Enhancing Salt Diffusivity of Battery Electrolytes: Insights from Dynamic Ion Correlation Analysis

Playing a critical role in electrolyte ion transport, ion correlations influence concentration polarization and ultimately contribute to the rate performance of Li-ion batteries. This study demonstrates that introducing monoglyme co-solvent into a carbonate-based electrolyte enhances salt diffusivity (D_salt) by 30%, effectively mitigating concentration polarization and improving the rate capability of a LiCoO₂ symmetric cell by 70% at 4C operation. Dynamic ion correlation analysis reveals that the improvement in D_salt is driven by increased positive self-correlations of cations and anions, which represent enhanced random movement of ions, facilitated by reduced viscosity and smaller solvation shell sizes. Conversely, introducing diglyme co-solvent reduces D_salt and deteriorates the rate capability. Although diglyme also improves self-correlations through reduced viscosity, its stronger Li-ion solvation increases cation-cation anti-correlation, resulting in an unfavorable balance between the correlation terms that suppresses overall salt diffusivity. These findings underscore the significant influence of microscopic ion correlations on macroscopic transport properties, a crucial aspect in optimizing battery performance.