Nowadays lithium ion batteries (LIB) became one of the key technologies for energy storage in different applications because of their high energy density and their advanced stage of development. Therefore, LIB’s are not simply rated by their performance parameters but also by issues of safety. With respect to the interaction of electrical and chemical hazards as well as emergence of fire and explosions, the thermal runaway represents the main risk potential related to the extended use of LIB’s. For safety studies a thermal runaway can be provoked by different events which can be analyzed via temperature and voltage monitoring, as well as measurements of gaseous products and post mortem studies.
The safety assessment of LIB’s is examined using various standards before the battery is approved on the respective market. The requirements of the standards for the individual tests can be very different. Therefore, it is necessary to identify the crucial parameters to unify a particular test. In order to understand the crucial parameters of a crush test, this study presents the results of different external bracing pressures, punch diameters and punch speeds during thermal runaway of LIB’s in a custom-made battery cell investigation chamber. This chamber allows the determination of parameters which influence the response of LIB’s to internal short circuits. The response is analyzed via measurement of cell voltage, temperatures, as well as camera recording, and FTIR spectroscopy to identify and quantify infrared active gas species.
The similarities and differences of the various test procedures on the electrical, thermal and chemical response of the cells are explained and the changes in the mechanical behavior is discussed. If a bracing device is used during the crush test, the cell is not able to inflate which results in a influence on the electrical as well as chemical reaction process. The reaction time is shortened, there is a greater mass loss during the thermal runaway and differences in various reaction gas concentrations can be measured. For the increase of the punch diameter there is a stronger initial voltage drop, a faster decrease of the measured cell surface temperature and a linear increase of the force as well as of the deformation required for the thermal runaway. On the other hand, the battery cell exhibits a much stiffer behavior when the crush speed is increased. Therefore, lower deformations und loads are necessary to initiate a thermal event. Both test series mainly influence the mechanical response of the LIB’s. With the dependencies of the required load and deformation for initiating a thermal runaway at different test parameters, a maximum acceptable deformation can be calculated to avoid an internal short circuit in the cell.
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