Linking Solvation Equilibrium Thermodynamics to Electrolyte Transport Kinetics for Lithium Batteries

Correlating the solvation structure and thermodynamic properties with transport properties serves as the foundation for electrolyte design. While various physicochemical properties, such as relative solvating power, solvation energy, and spectroscopies have been used to study ion solvation, fundamental investigations in thermodynamic properties of solvation equilibrium across broad temperature ranges are not available. In this work, we combined temperature-resolved Infrared and Raman spectroscopies to systematically pinpoint the dynamic evolution of Li⁺-solvent and Li⁺-anion local coordination in typical ether and carbonate electrolytes from -60 to 60 °C. We identified a trend of temperature-driven equilibrium among electrolyte components. As the temperature increases, solvent-separated ion pairs (SSIP) are prone to converting to contact ion pairs (CIP), and CIP reverts to SSIP reversibly as the temperature decreases. By quantifying the temperature-responsive mean coordination number and solvate species concentrations, we reveal a preferential CIP association in carbonates compared to that in ethers. Gibbs free energy changes in diverse electrolytes exhibit a strong correlation with their respective Li⁺ transference number. The thermodynamic properties of solvation equilibrium offer new descriptors for quantifying dynamic solvation structure, and the solvation-property knowledge gained from these model electrolytes can serve as a benchmark reference for a broad spectrum of battery electrolytes.