Role of Solvent Isomerism in Mixed Carbonate Electrolytes for Li-Ion Batteries

Current Li-ion battery (LIB) electrolytes employ mixed solvents consisting of ethylene carbonate (EC) and linear carbonates (LCs). Notably, the ion conductivities of the EC/LC electrolytes follow the order dimethyl carbonate > ethyl methyl carbonate > diethyl carbonate despite the similar physicochemical properties of the three LCs. However, the origin of this order remains elusive. In this study, we elucidated the important role of conformational isomerism of the LC solvent on salt dissociation in 0.1–3.0 M LiPF6 EC/LC solutions, using Raman spectroscopy and dielectric relaxation spectroscopies, along with first-principles calculations. The conductivity trend is closely related to the difference in the degree of salt dissociation, which, in turn, is determined by the fraction of the polar cis–trans-LC conformer, as this conformer participates in Li-ion solvation as readily as EC does. This study demonstrates the critical role of the conformational isomerism of the solvents in the electrolyte conductivity, indicating the feasibility of utilizing solvent isomerism to tune the bulk transport properties of electrolytes.