What Dictates the Optimal Concentration of Li-Based Battery Electrolytes? A Combined Experimental and Simulation Study

The performance of lithium-ion battery (LIB) electrolytes is governed by the complex interaction between Li+ ions and solvent molecules. Usually, a mixture of two organic solvents with certain composition are used to form the non-aqueous LIB electrolytes. In this study, we investigate what dictates the optimal solvent composition for LIB electrolytes. In this contest, we systematically explore the co-solvent (ethylene carbonate, EC) dependent phase transition, dielectric relaxation, structural, and transport properties of 1M LiTFSI in xEC+(1-x)ADN electrolytes using combination of differential scanning calorimetry (DSC), dielectric relaxation spectroscopy (DRS), and classical molecular dynamics (MD) simulation. From DSC analysis, the lowest melting temperature (250K) was found at 0.6 EC mole fraction electrolyte. This could enable extended low-temperature operation at this composition. Conductivity measurements exhibit an inflection point at the same 0.6 EC mole fraction, which correlates remarkably with solvent relaxation dynamics observed in DRS studies. MD simulations offer molecular-level insights into ion solvation structure and coordination number, self-diffusion, and correlated ion-ion motion, highlighting the influence of dielectric screening, ion-solvent interactions, and ion-ion correlations in governing the structure-dynamics relationship in the electrolytes. We have also shown that the 0.6 mole fraction EC composition electrolyte achieves an optimal balance between structural stability, ion transport efficiency, and ionic conductivity. These findings establish key molecular level insights for successful electrolyte engineering to enhance the operational stability and efficiency of LIBs.