Solvation Structure and Dynamics of the Lithium Ion in Organic Carbonate-Based Electrolytes: A Time-Dependent Infrared Spectroscopy Study

The structure and dynamics of electrolytes composed of lithium hexafluorophosphate and either butylene carbonate or dimethyl carbonate were investigated using steady state and two-dimensional infrared spectroscopies. This study was focused on lithium ion compositions similar to that of commercial batteries (i.e., X(Li+) = 0.09) and higher. Experiments provide sufficient evidence to demonstrate that both organic carbonates form tetrahedral solvation complexes around the lithium ion. Ab initio computations confirmed that the IR spectroscopic signatures derived from experiments correspond to a tetrahedral arrangement of carbonate molecules in the lithium ion solvation shell. Time resolved experiments further revealed that the solvation shell formed by cyclic carbonates is more rigid than that of its linear carbonate analogue. In addition, butylene carbonate was found to present a more organized overall solvent structure than dimethyl carbonate. At lithium salt concentrations beyond that of a conventional electrolyte, the electrolytes displayed changes in the dynamics and structure of their molecular components due to the presence of ion pairs. Cyclic and linear carbonates were found to preferentially form ion pairs with two distinct structures: contact and solvent separated ion pairs, respectively. The formation of distinct ion pairs by butylene carbonate and dimethyl carbonate is predicted to arise from the different solvation shells formed by the two carbonates; this was confirmed by ab initio frequency calculations and FTIR. Results of this study shed some light on the characterization of the solvation structure of the lithium ion in the electrolyte which could help to rationalize the importance of the electrolyte composition on the performance of the lithium ion battery.