On the nature of ion aggregation in EC-LiTFSI electrolytes

Abstract

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, P(n), which quantifies the probability of n anions aggregating around a cation, peaks at n = 0 for dilute salt concentrations, n = 1 for intermediate salt concentrations, and n = 2 or n = 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.