NiMH No.7 batteries

release time:2024-06-05 Hits: Popular:AG11 battery

Analysis of the physical and chemical properties and production process of NiMH No.7 batteries separators

Lithium-ion batteries are the representative of modern high-performance batteries. They are composed of four main parts: positive electrode, negative electrode, separator and electrolyte. Among them, the separator is a thin film with a microporous structure. It is the key inner layer component with the most technical barriers in the lithium-ion battery industry chain. It plays the following two main roles in lithium batteries: 1) Separating the positive and negative electrodes of the NiMH No.7 batteries to prevent the positive and negative electrodes from contacting and forming a short circuit. 2) The micropores in the film allow lithium ions to pass through to form a charge and discharge circuit. The basic properties that the separator should have 1) Avoid physical contact between the positive and negative electrode materials to prevent short circuits. 2) Easy to wet and have good liquid retention. 3) Have electrolyte ion permeability and low ion resistance. 4) Have chemical and electrochemical stability. 5) The separator is as thin as possible. 6) The separator must have a certain strength and sufficient durability of physical and mechanical properties. 7) The separator does not contain particles and metals that can be dissolved by the electrolyte and substances that are harmful to the battery. The role of the separator 1) Isolate the positive and negative electrodes of the battery to prevent short circuits. 2) Adsorb the electrolyte solution necessary for the electrochemical reaction in the battery to ensure high ion conductivity. 3) Ensure the function of stopping the battery reaction to improve the safety of the battery when an abnormality occurs in the battery. Requirements for the diaphragm 1) Have a certain mechanical strength to ensure that it will not break under the condition of battery deformation. 2) Have good ion permeability to reduce the internal resistance of the battery. 3) Excellent electronic insulation to ensure effective isolation between electrodes. 4) Have the ability to resist chemical and electrochemical corrosion and have good stability in the electrolyte. 5) Strong ability to absorb electrolyte. 6) Low cost, suitable for large-scale industrial production. 7) Low impurity content and uniform performance. Structural characteristics of the diaphragm 1) Thickness. The thickness of the lithium-ion battery diaphragm is generally ≤25μm. Under the premise of ensuring a certain mechanical strength, the thinner the diaphragm, the better. At present, most new high-energy batteries use a single-layer diaphragm with a thickness of 20μm or 16μm; the diaphragm used in batteries for electric vehicles (EV) and hybrid electric vehicles (HEV) is about 40μm, which is required for high current discharge and high capacity of the battery. Moreover, the thicker the diaphragm, the better its mechanical strength and the less likely it is to short-circuit during battery assembly. 2) Pore size and distribution. As a battery diaphragm material, it has a microporous structure that allows the absorption of electrolyte; in order to ensure consistent electrode/electrolyte interface properties and uniform current density in the battery, the distribution of micropores in the entire diaphragm material should be uniform. The size and uniformity of the pore size have a direct impact on battery performance: if the pore size is too large, it is easy for the positive and negative electrodes to contact directly or be pierced by lithium dendrites to cause a short circuit; if the pore size is too small, the resistance will increase. Uneven distribution of micropores will form excessive local current during operation, affecting the performance of the battery. 3) Porosity. Permeability can be characterized by the amount of gas passing through the diaphragm under a certain time and pressure, which mainly reflects the patency of lithium ions passing through the diaphragm. Porosity is very important for membrane permeability and electrolyte holding capacity. The porosity of most commercial lithium-ion battery separators is between 40% and 50%. Soak the weighed microporous membrane (Wd) in n-butanol for 2 hours, then take it out, gently absorb the liquid on its surface with filter paper, and weigh it again (Ww), and you can get the mass of n-butanol absorbed by the microporous membrane Wb=Ww-Wd. Where Wd—microporous membrane weight (g); Ww—weight after soaking (g); Wb—mass of n-butanol (g); ρb—density of n-butanol (g/cm3); Vp—dry membrane volume (cm3) 4) Permeability. The time required for a certain amount of air to pass through the membrane under certain conditions (pressure, measurement area) is called the Gurly value. The size of the membrane permeability is the result of the comprehensive influence of the membrane's internal pore structure, such as membrane porosity, pore size, pore shape and pore tortuosity. 5) SEM membrane surface morphology structure. 6) The basis weight of the membrane. 1) Cut three 30cm long membrane samples. 2) Stack and fold the three samples together. 3) Weigh and record the mass of the sample (mg). BW (mg/cm2) = weight (mg) / [3 × 30cm × width (cm)] Mechanical properties of membrane 1) Tensile strength: The tensile strength of the membrane is related to the manufacturing process of the membrane. Tensile strength: MDtension, TDtension The tensile strength of Dongran membrane is less than that of celgard, but the elongation is greater than that of celgard. When uniaxial stretching is used, the strength of the membrane in the stretching direction and the perpendicular stretching direction are different, while the strength of the membrane prepared by biaxial stretching is basically the same in both directions. 2) Puncture strength. Puncture strength refers to the mass applied to a given needle-shaped object to pierce a given membrane sample, which is used to characterize the tendency of short circuits to occur during the assembly of the membrane. Empirically, the puncture strength of lithium-ion battery membranes is at least 11.38kg/mm. Since the electrode is made by mixing active substances, carbon black, plasticizer and PVDF, uniformly coating it on metal foil, and then vacuum drying it at 120°C, the electrode surface is a convex and concave surface composed of tiny particles of active substances and carbon black mixture. The diaphragm material sandwiched between the positive and negative plates needs to withstand a lot of pressure. The physical and chemical properties of the diaphragm 1) Wettability and wetting speed. Poor wettability of the diaphragm will increase the resistance of the diaphragm and the battery, affecting the cycle performance and charge and discharge efficiency of the battery. The wetting speed of the diaphragm refers to the speed at which the electrolyte enters the micropores of the diaphragm, which is related to the surface energy, pore size, porosity, tortuosity and other characteristics of the diaphragm. 2) Liquid absorption rate of the diaphragm. Since the battery diaphragm material has the function of electrolyte, it must meet the following conditions: sufficient liquid absorption rate to ensure that the ion channel is unobstructed, and in the battery system, a large number of side reactions will inevitably occur, consuming a large amount of electrolyte, so there must be sufficient reserves, otherwise the interface resistance will increase due to the lack of electrolyte, and the consumption of electrolyte will be accelerated, which will be a vicious cycle, so the liquid absorption rate is a very important diaphragm parameter. Determination of membrane absorption: Determined with electrolyte. Take a small piece of membrane, extract the plasticizer and dry it to weigh the dry weight M1. Then soak the membrane in the electrolyte for 30 minutes and take it out after the membrane fully absorbs the electrolyte. Use filter paper to gently absorb the electrolyte on the surface of the membrane and weigh M2. 3) Chemical stability. The diaphragm should maintain long-term stability in the electrolyte, and should not react with the electrolyte and electrode materials under strong oxidation and strong reduction conditions. The chemical stability of the diaphragm is evaluated by measuring the corrosion resistance and expansion and contraction rate of the electrolyte. The ability to resist electrolyte corrosion is to immerse the diaphragm in the electrolyte for 4 to 6 hours after heating it to 50°C, then take it out, wash it, dry it, and finally compare it with the original dry sample. The expansion and shrinkage rate is to immerse the diaphragm in the electrolyte for 4 to 6 hours and then detect the size change, and calculate the percentage of the difference. ExpansionTMA (MD)ExpansionTMA (TD)4) Thermal stability. The battery releases heat during the charging and discharging process, especially when it is short-circuited or overcharged, a large amount of heat will be released. Therefore, when the temperature rises, the diaphragm should maintain its original integrity and a certain mechanical strength, continue to play the role of isolating the positive and negative electrodes, and prevent the occurrence of short circuits. TMA (thermal mechanical analysis) technology is a method to measure the integrity of the diaphragm at high temperatures. It can measure the change of the diaphragm shape with temperature. TMA measures the deformation of the diaphragm under load when the temperature rises linearly. Usually, the diaphragm first shrinks, then begins to stretch, and finally breaks. 5) The resistance of the diaphragm. The resistivity of the diaphragm is actually the resistivity of the electrolyte in the micropores, which is related to many factors, such as porosity, pore tortuosity, electrolyte conductivity, film thickness and the degree of wetting of the electrolyte on the diaphragm material. The more commonly used method for testing the resistance of the diaphragm is the alternating current impedance method (EIS). A sinusoidal alternating voltage signal is applied to the measuring device, and the impedance values of different frequencies within a certain range are measured. The data is then analyzed using an equivalent circuit to obtain information about the interface between the diaphragm and the electrode. Since the film is very thin, there are often defects that increase the error of the measurement result. Therefore, multi-layer samples are often used, and the average value of the measurement is taken. 6) Self-closing performance. Above a certain temperature, the components in the battery will undergo an exothermic reaction and cause "self-heating". In addition, due to charger failure, safety current failure, etc., overcharging or external short circuit of the battery will occur, which will generate a lot of heat. Due to the thermoplastic properties of polyolefin materials, when the temperature approaches the melting point of the polymer, the porous ion-conducting polymer film will become a non-porous insulating layer, and the micropores will close to produce a self-closing phenomenon, thereby blocking the continued transmission of ions and forming a short circuit, which plays a role in protecting the battery. Therefore, polyolefin separators can provide additional protection for the battery. The cost structure of lithium batteries. The production process of NiMH No.7 batteries separators 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 NiMH No.7 batteries. The excellent performance of the separator plays an important role in improving the comprehensive performance of the battery. The many characteristics of NiMH No.7 batteries separators and the difficulty in taking into account their performance indicators determine that their production process has high technical barriers and is difficult to develop. The separator production process includes many processes such as raw material formula and rapid formula adjustment, micropore preparation technology, and independent design of complete sets of equipment. Among them, micropore preparation technology is the core of the NiMH No.7 batteries separator preparation process. According to the difference in the micropore formation mechanism, the separator process can be divided into dry and wet processes. 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, form a lamellar structure under tensile stress during extrusion, and heat-treat the lamellar structure to obtain a hard and elastic polymer film. Then, it is stretched at a certain temperature to form slit-shaped micropores, and a microporous membrane is obtained after heat setting. At present, the dry process mainly includes two processes: dry unidirectional stretching and bidirectional stretching. Dry single-stretching uses polyethylene (PE) or polypropylene (PP) polymers with good fluidity and low molecular weight. Using the manufacturing principle of hard elastic fibers, a polyolefin casting with high orientation and low crystallinity is first prepared. After low-temperature stretching to form micro-defects such as silver streaks, high-temperature annealing is used to open the defects, thereby obtaining a microporous film with uniform pore size and uniaxial orientation. The dry single-drawing process is as follows: 1) Feeding: PE or PP and additives and other raw materials are pre-treated according to the formula and then 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 to form a melt 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 nanoporous membrane. 5) Slitting: The nanoporous membrane is cut into finished films according to the customer's specifications. 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 it 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 be transformed into a more dense and stable α crystal, and after absorbing a large amount of impact energy, holes will be generated inside the material. This process adds a β-crystal modifier with a nucleating effect to PP, and utilizes the density difference between different phases of PP to form micropores during the stretching process. The dry double-stretching 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: The sheet is longitudinally stretched at a certain temperature, and the pores are formed by utilizing the property that β-crystals are easy to form pores under tensile stress. 4) Transverse stretching: The sample is stretched transversely at a higher temperature to expand the pores and improve the uniformity of the pore size distribution. 5) Shaping and winding: By heat treating the diaphragm at high temperature, its thermal shrinkage rate is reduced and dimensional stability is improved. Wet-process diaphragms are divided into asynchronous and synchronous processes according to whether the stretching orientation is simultaneous. The wet process uses the principle of thermally induced phase separation to mix plasticizers (hydrocarbon liquids with high boiling points or some substances with relatively low molecular weight) with polyolefin resins, and uses the solid-liquid phase or liquid-liquid phase separation phenomenon that occurs during the cooling process 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 chains oriented in the same direction. After keeping it warm for a certain period of time, use volatile solvents (such as dichloromethane and trichloroethylene) to extract the plasticizer from the film, and then obtain mutually interpenetrating submicron-sized microporous membrane materials. The wet process is suitable for the production of thinner single-layer PE diaphragms. It is a preparation process with better thickness uniformity, better 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 pretreated raw materials are melted and plasticized in a twin-screw extruder system and then extruded from the die head 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) Shaping: The base film without a pore-forming agent is dried and shaped to obtain a nanoporous membrane. 7) Cutting: The nanoporous membrane is cut into finished films according to the customer's specifications. Wet asynchronous 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 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 poor adjustability. Only the transverse stretching ratio is adjustable, while the longitudinal stretching ratio is fixed. Wet synchronous stretching process Wet coating is the development direction of NiMH No.7 batteries diaphragms The overall performance of wet diaphragms is better than that of dry diaphragms 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 thickness uniformity, mechanical properties (tensile strength, puncture resistance), air permeability, physical and chemical properties (wettability, chemical stability, safety) and other material properties, 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 diaphragms also have disadvantages. In addition to the poor thermal stability due to the limitation of the base material, most of them are non-product factors, such as the need for a large amount of solvents, which are easy to 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 large, the production is difficult, and the production efficiency is low. In wet-process diaphragms, bidirectional synchronous stretching technology can be oriented in both the horizontal and vertical directions at the same time, eliminating the need for a separate longitudinal stretching process and enhancing the uniform thickness of the diaphragm.The product has high transparency, no scratches, excellent optical properties and surface properties. It is the best diaphragm with comprehensive performance. It occupies an important position in the high-end diaphragm market and is also the best NiMH No.7 batteries diaphragm in the market at this stage. Comparison of dry and wet processes for NiMH No.7 batteries diaphragms Performance comparison of dry and wet process diaphragms In terms of product performance, compared with dry diaphragms, wet 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 exposure of the pole piece caused by the large shrinkage of the diaphragm can be avoided, which makes up for the only shortcoming of thermal stability. The product performance has been comprehensively ahead of dry film. Coated diaphragm and conventional diaphragm under high temperature conditions Ceramic coated diaphragm Ceramic particle coated diaphragm uses base film as the matrix, 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 matrix, 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 problem of thermal runaway caused by thermal shrinkage of PP and PE diaphragms, thereby causing battery combustion and explosion; on the other hand, the ceramic composite diaphragm has good infiltration and liquid absorption and retention capabilities with electrolytes 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 NiMH No.7 batteries electrodes 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 infiltration 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 Aramid fiber is a high-performance fiber with 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 properties 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.