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 + BF 4 − ) 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 + BF 4 − salt is exothermic, 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 + BF 4 − ), are calculated to be 91.5, 89.8, 87.2, and 84.0 kcal mol −1 (per EC molecule), respectively. The intermolecular distances between Li + and the F atom of BF 4 − are elongated: 1.773 Å ( n =0), 1.820 Å ( n =1), 1.974 Å ( n =2), 1.942 Å ( n =3), and 4.156 Å ( 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 BF 4 − 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 + BF 4 − ) and the Li + ion is stripped from the salt. The following substitution reaction takes place: EC+(EC) 3 (Li + BF 4 − )→(EC) 4 Li + −(BF 4 − ). The reaction mechanism is discussed on the basis of the theoretical results.