Diffusion and Ion-Ion Correlations in EC-LiTFSI Electrolytes

Liquid electrolytes with high salt concentrations offer better chemical and thermal stability for rechargeable battery applications. In this work, we study the effect of salt concentration on the diffusion and ion-ion correlations in ethylene carbonate lithium bis(trifluoromethane sulfonyl)imide (EC-LiTFSI) electrolytes using molecular dynamics simulations. Our simulations show that the ionic diffusivities and Nernst-Einstein conductivity decrease with the concentration of the LiTFSI salt. We also investigated the influence of salt concentration on the structural properties by calculating the radial distribution functions and coordination numbers for different pairs, such as ion-ion and ion-EC. As the salt concentration increases, we observe a systematic decrease in the peak of radial distribution functions and an increase in the corresponding coordination numbers for the Li-TFSI pairs. Consistent with the structural features, the ion-pair time correlation functions are observed to decay slowly and monotonically with the addition of salt. We further investigated the interplay of different parameters that affect diffusivity and provide mechanistic insights into ion transport in EC-LiTFSI electrolytes.