Thermal runaway discrete propagation mechanisms and fire characteristics of lithium-ion battery modules with typical electrical structures

Conflagrations originating from the thermal instability of lithium-ion batteries (LIBs) have posed a serious hazard to public safety. The fire dynamics of the system-level LIB pack with complex electrical topologies is still unclear. This paper focused on the phenomenon of thermal runaway (TR) discrete propagation, which spreads TR among the LIB pack leapingly, and conducted thermophysical experiments. Laboratory-scale LIB modules with typical electric structures were constructed to reveal the mechanisms and patterns of the phenomenon from the cell component level, as well as the fire behaviors and characteristics. The horizontally insulated calorimetric wind tunnel provided the LIB modules with forced air cooling. Results showed that smooth occurrence of TR discrete propagation required the overcharge current that facilitated the steady and concentrated growth of lithium dendrites and the overcharged battery interior that maintained the low temperature and gas pressure before the lithium dendrites pierced the separator. Under these experimental conditions, it occurred when the charge state of the battery remote from the heat source exceeded 135.56 %, with a minimum onset temperature of 64.6 °C. Its concomitant electricity transmission from external short circuits lowered the TR onset temperature of the LIB submodule and hastened the spread rate of TR. The heat release rate of the burning LIB module peaked at 35.612 kW, while each cell reached a total heat release of 107.468 kJ, and the duration of each flaming was not affected by TR discrete propagation. The results provide insight into the fire mechanisms and characteristics of the high LIB concentration scenario.