Mechanism of Dissolution of a Lithium Salt in an Electrolytic Solvent in a Lithium Ion Secondary Battery: A Direct Ab Initio Molecular Dynamics (AIMD) Study

The mechanism of dissolution of the Li+ ion in an electrolytic solvent is investigated by the direct ab initio molecular dynamics (AIMD) method. Lithium fluoroborate (Li+BF4-) and ethylene carbonate (EC) are examined as the origin of the Li+ ion and the solvent molecule, respectively. This salt is widely utilized as the electrolyte in the lithium ion secondary battery. The binding of EC to the Li+ moiety of the Li+BF4- salt is exo-thermic, and the binding energies at the CAM-B3LYP/6-311++G(d,p) level for n= 1, 2, 3, and 4, where n is the number of EC molecules binding to the Li+ ion, (EC) n(Li+BF4-), are calculated to be 91.5, 89.8, 87.2, and 84.0 kcalmol(-1) (per EC molecule), respectively. The intermolecular distances between Li+ and the F atom of BF4- are elongated: 1.773 angstrom (n= 0), 1.820 angstrom (n= 1), 1.974 angstrom (n= 2), 1.942 angstrom (n= 3), and 4.156 angstrom (n= 4). The atomic bond populations between Li+ and the F atom for n= 0, 1, 2, 3, and 4 are 0.202, 0.186, 0.150, 0.038, and 0.0, respectively. These results indicate that the interaction of Li+ with BF4- becomes weaker as the number of EC molecules is increased. The direct AIMD calculation for n= 4 shows that EC reacts spontaneously with (EC) 3(Li+BF4-) and the Li+ ion is stripped from the salt. The following substitution reaction takes place: EC+(EC)(3)-(Li+BF4-)->(EC)(4)Li+-(BF4-). The reaction mechanism is discussed on the basis of the theoretical results.