26650 Lithium ion Battery Manufacturing Process.
1. Soft pack battery cell
The so-called soft pack battery cell is actually a battery cell that uses aluminum-plastic packaging film as the packaging material. Relatively speaking, the packaging of lithium-ion batteries can be divided into two categories: soft pack cells and metal shell cells. Metal shell cells include steel shells, aluminum shells, and so on. In recent years, due to special needs, some cells that use plastic shells can also be classified as such.
The difference between the two lies in the different shell materials, which determine their packaging methods. Soft pack cells use heat sealing, while metal shell cells generally use welding (laser welding). The reason why soft pack battery cells can be heat sealed is that they use aluminum-plastic packaging film as a material.
2. Aluminum plastic packaging film
The composition of aluminum-plastic packaging film (referred to as aluminum-plastic film) is shown in the figure, which has three layers in cross-section: nylon layer, Al layer, and PP layer.
Each of the three layers has its own function. Firstly, the nylon layer ensures the shape of the aluminum-plastic film, preventing deformation before manufacturing into lithium-ion batteries.
The Al layer is composed of a layer of metallic Al, which serves to prevent the infiltration of water. Lithium ion batteries are very sensitive to water, and generally require the moisture content of the electrodes to be at PPM level. Therefore, the packaging film must be able to block the infiltration of water vapor. Nylon is not waterproof and cannot provide protection. At room temperature, metal Al reacts with oxygen in the air to form a dense oxide film, preventing water vapor from penetrating and protecting the interior of the battery cell. The Al layer also provides plasticity for punching pits during the formation of aluminum-plastic film, as detailed in point 3.
PP is an abbreviation for polypropylene, which is characterized by melting at temperatures of over 100 degrees Celsius and possessing viscosity. So the thermal packaging of batteries mainly relies on the PP layer melting and bonding together under the heating effect of the head, and then the head is removed and cooled to solidify and bond.
Aluminum plastic film looks simple, but in reality, it is not easy to uniformly and firmly bond the three layers of materials together. Unfortunately, most of the high-quality aluminum-plastic films are now imported from Japan, and there are also some domestically produced ones, but the quality still needs improvement.
3. Aluminum plastic film forming process
Soft pack battery cells can be designed in different sizes according to customer needs. After the external dimensions are designed, corresponding molds need to be issued to form the aluminum-plastic film. The molding process is also known as punching hole (in my opinion, it should be called "gun hole", but everyone writes it as follows). As the name suggests, it is to use a molding mold to punch a hole on the aluminum-plastic film under heating, which can hold the core. See the following figure for details.
After the aluminum-plastic film is punched and cut into shape, it is generally called a Pocket bag, as shown in the following figure. Generally, when the battery cell is thin, choose to punch a single hole (left in the figure below), and when the battery cell is thick, choose to punch a double hole (right in the figure below), because excessive deformation on one side can exceed the deformation limit of the aluminum-plastic film and cause rupture.
Sometimes, according to design requirements, a small hole may be punched at the position of the airbag to expand its volume.
4. Top side sealing process
Finally getting to the point, the top side sealing process is the first packaging process for soft pack lithium-ion batteries. The top side sealing actually includes two processes, top sealing and side sealing. Firstly, place the wound core into the punched hole, and then fold the packaging film in half along the dotted line position, as shown in the following figure.
The following diagram shows several positions that need to be sealed after the aluminum-plastic film is loaded into the core, including the top sealing area, side sealing area, first sealing area, and second sealing area. Below are separate introductions.
Let you quickly understand the specifications and parameters of soft pack lithium batteries.
After placing the core in the pit, the entire aluminum-plastic film can be placed in the fixture and sealed on the top and side in the top sealing machine. The top side sealing machine looks like this:
Let you quickly understand the specifications and parameters of soft pack lithium batteries.
The top side sealing machine of this model in the picture comes with four fixtures, with the left workstation for top sealing and the right workstation for side sealing. The two yellow metal pieces are the upper end caps, and there is also a lower end cap underneath. During packaging, the two end caps have a certain temperature (usually around 180 ℃), and when they are closed, they are pressed onto the aluminum-plastic film, and the PP layer of the aluminum-plastic film melts
It was melted and bonded together, so the packaging is OK.
There's not much to say about the side seal, let's mainly talk about the top seal. The schematic diagram of the top seal area is shown in the following figure. The top seal needs to seal the electrode ears, which are made of metal (positive aluminum, negative nickel). How can they be packaged together with PP? This needs to be done with a small component on the ear - ear glue
It's done. I am not very clear about the specific structure of the ear gel, and I hope someone knowledgeable can supplement it. I only know that it also has the cost of PP, which means it can melt and bond when heated. The packaging at the extreme ear position is shown in the circle in the figure below. During packaging, the PP in the ear glue melts and bonds with the PP layer of the aluminum-plastic film, forming an effective packaging
Structure.
5. Injection and pre sealing process
After sealing the top side of the soft pack battery cell, X-ray inspection is needed to check the parallelism of its core winding, and then it is sent to the drying room for dehumidification. After a certain amount of time in the drying room, the injection and pre sealing process begins.
This page provides information on the specification table of soft pack lithium batteries from all over the country.
From the above introduction, we know that after the top sealing of the battery cell is completed, there is only one opening left on the side of the airbag, which is used for liquid injection. After the injection is completed, it is necessary to immediately perform pre sealing of the airbag edge, also known as sealing. After a package is completed, theoretically speaking, the battery cell is completely isolated from the external environment inside
great. The encapsulation principle of a single package is the same as that of a top side package, so it will not be elaborated here.
6. Static, chemical, and fixture shaping processes
After the injection and sealing are completed, the battery cell needs to be left to stand first. Depending on the process, it can be divided into high temperature standing and room temperature standing. The purpose of standing is to allow the injected electrolyte to fully infiltrate the electrode piece. Then the battery cells can be used for chemical conversion.
The above picture is a conversion cabinet for soft pack battery cells, which is actually a charging and discharging device. I have been searching for a long time but have not found a picture of the charged cells. Just think about the picture of the battery cells clamped on top. Chemical conversion is the initial charging of the battery cell, but it will not reach the maximum voltage required for use, and the charging current is also very small. The purpose of transformation is to make the electrode
The formation of a stable SEI film on the surface is equivalent to a process of "activating" the battery cell. During this process, a certain amount of gas will be generated, which is why aluminum-plastic film needs to reserve a gas bag. Some factories use fixture forming in their processes, which involves clamping the battery cells into the fixture (sometimes a glass plate is used for simplicity, and then steel is added)
The clamp is then loaded onto the cabinet and transformed, so that the generated gas can be fully squeezed into the nearby gas bag, and the electrode interface after transformation is also better.
After formation, some battery cells, especially thick ones, may undergo certain deformations due to high internal stress. So some factories will set up a fixture shaping process after formation, also known as fixture baking.
These three processes are not related to packaging, let's briefly introduce them here.
7. Second sealing process
Just now we mentioned that gas will be generated during the formation process, so we need to extract the gas and then perform a second encapsulation. Some companies here have two processes: Degassing (exhaust) and sealing, as well as the subsequent process of cutting airbags. Here, I will refer to them collectively as sealing.
When sealing, the airbag is first punctured by a guillotine and evacuated, so that the gas and a small portion of the electrolyte in the airbag can be extracted. Then immediately seal the second sealing head in the second sealing area to ensure the airtightness of the battery cell. Finally, cut off the airbag from the packaged battery cell, and a soft pack battery cell is basically formed. The second seal is for lithium-ion batteries
The principle of the last packaging process is still the same as the previous hot packaging, and will not be repeated here.
8. Subsequent processes
Because the main question is about packaging, the rest of it has little to do with packaging, so I will explain the process after the second packaging together.
After cutting the airbag, it is necessary to cut and fold the edges of the first and second seals to the appropriate width, and then fold them up to ensure that the width of the battery cell does not exceed the standard. After folding, the battery cells can be divided into capacity cabinets for capacity testing, which is actually a capacity test to see if the capacity of the battery cells has reached the specified minimum value. In principle
It is said that all battery cells need to undergo capacity testing before leaving the factory to ensure that cells with unqualified capacity will not be delivered to customers. But when the production volume of battery cells is large, some companies will perform partial capacity division to determine the qualification rate of the batch of battery cell capacity based on statistical probability.
After capacity division, cells with qualified capacity will enter the subsequent process, including inspection of appearance, yellow glue application, edge voltage testing, pole ear soldering, etc. Several processes can be added or removed according to customer needs. Finally, there is the OQC inspection, and then the packaging is shipped.
Soft pack lithium battery specification inquiry
Note that up to this point, they are still just battery cells. The battery cells will be sent to the Pack factory for further processing, including soldering circuit boards, packaging, and so on. The downstream of the Pack Factory is various user manufacturers, such as Apple, Lenovo, Huawei, etc. They will take the battery pack and install it into the terminal before it reaches us consumers.
