Experimental study on the vertical thermal runaway propagation in cylindrical Lithium-ion batteries: Effects of spacing and state of charge

Understanding thermal runaway propagation contributes to the fire safety of lithium-ion battery packs. The vertical propagation is a possible path due to jet flames, but this path has rarely been studied. In this paper, the effects of the state of charge (SOC) (50%, 80%, 100%) and spacing (0, 4, 6, 8 mm) on vertical propagation between two 18,650 cells are studied experimentally, by igniting the lower battery using a heater. We use a copper tube that has the same shape as the battery to replace the upper one, to obtain the heat transfer coefficient of 11.57 W/m(2).K in our experiments and the battery specific heat capacity of 1120.2 J/kg.K. Results show that thermal runaway propagation occurs only when the SOC is larger than 50%. The critical spacing triggering propagation is 4 mm and 6 mm for 80% and 100% SOC batteries, respectively. We find the flame (radiation and convection) is the key reason for propagation while conduction is not important. Regarding the critical spacings, the upper battery is ignited by the stable burning of lower battery, rather than the first jet flame, which causes propagation immediately when reducing spacing. We calculate the heat generation of the lower battery is 29.59, 28.22 and 26.41 kJ for 100%, 80% and 50% SOC. The minimum required energy triggering propagation for the upper battery is 5 kJ. This study reveals the heat transfer path, the minimum required energy, and the critical SOC and spacing of vertical thermal runaway propagation, contributing to preventing propagation.

Automated summary

This study experimentally investigated the effects of SOC and spacing on vertical thermal runaway propagation between lithium-ion battery cells. Results showed that thermal runaway only occurred when SOC was above 50%, with critical spacings triggering propagation at 4 mm and 6 mm for 80% and 100% SOC batteries. The flame was identified as the key reason for propagation, rather than conduction, and the minimum energy required to trigger propagation for the upper battery was found to be 5 kJ.