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First and second degassing of automotive Lithium-Ion batteries at overtemperature experiments


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In normal usage operation, lithium-ion batteries (LIB) do not exchange hazardous gases with the environment. But, if there is a defect within the cell, leakage of the LIB with vaporizing electrolyte and serious potential risks1,2,3 are possible. During internal or external battery failures, the pressure inside the battery cell increases due to gas generation until the cell opens and releases toxic and flammable gas.
At overtemperature triggered thermal runaway tests for pouch cells and metal can cells, two degassing events can be identified: A first venting and opening of the cell due to gas generation inside the failing cell. The first degassing appears at lithium nickel manganese cobalt oxide (NMC) – graphite cells at about 120 -140°C cell surface temperature, depending on cell geometry. If the cell further heats up, a second venting at the uncontrollable exothermic chemical reaction, the thermal runaway (TR), can be observed. Both degassing events can be measured with temperature and gas sensors.
First the failing behavior of state-of-the-art LIB is tested with overtemperature abuse tests in a custom-made thermal runaway reactor at VIRTUAL VEHICLE. Experiments are done in nitrogen atmosphere. For all battery tests, state of the art automotive NMC cells are used. Special focus is set on gases produced at first and second venting. Evolving gases are detected and analysed with Fourier Transform Infrared (FTIR) spectrometer and gas chromatograph in parallel. The measured gas composition during first and second degassing inside the nitrogen flushed reactor is shown in comparison and gives information about which gases can be detected first.
Temperature measurements were applied close to the cell opening to measure the vent gas temperature. The experiments show the response of temperature and gas analyzers at first and second venting event.
In this presentation the results of failing automotive batteries in overtemperature test conditions will be presented: the temperature response on cell surface and vent positions, the pressure signal in the test reactor and the gas measurements at first and second degassing.
By focusing on the first venting event early failure detection (i.e. before serious battery failure) incidents might be achieved with temperature and gas measurements.

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