How To Use Graphite Sheets with High Thermal Conductivity To Conduct Heat for Battery Cells?

1. Preparation for bonding when used as the cell shell

Before using graphite sheets as the cell shell, it is necessary to clean the surface of the cell to ensure that the surface is flat, free of oil stains and impurities. Special cleaning agents (such as electronic grade isopropyl alcohol) can be used to wipe the surface of the cell, and then it is dried with dry nitrogen gas. Cut the graphite sheets accurately according to the size and shape of the cell. If the cell is in the shape of a cuboid, the cut size of the graphite sheet should be slightly larger than the size of each face of the cell. Generally, a margin of 2-3 mm is reserved on each side for subsequent pasting and packaging.

Pasting process

Select an appropriate thermal conductive adhesive. The thermal conductivity of the thermal conductive adhesive should be relatively high. For example, some thermal conductive adhesives based on silicone rubber can have a thermal conductivity reaching 3-5 W/(m·K). Apply the thermal conductive adhesive evenly on the surface of the cell or the inner side of the graphite sheet, and the application thickness is about 0.1-0.2 mm. Carefully attach the graphite sheet to the surface of the cell. Use tools (such as soft rubber rollers) to slowly roll from the center of the graphite sheet to the edge to expel air and ensure that the graphite sheet is closely attached to the cell. During the rolling process, the pressure should be moderate to avoid excessive pressure that may cause the graphite sheet to break or the cell to be damaged. Generally, the rolling pressure is controlled within 0.2-0.3 MPa.

Encapsulation and reinforcement

After the graphite sheet is properly attached, in order to protect the graphite sheet and the cell, an insulating protective film can be wrapped around the graphite sheet. For example, polyimide (PI) film has good insulating properties and high temperature resistance (it can withstand high temperatures of 300 - 400 °C). Use a heat sealing machine to tightly encapsulate the PI film on the outer layer of the graphite sheet. The heat sealing temperature is set according to the characteristics of the PI film, generally between 200-300 °C, and the heat sealing time is 3-5 seconds. For some cells that require additional mechanical protection, a plastic protective shell or a metal protective shell can be put on the outside of the encapsulated cell. The plastic protective shell can be manufactured by the injection molding process, and the metal protective shell can be manufactured by the stamping and welding processes to ensure that there is a certain buffer between the shell and the cell and avoid external forces directly acting on the cell and the graphite sheet.

2. Use as an intermediate thermal conductive layer

Determination of placement position: when graphite sheets are used as an intermediate thermal conductive layer, it is necessary to accurately determine their placement position inside the battery cell.

In the case of cylindrical battery cells, the graphite sheets can be placed along the axial or circumferential direction of the cell. For example, placing annular graphite sheets between the positive and negative electrodes of the battery can effectively conduct the heat generated by the positive and negative electrodes out.

For square battery cells, the graphite sheets can be placed between regions with relatively concentrated heat generation in a tiled or folded manner according to the distribution of heating elements. For instance, graphite sheets can be placed between the battery management chip and the battery module of the cell to quickly conduct the heat generated by the chip.

Optimization of contact: To ensure good heat conduction between the graphite sheets and the upper and lower components, it is necessary to ensure their close contact. Thin thermal conductive gaskets, such as copper foil or aluminum foil gaskets with a thickness of about 0.05-0.1 mm, can be added between the graphite sheets and adjacent components. These thermal conductive gaskets can fill tiny gaps and improve the contact thermal conductivity. During the assembly process, appropriate pressure devices are used to assemble the battery cells so that the graphite sheets are subjected to a certain pressure, generally within the range of 0.1-0.2 MPa, to ensure close fitting between the graphite sheets and adjacent components. For example, during the stacking process of multi-layer battery cells, clamps with pressure control functions are used to clamp the battery cells and graphite sheets together to ensure good heat conduction contact.