Intra-cell thermal runaway propagation within a cylindrical battery induced by nail penetration

The thermal runaway in batteries presents significant safety concerns. This study investigates the characteristics of intra-cell thermal runaway propagation in the 18650 cylindrical batteries triggered by nail penetration from the top cap. The experiment changes the nail penetration depth and speed in parallel to the cylindrical axis. The minimum penetration depth required to trigger the thermal runaway is found to be around 10 mm from the top, when the nail tip is just fully embedded in the jelly roll, reaching the largest speed of top-to-bottom intra-cell thermal runaway propagation (about 100 mm/s). Further increasing the nail-penetration depth, the propagation rate continuously decreases tenfold to about 10∼20 mm/s when the cell is pierced, because of the cooling effect of metal nail. The intra-cell propagation rate increases from 9 mm/s along with the nail-penetration speed and eventually approaches the nail-penetration speed, because the propagation mechanism changes from heat-transfer controlled to structure-damage controlled. Both reaction-propagation and fire-spread models are adopted to explain the effect of nail penetration on the process of intra-cell thermal runaway propagation. These findings help understand thermal runaway behaviors inside battery cells and facilitate the development of safer battery designs and effective safety management strategies.