Ion Association in Aprotic Solvents for Lithium Ion Batteries Requires Discrete–Continuum Approach: Lithium Bis(oxalato)borate in Ethylene Carbonate Based Mixtures

Abstract

Ion association in solutions of lithium salts in mixtures of alkyl carbonates carries significant impact on the performance of lithium ion batteries. Focusing on lithium bis(oxalato)borate, LiBOB, in binary solvents based on ethylene carbonate, EC, we show that neither continuum nor discrete solvation approaches are capable of predicting physically meaningful results. So-called mixed or the discrete–continuum solvation approach, based on explicit consideration of an ion solvatocomplex combined with estimation of the medium polarization effect, is required in order to characterize the ion association at the quantitative level. The calculated changes of the Gibbs free energy are overestimated by nearly an order of magnitude by the purely continuum and purely discrete approaches, with the values having the opposite signs. The physically balanced discrete–continuum description predicts weak ion association. The numerical data obtained with density functional theory are validated using coupled-cluster calculations and experimental X-ray data. The study contributes to resolution of the challenge in solvation modeling in general, and develops a reliable, practical method that can be used to screen ion association in a broad range of ion–molecular mixtures for lithium ion batteries, especially for the solutions of LiBOB in EC based mixtures.