PHYSICS BASED ELECTROLYTE EVAPORATION MODEL FOR USE IN LI-ION THERMAL RUNAWAY SIMULATIONS

Numerical analysis of thermal runaway and propagating failures often relies on solving the transient heat conduction equation. Exothermic decomposition reactions create a heat source within the active material. The generated heat diffuses through the cell via heat conduction and is carried away from the cell via conduction, convection, and radiation. However, many studies neglect the evaporation of liquid electrolyte from the battery cell after the vent mechanism activates despite evidence that evaporation significantly affects the time-to-thermal-runaway. In studies which have considered evaporation of electrolyte, the rate of evaporation was modeled using a semi-empirical relationship instead of rigorous theoretical analysis. This work presents a new, physics-based evaporation model in which the mass transfer of electrolyte in the jellyroll is treated as a 1-D liquid diffusion process. Experiments were conducted to determine the diffusion coefficient of electrolyte through the porous separator material. The 1-D liquid diffusion model was then integrated within an existing battery thermal abuse model. The thermal abuse model with physics-based electrolyte evaporation model was exercised for oven tests at 150 degrees C and 155 degrees C and compared with published experimental data. Simulated results agreed well with experimental data, capturing the characteristic temperature drop after vent activation. The model may be used to further investigate the role of electrolyte evaporation on thermal runaway and propagating failures.