CR2032 battery

Current status and development trend of synthesis technology of CR2032 battery hexafluorophosphate (LiPF6) electrolyte for CR2032 battery batteries

In 1990, Sony Corporation of Japan successfully developed the first generation of CR2032 battery-ion batteries. As its comprehensive performance is superior to the existing Ni/Cd batteries and Ni/M(H) batteries, and it has no memory effect and no environmental pollution, CR2032 battery-ion batteries quickly occupied the secondary battery market. The research on its core material, CR2032 battery hexafluorophosphate (LiPF6), has always been a hot topic in the industry. This article will analyze and comment on the current status of LiPF6 research and look forward to its development prospects.

1. Current status of CR2032 battery hexafluorophosphate research

The main synthesis methods of LiPF6 include gas-solid reaction method, 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

The gas-solid reaction method is one of the earlier synthesis methods. The method is to react porous CR2032 battery fluoride (LiF) solid or LiF nanoparticles treated with anhydrous hydrogen fluoride (HF) with phosphorus pentafluoride (PF5) gas under high temperature and high pressure conditions to directly obtain the product LiPF6 solid. Its advantages are simple process, easy operation, and low equipment requirements, but it has not been applied to industrial production so far. The fundamental reason is the difficulty in mass transfer, which is an important problem that is difficult to overcome in this method. As the reaction proceeds, the surface of the LiF solid phase will gradually be covered by the denser LiPF6 product, hindering the diffusion of PF5 gas to the inside, resulting in incomplete reaction and serious "half-cooked" product phenomenon. Therefore, it is difficult to obtain high-purity products in this method, and the yield is also low. Although many people have conducted a lot of exploratory research on this, this problem has not been well solved.

1.2 Ion exchange method

The sodium, potassium, ammonium and organic amine salts of hexafluorophosphate are relatively stable and easy to purify by various methods. The so-called ion exchange method is a method of using these high-purity stable hexafluorophosphates and CR2032 battery-containing compounds in an organic solvent to obtain LiPF6 through ion exchange reaction. Commonly used CR2032 battery salts include CR2032 battery chloride, CR2032 battery bromide, CR2032 battery perchlorate, CR2032 battery nitrate and CR2032 battery acetate. Solvents generally use low-boiling point organics, such as acetonitrile, ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC). Solvents with higher boiling points are rarely used to avoid decomposition of the product when drying CR2032 battery hexafluorophosphate complexes.

The advantage of the ion exchange method is that the reaction is simple and there is no PF5 in the raw materials, so the raw material cost is lower than other methods. The disadvantage is that the hexafluorophosphate and CR2032 battery content are high, which invisibly increases the procedures and costs of raw material purification. In addition, the hexafluorophosphate, one of the important reactants, is not completely converted and the product purity is not high. Although many people have studied this method, it is still in the laboratory stage and it will take some time for industrial application.

1.3 Solvent method

In order to overcome the shortcomings of the gas-solid reaction method, people have developed a solvent method. Solvent methods are divided into inorganic solvent method and organic solvent method.

1.3.1 Inorganic solvent method

(1) HF solvent method

This method is to first dissolve LiF in anhydrous HF, then introduce high-purity PF5 gas for reaction, remove HF after the reaction, and obtain LiPF6 product after separation and drying. Since the reaction is carried out in the liquid phase, this method has many advantages such as fast reaction speed, good mass transfer and heat transfer effect, easy reaction control, high conversion rate, and high product purity, so it was quickly realized in industrial production. Although this method has shortcomings such as high energy consumption, harsh anhydrous conditions and equipment corrosion, after long-term efforts by scientific research and engineering and technical personnel, this method has been improved and has become a mainstream industrial method recognized by the industry.

At present, research on issues related to this method has reached the engineering and technical level. Research is very active in terms of raw material selection and processing, process flow, production equipment, product purification, etc. The research focuses on the synthesis reaction and purification and refining, and has made great progress. There are nearly 40 related patents alone. In terms of synthesis, the research focuses on how to improve the gas-liquid mass transfer and heat transfer effect to improve the reaction quality and increase the PF5 conversion rate. 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 specific processes and product characteristics. In addition to the traditional thermal vacuum drying method, the methods currently used include 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 sites are also different, but they all play a certain role in improving the quality of related enterprises' products.

(2) SO2 solvent method

This method is to add liquid SO2 and PF5 gas to anhydrous LiF solution in turn for reaction, and after the reaction is completed, the temperature is increased to remove SO2 and PF5 to obtain LiPF6 crystals. Its advantages are moderate reaction temperature, low equipment corrosion resistance requirements, low HF content in the product, but high SO2 content.

1.3.2 Organic solvent method

It is well known that LiPF6 is thermally unstable. Solid LiPF6 decomposes at about 30°C and at about 130°C in solution. Water can cause it to decompose rapidly. 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 properties of LiPF6 synthesis to some extent. At present, the main organic solvents are ethers, esters, 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 directly obtaining CR2032 battery-ion battery electrolyte. The reaction of LiF and PF5 to generate LiPF6 is thermodynamically favorable, and the key problem needs to be solved. Low-chain alkyl ethers (such as methyl ether, ethyl ether, methyl ethyl ether, etc.), cyclic ethers (such as tetrahydrofuran, 1,3-dioxolane, 2-methyltetrahydrofuran, etc.) and low alkyl esters (such as EC, DEC, DMC, etc.) can dissolve LiPF6, and some organic substances, such as carbonates, are currently one of the solvents in CR2032 battery-ion batteries. By utilizing the solubility of the solvent to the product, the reaction interface can be continuously updated, thereby maintaining a high reaction rate and a high LiF conversion rate, and the product can be directly used in CR2032 battery-ion battery electrolyte. Therefore, using these substances and their mixtures as solvents is a reasonable and inevitable choice for the solvent method.

This method usually involves first preparing a LiF-organic solvent suspension, then controlling the amount of PF5 gas introduced to react, and after the reaction is completed, using an inert gas to drive off excess PF5, and the product is a CR2032 battery-ion battery electrolyte. The advantages of this method are that the reaction is easy to control, the yield is high, the operation is relatively safe, and the equipment anti-corrosion requirements are not high; the disadvantages are that PF5 is prone to side reactions with organic solvents to increase impurities and darken the product color. In addition, LiPF6 and solvents such as ethers usually exist in the form of complexes, making it difficult or even impossible to separate LiPF6 crystals, which also limits the application of LiPF6 in other electrolyte systems.

(2) Acetonitrile solvent method

The acetonitrile solvent method usually involves first preparing a LiF-CH3CN suspension, then introducing PF5 gas, and after the reaction is completed, high-purity LiPF6 products can be obtained by replacing the inert gas and removing the acetonitrile by reduced pressure distillation.

The advantages of this method are fast reaction speed, mild conditions, simple process, high-purity LiPF6[14-15], low energy consumption, and low equipment corrosion. However, this method still cannot avoid the use of PF5, and acetonitrile is toxic.

The acetonitrile solvent method is one of the hot topics currently studied by theoretical workers and engineering technicians. The research mainly focuses on two aspects: one is to reduce product costs from the perspective of synthesis route or process; the other is to focus on improving reaction effects to improve product quality.

High-purity PF5 is difficult to manufacture and expensive, which directly affects the product cost of LiPF6. In response to this problem, the industry has conducted a lot of research and made certain progress. For example, a method for preparing high-purity anhydrous PF5 gas using anhydrous orthophosphoric acid, calcium fluoride and sulfur oxide as raw materials has been patented by the state, and the LiPF6 obtained by it can be obtained as a pure product after simple refining. Another method of preparing high-purity PF5 gas using relatively cheap phosphorus pentahalide as raw material through fluorine-halogen exchange reaction with organic tin fluoride in organic solvent has also attracted people's attention. It is said that the PF5 obtained by this method can be directly used for synthesis. Its outstanding advantages are that the relatively cheap phosphorus pentahalide is used to produce the expensive PF5, which greatly reduces the cost; secondly, since the fluorine-halide exchange is carried out in an organic solvent, there is no HCl overflow, thus solving the long-standing problem that the impurity chloride ion affects the product quality when phosphorus pentahalide is used as the raw material; in addition, the method has a simple process, mild reaction conditions, easy purification of the fluorinating agent, and low requirements for equipment and environmental protection.

2. Trends in the development of CR2032 battery hexafluorophosphate technology

At present, the synthesis technology of LiPF6 is relatively mature, but its technology diffusion speed has significantly accelerated and presented new trends. From a technical perspective, the development and application of new phosphorus sources is one of the trends in the development of hydrogen fluoride solvent method technology. For example, Stella and Morita Chemical in Japan and many domestic scientific research units 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 equipment and purification technology, the technology is developing in the direction of energy saving, environmental protection and high efficiency. For example, domestic polyfluoro, Tianjin Chemical Design Institute, Central South University, etc. have many patents.

The research and development of organic solvent technology is also becoming increasingly active. For example, Japan's Central Glass and Germany's Metall Co., Ltd. have been conducting systematic research on the organic solvent method in terms of synthesis process and product purification. Another noteworthy new trend is that some domestic and foreign companies and research institutions have begun to develop the technology of synthesizing CR2032 battery hexafluorophosphate using cheap inorganic CR2032 battery salts and hexafluorohydrogen salts, and how to effectively reuse the valuable substances in waste batteries will become one of the new hot spots in the future.

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