When engineering a Battery Thermal Management System (BTMS) for electric vehicles or energy storage, one of the most critical factors is hidden within the microscopic architecture of the cell itself. Whether a cell utilizes a wound (jelly roll) or stacked electrode assembly, its physical construction fundamentally dictates its thermal behavior.
Heat does not radiate uniformly inside a battery. Because of how the internal layers are arranged, cells exhibit anisotropic (directional) thermal conductivity. To understand why modern cooling systems are meticulously designed to extract heat along specific physical vectors, we must view the cell’s internal layers not just as mechanical components, but as a structural thermal circuit.
1. Through-Plane Heat Flow: The “Bulletproof Vest” Effect (Series Resistance)
An electrode assembly is essentially a repeating sandwich of materials with vastly different thermal properties: highly conductive copper and aluminum foils, porous active materials (graphite, metal-oxides), liquid-filled polymer separators, and microscopic inter-facial gaps.
When heat attempts to move perpendicular to these layers for instance, from the core of a cylindrical cell outward to its radial surface it must pass through every single layer sequentially. In thermal engineering, this behaves exactly like resistors in series:
In a series circuit, the component with the highest resistance acts as a bottleneck for the entire system. In a battery, the polymer separators ( 0.3 W/m·K) and the microscopic voids between layers (thermal contact resistance) act as massive thermal insulators.
We can compare this architectural hurdle to the mechanics of a bulletproof vest. A Kevlar vest does not stop a bullet with a single, solid metal plate. Instead, it relies on dozens of overlapping layers of fibrous material to dampen and trap kinetic energy, preventing penetration.
A jelly roll or electrode stack behaves exactly the same way, but rather than stopping kinetic energy, it dampens thermal energy. The repeating layers and physical interfaces act as a thermal shield, severely trapping heat inside the core. Because of this “bulletproof” layer-by-layer resistance, the through-plane thermal conductivity of a typical lithium-ion cell is remarkably low, usually hovering around 0.5 to 1.5 W/m·K (comparable to wood or plastic).
2. In-Plane Heat Flow: The Thermal Highways (Parallel Resistance)
The thermal dynamics change completely when we observe heat traveling parallel to the winding or stacking direction—meaning heat moving along the length of the layers toward the tabs or the top/bottom faces of the cell.
In this direction, the heat does not have to fight its way through the insulating separator or the inter-facial gaps. Instead, it can travel along the materials simultaneously. This mirrors an electrical circuit with resistors in parallel:
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n a parallel circuit, energy naturally takes the path of least resistance. Inside the cell, the highly conductive metallic current collectors—Copper (390 W/m·K) and Aluminum (235 W/m·K)—act as internal thermal highways.
Instead of fighting through the polymer “bulletproof vest,” the thermal energy completely bypasses the insulators. It rides the continuous metallic foils, rapidly transporting heat from the deep core to the edges of the electrode assembly.
Thanks to this parallel metallic network, in-plane thermal conductivity is typically 25 to 35 W/m·K—an astonishing 20 to 30 times higher than its through-plane counterpart.
Rethinking BTMS: Designing for the Path of Least Resistance
This extreme thermal anisotropy (typically a 20:1 to 30:1 ratio) dictates the modern philosophy of BTMS design. If engineers attempt to cool a cell on the faces where it acts as a thermal insulator, they are fighting the cell’s fundamental physics. This inevitably leads to a high internal temperature gradient , accelerating localized aging, causing unequal current distribution, and promoting lithium plating on the colder outer layers.
To achieve homogeneous temperatures and enable ultra-fast charging, cooling architectures are increasingly designed to align with the in-plane thermal highways:
- Cylindrical Cells: Historically, cylindrical cells (like 18650s and 2170s) were cooled radially using serpentine ribbons woven between the cells. However, extracting heat radially forces it to cross the jelly roll’s high-resistance layers (through-plane). As cells grow larger such as the 4680 format radial cooling traps dangerous amounts of heat in the core. This is why the industry is rapidly shifting toward tabless designs combined with top/bottom (axial) cooling. By extracting heat axially, the cold plate interfaces directly with the parallel thermal circuit of the foils, pulling heat straight out of the core.
- Pouch and Prismatic Cells: Cooling a pouch or prismatic cell through its largest flat faces forces heat to cross the stacked layers (through-plane). While geometrically convenient for packing, it is thermally inefficient. Advanced pack designs are increasingly utilizing edge cooling or direct terminal/tab cooling. By interfacing the heat sink directly with the edges where the continuous foils align, the system leverages the parallel thermal circuit to wick heat away efficiently.
The Bottom Line
A battery cell’s mechanical architecture cannot be separated from its thermal behavior. The overlapping material layers inherently act like a thermal bulletproof vest, resisting heat flow perpendicular to the stack.
The most efficient, fast-charging, and long-lasting thermal management systems do not waste energy trying to push heat through the insulators. Instead, they acknowledge the thermal circuit, routing heat down the highly conductive parallel highways of the current collectors. Ultimately, superior battery thermal management isn’t just about cooling harder; it’s about cooling in the right direction.
Kerem Buğra Batuş, Cell Design & Process Development
At AION Engineering, we accelerate lithium-ion cell development using AI-driven design and advanced technologies, delivering innovative, sustainable energy solutions for a better future