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

Abstract

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(Q), between neutron scattering cross sections observed for ⁶Li/⁷Li isotopically substituted 10 and 25 mol % *LiTFSA-ECd₄ 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^-.