Meaning
Permanent increase in the thickness or volume of a lithium-ion cell after extended cycle or calendar aging. End of life swelling results from the accumulation of solid electrolyte interphase products and gas generation from electrolyte decomposition. The expansion must be accommodated by the structural design of the battery module.
Mechanical Force
Expansion of the active materials generates pressure against the surrounding retention plates. This pressure increases the stress on tie rods and compression pads, which must be rated for these final forces. Failure to account for this expansion can lead to structural damage to the module housing.
Cell Degradation
Internal stresses within the cell assembly during swelling can compress separators, increasing the risk of micro-short circuits. The resulting high pressure can also force electrolyte out of the active areas, accelerating capacity loss. Maintaining an optimal preload helps to mitigate these degradation modes over the cell lifespan.
Pack Sourcing
Procurement contracts require suppliers to provide maximum expansion curves simulated to the target cycle limit. Sourcing engineers use this data to determine the thickness of the compression sheets and the strength of the outer bracketry. Accurate predictions prevent the need for expensive pack redesigns late in the vehicle development cycle.
Incorporating these swelling allowances ensures that the battery enclosure remains intact and watertight under all conditions.
