Microscopic Factors That Dictate the Optimal Compositions of Mixed Solvent Systems for Lithium Ion Battery...

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.