Structural Variation in Carbonate Electrolytes by the Addition of Li Salts Studied by X-Ray Total Scattering

X-ray total scattering measurements are carried out for the typical electrolyte used in lithium-ion batteries, that is, the mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) electrolyte with LiPF6 salt. The obtained pair distribution function is compared with that of the EC/EMC solvent without LiPF6, to investigate the effect of the Li+ cation on the intramolecular structure. It is found that the peak shift corresponding to the shrinking of the C-O bonds is induced by dissolving LiPF6. Also, the peak height becomes lower by the addition of LiPF6. These behaviors are also observed by the X-ray total scattering measurements for the mixture of EC and dimethyl carbonate (DMC) with and without LiPF6. Also, it is ascertained that such a shrinking is present in EC/DMC electrolytes by means of neutron total scattering. First-principles calculations on the basis of the density functional theory for isolated EC, EMC, and DMC molecules with the Li+ cation confirm that the shrinking of the C-O bond is accompanied by the formation of the solvation structure. The observed structural variations induced by adding LiPF6 will be a good indicator for the formation of the solvation structure.