Internal temperature detection of thermal runaway in lithium-ion cells tested by extended-volume accelerating rate calorimetry

•The maximum internal temperatures of cells during thermal runaway are hundreds of degrees higher than surface temperatures.•The real temperature releasing rate inside cell during thermal runaway is much more critical than that on surface.•Measurement of temperature between two identical cells can simulate the internal temperature for pouch cells.•The surface temperature is sufficient for evaluating the adiabatic reactions before the triggering temperature of cell thermal runaway in extended-volume accelerating rate calorimetry test. This paper presents a method of internal temperature detection of thermal runaway in lithium-ion cells during extended-volume accelerating rate calorimetry (EV-ARC) tests. Thermocouples were inserted into pouch, prismatic, and cylindrical cell samples with only moderate breakage of the cell packages. The temperatures measured at cell surface were compared with the internal temperatures. And for the pouch and prismatic cell samples, a common alternative method of simulating internal temperature by measuring the temperature between two identical cells was also evaluated for comparison. The results demonstrate that the surface measurements can be used to investigate the mild side reactions from the onset temperature, where the rate of temperature increase is slow enough to allow equilibration, to the triggering temperature, beyond which thermal runaway cannot be arrested. However, after the triggering temperature, the internal temperature detection is the most accurate for evaluating the maximum temperature and rate of increase. For the pouch cell, the simulated internal temperature detection method was sufficiently accurate, whereas for the prismatic and cylindrical cells, the simulated temperatures were inaccurate.