Meaning
Measure of the change in the three dimensional size of a body relative to its original state. Lithium ion cells expand and contract as ions move between the anode and the cathode during charging and discharging. Monitoring volumetric strain is critical for designing the battery housing, which must accommodate these physical changes without failing.
This value represents the total change in volume divided by the initial volume of the object.
Internal Pressure
Excessive swelling can damage the internal layers of a cell or put pressure on the neighboring units in a pack. High volumetric strain often correlates with the formation of a solid electrolyte interphase or the onset of lithium plating. Managing this expansion through mechanical constraints helps maintain the electrical performance and safety of the system.
An increase in pressure within the cell can also alter the ion transport rates and increase the internal resistance. Constant monitoring of this expansion during cycling provides a baseline for the expected aging of the cell.
Material Expansion
Silicon based anodes typically exhibit much higher displacement than graphite based versions. Calculating the volumetric strain allows engineers to size the gaps between cells correctly. The choice of active materials and the stiffness of the casing influence the final magnitude of the expansion.
Packaging Constraint
Rigid enclosures limit the ability of the cell to swell, which can increase the internal pressure and affect the chemistry. If the volumetric strain is too high, it may lead to the premature failure of the seals or the deformation of the pack structure. Thermal changes within the cell also contribute to the total volume variation measured during the test.
