Evaluating Polyolefin Separator Friction Limits in Lubricated Cell Stacks
Sufficient stack pre-load retention and ceramic coating friction prevent wetted polyolefin separator sliding and tab shear in lubricated cell stacks.
Tab Fatigue Failure
Meaning
Structural breakage of the thin metal electrical connections protruding from a battery cell due to repeated mechanical or thermal stress defines a primary durability risk. Through tab fatigue failure, the continuous vibration from vehicle motion or the cyclic swelling of the electrodes causes microscopic cracks to develop in the copper or aluminum tabs. These cracks grow over time until the terminal can no longer carry the electric current, leading to a sudden loss of module power or a localized hot spot.
It is a primary concern in high-vibration applications such as electric vehicles.
Fatigue Driver
Dynamic loads experienced during road driving and the thermal cycling of the cell layers are the main causes of this structural fatigue. As the vehicle travels, road inputs are transmitted through the pack structure, causing the individual cells to vibrate relative to the fixed busbars. This movement bends the terminal tabs back and forth, introducing localized tensile and compressive stresses.
Additionally, the periodic expansion and contraction of the cells during rapid charge and discharge cycles create a slow, high-amplitude bending force on the connections.
Failure Mode
The fracture typically initiates at the edge of the ultrasonic weld joint, where the tab is thinned or embrittled by the heat of the welding process. This microscopic notch serves as a concentration point for mechanical stress under cyclic load. Once a crack begins, it propagates across the width of the tab, reducing the cross-sectional area and causing the electrical resistance to rise.
The increased resistance generates localized heat during high-current charging or discharging, which accelerates the degradation of the nearby separator and polymer seals.
Risk Management
Selecting ductile materials and adding strain-relief loops to the tab geometry minimizes this failure risk. Sourcing guidelines mandate the use of high-purity copper and aluminum alloys with high elongation-at-break values to resist crack propagation. These flexible geometries allow the tabs to absorb the relative movement without accumulating fatigue damage.