The thermal behavior of lithium-ion batteries is a critical factor in their performance, safety, and longevity. Among the key design variables, cell geometry—particularly cylindrical and prismatic formats—plays a significant role in determining thermal profiles. Simulations reveal how differences in tab design, cooling surface area, and material anisotropy influence heat distribution and dissipation, ultimately impacting battery efficiency and reliability.

Cylindrical cells, characterized by their spiral-wound electrode assembly, exhibit distinct thermal properties compared to prismatic cells, which feature stacked or folded layers. The cylindrical design inherently creates a more uniform thermal gradient radially but faces challenges in axial heat dissipation due to limited surface area for cooling. In contrast, prismatic cells offer larger flat surfaces, enabling more efficient heat transfer to external cooling systems but may suffer from localized hot spots due to uneven current distribution.

Tab design is a major contributor to thermal non-uniformity. In cylindrical cells, tabs are typically located at either end, creating a bipolar thermal flow where heat generation is concentrated near the terminals. Simulations show that this configuration leads to higher temperatures at the poles compared to the cell's midsection. Prismatic cells often employ multiple tabs distributed along the electrode edges, reducing current path lengths and minimizing resistive heating. However, improper tab placement in prismatic designs can result in uneven current density, exacerbating thermal gradients. For instance, a study comparing 21700 cylindrical cells with 50 Ah prismatic cells demonstrated that optimized multi-tab designs in prismatic configurations reduced peak temperatures by up to 15% under 3C discharge rates.

Cooling surface area is another critical factor. Cylindrical cells, due to their curved geometry, have less effective contact with cooling plates unless arranged in tightly packed modules with thermal interface materials. Even then, the air gaps between cells can impede heat transfer. Prismatic cells, with their flat faces, allow for direct attachment to liquid-cooled plates or cold bars, significantly improving thermal management. Computational models indicate that prismatic cells can achieve up to 30% better heat dissipation efficiency when coupled with active cooling systems, compared to similarly sized cylindrical cells relying on passive cooling.

Material anisotropy further complicates thermal behavior. Electrode layers in cylindrical cells are wound under tension, creating mechanical stresses that influence thermal conductivity. The radial thermal conductivity of a cylindrical cell is typically higher than the axial conductivity due to the layered structure, leading to preferential heat flow outward rather than along the length. Prismatic cells, with their stacked electrodes, exhibit more isotropic thermal properties, but the presence of separators and uneven compression can introduce localized variations. Simulations accounting for anisotropic thermal conductivity reveal that cylindrical cells may develop higher core temperatures during fast charging, while prismatic cells show more predictable surface temperature distributions.

The interplay between geometry and cooling strategies is evident in high-power applications. For example, cylindrical cells in electric vehicle battery packs often rely on interstitial cooling with air or liquid channels between cells. While this approach works for moderate discharge rates, simulations show that under extreme conditions, such as rapid acceleration or regenerative braking, the central regions of cylindrical cells can exceed safe temperature thresholds. Prismatic cells, with their larger cooling surfaces, integrate more effectively with direct liquid cooling, maintaining lower average temperatures even at high C-rates.

Tabless designs, an emerging innovation primarily in cylindrical cells, aim to mitigate thermal issues by reducing internal resistance. By eliminating traditional welded tabs and instead using continuous current collectors, heat generation is distributed more evenly. Simulations of tabless cylindrical cells demonstrate a 20% reduction in peak operating temperatures compared to conventional tabbed designs, narrowing the thermal performance gap with prismatic cells.

Thermal runaway propagation risks also vary by geometry. Cylindrical cells, when arranged in modules, can experience slower propagation due to physical gaps between cells acting as thermal barriers. However, once runaway initiates in one cell, the radial heat emission can accelerate neighboring cell failures. Prismatic cells, often packed closely together, may facilitate faster propagation if cooling systems fail, but their flat surfaces allow for more effective integration of flame-retardant materials and heat shields.

Advanced simulation tools, such as finite element analysis and computational fluid dynamics, enable detailed comparisons of these geometries. Key parameters include:

| Parameter | Cylindrical Cell | Prismatic Cell |
|-------------------------|--------------------------|--------------------------|
| Cooling Surface Efficiency | Lower | Higher |
| Tab-Induced Heating | Concentrated at poles | Distributed, design-dependent |
| Thermal Anisotropy | High radial, low axial | More isotropic |
| Peak Temperature Under Load | Higher core temps | More uniform surface temps |

In summary, the choice between cylindrical and prismatic geometries involves trade-offs in thermal management. Cylindrical cells benefit from structural robustness and slower thermal runaway propagation but struggle with cooling efficiency. Prismatic cells excel in heat dissipation and uniform temperature distribution but require careful tab design to avoid hotspots. Simulations underscore the importance of geometry-specific thermal optimization to enhance battery performance and safety across applications. Future advancements in tabless designs and anisotropic material engineering may further bridge the thermal performance gap between these two dominant cell formats.