It turns out to be the core technology of lithium-ion batteries!

　　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 film with a microporous structure and is the key inner component with the most technical barriers in the lithium-ion battery industry chain. 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, which directly affects the battery's capacity, cycle, safety performance and other characteristics. Excellent separators play an important role in improving the overall performance of the battery. The main functions of the separator in lithium batteries: 1. Separate the positive and negative electrodes of the lithium battery to prevent the positive and negative electrodes from contacting and forming a short circuit; 2. The micropores in the film can allow lithium ions to pass through and form a charge and discharge circuit. The lithium ion battery separator Types Based on differences in physical and chemical properties, lithium battery separators can be divided into: woven membranes, non-woven membranes (non-woven fabrics), microporous membranes, composite membranes, separator paper, rolled membranes, etc. Although there are many types, commercial lithium battery separator materials so far mainly use polyethylene and polypropylene microporous films. The performance requirements of the lithium-ion battery separator are: 1. It has electronic insulation to ensure the mechanical isolation of the positive and negative electrodes; 2. It has a certain pore size and porosity to ensure low resistance and high ion conductivity, and has good resistance to lithium ions. Permeability; 3. Resistant to electrolyte corrosion, with sufficient chemical and electrochemical stability, because the solvent of the electrolyte is a highly polar organic compound; 4. It has good electrolyte wettability, and absorbs liquid and moisturizes Strong ability; 5. High mechanical stability, including puncture strength, tensile strength, etc., but the thickness is as small as possible; 6. Good spatial stability and flatness; 7. Good thermal stability and automatic shutdown protection performance; 8. The thermal shrinkage rate is small, otherwise it will cause short circuit and cause thermal runaway of the battery. In addition, power batteries usually use composite membranes, which have higher requirements for separators. Internal short circuit reduction technology and thermal shutdown performance of lithium-ion batteries In lithium batteries, after the separator absorbs the electrolyte, it can isolate the positive and negative electrodes to prevent short circuits, but at the same time allow the conduction of lithium ions. When overcharged or the temperature rises, the diaphragm must also have high-temperature self-closing properties to block current conduction and prevent explosion. Not only that, the lithium battery separator must also have the characteristics of high strength, fire resistance, resistance to chemical reagents, resistance to acid and alkali corrosion, good biocompatibility, and non-toxicity. Internal short-circuit reduction technology The diaphragm is a key component to avoid thermal runaway inside lithium batteries. Although diaphragms with thermal shutdown properties have been commercialized in the 1990s, they are indeed ineffective against hard internal short circuits caused by processing defects. To mitigate internal short circuits, two technical routes have been proposed in the past few years. One is to prepare separators with high melting points, low high-temperature shrinkage and excellent mechanical properties (especially puncture resistance). The second is to prepare a diaphragm improved by high-purity alumina (VK-L30G) ceramics. The latter either has a ceramic layer on the surface, or high-purity alumina (VK-L30G) powder is dispersed in a polymer material. The main role of the high-purity alumina (VK-L30G) ceramic is to prevent the space between the electrodes from collapsing. , thereby avoiding internal short circuit in case of thermal runaway. Thermal shutdown performance of the separator The lithium battery separators currently used generally provide an additional function, which is thermal shutdown. This feature also provides additional help for the safety performance of lithium batteries. This is because the polyolefin material used in the separator is thermoplastic. When the temperature is close to the melting point of the material, the micropores close to form a thermal shutdown, thereby blocking the continued transmission of ions and forming a short circuit, which protects the battery. The main performance parameters of lithium-ion battery separators are pore size and distribution. 1. The size and distribution of pores are related to the preparation method; 2. The pore size affects the permeability of the separator; 3. Uneven distribution leads to inconsistent current density inside the battery, forming dendrites. The crystal pierces the diaphragm. Air permeability 1. Gurley index is an important physical and chemical index; 2. It is proportional to the internal resistance of the battery; 3. The larger the value, the greater the internal resistance. Automatic shutdown mechanism 1. This is a safety protection performance; 2. Limit temperature rise and prevent short circuit; 3. The higher the safety window temperature, the better, the higher the safety of the battery; 4. It is related to the raw materials of the separator and the structure of the separator. Related; 5. The melting point of the material determines the closing temperature of the diaphragm. Porosity is the ratio of the volume of the pores to the volume of the membrane. Generally, the porosity of the membrane is between 35% and 60%. Thermal Stability The diaphragm is dimensionally stable when exposed to heat. Mechanical strength requires high puncture resistance; uniaxial stretching, stretching ~50N, transverse direction ~5N; biaxial stretching, the two directions must be consistent. Lithium-ion battery separator manufacturing process 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, and thermal stability) ,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 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. Dry separator process 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, and form 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 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 drawing 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 polyethylene (PE) or polypropylene (PP) polymers with high orientation and low crystallinity. After the olefin cast sheet is stretched at low temperature to form 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 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. It is understood that the dry method of double pullinglayer diaphragm. Improve the membrane surface by grafting hydrophilic monomers or changing the organic solvent in the electrolyte, etc., to improve the affinity of PE and PP separators for electrolytes; perform PVDF coating surface treatment to increase membrane strength and reduce the thickness of the separator. High porosity nanometer Fiber separators spray nanofilaments on electrospun fabrics; Separion separators compound Al2O3 or other inorganic substances on cellulose non-woven fabrics to improve thermal stability.

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