The production process of 6LR61 alkaline batteryseparators is complex

Lithium-ion batteries are the representative of modern high-performance batteries. They are composed of four main parts: positive electrode materials, negative electrode materials, separators, and electrolytes. 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 electrodes of the 6LR61 alkaline batteryto prevent the positive and negative electrodes from contacting and forming a short circuit; b. The micropores in the film allow lithium ions to pass through to form a charge and discharge circuit.

The production process of 6LR61 alkaline batteryseparators is complex and the technical barriers are high

High-performance lithium batteries require the separator to have uniform thickness and excellent mechanical properties (including tensile strength and puncture resistance), air permeability, 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, and safety performance of the lithium battery. The excellent performance of the separator plays an important role in improving the comprehensive performance of the battery.

The many characteristics of 6LR61 alkaline batteryseparators and the difficulty in balancing their performance indicators determine that their production process has high technical barriers and is difficult to develop. The diaphragm production process includes many processes such as raw material formula and rapid formula adjustment, micropore preparation technology, and independent design of complete equipment. Among them, micropore preparation technology is the core of the 6LR61 alkaline batterydiaphragm preparation process. According to the difference in the micropore formation mechanism, the diaphragm process can be divided into dry and wet processes.

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

The dry diaphragm process is the most commonly used method in the diaphragm preparation process. This process is to mix raw materials such as polymers and additives to form a uniform melt, form a lamellar structure under tensile stress during extrusion, heat-treat the lamellar structure to obtain a hard and elastic polymer film, and then stretch it at a certain temperature to form slit-shaped micropores, and obtain a microporous membrane after heat setting. At present, the dry process mainly includes two processes: dry unidirectional stretching and bidirectional stretching.

Dry single drawing

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

The process flow of dry single drawing is as follows:

1) Feeding: PE or PP and additives and other raw materials are pre-treated according to the formula and transported to the extrusion system.

2) Casting: The pre-treated raw materials are melted and plasticized in the extrusion system and then extruded from the die head. The melt is cast to form a base film with a specific crystal structure.

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-microporous film.

5) Slitting: The nano-microporous film is cut into finished films according to the customer's specifications.

6LR61 alkaline batterydiaphragm production process

Dry single-drawing process

Dry double-drawing

It is understood that the dry double-drawing process is a process with independent intellectual property rights developed by the Institute of Chemistry of the Chinese Academy of Sciences, and is also a unique diaphragm manufacturing process in China. Since the β-crystal form of PP is a hexagonal crystal system, the single crystal nucleation and the arrangement of the wafers are loose, and it has a lamellar structure that grows radially into a divergent bundle while not having a complete spherulite structure. Under the action of heat and stress, it will transform into a more dense and stable α-crystal, and after absorbing a large amount of impact energy, it will produce holes inside the material. This process adds a β-crystal modifier with a nucleating effect to PP, and uses the difference in density between different phases of PP to form micropores during the stretching process.

The dry double-drawing process is as follows:

1) Feeding: PP and pore-forming agents and other raw materials are pre-treated according to the formula and then transported to the extrusion system.

2) Casting: PP cast sheets with high β-crystal content and good β-crystal morphology uniformity are obtained.

3) Longitudinal stretching: longitudinally stretch the cast sheet at a certain temperature, and use the property of β crystals that they are easy to form pores under tensile stress to form pores.

4) Transverse stretching: transversely stretch the sample at a higher temperature to expand the pores and improve the uniformity of pore size distribution.

5) Forming and winding: heat treat the diaphragm at high temperature to reduce its thermal shrinkage and improve dimensional stability.

Wet-process diaphragms are divided into asynchronous and synchronous according to whether the stretching orientation is simultaneous.

The wet process uses the principle of thermally induced phase separation to mix plasticizers (high-boiling hydrocarbon liquids or some relatively low molecular weight substances) with polyolefin resins, and uses the solid-liquid or liquid-liquid phase separation phenomenon during the cooling of the molten mixture to press the membrane, heat it to a temperature close to the melting point, and then stretch it to make the molecular chain orientation consistent. After keeping warm for a certain period of time, use volatile solvents (such as dichloromethane and trichloroethylene) to extract the plasticizer from the film, thereby obtaining a mutually interpenetrating submicron-sized microporous membrane material.

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

The process flow of wet asynchronous stretching is as follows:

1) Feeding: PE, pore-forming agent and other raw materials are pre-treated according to the formula and transported to the extrusion system.

2) Casting: The pre-treated raw materials are melted and plasticized in the twin-screw extrusion system and then extruded from the die head. The melt is cast to form a cast thick sheet containing a pore-forming agent.

3) Longitudinal stretching: The cast thick sheet is stretched longitudinally.

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 by solvent to form a base film without a pore-forming agent.

6) Forming: The base film without a pore-forming agent is dried and formed to obtain a nano-microporous membrane.

7) Slitting: Cut the nanoporous membrane into finished membranes according to the customer's 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 the horizontal and vertical directions during stretching, eliminating the process of longitudinal stretching alone and enhancing the uniformity of the thickness of the diaphragm. However, the problems with synchronous stretching are firstly slow speed and secondly slightly poor adjustability. Only the transverse stretching ratio is adjustable, while the longitudinal stretching ratio is fixed.

The performance of diaphragm products is jointly affected by the base material and the manufacturing process. The stability, consistency and safety of the diaphragm have a decisive influence on the discharge rate, energy density, cycle life and safety of lithium batteries. Compared with dry diaphragms, wet diaphragms are better in material properties such as thickness uniformity, mechanical properties (tensile strength, puncture resistance), air permeability, physical and chemical properties (wettability, chemical stability, safety), etc., which are conducive to the absorption and retention of electrolytes and improve the charging, discharging and cycle capacity of batteries, and are suitable for high-capacity batteries. From the perspective of product strength, the comprehensive performance of wet diaphragms is stronger than that of dry diaphragms.

Wet-process diaphragms also have disadvantages. In addition to poor thermal stability due to limitations on the base material, most of them are non-product factors, such as the need for a large amount of solvents, which can easily cause environmental pollution; compared with the dry process, the equipment is complex, the investment is large, the cycle is long, the cost is high, the energy consumption is high, the production is difficult, and the production efficiency is low. In wet-process diaphragms, the bidirectional synchronous stretching technology can be oriented in both the horizontal and vertical directions, eliminating the process of longitudinal stretching alone, enhancing the uniformity of the diaphragm thickness, and the product has high transparency, no scratches, excellent optical properties and surface properties. It is the diaphragm with the best comprehensive performance, occupies an important position in the high-end diaphragm market, and is also the best 6LR61 alkaline batterydiaphragm in the market at this stage.

In terms of product performance, compared with dry-process diaphragms, wet-process diaphragms have certain advantages in mechanical properties, air permeability, and physical and chemical properties. By coating ceramic alumina, PVDF, aramid and other adhesives on the base film, the thermal stability of the diaphragm can be greatly improved, the high-temperature shrinkage rate can be reduced, and the pole piece exposure caused by the large shrinkage of the diaphragm can be avoided, which makes up for the only short board of thermal stability. The product performance has been comprehensively ahead of dry-process films.

The ceramic particle coated diaphragm is based on the base film, and a layer of Al2O3, SiO2, Mg(OH)2 or other inorganic ceramic particles with excellent heat resistance are coated on the surface. After special process treatment, it is tightly bonded to the base, stably combining the flexibility of organic matter and the thermal stability of inorganic matter, improving the high temperature resistance, heat shrinkage resistance and puncture strength of the diaphragm, and thus improving the safety performance of the battery. It is understood that the ceramic composite layer can solve the safety problems of thermal runaway caused by thermal shrinkage of PP and PE diaphragms, which may cause battery combustion and explosion; on the other hand, the ceramic composite diaphragm has good infiltration and liquid absorption and retention capabilities with electrolyte and positive and negative electrode materials, which greatly improves the service life of the battery. In addition, the ceramic coated diaphragm can also neutralize a small amount of hydrofluoric acid in the electrolyte to prevent battery bloating.

PVDF coated diaphragm

PVDF, or polyvinylidene fluoride, is a white powdery crystalline polymer with a melting point of 170°C, a thermal decomposition temperature of more than 316°C, and a long-term use temperature of -40 to 150°C. It has excellent chemical corrosion resistance, high temperature color change resistance, oxidation resistance, wear resistance, flexibility, and high swelling strength and impact resistance. PVDF coated diaphragm has the characteristics of low internal resistance, high (thickness/porosity) uniformity, good mechanical properties, and good chemical and electrochemical stability. Due to the presence of nanofiber coating, the new diaphragm has better compatibility and adhesion to 6LR61 alkaline batteryelectrodes than ordinary battery diaphragms, and can greatly improve the high temperature resistance and safety of the battery. In addition, the new diaphragm has good absorption of liquid electrolytes, good wetting and liquid absorption and retention capabilities, which prolongs the battery cycle life, increases the battery's high-rate discharge performance, and increases the battery's output capacity by 20%. It is particularly suitable for high-end energy storage batteries and automotive power batteries.

Aramid coated diaphragm

As a high-performance fiber, aramid fiber has heat resistance that can withstand temperatures above 400°C and excellent fire retardancy, which can effectively prevent the fabric from melting when exposed to heat. The coating obtained by composite treatment with high heat-resistant aramid resin can greatly improve the heat resistance of the diaphragm 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 electrolyte, the diaphragm has good wetting and liquid absorption and retention capabilities, 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 diaphragm, thereby achieving high potential and thus increasing energy density.

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