Local Structure of Li in Concentrated Ethylene Carbonate Solutions Studied by Low-Frequency Raman Scattering and Neutron Diffraction with Li/Li 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, 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(Q), between neutron scattering cross sections observed for Li/Li isotopically substituted 10 and 25 mol % *LiTFSA-ECd 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.