Mechanical Abuse Simulation and Thermal Runaway Risks of Large-Format Li-ion Batteries

Abstract

Internal short circuit of large-format Li-ion cells induced by mechanical abuse was simulated using a modified mechanical pinch test. A torsion force was added manually at ~40% maximum compressive loading force during the pinch test. The cell was twisted about 5 degrees to the side by horizontally pulling a wire attached to the anode tab. The combined torsion-compression force created small enough failure at the separator and allowed testing of fully charged large format Li-ion cells without triggering thermal runaway. Two types of commercial cells were tested using 4-6 cells at each state-of-charge (SOC). The 18 Ah LiFePO4 (LFP) and 25 Ah Li(NiMnCo)1/3O2 (NMC) cells were tested and the thermal runaway risk (TRR) score system was used to evaluate the safety risk of the cells under the same testing conditions. The aim is to provide the cell manufacturers and end users a tool to compare different designs and safety features.