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What are the advantages and disadvantages of PP, PE, and AGM separators?

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[Abstract]:
1. Overview of the separator The separator is an important component of the battery and is not an active substance.
1. Overview of the separator The separator is an important component of the battery and is not an active substance. In some cases it even plays a decisive role. Its material is an electronic insulator, and its porosity makes it ionic conductive. The resistance of the separator is an important property of the separator. It is determined by the thickness of the separator, the porosity, and the tortuosity of the pores. It has an important influence on the capacity and terminal voltage level of the high-rate discharge of the battery; the stability of the separator in sulfuric acid. It directly affects the life of the battery; the elasticity of the separator can delay the falling off of the active material of the positive electrode; the pore size of the separator affects the degree of short circuit of the lead dendrites. Due to the multi-layer effect of the separator on the performance of the lead storage battery, the improvement of the quality of the separator is accompanied by an improvement in the performance of the lead storage battery. The main function of the separator is to prevent the short circuit of the positive and negative electrodes, but it can not significantly increase the internal resistance of the battery. Therefore, the separator should be porous, allowing free diffusion and ion migration of the electrolyte and having a relatively small electrical resistance. When the active material is somewhat detached, it should not pass through the pores to reach the opposite plate, that is, the pore size should be small, the number of holes should be large, and the total area of ​​the gap should be large; in addition, mechanical strength, acid corrosion resistance, oxidation resistance, and Does not precipitate substances harmful to the plates. In the 1950s, the starting battery used mainly used wooden partitions. Because it must be used under wet conditions, the negative electrode plate is easily oxidized, and the initial charging time is long, and it cannot be used for dry-charged lead storage batteries. In particular, the wood separator is not resistant to oxidative corrosion in sulfuric acid, resulting in a short battery life. In order to improve the life of the lead battery, the wooden separator and the glass wool are used together to make the battery life increase exponentially, but the internal resistance of the battery increases, which has an adverse effect on the battery capacity and the starting discharge, and can also meet the standard requirements at that time. In the mid-1960s, microporous rubber separators appeared, which significantly improved battery life due to their good acid resistance and oxidation resistance. And promote the improvement of the battery structure, reduce the center distance of the plate, so that the battery start discharge performance and volume specific energy are greatly improved. Because of the excellent performance of microporous rubber separators, from the 1970s to the early 1990s, lead batteries were dominant in the industry. The disadvantage of the microporous rubber separator is that the impregnation by the electrolyte is slow, and in addition to the tropical region, there is a lack of resources, the manufacturing process is complicated, and the cost is expensive. In addition, it is not easy to make a thin finished product (it is difficult to have a thickness of 1 mm or less). At the same time as the production of the microporous rubber separator, a sintered PVC separator and a subsequent soft polyvinyl chloride separator are also appeared. The type of partition is similar to the rubber partition, but it was very popular in the 1980s.
Since 1993, due to the increased cost of microporous rubber separators, the formation of PVC separators has been in short supply. In the 1990s, PP (polypropylene) separators, PE (polyethylene) separators, and ultrafine glass fiber separators (10-G each) and their composite separators were successively appeared. There have also been fiber-paper separators, which have good electrical resistance and porosity, but have poor corrosion resistance and mechanical strength, and have large pore diameters, so they cannot be used in large quantities. At present, polyethylene bags are widely used in the world, especially in the United States and Western Europe. The PE separator has a small pore size, extremely low electrical resistance and a very thin substrate, and is easy to be made into a bag type, which is suitable for continuous production of a battery. However, at present, there is no domestically produced mass production in China, and the assembly line (including the matching machine) suitable for this partition is also limited, so the use is not widespread; PP separator and 10-G are gradually accepted by automobile battery manufacturers. Closed valve-regulated lead-acid batteries are mainly made of AGM (adsorbed glass fiber separator). Below we mainly introduce AGM separators. 2. Ultra-fine glass fiber separators are currently used in valve-regulated lead-acid batteries. Ultra-fine glass fiber separator (AGM), the main function of the separator is to make ions flow between the electrodes, and has a very high porosity; a large specific surface area and good wettability are capable of adsorbing the maximum amount of electrolyte The main characteristics of the partition. The separator must have long-term stable chemical and electrochemical corrosion resistance in the battery. It cannot release any substances that increase the gas evolution rate, corrosion or self-discharge, and also has good tensile strength to ensure the separator in the battery. The production assembly process is not pierced by sharp edges or small particles. The separator is an important component in the production of batteries. Its advantages and disadvantages directly affect the discharge capacity of the battery and the life of the charge and discharge cycle. Therefore, the selection and research of the battery separator must be taken seriously. In general, the separator should have the following requirements in the battery: I. Prevent the internal and external short circuit of the battery when the positive and negative electrode plates are in contact with each other; II. Make the battery assembly tight and reduce the battery volume; III. Prevent the deformation of the plate and bend And the active material is detached; IV. Store the necessary amount of electrolyte in the porous separator between the plates to ensure high conductivity and battery reaction requirements; V. Prevent some substances harmful to the electrode from migrating through the partition And diffusion. In order to ensure that the separator can smoothly perform the above functions in the battery, there must be certain requirements for the separator itself. The various requirements described below tend to be different depending on the battery's action conditions. In general, the quality of the separator has the following requirements: I. The separator material itself is an insulator, but the separator must have a loose porous structure and can absorb a large amount of the electrolyte solution; II. Chemical stability is good, must be resistant to sulfuric acid corrosion, oxidation and aging; III. The separator should have greater mechanical strength and elasticity, easy to install in production; IV. The separator should have good wettability, ie it should Can be quickly soaked by the electrolyte sulfuric acid; V. The separator can not be leached in the sulfuric acid solution can be harmful to the battery; VI. The surface color of the separator should be basically the same, no cracks and perforations are allowed; VII. The resistance of the plate immersed in the electrolyte sulfuric acid solution is small; VIII. The separator should have a wide temperature range of use; IX. The separator should have a certain porosity and the consistency of the pore diameter is high; X. The separator shall have the shrinkage or expansion ratio according to the requirements; XI. It shall have good folding resistance to the soft partition; XII. The dry thickness and uniformity of the separator shall meet the requirements of the index; 3. Ultra-fine glass fiber partition The structure and characteristics of the board Any organic binder containing a diameter of 0.5 ~ 4um microglass fibers are composed. The uncompressed glass fiber paper is formed by a papermaking method, and has a multi-layered felt structure, and a relatively small and high-diameter free passage is formed by the disorderly arranged glass fibers. The separator has properties that are significantly superior to conventional battery separators in many respects. In general, it has the following main characteristics: I. High liquid absorption, fast liquid absorption, good hydrophilicity, absorbs and maintains the electrolyte required for the rated capacity of the battery, and maintains its high life throughout its life. Liquid absorption rate; II. Large surface area and high porosity. As long as the electro-hydraulic is lacking, it can ensure that the oxygen generated by the positive electrode diffuses to the negative electrode through the separator, and combines with the sponge lead on the negative electrode; III. The small pore size can effectively prevent the battery short circuit and dendrite penetration; IV. High chemical purity, harmful impurities Less; V. has very good acid and oxidation resistance; VI. low resistivity. 4. Main factors affecting the performance of ultra-fine ultra-fine glass fiber separator: I. Influence of chemical composition of ultra-fine glass fiber The chemical composition of glass wool is a key factor affecting the performance of the separator, which directly affects the chemical properties of the separator. II. Influence of diameter and length of ultra-fine glass fiber cotton The smaller the diameter of ultra-fine glass fiber, the larger the surface area and the higher the wettability, the larger the liquid absorption rate, the smaller the pore size of the separator and the stronger the resistance to dendritic penetration. The resistance value will increase accordingly, so an optimal combination must be chosen. The length of the fiberglass cotton also affects the performance of the separator. The length of the cotton, the fibers are not easily dispersed and flocculate, making the separator uneven; the cotton is short, the uniformity of the separator is improved, but the strength is low, so an optimum length range should also be selected. III. Effects of Harmful Impurities in Ultrafine Glass Fiber Cotton Impurities in glass fiber cotton have a direct impact on the performance of the separator. The presence of metal or metal ions such as iron, copper, nickel, etc. in the glass fiber cotton will increase the self-discharge and gassing of the battery. Therefore, it is necessary to select raw materials with less harmful impurities to ensure good performance of the separator. 5. Influence of the separator on the battery performance The quality of the separator will directly affect the battery capacity, charge and discharge cycle life and self-discharge performance. The analysis results of the battery show that the main reason for the low cycle life of the battery is that the pore size of the separator is relatively large, the pore size distribution and the thickness are not uniform, so the positive lead powder gradually passes through a small amount as the charge and discharge progresses. The separator is on the negative side, and the negative lead dendrite may penetrate the separator, and finally cause a chronic short circuit of the battery. Therefore, as the charge and discharge progress, the capacity of the battery gradually decreases and fails. It can be seen from the battery dissection after the end of life that the separator turns reddish brown on the negative side, indicating that a small amount of lead powder has passed through the separator. It is worth emphasizing that the quality of the separator chosen plays a crucial role in preventing chronic short circuits in the battery. The better separator has good resistance to dendrite penetration and oxidation resistance, and the pore diameter is fine and uniform, and the pore volume is moderate, which can significantly reduce the charge termination current, prolong the battery electrical life and reduce the rate of self-discharge of the battery. The resistance of the separator directly affects the operating voltage and discharge capacity during discharge. The separator with a large resistance causes the working pressure drop of the battery to be discharged, and the discharge capacity of the battery is also low. Therefore, the battery separator resistance we use must be small. In summary, we believe that the advantages and disadvantages of battery separators are an extremely important factor affecting the charge and discharge life of lead batteries, the size of self-discharge, and the level of capacity. Therefore, in the battery production process, according to the characteristics of different batteries, Reasonably select the required battery separator.