A Physics-Based Piecewise Kinetic-Diffusion Control Mechanism for SEI Formation during Electrochemical Degradation

The growth of the solid electrolyte interphase (SEI) on anode is the main cause for electrochemical degradation of lithium-ion batteries. While the growth rate of the SEI is influenced by reaction kinetics and solvent diffusion limits, these two effects have been traditionally combined using the sum of reciprocals. Here, a physics-based methodology is proposed considering a two-stage process, with a piecewise kinetic-diffusion (PKD) control mechanism for the SEI formation in the aging model. The kinetic and diffusion limits are separately determined by calculating the molar fluxes of Li + and ethylene carbonate (EC) solvent, which are the two reactant species for SEI. They are compared at the reaction interphase to identify the limiting mechanism. The simulation results are validated with both calendar and cycle life data, under different SOCs, temperatures, and charging profiles. The PKD method more accurately captures the temperature and SOC dependency of capacity and voltage fade, as compared to the empirical and sum-of-reciprocal assumptions. The switch point (SP) between kinetic and diffusion limited process is identified as an optimizable parameter and its impact on battery life is studied. The analysis shows that the SEI-related electrochemical degradation is suppressed when the SP occurs late in the cycle.