What are the advantages and disadvantages of the lithium battery separator manufacturing process?
The base material used in lithium ion battery separators is polyolefin, which has excellent mechanical properties, chemical stability and relatively inexpensive characteristics. The diaphragm base material mainly includes polypropylene, polyethylene materials and additives. The matrix material used in the diaphragm has a direct relationship with the mechanical properties of the diaphragm and the degree of infiltration of the electrolyte. Although in recent years there have been studies using polyvinylidene fluoride, cellulose composite membranes and other materials to prepare lithium battery pack separators, the commercial battery separator materials still mainly use polyethylene and polypropylene microporous membranes.
Li-ion battery separator pore-making engineering technology mainly includes wet method and dry method, and dry method has two kinds of uniaxial stretching process and biaxial stretching process.
1. The wet process of lithium ion battery diaphragm production
The wet process is also called phase separation method or thermally induced phase separation method, in which small molecules with high boiling points are added as porogens to In the polyolefin, it is heated and melted to form a uniform system, and then the temperature is lowered to cause phase separation. After stretching, the organic solvent is used to extract small molecules to prepare interpenetrating microporous membrane materials.
The representative companies that adopt this method include Japan's Asahi Kasei, Tonen and Entek of the United States, etc., and they are mainly used for single-layer PE diaphragms.
Although the porosity and air permeability can be controlled in a wide range, the wet process needs to consume a large amount of organic solvents. On the one hand, the recovery and utilization of solvents should be considered, and the complexity of the process will increase, which will increase the cost. On the one hand, it pollutes the environment.
2. The dry biaxial stretching process for the production of lithium battery pack separators
The dry biaxial stretching process was developed by the Institute of Chemistry, Chinese Academy of Sciences in the early 1990s. Process with independent intellectual property rights. By adding
β crystal modifier with nucleation effect to polypropylene, the difference in density between different phases of polypropylene is used to transform the crystal form to form micropores during the stretching process.
3. Dry uniaxial stretching process for the production of lithium ion battery separators
The dry uniaxial stretching process is a method of producing hard elastic fibers to produce low crystallinity The highly oriented polypropylene or polyethylene film is annealed at a high temperature to obtain an oriented film with high crystallinity. This film is first stretched at low temperature to form micro-defects, and then at high temperature, the defects are pulled apart to form micro-pores.
After decades of development, the process has been very mature in the United States and Japan. Now the Celgard company in the United States and the UBE company in Japan use this process to produce single-layer PP, PE and three-layer PP/PE/PP composites. membrane. Due to the restrictions of foreign patent protection and intellectual property rights, the domestic industrialization of diaphragms prepared by uniaxial stretching methods has been slow.
The membrane produced by this method has an elongated microporous structure. Because it is only stretched in one direction, the lateral strength of the membrane is poor, but it is precisely because the transverse stretch is not carried out that the transverse direction is almost There is no heat shrinkage.
From the perspective of dry and wet methods, the separator produced by the dry biaxial stretching process has more advantages in physical and mechanical properties and can meet the requirements of high-current charging and discharging of power batteries. Therefore, the diaphragm produced by the dry biaxial stretching process is more suitable for use in power batteries for electric vehicles.
This kind of diaphragm has a two-layer (PP/PE) diaphragm and a three-layer (PP/PE/PP) diaphragm. When the temperature of the three-layer film rises, the PE in the middle part melts and shrinks at 130 degrees, causing thermal shutdown, but because the outer PP melting temperature is 160 degrees, the diaphragm can maintain a certain degree of safety, so the three-layer film is also more suitable for power battery. Currently Celgard and UBE have the technology and patent rights.
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