Solvent Chemistry Manipulated Iodine Redox Thermodynamics For Durable Iodine Batteries

The diverse valences of iodine enable it with multi-electron transfer capability for energy dense batteries. However, previous studies indicate that the primary I^-/I₂ redox couple exhibits distinct behaviors depending on electrolyte choice, with the mechanistic basis of aqueous versus nonaqueous systems remaining unclear. Here, we elucidated the solvent effect on iodine redox, particularly focusing on polyiodide formation and their molecular interaction correlations. We validate that a thermodynamically one-step conversion reaction (I₂ ↔ I^-) occurs in the protic solvents, while it is a two-step transformation (I₂ ↔ I₃ ^- ↔ I^-) in aprotic solvents. This distinction arises from strong electron-donating properties in aprotic solvents that facilitate charge transfer with iodine, promoting complexation with iodide as solvent⋅I₃ ^- species. Conversely, protic solvents form additional hydrogen bonds with iodine, alleviating polarization and reducing interaction with iodide. Furthermore, to address the limitations of single protic electrolytes - characterized by sluggish dissolution-precipitation and slow ion migration rates - we propose a hybrid electrolyte combining water and ethylene glycol. These hybrids enhance iodine redox kinetics, inhibits I₃ ^- generation, and modifies the Zn²⁺ solvation structure to mitigate zinc anode corrosion and dendrite. The Zn-I₂ batteries demonstrates exceptional long-term cycling stability in a wide temperature range, highlighting its potential for practical applications.