The importance of aluminum plastic film from the packaging process of soft-pack lithium battery

Currently, the soft-pack batteries we commonly encounter are all made using aluminum-plastic film. Generally speaking, lithium-ion soft-pack batteries can be categorized into two types: one uses aluminum-plastic film as the packaging material for the battery core, while the other type employs a metal casing, which includes steel shells and aluminum shells, among others. Different packaging materials determine the use of different packaging methods; polymer soft-pack batteries are heat-sealed, whereas metal-packaged batteries are soldered. When it comes to packaging soft films—specifically aluminum-plastic film—the composite film can roughly be divided into three layers: the inner layer serves as a bonding layer, typically made of polyethylene or polypropylene, which helps seal the joint; the middle layer consists of aluminum foil, preventing moisture from infiltrating the battery while also stopping the electrolyte from leaking out; the outer layer acts as a protective layer, often made of high-melting-point polyester or nylon, offering strong mechanical properties to shield the battery from external damage and providing additional protection. The development of high-quality aluminum-plastic composite film is crucial for the successful advancement of high-tech liquid lithium-ion batteries. As the outer casing of liquid flexible-packaging lithium-ion batteries, the aluminum-plastic composite film is no longer just a simple outer packaging for the battery—it has become an indispensable and vital component of these batteries. If the importance of this flexible packaging material isn't emphasized enough, it could negatively impact the design and development of flexible-packaging batteries. Given its critical role in the development of liquid flexible-packaging lithium-ion batteries, it's clear that this product has a high level of technical complexity. There is a significant 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 production technology of aluminum-plastic composite film is also under constant research and development. The soft-pack batteries can be designed into various sizes according to customer requirements. When designing the external dimensions, corresponding molds must be created to punch holes in the aluminum-plastic film for winding. This process, also known as punching, involves using a formed mold to create a hole that can be wound around the aluminum film. After washing the aluminum-plastic film, it is cut and formed, generally referred to as a pocket bag. For thinner battery cores, a single hole is selected, while thicker cores require a double hole. When using a double hole punch, the deformation on one side may change, and during assembly, exceeding the deformation limit of the aluminum-plastic film can cause cracking. Sometimes, based on design requirements, a small indentation is added near the punching 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 in the punched hole, and then the unpunched side is folded in half along the edge of the hole. After placing the wound core in the aluminum-plastic film, areas to be sealed include the top sealing area, the side sealing area, the first sealing area, and the second sealing area. After the core is placed in the aluminum-plastic film, it can be positioned in a jig and sealed using a top sealer and a side sealer. Top sealing refers to sealing the tabs, which are made of metal (positive aluminum, negative nickel). How does one seal these tabs with PP? This is achieved by adding a small portion of tab gel. I'm unsure about the specific structure of the tab gel, so I hope someone knowledgeable can provide more details. I only know that it contains PP, meaning it can melt and bond when heated. The tab gel is shown circled 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 form an effective seal. After the soft-pack battery is sealed on the top, an X-ray is performed to check the alignment of the core, followed by placement in a drying room to remove moisture. After standing in the drying room for a set period, it moves on to the injection and pre-sealing process. After completing the top seal, the battery core has an opening on the side of the airbag. This opening is used for liquid injection. Once the injection is complete, immediate pre-sealing of the airbag is required, also known as "lettering." Once the pre-seal is finished, the interior of the cell is completely isolated from the external environment. Following the injection and pre-sealing, the battery core needs to be left to stand. Depending on the production process, this can involve either high-temperature or room-temperature static settings. The purpose of the static setting is to allow the injected electrolyte to fully wet the electrode components before further processing. The formation process involves the initial charging of the battery but does not charge it to full capacity, and the charging current is kept low. The goal of formation is to create a stable SEI film on the electrode surfaces, essentially activating the cell. A certain amount of gas is generated during this process, which is why an airbag is reserved. Some factories use fixtures to place the cells in cabinets. The gas produced during formation is squeezed into the airbag, resulting in better electrode interfaces. After formation, some batteries, particularly thicker ones, may experience deformation due to internal stresses. Thus, some factories implement a shaping process after formation, also known as fixture baking. In the second seal, the airbag is first punctured with a file and a vacuum is applied, extracting the gas and some electrolyte from the airbag. Immediate double sealing ensures the airtightness of the battery. Finally, the airbag is cut off, completing the basic formation of a soft-pack battery. After cutting the airbag, trimming and folding are required to ensure the battery width does not exceed the specified limit. After hemming, the battery enters the sub-container for capacity division, which is essentially a capacity test. Once the capacity test is complete, qualified batteries move on to subsequent processes, including visual inspections, yellowing checks, side voltage detection, and ear transfer welding. These steps can be adjusted based on customer requirements, followed by OQC inspection. Packaging and shipping follow. [Attached images illustrate the various stages of the battery production process.]

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