Simulation and Modelling of the Solid Electrolyte Interphase with Varying Porosity

Abstract

Continuous capacity fade during the lifetime of lithium ion batteries is mainly attributed to the growth of the solid electrolyte interphase (SEI). This layer forms on the negative electrode and consists of electrolyte reduction products. Active lithium that is consumed in this reduction reaction is no longer available for the primary battery function, leading to capacity fade. By preventing direct electrolyte/electrode contact, the SEI layer can suppress the electrolyte reduction reaction to a large degree. However, capacity fade and SEI growth still continue with a decreasing speed [1,2]. The fundamental mechanisms behind this continuous SEI formation are still subject of active research. Numerous models attempt to describe this effect [2-4]. They typically assume that the formation rate is limited by a single transport mechanism that allows one reactant to cross the SEI layer. While most of these models predict the experimentally observed SEI thickness and capacity decay, they remain inconclusive with respect to the limiting transport process. We present and evaluate the results of a new model to describe the SEI growth. Our model instead is based on two rate limiting processes and contains a spatially-varying porosity. This novel approach allows us to track the growth of the SEI morphology distribution. Our simulations agree nicely with experiments [2] after adjusting the limiting transport parameter. Most importantly, the SEI thickness shows the characteristic √t growth behavior. The porosity inside the SEI is found to be constant. We derive analytic expressions for the thickness evolution and the attained porosity. These well justified estimates are in very good agreement with simulations.