Title

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

Document Type

Article

Publication Date

10-17-2019

Abstract

Copyright © 2019 American Chemical Society. 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 environment. Our experimental finding shows that the anion forms contact ion pairs, but the amount of contact ion pairs is strongly dependent on the type of carbonate. In particular, TDI- has strong propensity of forming contact ion pairs in linear carbonates, while in cyclic carbonates, it is primarily not directly interacting with Li+. The dynamics of the environment supports our speciation of the anion in both solvents. In addition, the concentration of contact ion pairs shows a monotonic and nonlinear increase with the concentration of linear carbonate in the mixture. Ab initio computations reveal that the nonlinear behavior is related to the energetics of ion pair formation, which appears to be much more favorable when the lithium solvation shell does not contain cyclic carbonates. Overall, our results show that the concentration of linear carbonates in a mixture of organic carbonates directly influences the speciation of the lithium ion.

Publication Source (Journal or Book title)

Journal of Physical Chemistry C

First Page

25102

Last Page

25112

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