Study on the extreme early warning method of thermal runaway utilizing li-ion battery strain

Effective early warning of li-ion battery (LIB) thermal runaway (TR) is essential for the safety utilization, especially before the fire risk has been formed. In this work, a new warning method with strain parameters is proposed and discussed, which presents an ultra-early warning signal with high reliability. The physical model of strain and internal state of battery has been initially established based on theoretical analysis, which clarifies the close relationship between battery strain with internal gas production. Subsequently, overheating tests of prismatic LIBs have been conducted considering the influence of state-of-charges, and the battery strain, expansion force and temperature parameters have been analyzed throughout the TR process. The results reveal that the strain of battery surface center shows the most pronounced variation, reaching the maximum of 8137.6 με. Once the strain change rate reaches 10 με/s, a turning point can be observed, which is derived mainly caused by the variation of internal gas accumulation rate. Simultaneously, the expansion force rate also reaches a critical point of 7.5 N/s. The strain and strain change rate successively reach their peaks before the safety valve opens, demonstrating that strain parameters are more sensitive to TR compared to temperature and expansion force. On this basis, a coupling model between state-of-charge and strain parameters has been developed, and a comprehensive five-level early warning model has been established by integrating expansion force and temperature parameters to cover the entire TR process. This research provides a theoretical foundation and technical support for enhancing LIB early warning system.