Helium Mass Spectrometry Principles for Battery Seal Hermeticity Testing
Helium mass spectrometry isolates sub-micron battery seal defects down to 10-12 mbar·L/s, preventing field electrolyte drying and moisture ingress.
Cell Seal Hermeticity
Meaning
Barrier integrity of a battery enclosure that prevents the ingress of moisture and the egress of volatile electrolyte components. Maintaining cell seal hermeticity is required for the electrochemical stability of lithium ion chemistries over a ten year lifespan. It stops at the physical boundary where the lid meets the can or where the pouch material is thermally fused.
Leakage Rate
Quantification of this property involves measuring the mass flow of a tracer gas across the seal under a pressure differential. Standard cell seal hermeticity levels are expressed in millibar liters per second. A value of ten to the power of minus seven often marks the limit for high performance automotive cells.
Environmental Influence
Mechanical stress and thermal cycling during operation can degrade the interface between the metal terminal and the insulating gasket. Loss of cell seal hermeticity leads to electrolyte dry out and internal pressure build up. As the electrolyte evaporates, the ion transport within the cell slows down, which causes an increase in internal resistance.
Eventually the chemistry fails to support the required discharge current and the cell becomes useless. These failures often occur silently until the entire pack performance drops below the rated capacity.
Quality Standard
Manufacturers perform vacuum decay or helium mass spectrometry on every unit during the final assembly phase. This verification of cell seal hermeticity ensures that only units with verified gas tight seals enter the module assembly line. It also acts as a filter for defective raw materials such as inconsistent gasket polymers or contaminated metal surfaces.