The aluminum plastic film is a multilayer film composed of an outer nylon layer, an adhesive, an intermediate aluminum foil (Al), an adhesive, and an inner heat-sealing layer (CPP), and is a packaging material for a soft-packaged lithium battery. Aluminum plastic film is required to have the following characteristics:
(1) High barrier properties;
(2) Has good heat seal performance;
(3) Material resistant to electrolyte and strong acid corrosion;
(4) Good ductility, flexibility and mechanical strength.
The main component of aluminum plastic film is ON/AL/CPP three-layer material. Each layer is bonded with adhesive. According to different composite processes, aluminum plastic film can be divided into dry and thermal methods, as shown in Figure 1. The dry process is made by bonding aluminum and polypropylene directly after adhesive bonding. The thermal process consists of bonding aluminum and polypropylene together with MPP. The main advantages of the dry-process aluminum-plastic film are: deep-draw forming effect, good appearance consistency, poor appearance of pinholes, fish eyes, impurities, etc., but poor electrolyte resistance. The thermal process can improve the Al layer. The adhesion between the PP layers greatly improves the ability of the inner surface layer to resist swelling and detachment of the electrolyte. However, this special treatment of the MPP requires a higher temperature to melt it before it can play a bonding role. After cooling, due to the PP There is a large difference in the coefficient of contraction and it is easy to curl inward.
Figure 1 two kinds of aluminum plastic film schematic
1 role of each part
Nylon: It can effectively prevent the penetration of air, especially oxygen, and maintain the internal environment of the battery. At the same time, it can ensure that the packaging aluminum foil has good deformability.
Al: It can effectively prevent the penetration of moisture in the air, maintain the internal environment of the cell, and has a certain thickness strength to prevent external damage to the cell.
PP: It will not be dissolved or swelled by the organic solvent inside the cell, it is the most direct packaging protection of the cell's internal environment, and it will not only prevent the internal electrolyte from contacting Allayer, but also prevent Allayer from being corroded.
2 evaluation methods
In order to fully understand the performance of lithium battery aluminum plastic film, establish the performance evaluation standard of aluminum plastic film, screen out the cost-effective aluminum plastic film products, and summarize the performance evaluation method of aluminum plastic film.
(1) Appearance of aluminum plastic film material
Visually check whether there are pits, scratches, etc. on the surface of the aluminum plastic film. Whether there is any debris or air bubbles between the composite layers. Use a micrometer to take no less than 10 points to measure the thickness and width of the aluminum plastic film, and determine its uniformity.
(2) Heat seal strength
Under certain temperature, time, and pressure encapsulation conditions, two pieces of aluminum plastic film or bonded to the tabs, cut to a certain width and cut into strips, using a tensile test equipment to test the peel strength. Observed after the peeling of aluminum plastic film Whether the inner surface layer is uniform milky white, and it is required that the Al layer and the inner surface layer do not delaminate. The heat sealing process parameters of the flexible packaging material mainly include the heat sealing time, the heat sealing pressure and the heat sealing temperature. Under normal circumstances, it is necessary to re-optimize these three parameters. , To achieve the best heat sealing effect.
(3) Drilling capability
Aluminium plastic film with a good mold die punching depth, within a certain range of value adjustment molding depth, until the plastic film edge cracks, record the depth of the maximum depth before the crack. After the depth of the aluminum plastic film must meet the following conditions : The thinnest corner of the four corners of the crater is not less than 50% of the original thickness. The thickness of the inner PP layer is 60~65 microns. Otherwise, it will cause leakage after battery molding, which will seriously affect the battery performance.
(4) Electrolytic corrosion resistance
The plastic film in the aluminum plastic film is in contact with the inner layer of the PP layer and the peripheral heat seals. These places must have good resistance to electrolyte corrosion. The molded aluminum film should be sealed on both sides without the tabs. Cell, inject a certain amount of electrolyte, baked at a certain temperature for a certain period of time after observation. Observe whether the aluminum plastic film after sealing and forming is corrosive, leakage, expansion, stratification.
(5) Barrier properties
Place the packaging bag in a water bath or in a high-temperature and high-humidity environment. Take it out after a period of time. Observe whether the aluminum-plastic film is abnormal and determine its water absorption through weighing or moisture test to characterize the barrier properties of the aluminum-plastic film.
3 stamping process
As shown in the schematic diagram of the forming process of aluminum-plastic film punching pits, the upper and lower stencils (die) clamp the aluminum-plastic film to maintain a certain pressure P, and the punching mold runs at a gentle speed to make the aluminum-plastic film. Meet the required depth.
Figure 2 Aluminium Plastic Film Stamping Schematic
As shown in Figure 3, the actual stamping process used in the current production of the enterprise has the following two types: (1) Extensive punching depth: The blank holder has large pressure on the aluminum plastic film, and the edge of the aluminum plastic film is fixed, and the fixed part is not generated. Deformation, it is difficult to compensate for the deep part of the hedging, the edge of the part is completely compensated by the bottom when forming, the depth of punching is shallow, and the adjustability is poor. At present, there is less adjustment; (2) The compensating depth: The pressure of the blank holder is adjustable on the aluminum plastic film. The deep part of the plastic film can be compensated by the material of the edge and the bottom. The whole movement of the aluminum plastic film is deformed, the thickness is relatively uniform, and the punching depth is strong.
Figure 3 Comparison of aluminum and plastic film stamping process
Formed aluminum plastic film shell shown in Figure 4, the most serious reduction in the corner position, is the key factor affecting the overall quality of the aluminum plastic film shell, it should be used as a key point to measure the quality of lithium-ion battery aluminum plastic film shell forming The process parameters that affect the stamping forming quality of aluminum plastic film mainly include the design of the die (punch and die), blank holder force and punching speed.
Figure 4 stamping aluminum plastic film
The mold design includes the length and width of the mold, the R angle of the punch and die, the surface roughness of the mold, and the punching clearance between the upper punch and the lower punch. Selection of the fillet radius of the punch and the friction coefficient of the die The quality of aluminum plastic film stamping process has a great influence. When the surface roughness of the mold is too large, large roughness causes large frictional force and correspondingly large flow resistance, which hinders material flow and is not conducive to flow supplementation during the forming process of the packaging bag. Causes excessive thinning of aluminum-plastic film stamped aluminum layers. Punching hole size, depth, R-angle of four corners, edge R-angle are based on the size of the cell, and the characteristics of aluminum-plastic film are related to the punching die. The depth of punching and R-angle are optimized. The relationship between the R-corner design and the size of the cell should be matched to avoid damage to the cell corner caused by the aluminum-plastic film package.
