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The four key materials of lithium-ion batteries include positive electrode, negative electrode, separator, and electrolyte. The electrolyte transports ions and ion compounds between the positive and negative electrodes of the battery, and its performance directly determines the conductivity, capacity, and output voltage of the lithium-ion battery. Electrolytes are generally prepared from high-purity organic solvents, solutes, and small amounts of additives in a certain proportion, and their main components are shown in the figure.

Lithium hexafluorophosphate is the most important component of electrolyte cost, accounting for approximately 43% of the total electrolyte cost. The production technology threshold for lithium hexafluorophosphate is high, especially for the production of high-purity crystalline lithium hexafluorophosphate. It can be said that lithium hexafluorophosphate, as a cutting-edge material in the lithium battery industry, is undoubtedly the soul of electrolytes.

As an electrolyte material, it has good overall performance, but its disadvantage is poor thermal stability and easy deliquescence. Therefore, it needs to be stored in a low-temperature isolated air environment

The irreplaceability of lithium hexafluorophosphate

Lithium hexafluorophosphate has moderate ion migration number, moderate dissociation constant, good antioxidant performance, and good aluminum foil passivation ability in commonly used organic solvents. It can also be matched with various positive and negative electrode materials, making it the most important electrolyte lithium salt for commercial lithium-ion batteries. Researchers are constantly trying to develop new lithium salts in order to replace lithium hexafluorophosphate, but so far, it has not been successful. Therefore, it is expected that for a long period of time in the future, lithium hexafluorophosphate will still be the only electrolyte salt widely used, and its uniqueness mainly depends on the composition of lithium, phosphorus, and fluorine.

Lithium is the lightest alkali metal element and the metal element with the smallest molar mass. It is also the metal element with the lowest redox potential, the highest mass energy density, and the highest electrochemical equivalent. These characteristics determine that lithium is a high specific energy electrode material; Fluorine is the element with the strongest electronegativity and activity among non-metallic elements in nature, and it is also the element with the highest standard electrode potential. The combination of fluorine and lithium forms an electrochemical reversible battery, with a maximum potential of 5.93V and the highest specific energy of the battery. At the same time, the radii of lithium and fluorine are extremely small, making them suitable as electrode materials for lithium batteries.

In addition, the binding ability of hexafluorophosphate ions is poor, so the conductivity of its electrolyte is higher than that of general inorganic lithium salts. Lithium hexafluorophosphate has strong electrochemical stability, with a stable cathode voltage of 5.1V, which is much higher than the 4.2V required for lithium-ion batteries. It does not corrode the current collector and has stronger comprehensive performance than other lithium salts.

Preparation process and application of lithium hexafluorophosphate

Lithium hexafluorophosphate has very unstable properties and decomposes at around 60 ℃. It is also easily deliquescent. Generally, the preparation of potassium hexafluorophosphate products should be carried out in non-aqueous solvents such as anhydrous hydrogen fluoride and low alkyl ether. Moreover, if lithium hexafluorophosphate is developed towards lithium-ion batteries and power batteries, it requires very high purity, stability, and consistency. Meanwhile, the production process of lithium hexafluorophosphate involves harsh working conditions such as low temperature, strong corrosion, and no water or dust, making the process extremely difficult.

The preparation methods of lithium hexafluorophosphate mainly include gas-solid reaction method, organic solvent method, and hydrogen fluoride solvent method. At present, the mainstream method for preparing lithium hexafluorophosphate at home and abroad is the hydrogen fluoride solvent method, which accounts for more than 80% of all industrial production methods. Large enterprises such as Morita Chemical, Jinniu Chemical, Duofuduo Chemical, and Jiangsu Jiujiu all use this method to achieve industrial production. Therefore, we mainly introduce the hydrogen fluoride solvent method that achieves continuous and automated production.

1. Gas solid reaction method

The gas-solid reaction method is the earliest method for preparing lithium hexafluorophosphate, proposed by American scientists in 1950. The preparation process of gas-solid reaction method mainly includes two steps:

LiF (solid)+HF (gas) → LiHF2 (solid) → LiF (porous)+HF (gas)

LiF (porous)+PF5 (gas) → LiPF6

This synthesis method is simple to operate and operates at high temperatures, but the generated lithium hexafluorophosphate will cover the surface of the decomposed lithium to form a dense protective film, preventing further reaction and resulting in a large amount of unreacted lithium fluoride in the final product, resulting in a relatively low purity of the product. If further purification is carried out, it will increase the process and cost, and the purity will not be easily guaranteed. If porous LiF is used to react with high-purity PF5 gas, a purity of 99.9% lithium hexafluorophosphate can be obtained, but the preparation cost is relatively high.

2. Hydrogen fluoride solvent method

The hydrogen fluoride solvent method is currently the most widely used method for preparing lithium hexafluorophosphate. The hydrogen fluoride solvent method involves dissolving lithium halide in anhydrous hydrogen fluoride, then reacting with high-purity PF5 gas to generate potassium hexafluorophosphate crystals, which are then separated and dried to obtain lithium hexafluorophosphate products.

Morita New Energy Materials Co., Ltd. (Japanese controlled) uses hydrogen fluoride liquid to react with phosphorus pentachloride to obtain a mixed gas of PF5 and hydrogen chloride, which is then introduced into hydrogen fluoride and LiF to produce a potassium hexafluorophosphate solution. The obtained potassium hexafluorophosphate solution is filtered to remove insoluble impurities, the filtrate is stirred for crystallization, and finally dried to obtain lithium hexafluorophosphate product. This method is easy to implement and control, and the process flow is shown in the figure.

Purified PF5 was introduced into an anhydrous hydrofluoric acid solution dissolved in LiF to obtain a lithium hexafluorophosphate solution. Then, ultrasound with a power of 200-400W and a frequency of 15-40KHz was applied to the crystallized lithium hexafluorophosphate solution- Crystallization at 30-20 ℃ for 2-3 hours, followed by separation and drying to obtain lithium hexafluorophosphate. This method can effectively shorten the induction period, accelerate the crystallization rate, thereby improving product yield and reducing production costs; It can narrow the particle size distribution range of the product and reduce the impurity content wrapped in the product, thereby obtaining lithium hexafluorophosphate with uniform particles and high purity. The process flow is shown in the figure

3. Organic solvent method

The preparation process of organic solvent method is similar to that of hydrogen fluoride solvent method. The difference in preparation is that the purity of the product prepared by organic solvent is only 90%~95%, and the product is easy to adsorb organic solvent. Further removal is difficult and it is not easy to produce solid lithium hexafluorophosphate.

The process flow diagram of preparing lithium hexafluorophosphate using organic solvent method is shown in the figure.

Summary

Batteries are the heart of new energy vehicles, and key materials such as positive and negative electrode materials, separators, and electrolytes deeply influence the development of lithium battery technology. Lithium hexafluorophosphate, as the most basic material for electrolyte production, is easily overlooked. However, its particle size distribution and impurity content restrict the development of lithium batteries, especially for impurity content, such as metal elements such as sodium, potassium, iron, as well as sulfate and nitrate ions, which are required to be below one hundred thousandth (mass fraction). This has forced the industry to invest more funds in the optimization of the production process of lithium hexafluorophosphate and its raw material purification technology. From the success of multifluorocarbon in recent years, its technological reserves for lithium extraction from salt lakes, lithium carbonate, and hydrofluoric acid purification are the key to its increasing strength.

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