Meaning
Permanent volumetric growth in a battery cell occurs as a result of side reactions and structural changes in the electrodes. This irreversible expansion differs from the temporary swelling seen during normal charge cycles. It accumulates over time and can eventually deform the cell housing or the module frame.
Cycle Degradation
Lithium ions entering the anode cause a physical increase in size that should reverse when the cell discharges. However, irreversible expansion builds up as the solid electrolyte interphase layer thickens and consumes active lithium. This growth takes up more space inside the can and puts constant pressure on the internal components.
Internal Pressure
High mechanical force within the cell can crush the separator and lead to an internal short circuit. Managing irreversible expansion involves leaving physical gaps in the module design or using compressible foam pads. These pads absorb the growth to prevent the outer shell of the battery pack from warping.
Cell Safety
Excessive growth often indicates that the battery has been operated outside its temperature or voltage limits. If irreversible expansion is not monitored, it can lead to the rupture of the safety vent or the leakage of the electrolyte. Engineers use this metric to determine the end of life for a battery system.
