Local Structure of Li⁺ in Concentrated Ethylene Carbonate Solutions Studied by Low-Frequency Raman Scattering and Neutron Diffraction with ⁶Li/⁷Li Isotopic Substitution Methods

Isotropic Raman scattering and time-of-flight neutron diffraction measurements were carried out for concentrated LiTFSA-EC solutions to obtain structural insight on solvated Li⁺ as well as the structure of contact ion pair, Li⁺···TFSA^–, formed in highly concentrated EC solutions. Symmetrical stretching vibrational mode of solvated Li⁺ and solvated Li⁺···TFSA^– ion pair were observed at ν = 168–177 and 202–224 cm^–1, respectively. Detailed structural properties of solvated Li⁺ and Li⁺···TFSA^– contact ion pair were derived from the least-squares fitting analysis of first-order difference function, Δ_Li(<i>Q</i>), between neutron scattering cross sections observed for ⁶Li/⁷Li isotopically substituted 10 and 25 mol % *LiTFSA-EC<i>d</i>₄ solutions. It has been revealed that Li⁺ in the 10 mol % LiTFSA solution is fully solvated by ca. 4 EC molecules. The nearest neighbor Li⁺···O­(EC) distance and Li⁺···O­(EC)C­(EC) bond angle are determined to be 1.90 ± 0.01 Å and 141 ± 1°, respectively. In highly concentrated 25 mol % LiTFSA-EC solution, the average solvation number of Li⁺ decreases to ca. 3 and ca. 1.5. TFSA^– are directly contacted to Li⁺. These results agree well with the results of band decomposition analyses of isotropic Raman spectra for intramolecular vibrational modes of both EC and TFSA^–