Multi-Thousand-Atom DFT Simulation of Li-Ion Transfer through the Boundary between the Solid-Electrolyte Interface and Liquid Electrolyte in a Li-Ion Battery

Microscopic mechanisms of the Li-ion transfer through the boundary between the solid-electrolyte interface (SEI) formed on the graphite anode and liquid electrolyte in the Li-ion battery are investigate theoretically. A simulation system (about 2400 atoms) for the boundary is modeled using dilithium ethylene dicarbonate (Li2EDC), ethylene carbonate (EC), and LiPF6 for the SEI, solvent, and salt, respectively. After inserting Li-ions in the Li2EDC region, we perform the first-principles molecular dynamics simulation for 4.8 Ps using the divide-and-conquertype real-space grid DFT. Enhanced stability of the Li-ions at the boundary where EDC2- and EC contact with each other is thereby found in the runs without salt, which acts to impede the Li-ion transfer through the boundary. It is also found that inclusion of 1.0 M LiPF6 salt in the liquid EC weakens such impedance effect significantly. Physical reasons for those phenomena are explained in combination with separate DFT calculations.