Lithium-ion battery separator process -Lithium - Ion Battery Equipment

What is the use of lithium-ion battery separator for lithium-ion batteries? What is its processing technology? -Lithium - Ion Battery Equipment



Lithium-ion batteries are representatives of modern high-performance batteries and are composed of four important 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 important purposes in lithium-ion batteries: a. Separating lithium-ion batteries. The positive and negative electrodes, be careful of the contact between the positive and negative electrodes to form a short circuit; b. The micropores in the film can allow lithium ions to pass through, forming a charge and discharge circuit.

Lithium-ion battery separator processing technology is complex and technical barriers are high

High-performance lithium-ion 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 quality of the separator directly affects the capacity, cycle capacity, safety performance and other characteristics of lithium-ion batteries. Excellent separators play an important role in improving the overall performance of the battery.

The many characteristics of lithium-ion battery separators and the difficulty in balancing their performance indicators determine that its processing technology barriers are high and research and development is difficult. The separator processing technology 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 lithium-ion battery separator preparation process. Based on 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 structure is formed under tensile stress, and the lamellae are thermally resolved. The structure obtains a hard elastic polymer film, which is then stretched at a certain temperature to form slit-like micropores, and the microporous film is obtained after heat setting. 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 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-solved 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.

Lithium-ion battery separator processing technology

Dry single drawing process

Dry double pull

It is understood that the dry double drawing process is a process with independent intellectual property rights developed by the Institute of Chemistry, Chinese Academy of Sciences, and is also a unique separator manufacturing process in my country. Since the beta crystal form of PP is a hexagonal crystal system, single crystal nucleation, wafers are loosely arranged, and it has a lamellae structure that grows in a radial direction into divergent bundles and does not have a complete spherulite structure. It is suitable for thermal and stress applications. It will transform into a more dense and stable alpha crystal, and after absorbing a large amount of impact energy, holes will form inside the material. This process adds a beta crystal modifier with nucleation purpose into PP and utilizes the density difference between different phases of PP to cause crystal transformation to form micropores during the stretching process.(Lithium - Ion Battery Equipment)

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 tape cast sheets with high beta crystal content and good beta crystal morphology uniformity.

3) Longitudinal stretching: The cast sheet is stretched longitudinally at a certain temperature, and the beta crystal's characteristic of easily forming holes under tensile stress is used to cause pores.

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

5) Shaping and winding: By heat treatment of the separator at high temperature, its thermal shrinkage rate is reduced and the dimensional stability is improved.

Is the wet separator divided into asynchronous and synchronous at the same time 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 It is extracted from the film and then made into interconnected sub-micron size microporous membrane materials.

The wet process is suitable for processing thin single-layer PE separators. It is a preparation process with better thickness uniformity and better 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: Pre-process PE, pore-forming agent and other raw materials according to the formula and transport them to the extrusion system.

2) Casting: The pre-solved 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 separator thickness. 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-ion 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 base 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 dry process, the equipment is complex and the investment is large. , long cycle, high cost, high energy consumption, difficult processing, low processing 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 and enhancing the uniformity of the separator thickness. The product has high transparency, no scratches, and excellent optical properties. It has excellent surface properties and is the separator with the best comprehensive performance. It occupies an important position in the high-end separator market and is also the best-performing lithium-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 is a composite solution of high heat-resistant aramid resin. On the one hand, it 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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