With the constant increase of energy density requirements, various manufacturers are working hard to improve the parameters of their own products. The main efforts are along the two half routes: one is to step up research and development of high specific energy cathode materials, and the other is to increase battery cell capacity. , Reduce the share of the auxiliary structure such as the housing in the monomer. The other half is to consider the transformation of the soft pack battery, which can be seen from the current mainstream battery factory to undertake the research project of the national soft pack battery direction. The soft pack battery is Isn't the most suitable power battery technology line? Let's put together some soft battery packs today to predict the future technology roadmap for power batteries.
Soft pack battery basic structure
The basic structure of the soft pack battery is similar to that of the cylinder and the square. They are the positive electrode, the negative electrode, the separator, the insulating material, the positive and negative electrode tabs and the housing, but the subject of the soft battery is the aluminum plastic film.
Aluminum plastic film is required to have the following characteristics: With high barrier properties; With good heat sealing properties; Material resistant to electrolyte and strong acid corrosion; With good ductility, flexibility and mechanical strength.
Soft pack battery advantages
Soft pack battery, aluminum plastic film deformation space is larger, unlike the steel shell aluminum shell battery when the thermal runaway will explode; The shell is a layer of aluminum plastic film, light weight, the proportion of inactive part is small, soft package The weight of the battery is 40% lighter than that of the same capacity steel shell lithium, 20% lighter than the aluminum shell battery; Compared with the same size specifications, the capacity is larger, and the soft-pack battery is 10~15% higher than the steel battery with the same size. Aluminum shell batteries are 5 to 10% higher; shell strength is low, mechanical stress on the internal structure during the cycle is small, useful for cycle life (in the case of a group design, of course, no additional stress is applied); The location is abundant, the heat is evenly distributed during charge and discharge.
Disadvantages of soft pack batteries
The strength of the shell is low, and the dependence on the group technology is strong. Compared with the winding production method, the lamination production efficiency is relatively low;
Because the shell is aluminum plastic film, its production process is different from other two kinds of commercial batteries in some aspects, such as the encapsulation process of aluminum plastic film, such as the shaping process in the chemical conversion process.
Soft pack battery packaging process
Aluminum plastic film forming process, soft package batteries can be designed according to customer needs into different sizes, When the external dimensions are designed, you need to issue the corresponding mold, so that aluminum plastic film molding. Forming process is also known as red pit, as the name suggests In the case of heating with a forming mold, a pit capable of loading the core is punched out of the aluminum plastic film, as shown in the figure below.
After the aluminum plastic film has been punched and cut, it is generally called a pocket bag. See the figure below. Generally, when the battery is thin, choose the punched hole (the left picture below). When the battery core is thick, choose the punching double. The pit (below in the figure below) will break through the deformation limit of the aluminium plastic film because the deformation of one side is too large.
Sometimes according to the design needs, a small hole will be punched in the position of the air bag to enlarge the volume of the air bag.
The top side sealing process, the top side sealing process is the first encapsulation process of the soft lithium-ion battery. The top side seal actually contains two processes, the top seal and the side seal. The roll core should be placed first. In the cratered hole, fold the packaging film in half along the dotted line, as shown in the figure below.
The following figure shows several locations where the aluminum-plastic film needs to be packaged after it is loaded into the core, including the top seal area, side seal area, one area and two seal areas.
After putting the core into the pit, the whole aluminum plastic film can be put into the fixture, and the top and side seals are made in the top side sealer. The top side sealer is like this:
In the figure, this type of top side sealer has four clamps. The left side station is the top seal, and the right side station is the side seal. The two yellow metals are the top head and the bottom head is below. When the package is sealed, the two heads have a certain temperature (generally around 180°C). When they are closed, they are pressed on the aluminum plastic film. The PP layer of the aluminum plastic film is melted and then bonded together. The package is OK.
In the main, the top seal, the schematic diagram of the top seal area is shown in the figure below. The top seal is to seal the ear, the ear is metal (positive aluminum, negative nickel), how to encapsulate it together with PP? It's done by a small part on the ear tip—a lug glue. The specific structure of the lug glue is not very clear to me. I hope someone with knowledge can supplement it. I only know that it also has the cost of PP, that is to say heating It can be melted and bonded. The encapsulation in the ear position is shown in the circled part of the figure. When encapsulated, the PP in the lug adhesive melts and bonds with the PP layer of the aluminum plastic film to form an effective package structure.
Injecting liquid, pre-sealing process, after the soft pack core is sealed on the top side, X-ray needs to check the parallelism of the core, and then enter the drying room to remove water. When the drying room is left standing for some time, Into the liquid injection and pre-sealing process.
From the above introduction, we know that after the top side seal is completed in the cell, there is only one opening on the side of the airbag. This opening is used for liquid injection. After the liquid injection is completed, the airbag needs to be carried on immediately. Pre-sealed, also known as a. After a package is completed, the cell is theoretically completely isolated from the external environment. The principle of a package is the same as that of the top-side package, so we will not go into details here.
After standing, forming, and shaping the fixture, after the injection of liquid and the completion of the package, the cell first needs to be left standing. According to the process, it is divided into high-temperature standing and standing at room temperature. The purpose of standing is to let the injected The electrolyte fully infiltrates the pole piece. The battery can then be put into the process.
The figure above shows the cabinet of the soft pack battery. It is actually a charging and discharging device. I haven't found a picture with a charged battery for a long time. Let's think about the picture of the battery clipped on the OK. The achievement is for the battery cell. The first charge, but will not charge to the highest voltage used, the charging current is very small.
The purpose of chemical conversion is to form a stable SEI film on the surface of the electrode, which is equivalent to a process of 'activation' of the cell. In this process, a certain amount of gas will be generated. This is why the aluminum-plastic film has to be reserved. Airbags. Some factory processes use jigs to form clamps. That is, the batteries are clamped in the clamps (sometimes the glass is simple and then the steel clamps are used). Next to the airbag, the electrode interface after forming is also better.
After forming, some cells, especially thick ones, may have a certain deformation due to the large internal stress. Therefore, some factories will set up a fixture shaping process after forming, also known as fixture baking.
The two sealing process, just said that gas will be generated during the formation process, so we have to extract the gas and then carry out the second packaging. Here some companies become two processes: exhaust and two seals, and a back gas bag The process, here I'm all together called the two closures.
In the second seal, the bag is first punctured by a guillotine blade and a vacuum is applied at the same time, so that gas and a small portion of the electrolyte in the bag will be extracted. Then the two seals are immediately sealed in the second seal area to ensure the batteries. The airtightness. Finally cut the encapsulated cell to the air bag, a soft battery core is basically formed. The second is the final packaging process of lithium-ion battery, the principle is still the same with the previous thermal package, not Then repeat.
Pressurization process during chemical formation
Because the flexible packaging battery adopts aluminum-plastic packaging structure, its exterior structure determines that the pole pieces cannot be tightly arranged, and gaps are easily generated between the pole pieces. The gases generated during the battery formation process are also likely to remain between the pole pieces, and the subsequent sealing is performed. Medium gas can not be completely discharged, thus affecting the battery performance, so consider using a rolling process between the two charges into the pole piece between the gas exclusion.
After comparing voltages twice a month ago, it was found that the two differential pressures of the unrolled and properly rolled batteries are very low, indicating that the capacity attenuation is very low, and that the battery with the highest rolling pressure has a large pressure difference, indicating a capacity reduction. Large, probably due to the excessive pressure of the rolling of the battery, causing local puncture of the diaphragm, which causes micro-short circuit inside the battery, resulting in capacity attenuation.
It can be seen that applying the proper rolling pressure at an appropriate time is a unique link in the soft bag forming process.
Soft pack lithium battery module pressure
The soft-package lithium-ion battery module generally adopts a stacking method to form a module, in which the laminated battery is pressed by a certain force, and the function thereof is mainly to constrain the battery. If the pressure is too small, the battery vibrates with the vehicle body. When it is too easy to be damaged; too large will have a negative impact on the battery life. Therefore, the design of soft-packaged lithium-ion battery modules must be considered to apply a suitable pressure to the battery. Small stacking pressure can prevent the layer from breaking off. The long-term battery life is beneficial. However, due to the loss of circulating lithium ions, larger pressures result in larger capacity decay rates. In addition, stacking pressure can also lead to local deformation of the diaphragm and chemical degradation. When the battery capacity changes from 0 to 100% , Lithium battery thickness will increase by about 1.2%. From the results of these studies, lithium battery external pressure on the performance of the battery has a significant impact, in the design of lithium battery module must be considered.
