Method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate

Abstract

The invention belongs to the technical field of lithium ion battery materials, and particularly relates to a method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate. The synthesis method comprises the following steps: uniformly mixing lithium tripolyphosphate and phosphorus pentoxide in an ether solvent to obtain a first mixture; introducing a mixed gas of hydrogen fluoride and nitrogen into the first mixture, and carrying out a mixed reaction to obtain a second mixture; and carrying out solid-liquid separation on the second mixture, and carrying out reduced-pressure concentration, recrystallization and reduced-pressure drying on the filtrate after solid-liquid separation to obtain lithium hexafluorophosphate. The lithium tripolyphosphate, the phosphorus pentoxide and the hydrogen fluoride are used as raw materials, the hydrogen fluoride gas is uniformly introduced for reaction, the lithium hexafluorophosphate is directly synthesized in the ether solvent, and the method has the advantages of mild reaction conditions, high reaction efficiency, high product purity and high yield. The problems of more byproducts, high cost and the like in the industrial production process of lithium hexafluorophosphate are solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium ion battery materials, and particularly relates to a method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate. Background Art

[0002] Hexafluorophosphoric acid and its derivatives (such as lithium hexafluorophosphate) have important applications in fields such as lithium ion battery electrolytes. As a new type of mobile and portable power source, lithium ion batteries have higher specific capacity and discharge voltage than traditional lead-acid batteries and alkaline batteries, and cause less environmental pollution. They are widely used in fields such as electric vehicles, energy storage systems, computers, communications, and consumer electronic products. With the expansion of the new energy vehicle market and the large-scale application of mobile devices and energy storage systems, the demand for lithium ion batteries and their key material lithium hexafluorophosphate will be increasing.

[0003] In the prior art, lithium hexafluorophosphate is usually prepared by gas-solid reaction method, hydrogen fluoride solvent method, organic solvent method, ion exchange method, etc. The gas-solid reaction method mainly treats lithium fluoride (LiF) with anhydrous hydrogen fluoride (HF) to form porous LiF, and then introduces phosphorus pentafluoride (PF5) gas to react with the porous LiF to generate lithium hexafluorophosphate. This method is relatively simple to operate, but requires protection by dry inert gas, has high requirements for equipment sealing, and the reaction occurs only on the solid surface, having defects such as low conversion rate and low product purity. The hydrogen fluoride solvent method is the method most easily realized for industrial production. First, phosphorus pentafluoride is generated by reacting anhydrous hydrogen fluoride with a phosphorus source (such as phosphorus pentoxide). Then, phosphorus pentafluoride reacts with an anhydrous hydrogen fluoride solution of lithium fluoride to generate lithium hexafluorophosphate. After purification and other steps, a high-purity product is obtained. This method is carried out in the liquid phase, is easy to control, and is suitable for large-scale production. However, it has high requirements for the corrosion resistance and safety of equipment, high energy consumption, and it is difficult to completely remove the residual HF in the product, which will affect the purity. The organic solvent method uses organic solvents (such as propylene carbonate, acetonitrile, etc.) as reaction media to avoid using highly corrosive hydrogen fluoride, but the reaction efficiency is low, the product purity is difficult to meet high requirements, and the organic solvent may undergo side reactions with the reactants. The ion exchange method performs an ion exchange reaction between hexafluorophosphate (such as NaPF6 or KPF6) and a lithium-containing compound (such as LiCl) in an organic solvent to generate lithium hexafluorophosphate. The reaction is a one-step process and is simple to operate, but the product purity is not high, impurities are easily introduced, and the cost is relatively high. Therefore, developing an efficient, low-cost and environmentally friendly method for preparing lithium hexafluorophosphate has important industrial significance. Summary of the Invention

[0004] Aiming at the disadvantages and deficiencies of the above existing technologies, the purpose of the present invention is to provide a method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate. The method of the present invention uses lithium tripolyphosphate, phosphorus pentoxide and hydrogen fluoride gas to react in one step in an ether solvent to synthesize lithium hexafluorophosphate, and solves the problems of many by-products and high costs in the production process of lithium hexafluorophosphate.

[0005] The purpose of the present invention is achieved through the following technical solutions: A method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate, comprising the following steps: Mix lithium tripolyphosphate and phosphorus pentoxide evenly in an ether solvent to obtain a first mixture; Pass a mixed gas of hydrogen fluoride and nitrogen into the first mixture for mixing reaction to obtain a second mixture; Perform solid-liquid separation on the second mixture, and take the filtrate after solid-liquid separation for concentration under reduced pressure, recrystallization, and drying under reduced pressure to obtain lithium hexafluorophosphate.

[0006] Further, in the above synthesis method, the purity of the lithium tripolyphosphate used is ≥99%, and the purity of the hydrogen fluoride is ≥99%.

[0007] Further, in the above synthesis method, the ether solvent includes at least one of ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, diethylene glycol methyl ethyl ether, and diethylene glycol diethyl ether.

[0008] Further, in the above synthesis method, the mass ratio of the lithium tripolyphosphate to the ether solvent added is 1:5 to 50.

[0009] Further, in the above synthesis method, the molar ratio of the lithium tripolyphosphate to the phosphorus pentoxide is 1:1 to 5; more preferably 1:2 to 4. By using an excessive amount of phosphorus pentoxide, the conversion of lithium tripolyphosphate can be promoted, the residue of lithium phosphate that is difficult to separate in the product can be reduced, and the product purity can be improved; at the same time, it can be used as a dehydrating agent in the reaction process to reduce the occurrence of side reactions and the water content in the product, and improve the product yield and product quality.

[0010] Further, in the above synthesis method, the volume ratio of the hydrogen fluoride to the nitrogen in the mixed gas is 1:3 to 6.

[0011] Further, in the above synthesis method, the molar ratio of the amount of hydrogen fluoride introduced to the lithium tripolyphosphate is 30 to 50:1; more preferably 30 to 40:1.

[0012] Further, in the above synthesis method, at least one of the following conditions is also satisfied: Before mixing the lithium tripolyphosphate and phosphorus pentoxide evenly in the ether solvent, it further includes: performing dehydration pretreatment on the ether solvent; Mix lithium tripolyphosphate and phosphorus pentoxide evenly in an ether solvent and carry out the reaction in an inert gas; The temperature of the mixing reaction is 20~90°C, the time for introducing the mixed gas is 0.5~4h, and the mixing reaction time is 0.5~1h.

[0013] Furthermore, in the above synthesis method, the temperature of the reduced-pressure concentration and the reduced-pressure drying is 40~60°C, and the pressure of the reduced-pressure concentration and the reduced-pressure drying is -0.095~-0.1MPa; the recrystallization solvent is an ether solvent. Recrystallization can reduce the water content in the product lithium hexafluorophosphate and prevent the hydrolysis reaction of lithium hexafluorophosphate during the subsequent drying process; by using the same ether solvent as the reaction solvent for recrystallization, no other organic phases are introduced, and the recrystallization solvent can be used as the reaction solvent repeatedly after simple dehydration pretreatment.

[0014] Furthermore, in the above synthesis method, the purity of the obtained lithium hexafluorophosphate is ≥99.5%, the water content is ≤20ppm, and the acid value is ≤30ppm.

[0015] The principle of the present invention is as follows: The reaction equations involved in synthesizing lithium hexafluorophosphate using lithium tripolyphosphate, phosphorus pentoxide and hydrogen fluoride are as follows: Li5P3O 10 + 30HF + P2O5→ 5LiPF6+ 15H2O; P2O5 + 3H2O→2H3PO4.

[0016] By using an ether that has a high solubility for the product lithium hexafluorophosphate and is inert to the reaction raw materials as the reaction solvent, and reacting lithium tripolyphosphate, phosphorus pentoxide, hydrogen fluoride and a mixed gas of nitrogen under anhydrous conditions, the reaction efficiency can be significantly improved and the occurrence of side reactions can be reduced. After the reaction is completed, the unreacted solid raw materials and insoluble by-products are removed by filtration, and then high-purity lithium hexafluorophosphate can be obtained through reduced-pressure concentration, recrystallization and reduced-pressure drying.

[0017] Compared with the prior art, the beneficial effects of the present invention are: (1) The present invention uses lithium tripolyphosphate, phosphorus pentoxide and hydrogen fluoride as raw materials to directly synthesize lithium hexafluorophosphate in an ether solvent, which has the advantages of simple reaction conditions, high reaction efficiency and high yield, and significantly reduces the production cost.

[0018] (2) The present invention uses anhydrous ether as the reaction solvent, which has better solubility for the reaction products. At the same time, by uniformly introducing hydrogen fluoride gas into the reaction, compared with the existing high-concentration hydrofluoric acid solution reaction system, the corrosion of the reaction equipment can be significantly reduced and the occurrence of side reactions can be reduced. It has the advantages of mild reaction conditions, high reaction efficiency, high product purity and high yield. Specific Embodiments

[0019] The present invention will be further described in detail below in conjunction with embodiments, but the embodiments of the present invention are not limited thereto.

[0020] Embodiment 1 A method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate, the specific steps are as follows: Under the protection of inert gas, 0.1 mol of lithium tripolyphosphate (purity ≥ 99%), 0.1 mol of phosphorus pentoxide and 300 g of dehydrated ethylene glycol dimethyl ether solvent are put into the reaction kettle. Under stirring, a mixed gas of hydrogen fluoride (purity ≥ 99%) and nitrogen with a volume ratio of 1:3 is uniformly introduced through a gas distributor, wherein the molar ratio of hydrogen fluoride to lithium tripolyphosphate is 30:1. The reaction temperature is controlled within the range of 20 - 30 °C, the gas passing time is 2 h, and after the gas passing is completed, the reaction is continued to be kept warm for 0.5 h. After the reaction is complete, the reaction solution is depressurized to -0.095 MPa, and nitrogen is introduced to discharge the gas phase part from the system. The reaction solution is filtered to remove unreacted solid raw materials and insoluble by-products, and the filtrate is concentrated under reduced pressure (-0.1 MPa, 60 °C) to obtain a lithium hexafluorophosphate solution. Then, ethylene glycol dimethyl ether is added for recrystallization, and the crystalline product is dried under reduced pressure (-0.1 MPa, 60 °C) to obtain a solid lithium hexafluorophosphate product.

[0021] The lithium hexafluorophosphate product obtained in this embodiment is detected to have a purity of 99.63% (detected by ion chromatography), a water content of 20 ppm (detected by the Karl Fischer method), and an acid value of 17 ppm (detected by potentiometric titration); the product yield is 80.30% (the actual mass of the obtained lithium hexafluorophosphate product / the theoretical mass of the lithium hexafluorophosphate product obtained by complete reaction of lithium tripolyphosphate).

[0022] Embodiment 2 In this embodiment, the addition amounts of phosphorus pentoxide are adjusted to 0.05 mol, 0.2 mol, 0.3 mol, 0.4 mol and 0.5 mol respectively, and the other conditions are the same as those in Embodiment 1.

[0023] The detection results and yield results of the lithium hexafluorophosphate products obtained in this embodiment under different addition amounts of phosphorus pentoxide are shown in Table 1 below.

[0024] Table 1 Detection Results and Yield Results Dosage of phosphorus pentoxide Purity, % Moisture, ppm Acid value, ppm Yield, % 0.05 mol 99.05 28 15 67.93 0.2 mol 99.87 19 18 89.14 0.3 mol 99.92 15 23 92.25 0.4 mol 99.96 13 27 94.76 0.5 mol 99.95 12 32 95.13

[0025] As can be seen from the results in Table 1, increasing the dosage of phosphorus pentoxide can improve the product purity and yield, and reduce the moisture content in the product. The reason is that excessive phosphorus pentoxide can promote the conversion of lithium tripolyphosphate, reduce the residue of lithium phosphate that is difficult to separate in the product, and the unreacted phosphorus pentoxide can be separated by filtration in the ether solvent to improve the product purity; at the same time, phosphorus pentoxide can be used as a dehydrating agent in the reaction process to reduce the occurrence of side reactions and the moisture content in the product, thereby improving the product yield and product quality. However, too much dosage of phosphorus pentoxide will lead to a high acid value of the product, which may be due to the residual phosphoric acid after reacting with water.

[0026] Example 3 In this example, the molar ratios of the introduced amount of hydrogen fluoride to lithium tripolyphosphate were adjusted to 35:1, 40:1, 45:1, and 50:1 respectively, and the other conditions were the same as those in Example 1.

[0027] The detection results and yield results of the lithium hexafluorophosphate products obtained in this example under different introduced amounts of hydrogen fluoride are shown in Table 2 below.

[0028] Table 2 Detection Results and Yield Results Flow rate of hydrogen fluoride introduced Purity, % Moisture, ppm Acid value, ppm Yield, % 35:1 99.75 21 19 85.22 40:1 99.90 20 26 90.56 45:1 99.89 17 31 91.30 50:1 99.82 16 36 91.32

[0029] As can be seen from the results in Table 2, the yield of the product obtained by increasing the introduced amount of hydrogen fluoride shows an increasing trend, and the product purity first increases and then decreases. The reason may be that excessive hydrogen fluoride will increase the generation of by-products, and at the same time, excessive hydrogen fluoride will lead to a high acid value of the product.

[0030] Example 4 In this example, the reaction temperatures were adjusted to 40 - 50 °C, 60 - 70 °C, and 80 - 90 °C respectively, and the other conditions were the same as those in Example 1.

[0031] The detection results and yield results of the lithium hexafluorophosphate products obtained in this example at different reaction temperatures are shown in Table 3 below.

[0032] Table 3 Detection Results and Yield Results Reaction temperature Purity, % Moisture, ppm Acid value, ppm Yield, % 40~50℃ 99.70 18 20 82.36 60~70℃ 99.62 17 23 82.88 80~90℃ 99.54 15 28 83.51

[0033] As can be seen from the results in Table 3, as the reaction temperature increases, the product yield increases slightly, the product purity decreases slightly; the product moisture decreases slightly, and the product acid value increases slightly.

[0034] Example 5 In this example, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, diethylene glycol methyl ethyl ether, and diethylene glycol diethyl ether were used to replace the ethylene glycol dimethyl ether solvent respectively, and the other conditions were the same as those in Example 1.

[0035] The detection results and yield results of the lithium hexafluorophosphate products obtained in this example under different reaction solvents are shown in Table 4 below.

[0036] Table 4 Detection Results and Yield Results Reaction solvent Purity, % Moisture, ppm Acid value, ppm Yield, % Ethylene glycol diethyl ether 99.69 15 16 81.79 Diethylene glycol dimethyl ether 99.72 19 17 80.93 Diethylene glycol methyl ethyl ether 99.80 18 15 81.73 Diethylene glycol diethyl ether 99.75 17 17 81.64

[0037] It can be seen from the results in Table 4 that the ether solvents with high solubility of the product lithium hexafluorophosphate and inertness to the reaction raw materials adopted in the present invention can all achieve good product yields and product qualities.

[0038] Comparative Example 1 A method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate. Compared with Example 1, an aqueous hydrofluoric acid solution is used as the reaction system, and the specific steps are as follows: Under the protection of an inert gas, 0.1 mol of lithium tripolyphosphate (purity ≥ 99%) and 0.2 mol of phosphoric acid are put into a reaction kettle containing an aqueous hydrofluoric acid solution and stirred and mixed evenly. The molar ratio of hydrogen fluoride contained in the aqueous hydrofluoric acid solution to lithium tripolyphosphate is 30:1. The reaction temperature is controlled to be kept within the range of 20 - 30 °C for 2.5 h. After the reaction is complete, the reaction solution is decompressed to -0.095 MPa, and nitrogen is introduced to discharge the gas phase part from the system. The reaction solution is filtered to remove unreacted solid raw materials and insoluble by-products. The filtrate is concentrated under reduced pressure (-0.1 MPa, 60 °C) to obtain a lithium hexafluorophosphate solution, and then ethylene glycol dimethyl ether is added for recrystallization. The crystalline product is dried under reduced pressure (-0.1 MPa, 60 °C) to obtain a solid lithium hexafluorophosphate product.

[0039] The lithium hexafluorophosphate product obtained in this comparative example has a detected purity of 99.41%, a water content of 27 ppm, and an acid value of 54 ppm; the product yield is 76.26%.

[0040] It can be seen from the comparison results between this comparative example and Example 1 that the method for synthesizing lithium hexafluorophosphate by one-step reaction of lithium tripolyphosphate, phosphorus pentoxide, and hydrogen fluoride gas in an ether solvent in the present invention can significantly improve the reaction efficiency and reduce the occurrence of side reactions compared with the existing method of reacting in an aqueous hydrofluoric acid solution, thereby improving the product yield and purity, and significantly reducing the acid value and water residue of the product.

[0041] The above embodiments are the preferred embodiments of the present invention, but the embodiments of the present invention are not limited by the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and are all included in the protection scope of the present invention.

Claims

1. A method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate, characterized in that, It includes the following steps: Mix lithium tripolyphosphate and phosphorus pentoxide evenly in an ether solvent to obtain a first mixture; Introduce a mixed gas of hydrogen fluoride and nitrogen into the first mixture for mixing reaction to obtain a second mixture; Perform solid-liquid separation on the second mixture, take the filtrate after solid-liquid separation, concentrate it under reduced pressure, recrystallize it, and dry it under reduced pressure to obtain lithium hexafluorophosphate.

2. The method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate according to claim 1, characterized in that, The purity of the lithium tripolyphosphate is ≥99%, and the purity of the hydrogen fluoride is ≥99%.

3. A method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate according to claim 1, characterized in that, The ether solvent includes at least one of ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, diethylene glycol methyl ethyl ether, and diethylene glycol diethyl ether; the mass ratio of the lithium tripolyphosphate to the ether solvent is 1:5 to 50.

4. A method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate according to claim 1, characterized in that, The molar ratio of the lithium tripolyphosphate to the phosphorus pentoxide is 1:1 to 5.

5. A method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate according to claim 1, characterized in that, The molar ratio of the lithium tripolyphosphate to the phosphorus pentoxide is 1:2 to 4.

6. The method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate according to claim 1, characterized in that, The volume ratio of the hydrogen fluoride to the nitrogen in the mixed gas is 1:3 to 6.

7. A method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate according to claim 1, characterized in that, The molar ratio of the introduced amount of hydrogen fluoride to the lithium tripolyphosphate is 30 to 50:

1.

8. A method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate according to claim 1, characterized in that, It also satisfies at least one of the following conditions: Before mixing the lithium tripolyphosphate and phosphorus pentoxide evenly in the ether solvent, it further includes: performing dehydration pretreatment on the ether solvent; Mixing the lithium tripolyphosphate and phosphorus pentoxide evenly in an inert gas; The temperature of the mixing reaction is 20 to 90 °C, the time for introducing the mixed gas is 0.5 to 4 h, and the mixing reaction time is 0.5 to 1 h.

9. A method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate according to claim 1, characterized in that, The temperature of the concentration under reduced pressure and the drying under reduced pressure is 40 to 60 °C, the pressure of the concentration under reduced pressure and the drying under reduced pressure is -0.095 to -0.1 MPa; the recrystallization solvent is an ether solvent.

10. A method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate according to claim 1, characterized in that, The purity of the obtained lithium hexafluorophosphate is ≥99.5%, the water content is ≤20 ppm, and the acid value is ≤30 ppm.