Safety assessment of thermal runaway behavior of lithium-ion cells with actual installed state

To quantitatively evaluate the overcharge-to-thermal runaway (OC-to-TR) behaviors of lithium-ion (Li-ion) cells in practice, here reported are overcharge tests on Li-ion pouch cells with NCM811 cathode under actual restricted condition. Such a restriction results from two splints during the overcharging process to simulate their actual installed state in the battery pack of electric vehicles (EVs). From the monitoring of the cell voltage, temperature, and expansion force, a unique feature that is discovered in this work is that the OC-to-TR process is strongly correlated to the evolution of expansion force, which can further be divided into five stages with distinctly different features. Moreover, this unique correlation depends critically on the current rate (C-rates) and preload. It is concluded that the Li-ion cells were more hazardous at higher C-rates; when a larger preload was applied to the cell, it exhibited better tolerance to overcharge, but the TR reactions became more hazardous (i.e., higher temperature and more severe TR reactions). These findings allow us to propose a safety evaluation method to evaluate the TR risks and TR hazards for Li-ion cells under different restricted conditions. In addition, a safety management method was proposed based on the evolution of several key parameters. The research results can provide some useful information to the safety design of Li-ion pouch cells and the enhancement of the safety function of battery management system.