Advanced abuse modelling of Li-ion Cells – A novel description of cell pressurisation and simmering reactions

Thermal runaway (TR) is a significant safety concern for Li-ion batteries (LIBs), which, through computational modelling can be better understood. However, TR models for LIBs lack a proper representation of the build-up of pressure inside a cell under abuse, which is integral to predicting cell venting. Here, an advanced abuse model (AAM) is developed and compared to a classical TR model, considering a lithium iron phosphate (LFP) cell case study. The AAM accounts for two additional features: 1) venting, with a novel description of the internal cell pressure governed by the bubble point of the electrolyte/decomposition-gas mixture, and 2) simmering reactions. The novel bubble pressure assumption is validated against experimental data, and we show that the AAM significantly improves the predictions of time to TR and of temperatures after TR. Further, it is shown that there is significant uncertainty in the parameters defining the decomposition reactions for LFP cells. Importantly, cell pressurisation is most dependent on the gases released by the solid electrolyte interphase reaction, and venting is dependent on cell burst pressure and reaction activation energies. The AAM is essential for accurate abuse modelling, due to its improved temperature predictions, and considerably enhances the LIB TR field of study

Electrochemical-thermal modeling and microscale phase change for passive internal thermal management of lithium ion batteries.

A fully coupled electrochemical and thermal model for lithium-ion batteries is developed to investigate the impact of different thermal management strategies on battery performance. In contrast to previous modeling efforts focused either exclusively on particle electrochemistry on the one hand or overall vehicle simulations on the other, the present work predicts local electrochemical reaction rates using temperature-dependent data on commercially available batteries designed for high rates (C/LiFePO{sub 4}) in a computationally efficient manner. Simulation results show that conventional external cooling systems for these batteries, which have a low composite thermal conductivity ({approx}1 W/m-K), cause either large temperature rises or internal temperature gradients. Thus, a novel, passive internal cooling system that uses heat removal through liquid-vapor phase change is developed. Although there have been prior investigations of phase change at the microscales, fluid flow at the conditions expected here is not well understood. A first-principles based cooling system performance model is developed and validated experimentally, and is integrated into the coupled electrochemical-thermal model for assessment of performance improvement relative to conventional thermal management strategies. The proposed cooling system passively removes heat almost isothermally with negligible thermal resistances between the heat source and cooling fluid. Thus, the minimization of peak temperatures and gradients within batteries allow increased power and energy densities unencumbered by thermal limitations