Modeling of SEI Layer Growth and Electrochemical Impedance Spectroscopy Response using a Thermal-Electrochemical Model of Li-ion Batteries

Lithium-ion batteries (LIBs) suffer severe performance degradation during their lifetime application because of the undesired chemical reactions, ageing, corrosion, structural integrity compromise, and the threat of thermal runaway.  Among those degradation mechanisms, much attention has been focused on LIBs materials decomposition, new components formation and growth due to undesired side reactions.  The electrode is covered by a passivation layer named as the solid electrolyte interface (SEI), which is one of dominant LIBs degradation mechanisms.  The SEI layer affect the LIBs cyclability, life time, power and rate capability, and even their safety.  In order to comprehensively investigate the effects of SEI growth on battery performance, a one-dimensional thermal-electrochemical model is developed (1). This model is equipped with a growth mechanism of the SEI layer coupled with thermal evolution, based on the diffusional process of the solvent through the SEI layer and the kinetic process at the interface between the solid phase and liquid phase. The developed model reveals the effects of diffusivity, kinetics, and temperature on SEI layer growth and cell capacity fade.  In our model, the SEI growth is accompanied by both diffusion-limited and kinetics-limited processes. With the layer becoming thicker, its growth rate slows down gradually possibly due to increased diffusion resistance.  SEI grows more quickly during charge than discharge due to the difference in electron flux through the SEI layer and the temperature change during cycling.  Temperature rise due to reactions and joule heating accelerates the SEI layer growth and causes more capacity loss. This model can also provide insights on position-dependent SEI growth rate and be used to guide the strategic monitoring location.  Based on the model developed, one more mathematical model is developed to simulate the impedance response of LIBs with considering the double layer capacitance effect (2).  The simulated impedance response is analyzed to reveal LIBs impedance behaviors, which can be used to predict the degradation mechanism of LIBs.  With combining the two mathematical models, a more accurate equivalent circuit model can be proposed to illustrate the features of LIBs impedance behaviors and degradation mechanism.  This work can provide us the insights of combining LIBs EIS analysis with degradation analysis.  Currently, the experimental validation is ongoing.  

1.             L. Liu, W. Lu and A. M. Sastry, Journal of Power Sources, Submitted and under review

2.             I. J. Ong and J. Newman, Journal of The Electrochemical Society, 146, 4360 (1999).