A Raman spectroscopic study of organic electrolyte solutions based on binary solvent systems of ethylene carbonate with low viscosity solvents which dissolve different lithium salts

Abstract

The ionic structure of organic electrolyte solutions has been investigated, by means of Raman spectroscopy for mixed aprotic solvents that dissolve lithium salts. The solutions consisted of binary solvent systems of a high permittivity solvent (ethylene carbonate, EC) mixed with low viscosity alkyl carbonates (dimethyl carbonate, DMC, and diethyl carbonate, DEC) or a linear alkyl ether (1,2-dimethoxyethane, DME) and of LiCF3SO3, LiPF6 and LiN(C2F5SO2)2 as the solute. The Raman shifts based on the solvents varied with the sort of dissolved lithium salts and their concentration. The ion solvation was estimated from the side-bands of the Raman scattering for the C–O stretch of single bonds of the carbonate groups and for the symmetric ring deformation of EC. The number of EC molecules interacting with the lithium cation (Li+) was higher than that of DMC in a mixed EC+DMC (50:50 by volume) system. The apparent solvation number of Li+ in 1.5 mol dm-3 LiCF3SO3 solution was about 2.8 in EC+DMC, whereas that in 1.5 M LiPF6 solution was about 3.9. Specific solvation of Li+ was distinguished in EC+DME (50:50), where DME predominantly coordinates to Li+. However, it was also confirmed that EC, which has a lower donicity, interacts with Li+ even in the EC+DME system. That is, the solvation number of EC in LiCF3SO3/(EC+DME) remained ca. 1.0 even in high salt concentrations.