Quantifying Micro Leak Thresholds for Lithium Ion Envelope Integrity Assurance
Micro leak rates above 1.0e-6 mbar L/s drive moisture ingress and hydrofluoric acid formation, degrading lithium ion cell capacity and triggering transport failure.
Seal Land Width
Meaning
Dimensional measure of the contact surface area of a pouch cell’s heat seal, extending from the internal cavity to the external edge. This dimension dictates the mechanical strength and the path length for moisture and gas ingress. Optimizing the seal land width ensures the long-term hermetic isolation of the internal cell chemistry.
Physical Barrier
The polymer-aluminum laminate relies on this width to create a tortuous path for moisture diffusion into the cell. Water molecules that penetrate the outer edge must travel across the entire seal land width before reacting with the electrolyte. Maintaining a minimum seal width is critical because moisture reaction with lithium salts produces corrosive hydrofluoric acid that degrades the cell from within.
Process Variable
The heat sealing process uses heated bars under pressure to melt the polymer layers together. During this operation, the polymer flows, and some of it is squeezed out of the sealing area. Measuring the final seal land width after this compression step ensures that the heat sealing parameters were held within acceptable limits.
If the width is too narrow due to excessive squeeze-out, the joint will have insufficient polymer thickness and strength.
Mechanical Force
Mechanical stresses during cell handling and cycling put the heat seal under constant tension. A wider seal distributes these forces over a larger area, reducing the risk of localized peeling or joint failure. Battery sourcing specifications mandate a strict minimum seal land width to guarantee that the pouch maintains its hermetic seal under high storage and operation pressures.