Understanding Li-Ion Cell Thermal Runaway through in Situ Measurement of Temperature Distribution

Abstract

Thermal runaway is a critical safety challenge for Li-ion cells, characterized by rapid temperature rise and potentially rapid energy release. Numerical modeling studies revealed that temperature distribution in Li-ion cells is highly non-uniform during thermal runaway1 , 2. However, there is a lack of experimentally measured temperature distribution results for model validation or insightful understanding of thermal runaway. Here we report development of a Li-ion cell with multiple embedded thermocouples and in situ measurement of its temperature distributions during thermal runaway.As schematically shown in Figure 1, seven micro thermocouples are embedded inside a 2.5 Ah Li-ion pouch cell, enabling in situ measurement of temperatures at different locations. Various testing is performed, including nail penetration testing using a 3 mm-diameter stainless steel nail to trigger thermal runaway. Following our earlier practice3, the nail speed utilized is lower than 0.1 mm/s and the voltages between the nail and cell tabs are monitored.Figure 2 shows the spatial temperature distribution along the centerline during 5C (12.5 A constant current) discharging in comparison with that during thermal runaway as triggered by slow nail penetration. It can be seen that the maximum temperature rise during 5C discharging is about 25 °C and that local temperatures closer to tabs are always slightly higher than those away from tabs. In comparison, maximum temperature rise during thermal runaway is up to 700 °C. More interestingly, the pattern of spatial temperature distribution changes dramatically and is highly non-uniform during thermal runaway. Before 238s, there is little temperature rise and temperature distribution is quite uniform, indicating very slow heat generation near the nail. At 240s, the rise of local temperature closest to the nail is more than 400 °C while the rise of local temperature near the tab is only 20 °C, indicating highly non-uniform temperature distribution and rapid heat generation near the nail. Note that temperature gradient is directionally opposite to that during 5C discharging. Local temperature farther away from tabs but closer to penetration location rises more and faster than that closer to tabs. All of the local temperatures then continue to rise while the temperature gradient becomes smaller, suggesting propagation of thermal runaway from nail penetration location to entire cell. Such detailed spatial-temporal experimental results will help in better understanding of thermal runaway behavior and the development of safer Li-ion cells.