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

•Vertical thermal runaway propagation occurs only if the SOC is larger than 50%.•The critical spacing is 4 mm and 6 mm for 80% and 100% SOC batteries, respectively.•Flame is the key reason for vertical propagation and conduction is not important.•The minimum required energy triggering propagation is 5 kJ. 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/m2·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.