Lithium battery separator production technology—dry and wet methods. The production process of lithium battery separators is complex. The many characteristics of lithium battery separators and the difficulty in taking into account their performance indicators determine that the technical barriers to its production process are high and its research and development is difficult. According to the difference in micropore formation mechanism, the membrane process can be divided into two types: dry method and wet method.

　　The many characteristics of lithium 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 lithium 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.

　　1. Dry separator process

　　The dry separator process is the most commonly used method in the preparation process of lithium battery separators. This process is to mix polymers, additives and other raw materials to form a uniform melt, and form a lamellar structure under tensile stress during extrusion. The heat-treated sheet The crystal structure is obtained into a hard elastic polymer film, which is then stretched at a certain temperature to form slit-like micropores, and a 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.

　　2. Wet separator process

　　The wet process uses the principle of thermally induced phase separation to mix plasticizer and polyolefin resin, and utilizes the phenomenon of solid-liquid phase or liquid-liquid phase separation that occurs during the cooling process of the molten mixture. The film is pressed and heated to close to the melting point. Temperature post-stretching makes the molecular chain orientation consistent, and after a certain period of heat preservation, the plasticizer is extracted from the film with a volatile solvent, thereby producing an interconnected sub-micron size microporous membrane material.

　　The wet process is suitable for producing thinner single-layer PE separators. It is a preparation process with better thickness uniformity, better physical and chemical properties and mechanical properties of lithium battery 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.

　　Wet simultaneous stretching process

　　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.

　　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 lithium batteries, and 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 coating is the development direction of lithium battery separators.

　　The main steps and principles of dry and wet lithium battery separator processes

　　1. Dry method - first melt, extrusion and blowing the polyolefin resin to form a crystalline polymer film, then perform crystallization heat treatment and annealing operations to obtain a highly oriented film structure, and then stretch it at high temperature , test the crystal cross-section separation to form a porous structure battery separator. The dry process can also be divided into uniaxial stretching and biaxial stretching.

　　2. Wet method - Traditional wet method preparation is mainly based on phase inversion method, and in recent years, TIPS thermally induced phase separation method is mainly used. The principle is to mix crystalline polymers, thermoplastic polymers and small molecule chemical diluents with high boiling points (such as paraffin oil) to form a homogeneous solution at high temperature, and then lower the solution temperature to cause solid-liquid phase separation of the mixture. Or liquid-liquid separation, extracting and removing small molecule chemical diluents to form a porous membrane of thermoplastic and crystalline polymers.

　　The main characteristics of the wet process are high cost, large investment, high equipment requirements, long construction and production cycle, large energy consumption during the production process, and the use of organic solvents. However, the wet process can better control the pore size, distribution and porosity, so it is generally used to manufacture high-end films.

　　Compared with foreign companies, the separators produced by domestic companies were mainly dry-stretched, and most of them were mid- to low-end products. High-end wet-process separators have been monopolized by foreign companies for many years. However, in recent years, as domestic separator manufacturers have continued to improve their wet production processes, the output and performance of wet separators have become closer and closer to the levels of foreign companies. Domestic companies have rapidly expanded the production of wet separators, and the pattern of the separator market has also changed. Variety.

　　As one of the four major materials of lithium batteries, separators do not participate in the electrochemical reactions in the battery, but key properties such as battery capacity, cycle performance, and charge and discharge current density are directly related to the separator.

　　The separator is an important part of the lithium-ion battery and an important component that supports the electrochemical process of charging and discharging the lithium-ion battery. It is located between the positive and negative electrodes inside the battery, ensuring the passage of lithium ions while blocking electron transmission. The performance of the separator determines the interface structure, internal resistance, etc. of the battery, and directly affects the battery's capacity, cycle, safety performance and other characteristics. Excellent separators play an important role in improving the overall performance of lithium batteries.

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