Soft-pack batteries are actually batteries that use aluminum-plastic packaging film as the packaging material. Relatively speaking, the packaging of lithium-ion batteries is divided into two categories, one is soft-packed batteries, and the other is metal shell batteries. Metal shell batteries also include steel shells and aluminum shells, etc. In recent years, some batteries that use plastic shells due to special needs can also be classified as this type.
The difference between the two, except for the shell material, determines that the packaging method is also different. Soft-packed batteries are thermally packaged, while metal-cased batteries are generally welded (laser welding). The reason why soft-pack batteries can be thermally packaged is that they use aluminum-plastic packaging film.
Aluminum plastic packaging film
The composition of aluminum-plastic packaging film (abbreviated as aluminum-plastic film) is shown in the figure, and its cross-section has three layers: nylon layer, Al layer and PP layer.
Each of the three layers has its own function. First of all, the nylon layer guarantees the shape of the aluminum-plastic film, ensuring that the film will not be deformed before being manufactured into a lithium ion battery.
The Al layer is made up of a layer of metallic Al, and its function is to prevent the penetration of water. Lithium-ion batteries are very afraid of water, and generally require the water content of the pole pieces to be PPM level, so the packaging film must be able to block the infiltration of water vapor. Nylon is not waterproof and cannot provide protection. The metal Al will react with oxygen in the air to form a dense oxide film at room temperature, which prevents water vapor from penetrating and protects the inside of the cell. The Al layer also provides the plasticity of the punching hole when the aluminum plastic film is formed. For details, see point 3.
PP is the abbreviation of polypropylene, this material is characterized by melting at a temperature of more than one hundred degrees Celsius, and is viscous. Therefore, the thermal packaging of the battery mainly relies on the PP layer being melted and bonded together under the action of the heating of the head, and then the head is removed, and the temperature is cooled to solidify and bond.
The aluminum-plastic film looks very simple. In practice, how to evenly and firmly combine the three layers of materials is not so easy. It is a pity that most of the aluminum-plastic films with good quality are imported from Japan, and the domestic ones are not available, but the quality needs to be improved.
Aluminum plastic film forming process
The soft-pack batteries can be designed into different sizes according to the needs of customers. After the outer dimensions are designed, the corresponding molds need to be opened to form the aluminum-plastic film. The molding process is also called punching (in fact, I think it should be a "punk pit", but everyone writes it like that). As the name suggests, it is to punch a mold that can be installed on the aluminum-plastic film under heating. The pit of the core.
After the aluminum-plastic film is punched and cut into shape, it is generally called a Pocket bag, as shown in the figure below. Generally, the single hole is selected when the battery is thin, and the double hole is selected when the battery is thick, because too much deformation on one side will break through the deformation limit of the aluminum-plastic film and cause rupture.
Sometimes, according to the needs of the design, a small hole is punched in the position of the air bag to expand the volume of the air bag. Zhejiang Hangke Technology Incorporated Company produces quality High Temperature Pressing JIG Formation System for Pouch Lithium Power Cell.