Ion Correlation and Collective Dynamics in BMIM/BF_4 Based Organic Electrolytes: From Dilute Solutions to the Ionic Liquid Limit

Quantifying ion association and collective dynamical processes in organic 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 sufficiently 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 organic electrolytes composed of binary mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM^+][BF^4–] and 1,2-dichloroethane, acetone, acetonitrile, and water solvents. We illustrate different physical regimes of characteristically distinct ion correlations for the systematic range of electrolyte concentrations and solvent dielectric strengths. Dilute electrolytes composed of low-dielectric solvents exhibit significant counterion correlation in the form of ion pairing and clustering driven by both weak screening and relatively low solvation energies. This regime is characterized by enhanced ion coordination numbers and near equality of cation and anion diffusion coefficients, despite the significantly different ion sizes. In contrast, ion correlation in highly concentrated electrolytes is dominated by the anti-correlated motion of both like-charge and opposite-charge ions, approaching neat ionic liquid behavior. We show that the cross-over of these correlation regimes is clearly illuminated by quantifying the fractional self and distinct contributions to the net ionic conductivity. For organic electrolytes composed of low-dielectric solvents, we conclude that significant ion correlation exists at all concentrations but the nature of the correlation changes markedly from the dilute electrolyte to the pure ionic liquid limit.