This research study investigates how mechanical stress affects lithium-ion battery degradation, a factor less understood than thermal or electrical aging mechanisms. Lithium-ion batteries used in electric vehicles and stationary storage systems are continuously exposed to vibrations and shocks during operation and transport, yet the effects of such mechanical stresses on cell aging and performance have received less attention than thermal or electrical factors.
Cylindrical 18650 NMC/graphite lithium-ion cells (LG INR18650 MH1) were subjugated to controlled mechanical agitation using a custom sled-based acceleration platform for 21 hours. The study tested two cell orientations, axial and radial, to evaluate orientation-dependent sensitivity to mechanical stress. The acceleration sled achieved top speeds of 2 m/s, equivalent to dropping the cell from approximately 20 cm height. Following mechanical stress exposure, cells underwent 100 charge-discharge cycles at 1C/1C rate at 25°C within manufacturer-specified voltage limits, followed by comprehensive post-mortem analysis including half-cell testing and physical inspection.
Cells exposed to 21 hours of mechanical agitation exhibited significant discharge capacity reduction during subsequent cycling compared to control cells. Notably, no significant dependency on cell orientation (axial vs. radial) was observed within standard deviation, suggesting mechanical stress effects are relatively uniform regardless of vibration direction. Post-mortem half-cell testing revealed marginal decreases in anode delithiation capacity while cathode performance remained essentially unaffected, indicating that the anode is more susceptible to mechanical perturbation than the cathode. Physical inspection showed no significant damage in electrode thickness or weight measurements of the cut-out electrode disks.
These findings emphasize the critical importance of incorporating mechanical stress considerations into battery design and testing protocols, particularly for automotive applications where batteries experience continuous vibration and shock events.