The thermal behavior of a lithium-ion cell is inseparable from its geometry. Cylindrical cells are discussed extensively in this blog because they are the dominant format in Indian two-wheeler packs today, but pouch cells and prismatic cells are present in the market and are gaining share, particularly in larger three-wheeler and light commercial vehicle applications where energy density and pack integration requirements differ. The thermal management challenges for pouch and prismatic formats are different from cylindrical cells in ways that require different modeling approaches and different charge protocol logic.
The comparison is not about which format is better. Pouch cells, cylindrical cells, and prismatic cells each have tradeoffs in energy density, cost, thermal management complexity, and cycle life under specific operating conditions. The engineering question is how to design a charge protocol that is appropriately calibrated for the thermal characteristics of whichever format you are working with.
Pouch cell thermal geometry
A pouch cell is a flat stack of electrode layers in a flexible foil package. The electrodes are stacked (not wound), and the two large flat faces of the cell provide the primary thermal surfaces. Heat generated in the electrode stack must travel through the thickness of the stack to reach either the top or bottom large face, or alternatively through the narrow edges.
The thermal conductivity in the through-thickness direction (perpendicular to the electrode layers, which is the short dimension of the cell) is similar to the radial direction in a cylindrical cell: low, in the range of 0.3 to 1.0 W/m per Kelvin, because you are conducting through the separator and electrolyte layers. The in-plane conductivity along the electrode surface is much higher, in the range of 15 to 30 W/m per Kelvin, because you are conducting through the metal current collector foil.
This anisotropy has a direct consequence for heat accumulation in pouch cells: if the large faces are cooled (as in a liquid cooling plate design), the through-thickness gradient is the dominant thermal driver, and it is relatively short path for larger cells (maybe 5 to 15 mm through-thickness). If only the edges or tabs are cooled (more common in passive cooling designs), heat must travel the full in-plane distance from the center of the electrode to the cooled edge, which can be 50 to 100 mm for medium-sized pouch cells. The center of a large pouch cell with edge-only cooling runs significantly hotter than the periphery.
Prismatic cell thermal geometry
Prismatic cells use a wound or stacked electrode configuration inside a rigid aluminum housing. The housing acts as both structural support and thermal conductor. In liquid-cooled applications, the aluminum case makes direct contact with the cooling plate, providing a well-defined thermal interface. In passive cooling designs, the large flat face of the prismatic case is the primary heat rejection surface.
The electrode-to-case thermal resistance in a prismatic cell depends on the internal construction. A wound prismatic (where the electrode strip is wound into a flat oval) has similar radial resistance to a cylindrical cell in the direction perpendicular to the winding axis. A stacked prismatic has the through-thickness resistance of the electrode stack to the nearest case wall. In either design, the distance from the internal electrode stack to the case surface is typically 10 to 30 mm, shorter than the depth of the electrode center in a large pouch with edge cooling.
An important characteristic of prismatic cells that differs from pouch cells is the rigid case. The aluminum housing constrains the swelling of the electrode stack during cycling and at elevated temperature. Pouch cells, with their flexible foil packaging, allow the electrodes to swell more freely. This matters for heat accumulation because electrode swelling during fast charge compresses the internal contact resistance, which affects the local heat generation distribution. In a pouch cell stack mounted in a module, the compression state changes with charge and temperature in ways that shift the contact resistance and local heat distribution across the stack. In a rigid prismatic cell, this effect is more constrained.
Different failure signatures under fast charge
Given these geometric differences, pouch cells and prismatic cells develop different failure signatures under identical fast-charge protocols, and understanding the distinction helps in root cause analysis during cell development and field failure investigation.
Pouch cells with center-biased heat accumulation (from edge-only cooling) develop non-uniform aging across the electrode area. The center of the cell runs hotter and ages faster. This shows up in capacity fade as a gradual loss concentrated in the high-resistance center region, which is not easily diagnosed from terminal measurements alone. Swelling in the center region may be visible on physical inspection of a degraded cell but is not present early in cell life. In failure analysis, cross-sectioning a degraded pouch cell and looking at the electrode surface across the area (SEI thickness, evidence of plating or active material particle cracking) will reveal the center-biased aging pattern.
Prismatic cells under fast charge and passive cooling tend to show tab-proximate degradation. The current collection path in a prismatic cell routes current through a relatively small number of tab contact points. Near the tabs, both current density and heat generation per unit area are elevated compared to the bulk of the electrode. Under high C-rate charging, these tab-proximate regions age faster. This can show up as localized capacity loss and increased impedance near the tab connections, which in severe cases can lead to tab weld failure or tab corrosion as secondary failure modes.
Protocol implications for each format
For pouch cells in passively cooled two-wheeler pack configurations (which are not the dominant format but do appear in some three-wheeler and delivery vehicle applications), the charge protocol thermal limit should be derived from the estimated temperature at the center of the electrode area, not from the thermistor at the cell surface near the edge. A center-temperature model requires knowledge of the in-plane thermal conductivity and the heat generation distribution, which is more complex than the 1D radial model for cylindrical cells but follows the same physical principles.
For prismatic cells, the protocol should account for the tab-proximate heat generation asymmetry if the cell design has single-ended tabs (tabs at only one end of the cell). The current density and heat generation near the single-ended tab can be significantly higher than at the distal end during high-rate charging. A protocol that sets its thermal limit based on a cell-average temperature estimate will be optimistic about the tab-region temperature. A thermistor placed near the tab, rather than at the cell surface center, gives a better proxy for the hottest region in a single-ended tab prismatic cell design.
The modeling approach is format-specific, not format-agnostic
The larger point from this comparison is that the thermal model architecture needs to be appropriate for the cell format. A cylindrical cell model that uses a 1D radial heat equation in cylindrical coordinates is a reasonable approximation for that geometry. Applying the same model structure to a pouch cell by treating it as an equivalent cylinder is an approximation that can be misleading if the in-plane gradient is the dominant feature of the pouch cell's thermal behavior.
We do not do this because it is complicated for its own sake. We do it because the protocol decisions that come out of the model depend on which gradient is being managed. If your pouch cell model does not capture the center-to-edge in-plane gradient, the charge protocol it produces will not adequately protect the cell center from thermal stress. The model complexity is in service of protocol correctness, not complexity for its own sake.