Cooling Along the Path of Least Resistance

A cylindrical lithium-ion cell is not a thermally uniform object. Its internal structure is a spiral wound assembly, commonly called a jelly roll, formed by rolling long strips of anode foil, cathode foil, and separator into a cylinder. This construction is what gives cylindrical cells their high active surface area and low internal resistance, and it is also what makes their thermal behavior strongly directional.

The mechanism is straightforward. Heat traveling along the cell axis, or circumferentially around the winding, moves parallel to the layers. It travels through continuous metal foil, which conducts well. Heat traveling radially, from the center of the roll outward toward the can wall, must cross every layer interface in the stack: foil, separator, electrolyte, foil, and so on, dozens of times. Each interface adds contact resistance, and the separator and electrolyte are poor conductors to begin with.

The measured consequence is large. Published measurements of cylindrical cells consistently report radial thermal conductivity between roughly one and two orders of magnitude below axial conductivity, depending on cell format, chemistry, and measurement technique. Drake et al. reported radial conductivity approximately two orders of magnitude below axial for 18650 and 26650 cells. Modeling literature commonly treats the jelly roll as a homogenized anisotropic solid for precisely this reason, because ignoring the anisotropy produces substantially wrong peak temperature predictions.

For a pack designer, this is not an academic detail. It means the cell is presenting two very different thermal paths, and the choice of which one to use has a first-order effect on how well the pack manages heat.

Given the above, extracting heat axially is the obvious choice. Most designs do not, and the reason is geometric rather than thermal.

In a conventional cylindrical cell, the positive terminal is the top cap and the negative terminal is the can, with the can bottom serving as the accessible negative contact surface. Interconnection therefore happens at both ends: busbars or welded tabs at the top for positive, and at the bottom for negative. Both axial ends are consumed by electrical connection hardware.

That leaves only the sidewall available for thermal contact. So the designer, having no other option, routes the cooling path through the radial direction, which is the worst-conducting direction the cell has. Cold plates run alongside the cells, or coolant flows through the interstitial space, or thermally conductive potting fills the gaps.

These approaches work. They are used successfully in production vehicles. But they are compensating for a geometric constraint, not optimizing for the cell's thermal physics.

The MonoLith architecture resolves the constraint by moving both electrical contacts to the same axial end.

The negative contact connects to the top rim of the cell can. The positive contact connects to the center portion of the top cap. The two are separated by the cell's existing gasket. Because the can is electrically continuous, contacting it at the top rim is electrically equivalent to contacting it at the base, so no modification to standard cell construction is required.

With both contacts at the top, the entire bottom axial end of every cell becomes available. Cells are seated with their bases against a thermally conductive surface, with a thermal interface layer between the cell base and the plate to accommodate surface irregularity and ensure consistent contact.

Interconnection at the top is handled by collector plates that overlie the cell terminals and are ultrasonically wire bonded to them. Because the collector plates are planar and the terminals are co-planar, a single collector plate assembly can address every cell in a module, and multiple electrically isolated collector plates can address different subsets of cells to configure series and parallel arrangements without changing the mechanical design.

Once the cell bases are the thermal interface, the surface they sit against can be given a second job.

In the MonoLith architecture, the thermal plate is not a component bolted into a chassis. It is the chassis. The plate is I-shaped in cross section, carries internal coolant passages along its length, and presents thermally conductive faces on both sides so that two rows of modules can be mounted back to back on a single plate. Mounting channels run lengthwise down each face, and modules attach to those channels in a way that holds them in compressive thermal contact with the plate face.

This consolidation has several consequences that matter more at the system level than at the component level.

Two further characteristics follow from the architecture rather than being added to it.

The architecture suits applications where volumetric packaging is constrained, where thermal uniformity across a large cell count matters, and where field serviceability has real value. Development and prototype programs benefit specifically from the configuration flexibility, since energy and voltage targets frequently move during a program and the plate-length variable absorbs those changes without a structural redesign.

It suits high-cell-count cylindrical architectures better than low-count or large-format prismatic designs, where the anisotropy argument does not apply in the same form.