In this paper, tests and analysis of thermal runaway propagation for commercial modules consisting of four 41 Ah Li-ion pouch cells are presented. Module samples were tested at 100% state-of-charge and mechanically constrained between two steel plates to provide thermal and mechanical contact between the parts. Voltage and temperature of each cell were monitored during the whole experiment. The triggering of the exothermal reactions was obtained by overheating one cell of the stack with a flat steel heater. In preliminary studies, the melting temperature of the separator was measured (from an extracted sample) with differential scanning calorimetry and thermogravimetric analysis techniques, revealing a tri-layers separator with two melting points (≈135 °C and ≈170 °C). The tests on module level revealed 8 distinct phases observed and analyzed in the respective temperature ranges, including smoking, venting, sparkling, and massive, short circuit condition. The triggering temperature of the cells resulted to be close to the melting temperature of the separator obtained in preliminary tests, confirming that the violent exothermal reactions of thermal runaway are caused by the internal separator failure. Postmortem inspections of the modules revealed the internal electrical failure path in one cell and the propagation of the internal short circuit in its active material volume, suggesting that the expansion of the electrolyte plays a role in the short circuit propagation at the single cell level. The complete thermal runaway propagation process was repeated on 5 modules and ended on average 60 s after the first thermal runaway triggered cell reached a top temperature of 1100 °C.
Understanding Li-Ion Cell Internal Short Circuit and Thermal Runaway through Small, Slow and In Situ Sensing Nail Penetration