Effects of injection methods and occasions of liquid nitrogen on thermal runaway characteristics in 65 Ah LiFePO4 batteries

Lithium-ion batteries (LIBs) pose a significant risk of thermal runaway (TR), and existing methods cannot effectively prevent temperature rise and reignition. In this work, a series of experiments were carried out to investigate the TR characteristics of 65 Ah LiFePO4 batteries and explore the potential of liquid nitrogen (LN) to suppress TR. The effects of different LN injection methods (intermittent and continuous) and occasions are analyzed for the first time. The results show that the hazard of LIB TR increases with the state of charge (SOC). The self-generated heat of 100 % SOC batteries during TR is 4.6 times greater than that of 25 % SOC batteries. Both intermittent and continuous LN injection methods can effectively prevent TR, with intermittent injection yielding about a 10 % higher effective heat absorption rate than continuous injection. Injecting 1.76 kg LN can prevent TR before the safety valve opens at approximately 95 degrees C. However, if TR has already occurred at around 135 degrees C, 1.76 kg LN can only reduce the battery's temperature but not interrupt the TR process. By increasing the mass of LN to 6.66 kg, TR can be successfully suppressed. The work is expected to provide a guideline for suppressing TR in large-capacity LIBs.