Current situation and Prospect of lithium hexafluo

2022-09-27
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Lithium ion battery electrolyte lithium hexafluorophosphate synthesis technology status and development trend

in 1990, Sony Company of Japan successfully developed the first generation of lithium ion battery. Because its comprehensive performance is better than the existing ni/cd batteries and ni/m (H) batteries, and there is no memory effect and no environmental pollution, lithium-ion batteries soon occupied the secondary battery market. The research on its core material lithium hexafluorophosphate (LiPF6) has always been a hot spot in the industry. This paper will analyze and comment on the research status of LiPF6, and prospects its development prospects

1. The synthesis methods of LiPF6 mainly include gas-solid reaction, hydrogen fluoride (HF) solvent method, organic solvent method and ion exchange method. In industry, hydrogen fluoride solvent method is the main method, followed by organic solvent method

1.1 gas solid reaction method

gas solid reaction method is one of the earlier synthesis methods. In this method, porous lithium fluoride (LIF) solid or LIF nanoparticles treated with anhydrous hydrogen fluoride (HF) are reacted with phosphorus pentafluoride (PF5) gas under high temperature and high pressure to directly produce product l ipf6 solid since 1988. Its advantage is that the process is simple, easy to operate, open the oil return valve to make the piston fall back, and the equipment requirements are not high, but it has not been applied to industrial production so far. The fundamental reason lies in the difficulty of mass transfer, which is an important problem that this method is difficult to overcome. As the reaction goes on, the solid surface of LIF will gradually be covered by relatively dense LiPF6 products, which will prevent the diffusion of PF5 gas to the interior, resulting in incomplete reaction and serious "entrainment" of products. Therefore, this method is difficult to produce high purity products, and the yield is also low. Although many people have done a lot of exploratory research on this, they still haven't solved this problem well

1.2 ion exchange method

the properties of sodium, potassium, ammonium and organic amine salts of hexafluorophosphate are relatively stable, which is convenient for purification by various methods. The so-called ion exchange method is a method to prepare LiPF6 by ion exchange reaction between these stable high-purity hexafluorophosphate and lithium containing compounds in organic solvents. Commonly used lithium salts include lithium chloride, lithium bromide, lithium perchlorate, lithium nitrate, lithium acetate, etc. solvents generally use low boiling point organics, such as acetonitrile, vinyl carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), etc. solvents with high boiling points are rarely used to avoid decomposition of products when drying lithium hexafluorophosphate complexes

the advantage of ion exchange method is that the reaction is simple and there is no PF5 in the raw material, so the raw material cost is lower than other methods. The disadvantage is the high demand for hexafluorophosphate and lithium, which virtually increases the procedures and costs of raw material purification. In addition, as one of the important reactants, hexafluorophosphate is not completely converted, and the purity of the product is not high. Although many people have studied this method, it is still in the laboratory stage, and its industrial application will take time

1.3 solvent method

to overcome the shortcomings of gas-solid reaction method, people have developed solvent method. There are two kinds of solvent methods: inorganic solvent method and organic solvent method

1.3.1 inorganic solvent method

(1) HF solvent method

this method is to dissolve LIF in anhydrous HF first, then inject high-purity PF5 gas for reaction, remove HF after reaction, and obtain LiPF6 product after separation and drying. Because the reaction is carried out in the liquid phase, this method has many advantages, such as fast reaction speed, good mass and heat transfer effect, easy reaction control, high conversion rate, high product purity, etc., so it quickly realizes industrial production. Despite the shortcomings of this method, such as high energy consumption, harsh anhydrous conditions and equipment corrosion, through the long-term efforts of scientific research and engineering technicians, this method has become increasingly perfect and has become the mainstream industrialized method recognized in the industry

at present, the research on the related problems of this method has gone deep into the engineering technology level. Research is very active in raw material selection and treatment, process flow, production equipment, product purification and other aspects. The research focuses on the synthesis reaction and purification, and has made great progress, with nearly 40 related patents alone. In the aspect of synthesis, the main focus of research is how to improve the effect of gas-liquid mass transfer and heat transfer, so as to improve the reaction quality and improve the conversion of PF5. Typical technical representatives include LIF HF solution atomization process, microporous aeration process, tubular reactor process, etc. In terms of purification and refining technology, the relevant research has obvious personalized characteristics, mainly focusing on the specific process and product characteristics. At present, in addition to the traditional thermal vacuum drying method, there are chemical reaction method, microwave radiation drying method, solvent recrystallization method and ultrasonic induced crystallization method. These methods have their own advantages and disadvantages, and the application places are different, but they all play a certain role in improving the product quality of relevant enterprises

(2) SO2 solvent method

this method is to add liquid SO2 and PF5 gas into anhydrous LIF solution successively for reaction, and then heat up after the reaction to remove SO2 and PF5 to prepare LiPF6 crystal. Its advantages are that the reaction temperature is moderate, the anti-corrosion requirements of the equipment are not high, and the HF content in the product is low, but the SO2 content is high

1.3.2 organic solvent method

as we all know, LiPF6 is thermally unstable. Solid LiPF6 decomposes at about 30 ℃ and 130 ℃ in solution. Water can lead to its rapid decomposition. Therefore, in terms of the reaction system with LIF and PF5 as raw materials, the organic solvent method can be said to be a regression of the technical attributes of LiPF6 synthesis to some extent. At present, organic solvents mainly include ether, ester, pyridine and acetonitrile (CH3CN)

(1) ether and ester solvent method

the starting point of the study of ether and ester solvent method is mostly based on the direct acquisition of lithium-ion battery electrolyte. The reaction between lif and PF5 to produce LiPF6 is thermodynamically advantageous, and the key point is to solve the kinetic problem. Low chain alkyl ethers (such as methyl ether, ether, methyl ether, etc.), cyclic ethers (such as tetrahydrofuran, 1,3-dioxane, 2-methyltetrahydrofuran, etc.) and low alkyl esters (such as EC, Dec, DMC, etc.) can dissolve LiPF6, and some organic substances, such as carbonate, are one of the solvents in lithium-ion batteries at present. Using the solubility of the solvent to the product, the reaction interface can be constantly updated, so as to maintain a high reaction rate and high LIF conversion. Moreover, the product can be directly used in the electrolyte of lithium-ion batteries. Therefore, using these substances and their mixtures as solvents is a reasonable and inevitable choice for solvent method

in this method, LIF organic solvent suspension is usually prepared first, and then the amount of PF5 gas is controlled for reaction. After the reaction, the excess PF5 is removed with inert gas, and the product is the electrolyte of lithium ion battery. The advantages of this method are that the reaction is easy to control, the yield is high, the operation is relatively safe, and the anti-corrosion requirements of the equipment are not high; The disadvantage is that PF5 is easy to react with organic solvents, which increases impurities and darkens the color of the product. In addition, LiPF6 and ethers and other solvents usually exist in the form of complexes, so it is difficult or even impossible to separate LiPF6 crystals, which also limits the application of LiPF6 in other electrolyte systems

(2) acetonitrile solvent method

acetonitrile solvent method usually makes lif-ch3cn suspension first, then adds PF5 gas, and after the reaction is completed, it can get high-purity LiPF6 product after inert gas replacement and vacuum distillation to remove acetonitrile

this method is a lightweight bioplastics method. Its advantages are fast reaction speed, mild conditions, simple process, high purity lipf6[] can be prepared, low energy consumption, low equipment corrosion, but this method still cannot avoid the use of PF5, and acetonitrile is toxic

the acetonitrile dissolution method, which stipulates that only one of the three hardness values can be measured, is one of the research hotspots of theorists and engineering technicians at present. The research mainly focuses on two aspects: one is to reduce the product cost from the synthesis route or process; Second, focus on improving the reaction effect to improve product quality

high purity PF5 is difficult to manufacture and expensive, which directly affects the product cost of LiPF6. To solve this problem, the industry has carried out a lot of research and made some progress. For example, a method of preparing high-purity anhydrous PF5 gas with anhydrous orthophosphoric acid, calcium fluoride and sulfur oxide as raw materials has obtained a national patent. The LiPF6 prepared with it can be purified after simple refining. Another method of preparing high-purity PF5 gas by fluorine halogen exchange reaction with organotin fluoride in organic solvent using relatively cheap phosphorus pentahalide as raw material has also attracted people's attention. It is said that PF5 prepared by this method can be directly used for synthesis. One of its outstanding advantages is that the relatively cheap phosphorus pentahalide is used to prepare the expensive PF5, which greatly reduces the cost; Second, because the fluorine halogen exchange is carried out in organic solvents, there is no HCl overflow, which solves the long-term problem that the impurity chloride ion affects the product quality when phosphorus pentahalide is used as raw material; In addition, this method has the advantages of simple process, mild reaction conditions, easy purification of fluorinating agent, and low requirements for equipment and environmental protection

2. Research and development trend of lithium hexafluorophosphate technology

at present, the synthesis technology of LiPF6 has been relatively mature, but its technology diffusion speed has significantly accelerated, and presents a new trend. In terms of technology, the development and application of new phosphorus sources is one of the trends in the development of hydrogen fluoride solvent technology. For example, Stella in Japan, Morita chemical and many domestic scientific research institutions are currently committed to the selection and preparation of phosphorus sources, trying to make breakthroughs in raw materials or process methods; In addition, in terms of process devices and purification technology, the technology is developing towards energy conservation, environmental protection and high efficiency. For example, many patents have appeared in domestic polyfluorocarbons, Tianjin Chemical Design Institute, Central South University, etc

the research and development of organic solvent method technology is also becoming increasingly active. For example, central nitrate in Japan and metal shares in Germany have been conducting systematic research on the synthesis process and product purification around the organic solvent method. Another noteworthy new trend is that some enterprises and research institutions at home and abroad have begun to research and develop the technology of synthesizing lithium hexafluorophosphate with cheap inorganic lithium salt and hexafluorohydrofluoric acid salt, and how to effectively reuse valuable substances in waste batteries will become one of the new hot spots in the future

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