Impact of State of Charge and Cell Arrangement on Thermal Runaway Propagation in Lithium Ion Battery Cell Arrays

Lithium ion batteries are increasingly used in electrical energy storage systems despite their thermal and chemical hazards on failure. These hazards are magnified when multiple cells are combined together in a pack because of the risk of cascading failure. An experimental setup was utilized to investigate the dynamics, gaseous emissions, and energetics associated with thermal failure of cylindrical 18650 form factor, 2600 mAh, lithium cobalt oxide cathode cell arrays in an inert atmosphere. Cell state of charge (SOC) and arrangement were altered to investigate mitigation strategies and provide recommendations for safer battery pack design, transportation, and storage. Complete failure propagation was not prevented in any test, but decreasing to 50% SOC did result in propagation speeds 8.5 times slower than at 100% SOC. Introducing a 5 mm gap between cell rows also slowed propagation somewhat, but to a lesser degree than lowering SOC. Maximum temperatures, hazardous gas yields, and chemical heat generation were all reduced for cells at 50% SOC compared with 100% SOC, but introducing 5 mm gaps had little impact on these quantities. Limiting cells to 50% SOC was by far the most effective mitigation strategy tested, but no strategy was able to eliminate the failure propagation risk. Installing battery packs in inert environments, limiting SOC, and instituting gaps between certain cells can all be effective strategies to lessen the severity of cascading failure, but none of these strategies are individually sufficient for safe transportation and storage of battery packs.