Hybrid Quantum-Classical Simulation of Li Ion Dynamics and the Decomposition Reaction of Electrolyte Liquid at a Negative-Electrode/Electrolyte Interface

Solid electrolyte interphase (SEI) films play a key role in the operation of Li ion batteries; however, the formation mechanism of SEI remains unclear. In this study, the dynamics of Li ions inserted between a negative graphite electrode and an ethylene carbonate (EC) electrolyte solvent were investigated using a hybrid quantum-classical (QM-CL) molecular dynamics (MD) simulation. The activation energy of the Li ion insertion process was estimated to be 0.73 eV, which is nearly equal to the experimental one of the Li ion desolvation process. This was converted to 0.22 V/angstrom (the critical electric field strength required to overcome the activation energy of the 3.29 angstrom distance between the transition and equilibrium states). Upon applying an electric field of 0.3 V/angstrom in the hybrid QM-CL MD simulation, the desolvated Li ion was rapidly inserted into the graphite. On the other hand, when an electric field strength of 0.2 V/angstrom was applied, the EC molecule received an electron from the graphite electrode at the interface coexisting with a Li ion and decomposed then to C2H4 and Li-CO3. This indicated that the Li ions catalyzed the decomposition of the EC, thereby clarifying the initial stage of the formation of a SEI by the reductive decomposition of the EC molecules.