The performance of Li-ion batteries (LIB) is highly dependent on the electrochemical reactions occurring at the Solid Electrolyte Interphase (SEI). However, its nanometric scale thickness combined with its complex structure makes the SEI remains “the most important but the least understood in LIB”. Simulations can provide critical insights to the electrochemical reactions at the electrode/SEI/electrolyte interface. Density Function Theory (DFT) and Density Functional Tight Binding (DFTB) methods were used to model the charge transfer reaction at a Li/Li₂CO₃/liquid Ethylene carbonate (EC) electrolyte interface. In this study, we first developed a new set of DFTB parameters for Li-X (X=Li, H, C, O) interactions by fitting and calibrating with various DFT calculations. Then, the lithiation and delithiaiotn processes were simulated.

The simulation clearly showed four layers Li₂CO₃ can effectively protect the Li electrode surface within the simulation time. During de-lithiation, it was energetically favorable to form a void at the Li/Li₂CO₃ interface. The lithiation process was simulated as three steps: Li⁺ desolvation at the SEI/Electrolyte interface, Li⁺ diffusion through the Li₂CO₃ layer, and annihilation of e^- and Li⁺ at the Li surface. The simulation clearly demonstrated the charge transfer reaction of Li⁺ + e^- -> Li⁰ occurs at the Li/Li₂CO₃ interface or beneath the SEI layer. The energy profile of this reaction was simulated under different applied voltage. It was found that the experimentally defined zero voltage of Li⁺/Li⁰ corresponds to a negatively charged Li metal surface at a charge density of ~ 1e/nm². When the electron density is larger than this (more negative potential), Li plating occurs; while when the electron density is lower than this (more positive potential), Li stripping occurs. The excess electrons on the Li surface will trigger more electrolyte decomposition and Li-dendrite formation when SEI fractures.