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+center dot center dot center dot TFSA(-), formed in highly concentrated EC solutions. Symmetrical stretching vibrational mode of solvated Li+ and solvated Li+center dot center dot center dot TFSA(-) ion pair were observed at nu = 168-177 and 202-224 cm(-1), respectively. Detailed structural properties of solvated Li+ and Li+center dot center dot center dot TFSA(-) contact ion pair were derived from the least-squares fitting analysis of first-order difference function, Delta(Li)(Q), between neutron scattering cross sections observed for Li-6/Li-7 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+center dot center dot center dot O(EC) distance and Li+center dot center dot center dot(EC)=C(EC) bond angle are determined to be 1.90 +/- 0.01 angstrom and 141 +/- 1 degrees, 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(-).