Study on the Characteristics of Thermal Runaway Expansion Force of LiNi0.5Co0.2Mn0.3O2/Graphite Lithium-ion Batteries with Different SOCs

In recent years, lithium-ion batteries have been widely adopted by the automotive industry because of their high energy density and environmentally friendly nature. However, the thermal runaway of lithium-ion batteries poses a significant risk of explosions, fires, and other hazards. Therefore, it is crucial to have effective thermal runaway warning systems to enhance the safety of battery applications. Currently, the main warning signals for thermal runaway include voltage, temperature, internal resistance, gas composition, and smoke. However, these signals suffer from issues such as low accuracy and delayed warnings. During thermal runaway, voltage, temperature, internal resistance, expansion force, and smoke undergo abnormal changes at varying times, with expansion force abnormalities detected notably earlier. To improve the accuracy and timeliness of thermal runaway warnings, it is crucial to quantitatively measure the changes in expansion force and signal progression related to voltage and temperature during thermal runaway through experiments. In this study, the 51 Ah LiNi0.5Co0.2Mn0.3O2/Graphite commercialized Li-ion batteries were used to study the characteristics of thermal runaway expansion force at different states of charge (SOCs) (25%, 50%, 100%, 110%). The variation patterns of thermal runaway parameters such as temperature, voltage, internal resistance, expansion force, and flame were analyzed. The test results indicate that the expansion force in lithium-ion batteries is related to the lithium-ion concentration in the negative electrode and remains below 2000 N with a rate of change under 1.8 N/s during normal charging and discharging. However, it surpasses 5000 N for thermal runaway. This paper suggests using a 2000 N expansion force as an early warning signal for thermal runaway, which precedes approximately 11.6 s earlier than the voltage signal and 10 s earlier than the internal resistance and temperature signals. Adopting a 1.8 N/s growth rate can further enhance warning time, issuing alerts 134.2 s before thermal runaway. Research confirms that using expansion force as the main signal significantly improves warning time and alarm accuracy in lithium-ion battery safety.