Power battery future technology line: comprehensive analysis of soft battery

With the continuous increase in energy density requirements, various manufacturers are working hard to improve the parameters of their own products, mainly along the two half-line efforts: one is to step up research and development of high specific energy cathode material, and the other is to increase the battery cell capacity , reduce the proportion 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 actively undertake the research project of the national soft pack battery direction. Soft pack battery is not the most suitable power battery technology line? Organize some soft pack batteries today and predict the future technical path of the power battery.

The basic structure of the soft pack battery is similar to that of the cylinder and the square, and it is 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 pack battery is the aluminum plastic film.

Aluminum plastic film structure as shown above: The outer layer is a nylon layer, the middle is aluminum foil, the inner layer is a heat seal layer, each layer is bonded with a binder.

Aluminum plastic film is required to have the following characteristics: high barrier properties; good heat sealing properties; material resistant to electrolyte and strong acid corrosion; good ductility, flexibility and mechanical strength.

Soft pack battery advantages

Soft pack battery, aluminum plastic film deformation space is larger, unlike steel shell aluminum shell batteries when the thermal runaway will explode; shell is a layer of aluminum plastic film, light weight, the proportion of non-active part of the small, soft package The battery weight is 40% lighter than the same capacity of steel Lithium Battery, 20% lighter than aluminum shell battery; compared with the same size specifications, the capacity is larger, and the soft battery is 10~15% higher than the steel battery with the same size. The aluminum shell battery is 5 to 10% higher; the shell has low strength and the mechanical stress on the internal structure during the cycle is small, which is beneficial to the cycle life (in the case of no extra stress applied to the group design, of course); The location is abundant, and the heat distribution is even during the charging and discharging process.

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.

Soft pack battery general production process

Because the shell is an aluminum plastic film, its production process is different from other two types of commercial batteries in some aspects. Such as aluminum plastic film packaging process, such as the plasticization process in the formation 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 size of the design is good, you need to open the appropriate mold, aluminum plastic film molding. The forming process is also referred to as cratering. As the name implies, it is to use a forming mold to heat a plastic mold on the aluminum plastic film to unload a pit that can hold the core, as shown in the figure below.

After the aluminum plastic film is punched and cut into shape, it is generally called a pocket bag, as shown in the figure below. Usually when the battery is thin, choose to punch a single hole (lower left in the figure below). When the battery is thick, choose a double hole (below in the figure below), because the deformation of one side will break through the deformation limit of the aluminum-plastic film. Causes cracking.

Sometimes according to the design needs, a small hole will be punched in the position of the airbag to expand the volume of the airbag.

The topside sealing process and the topside sealing process are the first packaging steps for a soft-packaged lithium-ion battery. The top side seal actually contains two processes, the top seal and the side seal. First, place the wound core into the punched hole, then fold the film in half along the dashed 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 winding core, including the top seal area, side seal area, and one seal area and two seal areas. The following are introduced separately.

After placing the core in the pit, the entire aluminum plastic film can be placed in the fixture and the top and side seals are applied 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 station is the top seal and the right side station is the side seal. The two pieces of yellow metal are the upper heads, and the lower ones are also underneath. When the package is sealed, the two heads have a certain temperature (generally about 180°C), and they are pressed against the aluminum plastic film when they are closed. The PP layer of the film melts and sticks together, so the package is OK.

It is mainly 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 ear, the ear is the metal (positive aluminum, negative nickel), how to encapsulate it with PP? This is accomplished with a small piece of tab-ear glue. The specific structure of the lug glue is not very clear to me and I hope someone who knows how to supplement it. I only know that it also has the cost of PP, which means it can melt and bond when heated. The package in the lobe position is shown in the circled part of the figure below. 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.

The liquid injection and pre-sealing processes require the X-ray to check the parallelism of the core after the soft pack core is sealed on the top side, and then enter the drying room to remove water. When the drying room is left standing for several hours, it enters the filling and pre-sealing process.

Through the above introduction, we know that after the top side seal is completed, only one opening on the side of the airbag is left. This opening is used for liquid injection. After the injection is complete, the airbag must be sealed immediately before it is also called a letter. After a package is completed, the cell is theoretically completely isolated from the external environment. The principle of one 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 battery first needs to be left standing. According to the different processes, it will be divided into high temperature static and standing at room temperature. The purpose of standing is to allow the injection of The electrolyte fully infiltrated the pole piece. Then the battery can be turned into a finished one.

Pictured above is a soft packet battery forming cabinet, in fact, is a charge and discharge device, I have not found a long time to find the picture with a battery, we think about the battery clip in the above picture is OK. The achievement is to charge the battery for the first time, but it will not be charged to the highest voltage used, and the charging current is also very small.

The purpose of the formation 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, which is why the aluminum plastic film has to reserve a gas bag. Some factories' processes use jigs to form, that is, to clamp the batteries in the fixture (sometimes the glass is easy to use, and then the steel clips) and then turn it into a cabinet. The resulting gas will be squeezed into the next gas. In the bag, the electrode interface is also better after the formation.

After forming, some cells, especially thick cells, may have a certain deformation due to the large internal stress. So some factories will set up a fixture shaping process after forming, also known as fixture baking.

The two sealing processes, just said that gas will be generated during the formation process, so we have to extract the gas and then carry out the second package. Here some companies become two processes: exhaust and two seals, as well as the back of a gas bag process, where I together are called two general closure.

In the second seal, the bag is first punctured by a guillotine blade and a vacuum is drawn at the same time, so that gas and a small part of the electrolyte in the bag will be extracted. Then two packages are sealed immediately in the two sealed areas to ensure the airtightness of the batteries. Finally, the encapsulated core is cut off to the air bag, and a soft pack core is basically formed. The second seal is the last encapsulation process of a lithium-ion battery. The principle is the same as that of the previous thermal package and will not be repeated.

Pressurization process during chemical formation

Since the flexible packaging battery adopts aluminum-plastic packaging structure, its exterior structure determines that the pole pieces cannot be closely arranged, and gaps are easily generated between the pole pieces. The gas generated during the battery formation process is also likely to remain between the pole pieces, and the subsequent sealing is performed. The gas can not be completely discharged, thereby affecting the battery performance, so consider using a rolling process between the two charges into the gas between the pole pieces to exclude.

The following figure shows the dissected picture of two types of batteries that have not been rolled and applied with rolling after the completion of the chemical conversion.

Batteries using a rolling process

In the previous figure, there is a lot of dead zone in the negative electrode of the battery, while in the following figure, the surface of the negative electrode of the battery is complete and no dead zone phenomenon is found. The soft pack battery will generate gas in the process of chemical conversion, which will increase the distance between the positive electrode, the separator, and the negative electrode during the formation process and hinder the transmission of lithium ions from the positive electrode sheet to the negative electrode sheet. In addition, the presence of gas will also hinder the electrolyte. Contact with the positive electrode and the negative electrode causes the local wetting performance of the negative electrode to deteriorate, eventually resulting in a large amount of unreacted dead space on the negative electrode sheet.

By comparing the voltages twice before and after one month, 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 the battery with the largest rolling pressure has a large pressure difference, indicating the capacity attenuation. Large, it may be due to the pressure of the battery rolling too much, resulting in local puncture of the diaphragm, resulting in micro-short circuit inside the battery, resulting in capacity attenuation.

It can be seen that applying proper rolling pressure at a proper time is a unique link in the soft bag molding process.

Soft pack lithium battery module pressure

The soft-package lithium-ion battery module generally adopts a stacking manner to form a module, and a certain force is applied to the laminated battery in the module to compress the battery. The function of the battery is mainly to restrain the battery. If the pressure is too small, the battery is easily damaged when the vehicle body vibrates; if it is too large, the battery life will be adversely affected. Therefore, when designing the structure of a soft-packaged lithium-ion battery module, it must be considered that a suitable pressure is applied to the battery. A small stacking pressure can prevent the layer from delaminating, which is beneficial for long-term battery life. However, due to the loss of circulating lithium ions, larger pressures result in larger capacity decay rates. In addition, stacking pressure can also cause local deformation and chemical degradation of the diaphragm. When the battery capacity changes from 0 to 100%, the thickness of the lithium battery increases by about 1.2%. From the results of these studies, the external pressure of the lithium battery has a significant impact on the performance of the battery and must be considered when designing the lithium battery module structure.