Thermal Runaway Induced Casing Rupture: Formation Mechanism and Effect on Propagation in Cylindrical Lithium Ion Battery Module

With the wide-ranging and ever-increasing applications of lithium-ion batteries in electric vehicles (EV), thermal runaway (TR)-induced safety issues, such as fires and explosions, are raising more and more concerns. In this work, cylindrical 21700 batteries were externally heated to conduct the TR experiment, and the casing rupture in the form of melting holes and tearing cracks was found to be one of the key factors that caused cell-to-cell TR propagation. The appearance and the cross-section microstructure of the ruptured casing showed that the melting hole is formed because of a large current short circuit and the tearing crack is due to a decrease in the mechanical strength at high temperatures. Experimental simulations were conducted to further demonstrate the casing rupture mechanism. In addition, new designs with increased casing thickness were implemented to inhibit the occurrence of casing rupture, and their effectiveness was analyzed by performing numerous TR experiments. The improved casing was used in a commercial battery pack, and no TR propagation occurred during operation. Thus, in this study, the casing rupture mechanism was first elucidated, and guidance for the design of lithium-ion batteries with improved safety was provided.