The Key Material Of Lithium Battery – Lithium Hexafluorophosphate

1. Main components of lithium battery electrolyte
2. The irreplaceability of lithium hexafluorophosphate
3. Preparation process and application of lithium hexafluorophosphate
   1. Gas-solid reaction method
   2. Hydrogen fluoride solvent method
   3. Organic solvent method
4. Analysis and forecast of lithium hexafluorophosphate demand
5. Summary

Lithium hexafluorophosphate is the most important part of the cost of the electrolyte, accounting for about 43% of the total cost of the electrolyte. The production technology threshold of lithium hexafluorophosphate is relatively high, especially the production of high-purity crystalline lithium hexafluorophosphate. The four key materials of lithium-ion batteries include positive electrode, negative electrode, diaphragm, and electrolyte. The electrolyte transports ions and ionic compounds between the positive and negative electrodes of the battery. Its performance directly determines the conductivity, capacity and output of lithium ion battery voltage. Electrolyte is generally prepared by high-purity organic solvent, solute and a small amount of additives in a certain proportion, and its main components are shown in the figure.

Main components of lithium battery electrolyte

Lithium hexafluorophosphate is the most important part of the cost of the electrolyte, accounting for about 43% of the total cost of the electrolyte. The production technology threshold of lithium hexafluorophosphate is relatively high, especially 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 worthy of the name of the soul of the electrolyte. As an electrolyte material, the overall performance is good, but the disadvantage is that the thermal stability is not good, and it is easy to deliquescence, so it needs to be stored at low temperature and isolated from the air.

The irreplaceability of lithium hexafluorophosphate

Lithium hexafluorophosphate has moderate ion transfer number, moderate dissociation constant, good oxidation resistance and good aluminum foil passivation ability in commonly used organic solvents, and can be matched with various positive and negative electrode materials, thus becoming a commercial lithium salt is the most important electrolyte used in batteries. Researchers are constantly trying to develop new lithium salts in order to replace lithium hexafluorophosphate, but so far they have 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 used on a large scale, and its uniqueness mainly depends on the three elements 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 largest mass energy density, and the highest electrochemical equivalent. These characteristics determine that lithium is a high specific energy electrode material. Fluorine is the most electronegative element in nature and the most active element among non-metallic elements, and it is also the element with the highest standard electrode potential. The combination of fluorine and lithium constitutes an electrochemical reversible battery, with a maximum potential of 5.93V and the highest specific battery energy density. 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 association ability of hexafluorophosphate is poor, so the conductivity of its electrolyte is relatively high, which is higher than that of general inorganic lithium salts. Lithium hexafluorophosphate has strong electrochemical stability, and the stable voltage of the cathode reaches 5.1V, which is much higher than the 4.2V required by lithium-ion batteries. It does not corrode the current collector, and its comprehensive performance is stronger than other lithium salts.

Preparation process and application of lithium hexafluorophosphate

Lithium hexafluorophosphate is very unstable, decomposes at around 60°C, and is also prone to deliquescence. Generally, potassium hexafluorophosphate products should be prepared in non-aqueous solvents such as anhydrous hydrogen fluoride and low alkyl ethers. Moreover, if lithium hexafluorophosphate develops in the direction of lithium-ion batteries and power battery, the requirements for its purity, stability, and consistency are very high. At the same time, the production process of lithium hexafluorophosphate involves harsh working conditions such as low temperature, strong corrosion, no water and no dust, and the process is 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 lithium hexafluorophosphate preparation method at home and abroad is the hydrogen fluoride solvent method, which accounts for more than 80% of all industrial production methods. Large-scale enterprises such as Morita, Jinniu Chemical, DFD Chemical, and Jiangsu Jiujiujiu all adopt this method to achieve industrialization. Therefore, the hydrogen fluoride solvent method that realizes continuous and automatic production is mainly introduced.

Gas-solid reaction method

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

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

LiF (porous) + PF5 (gas) → LiPF6

The synthesis method is simple to operate and carried out at high temperature and high temperature, but the generated lithium hexafluorophosphate will cover the surface of the corroded lithium to form a dense protective film, which prevents the further progress of the reaction, resulting in a large amount of unreacted lithium fluoride in the final product, the product purity is relatively low. If it is further purified, the process and cost will be increased, and the purity is not easy to guarantee. If porous LiF is used to react with high-purity PF5 gas, lithium hexafluorophosphate with a purity of 99.9% can be prepared, but the preparation cost is relatively high.

Hydrogen fluoride solvent method

The hydrogen fluoride solvent method is currently the most widely used preparation method for lithium hexafluorophosphate. The hydrogen fluoride solvent method is to dissolve lithium halide in anhydrous hydrogen fluoride, and then introduce high-purity PF5 gas to react to generate potassium hexafluorophosphate crystals, and then obtain lithium hexafluorophosphate products through separation and drying. Morita (Japanese-owned holding company) uses hydrogen fluoride liquid to react with phosphorus pentachloride to obtain a mixed gas of PF5 and hydrogen chloride, and then passes the mixed gas into hydrogen fluoride and LiF to prepare a potassium hexafluorophosphate solution. The obtained potassium hexafluorophosphate solution is filtered to remove insoluble impurities, the filtrate is stirred and crystallized, and finally dried to obtain a lithium hexafluorophosphate product.

Organic solvent method

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

Analysis and forecast of lithium hexafluorophosphate demand

The data shows that the demand for lithium hexafluorophosphate in 2020 is about 35,000 tons, the predicted demand is about 55,000 tons by 2022, and the predicted demand for lithium hexafluorophosphate is about 100,000 tons by 2025. In terms of supply, the world’s lithium hexafluorophosphate production capacity is mainly concentrated in China, Japan and South Korea, with China accounting for the highest proportion. In 2020, the total production capacity of China’s lithium hexafluorophosphate industry will be about 56,500 tons. China is the second country to industrialize lithium hexafluorophosphate after Japan. With the rapid expansion of China’s lithium hexafluorophosphate production capacity, China’s import dependence on lithium hexafluorophosphate has dropped significantly, and China has become the largest producer of lithium hexafluorophosphate. According to data, China’s lithium hexafluorophosphate shipments account for about 70% of the world. At present, the production enterprises of lithium hexafluorophosphate in China with a production capacity of more than 5,000 tons are mainly Tinci, DFD,  Xintai, Zhonglan Hongyuan, Jiangsu Jiujiujiu, Morita, Kanto Electrochemical.

Summary

The new energy industry is a hot area for the future development of the world. Lithium-ion batteries are an important part of the new energy industry. The demand will also face high growth in the next few years. As the main electrolyte, lithium hexafluorophosphate still has a large gap between demand and production capacity.

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