Background and Overview[1][2]
LiPF6 is a white crystal that is soluble in non-aqueous solvents such as anhydrous hydrogen fluoride, low alkyl ethers, nitriles, pyridine and alcohols, but is insoluble in organic solvents such as alkanes and benzene. It easily reacts with acid to form PF5 and lithium salt. LiPF6 has poor thermal stability and begins to decompose when heated to 60 ℃. The products are LiF and PF5, which will decompose in large quantities when heated to 175 ℃ ~ 185 ℃. . LiPF6 has strong coordination ability and can form stable crystalline complexes with polyethers. The conductivity of LiPF6 in non-aqueous solvent systems is similar to that of LiAsF6, and is higher than that of other lithium salts such as LiClO4 and LiBF4 , LiCF3SO3 and Li(CF3SO2)2N, etc. are all high because the organic electrolyte solution containing LiPF6 has good conductivity and electrochemical stability. Currently widely used lithium-ion batteries generally use LiPF6 as their electrolyte, which has particularly high purity requirements and contains impurities such as Na, K, Fe, Ni, Pb, Zn, SO4 2-, NO3–, Cl–, HF and H2O etc. are required to be below 10 -5 (mass fraction). Since LiPF6 reacts easily with water, the preparation process of LiPF6 generally uses anhydrous hydrogen fluoride, low alkyl ethers and non-water-soluble materials such as nitriles and pyridine. LiPF6 can also be used to make catalysts.
Synthesis method[1][2][3][4]
The preparation process of LiPF6 has extremely high environmental requirements and needs to be carried out under the protection of an atmosphere of high-purity nitrogen or high-purity neon. LiPF6 is mainly used as a battery electrolyte, but the product requires extremely high purity, which largely depends on the solvent used in the reaction and purification process. At present, the industrial production of LiPF6 is mainly based on the preparation method of anhydrous hydrogen fluoride as the solvent. It is difficult to avoid the impact of residual HF in the product on batteries and other products. The research and development of LiPF6 Poloxamer 407 mainly involves the search for lower-cost phase transfer catalysts. The reaction is easy to proceed under mild conditions and uses low-cost raw materials to generate LiPF6 at high temperature and high pressure. After that, the product is separated with ether and then purified.
Synthetic path 1:
Using LiF and BrF3 as starting materials, react with excess P2O5 to generate LiPF6 sub>, the synthesis mechanism is as follows. LiPF6 prepared by this method generally contains LiF impurities.
Synthesis path 2:
Using LiF and PF5 as starting materials, react LiF and PF5 in anhydrous hydrogen fluoride liquid to prepare LiPF6 , the reaction mechanism is as follows. The general process is to put a certain flow rate of PF5 gas into countercurrent contact with anhydrous hydrogen fluoride solution containing LiF in the tower. After the kettle liquid discharged from the bottom of the tower is filtered and evaporated to evaporate the hydrogen fluoride, LiPF6Crude product. The purity of LiPF6 prepared by this method is only 90% to 95%. The impurities are mainly HF, oxyfluoride, LiF, metal ions, etc., and generally require further purification.
For crude LiPF6, which contains a lot of LiF and metal salts, it can be dissolved in organic solvents such as carbonate and ether to form a saturated liquid. After filtration, evaporation, concentration or further treatment, a high-quality product can be obtained. Pure LiPF6 product; for crude products with more oxygen-containing fluoride and LiF, high-purity PF5 or F2 can be passed into it sub>, a high-purity product can be obtained by reacting with impurities in the crude product at a certain temperature; for the crude product LiPF6 with a higher HF content, the product can be heated to 80 ℃ ~ 100 ℃ and controlled A certain degree of vacuum can separate and remove most of the HF. Finally, a high-purity product with a HF content of less than 10 × 10-5 (mass fraction) can be obtained. The HF content must be less than 10 × 10 -6 (mass fraction), which is difficult to achieve using general methods. The process of using anhydrous HF as a solvent to produce lithium hexafluorophosphate is a cryogenic process, which consumes a lot of energy. The biggest drawback is that the HF remaining in the product exists in the form of LiPF6·HF, and the residual HF is corrosive to battery materials, thus affecting battery performance.
Synthetic path 3:
Dissolve a salt in a solution containing ether. This salt consists of an anion containing PF6— and a cation containing a cation. A proton and a Lewis acid group, and the ether can form an isolable compound with LiPF6, and then a calcium phosphate lithium group is added to this solution, so LiPF6 Sub> and its compounds with ethers can naturally form the isolable LiPF6 monoether compound. Moreover, this method also includes dissolving this compound in a non-aqueous solvent to a required concentration to form a stable LiPF6 electrolyte, and this electrolyte is stable even in the absence of ether compounds. Separation occurs, which can well solve the problem that LiPF6 is easily decomposed into LiF and PF5 when the purity is very high. The specific mechanism is as follows.
Application fields[5][6][7]
1. Preparation of low-temperature lithium-ion battery electrolyte
Lithium-ion battery electrolytes are usually composed of carbonate organic solvents (such as ethylene carbonate, dimethyl carbonate, propylene carbonate), electrolyte lithium salts (such as LiPF6, LiBF 4) and other components. Lithium hexafluorophosphate is the most commonly used electrolyte lithium salt. Lithium ion battery electrolyte is prepared using lithium hexafluorophosphate LiPF6 and a mixed solvent. The concentration of LiPF6 is 0.8~1.5mol/L. The mixed solvent includes ethylene carbonate and methyl vinyl carbonate. , methyl acetate and vinylene carbonate, lithium-ion batteries with excellent low-temperature performance and cycle performance can be prepared. The discharge capacity retention rate at -40℃ 0.2C is 54.1%, and the cycle efficiency at 25℃ 1C for 300 cycles reaches 90.6%.
2. Preparation of high-voltage electrolyte for lithium-ion batteries
The high-voltage electrolyte of lithium-ion battery is composed of solvent, electrolyte lithium salt and negative electrode film-forming additive. The organic solvent is a mixed solvent composed of fluoroethylene carbonate, trifluoropropylene carbonate and hydrofluoroether. The electrolyte lithium salt is A mixed lithium salt composed of lithium hexafluorophosphate and lithium bisoxaloborate. Lithium hexafluorophosphate has poor thermal stability and will decompose at higher temperatures to produce PF5. PF5 has high hydrolysis sensitivity to trace amounts of moisture in the electrolyte. Water will produce HF and clog the aluminum foil of the cathode current collector, reducing the cycle performance of the battery. At the same time, due to the increase in charging voltage, the cathode material will undergo structural distortion in the charge and discharge system, and the anions in lithium bisoxalate borate can interact with Al The 3+ reaction forms a very stable network structure passivation film, which can not only prevent the structural change of the cathode material, improve the high-voltage resistance of the cathode system, but also inhibit the corrosion of aluminum foil by HF. Through the synergy of lithium hexafluorophosphate and lithium bisoxaloborate, the conductivity of the electrolyte and the cycle performance of the battery are ensured.
3. Preparation of up-conversion luminescent materials
Using waste lithium battery electrolyte to prepare lithium-doped CaF2-based up-conversion luminescent materials. The organic solvent ethylene carbonate in used lithium-ion batteries is used as a surfactant to control the grain size and morphology of fluoride. The lithium fluoride salt LiPF6 can also be used to form CaF 2 is a fluorine source, and its lithium ions can be used as dopants to improve the rare earth Er3+/Tm3+/Yb3+Up-conversion luminescence performance of doped CaF2.
Main reference materials
[1] Zhuang Quanchao, Wu Shan, Liu Wenyuan, et al. Research on the production process of lithium hexafluorophosphate [J]. Battery Industry, 2005, 10(3): 169-172.
[2] Sha Shunping. Preparation and performance study of lithium hexafluorophosphate as lithium ion battery electrolyte material [D]. Xining: Qinghai Lake Research Institute, Chinese Academy of Sciences, 2005.
[3] Li Lingyun, Zhang Zhiye, Chen Xin. New progress in the preparation process of lithium hexafluorophosphate [D]. , 2005.
[4] Zhang Jiangang, Wang Yao. Analysis of preparation progress and difficulties of electrolyte lithium hexafluorophosphate [J]. Inorganic Salt Industry, 2012, 44(6): 57-60.
[5] Li Shijiang, Hou Hongjun, Yang Huachun, a low-temperature lithium-ion battery electrolyte and lithium-ion battery, CN 201210029082, application date 2012-02-09
[6] Yan Hong, a high-voltage electrolyte for lithium-ion batteries, CN 201310276370, application date 2013-07-02
[7] Zhu Nanwen, Huang Shouqiang, Sun Tonghua, Method for preparing upconversion luminescent materials using waste lithium battery electrolyte, CN 201610161600, application date 2016-03-21
�Up-conversion luminescence properties of Er3+/Tm3+/Yb3+ doped CaF2.
Main reference materials
[1] Zhuang Quanchao, Wu Shan, Liu Wenyuan, et al. Research on the production process of lithium hexafluorophosphate [J]. Battery Industry, 2005, 10(3): 169-172.
[2] Sha Shunping. Preparation and performance study of lithium hexafluorophosphate as lithium ion battery electrolyte material [D]. Xining: Qinghai Lake Research Institute, Chinese Academy of Sciences, 2005.
[3] Li Lingyun, Zhang Zhiye, Chen Xin. New progress in the preparation process of lithium hexafluorophosphate [D]. , 2005.
[4] Zhang Jiangang, Wang Yao. Analysis of preparation progress and difficulties of electrolyte lithium hexafluorophosphate [J]. Inorganic Salt Industry, 2012, 44(6): 57-60.
[5] Li Shijiang, Hou Hongjun, Yang Huachun, a low-temperature lithium-ion battery electrolyte and lithium-ion battery, CN 201210029082, application date 2012-02-09
[6] Yan Hong, a high-voltage electrolyte for lithium-ion batteries, CN 201310276370, application date 2013-07-02
[7] Zhu Nanwen, Huang Shouqiang, Sun Tonghua, Method for preparing upconversion luminescent materials using waste lithium battery electrolyte, CN 201610161600, application date 2016-03-21
