Ion Correlation and Collective Dynamics in Organic Electrolytes and Ionic Liquid Mixtures: From Dilute Solutions to the Ionic Liquid Limit*

Quantifying ion association and collective dynamics in electrolytes is essential for fundamental property interpretation and optimization for electrochemical applications. The extent of ion correlation depends on both the ion concentration and dielectric strength of the solvent; ions may be largely uncorrelated in high-dielectric solvents at low concentration, but properties of concentrated electrolytes are dictated by correlated and collective ion processes. In this work, we utilize molecular dynamics simulations to characterize ion association and collective ion dynamics in electrolytes composed of binary mixtures of BMIM⁺BF₄⁻ and various organic solvents, water, and LiBF₄ and LiTFSI salts. We illustrate different physical regimes of distinct ion correlation character. Electronic polarization and solvent dielectric controls the extent of ion pairng and clustering, changing the dominant ion correlation mechanism characterized by quantifying the fractional self and distinct contributions to the net ionic conductivity. The analysis also shed light on understanding the negative transference number observed in ionic liquid mixtures and concentrated polymer electrolytes.