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

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.