How to distinguish the so-called "soft pack lithium battery" and "polymer battery"

Issuing time:2020-07-20 08:47

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

   Soft-pack batteries are actually batteries that use aluminum-plastic packaging film as the packaging material. Relatively speaking, the packaging of lithium-ion batteries is divided into two categories, one is soft-packed batteries and the other is metal shell batteries. Metal shell batteries also include steel shells and aluminum shells, etc. In recent years, some batteries with plastic shells due to special needs can also be classified as this type.

   The difference between the two is not only the case material, but also the different packaging method. Soft-packed cells are thermally packaged, while metal shell cells are generally welded (laser welding). The reason why soft-pack batteries can be thermally sealed is that they use aluminum-plastic packaging film.

   aluminum plastic packaging film

The constitution of    aluminum-plastic packaging film (abbreviated as aluminum-plastic film) is shown in the figure, its cross-section has three layers: nylon layer, Al layer and PP layer.

   Each of the three layers has its own function. First of all, the nylon layer ensures the shape of the aluminum-plastic film and ensures that the film will not be deformed before being manufactured into a lithium ion battery.

   The Al layer is composed of a layer of metallic Al, and its function is to prevent the penetration of water. Lithium-ion batteries are very afraid of water. Generally, the water content of the pole pieces is required to be at PPM level, so the packaging film must be able to block the infiltration of moisture. Nylon is not waterproof and cannot provide protection. The metal Al will react with oxygen in the air to form a dense oxide film at room temperature, which prevents water vapor from penetrating and protects the inside of the cell. The Al layer also provides punching plasticity when forming the aluminum-plastic film. See point 3 for details.

  PP is the abbreviation of polypropylene. The characteristic of this material is that it will melt at a temperature of more than one hundred degrees Celsius and is sticky. Therefore, the thermal packaging of the battery mainly relies on the PP layer being melted and bonded together under the action of the heating of the head, and then the head is removed, and the temperature is cooled to solidify and bond.

  Aluminum plastic film looks very simple. In practice, how to evenly and firmly combine the three layers of materials is not so easy. It is a pity that most of the aluminum-plastic films with good quality are imported from Japan, and the domestic ones are not available, but the quality needs to be improved.

   aluminum plastic film forming process

  The soft-packed batteries can be designed into different sizes according to the needs of customers. After the external dimensions are designed, the corresponding mold needs to be opened to form the aluminum plastic film. The molding process is also called punching (in fact, I think it should be a "bump pit", but everyone writes it like that). As the name suggests, it is to punch out a mold that can be installed on the aluminum plastic film under heating. The pit of the core is shown in the figure below.

After    aluminum plastic film is punched and cut into shape, it is generally called Pocket bag, as shown in the figure below. Generally, when the battery core is thin, choose to punch a single pit (left in the figure below), and when the battery core is thick, choose to punch a double pit (right in the figure below), because too much deformation on one side will break the deformation limit of the aluminum-plastic film. Cause rupture.

   Sometimes according to the needs of the design, a small hole is punched in the position of the air bag to expand the volume of the air bag.

   top side sealing process

   has finally come to the topic (how good are you digressing!), the top-side sealing process is the first packaging process for soft-packed lithium-ion batteries. The top side seal actually includes two processes, the top seal and the side seal. First, put the wound core into the punched pit, and then fold the packaging film in half along the dotted line, as shown in the figure below.

   The following picture shows several positions that need to be packaged after the aluminum-plastic film is loaded into the core, including the top sealing area, side sealing area, one sealing area and second sealing area. They are introduced separately below.

   After placing the core in the pit, the entire aluminum plastic film can be placed in the fixture, and the top and side sealing are performed in the top side sealing machine. The top side sealing machine looks like this:

   The top side sealing machine of this model in the picture is equipped with four clamps. The left station is for top sealing and the right station is for side sealing. The two pieces of yellow metal are the upper head, and there is a lower head underneath. The two heads have a certain temperature (usually around 180℃) when they are packaged, and they are pressed on the aluminum plastic film when they are closed. The PP layer of the film is melted and then bonded together, and the package is OK.

   There is not much to say about the side seal (the side voltage is too far to talk about it), mainly talk 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 lugs. The lugs are made of metal (aluminum anode and nickel cathode). How can they be packaged with PP? This is done by a small part on the lugs-lug glue. I am not very clear about the specific structure of the tab, I hope someone who knows how to do it will add it. I only know that it also has the cost of PP, which means it can melt and bond when heated. The package at the tab position is shown in the circled part in the figure below. During packaging, the PP in the tab glue and the PP layer of the aluminum-plastic film melt and bond to form an effective packaging structure.

   liquid injection, pre-sealing process

   After the soft-pack battery core is sealed on the top side, X-ray is required to check the parallelism of the core, and then go into the drying room to remove moisture. After being left in the drying room for a certain period of time, the liquid injection and pre-sealing processes are entered.

  We know from the above introduction that after the cell is sealed on the top side, there is only one opening on the air bag side, which is used for liquid injection. After the injection is completed, it is necessary to pre-seal the side of the air bag immediately, which is also called one. After a package is completed, the core is theoretically completely isolated from the external environment. The packaging principle of a letter is the same as that of the top side seal, so I won't repeat it here.

  Standing, chemical conversion, fixture shaping process

  After the liquid injection and the seal are completed, the battery cell needs to be left to stand. According to the different processes, it is divided into high temperature standing and normal temperature standing. The purpose of standing is to allow the injected electrolyte to fully soak the pole piece. Then the batteries can be used to make them.

   The picture above is a conversion cabinet with soft-packed batteries, which is actually a charging and discharging device. I have been searching for a long time and haven't found a picture of a charged core. Think about the picture of the battery sandwiched on it. The chemical achievement is the first charging of the battery, but it will not be charged to the highest voltage used, and the charging current is also very small.

  The purpose of chemical transformation is to form a stable SEI film on the electrode surface, which is equivalent to a process of "activating" the battery cell. In this process, a certain amount of gas will be generated, which is why the aluminum plastic film needs to reserve an air bag. In some factories, the process will use fixture formation, that is, the cell is clamped in the fixture (sometimes it is simple to use a glass plate, and then a steel clamp is used) and then the cabinet is formed, so that the gas generated will be fully squeezed to the side of the gas The electrode interface after chemical formation is also better.

  Some cells, especially thick cells, may undergo certain deformation due to large internal stress after formation. Therefore, some factories will set up a fixture shaping process after the formation, also called fixture baking (baking).

  Second Sealing Process

   I just said that gas is generated during the formation process, so we have to extract the gas and then perform the second encapsulation. Some companies here have two processes: Degassing (exhaust) and two seals, and the next process of cutting the air bag. Here I call them two seals in general.

In the second seal, the air bag is first pierced with a guillotine and a vacuum is drawn at the same time, so that the gas and a small amount of electrolyte in the air bag will be drawn out. Then immediately the second sealing head is encapsulated in the second sealing area to ensure the air tightness of the cell. Finally, cut off the air bag of the encapsulated cell, and a soft-package cell is basically formed. The second seal is the last packaging process of the lithium-ion battery, and its principle is still the same as the previous thermal packaging, so I will not repeat it.

   Follow-up process

   Because the subject of the question is about packaging, the latter has little to do with packaging, so I will talk about the process after the second seal.

   After cutting the air bag, it is necessary to trim and fold the edges, that is, cut the one edge and the second edge to the appropriate width, and then fold it to ensure that the width of the battery does not exceed the standard. After folding, the battery cell can be installed in the capacity-dividing cabinet for capacity distribution. In fact, it is a capacity test to see if the capacity of the battery core reaches the specified minimum value. In principle, all batteries need to be tested before leaving the factory to ensure that batteries with unqualified capacity will not be delivered to customers. However, when the battery production volume is large, some companies will perform partial capacity division and use statistical probability to judge the pass rate of the batch of battery capacity.

  After the capacity is divided, the cells with qualified capacity will enter the subsequent processes, including appearance inspection, yellow glue, edge voltage detection, tab transfer welding, etc. Several processes can be added or removed according to customer needs. The last is OQC inspection, and then packaged and shipped.

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