As an important component of lithium-ion batteries, the position and function of the diaphragm are relatively special. It is located between the positive and negative electrodes and is soaked in electrolyte. Its main function is to block the passage of electrons between the positive and negative electrodes to prevent short circuits. At the same time, it requires that the lithium ions inside the battery can pass freely to protect the battery's charging and discharging. Although the diaphragm itself does not participate in any battery reaction, its morphology, structure and physical and chemical properties can have an important impact on the interface and impedance inside the lithium-ion battery, which further affects the battery's operating conditions, charge and discharge capacity, cycle performance and even safety performance. Therefore, the research and development and manufacture of high-performance diaphragms are of great significance to improving the overall performance of lithium-ion batteries.
At present, the production and manufacturing methods of lithium-ion battery diaphragms are nothing more than dry and wet methods. Although the process of these two preparation methods is different, they both include an indispensable orientation process, which is used to produce pores in the diaphragm.
1.Dry diaphragm process
The so-called dry diaphragm process (Melt Spinning and Cold Stretching, referred to as MSCS) is a process that mixes high molecular polymers (such as polyethylene cation, polypropylene P) and additives to form a homogeneous melt. After extrusion, casting, heat treatment and other steps, it is stretched at a certain temperature to form slit-shaped micropores, and finally heat-set to obtain a microporous membrane. The advantages of the dry diaphragm process include simple production process, low cost, and good mechanical and physical properties of the product.
The multi-layer co-extrusion casting dry composite test line developed by Guangzhou Putong consists of three single-screw extruder main machines and a set of casting molding auxiliary machines. It is mainly used for the blending and modification of polymer materials, the development of new material formulas, process optimization, quality control and small-scale production.
The multi-layer co-extrusion casting dry composite test line process is to melt and extrude resin raw materials with different functions by connecting multiple extruders in parallel, and then realize the composite casting molding of multi-layer melts in the multi-layer co-extrusion casting die head. The film prepared by this process has the excellent properties of various materials, such as high strength, high barrier properties, high transparency, etc., and has relatively low cost and a wide range of applications.
2.The wet process
The wet process is also known as Thermally Induced Phase Separation (TIPS). Its preparation principle is: mix the polymer with a diluent with a small molecular weight and a high boiling point at high temperature to form a homogeneous solution, then extrude and cast it, and then cool it down at a certain cooling rate. Due to the crystallization of the polymer and the decrease in the solubility of the diluent during the cooling process, the system undergoes solid-liquid phase separation or liquid-liquid phase separation, so that the diluent in the mixture is dispersed in the polymer phase. Finally, the diluent is extracted with a low boiling point extractant, and the volatilized extractant is dried to finally obtain a polymer microporous membrane for lithium-ion battery separators.
The basic steps for preparing microporous membranes by TIPS method are:
(1) Dissolve the polymer in a high boiling point, low molecular weight diluent at a high temperature, usually close to or higher than the melting point of the polymer, to form a uniform melt blend;
(2) Cast the membrane liquid into a shape, and select appropriate process conditions to produce flat plates, tubes, hollow fibers, etc. according to production requirements;
(3) Cool or refrigerate the membrane liquid under certain conditions to cause phase separation and solidification;
(4) Finally, extract the diluent in the membrane precursor and other steps to obtain a microporous membrane.
Characteristics of Thermally Induced Phase Separation
Compared with other pore-forming methods, thermally induced phase separation has some unique advantages:
(1) The material has wide applicability. This method can be used to prepare microporous membranes for most crystalline polymers that have no suitable solvent at room temperature and are difficult to prepare by melt extrusion.
(2) It can prepare microscopic pores with different morphological structures, such as honeycomb structure, lace structure, spherical structure, leaf structure, etc.
(3) It can prepare isotropic or anisotropic microporous membranes.
(4) The microporous membrane is prepared by thermally induced phase separation, which requires fewer parameters to be controlled, and the membrane preparation process is relatively easier to control, and simple continuous production can be achieved.
(5) Compared with melt stretching, the microporous membrane prepared by this method has higher porosity, more uniform pore size distribution, and higher product stability.
The wet diaphragm casting test line developed by Guangzhou POTOP is also called the thermally induced phase separation twin-screw extrusion casting test line. It is mainly composed of a 22 twin-screw extruder main machine, exquisite casting auxiliary machine, liquid metering pump, chiller, water temperature machine and measurement and control system. The extruder adopts advanced partial design. The screw is composed of various interchangeable threaded elements. The threaded elements are made of high-quality alloy tool steel and the surface is hardened. It is suitable for various mixing operations. The test line can realize the functions of plasticizing extrusion, casting, traction and winding of polymer materials.
Application of wet process diaphragm
Wet-process diaphragms have broad application prospects in lithium-ion batteries, optoelectronics, electronics and biomedicine due to their unique microporous structure and excellent performance.
(1) Lithium-ion battery: The wet-process diaphragm is an important component of lithium-ion battery. Its pore structure and air permeability directly affect the battery's charge and discharge performance and cycle life. The diaphragm prepared by the wet-process diaphragm casting test line has the advantages of suitable pore size, uniform pore size distribution, few defects, and thin thickness, which can meet the needs of high-performance lithium-ion batteries.
(2) Optoelectronics: The wet-process diaphragm plays an important role in the preparation of devices such as photodetectors and solar cells. Its excellent light transmittance and electrical properties help to improve the sensitivity and conversion efficiency of the device.
(3) Electronics: In the preparation of electronic components such as integrated circuits and capacitors, the wet-process diaphragm can provide stable electrical properties and good mechanical strength as an insulating layer or protective layer.
Biomedical: The wet-process diaphragm has potential application value in the preparation of biomedical materials such as biosensors and drug carriers. Its good biocompatibility and controllable pore structure help to achieve precise drug release and efficient detection of biomolecules.
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