18650 li ion battery

　　What are the functions of 18650 li ion battery separators for lithium batteries? What are its production processes?

　　Lithium-ion batteries are representatives of modern high-performance batteries and are composed of four main parts: positive electrode material, negative electrode material, separator, and electrolyte. Among them, the separator is a thin film with a microporous structure. It is the key inner component with the most technical barriers in the lithium-ion battery industry chain. It plays the following two main roles in lithium batteries: a. Separating the positive and negative sides of the 18650 li ion battery. , negative electrode to prevent the positive and negative electrodes from contacting and forming a short circuit; b. The micropores in the film can allow lithium ions to pass through to form a charge and discharge circuit.

　　18650 li ion battery separator production process is complex and technical barriers are high

　　High-performance lithium batteries require separators with uniform thickness and excellent mechanical properties (including tensile strength and puncture resistance), breathability, and physical and chemical properties (including wettability, chemical stability, thermal stability, and safety).

　　It is understood that the excellence of the separator directly affects the capacity, cycle capacity, safety performance and other characteristics of lithium batteries. Excellent separators play an important role in improving the overall performance of the battery.

　　The many characteristics of 18650 li ion battery separators and the difficulty in balancing their performance indicators determine that the technical barriers to its production process are high and its research and development is difficult. The separator production process includes raw material formula and rapid formula adjustment, micropore preparation technology, independent design of complete sets of equipment, and many other processes. Among them, micropore preparation technology is the core of the 18650 li ion battery separator preparation process. According to the difference in micropore formation mechanism, the separator process can be divided into two types: dry method and wet method.

　　Dry separators are divided into single-stretch and double-stretch according to the stretching orientation.

　　The dry separator process is the most commonly used method in the separator preparation process. This process is to mix high molecular polymers, additives and other raw materials to form a uniform melt. During extrusion, a lamellar crystal structure is formed under tensile stress. The lamellar crystal structure is heat treated. A hard elastic polymer film is obtained, which is then stretched at a certain temperature to form slit-like micropores, and then heat-set to produce a microporous film. At present, dry processes mainly include dry uniaxial stretching and biaxial stretching.

　　Dry single pull

　　Dry single drawing uses polyethylene (PE) or polypropylene (PP) polymers with good fluidity and low molecular weight. It uses the manufacturing principle of hard elastic fibers to first prepare polyolefin cast sheets with high orientation and low crystallinity. After low-temperature stretching forms micro-defects such as silver streaks, high-temperature annealing is used to pull the defects apart, thereby obtaining a microporous film with uniform pore size and uniaxial orientation.

　　The dry single drawing process flow is:

　　1) Feeding: After preprocessing raw materials such as PE or PP and additives according to the formula, they are transported to the extrusion system.

　　2) Casting: The pretreated raw materials are melted and plasticized in the extrusion system and then the melt is extruded from the die. The melt forms a base film with a specific crystal structure after casting.

　　3) Heat treatment: The base film is heat treated to obtain a hard elastic film.

　　4) Stretching: The hard elastic film is cold stretched and hot stretched to form a nano-porous film.

　　5) Cutting: Cut the nano-microporous membrane into finished membranes according to customer specifications.

　　18650 li ion battery separator production process

　　Dry single drawing process

　　Dry double pull

　　The dry double drawing process flow is:

　　1) Feeding: Raw materials such as PP and pore-forming agent are pretreated according to the formula and then transported to the extrusion system.

　　2) Tape casting: Obtain PP cast sheets with high β-crystal content and good uniformity of β-crystal morphology.

　　3) Longitudinal stretching: The cast sheet is stretched longitudinally at a certain temperature, and the characteristics of β crystals that are prone to pore formation under tensile stress are used to cause pores.

　　4) Transverse stretching: The sample is stretched transversely at a higher temperature to expand the pores while improving the uniformity of pore size distribution.

　　5) Shaping and winding: By heat-treating the separator at high temperatures, the thermal shrinkage rate is reduced and the dimensional stability is improved.

　　Whether wet separators are divided into asynchronous and synchronous according to the stretching orientation?

　　The wet process uses the principle of thermally induced phase separation to mix plasticizers (high-boiling hydrocarbon liquids or some substances with relatively low molecular weight) with polyolefin resins, and utilizes the solid-liquid phase or the occurrence of solid-liquid phases during the cooling process of the molten mixture. The phenomenon of liquid-liquid phase separation is to press the film, heat it to a temperature close to the melting point and then stretch it to make the molecular chain orientation consistent. After keeping it warm for a certain period of time, use a volatile solvent (such as methylene chloride and trichlorethylene) to remove the plasticizer from the film. It is extracted from the water and then made into interconnected sub-micron size microporous membrane materials.

　　The wet process is suitable for producing thinner single-layer PE separators. It is a preparation process with better thickness uniformity, physical, chemical and mechanical properties of separator products. Depending on whether the orientations are simultaneous during stretching, the 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: Preprocess PE, pore-forming agent and other raw materials according to the formula and transport them to the extrusion system.

　　2) Casting: The pretreated raw materials are melted and plasticized in a twin-screw extrusion system and then the melt is extruded from the die. After casting, the melt forms a cast thick sheet containing a pore-forming agent.

　　3) Longitudinal stretching: Longitudinal stretching of the cast thick sheet.

　　4) Transverse stretching: The cast thick sheet after longitudinal stretching is stretched transversely to obtain a base film containing a pore-forming agent.

　　5) Extraction: The base film is extracted with a solvent to form a base film without pore-forming agents.

　　6) Shaping: Dry and shape the base film without pore-forming agent to obtain a nano-porous membrane.

　　7) Cutting: Cut the nano-microporous membrane into finished membranes according to customer specifications.

　　The process flow of wet synchronous stretching technology is basically the same as that of asynchronous stretching technology, except that it can be oriented in both transverse and longitudinal directions simultaneously during stretching, eliminating the need for a separate longitudinal stretching process and enhancing the uniformity of the thickness of the separator. However, the problems with synchronous stretching are firstly the slow speed of the vehicle, and secondly the slightly poor adjustability. Only the transverse stretch ratio is adjustable, while the longitudinal stretch ratio is fixed.

　　The performance of diaphragm products is affected by the base 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 dry separators, wet separators have better material properties in terms of thickness uniformity, mechanical properties (tensile strength, puncture resistance), air permeability, and physical and chemical properties (wettability, chemical stability, safety). It is excellent and is conducive to the absorption and retention of electrolyte and improves the charge, discharge and cycle capabilities of the battery. It is suitable for high-capacity batteries. From the perspective of product strength, the comprehensive performance of wet separators is stronger than that of dry separators.

　　Wet process separators also have shortcomings. In addition to poor thermal stability due to limitations in the matrix material, most of them are non-product factors. For example, a large amount of solvent is required, which can easily cause environmental pollution. Compared with the dry process, the equipment is complex and the investment is larger. , long cycle, high cost, high energy consumption, difficult production, low production efficiency, etc. In wet separators, the two-way synchronous stretching technology can orient in both the transverse and longitudinal directions simultaneously, eliminating the need for a separate longitudinal stretching process, enhancing the uniformity of the thickness of the separator. The product has high transparency, no scratches, and excellent optical properties. It has excellent surface properties and is the separator with the best overall performance. It occupies an important position in the high-end separator market and is also the best-performing 18650 li ion battery separator in the market at this stage.

　　Aramid coated separator

　　As a high-performance fiber, aramid fiber has heat resistance that can withstand high temperatures above 400°C and excellent fire retardancy, which can effectively prevent fabrics from melting when exposed to heat. The coating obtained by composite processing using high heat-resistant aramid resin can greatly improve the heat resistance of the separator and achieve a comprehensive combination of closed cell characteristics and heat resistance;

　　On the other hand, due to the high affinity of aramid resin to the electrolyte, the separator has good ability to wet, absorb and retain liquid, and this excellent high wettability can extend the cycle life of the battery. In addition, aramid resin plus fillers can improve the oxidation resistance of the separator, thereby achieving high potential and thus increasing energy density.

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