Soft-pack lithium-ion battery: Introduction of heat-sealing process and influencing factors -Lithium - Ion Battery Equipment
The design and manufacturing technology requirements of aluminum-plastic film are relatively high. At present, 90% of the domestic market is mainly monopolized by Japanese and Korean companies such as DNP/Showa Denko of Japan and Kurimura of South Korea. Domestic companies such as Selen Technology and Fosu Technology are also developing Accelerate the development and processing of aluminum-plastic film, but there is still a certain gap compared with Japanese and Korean companies.
At present, the packaging forms of lithium-ion batteries on the market are mainly divided into square packaging, cylindrical packaging and aluminum-plastic film packaging. Square packages and cylindrical packages are mainly metal shells, including aluminum shells and steel shells. Aluminum-plastic film packaging is mainly used in the packaging of soft-pack lithium-ion batteries. Soft-pack batteries packaged in aluminum-plastic film are mainly used in the 3C field. In recent years, they have gradually penetrated into the new energy vehicle industry, providing safe and stable power output for various types of vehicles.(Lithium - Ion Battery Equipment)
With the continuous improvement of power lithium-ion battery processing technology and the continuous acceleration of the research and development of new batteries, the country's various performance requirements for power lithium-ion batteries are becoming more and more stringent. The government subsidy standards continue to increase, and the amount of subsidies is gradually reduced until no subsidies are formed. Push technology forward. 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 kinds of lithium-ion batteries, soft-pack batteries have the advantages of small mass, low internal resistance, high safety, and high energy density, and occupy an important position in the field of power lithium batteries.
1. Introduction of aluminum plastic film
The packaging material used in soft-pack lithium-ion batteries is aluminum-plastic composite film, referred to as aluminum-plastic film. The design and manufacturing technology requirements of aluminum-plastic film are relatively high. At present, 90% of the domestic market is mainly monopolized by Japanese and Korean companies such as DNP/Showa Denko of Japan and Kurimura of South Korea. Domestic companies such as Selen Technology and Fosu Technology are also developing Accelerate the development and processing of aluminum-plastic film, but there is still a certain gap compared with Japanese and Korean companies. The thickness of aluminum-plastic film has different specifications, and its structure is mainly composed of three materials, which are CPP layer, Al layer and nylon layer from inside to outside.
Before packaging the bare cells, the aluminum-plastic film must be punched to meet the required specifications before packaging the cells. The innermost material of the die-cut pit is a cast polypropylene film (CPP), which has a certain melting point. During packaging, the CPP layer materials on both sides are relatively melted and bonded together under a certain temperature and pressure, as shown in Figure 2. The CPP layer material has good heat-sealing adhesion with metal Ni, Al and tab glue, and it has good electrolyte and HF resistance performance. At the same time, the CPP layer also has good insulation and physical protection performance, which can effectively guard against Corrosion of the aluminum-plastic film caused by the short circuit between the electrode and the soft package material of the lithium-ion battery.
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 cell, it will react with the electrolyte to generate HF and form a side reaction to generate a large amount of gas, causing the lithium-ion battery to bulge, which is one of the major hidden dangers of lithium-ion batteries. . The Al layer has a certain strength and can guard against damage to the battery core by external force. The outermost nylon layer is mainly to prevent the penetration of oxygen in the air and maintain the internal environment of the cell. The outermost layer is made of nylon because it has good drawing performance. In the early aluminum-plastic film, the outer layer was made of PET, but because of the high tension of PET itself, it caused bending and poor formability, so it was discarded.
The manufacturing methods of aluminum-plastic film mainly include thermal method and dry method. The thermal method is that the aluminum layer and the CPP layer are bonded with MPP and synthesized by hot pressing at a certain temperature. The van der Waals force in MPP is destroyed at high temperature, aging, and the short-circuit resistance performance 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 an adhesive between the PP and the aluminum layer and combine them vertically, using an insulating adhesive, which does not require high temperature treatment, so the anti-short circuit performance is better than that of the thermal process. And the ductility of the adhesive itself is better than that of the PP layer, so it does not affect the molding without high temperature treatment.
2. Discussion on the influencing factors of heat sealing
First of all, it is necessary to understand that the purpose of aluminum-plastic film heat sealing is to completely isolate the battery cell from the external environment, that is, any poor packaging that may affect external moisture, air infiltration, and electrolyte leakage should not occur. Soft-pack battery packaging is mainly divided into top-side sealing and final sealing processes.
The top sealing process is mainly the packaging of aluminum-plastic film, tab glue, and straight copper-nickel tabs, which has a greater possibility of damage. The side sealing is the packaging between the CPP layers of the aluminum-plastic film. There are not too many problems. It is important to watch out for wrinkles and air bubbles. The final seal is a vacuum package. Since the lithium-ion battery is pre-charged, gas will be generated, and the electrolyte will easily stick to the PP layer of the aluminum-plastic film, resulting in poor packaging.
In the process of processing, technicians often use man-machine-material method to test six aspects to analyze the failure mode. First of all, we need to eliminate the secondary influencing factors, which include personnel, environment and detection methods. The operating level and work-related experience of the personnel have a certain impact on the packaging effect. Here we set the operating level of the personnel to be certain and there is no problem. The detection method is certain, and the inspection of the encapsulation effect is obvious, and abnormal decomposition is not required. Before the packaging is carried out, the workshop environment must be cleaned and confirmed, including foreign objects on the desktop, impurities, and ambient temperature and humidity, etc., and the operation can only be carried out after everything is normal, and there will be no separate discussion here. The following will break down the important factors that affect the heat sealing effect, namely materials, equipment and process.
1. Materials
Incoming material inspection is an important method to guard against abnormal inflow, and it is also an important means to reduce the rate of unqualified products and reduce processing costs. There are two important aspects of incoming materials in the packaging process: one is the aluminum-plastic film itself, and the other is the incoming materials of bare cells. The problems of the aluminum-plastic film itself are very important, including the abnormalities caused by the aluminum-plastic film itself and the introduction of punching pits. The aluminum-plastic film itself should be observed to see if its color is normal, and whether there are bubbles, stains, scratches, etc. on the surface of the appearance. Other properties of the aluminum-plastic film itself will be provided by the supplier with corresponding detection data, such as the peel strength of the aluminum foil and the PP layer, production performance, impermeability and so on. Lithium-ion battery companies will also conduct corresponding inspections on incoming materials to confirm that they are put into use. The pitting of the aluminum-plastic film is for the good and beautiful packaging. The defects introduced by the pitting mainly include CPP damage and abnormal pits. The key to the quality of the punching pit is the design of the punching abrasive tool. The punching abrasive tool mainly includes dies, punches, pressure plates and other auxiliary devices. The length, width and depth of the mold should be designed according to the shape and size of the battery. Then design the gap between the concave and convex molds according to the depth of the punching pit, and design the surface roughness of the concave and convex molds according to the production characteristics of the aluminum-plastic film. The main problem in the raw material of the bare cell is that the surface of the tab is polluted, which may cause poor packaging.
2. Equipment
The design and structural design of the equipment mold are undoubtedly critical to the packaging effect. In the top sealing process, due to the existence of positive and negative tabs, there is an uneven contact surface 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 bonded and sealed well. , but also to ensure that the PP layers of the aluminum-plastic film in the non-tip area are bonded and sealed well. The equipment packaging mold design and the selection of the head are more critical. When using the hard seal, you can add silica gel to the hard seal head, and use the deformation of the silica gel to compensate for the lack of the top seal. You can also design the mold for the hard seal mold. Dig a groove to match the tab for packaging. Don’t think so much when using soft seals, but pay attention to the packaging process of soft seals, verify the sealing of the package, and the aging of high-temperature-resistant adhesives. Too large or too small packaging thickness is not conducive to the packaging of aluminum-plastic film. Excessive thickness is essentially 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 thickness of the package is too small, it means that it is over-sealed, which may cause the PP layer to melt and expose the aluminum layer. The corresponding limit device 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 and cause over-seal or under-seal, resulting in poor packaging.
3. Craft
The packaging process is the most critical link, and the key factors in the packaging process are temperature, pressure and time. The best state of encapsulation is that after the PP film reaches the melting point, it is bonded together, the appearance of the encapsulation is good, there are no bubbles and no wrinkles, the encapsulation strength is high, the airtightness is good, and the insulation detection is passed. The encapsulation strength is detected 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 way of sample stretching. Tensile testing machines include horizontal tensile testing machines and vertical tensile testing machines. Generally speaking, vertical testing machines are generally electronic. Universal testing machine, the sensor is on the mobile side, so the vertical one can do peeling, stretching, compression, and sharpness experiments. In terms of stability, the special horizontal testing machine for peeling force is better than the vertical electronic universal testing machine, and the appropriate sensor range must be selected for both. The appearance is checked by visual inspection, and the tightness is detected by vacuuming. Insulation detection is to apply instantaneous high voltage to tabs and aluminum-plastic film to detect whether the packaging is good, detect defective products in advance, and guard against the outflow of defective products.
At present, the packaging forms of lithium-ion batteries on the market are mainly divided into square packaging, cylindrical packaging and aluminum-plastic film packaging. Square packages and cylindrical packages are mainly metal shells, including aluminum shells and steel shells. Aluminum-plastic film packaging is mainly used in the packaging of soft-pack lithium-ion batteries. Soft-pack batteries packaged in aluminum-plastic film are mainly used in the 3C field. In recent years, they have gradually penetrated into the new energy vehicle industry, providing safe and stable power output for various types of vehicles.(Lithium - Ion Battery Equipment)
With the continuous improvement of power lithium-ion battery processing technology and the continuous acceleration of the research and development of new batteries, the country's various performance requirements for power lithium-ion batteries are becoming more and more stringent. The government subsidy standards continue to increase, and the amount of subsidies is gradually reduced until no subsidies are formed. Push technology forward. 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 kinds of lithium-ion batteries, soft-pack batteries have the advantages of small mass, low internal resistance, high safety, and high energy density, and occupy an important position in the field of power lithium batteries.
1. Introduction of aluminum plastic film
The packaging material used in soft-pack lithium-ion batteries is aluminum-plastic composite film, referred to as aluminum-plastic film. The design and manufacturing technology requirements of aluminum-plastic film are relatively high. At present, 90% of the domestic market is mainly monopolized by Japanese and Korean companies such as DNP/Showa Denko of Japan and Kurimura of South Korea. Domestic companies such as Selen Technology and Fosu Technology are also developing Accelerate the development and processing of aluminum-plastic film, but there is still a certain gap compared with Japanese and Korean companies. The thickness of aluminum-plastic film has different specifications, and its structure is mainly composed of three materials, which are CPP layer, Al layer and nylon layer from inside to outside.
Before packaging the bare cells, the aluminum-plastic film must be punched to meet the required specifications before packaging the cells. The innermost material of the die-cut pit is a cast polypropylene film (CPP), which has a certain melting point. During packaging, the CPP layer materials on both sides are relatively melted and bonded together under a certain temperature and pressure, as shown in Figure 2. The CPP layer material has good heat-sealing adhesion with metal Ni, Al and tab glue, and it has good electrolyte and HF resistance performance. At the same time, the CPP layer also has good insulation and physical protection performance, which can effectively guard against Corrosion of the aluminum-plastic film caused by the short circuit between the electrode and the soft package material of the lithium-ion battery.
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 cell, it will react with the electrolyte to generate HF and form a side reaction to generate a large amount of gas, causing the lithium-ion battery to bulge, which is one of the major hidden dangers of lithium-ion batteries. . The Al layer has a certain strength and can guard against damage to the battery core by external force. The outermost nylon layer is mainly to prevent the penetration of oxygen in the air and maintain the internal environment of the cell. The outermost layer is made of nylon because it has good drawing performance. In the early aluminum-plastic film, the outer layer was made of PET, but because of the high tension of PET itself, it caused bending and poor formability, so it was discarded.
The manufacturing methods of aluminum-plastic film mainly include thermal method and dry method. The thermal method is that the aluminum layer and the CPP layer are bonded with MPP and synthesized by hot pressing at a certain temperature. The van der Waals force in MPP is destroyed at high temperature, aging, and the short-circuit resistance performance 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 an adhesive between the PP and the aluminum layer and combine them vertically, using an insulating adhesive, which does not require high temperature treatment, so the anti-short circuit performance is better than that of the thermal process. And the ductility of the adhesive itself is better than that of the PP layer, so it does not affect the molding without high temperature treatment.
2. Discussion on the influencing factors of heat sealing
First of all, it is necessary to understand that the purpose of aluminum-plastic film heat sealing is to completely isolate the battery cell from the external environment, that is, any poor packaging that may affect external moisture, air infiltration, and electrolyte leakage should not occur. Soft-pack battery packaging is mainly divided into top-side sealing and final sealing processes.
The top sealing process is mainly the packaging of aluminum-plastic film, tab glue, and straight copper-nickel tabs, which has a greater possibility of damage. The side sealing is the packaging between the CPP layers of the aluminum-plastic film. There are not too many problems. It is important to watch out for wrinkles and air bubbles. The final seal is a vacuum package. Since the lithium-ion battery is pre-charged, gas will be generated, and the electrolyte will easily stick to the PP layer of the aluminum-plastic film, resulting in poor packaging.
In the process of processing, technicians often use man-machine-material method to test six aspects to analyze the failure mode. First of all, we need to eliminate the secondary influencing factors, which include personnel, environment and detection methods. The operating level and work-related experience of the personnel have a certain impact on the packaging effect. Here we set the operating level of the personnel to be certain and there is no problem. The detection method is certain, and the inspection of the encapsulation effect is obvious, and abnormal decomposition is not required. Before the packaging is carried out, the workshop environment must be cleaned and confirmed, including foreign objects on the desktop, impurities, and ambient temperature and humidity, etc., and the operation can only be carried out after everything is normal, and there will be no separate discussion here. The following will break down the important factors that affect the heat sealing effect, namely materials, equipment and process.
1. Materials
Incoming material inspection is an important method to guard against abnormal inflow, and it is also an important means to reduce the rate of unqualified products and reduce processing costs. There are two important aspects of incoming materials in the packaging process: one is the aluminum-plastic film itself, and the other is the incoming materials of bare cells. The problems of the aluminum-plastic film itself are very important, including the abnormalities caused by the aluminum-plastic film itself and the introduction of punching pits. The aluminum-plastic film itself should be observed to see if its color is normal, and whether there are bubbles, stains, scratches, etc. on the surface of the appearance. Other properties of the aluminum-plastic film itself will be provided by the supplier with corresponding detection data, such as the peel strength of the aluminum foil and the PP layer, production performance, impermeability and so on. Lithium-ion battery companies will also conduct corresponding inspections on incoming materials to confirm that they are put into use. The pitting of the aluminum-plastic film is for the good and beautiful packaging. The defects introduced by the pitting mainly include CPP damage and abnormal pits. The key to the quality of the punching pit is the design of the punching abrasive tool. The punching abrasive tool mainly includes dies, punches, pressure plates and other auxiliary devices. The length, width and depth of the mold should be designed according to the shape and size of the battery. Then design the gap between the concave and convex molds according to the depth of the punching pit, and design the surface roughness of the concave and convex molds according to the production characteristics of the aluminum-plastic film. The main problem in the raw material of the bare cell is that the surface of the tab is polluted, which may cause poor packaging.
2. Equipment
The design and structural design of the equipment mold are undoubtedly critical to the packaging effect. In the top sealing process, due to the existence of positive and negative tabs, there is an uneven contact surface 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 bonded and sealed well. , but also to ensure that the PP layers of the aluminum-plastic film in the non-tip area are bonded and sealed well. The equipment packaging mold design and the selection of the head are more critical. When using the hard seal, you can add silica gel to the hard seal head, and use the deformation of the silica gel to compensate for the lack of the top seal. You can also design the mold for the hard seal mold. Dig a groove to match the tab for packaging. Don’t think so much when using soft seals, but pay attention to the packaging process of soft seals, verify the sealing of the package, and the aging of high-temperature-resistant adhesives. Too large or too small packaging thickness is not conducive to the packaging of aluminum-plastic film. Excessive thickness is essentially 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 thickness of the package is too small, it means that it is over-sealed, which may cause the PP layer to melt and expose the aluminum layer. The corresponding limit device 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 and cause over-seal or under-seal, resulting in poor packaging.
3. Craft
The packaging process is the most critical link, and the key factors in the packaging process are temperature, pressure and time. The best state of encapsulation is that after the PP film reaches the melting point, it is bonded together, the appearance of the encapsulation is good, there are no bubbles and no wrinkles, the encapsulation strength is high, the airtightness is good, and the insulation detection is passed. The encapsulation strength is detected 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 way of sample stretching. Tensile testing machines include horizontal tensile testing machines and vertical tensile testing machines. Generally speaking, vertical testing machines are generally electronic. Universal testing machine, the sensor is on the mobile side, so the vertical one can do peeling, stretching, compression, and sharpness experiments. In terms of stability, the special horizontal testing machine for peeling force is better than the vertical electronic universal testing machine, and the appropriate sensor range must be selected for both. The appearance is checked by visual inspection, and the tightness is detected by vacuuming. Insulation detection is to apply instantaneous high voltage to tabs and aluminum-plastic film to detect whether the packaging is good, detect defective products in advance, and guard against the outflow of defective products.
