Network thermal-runaway propagation model – An experimental and modeling study

Thermal runaway is a major hazard of Li-ion batteries (LIBs). This study investigates thermal runaway propagation (TRP) of LIB modules made of 18650 form-factor LIB cells. This type of LIB cell is widely used in applications such as data center battery backup units and electric vehicles. TRP experiments were conducted with single cells or nine-cell arrays for two types of LIBs. The cathode materials for the two LIB types were lithium cobalt oxide (LiCoO2 or LCO) and lithium iron phosphate (LiFePO4 or LFP). Both battery types had graphite anodes and were tested at 100% state-of-charge. The TRP rates across the LCO cell arrays were significantly faster than those across the LFP cell arrays. Experiments also showed that heat transfer by ejected material within an enclosure can significantly accelerate TRP and needs to be included the TRP model. In addition to the TRP experiment, heat transfer between copper slugs with the same shape as an 18650 LIB cell was experimentally investigated to support the heat transfer modeling. The TRP modeling of the 18650 cell arrays included modeling LIB cell thermal runaway behavior and heat transfer between cells. A previous study developed a lumped thermal runaway model for single 18650 cells. Based on this approach, the current study developed a network TRP model coupling the lumped cell-level thermal runaway model with heat transfer among cells. The model successfully predicts the average TRP rate in LFP arrays and illustrates the importance of different energy release and heat transfer modes for different battery types. The modeled TRP rates for LCO arrays showed some deviations from the experiments due to the increased complexity and greater importance of heat transfer by ejected material.