Lithium-ion batteries are a proven energy storage device which continue to gain market share across a wide range of applications.  However, the safety of these devices is still a major factor in many applications.  In failures which result in thermal runaway, a series of chemical reactions are initiated which produce a large quantity of gas and heat.  The pressure and temperature inside the battery rise sharply and may cause a fire or explosion if the metal casing ruptures.  The lithium-ion battery cap is a key safety device used in 18650 format cells to mitigate an energetic failure of the metal casing.  The lithium-ion battery cap consists of several components, such as positive terminal, positive thermal coefficient (PTC) disk, current interrupt device (CID), sealing rings, etc.  In this paper, the cap structure and venting parameters of three cap designs are analyzed.  The caps are extracted from commercially available cells: MTI Corporation 18650, LG MJ1 18650, and K2 Energy 18650E.  The geometry and dimensional information of each component in the cap assembly are extracted using 3D x-ray scans and measurements from cap dissection.  Solid models are reconstructed from the x-ray images for each cap design to aid in geometric characterization.  Venting parameters, such as CID activation pressure, vent activation pressure, and discharge coefficient are characterized experimentally.  The activation pressure of the CID and the activation pressure of the vent mechanism are measured in one set of experiments.  Compressed nitrogen is used to pressurize the cap until the point of failure.  The experiments are conducted at ambient temperature and high temperature (100 ℃).  Results indicate that activation pressures vary among manufacturers and activation pressures are reduced as temperature increases.  The MTI cap has the largest CID and vent activation pressure, while K2 cap has the smallest CID and vent activation pressure.  After the CID and vent activation experiments, the structure change of the caps is observed from the 3D x-ray scanning images.  A second set of experiments is used to analyze the flow characteristics of each cap design.  Compressed air is used to simulate the gas flow through the cap.  Discharge coefficients are calculated from the measured mass flowrate and pressure ratio across the caps.  Experiments indicate that the LG cap allowed the most mass flow of the three designs, owed to a unique CID design which creates a large flow area after vent activation.  The discharge coefficients of the battery caps follow the same trend as a sharp-edge orifice, albeit at a reduced magnitude due to the more tortuous flow path.  The discharge coefficient is an important parameter needed in thermal runaway models which consider venting from the cell.


Analysis of Lithium-Ion Battery Cap Structure and Characterization of Venting Parameters

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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