Meaning
A physical deformation metric identifies the change in length or area of a porous membrane relative to its original dimensions when subjected to mechanical or thermal stress. Engineers monitor separator strain to ensure that the critical barrier between battery electrodes does not shrink or tear during the manufacturing and operation of a cell. Excessive deformation can lead to misaligned edges and increase the risk of internal short circuits.
Thermal Stability
High temperatures during the drying process or during an abnormal discharge event can cause the polymer to contract. The shrinkage is a specific type of separator strain that exposes the edges of the electrodes. Manufacturers measure the percentage change in both the machine direction and the transverse direction after exposing the material to heat for a set duration.
Mechanical Tension
Tension applied during the high-speed winding of cylindrical or prismatic cells must be carefully managed to avoid permanent stretching. If the material is pulled too hard, the resulting separator strain can distort the pore structure and decrease the ionic conductivity. Consistent tension control systems are necessary to keep the deformation within the limits specified by the material supplier.
Precise sensors on the production line provide real-time data to adjust the motor speeds and keep the load constant. This prevents the thinning of the membrane which could otherwise compromise the puncture resistance of the finished battery.
Performance Impact
Changes in the dimensions of the separator can alter the local current density within the cell. If one area exhibits higher separator strain than another, the resistance will vary and lead to uneven heat generation. Maintaining a uniform state across the entire surface area is a requirement for achieving long cycle life and reliable safety performance.
