Standard Equivalent Air Leak Rate Conversions for Sealed Commercial Pouch Cells
Standard air equivalent leak conversions transform empirical helium mass spectrometer readings into precise air ingress and solvent loss metrics for pouch cells.
Water Vapor Ingress
Meaning
Slow diffusion of atmospheric moisture into the active volume of a battery cell occurs through microscopic seal defects. This mass transport process is known as water vapor ingress, and it is a leading cause of long-term electrochemical degradation. The moisture reacts with the lithium salts to form hydrofluoric acid.
This process continues throughout the lifetime of the cell unless prevented by hermetic barriers.
Transport Driver
Differences in vapor pressure between the inside and outside of the cell drive the moisture penetration. Higher external humidity accelerates water vapor ingress because it increases the concentration gradient across the seal. Polmeric seal materials have a natural permeability that allows this transport even without physical seal cracks.
This flow is almost independent of external mechanical pressures.
Chemical Degradation
Internal reactions between moisture and the electrolyte generate corrosive species. During water vapor ingress, the incoming water molecules hydrolyze the lithium hexafluorophosphate salt. This chemical reaction produces hydrofluoric acid, which attacks both the positive and negative electrodes.
The resulting corrosion leads to gas generation, transition metal dissolution, and a rapid decline in usable cell capacity. This acid also dissolves the protective solid electrolyte interface layer on the anode, forcing the cell to consume more active lithium to rebuild it.
Industrial Prevention
Packaging designs use metal-coated laminate films to block the diffusion of moisture. High-quality pouch cells rely on metallic barrier layers to limit water vapor ingress over decades of operation. Sourcing agents evaluate the water vapor transmission rates of sealant materials during the cell design phase.
This selection process protects the investment in battery systems from premature chemical breakdown.