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 is directly related to 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-ion battery separators, the commercial battery separator materials still mainly use polyethylene and polypropylene microporous membranes.
The pore-making engineering technology of lithium ion battery diaphragm mainly includes wet method and dry method, and the dry method has two kinds of uniaxial stretching process and biaxial stretching process.
1. Wet process for the production of lithium ion battery diaphragm
The wet method is also called phase separation method or thermally induced phase separation method. High boiling point small molecules are added to polyolefin as porogen, heated and melted to form a uniform system, and then the temperature is reduced for phase separation. After stretching, the small molecules are extracted with organic solvent. Molecules can be used to prepare interpenetrating microporous membrane materials.
Representative companies using this method include Japan's Asahi Kasei, Tonen and Entek from the United States, etc., and they are currently mainly used for single-layer PE diaphragms.
Although the porosity and air permeability have a large controllable range, the wet process needs to consume a large amount of organic solvents. On the one hand, the recycling of solvents must be considered, the process complexity increases, which increases the cost, and on the other hand, it pollutes the environment.
2. Dry biaxial stretching process for the production of lithium ion battery diaphragm
The dry biaxial stretching process is a process with independent intellectual property rights developed by the Institute of Chemistry, Chinese Academy of Sciences in the early 1990s. By adding nucleating effect to polypropylene
The β crystal form modifier utilizes the difference in density between different phases of polypropylene to transform the crystal form to form micropores during the stretching process.
3. Dry uniaxial stretching process for lithium ion battery separator production
The dry uniaxial stretching process is a method of producing hard elastic fibers to prepare a highly oriented polypropylene or polyethylene film with low crystallinity and annealing at 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 composite films. 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 there is no transverse stretch that there is almost no thermal shrinkage in the transverse direction.
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 separator 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 melting temperature of the outer PP is 160 degrees, the diaphragm can also 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
Performance parameters of 18650 lithium battery separator
The factors that affect the performance of 18650 lithium battery separators are: thickness, air permeability, wettability, chemical stability, pore size, puncture strength, thermal shutdown temperature, and porosity. These factors directly affect the quality of lithium battery products. Let's take a look at the performance parameter requirements of these 18650 lithium battery separators.