On the nature of ion aggregation in EC-LiTFSI electrolytes

We investigate the structural and dynamic properties of concentrated ethylene carbonate (EC)-LiTFSI (lithium bis(trifluoromethanesulfonyl)imide) electrolytes using molecular dynamics (MD) simulations to elucidate the molecular mechanisms governing ion aggregation and transport. Increasing salt concentration induces a transition in the local solvation environment, marked by reduced radial distribution functions for ion-ion and ion-solvent interactions. This shift reflects the formation of ion pairs and larger ionic clusters, altering electrostatic interactions and weakening Li⁺-EC solvation. Ion aggregation probability, <i>P</i>(<i>n</i>), which quantifies the probability of <i>n</i> anions aggregating around a cation, peaks at <i>n</i> = 0 for dilute salt concentrations, <i>n</i> = 1 for intermediate salt concentrations, and <i>n</i> = 2 or <i>n</i> = 3 for high salt concentrations. These structural changes significantly impact dynamics, as ion aggregation slows ion mobility and reduces diffusion coefficients for Li⁺ and TFSI^- ions. We observe strong correlations between ion diffusion, ion-pair relaxation times, and viscosity signifying the interplay between ion pairing, cluster formation, and mobility. This study provides molecular-level insights into how salt concentration influences ionic transport, advancing the theoretical framework for transport in dense liquid systems and guiding the design of advanced electrolytes.