A multiphysics framework for modeling expansion force during thermal runaway propagation in lithium-ion battery packs

Early warning of thermal runaway is essential for ensuring the safety of lithium-ion batteries. Expansion force has emerged as an important early warning signal. Modeling cell expansion can provide critical insights into the underlying thermo-mechanical processes and offer a simulation tool for developing reliable warning strategies. However, existing models are generally limited to the single-cell level and lack dynamic coupling with TR propagation (TRP) in the battery pack. In this work, a multiphysics and multi-scale modeling framework is developed to capture the cell expansion behavior during TRP. The framework integrates the lumped electrochemical and fluid dynamic sub-models into the thermal resistance network, capturing the heat generation, gas generation, pressure evolution, heat transfer, and TRP across the electrode, cell, and battery pack scales. The dynamic pressure boundary links the thermal resistance network with the finite element model to simulate the mechanical behavior of batteries. The model accurately captures the transient evolution of cell temperature and expansion force during TRP, which is validated by experiments. Simulation results clarify the propagation modes and mechanism of TR and cell expansion, and further reveal the effects of triggering location and heating power on expansion force and warning performance. The expansion force is found to provide the earliest warning with the longest lead time, and its performance remains robust across different triggering locations. This work advances the prediction of cell expansion at the battery pack level, offering new insights into the design of early warning systems.