Electrochemical kinetics of SEI growth in porous electrodes

Growth of the solid electrolyte interphase (SEI) is a major driver of capacity fade in LIBs. Despite its importance, the fundamental mechanisms remain unclear, primarily because of the complicated reaction pathways involved. SEI growth can be both electrochemical and chemical in nature, and thus, it is a strong function of the potential and degree of lithiation of the electrode. In this work, we model the early-stage and long-term growth of SEI by accurately capturing the potential dependence of its formation kinetics as well as long term rate limiting steps, and validating it against extensive experimental data. This is done using the Multiphase Porous Electrode Theory (MPET) framework on graphite (phase separating) and carbon black (non phase separating) particles. Results indicate that the peak SEI-forming currents are higher for higher driving currents. Counterintuitively, despite higher peak SEI-forming currents, the highest differential capacities or ‘extent of SEI growth’ are seen for lower driving currents, which implies that SEI formation is a stronger function of potential and cycling time, than the driving current. This work holds promise for the predictive design of procedures for manufacture and formation of LIBs.