The four key materials of lithium-ion batteries include positive electrode, negative electrode, separator, and electrolyte. The electrolyte transports ions and ionic compounds between the positive and negative electrodes of the battery. Its performance directly determines the conductivity of the lithium battery. capacity and output voltage. The electrolyte is generally prepared from high-purity organic solvents, solutes and a small amount of additives in a certain proportion. Its main components are as shown in the figure.
Picture: Main components of lithium battery electrolyte
Lithium hexafluorophosphate is the most important component of electrolyte cost, accounting for approximately 43% of the total electrolyte cost. The technical threshold for the production 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 being called the soul of the electrolyte.
Picture: Lithium hexafluorophosphate is white crystal or powder at room temperature
As an electrolyte material, it has good overall performance, but its disadvantage is that it has poor thermal stability and is easy to deliquesce, so it needs to be stored at low temperature and isolated from air
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, and can be matched with various positive and negative electrode materials, thus becoming a commercial product Lithium salt is the main electrolyte used in lithium-ion batteries. Scientific 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 lithium hexafluorophosphate will remain the only electrolyte salt used on a large scale for a long time to come, and its uniqueness mainly depends on the three elements of lithium, phosphorus, and fluorine.
Lithium element 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 element is the most electronegative and most active element among non-metallic elements in nature, and it is also the element with the highest standard electrode potential. Fluorine and lithium are combined to form an electrochemically reversible battery, with a maximum potential of 5.93V. The battery antistatic agent Irgastat P18 has the highest specific energy. At the same time, the radii of the two elements 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 larger than that of general inorganic lithium salts. Lithium hexafluorophosphate has strong electrochemical stability. The stable voltage of the cathode reaches 5.1V, which is much higher than the 4.2V required for lithium-ion batteries. It does not corrode the current collector, and its overall performance is better than other lithium salts.
Preparation process and application of lithium hexafluorophosphate
Lithium hexafluorophosphate is very unstable, decomposes at around 60°C, and is easily deliquescent. Generally, potassium hexafluorophosphate products are 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 batteries, the requirements for its purity, stability, and consistency will be very high. At the same time, the production process of lithium hexafluorophosphate involves harsh working conditions such as low temperature, strong corrosion, and no water and 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 preparation method of 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 Japan Morita Chemical, Jinniu Chemical, Duofuo Chemical, and Jiangsu Jiujiujiu all use this method to achieve industrialization. Production. Therefore, we mainly introduce the hydrogen fluoride solvent method that realizes continuous and automated production.
1. Gas-solid reaction method
The gas-solid reaction method is the earliest preparation method for 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 is carried out at high temperatures. However, the generated lithium hexafluorophosphate will cover the surface of corroded lithium to form a dense protective film, preventing the further progress of the reaction, resulting in the final product containing a large amount of unreacted Lithium fluoride, product purity is relatively low. If further purified, the process and cost will be increased, and the purity will not be 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 obtained, but the preparationThe cost is higher.
2. 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 pass in high-purity PF5 gas for reaction to generate potassium hexafluorophosphate crystals, which are then separated and dried to obtain the lithium hexafluorophosphate product.
Morita New Energy Materials Co., Ltd. (Japanese-owned 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, and the filtrate is stirred and crystallized, and finally dried to obtain the lithium hexafluorophosphate product. The method is easy to carry out and easy to control, and the process flow is shown in the figure.
Figure Morita New Energy Co., Ltd.’s process flow chart for preparing lithium hexafluorophosphate
Polyfluoropolymer (002407) passes the purified PF5 into an anhydrous hydrofluoric acid solution dissolved with LiF to obtain a lithium hexafluorophosphate solution, and then applies ultrasonic waves with a power of 200~W and a frequency of 15~40KHz to the Lithium hexafluorophosphate solution to be crystallized. Crystallize at -30~-20℃ for 2~3 hours, then separate and dry to obtain lithium hexafluorophosphate. This method can effectively shorten the induction period and speed up the crystallization speed, thereby increasing product yield and reducing production costs; it can narrow the particle size distribution range of the product and reduce the content of impurities wrapped in the product, thereby obtaining lithium hexafluorophosphate with uniform particles and high purity. . The process flow is shown in the figure
Figure Duofludo independently developed process flow chart for preparing lithium hexafluorophosphate
3. Organic solvent method
The preparation process of the organic solvent method is similar to the hydrogen fluoride solvent method. The difference in preparation is that the purity of the product prepared by the organic solvent is only 90%~95%. The product easily absorbs the organic solvent, and further removal is difficult and it is not easy to produce solid lithium hexafluorophosphate. .
The process flow chart for preparing lithium hexafluorophosphate by organic solvent method is shown in the figure.
Process flow chart for preparing lithium hexafluorophosphate
Summary
The battery is the heart of new energy vehicles. Each of the key materials for lithium batteries, such as positive and negative electrode materials, separators, and electrolytes, profoundly affects the development of lithium battery technology. As the most basic material for electrolyte production, lithium hexafluorophosphate is easily overlooked, but its particle size distribution and impurity content restrict the development of lithium batteries, especially the impurity content of cosmetic raw materials, such as sodium, potassium, iron and other metal elements, as well as sulfate, Nitrate, etc. are all required to be below one hundred thousandth (mass fraction), which forces the industry to invest more in the optimization of the production process of lithium hexafluorophosphate and its raw material purification technology. Judging from the success of PTFE in recent years, its technical reserves for lithium extraction from salt lakes, lithium carbonate and hydrofluoric acid purification are the key to its growing strength.
