Prismatic Cell Solid State Volume Change during Lithiation
Solid-state prismatic expansion demands dynamic stack compression and strict header weld strain limits to prevent interfacial delamination and capacity fade.
Terminal Seal Strain
Meaning
Mechanical stress concentrated at the interface between an electrical terminal and the cell casing threatens the integrity of the moisture barrier and the structural bond. This localized deformation occurs due to the expansion of the cell internals or external forces acting on the busbars that connect the cells. Terminal seal strain is a critical safety parameter because a failure of the seal can lead to electrolyte leakage and the ingress of oxygen or moisture.
The seal is usually made of a polymer or a glass to metal transition that must remain hermetic throughout the life of the battery. Measurement of the strain in this area is used to validate the design of the cell closure and the terminal assembly.
Leakage Pathway
Progressive failure of the bond between the terminal and the casing allows for the escape of flammable electrolyte and the entry of contaminants. Once terminal seal strain leads to a breach, the chemical stability of the cell is compromised, often resulting in a rapid loss of capacity or a thermal event. The seal must be able to withstand the mechanical vibrations of the application while maintaining its insulating properties.
If the strain is too high, the polymer material may creep or crack, creating microscopic pathways for gas and liquid to pass through.
Stress Concentration
Geometry of the terminal and the method of attachment can focus mechanical loads in a small area, increasing the risk of failure.
Mechanical Test
Rigorous evaluation of the seal integrity involves subjecting the cell to thousands of hours of thermal cycling and mechanical vibration while monitoring for leaks. Terminal seal strain is often measured using high resolution cameras or digital image correlation to track the displacement of the material under load. This testing ensures that the seal can accommodate the expansion of the cell without reaching its breaking point.
Engineers use the data to refine the terminal design and the selection of the sealant materials. A robust seal is necessary for the long term safety and reliability of any energy storage system, especially those used in harsh environments. Ensuring that the strain remains within the elastic limits of the material is a requirement for the production of durable battery cells.