The purpose of thermal interface materials (TIM) is to transfer heat between two solid surfaces. In the case of a battery this is normally between the outer surface of the cell case and a cooling plate.
Production tolerances of the cell, cooling plate and the assembly will mean there are gaps between the two surfaces. If a cell runs hotter than another cell then it will age faster or even become unstable if it gets very hot. Therefore it is important to run all of the cells at the same temperature and one element of that is a consistent thermal connection to the cooling system.
TIM materials tend to be grouped based on how they are supplied/applied:
- Thermal paste
- Thermal adhesive
- Thermal gap filler
- Thermally conductive pad
- Thermal tape
There are a few key attributes for any thermal interface material:
- good thermal conductivity
- compliant
- resistant to chemicals and gases
- non-combustible
- easy to apply
- stable over lifetime
Good Thermal Conductivity
The thermal conductivity of aluminium = 236W/m.K, the thermal conductivity of a typical TIM ~ 2W/m.K a quite poor thermal conductor. However, no surfaces are flat and the thermal conductivity of air = 0.024W/m.K a good insulator.
Thermal Conductivity of the Active Layers
If we look at the active layers of a cell the thermal conductivity in the plane of the layers is approximately 10x to 100x that through the planes.
This should not be unexpected as the electrodes are made from sheets of aluminium and copper. Two of the best materials for thermal conductivity.
These values though have a large range:
- 15 to 160 W/mK In-Plane
- 0.2 to 8 W/mK Through-Plane
In the units for thermal conductivity you will see that this is per unit thickness of the material. Closing the distance between the two surfaces means you will improve the thermal conductivity. You will also reduce the amount of TIM required, a good thing as it can be expensive material.
Compliant
If you are using a gap pad type of TIM then you need it to be compressible so that it can comply to the variation in distance between the two surfaces. You will also need some spring force in the material so that it can remain in contact with both surfaces over the lifetime of the battery.
If the TIM is an adhesive material then it may not need the compliance of a pad if it remains adhered to both surfaces over the lifetime.
Resistant to Chemicals and Gases
In any battery pack design you need to consider all of the materials, chemicals and gases that might be present in the battery and in the surrounding environment. You should then look at the defined and possible interfaces of these materials and look at their compatibility.
This should include coolant fluid leaks and cell venting or even combustion.
Non-Combustible
In the case of a battery cell going into thermal runaway it is important that the surrounding materials do not add to the overall combustion. This includes the TIM materials.
If the pack is designed so that it can continue to operate even if one cell fails catastrophically then it is important that the TIM materials do not degrade or the result could be the overheating of other cells in normal use.
Easy to Apply
It is important that these materials interface with all surfaces as designed. Some materials will be easier to apply than others depending on your design.
Therefore it is important to design the application for the manufacturing environment and hence include the manufacturing engineers within the design process and material selection.
Stable Over Lifetime
The stability of the material over the lifetime of the product is very important as you need it to remain in contact with the two surfaces, but at the same time you don’t want it to migrate.
Consider: gravity, change in gap distance due to expansion and contraction, vibration and impact. All of these could move the material out of position or mean that it just loses contact with one or both of the surfaces.
Use the right material, check compatibility, check the tolerances of the design, ensure it can be applied as per the design and ensure it will survive the lifetime of the battery pack.
We will cover the different types of TIM and list the manufacturers of TIM in further posts. Please drop me a line if you would like to contribute, Nigel@batterydesign.net
Battery Cooling Options
There are many battery cooling options, which is better or best depends on the cell selection and application. There is no right and wrong. However, let’s look at them and at a first attempt list of advantages and disadvantages.