As energy density requirements continue to increase, various manufacturers are working to improve their product parameters through two main efforts: one is to enhance the development of high specific energy cathode materials, and the other is to increase cell capacity and reduce the proportion of monomer in auxiliary structures such as housings. The other half is considering the modification of flexible packaging batteries. This can be seen from the current mainstream battery factories actively undertaking national research projects on flexible packaging batteries. Is flexible packaging battery the most suitable power battery technology route? Today, I will sort out certain aspects of flexible batteries and predict the future technology route for power batteries.
The basic structure of a flexible pack cell is similar to a cylinder and a square, they are both positive, negative, spacer, insulation, positive and negative terminals and a housing. The only problem with flexible packaging batteries is the aluminum-plastic film.
The structure of the aluminum-plastic film: the outermost layer is nylon, the middle is aluminum foil, and the inner layer is a heat sealer, each layer being bonded with an adhesive.
The following properties are required: very high barrier properties; good heat sealing properties; electrolyte and acid resistant material; good ductility, flexibility and mechanical strength.
Advantages of flexible packaging batteries
Flexible packaging batteries, aluminum plastic film has a lot of room for deformation, and will not explode when out of control, as in the case of steel-cased aluminum-cased batteries; shell is a layer of aluminum plastic film, light weight, the proportion of inactive parts is small, flexible packaging batteries than the same capacity of steel batteries 40% lighter than aluminum batteries 20% lighter; compared with the same size specifications, the capacity of flexible packaging batteries steel-cased batteries than the same size capacity 10-15% higher capacity than aluminum cased cells, and 5-10% higher capacity than aluminum cased cells; low strength of the casing and less mechanical stress on the internal structure during cycling, which facilitates cycle life (of course (in the case of a group design without additional stress); location of the pole lugs
Disadvantages of flexible packaging cells
Low case strength and dependence on group formation technology; relatively low production efficiency of laminated layers compared to wound groups Production methods
General production process of flexible packaging cells
Since the shell is aluminum-plastic film, the production process differs from some aspects of the other two commercial batteries, such as the packaging process for aluminum-plastic film, for example, the forming process in the molding process
The aluminum plastic film forming process , so ft pack batteries can be designed to different sizes in inches according to the customer's needs. When designing the external dimensions, the corresponding mold needs to be opened to make the aluminum plastic film. As the name implies, the forming process, also known as stamping, involves using a forming die to heat a pit in the aluminum plastic film that can be loaded with a core.
Sometimes, depending on the design needs, a small pit is punched in the location of the airbag to expand the volume of the airbag.
Top sealing process, the top sealing process is the first packaging process for flexible lithium-ion batteries. Top sealing actually consists of two processes, top sealing and side sealing. First, the wound core is placed into the perforated recess and then the packaging film is folded in half along the dotted line, as shown in the figure below.
After placing the core in the recess, the entire aluminum plastic film can be placed into the fixture and top and side sealed in the top and side sealer.
This type of top sealer model has four fixtures, the left one for top sealing and the right one for side sealing. The two yellow metal pieces are the top head, and the bottom head. The two heads have a certain temperature during the packaging process (usually around 180°C). When they are closed, they are pressed against the aluminum-plastic film. The PP layer of the film melts and sticks together so the package can.
Mainly the top seal is used. A diagram of the top seal area is shown below. The top seal is used to seal the joints, which are metal (anodized aluminum, nickel anode), how is it encapsulated with PP? It depends on a very small part of the pole lug on the pole lug glue to complete. I am not very clear about the exact structure of the jelly glue. I hope someone with knowledge can add it. All I know is that it also has the cost of PP, which means it melts and bonds when heated. The packaging of the pole lug position is shown in the circled part of the picture below. The packaging process melts and combines the PP in the surface lug adhesive with the PP layer of the aluminum plastic film to form an effective packaging structure.
Liquid injection and pre-sealing process, after sealing the flexible packaging core at the top, need to check the core parallelism of X-rays, and then into the drying chamber to remove water vapor on the go. When the drying chamber rests for a certain time, enter the liquid injection and pre-sealing procedure.
From the above, we know that after sealing the top of the cell, there is only an opening on the side of the left lower airbag. This opening is used for fluid injection. After the liquid injection is complete, the edges of the airbag, also known as the letters, must be pre-sealed. Once the encapsulation is complete, it theoretically isolates the battery completely from the external environment. The encapsulation principle for one is the same as for the top seal, so it will not be repeated here.
The process of standing, molding and fixture forming. After the injection and one piece is completed, the cell first needs to be placed. Depending on the process, it is divided into high temperature and room temperature. The purpose is to make the injected electrolyte completely wet the pole boot. Then the cell can be used to make it.
The purpose of the formation is to form a stable SEI film on the surface of the electrode, which is equivalent to the process of "activating" the cell. During this process, a certain amount of gas is produced, which is why an airbag is reserved for the aluminum plastic film. Some factory processes will be formed using fixtures, where the cell is clamped in the fixture (sometimes using a glass plate followed by a steel clamp for simplicity) and then the cabinet is formed. The generated gas will be completely squeezed to one side. In the bag, the electrode interface is also better after formation.
Some cells, especially thick ones, may undergo some deformation due to higher internal stress after formation. Therefore, some factories set up a jig forming process, also called jig baking, after forming.
The second sealing process mentioned earlier is that gas is generated during the forming process, so we have to extract that gas and then encapsulate it again. Here some companies turn into two processes: venting and second sealing, and behind the airbag there is a process, which here I will call two seals.
In the second seal, the airbag is first punctured with a gate knife and simultaneously evacuated so that the gas and a small portion of the electrolyte in the airbag will be extracted. Then, the two sealing heads are immediately encapsulated in the second sealing area to ensure the gas tightness of the cell. Finally, the encapsulated cell is cut off to the airbag, essentially forming a flexible packaged cell. The second seal is the final packaging process for lithium-ion batteries, and its principle remains the same as that of the previous thermal packaging, so it will not be described in detail.
Forming process]
The pressure process in the flexible packaging battery because of the aluminum-plastic packaging structure, so its appearance structure determines that the pole boots can not be closely aligned, easy to produce gaps between the pole boots, and in the charging process will produce gas. The cell formation process also tends to remain between the pole shoes and the gas cannot be completely discharged in the subsequent sealing, which affects the performance of the cell, so a rolling process between the two charges is considered to eliminate the gas between the pole shoes.
The negative electrode of the cell has many dead zones In the negative, the negative electrode surface of the cell is intact and no dead zones are found. Flexible pack cells will generate gas during the formation process, which will increase the distance between the positive electrode, the spacer and the negative electrode during the formation process, thus hindering the transfer of lithium ions from the positive to the negative electrode sheet. In addition, the presence of gas will also hinder the electrolyte. contact with the positive and negative electrodes making the local wettability of the negative electrode poor and eventually leading to a large number of unreacted dead zones on the negative electrode sheet.
By comparing the voltages twice before and after one month, it was found that the voltage difference between the unfolded and properly wound cells was very low, indicating very low capacity decay, and that rolling the largest cell pressure with a large value of pressure difference A indicates high capacity decay, which may be caused by the excessive pressure of rolling the cell, which leads to localized punctures in the spacer, which in turn leads to micro-short circuits inside the cell. This can lead to capacity degradation.
As can be seen, applying the proper rolling pressure at the right time is a unique aspect of the flexible pouch forming process.
Pressure in flexible packaging lithium-ion battery modules
Flexible packaged Li-ion battery modules are usually formed using a stacking method and a certain force is applied to the stacked cells Compression is applied in the module. The effect is mainly to restrain the cells. If the pressure is too small, the battery will be easily damaged when it vibrates with the body. If it is too large, it will negatively affect the battery life. Therefore, when designing the structure of flexible packaging lithium-ion battery modules, it is important to consider the appropriate pressure to be applied to the cells. A small stacking pressure prevents the layers from separating, which contributes to the long-term life of the battery. However, larger pressures result in larger capacity decay rates due to the loss of cyclic lithium ions. In addition, the stacking pressure causes local deformation and chemical degradation of the diaphragm. 2 When the battery capacity changes from 0 to 100%, the thickness of the lithium battery will increase by 1.2% about. From these findings, it is clear that the external pressure of a lithium battery can have a significant impact on the performance of the battery and must therefore be considered when designing the structure of the lithium battery module.
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