The technical principle of lithium battery pack packaging process mainly focuses on isolating the battery cells from the external environment to maintain a stable internal electrochemical environment. Depending on the packaging materials and structures, the specific processes vary. The following takes soft-pack lithium batteries as an example for an introduction:

Encapsulation materials and structures

Outer layer: Generally a nylon layer, its function is to protect the middle layer, reduce scratches and dirt contamination, ensure the battery has a good appearance, prevent the penetration of air, especially oxygen, maintain the internal environment of the battery cell, and ensure that the packaging aluminum foil has good deformation capacity. Sometimes, PET is used instead of nylon to achieve better chemical corrosion resistance, but this will lead to a reduction in the pit depth of the aluminum-plastic film.

The middle layer: It has a certain thickness and strength, which can prevent water vapor penetration and external damage to the battery cell. The most mainstream material used is aluminum foil.

Inner layer: It mainly serves to encapsulate, insulate, and prevent the Al layer from coming into contact with the electrolyte, etc. It mainly adopts PP layer material.

Decoration or special protective layer: To improve the appearance luster of the battery, a matte layer is sometimes added outside the PET/ nylon, but this will cause a significant increase in the price of the aluminum-plastic film.

Packaging process flow

The forming process of aluminum-plastic film: According to the customer's requirements, the size of the soft-pack battery cell is designed. Once the external dimensions are designed, the corresponding mold is made to form the aluminum-plastic film. The forming process, also known as pit punching, involves using a forming mold to punch out a pit on the aluminum-plastic film that can accommodate the core under heating conditions. Sometimes, according to the design requirements, a small hole is punched at the position of the air bag to increase its volume.

Top and side sealing process: Top and side sealing actually consists of two processes, top sealing and side sealing. Place the wound core into the punched pit, fold the packaging film in half along the dotted line position, put the core into the pit, then place the entire aluminum-plastic film in the fixture and perform top sealing and side sealing in the top and side sealing machine. The top and side sealing machine is equipped with four fixtures. The left position is for top sealing and the right position is for side sealing. The two yellow metal pieces are the upper end caps, and there is another lower end cap below. During packaging, the two end caps are at a certain temperature (generally around 180°C). When they are closed, they press against the aluminum-plastic film, and the PP layer of the aluminum-plastic film melts and then adheres together. The top seal is to seal the tabs. The tabs are made of metal (aluminum for the positive pole and nickel for the negative pole), and the encapsulation with PP is accomplished by the TAB glue on the tabs. Ear glue also contains PP components and can melt and bond when heated. During packaging, the PP in the TAB glue melts and bonds with the PP layer of the aluminum-plastic film to form an effective packaging structure.

Liquid injection and pre-sealing process: After the top and side sealing of the pouch cells, X-ray inspection is required to check the parallelism of the wound cores, and then they are sent to the drying room to remove moisture. After the top and side sealing of the battery cell is completed, only one opening remains on the side of the air bag, which is used for liquid injection. After the liquid injection is completed, the edge of the air bag needs to be pre-sealed immediately, also known as sealing. After one package is completed, theoretically speaking, the battery cell is completely isolated from the external environment inside.

The process of static placement, formation, and fixture shaping: After the liquid injection and sealing are completed, the battery cells need to be static placed first. Depending on the process, it can be divided into high-temperature static placement and normal-temperature static placement. The purpose is to ensure that the injected electrolyte fully wets the electrode sheets. Formation is the first charging of the battery cell, but it will not reach the maximum voltage used, and the charging current is also very small. The purpose of formation is to create a stable SEI film on the electrode surface, which is equivalent to a process of "activating" the battery cell. During this process, a certain amount of gas will be produced, which is why a gas bag needs to be reserved for the aluminum-plastic film. Some factories' processes use fixture formation, that is, the battery cells are clamped in fixtures (sometimes glass plates are used for simplicity, followed by steel clamps) and then placed on the cabinet for formation. In this way, the generated gas is fully squeezed into the adjacent gas bag, and at the same time, the electrode interface after formation is also better. After formation, some battery cells, especially thick ones, may undergo certain deformation due to relatively large internal stress. So some factories will set up a fixture shaping process after formation, which is also called fixture baking.

Exhaust and secondary sealing process: First, the gas bag is punctured by a guillotine, and at the same time, the air is evacuated. In this way, the gas and a small part of the electrolyte in the gas bag will be extracted. Then immediately, the secondary seal heads are sealed in the secondary seal area to ensure the airtightness of the battery cells. Finally, the air pockets of the packaged battery cells are cut off, and a soft-pack battery cell is basically formed. The second packaging is the final packaging process of lithium-ion batteries, and its principle is still the same as the previous thermal packaging.

Subsequent processes: After sealing and cutting the air bags, edge trimming and folding are required. That is, the first sealing edge and the second sealing edge are cut to the appropriate width and then folded to ensure that the width of the battery cells does not exceed the standard. After the edges are folded, the battery cells can be placed in the capacity sorting cabinet for capacity sorting. In fact, it is a capacity test to see if the capacity of the battery cells has reached the specified minimum value. In principle, all battery cells need to undergo capacity separation tests before leaving the factory to ensure that cells with unqualified capacity will not be delivered to customers. However, when the production volume of battery cells is large, some companies will conduct partial capacity sorting and use statistical probabilities to determine the qualification rate of the capacity of that batch of battery cells. After capacity separation, the battery cells with qualified capacity will enter the subsequent processes, including appearance inspection, yellow glue application, edge voltage detection, TAB transfer welding, etc. Several processes can be added or removed according to the customer's requirements.

Confirmation of encapsulation parameters and effects

Packaging parameters: including temperature (melting points of PP and outer nylon, head structure and heat dissipation), time (heat transfer and PP fusion), pressure (PP lamination, affecting melt and heat transfer), vacuum degree (mainly vacuum standing and degassing process).

Effect confirmation: Check the parallelism of the package (On the basis that the parallelism of the stopper is ok, carry out the package, tear off the package surface and observe the foil melt adhesive showing milky white, seamless package defects); Tensile test (including TAB position and side seal position); Unsealed area inspection (visual inspection, including the inner and outer unsealed areas of the top seal, side seal and bottom seal, and check the internal PP condition); Youdaoplaceholder0 Exposure inspection (visual inspection, mainly negative TAB); Resistance test (After top and side sealing, check between the positive and negative tabs, and between the negative TAB and the Al layer in the pocket bag. After degassing, check between the negative TAB and the Al layer in the pocket bag); Hierarchical inspection (degassing followed by final edge sealing).