Solvate Structures and Computational/Spectroscopic Characterization of LiPF6 Electrolytes

Raman spectroscopy is a powerful method for identifying ion–ion interactions, but only if the vibrational band signatures for the anion coordination modes can be accurately deciphered. The present study characterizes the PF6– anion P–F Raman symmetric stretching vibrational band for evaluating the PF6–···Li+ cation interactions within LiPF6 crystalline solvates to create a characterization tool for liquid electrolytes. To facilitate this, the crystal structures for two new solvates—(G3)1:LiPF6 and (DEC)2:LiPF6 with triglyme and diethyl carbonate, respectively—are reported. DFT calculations for Li-PF6 solvates have been used to aid in the assignments of the spectroscopic signatures. The information obtained from this analysis provides key guidance about the ionic association information which may be obtained from a Raman spectroscopic evaluation of electrolytes containing the LiPF6 salt and aprotic solvents. Of particular note is the overlap of the Raman bands for both solvent-separated ion pair (SSIP) and contact ion pair (CIP) coordination in which the PF6– anions are uncoordinated or coordinated to a single Li+ cation, respectively.