Estimation of Reduced Partition Function Ratios of Lithium-Graphite Intercalation Compounds by Density Functional Theory

Abstract

The reduced partition function ratio (RPFR) of lithium in lithium-graphite intercalation compounds (Li-GICs) was evaluated at the UB3LYP=6-311G(d) level of theory. The partition functions were written in the usual rigid-rotor harmonic oscillator approximation.With a C24H12 coronene molecule as the model of graphene, lithium-coronene sandwich, and club sandwich compounds were considered as models of Li-GICs. The estimated value of the 6Li-to-7Li RPFR was 1.0402 at 25 °C, which yielded 1.034 as the value of the equilibrium constant, K, of the lithium isotope exchange reaction between a lithium ion in an ethylene carbonate=ethylmethyl carbonate mixed solvent and a lithium atom in interlayer space of graphite. The estimated value of K was larger than the experimental value of 1.025. The unsatisfactory agreement between the estimated and experimental K values suggested that larger molecules should be used as models of graphene and that the vibrational anharmonicity should also be taken into consideration.