Superhalide structure and iodide-proof interphase via electrolyte regulation enable ultrastable zinc-iodine batteries

Challenges lie in the sluggish redox kinetics and uncontrolled side reactions of traditional aqueous zinc–iodine batteries, making them inferior to other battery congeners. Herein, we introduce a superhalide solvation structure and an iodide-proof solid electrolyte interphase to realize a stable iodide redox reaction and zinc plating/stripping. This is achieved by electrolyte optimization synergy using an organic iodide source and a hydroxyl solvent. Notably, the dissociative electron-donating I⁻ anions can participate in the zinc solvation sheath and coordinate in the form of a superhalide, which increase electron transfer to Zn²⁺ and reduce the electron loss of the solvent, thus enhancing the reduction stability of the electrolyte. Meanwhile, a solvation-complex-triggered, inorganic/organic-rich, in situ-formed interphase featuring iodide-proof ability is induced, thereby suppressing the side reactions involved with the free shuttling of iodide and facilitating dendrite-free zinc deposition. The combination of metrics endows the battery with a superior rate performance of 127 mA h g⁻¹ at 5.0 A g⁻¹ and a high capacity retention of 86% for long-term 45 000 cycles. Finally, reversible operation is also obtained under practical conditions, including a small N/P of 4.7 or a low temperature of −18 °C. Our work provides new insights into fine-tuning electrolyte formulation for reliable halide conversion.