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.
Fickian Moisture Diffusion
Meaning
Mass transport kinetics describe the rate at which water molecules penetrate a solid material based on a concentration gradient. In battery pouch materials, fickian moisture diffusion predicts how quickly ambient humidity will move through the polymer layers to reach the sensitive electrolyte. The rate of this movement depends on the diffusion coefficient of the specific polymer and the thickness of the barrier.
Diffusion Coefficient
The numerical value representing the ease with which molecules move through a medium is determined by material chemistry. A lower coefficient in a sealant means moisture takes longer to penetrate the cell.
Gradient Effect
Under the laws of fickian moisture diffusion, the flow of moisture always moves from a region of high concentration outside the cell to a region of low concentration inside. High humidity environments increase the driving force behind this movement. This pressure accelerates the degradation of internal components.
Barrier Performance
Material selection for cell packaging relies on measuring the total moisture intake over a defined period. A thick aluminum layer typically stops the flow entirely, leaving the side seals as the only path for ingress. Designers use these calculations to estimate the shelf life of a cell before internal contamination occurs.