Meaning
Composite packaging materials provide a moisture and oxygen barrier for soft-shell battery cells by combining a metallic core with polymer layers for thermal sealing. This multilayered structure is the primary housing for pouch format lithium batteries and prevents the ingress of environmental contaminants that would degrade the electrolyte. The aluminum laminated film consists of an outer nylon layer for mechanical strength, a central aluminum foil for gas resistance, and an inner polypropylene layer for chemical stability.
It functions as a flexible container that accommodates the slight expansion of the cell during cycling while maintaining a hermetic seal. The application of this material ends at the external tab terminals where the polymer must bond to the metal leads.
Barrier Property
Resistance to the transmission of water vapor is the most critical function of the metallic layer within the laminate. Because lithium salts are highly reactive with moisture, the aluminum laminated film must provide a near zero transmission rate to ensure a long shelf life for the cell. This metallic core also prevents the escape of volatile organic solvents that make up the liquid electrolyte.
Even a small pinhole in the foil can lead to the premature failure of the electrochemical system. Manufacturers use specialized testing to ensure the foil remains intact after the deep drawing process that forms the cell pocket.
Thermal Sealing
Inner layers of the laminate are designed to melt and fuse together when heat and pressure are applied during the final assembly of the pouch. This heat sealable polypropylene must be thick enough to insulate the aluminum foil from the internal chemistry to prevent corrosion. When the aluminum laminated film is pressed at the edges, it creates a permanent bond that can withstand the internal pressure generated during normal operation.
The temperature at which this bond forms must be carefully controlled to avoid damaging the polymer structure. A consistent seal width is necessary to prevent leaks at the corners.
Mechanical Durability
External layers of the material protect the sensitive metal foil from physical abrasion and puncture during the handling of the battery. Nylon or polyester is used for this purpose because these polymers provide excellent stretch and resistance to tearing. The aluminum laminated film must remain flexible enough to be folded and tucked into a battery module without cracking the barrier layer.
During the life of a vehicle, the pouch will experience vibrations and temperature swings that test the fatigue resistance of the laminate. Proper selection of the outer coating prevents the environment from compromising the internal chemistry. Quality control checks involve checking the peel strength of these bonded layers.
