Whether in the digital or energy application market, soft-pack batteries are becoming a very important technical road.

　　Soft-packed batteries are actually batteries that use aluminum-plastic packaging film as packaging material. Relatively speaking, the packaging of lithium-ion batteries is divided into two categories, one is soft-packed cells and the other is metal-cased cells. Metal-cased batteries include steel shells, aluminum shells, etc. In recent years, due to special needs, some batteries use plastic shells, which can also be classified as this category.

　　The difference between the two not only has different shell materials, but also determines their packaging methods. Soft-packed batteries are thermally sealed, while metal-cased batteries are generally welded (laser welding). The reason why soft-packed batteries can be thermally sealed is that they use aluminum-plastic packaging film.

　　2. Aluminum plastic packaging film

　　The composition of aluminum-plastic packaging film (referred to as aluminum-plastic film) is shown in the figure. Its cross-section consists of three layers: nylon layer, Al layer and PP layer.

　　Each of the three layers has its own role. The first nylon layer ensures the shape of the aluminum-plastic film and ensures that the film will not deform before it is manufactured into a lithium-ion battery.

　　The Al layer is composed of a layer of metallic Al, and its function is to prevent water from entering. Lithium-ion batteries are very afraid of water. Generally, the moisture content of the pole pieces is required to be at the PPM level, so the packaging film must be able to block the entry of water vapor. Nylon is not waterproof and cannot be maintained. Metal Al will react with oxygen in the air at room temperature to form a dense oxide film, preventing water vapor from entering and protecting the inside of the battery core. The Al layer also provides the plasticity for punching when the aluminum plastic film is formed. See point 3 for details.

　　PP is the abbreviation of polypropylene. The characteristic of this material is that it melts at temperatures of more than 100 degrees Celsius and is sticky. Therefore, the thermal sealing of batteries mainly relies on the PP layer being melted and bonded together under the action of heating of the sealing head. Then the sealing head is removed and the temperature is reduced to solidify and bond.

　　Aluminum plastic film looks very simple, but in practice, it is not that simple to combine the three layers of materials evenly and firmly together. It is a pity that the good quality aluminum plastic films nowadays are basically imported from Japan. There are some domestically produced ones, but the quality still needs to be improved.

　　3. Aluminum plastic film forming process

　　Soft-packed batteries can be designed into different sizes according to customer needs. After the appearance size is planned, the corresponding mold needs to be opened to form the aluminum-plastic film. The molding process is also called punching (actually, I think it should be "gun pit", but if everyone writes it this way, just follow the convention), as the name suggests, it uses a forming mold to punch out a hole on the aluminum-plastic film under heating. The pit for installing the core is shown in the picture below for details.

　　Sometimes, according to design needs, a small pit will be punched in the position of the air bag to expand the volume of the air bag.

　　4. Top and side sealing process

　　Finally getting to the point (how great are you at digressing!), the top and side sealing process is the first packaging process for soft-packed lithium-ion batteries. The practice of top and side sealing includes two processes, top sealing and side sealing. First, place the rolled core into the punched pit, and then cut the packaging film in half along the dotted line, as shown in the picture below.

　　The picture below shows the several locations that need to be packaged after the aluminum-plastic film is loaded into the roll core, including the top sealing area, side sealing area, first sealing area and second sealing area. They are introduced separately below.

　　After placing the roll core in the pit, the entire aluminum-plastic film can be placed in the clamp, and top and side sealing is performed in the top and side sealing machine. The top side sealing machine looks like this:

　　There isn’t much to say about the side seals (the side voltage is too far-fetched so I won’t talk about it). Let’s mainly talk about the top seal. The schematic diagram of the top seal area is shown in the figure below. The top seal is to seal the tabs. The tabs are made of metal (aluminum for the positive electrode and nickel for the negative electrode). How to package it with the PP package? This is accomplished by a small component on the lug - the lug glue. I am not very clear about the detailed structure of the ear glue, and I hope someone knowledgeable can add to it. I only know that it also has the cost of PP, which means it can melt and bond when heated. The package at the tab position is shown in the circle in the figure below. During packaging, the PP in the tab glue and the PP layer of the aluminum-plastic film melt and bond, forming an effective packaging structure.

　　5. Liquid injection and pre-sealing process

　　After the soft-packed battery core is sealed on the top and side, X-ray is required to check the parallelism of the core, and then it is sent to the drying room to remove moisture. After letting it sit in the drying room for a certain period of time, it enters the liquid injection and pre-sealing process.

　　From the above introduction, we know that after the battery core is sealed on the top side, there is only an opening on the side of the air bag, which is used for liquid injection. After the liquid injection is completed, the air bag side needs to be pre-sealed immediately, also called a seal. After the package is completed, the battery core is theoretically completely isolated from the external environment. The sealing principle of an envelope is the same as that of top and side sealing, so I won’t go into details here.

　　6. Resting, forming, and fixture shaping processes

　　After the liquid injection and sealing are completed, the battery core first needs to be left to rest. Depending on the process, it can be divided into high-temperature resting and room-temperature resting. The purpose of resting is to allow the injected electrolyte to fully moisturize the pole pieces. Then the battery core can be used to make chemical products.

　　The picture above is a formation cabinet for soft-packed batteries. It is actually a charging and discharging device. I searched for a long time but could not find a picture of the battery cells. Just think about the image of the battery cells clamped on it. Formation is the first charge of the battery, but it will not charge to the highest voltage used, and the charging current is also very small.

　　The purpose of formation is to form a stable SEI film on the electrode surface, which is equivalent to a process of "activating" the battery core. During this process, a certain amount of gas will be generated, which is why the aluminum-plastic film needs to reserve an air bag. The process in some factories will use fixtures, that is, clamp the batteries in the fixtures (sometimes simply use a glass plate, and then put on steel clips) and then put it on the cabinet to make it, so that the gas generated will be fully squeezed into the surrounding gas. Put it in the bag, and the electrode interface after being formed together is also better.

　　After formation, some cells, especially thick cells, may undergo certain deformation due to large internal stress. Therefore, some factories will set up a fixture shaping process after formation, also called fixture baking.

　　7. Second sealing process

　　As mentioned just now, gas will be generated during the formation process, so we need to extract the gas and then perform the second packaging. Some companies here have two processes: degassing (exhaust) and second sealing, followed by a process of cutting the air bag. Here I will collectively refer to them as second sealing.

　　During the second sealing, the air bag is first punctured with a guillotine and vacuumed at the same time, so that the gas and a small part of the electrolyte in the air bag will be extracted. Then the second sealing head is immediately sealed in the second sealing area to ensure the airtightness of the battery core. Finally, the air bag is cut off from the packaged battery core, and a soft-packed battery core is basically formed. Second sealing is the last packaging process of lithium-ion batteries. Its principle is still the same as the previous thermal sealing and will not be described again.

　　8. Subsequent processes

　　Because the question is about packaging, and the rest of it has little to do with packaging, I will talk about the process after the second seal together.

　　After cutting the second sealing air bag, you need to trim and fold the edges. This means cutting the first and second sealing edges to the appropriate width, and then folding them to ensure that the width of the battery core does not exceed the budget. After folding, the battery core can be put into a capacity classification cabinet for capacity classification. In fact, it is a capacity test to see if the capacity of the battery core has reached the specified minimum value. In principle, all batteries need to undergo a capacity test before leaving the factory to ensure that batteries with unqualified capacity will not be sent to customers. However, when the battery cell production volume is large, some companies will divide the capacity into parts and use statistical probability to judge the qualification rate of the batch of battery cells.

　　After the capacity is divided, the cells with qualified capacity will enter the post-processing process, including appearance inspection, yellow glue application, edge voltage detection, tab transfer welding, etc. Several processes can be added or deleted according to customer needs. Finally, it is OQC inspection, and then packaged and shipped.

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