At present, the soft-pack batteries we commonly encounter are all made using aluminum-plastic film. Generally speaking, lithium-ion soft-pack batteries fall into two main categories: one uses aluminum-plastic film as the packaging material for the battery core, while the other type features a metal casing, which includes steel shells and aluminum shells.
Different packaging materials dictate different packaging methods. Polymer soft-pack batteries are thermally sealed, whereas metal-packaged batteries are typically soldered.
When discussing the packaging soft film—aluminum-plastic film—the aluminum-plastic composite film can be roughly divided into three layers: the inner layer serves as a bonding layer, often made from polyethylene or polypropylene, which helps seal the joint; the middle layer consists of aluminum foil, which prevents water vapor from infiltrating the battery while also stopping the electrolyte from leaking out; the outer layer acts as a protective layer, typically made from high-melting-point polyester or nylon, providing strong mechanical properties to prevent external damage to the battery and offering additional protection.
The development of high-quality aluminum-plastic composite films is crucial to the success of advanced liquid lithium-ion batteries. As the outer casing for liquid flexible-packaging lithium-ion batteries, the aluminum-plastic composite film is more than just a simple outer packaging—it's an essential and integral component of these batteries.
If this flexible packaging material isn't given enough attention, it could negatively impact the design and development of flexible-packaging batteries. Its significant role in the advancement of liquid flexible-packaging lithium-ion batteries indicates that the product has a high level of technical sophistication, and there's a noticeable difference in performance between the design, manufacturing, and application of ordinary composite packaging materials.
All domestic and international production facilities are continuously improving their products, and the technology behind aluminum-plastic composite films is always under development.
The soft-pack batteries can be customized into various sizes according to customer requirements. During the design phase, corresponding molds need to be created to punch holes in the aluminum-plastic film for forming. This process, known as stamping, involves using a shaped mold to punch a hole in the aluminum film that can later be wound.
After washing the aluminum-plastic film, it’s cut and formed, typically referred to as a pocket bag. For thinner battery cores, a single hole is chosen, while thicker cores require a double hole. When using a double-hole punch, the deformation on one side changes, which can cause the assembly to exceed the deformation limit of the aluminum-plastic film, leading to cracks.
Sometimes, based on design requirements, a small indentation is added beside the punched hole to increase the volume of the airbag.
The packaging process includes two stages: top sealing and side sealing. First, the wound core is placed inside the punched hole, and then the unpunched side is folded in half along the edge of the punched hole.
After loading the core into the aluminum-plastic film, areas to be sealed include the top sealing area, side sealing area, first sealing area, and second sealing area.
Once the core is placed in the aluminum-plastic film, it can be positioned in a jig and sealed using a top sealer and side sealer.
The top seal is used to seal the tabs, which are made of metal (positive aluminum, negative nickel). How do you seal these with PP? This is done by adding a small portion of the tab, called the tab gel.
I'm not sure about the exact structure of the tab gel, but I know it has the same cost as PP, meaning it can melt and bond when heated. The tab gel is shown in the circle in the image below.
During packaging, the PP in the tab gel melts and bonds with the PP layer of the aluminum-plastic film to create an effective seal.
After the soft-pack battery is sealed on the top side, an X-ray check is performed to ensure the core's parallelism, followed by drying in a drying room to remove moisture. After standing in the drying room for a certain period, it moves to the injection and pre-sealing process.
After completing the top seal, the battery core still has an opening on the side of the airbag. This opening is used for liquid injection. Once the injection is complete, the airbag must be pre-sealed immediately, also called "lettering."
After the pre-sealing is done, the inside of the cell is entirely isolated from the external environment.
Following the injection and pre-sealing, the battery core needs to stand still. Depending on the production process, this can involve either high-temperature or normal-temperature static settings. The purpose of static setting is to allow the injected electrolyte to fully saturate the components before further processing.
Achievement refers to the initial charging of the battery, which does not reach the maximum voltage, and the charging current is also very low. The purpose of formation is to form a stable SEI film on the electrode surface, which is essentially activating the cell.
A certain amount of gas is produced during this process, which is why an airbag is reserved.
Some factories use fixtures to place the cells in a cabinet. The gas produced is squeezed into the airbag, resulting in better electrode interfaces after formation.
After formation, some batteries, especially thick ones, may experience deformation due to high internal stress. Therefore, some factories implement a shaping process after formation, also known as fixture baking.
In the second sealing process, the airbag is first punctured with a file, and a vacuum is applied to extract the gas and some electrolyte from the airbag. Then, immediate double sealing ensures the battery's airtightness. Finally, the airbag is cut off, and a soft-pack battery is basically complete.
After cutting the airbag, trimming and folding are necessary to ensure the battery width does not exceed the limit. After hemming, the battery enters the sub-container for capacity division, which is essentially a capacity test.
After the capacity test, qualified batteries proceed to subsequent processes, including visual inspection, yellowing, side voltage detection, and ear transfer welding. These processes can be adjusted according to customer requirements, followed by OQC inspection. Packaging and shipping follow.
This is a comprehensive overview of the soft-pack battery production process.
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