402030 polymer battery.What is the core technology of lithium-ion batteries?

　　What is the core technology of lithium-ion batteries?

　　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 role of separators in lithium batteries:

　　1. Separate the positive and negative electrodes of lithium batteries to prevent short circuits caused by contact between the positive and negative electrodes;

　　2. The micropores in the film can allow lithium ions to pass through and form a charge and discharge circuit.

　　Types of lithium-ion battery separators

　　According to 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.

　　Performance requirements for lithium-ion battery separators

　　1. It has electronic insulation and ensures 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 permeability to lithium ions;

　　3. Resistant to electrolyte corrosion and has sufficient chemical and electrochemical stability. This is because the solvent of the electrolyte is a highly polar organic compound;

　　4. It has good electrolyte wettability and strong ability to absorb and moisturize liquid;

　　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, it must also 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.

　　Reduce internal short circuit 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.

　　Diaphragm Thermal Shutdown Performance

　　Currently used lithium battery separators 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.

　　Pore size and distribution

　　1. The size and distribution of pores are related to the preparation method; 2. The size of pores affects the permeability of the separator; 3. Uneven distribution leads to inconsistent current density inside the battery, forming dendrites that pierce the separator.

　　Breathability

　　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, and the higher the safety of the battery; 4. It is related to the raw material of the separator and the structure of the separator; 5 , the melting point of the material determines the closing temperature of the diaphragm.

　　Porosity

　　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

　　Diaphragm dimensional stability when heated.

　　Mechanical strength

　　It requires high puncture resistance; uniaxial stretching, stretching ~50N, transverse direction ~5N; biaxial stretching, the two directions must be consistent.

　　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 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. 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.

　　Dry double pull

　　It is understood that the dry double drawing process is a unique separator manufacturing process in China. Since the beta crystal form of PP is a hexagonal crystal system, single crystal nucleation, wafers are loosely arranged, and it has a lamellar structure that grows in a radial direction into divergent bundles and does not have a complete spherulite structure. Under the action of heat and stress, It will transform into a more dense and stable α crystal, which will produce holes inside the material after absorbing a large amount of impact energy. This process adds a β-crystalline modifier with nucleation effect into PP and utilizes the density difference between different phases of PP to cause crystalline transformation to form micropores during the stretching process.

　　The dry double drawing process flow is:

　　1. Feeding: Pretreat raw materials such as PP and pore-forming agent according to the formula and then transport them to the extrusion system.

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

　　3. Longitudinal stretching: Longitudinal stretching of the cast sheet at a certain temperature, using the characteristics of β crystals that are prone to pore formation under tensile stress to cause pores.

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

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

　　Wet separator process

　　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.

　　Wet asynchronous stretching process

　　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 cast thick sheets.

　　4. Transverse stretching: Stretch the cast thick sheet transversely after longitudinal stretching 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.

　　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.

　　Characteristics of wet process

　　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 comprehensive performance. It occupies an important position in the high-end separator market and is also the best-performing lithium battery separator in the market at this stage.

　　In terms of product performance, compared with dry separators, wet separators have certain advantages in mechanical properties, air permeability, and physical and chemical properties. By coating adhesives such as ceramic alumina, PVDF, and aramid on the base film, it can It greatly improves the thermal stability of the separator, reduces the shrinkage rate at high temperatures, and avoids the exposure of the pole pieces caused by the large shrinkage of the separator. This makes up for the only shortcoming in thermal stability, and the product performance is now ahead of the dry film.

　　ceramic coated diaphragm

　　The ceramic particle-coated separator uses the base film as the base film, and the surface is coated with a layer of Al2O3, SiO2, Mg(OH)2 or other inorganic ceramic particles with excellent heat resistance. After special processing, it is tightly bonded to the base body. Stably combines the flexibility of organic matter and the thermal stability of inorganic matter to improve the high temperature resistance, heat shrinkage resistance and puncture strength of the separator, thereby improving the safety performance of the battery. It is understood that on the one hand, the ceramic composite layer can solve the safety problem of battery combustion and explosion due to thermal runaway caused by thermal shrinkage of PP and PE separators; on the other hand, the ceramic composite separator has good infiltration and liquid absorption with electrolyte and positive and negative electrode materials. The ability to retain liquid greatly increases the service life of the battery. In addition, the ceramic-coated separator neutralizes small amounts of hydrofluoric acid in the electrolyte, preventing the battery from bloating.

　　PVDF coated separator

　　PVDF, polyvinylidene fluoride, is a white powdery crystalline polymer with a melting point of 170°C, a thermal decomposition temperature of over 316°C, and a long-term use temperature of -40 to 150°C. It has excellent resistance to chemical corrosion and high temperature discoloration. , oxidation resistance, wear resistance, flexibility and high tensile strength and impact resistance. PVDF coated separators have the characteristics of low internal resistance, high (thickness/void ratio) uniformity, good mechanical properties, and good chemical and electrochemical stability. Due to the existence of the nanofiber coating, this new separator has better compatibility and adhesion to lithium battery electrodes than ordinary battery separators, and can greatly improve the high temperature resistance and safety of the battery. In addition, the new separator has good absorbency to liquid electrolytes, has good infiltration and liquid absorption and retention capabilities, extends the battery cycle life, increases the battery's high-rate discharge performance, and increases the battery's output capacity by 20%. It is especially suitable for High-end energy storage batteries and automotive power batteries.

　　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 aramid resin's The electrolyte has high affinity, which enables the separator to have 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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