Predicting Transport Properties at Electrode/SEI/Electrolyte Interfaces in Li Ion Batteries

The electrode/solid-electrolyte-interphase (SEI)/electrolyte interface is rather complex and critially important in Li-ion battereis. Improving Li transport and preventing electron leakage through the solid electrolyte interphase (SEI) are critical to Li-ion batteries capacity drop and power loss.  Identification of the dominant diffusion carriers and their diffusion pathways in main components of SEI is a necessary step to understand the functionality of SEI before tailoring its properties. Therefore, dominant Li diffusion carriers and the ionic conductivity in perfect SEI components (such as Li₂CO₃ and LiF) were predicted with density functional theory (DFT) over a broad voltage range of the electrode materials that the SEI component covers. It was predicted that the main diffusion carriers in Li₂CO₃ on anode are excess Li ion interstitials (Li_i⁺), however, above 3.98V, Li ion vacancies (V_Li^-) become the dominant. In contrast, the main diffusion carriers in LiF was predicted to be Schottky pairs (V_Li^-  and V_F⁺), agreeing with experimental observations. While the V_Li^- diffuses through direct hopping, the Li_i⁺ diffuses by knocking off the Li⁺ ion from a lattice. Meso-scale Li⁺ ion diffusion equations were then formulated based on the diffusion mechanisms discovered by DFT and the boundary conditions of isotope exchange experiments in order to make direct comparison with TOF-SIMS measurements. Furthermore, by studying the point defect formation and diffusion, it is proposed that electron or polaron hopping can be facilitated by the diffusion of point defects, such as Li_i⁰ interstitial, when the SEI is thicker than electron tunneling length. Ab initio molecular dynamics of electrode/SEI/electrolyte interface simulations further demonstrated that when a Li_i⁰ interstitial transports into the electrolyte, ethylene carbonate (EC) reduction is observed immediately. This is a possible mechanism to explain electron leakage through thick SEI films into the electrolyte.