Multi-physics simulation and risk analysis of internal thermal runaway propagation in lithium-ion batteries

This study investigates internal thermal runaway propagation (TRP) mechanism in lithium-ion batteries (LIBs) triggered by hotspots, focusing on the TRP dynamics and thermal interactions between internal short circuits (ISC) and side reactions within the TRP front. An integrated electrical-electrochemical-thermal-chemical model, incorporating a novel ISC model, is developed within the in-house BatteryFOAM solver to simulate global thermal runaway initiation and TRP behaviors. A new TRP front multi-zone model is built to analyze the coupling between heat conduction, ISC-driven ignition, and side reactions. The results show that the TRP occurs when the separator melt failure temperature ($T_{sep}$) is reached before the maximum temperature gradient, allowing ISC Joule heating to maintain a high temperature gradient propagating from the hotspot to the normal zone. Therefore, a first-ever dimensionless risk coefficient ($f$) is introduced to quantify the balance between heat generation and dissipation, identifying high-risk TRP fronts where $f$ ranges from 1 to 1e5, with cathode reactions and electrolyte decomposition dominating TRP acceleration. Model validation against the experiments confirms the predictive accuracy. Simulations demonstrate a TRP velocity of 7.5 mm/s, a width of 2.8 mm, and a maximum temperature of 690 K. Notably, the TRP velocity is, for the first time, revealed to be correlated with the square root of the thermal diffusivity, and an equation linking velocity with $T_{sep}$ is derived to guide LIB safety implementations. This study provides quantitative insights for designing safer LIBs, particularly in electric vehicles and large-scale energy storage.