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

Authors

  • Dr. Hiroto Tachikawa

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    1. Division of Materials Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628 (Japan)
    • Division of Materials Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628 (Japan)===

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Abstract

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 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+BF4), 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 BF4 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 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)4Li+−(BF4). The reaction mechanism is discussed on the basis of the theoretical results.

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