves plating/stripping reversibility. As a result, Zn||I₂ cells achieve a high specific capacity of 391.0 mAh g^-1 at 0.1 A g^-1 (approaching the theoretical limit of 422 mAh g^-1), with an excellent rate performance (302.4 mAh g^-1 at 3 A g^-1), and long-term cycling stability (70% capacity retention over 2000 cycles). Practical viability is demonstrated by high-loading pouch cells delivering 190 mAh and powering electronic devices.