Meaning
The physical migration of water molecules through polymer seals or packaging materials represents a major route for environmental contamination in lithium batteries. In pouch cells with polymer laminate edges, moisture diffusion introduces water into the active cell volume over long storage periods. This transport process occurs continuously because of the vapor pressure differential between the cell interior and the outside environment.
Transport Mechanism
Fickian transport laws govern the rate at which water molecules penetrate the seal boundary. The rate of moisture diffusion depends heavily on the permeability of the polymer adhesive and the thickness of the seal seam. Higher ambient humidity accelerates this migration.
Degradation Severity
Ingressed water reacts with the electrolyte salt to form highly corrosive acid. This chemical reaction driven by moisture diffusion consumes active lithium and damages the electrode structures. Over time, it destroys the solid electrolyte interphase layer.
Prevention Method
Pouch laminate materials incorporate thin aluminum foil layers to block vapor transmission across the face of the cell. Minimizing moisture diffusion requires optimizing the width and formulation of the thermocompression seal at the edges of the pouch. These structural designs are verified by helium leak testing and weight gain trials in environmental chambers to ensure the cells meet their designated twenty-year shelf life.
