Solvation of Lithium Ion in Organic Electrolyte Solutions and Its Isotopic Reduced Partition Function Ratios Studied by ab initio Molecular Orbital Method

To explore local structures around lithium ions and to estimate lithium isotopic reduced partition function ratios (RPFRs) ofsolvated lithium ions in ethylene carbonate (EC), methylethyl carbonate (MEC) and EC/MEC mixed solvent systems, ab initio molecular orbital calculations at the HF/6-31G(d) level of theory were carried out. Both EC and MEC were coordinated to lithium ions using their carboxyl oxygens and the Li-O bond distance increase with increasing solvation number up to 4 in the primary solvation sphere both in EC and MEC systems. Binding energy calculations suggested that EC was preferentially coordinated to the lithium ion in the EC/MEC mixed solvent system. RPFRs of solvated lithium ions were convex functions of the solvation number between 1 and 4 and took the maxima at 3 both in EC and MEC systems. The RPFR value in EC/MEC mixed solvent system was estimated to be 1.07818 at 25°C.