Thermal interface materials for battery packs

What does a thermal interface material do between cell and casing?

Two solid surfaces that look flat only ever touch at their high points. The rest of the joint is air, and air is a poor conductor, so heat that should be leaving the cell sits in the cell instead. A thermal interface material is a soft, conformable layer that fills those voids and gives the heat a continuous solid path across the joint.

Inside a battery pack that job has an electrical condition attached to it. The same layer that conducts heat out of the cell must not conduct electricity into the casing. That is the pairing quoted for LJIPL Mr.Bond® Thermal Pads: good thermal conductivity with low thermal resistance, while being an excellent insulation material. It is also why the material is rated for flammability, the pads are UL 94V0 approved, the highest rating under the UL 94 classification.

The third job is mechanical and it is easy to overlook. Cells change dimension slightly as they charge and discharge, and a pack in a vehicle vibrates continuously. Because the pad is compressible it allows for that expansion and contraction rather than resisting it, keeps contact with both surfaces through the cycle, and aids in securing the cells in position when vibrations occur.

Thermal pad or thermally conductive tape?

The honest answer is that it depends on whether you are filling a gap or bonding a surface. A pad is specified by thickness and compressibility: it is cut to a footprint and squeezed into a measured gap between the cell and the casing, and it is the right choice when the gap is the problem. Stocks of Mr.Bond® Thermal Pads are held in roll form and sizes and other thicknesses can be customised as per requirements.

Prateek® Nitrile Foam Tape is the tape form of the same idea. It is used as a thermal interface material, helping transfer heat away from the battery cells and keeping thermal connections consistent within the pack. The LJIPL catalogue lists nitrile foam tape and thermal conductive tapes alongside its other products for the EV battery industry.

A third material is worth naming because it is often mistaken for a thermal one. Acrylic Foam Tape is a sealing and gasketing tape: it helps preserve a water-resistant seal between the casing and the rubber gasket and fills gaps between rigid components. It is doing structural and sealing work, not thermal work, so do not specify it where a heat path is what you need.

What does 1 to 8 W/m·K mean, and which do you need?

W/m·K, watts per metre kelvin, is the standard measure of how readily a material conducts heat. A higher figure means more heat crosses the same thickness of material for the same temperature difference across it. We can offer thermal conductivity from 1 W/m·K to 8 W/m·K as per your specification.

That range exists because a more conductive pad is not automatically the better pad. Conductivity, softness and thickness trade against each other in a real assembly: a very conductive pad that is too firm to conform may transfer less heat in practice than a softer one that actually touches both surfaces. The figure to order against is the one your own thermal model asks for, which is why the grade is specified per enquiry rather than sold as a single stock item. Our guide to what W/m·K means when you are specifying a thermal pad takes that further, with the arithmetic behind why thickness and contact pressure change the result as much as the conductivity figure does.

What to specify when you enquire

• Gap. The distance the material has to fill between the cell and the casing, and whether it varies across the assembly.
• Area. The footprint or pad shape you need covered, so the roll can be slit or cut to it.
• Operating temperature. The temperature the assembly actually reaches in service, not only at ambient.
• Thermal conductivity. The W/m·K figure your thermal model calls for, anywhere from 1 to 8.
• Compliance requirement. Any flame rating the assembly has to evidence, our pads are UL 94V0 approved, and any other material requirement in your specification.

Send those five to our engineering team and the pad will be specified against them, with samples available on request. If the enquiry covers more than the thermal layer, the EV battery tapes hub maps the insulation, shielding, bonding and sealing materials that sit around it in the same pack. For the compliance line in particular, what UL 94 V-0 means on a tape explains what the classification does and does not cover before you write it into a drawing.