Thermal and Mechanical Stress Degradation Mechanisms in Prismatic Formats
Constraining prismatic cells between 300 and 500 kPa prevents electrode delamination and suppresses localized lithium plating over long cycle life.
Ultrasonic Tab Fatigue
Meaning
Mechanical degradation occurring at ultrasonic solid-state weld joints under repeated shock, vibration and thermal expansion loads threatens electrical connection integrity. Evaluating ultrasonic tab fatigue allows interconnect engineers to design reliable busbar joints between thin electrode current collector tabs and heavy module conductors. High-frequency ultrasonic welding joins thin copper and aluminum foils through solid-state atomic diffusion without melting base metals.
Cyclic mechanical stresses from vehicle vibration and thermal cycling concentrate at weld nugget perimeters, initiating micro-cracks that propagate across thin foil layers. The analysis applies to ultrasonically welded thin-foil electrical joints, whereas laser-welded structural joints exhibit distinct fatigue mechanisms.
Fatigue Dynamics
High-cycle mechanical vibration and low-cycle thermal expansion induce complex multiaxial stress fields within thin foil weld zones. Vehicle motion transfers continuous high-frequency vibration through module structures, flexing unconstrained foil tab segments. Concurrent thermal expansion during charge-discharge cycles forces busbars and cell tops to shift relative to one another, imposing cyclic shear loads on weld nuggets.
Stress concentrations focus along the outer edge of the weld tool indentation mark, where material thickness changes abruptly. Micro-cracks initiate at these high-stress notches and grow progressively through individual foil layers under continuous dynamic loading.
Failure Consequences
Progressive crack propagation reduces the net conductive cross-sectional area of the tab joint, raising electrical contact resistance. Increased resistance causes localized Joule heating during high-current operations, elevating temperatures around the damaged joint. Thermal spikes accelerate localized oxidation and speed crack growth, driving a destructive thermal feedback loop.
Complete fatigue failure severs individual foil tabs from the collector stack, reducing usable cell capacity and forcing remaining tabs to carry higher current densities. Severed tabs can physically short against adjacent internal components, creating severe electrical fault hazards within the module envelope.
Mitigation Methods
Interconnect designers reduce fatigue risks through strain-relief geometry, optimized welding tool parameters and potting encapsulation. Forming s-bend flex loops in foil tabs absorbs relative physical motion between cell bodies and busbars, lowering mechanical stress on weld interfaces. Optimizing ultrasonic welding parameters, such as anvil pattern, clamping force and energy delivery, prevents over-thinning of delicate foil edges.
Structural potting resins or vibration-dampening foam blocks encapsulate delicate tab regions, restricting physical movement under shock loads. Standardized fatigue testing subjects tab assemblies to millions of vibration cycles to verify joint durability before production approval.