Lithium-ion polymer battery is small in size, light in weight, long in cycle life (more than 1000 times), and has no memory effect (can be used with charging without affecting performance). It is widely used in mobile phones and PDAs. Laptops, portable military tools, camcorders, and DVD players are being developed for use in electric vehicles (electric bicycles, motorcycles, and automobiles). It is the green chemical energy of the 21st century that is due to its excellent performance and its wide application prospects. Its successful development depends mainly on the solution of three major technical problems, namely film making technology, lamination technology and flexible packaging technology.
The flexible packaging technology here refers to the design and manufacturing process technology of the inner packaging (forming) material of the lithium ion polymer battery core and the application process technology of the material (hereinafter referred to as flexible packaging technology).
1 The importance of flexible packaging materials
Flexible packaging technology is one of the three major technical problems to be solved in the top-notch high-tech industry of lithium-ion polymer batteries. It is placed in such an important position that the product has high technical content, in design, manufacturing and There are qualitative differences in performance between the application and ordinary composite packaging materials. So far, there is still no company in the world that the project product can fully meet the comprehensive technical requirements of the lithium ion polymer battery for this product. Therefore, the lithium ion polymer battery core inner packaging molding material is not only a packaging problem of a lithium ion polymer battery, but also an indispensable and important component constituting a lithium ion polymer battery. If the importance of this flexible packaging material is not enough, it will be detrimental to its design and development.
(1) Lithium-ion polymer batteries require that the barrier properties (such as moisture and oxygen) of flexible packaging materials are 10,000 times higher than those of ordinary aluminum-plastic composite films. Such high barrier properties have exceeded the detection accuracy of the instrument.
(2) The product cannot exist without the package, and the service life of the package is the same as the inventory and service life of the product. Packaging has become an indispensable and important part of the product. The life cycle of the product is actually the process of gradual failure of the packaging. Without the packaging, there is no product, or the packaging is invalid, and the product is scrapped.
(3) The use process of the product (continuous charge and discharge process) is a dynamic electrochemical reaction process. The flexible packaging material itself can effectively resist the swelling, dissolution, penetration, absorption and electrochemical reaction of the internal electrolyte.
(4) The electrolyte in the battery is present in a variety of organic solvents and lithium salts that can rapidly produce highly corrosive hydrofluoric acid in contact with moisture.
A variety of organic solvents usually swell, dissolve and absorb soft packaging materials, especially they are good solvents for adhesives for general composite materials, which will destroy the bonding effect between the composite layers, change the concentration of each component in the electrolyte, and thus affect the battery. Electrical performance. The presence of highly corrosive hydrofluoric acid will severely corrode the aluminum foil, separating the inner film from the aluminum foil, and then corroding and perforating the aluminum foil, thereby destroying the entire package.
(5) The inner membrane of the flexible packaging material must be resistant to the puncture of the burrs around the battery cells and the insulation of the aluminum foil in the metal foil and the flexible packaging material under heat and pressure. The lithium ion polymer battery core has burrs around the copper mesh and the aluminum mesh. When vacuuming and shrinking, the burrs will spur the inner film and may pierce the inner film until the aluminum foil, so the hydrofluoric acid in the battery will directly pass through the aluminum foil. Pitting corrosion accelerates electrochemical corrosion and changes the composition of the electrolyte. In severe cases, the aluminum foil is corroded and leaked, and a short circuit is also caused, resulting in the battery being scrapped. When the thickness of the metal electrode sheet is about 100u, when the heat sealing temperature is about 170 Â° C and the pressure is about 3 kg / CM 2 , if there is no high temperature resistant insulating layer in the inner film, the metal electrode is often pressed onto the packaging aluminum foil. A short circuit is caused, the battery is scrapped, and the yield is reduced.
Due to this special high performance requirement, the flexible packaging technology of this product has become one of the three technical problems in the lithium ion polymer battery industry.
2 Structure and classification of flexible packaging materials
Lithium-ion polymer battery core packaging materials may have multiple classification methods in different ways. However, the most common classification methods are as follows: whether the cold stamping classification is required and whether the inner film heat seal layer has a performance classification directly with the metal electrode.
(1) According to whether cold stamping is required, it is divided into a cold stamped film and a non-cold stamped film.
The typical structure of a cold formed film is:
The non-cold stamped film structure is:
Note: NPP50-70 means that the heat seal layer is a special multifunctional layer of PP type, which is characterized by not being directly heat sealed with metal.
NPE50-70 means that the heat seal layer is a special multifunctional layer of modified polyethylene, which is characterized by being directly heat sealable with the metal electrode.
(2) According to whether the inner film heat seal layer has heat sealing properties with metal electrodes:
A can be directly sealed with a metal electrode:
B cannot directly seal the film with the metal electrode:
3 Technical requirements for flexible packaging materials
Since the lithium-ion polymer battery was successfully developed in the laboratory in 1996, but the batch production was only three or four years, the development of the packaging film was later than the battery, and no international standard was formed. The domestic in-depth research was insignificant. There are no standards to follow. The battery packaging film developed by our company has been gradually formed through the communication and cooperation of the domestic Lithium-ion Battery factory (Xiamen Baolong Industry, Huizhou TCL Almighty, Dongguan New Energy ATL, Shunde Jingjin Electronics, etc.). Some basic requirements for product quality. These basic requirements have become the technical indicators of current products, some of which can be quantitatively described, can be detected by appropriate methods, and some can not be detected by advanced methods. Some requirements can only be qualitatively described and no suitable method can be found. In addition, with the improvement of the process of the battery factory, the advancement of technology, and the improvement of the performance requirements of the battery, many indicators should be changed accordingly, especially as the battery membrane studies the electrical performance of the battery itself, and the updated technical requirements are also proposed. . These requirements will be an important part of future technical standards. The following describes the existing technical requirements of the product.
First, the appearance: no pinholes, foreign bodies, adhesions, uneven coating, separation of composite guides and obvious damage, bubbles, wrinkles, dirt and other defects.
Second, the thickness: the total thickness of the composite film within 150u (forming), within 120u (non-forming).
Third, barrier properties:
1. Water vapor transmission rate <10-4g/m2Â·d.1atm
2. Oxygen transmission rate <0.1cm3/m2Â·d.1atm
Fourth, cold stamping formability;
1, cold stamping depth is 3-8mm
2, does not allow delamination, cracking, pinhole leakage
3, shape retention, deformation degree a < 50
5. Heat seal strength: When the inner membrane is soaked by the electrolyte into three seals (at about 12 days), the seal strength is >40u/15mm.
Sixth, blister resistance: the finished battery is sealed at room temperature for 20 days, and the sealing is tight, and no gas is generated.
VII. Electrical insulation: heat-sealed pole piece resistivity>1&TImes; 107s2Â·cm-1 resistivity between electrode and composite film aluminum foil 8. Endometrial puncture resistance: When the inner membrane can withstand burr piercing and heat sealing around the cell The metal electrode is not shorted to the AL foil in the middle of the film.
The spot corrosion rate and short circuit rate of the finished product are <1â€°.
IX. Resistance to electrolyte stability: The electrolyte will swell, dissolve and absorb the inner composite film, especially the interlayer adhesive or bonding resin. The HF generated will corrode the AL foil in the middle of the composite film, resulting in AL foil and inner film. The separation, which causes leakage or bloating, causes the battery to be scrapped. When the three seals are used, the peel strength between the layers within the AL foil is >2.5N/15mm, thus ensuring the tightness of the seal.
X. Electrical performance: The electrical performance index is being explored further. The manufacturer is temporarily confidential and is not willing to disclose it in detail.
The essence of electrical performance is the effect of the packaging film on the charge and discharge rate of the battery. This effect includes electrical insulation, the influence on the balance of the electrolyte composition, and the electrical insulation of the composite film, especially the AL foil, after the inner membrane is soaked by the electrolyte. Wait.
Eleven, other: film thickness deviation, dimensional deviation, heat seal folding and peeling, initial peel strength between the original film layers, breaking strength (> 100N), elongation at break (> 35%) and so on.
In short: the film requires many indicators, the most important of which is different from ordinary composite film.
1 very high barrier;
2 good cold stamping formability;
3 puncture resistance;
4 resistance to electrolyte stability;
5 electrical properties (including insulation).
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