Microscopic Factors That Dictate the Optimal Compositions of Mixed Solvent Systems for Lithium Ion Battery Applications: Revelation through a Combined Experimental and Simulation Study

Optimization of the mixture composition for mixed solvent-based battery electrolyte systems and determination of the underlying governing factors are critical for the advancement of high-performance lithium-ion batteries (LIBs). In this study, we have made an attempt to identify the factors that dictate the optimal composition of a representative binary solvent mixture and electrolyte system containing 1 M LiTFSI in a mixture of ethylene carbonate (EC) and adiponitrile (ADN). For this, we have performed differential scanning calorimetry (DSC) and dielectric relaxation spectroscopy (DRS) measurements at different EC mole fractions ( X EC ). Subsequently, the molecular-level aspects related to composition optimization have been explored by examining the solvation structure, ion transport, and ion–ion dynamical correlations via molecular dynamics (MD) simulations. DSC measurements reveal a nonmonotonic dependence of the melting temperature ( T m ) on solvent composition, with a minimum value of 250 K at X EC = 0.6. Furthermore, both the ionic conductivity (σ) and average DR time (τ DR ) exhibit composition-dependent inflection points between X EC = 0.4 and 0.6 in the temperature range of 298 K–323 K. MD simulations have revealed identical self-diffusivities of cations and anions at X EC = 0.6, while both EC and ADN maintain a balanced presence inside the Li + solvation shell. Moreover, the composition dependence of the solvent coordination number inside the Li + solvation shell correlates with the experimentally observed crossovers in σ and τ DR . Notably, an optimized presence of both EC and ADN molecules inside Li + solvation shell facilitates balanced ion transport with Li + transference number t + ≈ 0.5 at X EC = 0.6, suitable for battery operation, while simultaneously suppressing the anion–anion dynamical anticorrelation.