What are the production processes for lithium battery separator production lines

日期:2023/5/5 14:37:12 / 閱讀: / 來源:本站

The microporous preparation technology is the core separator in the lithium battery separator preparation process. According to the difference in microporous pore formation mechanism, the separator process can be divided into two types: dry method and wet method.

Dry diaphragm is divided into single pull and double pull according to tensile orientation

The dry diaphragm process is the most commonly used method in the preparation of membranes. This process involves mixing polymer, additives, and other raw materials to form a uniform melt, forming a lamellar structure under tensile stress during extrusion. The lamellar structure is heat treated to obtain a hard elastic polymer film, which is then stretched at a certain temperature to form narrow micropores. After heat setting, the microporous film is prepared. At present, the dry process mainly includes two processes: dry unidirectional stretching and biaxial stretching.

Dry method single pull

Dry stretching is the use of polyethylene (PE) or polypropylene (PP) polymers with good flowability and low molecular weight. Using the manufacturing principle of hard elastic fibers, high orientation and low crystallization polyolefin cast sheets are first prepared, and micro defects such as silver lines are formed by low-temperature stretching. Then, high-temperature annealing is used to open the defects and obtain microporous films with uniform pore size and uniaxial orientation.

The dry method single drawing process is as follows:

1) Feeding: After pre processing raw materials such as PE or PP and additives according to the formula, they are transported to the extrusion system. 2) Casting: The pre processed raw materials are melted and plasticized in an extrusion system, and then extruded into a melt diaphragm from the die. The melt is then stretched to form a specific crystalline structure of the base film. 3) Heat treatment: The base film is heat treated to obtain a hard elastic film. 4) Stretch: The hard elastic film is subjected to cold and hot stretching to form a nano microporous film. 5) Cutting: Cut the nano microporous membrane into finished film according to the customer's specifications.

Dry Method Double Pull

The dry double pulling process is a process developed by the Institute of Chemistry, Chinese Academy of Sciences with independent intellectual property rights, and is also a unique diaphragm manufacturing process in China. Due to PP's β The crystal form is hexagonal crystal system, with single crystal nucleation and loose arrangement of wafers. It has a lamellar structure that grows into divergent bundles along the radial direction but does not have a complete spherulite structure, which will be transformed into more compact and stable under the action of heat and stress α Crystals, after absorbing a large amount of impact energy, will create pores inside the material. This process involves adding nucleating agents to PP β Crystal type modifiers utilize the density differences between different phases of PP to undergo crystal type transformation and form micropores during the stretching process.

The process flow of dry double pulling is as follows:

1) Feeding: Raw materials such as PP and pore forming agent are pre treated according to the formula and transported to the extrusion system. 2) Streaming: Obtaining β High crystal content β PP casting sheet with good crystal morphology uniformity. 3) Longitudinal stretching: Perform longitudinal stretching on the casting sheet at a certain temperature, using β The characteristic of crystals being easily porous under tensile stress leads to pore formation. 4) Transverse stretching: The sample is stretched horizontally at higher temperatures to expand the pores and improve the uniformity of pore size distribution. 5) Setting and winding: By heat treating the diaphragm at high temperature, the thermal shrinkage rate is reduced and the dimensional stability is improved.

Wet diaphragm can be divided into asynchronous and synchronous according to its tensile orientation

The wet process utilizes the principle of thermally induced phase separation to mix plasticizers (high boiling point hydrocarbon liquids or some relatively low molecular weight substances) with polyolefin resins. It utilizes the phenomenon of solid-liquid or liquid-liquid phase separation during the cooling process of the molten mixture, compresses the membrane, heats it to a temperature close to the melting point, and then stretches it to achieve consistent molecular chain orientation, After holding for a certain period of time, plasticizers are extracted from the film using volatile solvents such as dichloromethane and trichloroethylene, resulting in interconnected submicron sized microporous membrane materials.

The wet process is suitable for producing thinner single-layer PE membranes, which is a preparation process with better thickness uniformity, physical, chemical, and mechanical properties of membrane products. According to whether the orientation during stretching is the same, wet process can also be divided into two types: wet bidirectional asynchronous stretching process and bidirectional synchronous stretching process.

The wet asynchronous stretching process flow is:

1) Feeding: Pre treat raw materials such as PE and pore forming agents according to the formula and transport them to the extrusion system. 2) Casting: Melt and plasticize the pre treated raw materials in a twin screw extrusion system before extruding the melt from the die head. The melt then flows to form thick sheets containing pore forming agents. 3) Longitudinal stretching: Stretching the thick sheet in a longitudinal direction. 4) Horizontal stretching: Stretching the longitudinally stretched thick strip to obtain a substrate film containing a pore forming agent. 5) Extraction: The base film is extracted by solvent to form a base film without pore forming agents. 6) Shaping: Drying and shaping the base film without pore forming agent to obtain a nano microporous film. 7) Cutting: Cut the nano microporous membrane into finished film according to the customer's specifications.

The process flow of wet asynchronous stretching technology is basically the same as asynchronous stretching technology, except that it can be oriented simultaneously in both horizontal and vertical directions during stretching, eliminating the need for separate longitudinal stretching and enhancing the uniformity of membrane thickness. But the problem with synchronous stretching is slow vehicle speed, and the second is slightly poor adjustability. Only the lateral stretching ratio is adjustable, while the longitudinal stretching ratio is fixed.

Wet synchronous stretching process wet coating is the development direction of lithium battery separators

The overall performance of wet diaphragm is superior to that of dry diaphragm products, which is influenced by both the matrix material and manufacturing process. The stability, consistency, and safety of the separator have a decisive impact on the discharge rate, energy density, cycle life, and safety of lithium batteries. Compared with the dry diaphragm, the wet diaphragm is better in thickness uniformity, mechanical properties (tensile strength, puncture resistance), air permeability, physical and chemical properties (wettability, chemical stability, safety) and other material properties, which is conducive to the absorption and retention of electrolyte and improve the battery's charge discharge and circulation capacity. It is suitable for high-capacity batteries. From the perspective of product strength, the comprehensive performance of wet diaphragm is stronger than that of dry diaphragm.


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