<p>Safety is one of the most critical concerns for lithium-ion batteries. &nbsp;If a lithium-ion battery is mechanically, electrically, or thermally abused, thermal runaway may be initiated. &nbsp;During thermal runaway, a series of exothermal reactions produce a large amount of heat and gas in a short time which causes the temperature and pressure inside the cell to increase rapidly. &nbsp;Two pressure-activated safety devices, a current interrupt device (CID) and a vent mechanism, are commonly built into the cap of cylindrical 18650 lithium-ion cells to isolate the cell electrically and relieve internal cell pressure prior to case rupture, respectively, in an abuse or thermal runaway event. &nbsp;The activation pressure for these two mechanisms, and how it varies with temperature, is an important parameter for thermal runaway modeling when it incorporates electrolyte evaporation, gas generation and venting. &nbsp;This paper presents a method to extract the vent cap assembly from commercial 18650 lithium-ion cells and measure the CID- and vent-activation pressures with a customized experimental setup. &nbsp;The paper also presents a method, based on computed tomography (CT) imaging, to reconstruct 3D models of the vent cap assemblies and conduct finite element modeling (FEM) to simulate the CID- and vent-activation pressures.&nbsp; The experimental data were collected at ambient temperature and 100 ℃ and simulations were conducted over a wider temperature range, from ambient to 300 ℃.&nbsp; Using results for the LG M36 cap as an example, the experimentally measured CID- and vent-activation pressure at ambient temperature were 1.393 ± 0.113, and 2.363 ± 0.199 MPa, respectively. &nbsp;From the FEM, the CID- and vent-activation pressure obtained at the same temperature were 1.482 and 2.413 MPa, respectively, indicating good agreement between the model and experiments. &nbsp;The FEM simulations showed that the sensitivity of peak stress to internal pressure decreased as the peak stress was in the vicinity of the ultimate tensile strength of the material. &nbsp;This result helps explain the variation in experimentally measured vent-activation pressures. &nbsp;The simulation results also showed that the vent-activation pressure decreased with the temperature, which agreed with the experimental measurements. &nbsp;Using the MTI cap as an example, the simulated vent-activation pressure was 2.379 MPa at ambient temperature, 1.972 MPa at 100 ℃, and 0.448 MPa at 300 ℃. &nbsp;The experimental setup may be used in future studies to measure the activation pressures of future cylindrical cells and the validated FEM model may be used to design more reliable and effective CID and vent mechanisms.</p>

Experimental Measurement of CID- and Vent-Activation in Cylindrical Lithium-Ion Batteries

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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