What are the eight major processes for soft-pack lithium batteries?

Whether in the digital or power application market, soft pack batteries are becoming a very important technical route.

1. 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 a soft-packed battery core, and the other is a metal-clad battery core. The metal casing core includes a steel shell and an aluminum shell, etc. In recent years, since the battery core of a special need is made of a plastic outer casing, it can also be classified as such.

The difference between the two, in addition to the different shell materials, determines the way it is packaged. The soft-clad cells are made of a thermal package, while the metal-clad cells are usually soldered (laser welded). The reason why the soft-packed battery can be thermally packaged is that it uses an aluminum-plastic packaging film.

2. Aluminum plastic packaging film

The aluminum plastic packaging film (abbreviated as aluminum plastic film) is shown in the figure, and has a three-layer structure in cross section: a nylon layer, an Al layer and a PP layer.

The three layers each have their own functions. First, the nylon layer ensures the shape of the aluminum plastic film, ensuring that the film does not deform before being fabricated into a lithium ion battery.

The Al layer is composed of a layer of metal Al, which serves to prevent water from penetrating. Lithium-ion batteries are very afraid of water. Generally, the water content of the pole pieces is at the PPM level, so the packaging film must be able to block the infiltration of moisture. Nylon is not waterproof and does not provide protection. Metal Al reacts with oxygen in the air at room temperature to form a dense oxide film, which prevents moisture from penetrating and protects the inside of the cell. The Al layer also provides the plasticity of the crater when the aluminum film is formed. See point 3 for details.

PP is an abbreviation for polypropylene, which is characterized by melting at a temperature of more than one hundred degrees Celsius and is viscous. Therefore, the thermal packaging of the battery mainly relies on the PP layer being melted and bonded under the action of the heating of the head, and then the head is removed, and the temperature is solidified and bonded.

The aluminum plastic film looks very simple. Actually, how to combine the three layers of materials evenly and firmly is not so easy. It is a pity that the quality aluminum film is basically imported from Japan, and the domestic one is not without, but the quality still needs improvement.

3. Aluminum plastic film forming process

The soft-packed battery core can be designed into different sizes according to the customer's requirements. When the external dimensions are designed, it is necessary to open the corresponding mold to shape the aluminum plastic film. The molding process is also called a crater (in fact, the individual thinks it should be "squatting", but everyone writes it as usual.) As the name implies, it is a mold that can be loaded on the aluminum film with heating. The core of the core, see the figure below.

After the aluminum film is punched and cut, it is generally called the Pocket bag, as shown in the figure below. Generally, when the battery core is thin, select a single pit (the left side of the figure below), and when the battery core is thick, select the double pit (the right figure below), because the deformation amount of one side will break the deformation limit of the aluminum plastic film. Causes rupture.

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

4. Top side sealing process

Finally, I got to the point (you are awesome!), the top side sealing process is the first packaging process of soft-packed lithium-ion batteries. The top side seal actually contains two processes, a top seal and a side seal. First, put the wound core into the punched hole, and then fold the packaging film in half along the dotted line, as shown in the figure below.

The following figure shows several positions that need to be packaged after the aluminum plastic film is loaded into the core, including the top sealing zone, the side sealing zone, a zone and a second zone. The following is introduced separately.

After placing the core in the pit, the entire aluminum film can be placed in the fixture and the top and side seals are placed in the top side sealer. The top side sealer is like this:

In the figure, the top side sealer of this type has four clamps, the left station is the top seal, and the right side is the side seal, the two yellow metal are the upper head, and there is a lower head underneath when encapsulating, the two heads have a certain temperature (generally around 180 °C), and when pressed, they are pressed on the aluminum plastic film, aluminum plastic The PP layer of the film melts and sticks together, so the package is OK.

There is not much to say about the side seals (the side voltage is too far away to talk about it), mainly speaking, the top seal, 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 package with PP? This is done by a small part of the ear, the ear gel. I don't know the specific structure of the ear gel. I hope someone with knowledge can add it. I only know that it also has the cost of PP, which means that it can melt and bond when heated. The package in the polar is shown in the circle in the figure below. During packaging, the PP in the ear gel melts and bonds with the PP layer of the aluminum plastic film to form an effective package structure.

5. Injection, pre-sealing process

After the soft-packed battery is sealed on the top side, X-ray is required to check the parallelism of the core, and then the water is removed from the drying room. When the drying room is allowed to stand for a certain period of time, the liquid injection and pre-sealing steps are entered.

Through the above introduction, we know that after the top side seal is completed, only one opening on the side of the air bag is left. This opening is used for liquid injection. After the injection is completed, the pre-sealing of the airbag is required immediately, also called a letter. After a package is completed, the battery is theoretically completely isolated from the external environment. The principle of a package is the same as that of the top side seal, so I won't go into details here.

6. Static, chemical, fixture shaping process

After the liquid injection and the completion of the liquid, the battery core needs to be firstly placed, and it is divided into a high temperature static setting and a normal temperature depending on the process. The purpose of the static setting is to allow the injected electrolyte to fully wet the pole piece. Then the battery can be used to make it.

The picture above shows the formation of a soft-packed battery. In fact, it is a charging and discharging device. I haven't found a picture of the charged core for a long time. Let's think about the picture of the battery clip on it. The achievement is the first charge of the battery, but it will not charge to the highest voltage used, and the charging current is 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 "activating" the cell. In this process, a certain amount of gas is produced, which is why an aluminum film has to be reserved for an air bag. In some factories, the process is formed by using a fixture, that is, the battery is clamped in the fixture (sometimes the glass plate is used, and then the steel clamp is used) and then the cabinet is turned into a cabinet, so that the generated gas is fully squeezed to the side. The bag is also removed, and the electrode interface after the formation is also better.

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

7. Two processes

I just said that gas will be generated during the formation process, so we have to pump the gas out and then carry out the second package. Here some companies become two processes: Degassing (exhaust) and two, and the latter process of cutting air bags, here I will be called the general two.

In the second seal, the air bag is first pierced by a file, and a vacuum is applied, so that the gas in the air bag and a small portion of the electrolyte are extracted, then immediately two seals are sealed in the second zone to ensure the airtightness of the battery. Finally, the packaged battery core is cut into the air bag, and a soft package core is basically formed. The second is the last packaging process of the lithium-ion battery, and the principle is the same as the previous thermal package, and will not be described again.

8. Subsequent operations

Because the main question is the package, the latter relationship with the package is not big, so I will talk about the process after the second seal.

After cutting the air bag, the trimming and folding are required. The edge and the second edge are cut to the appropriate width, and then folded to ensure that the width of the battery is not exceeded. After folding the battery, the battery can be divided into sub-combined cabinets. In fact, it is the capacity test to see if the capacity of the battery has reached the specified minimum. In principle, all the batteries need to be tested separately before leaving the factory to ensure that the batteries with unqualified capacity will not be delivered to the customer. However, when the production capacity of the battery is large, some companies will do some partial capacity and judge the qualified rate of the batch capacity by statistical probability.

After the volume is divided, the qualified batteries will enter the post-process, including inspection appearance, yellow glue, side voltage detection, polar transfer welding, etc., and can increase or decrease several processes according to customer needs. The last is the OQC check, and then the package is shipped.

The page contains the contents of the machine translation.