Mechanical stress behavior in lithium-ion batteries varies significantly between cylindrical and prismatic cells due to differences in form factor, structural design, and material distribution. Under identical test conditions—such as compression and indentation—these cells exhibit distinct failure modes and stress responses, influencing their suitability for specific applications. Understanding these differences is critical for optimizing battery safety, performance, and longevity.

Cylindrical cells, characterized by their robust metal casing and spiral-wound electrode assembly, demonstrate high radial strength due to their symmetrical geometry. When subjected to external compression, the circular cross-section distributes stress evenly, minimizing localized deformation. However, under extreme mechanical loading, cylindrical cells tend to fail through buckling or radial collapse, particularly at the weakest point near the cell's mid-height. The spiral-wound design provides inherent resistance to internal electrode displacement, but excessive force can lead to internal short circuits as the separator becomes compromised.

In contrast, prismatic cells, with their rectangular shape and stacked or folded electrode configuration, exhibit anisotropic mechanical behavior. The flat surfaces of prismatic cells are more susceptible to deformation under compression, often leading to bending or warping of the casing. Stress concentrations typically occur at the corners and edges, where structural rigidity is lower. Under indentation tests, prismatic cells show a higher likelihood of separator breach near the indentation site due to the concentrated force on a smaller area. The stacked design can also lead to electrode misalignment under stress, increasing the risk of internal short circuits.

Failure modes differ notably between the two form factors. Cylindrical cells often experience jellyroll unraveling or can rupture at the end caps when over-compressed, releasing electrolyte and potentially triggering thermal runaway. Prismatic cells, meanwhile, are more prone to casing rupture along welded seams or delamination of electrode layers, which can propagate internal damage more rapidly. The rigid casing of cylindrical cells provides better containment of thermal events, whereas prismatic cells may vent gases more abruptly due to their larger surface area and thinner casing.

Design trade-offs between the two form factors become evident under mechanical stress. Cylindrical cells offer superior mechanical stability and thermal management due to their uniform shape, making them ideal for high-vibration environments like electric vehicles. However, their round shape leads to inefficient packing density in battery modules, reducing overall energy density. Prismatic cells optimize space utilization, enabling higher energy density in confined spaces, but their structural weaknesses require additional reinforcement to withstand mechanical abuse. The choice between the two often hinges on the application's priority—durability versus energy density.

Thermal behavior under mechanical stress also varies. Cylindrical cells, with their consistent heat dissipation profile, tend to localize thermal hotspots less than prismatic cells, where uneven stress distribution can create irregular heat generation. This difference impacts safety protocols, as prismatic cells may require more sophisticated thermal management systems to mitigate stress-induced thermal runaway risks.

Material properties further influence stress responses. The thicker casing of cylindrical cells absorbs more energy before deforming, while prismatic cells rely on thinner, lighter materials to save weight and cost. This trade-off affects not only mechanical resilience but also manufacturing complexity and cost-efficiency.

In summary, cylindrical cells excel in mechanical robustness and even stress distribution but sacrifice packing efficiency. Prismatic cells offer better space utilization and higher energy density but require careful engineering to counteract their susceptibility to deformation and localized failures. The decision between these form factors depends on balancing mechanical integrity, thermal safety, and energy density requirements for the intended application. Understanding these differences ensures informed design choices that enhance battery performance and safety under mechanical stress.