Effects of solvation structure, aggregation, and dynamic heterogeneity in highly concentrated electrolytes

Understanding ion transport in highly concentrated electrolytes requires establishing clear connections between local structural organization and dynamical response. Here, we performed molecular dynamics simulations of lithium and sodium bis(fluorosulfonyl)imide and fluoroborate salts (Li[FSA], Na[FSA], and LiBF4) dissolved in sulfolane and 3-methylsulfolane, over a temperature range of 350-450 K and solvent mole fractions between 0.5 and 0.91. Structural analyses show that heterogeneities in these mixtures reach a maximum near solvent mole fraction of 0.67 (corresponding to a solvent-to-salt ratio of 2:1), where the proportions of solvent and anion oxygen atoms in the cation coordination shell become comparable. At this composition, the mixtures exhibit extensive cation-anion and cation-sulfone aggregates, and the prepeak in the total x-ray structure factor (indicative of nanosegregation) reaches maximum intensity. Dynamical properties, characterized by time-correlation functions between cations and anions and between cations and solvent molecules, display non-exponential relaxation behavior. The stretching exponent shows an inflection at the same composition identified in the structural analyses. These combined structural and dynamical signatures identified a critical composition window separating vehicular and hopping transport mechanisms in sulfolane and 3-methylsulfolane. The observed crossover provides a microscopic framework for understanding concentration-dependent conductivity in sulfone-based electrolytes and offers a design guideline for tuning ion mobility through the control of the solvent-to-salt ratio.