The Effect of Solvation Shell Structure and Composition on Ion Pair Formation: The Case Study of LiTDI in Organic Carbonates

Organic carbonates are an integral part of commercial lithium-ion batteries due to their desirable properties, such as high dielectric constants. However, the high viscosity of organic carbonates requires the use of mixtures containing a cyclic and a linear carbonate. In binary mixtures, the linear and cyclic organic carbonates compete to solvate the lithium salt. While the preferential solvation has been extensively studied, the effect of the mixed solvation on the ionic speciation of the lithium ion is a topic that has not been studied in detail. In this study, infrared spectroscopies and ab initio chemical computations are utilized to study the change in the solvation structure and dynamics as a function of solvent composition for the model system: lithium 4,5-dicyano-2-(trifluoromethyl)imidazole (LiTDI). The TDI– nitrile bands are used to probe the changes in the ion speciation as well as in the molecular environment since nitriles are known to be good and sensitive IR spectroscopic reporters of the e...