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

Abstract

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 Å(-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.

Recent progress in fluorinated electrolytes for improving the performance of Li–S batteries

“…Similar local Li + -ion structures were found in the [Li(G4)][TFSI]/TTE electrolyte ( Fig. 9 ) [92] . Based on the Raman spectra and molecular dynamics (MD) simulations, the solvent shared ion pair (SSIP) of Li + -TFSI − and the monodentate or bidentate contact ion pair (CIP) of Li + -TFSI − were determined to co-exist in neat and TTE-diluted [Li(G4)][TFSI] SIL.…”

Categorizing Molten Salt Complexes as Ionic Liquids and Their Applications to Battery Electrolytes

“…Studies of the fundamentals and applications are further required to establish the concept of solvate ILs. The bulk liquid structure of the glyme-Li salt solvate IL was revealed by high-energy X-ray diffraction measurements, 31 neutron diffraction measurements, 32 and MD simulations. 16,33 Studies of the solid-liquid interface are also quite important to understand the electrode reactions in batteries, especially when desolvation/solvation of the complex cations occurs.…”

Physicochemical compatibility of highly-concentrated solvate ionic liquids and a low-viscosity solvent

“…Our result were consistent with those of a previous spectroscopic report that indicated the formation of a microscopic stable coordination microstructure between Li + and glyme molecules even if SIL was diluted with HFE. 25 In contrast, positive values of E r were observed for the samples with a ¼ 1 and a high molar fraction of HFE at 323.15 K. In the case of high temperature and low SIL concentration, slight solubility between the SIL and HFE might occur through colligative property. In the future, we will further investigate the nanoscopic relationships between LiTFSA, G4 using high energy X-ray diffraction and Raman spectroscopy measurements.…”

“…This results suggest that the interactions between [Li a (G4) 1 ]TFSA a and HFE were weak because no coordination changes were observed. 25,26 Fig . 3(a) TFSA a was less than one in both systems.…”