Li+ Local Structure in Hydrofluoroether Diluted Li-Glyme Solvate Ionic Liquid

Hydrofluoroethers have recently been used as the diluent to a lithium battery electrolyte solution to increase and decrease the ionic conductivity and the solution viscosity, respectively. In order to clarify the Li+ local structure in the 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE) diluted [Li(G4)] [TFSA] (G4, tetraglyme; TFSA, bis(trifluoromethanesulfonyl)amide) solvate ionic liquid, Raman spectroscopic study has been done with the DFT calculations. It has turned out that the HFE never coordinates to the Li+ directly, and that the solvent (G4) shared ion pair of Li+ with TFSA anion (SSIP) and the contact ion pair between Li+ and TFSA anion (CIP) are found in the neat and HFE diluted [Li(G4)] [TFSA] solvate ionic liquid. It is also revealed that the two kinds of the CIP in which TFSA anion coordinates to the Li+ in monodentate and bidentate manners (hereafter, we call them the monodentate CIP and the bidentate CIP, respectively) exist with the SSIP of predominant [Li(G4)](+) ion-pair species in the neat [Li(G4)] [TFSA] solvate ionic liquid, and that the monodentate CIP decreases as diluting with the HFE. To obtain further insight, X-ray total scattering experiments (HEXTS) were carried out with the aid of MD simulations, where the intermolecular force field parameters, mainly partial atomic charges, have been newly proposed for the HFE and glymes. A new peak appeared at around 0.6-0.7 angstrom(-1) in X-ray structure factors, which was ascribed to the correlation between the [Li(G4)][TFSA] ion pairs. Furthermore, MD simulations were in good agreement with the experiments, from which it is suggested that the terminal oxygen atoms of the G4 in [Li(G4)](+) solvated cation frequently repeat coordinating/uncoordinating to the Li+, although almost all of the G4 coordinates to the Li+ to form [Li(G4)](+) solvated cation in the neat and HFE diluted [Li(G4)][TFSA] solvate ionic liquid.