Method for preparing lithium hexafluorophosphate from lithium phosphate and hydrogen fluoride

Abstract

The invention belongs to the technical field of lithium ion battery materials, and particularly relates to a method for preparing lithium hexafluorophosphate from lithium phosphate and hydrogen fluoride, which comprises the following preparation steps: dissolving lithium phosphate in a phosphoric acid aqueous solution to form a mixed solution, and then introducing a mixed gas formed by hydrogen fluoride gas and nitrogen for reaction to obtain lithium hexafluorophosphate. Controlling the feeding molar ratio of the lithium phosphate to the phosphoric acid to the hydrogen fluoride gas to be (1.5-3): 2: (18-20), and after the reaction is completed, decompressing, degassing and concentrating to obtain a mixed crude product of lithium hexafluorophosphate and lithium phosphate; and dissolving the obtained mixed crude product in acetone, carrying out solid-liquid separation, and drying and crystallizing the obtained liquid phase to obtain the high-purity lithium hexafluorophosphate product. According to the method disclosed by the invention, the phosphoric acid aqueous solution is used as a reaction solvent system, meanwhile, the hydrogen fluoride gas is continuously introduced for reaction, and excessive lithium phosphate is adopted for reaction, so that corrosion to reaction equipment can be reduced, side reactions can be reduced, the production safety is improved, and the product yield and quality are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion battery materials, and particularly relates to a method for preparing lithium hexafluorophosphate by using lithium phosphate and hydrogen fluoride. Background Art

[0002] As one of the important electrolyte salts for lithium-ion batteries, lithium hexafluorophosphate is widely used in fields such as new energy vehicles and portable electronic products. At present, the main preparation methods of lithium hexafluorophosphate include gas-solid reaction method, ion exchange method, and solvent method, etc.

[0003] The gas-solid reaction method has problems such as low product purity and low yield, and it is difficult to achieve large-scale production; the ion exchange method has problems such as difficulty in obtaining metal hexafluorophosphate (ammonium salt), high difficulty in product purification, and difficult recovery and reuse of related cations, resulting in high costs. The solvent method is the main way for industrial production of lithium hexafluorophosphate at present, but the existing solvent method also has some deficiencies. When using anhydrous hydrogen fluoride or high-concentration hydrofluoric acid aqueous solution as the solvent, due to the corrosiveness and toxicity of anhydrous hydrogen fluoride and high-concentration hydrofluoric acid aqueous solution, high requirements are imposed on equipment, there are safety hazards, and it is easy to generate complexes of lithium hexafluorophosphate and hydrogen fluoride as well as promote the formation of by-products lithium fluoride, lithium hydrogen fluoride, and hexafluorophosphoric acid, affecting product quality and yield. When using organic solvents, due to the low solubility of lithium phosphate in organic solvents, a suspension is formed, resulting in low reaction efficiency, and the unreacted intermediate lithium fluoride will be carried into the product, resulting in a decrease in product purity. Therefore, the existing preparation methods of lithium hexafluorophosphate generally have problems such as high raw material costs, complex process routes, harsh reaction conditions, poor safety and environmental protection, low product purity and yield, and the use of toxic solvents, and there is an urgent need to develop a new method with simple process and green environmental protection. Summary of the Invention

[0004] Aiming at the above-mentioned shortcomings and deficiencies of the prior art, the purpose of the present invention is to provide a method for preparing lithium hexafluorophosphate by using lithium phosphate and hydrogen fluoride.

[0005] The purpose of the present invention is achieved by the following technical solutions: A method for preparing lithium hexafluorophosphate by using lithium phosphate and hydrogen fluoride, comprising the following preparation steps: (1) Dissolve lithium phosphate in phosphoric acid aqueous solution to form a mixed solution; (2) Introduce a mixed gas of hydrogen fluoride gas and nitrogen into the mixed solution in step (1) for reaction, control the molar ratio of the feed of lithium phosphate, phosphoric acid, and hydrogen fluoride gas to be (1.5~3):2:(18~20), and after the reaction is completed, carry out decompression degassing and concentration to obtain a mixed crude product of lithium hexafluorophosphate and lithium phosphate; (3) Dissolve the mixed crude product obtained in step (2) in acetone, perform solid-liquid separation, and dry and crystallize the obtained liquid phase to obtain a high-purity lithium hexafluorophosphate product.

[0006] Further, the mass concentration of the phosphoric acid aqueous solution in step (1) is 5% - 25%; the conditions for forming the mixed solution are: stirring and mixing at a temperature of 30 - 60 °C for 1 - 3 h.

[0007] Further, the volume ratio of the hydrogen fluoride gas to nitrogen gas mixed in step (2) is 1:3 - 6.

[0008] Further, the mixed gas in step (2) is introduced in a uniform manner using a gas distributor.

[0009] Further, the temperature of the reaction in step (2) is 0 - 30 °C, the time for introducing the mixed gas is 1.5 - 4 h, and after the gas introduction is completed, keep the temperature for reaction for 10 - 30 min.

[0010] Further, the vacuum degassing and concentration in step (2) means concentrating to a water content of less than 1 wt% under the conditions of a temperature of 40 - 60 °C and a pressure of -0.05 - -0.1 MPa.

[0011] Further, the addition amount of the acetone solvent in step (3) is 2 - 10 times the mass of the mixed crude product.

[0012] Further, the drying and crystallization in step (3) means vacuum drying and crystallization under the conditions of a temperature of 60 - 80 °C and a pressure of -0.05 - -0.1 MPa.

[0013] Further, the solid phase after the solid-liquid separation in step (3) is lithium phosphate, and the lithium phosphate is recycled as a reaction raw material.

[0014] Further, the purity of the high-purity lithium hexafluorophosphate product in step (3) > 99.9%, and the yield > 93%.

[0015] The principle of the present invention is: The reaction equation involved in preparing lithium hexafluorophosphate from lithium phosphate and hydrogen fluoride in a phosphoric acid solution is as follows: Li 3 PO 4 + 2H 3 PO 4 + 18HF = 3LiPF 6 + 12H 2 O.

[0016] The present invention uses lithium phosphate as a raw material, which has the advantages of good stability and low requirements for equipment. However, the solubility of lithium phosphate in water and organic solvents is relatively low, resulting in low reaction efficiency. Although the existing use of a high-concentration hydrofluoric acid aqueous solution as a reaction solvent can dissolve lithium phosphate better, it has strong corrosion to equipment and is prone to side reactions (under the reaction conditions of a high-concentration hydrofluoric acid solution, it will promote the side reactions of hydrofluoric acid and lithium phosphate to generate lithium fluoride and hexafluorophosphoric acid, as well as the side reaction of lithium fluoride and hydrofluoric acid to generate lithium bifluoride, resulting in a decrease in the purity and yield of the product). The present invention innovatively uses a simple phosphoric acid aqueous solution as the reaction solvent system. On the one hand, it can dissolve the lithium phosphate raw material better, and on the other hand, it is used to supplement the phosphorus source for the reaction. Therefore, the reaction efficiency can be significantly improved. However, due to the presence of water, a large amount of hydrogen fluoride contacting water to form a high-concentration hydrofluoric acid aqueous solution will also cause problems of equipment corrosion. Therefore, the present application also adopts the method of continuously introducing a mixed gas of hydrogen fluoride gas and nitrogen into the reaction. On the one hand, the gas-liquid reaction efficiency is high and the time is short; on the other hand, by controlling the gas flow rate, it can effectively avoid a large amount of reaction raw material hydrogen fluoride contacting the phosphoric acid aqueous solution at one time to form a high-concentration hydrofluoric acid solution, significantly reducing the concentration of hydrofluoric acid in the solution, reducing the corrosion of the reaction equipment and reducing the occurrence of side reactions, thereby improving the reaction efficiency, product quality and yield. Finally, by using an excessive amount of lithium phosphate, on the one hand, it can better promote the reaction conversion of phosphoric acid and hydrogen fluoride and reduce the residues of phosphoric acid and hydrogen fluoride in the product; on the other hand, under the conditions of using an excessive amount of lithium phosphate and a lower acid concentration, the generation of by-product hexafluorophosphoric acid can be effectively inhibited, thereby significantly reducing the acid value of the product and increasing the product purity. The main components of the crude product obtained by the reaction are lithium hexafluorophosphate and excessive lithium phosphate, and there are no complex impurity components and acidic components that are difficult to separate. By further adding acetone as a good solvent for lithium hexafluorophosphate and a poor solvent for lithium phosphate, lithium phosphate is precipitated, and the remaining solution is directly dried and crystallized to obtain high-purity lithium hexafluorophosphate. The separated lithium phosphate is recycled as a reaction raw material. Through the combined action of the above technical solutions, the lithium hexafluorophosphate product obtained by the present invention can achieve a lower acid value and higher purity without using a poor solvent for cooling crystallization to purify and remove impurities and acids, and significantly improve the product yield.

[0017] Compared with the prior art, the beneficial effects of the present invention are: (1) The present invention uses a phosphoric acid aqueous solution as the reaction solvent system without using toxic and harmful organic solvents, which has the advantage of environmental friendliness; at the same time, it adopts the method of uniformly introducing hydrogen fluoride gas into the reaction. Compared with the existing high-concentration hydrofluoric acid solution reaction system, it can significantly reduce the corrosion of the reaction equipment and the occurrence of side reactions, and has the advantages of mild reaction conditions, high reaction efficiency, high product purity and high yield.

[0018] (2) By further using excessive lithium phosphate for the reaction, the present invention can effectively promote the reaction conversion of phosphoric acid and hydrogen fluoride, reduce the residues of phosphoric acid and hydrogen fluoride in the product, and effectively inhibit the formation of by-product hexafluorophosphoric acid. The obtained crude product can be dissolved in acetone for impurity removal and then obtained high-purity lithium hexafluorophosphate product through one-step liquid-phase drying and crystallization, without further using the process of crystallization and impurity removal with a poor solvent, which simplifies the preparation process and improves the product yield. Detailed 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] The requirements for raw material reagents used in the following embodiments are as follows: The purity of lithium phosphate ≥ 99.9%; the purity of hydrogen fluoride gas ≥ 99.9%; the purity of nitrogen ≥ 99.5%; the purity of acetone ≥ 99%.

[0021] Example 1 (1) 58 g of lithium phosphate was mixed with 245 g of phosphoric acid aqueous solution with a concentration of 20 wt% to form a mixture, and the temperature was controlled at 45 °C and stirred for 2 h to obtain a mixed solution.

[0022] (2) At room temperature, 4.5 mol of hydrogen fluoride gas and nitrogen were formed into a mixed gas at a volume ratio of 1:5, and the mixed gas was introduced into the mixed solution in step (1) through a gas distributor at a flow rate of 3 L / min for stirring reaction. The ventilation time was 200 min, the temperature of the reaction solution was controlled at 20 °C, and after the ventilation was completed, the reaction was continued to be kept warm for 10 min until the reaction was complete. The reaction mixture was concentrated under reduced pressure at a temperature of 50 °C and a pressure of -0.096 MPa until the detected moisture content was below 1 wt% to obtain a mixed crude product of lithium hexafluorophosphate and lithium phosphate.

[0023] (3) The mixed crude product obtained in step (2) was dissolved in 1000 ml of acetone solvent and stirred for 30 min, then left to stand for precipitation and filtered. The obtained liquid phase was vacuum dried and crystallized at -0.095 MPa and 75 °C to obtain 109.0 g of lithium hexafluorophosphate finished product, and the product yield was 95.6% (calculated based on the theoretical reaction amount when phosphoric acid reacted completely, the same below). The obtained solid phase was unreacted lithium phosphate, and after detection, the purity was 99.92%, which could be recycled as a reaction raw material.

[0024] The lithium hexafluorophosphate finished product obtained in this example was detected to have a purity of 99.94% (detected by ion chromatography), a moisture content of 16 ppm (detected by Karl Fischer method), and an acid value of 11 ppm (detected by potentiometric titration method). It can be directly used for the preparation of electrolyte salts for lithium-ion batteries.

[0025] Example 2 (1) Mix 43.5 g of lithium phosphate with 490 g of a 10 wt% phosphoric acid aqueous solution to form a mixture, control the temperature at 60 °C and stir for 1 h to obtain a mixed solution.

[0026] (2) At room temperature, form a mixed gas of 5 mol of hydrogen fluoride gas and nitrogen with a volume ratio of 1:4, and introduce it into the mixed solution in step (1) through a gas distributor at a flow rate of 5 L / min for stirring reaction. The gas injection time is 110 min, control the temperature of the reaction solution at 10 °C, and continue the heat preservation reaction for 10 min after the gas injection ends until the reaction is complete. Concentrate the reaction mixture under reduced pressure at a temperature of 40 °C and a pressure of -0.098 MPa until the detected water content is below 1 wt% to obtain a crude product mixture of lithium hexafluorophosphate and lithium phosphate.

[0027] (3) Dissolve the mixed crude product obtained in step (2) in 800 ml of acetone solvent, stir for 30 min, let it stand for precipitation and then filter. The obtained liquid phase is vacuum dried and crystallized at -0.095 MPa and 70 °C to obtain 107.4 g of lithium hexafluorophosphate finished product, and the product yield is 94.2%. The obtained solid phase is unreacted lithium phosphate, and its detected purity is 99.91%, which can be recycled as a reaction raw material.

[0028] The lithium hexafluorophosphate finished product obtained in this example has a detected purity of 99.93%, a water content of 12 ppm, and an acid value of 18 ppm. It can be directly used for the preparation of electrolyte salts for lithium-ion batteries.

[0029] Example 3 (1) Mix 87 g of lithium phosphate with 490 g of a 10 wt% phosphoric acid aqueous solution to form a mixture, control the temperature at 60 °C and stir for 2 h to obtain a mixed solution.

[0030] (2) At room temperature, form a mixed gas of 4.75 mol of hydrogen fluoride gas and nitrogen with a volume ratio of 1:3, and introduce it into the mixed solution in step (1) through a gas distributor at a flow rate of 2 L / min for stirring reaction. The gas injection time is 210 min, control the temperature of the reaction solution at 5 °C, and continue the heat preservation reaction for 30 min after the gas injection ends until the reaction is complete. Concentrate the reaction mixture under reduced pressure at a temperature of 60 °C and a pressure of -0.095 MPa until the detected water content is below 1 wt% to obtain a crude product mixture of lithium hexafluorophosphate and lithium phosphate.

[0031] (3) Dissolve the mixed crude product obtained in step (2) in 1500 ml of acetone solvent, stir for 30 min, let it stand for precipitation and then filter. The obtained liquid phase is vacuum dried and crystallized at -0.095 MPa and 60 °C to obtain 109.7 g of lithium hexafluorophosphate finished product, and the product yield is 96.2%. The obtained solid phase is unreacted lithium phosphate, and its detected purity is 99.92%, which can be recycled as a reaction raw material.

[0032] The lithium hexafluorophosphate product obtained in this example was tested to have a purity of 99.95%, a water content of 15 ppm, and an acid value of 10 ppm. It can be directly used for the preparation of electrolyte salts for lithium-ion batteries.

[0033] Example 4 (1) 58 g of lithium phosphate was mixed with 980 g of a phosphoric acid aqueous solution with a concentration of 5 wt% to form a mixture, and the temperature was controlled at 30 °C and stirred for 1 h to obtain a mixed solution.

[0034] (2) At room temperature, 4.5 mol of hydrogen fluoride gas and nitrogen were formed into a mixed gas at a volume ratio of 1:6, and the mixed gas was introduced into the mixed solution in step (1) through a gas distributor at a flow rate of 5 L / min for stirring reaction. The gas introduction time was 140 min, the temperature of the reaction solution was controlled at 30 °C, and after the gas introduction was completed, the reaction was continued to be kept warm for 10 min until the reaction was complete. The reaction mixture was concentrated by vacuum degassing at a temperature of 50 °C and a pressure of -0.095 MPa until the detected water content was below 1 wt% to obtain a crude mixture of lithium hexafluorophosphate and lithium phosphate.

[0035] (3) The crude mixture obtained in step (2) was dissolved in 1200 ml of acetone solvent, stirred for 30 min, allowed to stand for precipitation and then filtered. The obtained liquid phase was vacuum dried and crystallized at -0.095 MPa and 80 °C to obtain 106.5 g of lithium hexafluorophosphate product, and the product yield was 93.4%. The obtained solid phase was unreacted lithium phosphate, and its detected purity was 99.95%, which could be recycled as a reaction raw material.

[0036] The lithium hexafluorophosphate product obtained in this example was tested to have a purity of 99.91%, a water content of 15 ppm, and an acid value of 19 ppm. It can be directly used for the preparation of electrolyte salts for lithium-ion batteries.

[0037] Example 5 (1) 46.5 g of lithium phosphate was mixed with 196 g of a phosphoric acid aqueous solution with a concentration of 25 wt% to form a mixture, and the temperature was controlled at 60 °C and stirred for 1 h to obtain a mixed solution.

[0038] (2) At room temperature, 4.5 mol of hydrogen fluoride gas and nitrogen were formed into a mixed gas at a volume ratio of 1:4, and the mixed gas was introduced into the mixed solution in step (1) through a gas distributor at a flow rate of 4 L / min for stirring reaction. The gas introduction time was 125 min, the temperature of the reaction solution was controlled at 15 °C, and after the gas introduction was completed, the reaction was continued to be kept warm for 25 min until the reaction was complete. The reaction mixture was concentrated by vacuum degassing at a temperature of 45 °C and a pressure of -0.096 MPa until the detected water content was below 1 wt% to obtain a crude mixture of lithium hexafluorophosphate and lithium phosphate.

[0039] (3) Dissolve the crude mixture obtained in step (2) in 900 ml of acetone solvent, stir for 30 min, let it stand for precipitation and then filter. The obtained liquid phase is vacuum dried and crystallized at -0.095 MPa and 75 °C to obtain 108.1 g of lithium hexafluorophosphate product, and the product yield is 94.8%. The obtained solid phase is unreacted lithium phosphate, and its detected purity is 99.93%, which can be recycled as a reaction raw material.

[0040] The lithium hexafluorophosphate product obtained in this example has a detected purity of 99.92%, a water content of 13 ppm, and an acid value of 20 ppm. It can be directly used for the preparation of electrolyte salts for lithium-ion batteries.

[0041] Comparative Example 1 A method for preparing lithium hexafluorophosphate using lithium phosphate and hydrogen fluoride. Compared with Example 1, an aqueous hydrofluoric acid solution is used as the reaction system, and the specific steps are as follows: (1) Add 58 g of lithium phosphate and 49 g of phosphoric acid to 286 g of an aqueous hydrofluoric acid solution with a concentration of 31.5 wt% to form a mixture. Control the temperature of the reaction solution at 20 °C and stir for 210 min until the reaction is complete. Decompress and concentrate the reaction mixture at 50 °C and -0.096 MPa until the detected water content is below 1 wt% to obtain a crude mixture.

[0042] (2) Dissolve the crude mixture obtained in step (1) in 1000 ml of acetone solvent, stir for 30 min, let it stand for precipitation and then filter. The obtained liquid phase is vacuum dried and crystallized at -0.095 MPa and 75 °C to obtain 100.8 g of lithium hexafluorophosphate product, and the product yield is 88.4%. The obtained solid phase is mainly unreacted lithium phosphate, and its detected purity is 99.18%. The main impurity is lithium fluoride, and it cannot meet the purity requirements for use as a reaction raw material.

[0043] The lithium hexafluorophosphate product obtained in this comparative example has a detected purity of 99.73%, the main impurity is hexafluorophosphoric acid (detected by ion chromatography), a water content of 15 ppm, and an acid value of 52 ppm. Its purity is relatively low and the acid value is relatively high, so it cannot be directly used for the preparation of electrolyte salts for lithium-ion batteries.

[0044] From the comparison results with Example 1, it can be seen that the method of continuously introducing hydrogen fluoride gas into the reaction in the present invention can reduce the occurrence of side reactions (the main by-products include lithium fluoride in the solid phase and hexafluorophosphoric acid in the liquid phase) compared with directly reacting in an aqueous hydrofluoric acid solution, and improve the product purity and product yield.

[0045] Comparative Example 2 A method for preparing lithium hexafluorophosphate using lithium phosphate and hydrogen fluoride. Compared with Example 1, an excessive amount of lithium phosphate was not added to the reaction, and the specific steps are as follows: (1) Mix 29 g of lithium phosphate with 245 g of phosphoric acid aqueous solution with a concentration of 20 wt% to form a mixture, control the temperature at 45 °C and stir for 2 h to obtain a mixed solution.

[0046] (2) Under room temperature conditions, form a mixed gas of 4.5 mol of hydrogen fluoride gas and nitrogen with a volume ratio of 1:5, and introduce it into the mixed solution in step (1) through a gas distributor at a flow rate of 3 L / min for stirring reaction. The ventilation time is 200 min, control the temperature of the reaction solution at 20 °C, and continue the heat preservation reaction for 10 min after the ventilation ends until the reaction is complete. Decompress and concentrate the reaction mixture at a temperature of 50 °C and a pressure of -0.096 MPa until the detected moisture content is below 1 wt% to obtain a mixed crude product.

[0047] (3) Dissolve the mixed crude product obtained in step (2) in 1000 ml of acetone solvent, stir for 30 min, let it stand for precipitation and then filter. The obtained liquid phase is vacuum dried and crystallized at -0.095 MPa and 75 °C to obtain 98.5 g of lithium hexafluorophosphate finished product, and the product yield is 86.4%. The obtained solid phase is mainly unreacted lithium phosphate, and its detected purity is 99.85%, and the main impurity is phosphoric acid.

[0048] The purity of the lithium hexafluorophosphate finished product obtained in this comparative example is detected to be 99.81%, the main impurities are hexafluorophosphoric acid and phosphoric acid, the moisture is 19 ppm, and the acid value is 96 ppm. Its purity is relatively low and the acid value is relatively high, so it cannot be directly used for the preparation of lithium-ion battery electrolyte salts.

[0049] From the comparison results with Example 1, it can be seen that using excessive lithium phosphate in the present invention can better promote the reaction conversion of phosphoric acid and hydrogen fluoride, reduce the residual phosphoric acid in the product, and effectively inhibit the generation of by-product hexafluorophosphoric acid. Without adding a poor solvent for the cooling crystallization process, it can still achieve the effects of reducing the acid value and increasing the product purity, and significantly improve the product yield.

[0050] Comparative Example 3 A method for preparing lithium hexafluorophosphate using lithium phosphate and hydrogen fluoride. Compared with Example 1, use conventional good solvents for lithium hexafluorophosphate: dimethyl carbonate, ethylene carbonate, acetonitrile, ethylene glycol dimethyl ether to replace acetone solvent to dissolve the mixed crude product, and the rest is the same.

[0051] The yield, purity, moisture, and acid value results of the lithium hexafluorophosphate finished products obtained by dissolving, filtering, and drying and crystallizing with different good solvents in this comparative example are shown in Table 1 below.

[0052] Table 1

[0053] As can be seen from the results in Table 1, the purity of the lithium hexafluorophosphate product obtained by dissolving and crystallizing with a conventional good solvent for lithium hexafluorophosphate is relatively low and cannot be directly used for the preparation of the electrolyte salt for lithium-ion batteries. The reason is that a small amount of unreacted lithium phosphate raw material is mixed and dissolved in the above-mentioned good solvent and forms an impurity phase during the subsequent drying and crystallization process.

[0054] The above embodiments are the preferred embodiments of the present invention, but the embodiments of the present invention are not limited to 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 preparing lithium hexafluorophosphate using lithium phosphate and hydrogen fluoride, characterized in that: The method comprises the following preparation steps: (1) dissolving lithium phosphate in a phosphoric acid aqueous solution to form a mixed solution; (2) introducing a mixed gas formed by hydrogen fluoride gas and nitrogen gas into the mixed solution of step (1) for reaction, controlling the molar ratio of lithium phosphate, phosphoric acid and hydrogen fluoride gas to be (1.5-3):2:(18-20), and degassing and concentrating under reduced pressure after the reaction is completed to obtain a crude mixed product of lithium hexafluorophosphate and lithium phosphate; (3) The mixed crude product obtained in step (2) is dissolved in acetone, and the solid-liquid separation is performed. The obtained liquid phase is dried and crystallized to obtain a high-purity lithium hexafluorophosphate product.

2. The method for preparing lithium hexafluorophosphate by using lithium phosphate and hydrogen fluoride according to claim 1, characterized in that: The mass concentration of the phosphoric acid aqueous solution in step (1) is 5% to 25%; the condition for forming the mixed solution is: stirring and mixing at a temperature of 30 to 60° C. for 1 to 3 hours.

3. The method for preparing lithium hexafluorophosphate by using lithium phosphate and hydrogen fluoride according to claim 1, characterized in that: The volume ratio of the mixture of hydrogen fluoride gas and nitrogen in step (2) is 1:3-6.

4. The method for preparing lithium hexafluorophosphate by using lithium phosphate and hydrogen fluoride according to claim 1, characterized in that: The mixed gas in step (2) is introduced evenly using a gas distributor.

5. The method for preparing lithium hexafluorophosphate by using lithium phosphate and hydrogen fluoride according to claim 1, characterized in that: The reaction temperature in step (2) is 0-30°C, the mixed gas is introduced for 1.5-4 hours, and the reaction is continued at a temperature of 10-30 minutes after the introduction of the mixed gas.

6. The method for preparing lithium hexafluorophosphate by using lithium phosphate and hydrogen fluoride according to claim 1, characterized in that: The reduced pressure degassing concentration in step (2) refers to concentration to a water content of less than 1 wt % under the conditions of a temperature of 40 to 60° C. and a pressure of -0.05 to -0.1 MPa.

7. The method for preparing lithium hexafluorophosphate by using lithium phosphate and hydrogen fluoride according to claim 1, characterized in that: The amount of acetone solvent added in step (3) is 2 to 10 times the quality of the mixed crude product.

8. The method for preparing lithium hexafluorophosphate by using lithium phosphate and hydrogen fluoride according to claim 1, characterized in that: The drying and crystallization in step (3) refers to drying and crystallizing under reduced pressure at a temperature of 60 to 80°C and a pressure of -0.05 to -0.1 MPa.

9. The method for preparing lithium hexafluorophosphate by using lithium phosphate and hydrogen fluoride according to claim 1, characterized in that: The solid phase after the solid-liquid separation in step (3) is lithium phosphate, and the lithium phosphate is recycled as a reaction raw material.

10. The method for preparing lithium hexafluorophosphate by using lithium phosphate and hydrogen fluoride according to claim 1, characterized in that: The purity of the high-purity lithium hexafluorophosphate product in step (3) is greater than 99.9%, and the yield is greater than 93%.

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