18650 lithium battery cells: analysis of factors affecting heat sealing process

　　The design and manufacturing technology of aluminum-plastic films require high standards. At present, 90% of the domestic market is mainly monopolized by Japanese and Korean companies such as Japan's DNP/Showa Denko and South Korea's Yulmura. Domestic companies such as Xinlun Technology and Fosu Technology are also developing Speed up the development and production of aluminum-plastic films, but there is still a certain gap compared with Japanese and Korean companies.

　　Currently, the packaging forms of lithium batteries on the market are mainly divided into square packaging, cylindrical packaging and aluminum-plastic film packaging. Square packages and cylindrical packages are mainly made of metal shells, including aluminum shells and steel shells. Aluminum-plastic film packaging is mainly used for packaging soft-pack lithium-ion batteries. Soft-pack batteries encapsulated in aluminum-plastic films are mainly used in the 3C field. In recent years, they have gradually penetrated into the new energy automobile industry, providing safe and stable power output for various types of cars.

　　As the production technology of power lithium-ion batteries continues to improve and the research and development of new batteries continues to accelerate, the country's performance requirements for power lithium batteries are becoming more and more stringent. Government subsidy standards continue to increase, and the amount of subsidies gradually decreases until there is no subsidy to form a market push. Technological development. Survival of the fittest is the law of the market, and key indicators such as energy density, safety, and cost of lithium-ion batteries must be strictly controlled. Among the three types of lithium-ion batteries, soft-pack batteries have the advantages of small mass, small internal resistance, high safety, and high energy density, and occupy an important position in the field of power batteries.

　　1. Introduction to aluminum plastic film

　　The packaging material used in soft-pack lithium batteries is aluminum-plastic composite film, referred to as aluminum-plastic film. The design and manufacturing technology of aluminum-plastic films require high standards. At present, 90% of the domestic market is mainly monopolized by Japanese and Korean companies such as Japan's DNP/Showa Denko and South Korea's Yulmura. Domestic companies such as Xinlun Technology and Fosu Technology are also developing Speed up the development and production of aluminum-plastic films, but there is still a certain gap compared with Japanese and Korean companies. The thickness of aluminum-plastic film has different specifications. Its structure is mainly composed of three materials, which are CPP layer, Al layer and nylon layer from the inside to the outside, as shown in Figure 1.

　　Before packaging the bare battery core, the aluminum plastic film must be punched and punched into the required specifications before the battery core is packaged. The innermost material of the die-cut pit is cast polypropylene film (CPP), which has a certain melting point. During packaging, the CPP layer materials on both sides melt relatively under a certain temperature and pressure and bond together, as shown in Figure 2. The CPP layer material has good heat sealing adhesion with metal Ni, Al and tab glue, and has good electrolyte resistance and HF resistance. At the same time, the CPP layer also has good insulation and physical protection properties, which can effectively prevent Corrosion of the aluminum-plastic film caused by a short circuit between the electrode and the lithium-ion battery soft packaging material.

　　The main purpose of the Al layer material in the middle is to prevent the infiltration of moisture in the air. When moisture penetrates into the battery core, it will react with the electrolyte to generate HF and form side reactions to produce a large amount of gas, causing the lithium battery to bulge, which is one of the important hidden dangers of lithium batteries. The Al layer has a certain strength that can prevent damage to the battery core from external forces. The outermost nylon layer mainly prevents the penetration of oxygen in the air and maintains the internal environment of the battery core. The nylon layer is used as the outermost layer because of its good punching performance. In the early aluminum-plastic films, PET was used as the outer layer. However, due to its high tension, PET itself caused bending and poor formability, so it was abandoned.

　　There are two main manufacturing methods for aluminum-plastic films: thermal method and dry method. The thermal method is to bond the aluminum layer and the CPP layer with MPP and heat and press them at a certain temperature. At high temperatures, the van der Waals forces in MPP are destroyed, aging, and the short-circuit resistance will drop sharply. At the same time, due to the destruction of its intermolecular structure, the toughness is reduced and it is easy to break during the molding process. The dry process is to add a binder between the PP and aluminum layers and directly composite it. It uses an insulating adhesive and does not require high-temperature treatment, so the short-circuit prevention performance is better than the thermal process. Moreover, the adhesive itself has better ductility than the PP layer and does not require high-temperature treatment without affecting the molding.

　　2. Discussion on factors affecting heat sealing

　　First of all, you need to understand that the purpose of aluminum-plastic film heat sealing is to completely isolate the battery core from the external environment, that is, any packaging defects that may affect external moisture, air infiltration, or electrolyte leakage should not occur. Soft-packed battery cell packaging is mainly divided into top and side sealing and final sealing processes.

　　The top sealing process is mainly the direct packaging of aluminum plastic film, tab glue and copper-nickel tabs, which has a greater possibility of damage. Side sealing is the packaging between the CPP layers of aluminum plastic film. There are not many problems. The main purpose is to prevent the occurrence of wrinkles and bubbles. The final sealing is a vacuum sealing. Since the lithium battery is precharged, gas will be generated, and the electrolyte will easily stick to the aluminum-plastic film PP layer, resulting in poor packaging.

　　During the production process, technicians often use six aspects of human-machine-material environmental testing to analyze failure modes. First, we need to eliminate secondary influencing factors. Secondary influencing factors include personnel, environment and testing methods. The operating level and work experience of personnel have a certain impact on the packaging effect. Here we set the operating level of personnel to be certain and there is no problem. The testing method is certain, and the verification of the packaging effect is obvious, and there is no need for abnormal analysis. Before packaging is carried out, the workshop environment needs to be cleaned and confirmed, including foreign objects on the table, impurities, environmental temperature and humidity, etc. The operation can only be carried out after everything is normal, and will not be discussed separately here. The following will analyze the main factors that affect the heat sealing effect, namely materials, equipment and processes.

　　1.Materials

　　Incoming material inspection is an important method to prevent abnormal inflow, and it is also an important means to reduce the rate of defective products and reduce production costs. There are two main aspects of incoming materials in the packaging process: one is the aluminum-plastic film itself, and the other is the incoming materials for bare cells. The abnormal problems of the aluminum-plastic film itself mainly include the abnormalities brought by the aluminum-plastic film itself and the abnormalities introduced by the punching pit. It is necessary to observe whether the color of the aluminum-plastic film itself is normal and whether there are bubbles, stains, scratches, etc. on the surface. Other properties of the aluminum-plastic film itself will be provided by the supplier with corresponding test data, such as aluminum foil and PP layer peel strength, processing performance, anti-penetration performance, etc. Lithium battery companies will also conduct corresponding inspections on incoming materials to confirm that they are put into use. Aluminum plastic film punching is for good and beautiful packaging. The defects caused by punching mainly include CPP damage, abnormal pits, etc. These abnormalities can lead to corrosion of the liquid-injected aluminum plastic film of lithium batteries and high corner stress. The key to the quality of punching is the design of the punching abrasives. The punching abrasives mainly include die, punch, pressing plate and other auxiliary devices. The length, width and depth of the mold need to be designed according to the shape and size of the battery. Then the gap of the concave and convex molds is designed according to the depth of the punching pit, and the surface roughness of the concave and convex molds is designed according to the processing characteristics of the aluminum plastic film. The main problem with incoming bare cell materials is contamination on the surface of the tabs, which may cause poor packaging.

　　2. Equipment

　　The design and structural design of the equipment mold are undoubtedly important to the packaging effect. In the top sealing process, due to the existence of positive and negative electrode tabs, there are uneven contact surfaces between the aluminum plastic film and the tab glue. It is necessary to ensure that the PP layer of the plastic film and the PP layer on the outer surface of the tab are well bonded and sealed. , and it is necessary to ensure that the aluminum-plastic film PP layer in the non-tab area is well bonded and sealed face-to-face. The design of the equipment packaging mold and the selection of the head are critical. When using hard sealing, you can add silica gel to the hard sealing head to compensate for the deficiencies in the top seal with the deformation of the silica gel. You can also design the hard sealing mold and dig it out. A groove to mate with the lug for packaging. When using soft sealing, you don’t need to think so much, but you have to pay attention to the packaging process of the soft sealing, verify the sealing performance of the packaging, and the aging of the high-temperature resistant glue. Too large or too small packaging thickness is not conducive to the packaging of aluminum-plastic film. Excessive thickness essentially means that the aluminum-plastic film is not packaged well. In terms of equipment, the compressed air pressure may be unstable due to low pressure or abnormal sensors. If the package thickness is too small, it means that it is over-sealed, which may cause the PP layer to over-melt and expose the aluminum layer. Corresponding limiting devices can be installed on the equipment to achieve the best packaging effect. In addition, when the temperature sensor is abnormal, it will not reach the set process temperature, causing over-sealing or under-sealing, resulting in poor packaging.

　　3. Craftsmanship

　　The packaging process is the most critical link. The key factors in the packaging process are temperature, pressure and time. The best state of the package is when the PP film reaches the melting point and is bonded together. The package has a good appearance, no bubbles and no wrinkles. The package has high strength, good sealing, and passes the insulation test. The packaging strength is tested with a tensile testing machine, the schematic diagram of which is shown in Figure 3. In addition, you can choose transverse stretching and longitudinal stretching in the sample stretching method. Tensile testing machines include horizontal tensile testing machines and vertical tensile testing machines. Generally speaking, vertical testing machines are generally electronic The universal testing machine has a sensor on the mobile end, so the vertical version can do tests such as peeling, tension, compression, and sharpness. In terms of stability, a special horizontal testing machine for peeling force is better than a vertical electronic universal testing machine. For both, the appropriate sensor range must be selected. The appearance is inspected by visual inspection, and the tightness is tested by vacuuming. The insulation test applies instantaneous high voltage to the tabs and aluminum-plastic film to test whether the packaging is good, detect defective products in advance, and prevent defective products from flowing out.

　　Figure 3. Tensile diagram of packaging strength

　　After selecting an aluminum-plastic film, it is necessary to conduct a three-factor DOE of temperature, pressure and time to confirm the best packaging process. The PP melting temperature of aluminum plastic film is generally 180℃~190℃. Several temperature points can be selected within a certain range to conduct heat sealing experiments under the same pressure and heat sealing time. The optimal packaging temperature can be determined by heat sealing strength and effect. . Correspondingly, the effects of heat sealing pressure and time on strength and encapsulation effects can also be determined. After correlation analysis, it can be determined which factor is the main influencing factor.

　　Generally speaking, the packaging strength increases as the packaging temperature increases. When the temperature reaches a certain height, it may cause over-sealing. The PP melt overflows the heat sealing area and forms cracks after cooling, thereby reducing the heat sealing strength. The optimal time for heat sealing is related to the temperature. When the temperature is low, the heat sealing time has little effect on the strength. This is because the PP layer does not reach the melting point at low temperature and the heat sealing layer is not fused together. When the temperature is high enough, there is a positive correlation between heat sealing time and heat sealing strength. Package pressure affects package thickness, but is a less important factor than temperature and time. During the daily use of packaging equipment, we should also pay attention to the flatness and parallelism of the head, confirm whether the head temperature and temperature sensor are normal, etc., and solve the corresponding problems when abnormal temperature is found.

　　Battery core packaging is a key process that affects the final performance of the battery core. The main influencing factors for aluminum-plastic film packaging include the selection of equipment and molds, and the determination of the packaging process. The packaging temperature and packaging time in the packaging process occupy a major position. Secondary influencing factors include the quality of incoming materials, the operator's ability and the workshop environment. Only by coordinating all aspects can the cell packaging work be done well. While preventing electrolyte corrosion, the interior of the cell is isolated from the external environment, preventing the penetration of water vapor, and ensuring the cycle life and use safety of lithium batteries.

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