1. Soft core
The so-called soft core is actually a core that uses aluminum-plastic packaging film as packaging material. Relatively speaking, the packaging of lithium polymer battery is divided into two categories, one is soft core and the other is metal shell core. Metal shell cell also includes steel shell and aluminum shell. In recent years, due to special needs, some cells with plastic shell can also be classified as such. The difference between the two shows that the shell material is different, which determines that the packaging method is also different. Soft core adopts thermal packaging, while metal shell core generally adopts welding (laser welding). The reason why the soft core can be thermally encapsulated is that it uses 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. From the cross section, it is composed of three layers: nylon layer, Al layer and PP layer. The three layers have their own functions. First, the nylon layer ensures the shape of the aluminum-plastic film and that the film will not deform before it is manufactured into a lithium polymer battery. Al layer is composed of a layer of metal Al, which is used to prevent the infiltration of water. Lithium polymer battery is very afraid of water. Generally, the water content of the pole piece is required to be ppm, so the packaging film must be able to block the infiltration of water vapor. Nylon is not waterproof and cannot play a protective role. The metal Al reacts with oxygen in the air at room temperature to form a dense oxide film, which prevents water vapor from penetrating and protects the interior of the cell. The Al layer also provides the plasticity of the scour pit 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 will melt at a temperature of more than 100 degrees Celsius and has viscosity. Therefore, the thermal packaging of the battery mainly depends on the melting and bonding of the PP layer under the heating of the head, and then the head is removed, and the aluminum-plastic film is solidified and bonded after cooling. It looks very simple. In practice, it is not so easy to combine the three layers of materials evenly and firmly. Unfortunately, the aluminum-plastic films with good quality are basically imported from Japan, and the domestic ones are not absent, but the quality needs to be improved.
3. Aluminum plastic film forming process
The soft core can be designed into different sizes according to the needs of customers. When the overall dimensions are designed, it is necessary to issue corresponding molds to form the aluminum-plastic film. The forming process is also called punching pit (in fact, I think it should be "gun pit", but let's follow the customs). As the name suggests, it is to punch a pit that can hold roll core on the aluminum-plastic film with the forming die under the condition of heating. After the aluminum-plastic film is punched and cut into shape, it is generally called pocket bag, as shown in the figure below. Generally, single pit punching (left in the figure below) is selected when the cell is thin, and double pit punching (right in the figure below) is selected when the cell is thick, because the deformation on one side will break through the deformation limit of aluminum-plastic film and lead to fracture. Sometimes, according to the design needs, a small pit will be punched at the position of the air bag to expand the volume of the air bag.
4. Top and side sealing process
To get to the point, the top side sealing process is the first packaging process of soft encapsulated lithium-ion cells. The top side seal actually includes two processes, top seal and side seal. First, put the wound core into the punched pit, and then fold the packaging film in half along the dotted line. The following figure shows several positions that need to be sealed after the aluminum-plastic film is loaded into the core, including top sealing area, side sealing area, first sealing area and second sealing area. The following are introduced respectively. After putting the roll core into the pit, the whole aluminum-plastic film can be put into the fixture for top and side sealing in the top and side sealing machine. The top side sealing machine is like this: this type of top side sealing machine is equipped with four clamps. The station on the left is top sealing and the station on the right is side sealing. The two yellow metals are the upper head and the lower head. During packaging, the two heads have a certain temperature (generally about 180 ℃), and when they are closed, they are pressed on the aluminum-plastic film. The PP layer of the aluminum-plastic film melts and binds together. In this way, the packaging is OK. There is not much to say about the side seal (I won't talk about it if the side voltage god horse pulls too far). I 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 pole ear. The pole ear is metal (positive aluminum, negative nickel). How can it be encapsulated with PP? This depends on a small part on the polar ear - polar ear glue. I'm not very clear about the specific structure of polar ear glue. I hope someone knowledgeable can supplement it. I only know that it also has the cost of PP, that is, it can melt the bond when heated. The packaging at the pole ear position is shown in the circle in the figure below. During packaging, PP in polar ear glue is melted and bonded with PP layer of aluminum-plastic film to form an effective packaging structure.
5. Liquid injection and pre sealing process
After the soft core is sealed at the top and side, it is necessary to check the parallelism of its winding core by X-ray, and then enter the drying room to remove water and gas. When the drying room stands for a certain time, it enters the liquid injection and pre sealing process. Through the above introduction, we know that after the top and side sealing of the cell is completed, there is only one opening on the side of the air bag, which is used for liquid injection. After the liquid injection is completed, it is necessary to pre seal the side of the air bag, also known as one seal. After a package is completed, the cell is theoretically completely isolated from the external environment. The packaging principle of one seal is the same as that of the top side seal, which will not be repeated here.
6. Static, chemical forming, fixture shaping process
After the completion of liquid injection and sealing, the cell needs to be left standing first. According to different processes, it can be divided into high-temperature standing and normal temperature standing. The purpose of standing is to make the injected electrolyte fully infiltrate the electrode. Then the cell can be made into. The conversion cabinet of soft packed electric core is actually a charging and discharging device. I haven't found the picture of the electric core for a long time. Just think about the picture of the electric core clamped on it. Formation is the first charge of the cell, but it will not be charged 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 cell. In this process, a certain amount of gas will be generated, which is why an air bag should be reserved for the aluminum-plastic film. In some factories, the process will be formed by using clamps, that is, the cell will be clamped in the clamp (sometimes glass plate will be used for simple drawing, and then steel clamp) and then put on the cabinet. In this way, the generated gas will be fully squeezed into the nearby air bag, and the electrode interface after formation is also better. After formation, some cells, especially thick cells, may produce certain deformation due to large internal stress. Therefore, some factories will set up a fixture shaping process after formation, which is also called fixture baking. These three processes have nothing to do with packaging, so let's briefly introduce them here.
7. Secondary sealing process
As mentioned earlier, gas will be produced during the formation process, so we need to extract the gas and then carry out the second packaging. Here, some companies have two processes: degassing and secondary sealing, and the subsequent process of cutting air bags. Here, I generally call them secondary sealing. During the second sealing, firstly, the air bag is punctured by the guillotine and vacuumized at the same time, so that the gas in the air bag and a small part of electrolyte will be extracted. Then, the second sealing head is immediately sealed in the second sealing area to ensure the air tightness of the cell. Finally, cut the air bag of the encapsulated cell, and a soft encapsulated cell is basically formed. Second sealing is the last packaging process of lithium polymer battery. Its principle is the same as the previous thermal packaging, so I won't repeat it.
8. Subsequent process
After cutting the air bag of the second seal, it is necessary to cut and fold the edge, that is, cut the edge of the first seal and the edge of the second seal to the appropriate width, and then fold it to ensure that the width of the cell does not exceed the standard. The folded cell can be put into the capacity sharing cabinet for capacity sharing. In fact, it is a capacity test to see whether the capacity of the cell reaches the specified minimum value. In principle, all cells need to be tested before leaving the factory to ensure that unqualified cells will not be delivered to customers. However, when the production volume of electric cells is large, some companies will do partial capacity division to judge the qualification rate of the capacity of this batch of electric cells by statistical probability. After capacity division, the qualified cells will enter the post process, including appearance inspection, yellow glue pasting, edge voltage detection, lug transfer welding, etc. several processes can be added or deleted according to the needs of customers. Finally, oqc inspection, packaging and shipment.
Note that it's just an electric cell here. The cells will be sent to the pack factory for further processing, including welding circuit boards, packaging, etc. The downstream of the pack factory is each user manufacturer
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