Solvate Structures and Computational/Spectroscopic Characterization of LiPF₆ Electrolytes

Abstract

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 PF 6 – anion P–F Raman symmetric stretching vibrational band for evaluating the PF 6 – ···Li + cation interactions within LiPF 6 crystalline solvates to create a characterization tool for liquid electrolytes. To facilitate this, the crystal structures for two new solvates—(G3) 1:LiPF 6 and (DEC) 2:LiPF 6 with triglyme and diethyl carbonate, respectively—are reported. DFT calculations for Li-PF 6 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 LiPF 6 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 PF 6 – anions are uncoordinated or coordinated to a single Li + cation, respectively.