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.