Preparation method of lithium difluorophosphate
Through the slow hydrolysis reaction and liquid separation treatment of lithium hexafluorophosphate in the presence of organic alkali, the problems of low purity and low yield in the preparation of lithium difluorophosphate were solved, and high purity and high yield preparation of lithium difluorophosphate was achieved.
Patent Information
- Application Number
- CN202510625472.4
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-05-15
- Publication Date
- 2025-07-11
AI Technical Summary
In the prior art, the preparation method of lithium difluorophosphate has problems such as difficult to obtain raw materials, complex operation, low product purity and low yield.
Lithium hexafluorophosphate is used to carry out a slow and controllable hydrolysis reaction in the presence of an organic alkali, and liquid separation is performed using an organic solvent system. The acid produced by the complex hydrolysis of the organic alkali is converted into lithium difluorophosphate, and impurities are separated by precision filtration and crystallization to obtain a high-purity product.
The high purity and high yield preparation of lithium difluorophosphate is achieved, the operation process is simplified, the equipment and environmental pressure is reduced, and the purity and yield of the product is improved.
Abstract
Description
Technical Field
[0001] The present invention belongs to the technical field of new lithium battery materials, and particularly relates to a preparation method of lithium difluorophosphate. Background Art
[0002] With the rapid development of mobile intelligent devices such as smart phones and tablet computers, and the increasing improvement of electric vehicle technology, the market demand for small and large lithium batteries has increased rapidly. It is expected that in the next few years, lithium batteries will maintain a growth trend. Since most lithium batteries are secondary batteries, there are technical difficulties such as poor stability, poor low-temperature resistance, and poor cycling characteristics.
[0003] As a new type of additive for lithium-ion battery electrolytes, lithium difluorophosphate can improve the high and low temperature performance, cycling stability and rate performance of the battery. With the increasing demand for lithium-ion batteries, it is of great significance to develop a process that can obtain lithium difluorophosphate with high quality and low cost.
[0004] In the prior art, the main preparation methods of lithium difluorophosphate are as follows: 1. Reacting Li2CO3 with LiPF6 or P2O5 to prepare lithium difluorophosphate, the disadvantage is that it will produce lithium fluoride impurities that are difficult to separate; 2. Using P4O 10 and LiF to carry out a solid-solid reaction to synthesize lithium difluorophosphate, the disadvantage is that the reaction is not easy to control, there are many by-products, and the yield is low; 3. Using Li3PO4 and PF5 to react to synthesize lithium difluorophosphate, the disadvantage is that it has high requirements for equipment, there are many by-products and they are not easy to separate; 4. Using organotin fluoride to displace dichlorophosphoric acid lithium to prepare lithium difluorophosphate, the disadvantage is that the raw material trimethyltin fluoride is not easy to obtain; 5. Using LiPF6 to directly hydrolyze to prepare lithium difluorophosphate, the disadvantage is that this reaction is not easy to control, it will produce impurities such as Li2PO3F, HF, and HPO2F2 that affect the battery performance, and it is not easy to purify, and it is difficult to obtain high-purity products.
[0005] Based on the deficiencies in the prior art, the present invention adjusts the synthesis route, raw material types, dosages, process parameters, etc. of lithium difluorophosphate, and provides a new preparation method of lithium difluorophosphate, aiming to improve the yield and purity of lithium difluorophosphate and meet its industrial requirements. Summary of the Invention
[0006] The purpose of the present invention is to provide a synthesis method of lithium difluorophosphate with easily available raw materials, convenient operation and high product purity in view of the deficiencies in the prior art, and improve the purity and yield of the prepared lithium difluorophosphate.
[0007] To achieve the above purpose, the present invention adopts the following technical solutions: A preparation method of lithium difluorophosphate, comprising the following steps: 1) Place lithium hexafluorophosphate and organic solvent I in a closed reactor, stir and mix evenly, then add organic base and water to the reactor, and stir and react for a period of time; 2) Let the reaction product of step 1) stand for liquid separation, discard the lower aqueous phase, retain the upper organic phase, filter the organic phase to remove insoluble particles, and obtain a filtrate; 3) Place the filtrate of step 2) in a crystallization kettle, carry out vacuum distillation to remove most of the solvent, then add organic solvent II, stir for crystallization, filter to obtain crystals, and dry to obtain the product.
[0008] The reaction equation of the present invention is: LiPF6 + 2H2O — 2LiPO2F2 + 4HF.
[0009] Preferably, the organic solvent I in step 1) is one of ethyl acetate, acetonitrile, ethyl formate, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, propylene glycol dimethyl ether, or a mixed solvent of two or more in any proportion; in step 1), the organic solvent I can make the reaction rate faster and the raw material conversion rate higher.
[0010] Preferably, in step 1), the mass ratio of the organic solvent I to lithium hexafluorophosphate is (9~16):1.
[0011] Preferably, the organic base in step 1) is at least one of propylamine, ethylenediamine, propanediamine, diethylamine, triethylamine; in the present invention, the organic base is mainly amines, which can complex the acid generated in the reaction and improve the reaction yield.
[0012] Preferably, in step 1), the molar ratio of the organic base to lithium hexafluorophosphate is (1~5):1, and more preferably (1~2.5):1.
[0013] Preferably, in step 1), the mass ratio of water to lithium hexafluorophosphate is (0.5~1):1.
[0014] Preferably, in step 1), the reaction temperature is 40 - 80°C; the reaction time is 1 - 4 hours. The reaction time set in the present invention is to ensure sufficient reaction progress.
[0015] Preferably, in step 2), when filtering, a nitrogen positive pressure filter equipped with a filter membrane is used to filter the organic phase.
[0016] More preferably, the pore diameter of the filter membrane in step 2) is not greater than 0.5 μm.
[0017] Preferably, in step 3), the vacuum distillation temperature is 60 - 90°C; the distillation pressure is -0.3~ -0.1 MPa; the distillation time is 1 - 2 hours.
[0018] Preferably, in step 3), the organic solvent II is at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, dichloromethane, chloroform, and carbon tetrachloride.
[0019] Preferably, in step 3), when adding the organic solvent II, the mass ratio of the organic solvent II to lithium hexafluorophosphate is (1.5 - 3):1.
[0020] Preferably, in step 3), the crystallization temperature is 0 - 6°C and the crystallization time is 1 - 2 hours.
[0021] More preferably, the specific steps of step 3) are as follows: place the filtrate obtained in step 2) in a crystallization kettle, carry out vacuum distillation at a temperature of 60 - 90°C and a pressure of -0.3 to -0.1 MPa to remove most of the solvent, then cool down to 15 - 25°C, add the organic solvent II, continue to cool down to 0 - 4°C, stir and crystallize for 1 - 2 hours, filter to obtain crystals, and dry to obtain the product.
[0022] In the present invention, by continuously complexing hydrogen fluoride generated in the reaction with an organic base, the hydrolysis process of lithium hexafluorophosphate can be continuously carried out to obtain a better hydrolysis effect. Since the organic base hydrofluoride salt in the reaction process has poor solubility in the organic solvent I but excellent solubility in water, the hydrofluoride salt produced in the reaction can be retained in the aqueous phase system and continuously accumulated, so that the lower salt solution and the upper organic solution are stratified due to the differences in different densities and dissolved components; in the lower aqueous phase solution, amines combine to form hydrofluoride salts and are continuously enriched. At the same time, due to the continuous increase in the equilibrium concentration of hydrofluoric acid in the aqueous phase, the excessive hydrolysis of difluorophosphate can be better inhibited.
[0023] And the binding energy between lithium difluorophosphate and the selected organic solvent II is relatively high, enabling it to better dissolve the product lithium difluorophosphate to form a relatively pure lithium difluorophosphate solution. Therefore, after the hydrolysis of lithium hexafluorophosphate is completed, a lithium difluorophosphate solution and an organic base hydrofluoric acid aqueous solution can be obtained in the upper and lower layers respectively. At the same time, due to the inevitable generation of fine lithium fluoride and lithium monofluorophosphate crystal particles during the hydrolysis reaction, which are suspended and distributed in the entire reaction system, they need to be separated by precision filtration. Finally, most of the obtained lithium difluorophosphate product is retained in the organic phase. Through concentration and recrystallization, a small amount of ammonium hexafluorophosphate salt dissolved in the system can be separated to obtain a better lithium difluorophosphate product.
[0024] The preparation method of lithium difluorophosphate described in the present invention can utilize lithium hexafluorophosphate to be slowly and controllably hydrolyzed in the presence of an organic base to gradually generate lithium difluorophosphate. The reaction gradually hydrolyzes lithium hexafluorophosphate into lithium difluorophosphate by selecting a suitable reaction solvent system and using an acid generated by organic base complex hydrolysis, and then separates the organic base hydrofluoride through a liquid separation process, so that lithium hexafluorophosphate can be converted into a lithium difluorophosphate product with high purity at a high conversion rate.
[0025] The invention has easy-to-obtain raw materials, convenient operation, high product purity and low environmental pressure. It solves the shortcomings of the prior art in preparing lithium difluorophosphate, such as difficulty in product extraction, low yield, difficulty in removing byproducts and low product purity. The invention has mild conditions, less side reactions, impurities can be removed during crystallization, simple process, low requirements on equipment and environmental protection, easy separation and extraction of products, and high product purity.
[0026] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention utilizes the property that an organic base can combine with hydrofluoric acid to form a stable, water-soluble salt, and continuously consumes the hydrofluoric acid produced by the hydrolysis of lithium hexafluorophosphate, so that the lithium hexafluorophosphate undergoes a slow and controllable hydrolysis, thereby achieving a higher hydrolysis efficiency. At the same time, the selected organic base has a large steric effect, and is not easy to form complex salts with hexafluorophosphate or difluorophosphate, thereby better avoiding the by-product ammonium salt impurities.
[0027] 2. Since the selected organic solution has a strong solubility for lithium difluorophosphate, most of the lithium difluorophosphate can be retained in the organic phase, and the aqueous phase has excellent solubility for the organic base hydrofluoride. After the reaction occurs, the two-phase solution will continue to separate due to the density difference, so that the organic phase lithium difluorophosphate solution and the organic base hydrofluoride solution are separated, most of the ammonium salt impurities can be removed, and a relatively pure lithium difluorophosphate solution can be obtained.
[0028] 3. Since the lower aqueous phase solution contains a high concentration of organic base hydrofluoride, there is a weak free equilibrium, so that hydrofluoric acid exists in the solution, thereby inhibiting the further hydrolysis of difluorophosphate in the aqueous phase and producing less monofluorophosphate.
[0029] 4. Since there are more free fluoride ions and trace amounts of monofluorophosphate in the reaction, lithium fluoride and lithium monofluorophosphate particles will inevitably precipitate in the organic phase solution. The present invention can effectively remove insoluble impurities in the organic phase through precision filtration to increase product purity.
[0030] 5. The present invention utilizes the extremely low solubility of lithium difluorophosphate in weakly polar solvents to cause lithium difluorophosphate to crystallize and precipitate. Meanwhile, the remaining very small amount of hexafluorophosphate still has weak solubility in the mixed solvent, and these impurities are thus separated.
[0031] 6. The produced aqueous solution of organic base hydrofluoric acid can react with sodium hydroxide solution to remove hydrofluoric acid, and the organic base and sodium fluoride solution can be obtained again. The organic base can be recycled by layering, and sodium fluoride can be produced as a by-product. Detailed implementation mode
[0032] The technical solutions of the present application will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present application.
[0033] For the experimental methods without specific conditions in the following embodiments, they are usually carried out according to conventional conditions or according to the conditions recommended by the manufacturer. The raw materials and reagents used are all conventional commercially available products without special instructions.
[0034] The index requirements for lithium hexafluorophosphate are that the acidity is less than 100 ppm, the content of common metal ions is lower than 1 ppm, and the chloride ion and sulfate ion are less than 10 ppm. Good purity can reduce the amount of lithium fluoride and lithium phosphate generated during the reaction, which helps to quickly carry out the filtration treatment. The lower anion and cation impurities help to improve the product quality.
[0035] The organic solvent used meets the battery grade standard, and the purity is greater than 99.99%.
[0036] The nitrogen positive pressure filter equipped with a filter membrane is a conventional device in the prior art, and its structure is not the inventive point of the present invention, so it will not be described in detail.
[0037] Example 1 A preparation method of lithium difluorophosphate, the specific steps are as follows: 1) Add 2736 g of propylene glycol dimethyl ether to a closed reaction kettle, control the temperature at 40 °C, add 304 g of lithium hexafluorophosphate while stirring, then add 303 g of triethylamine and 152 g of water, adjust the temperature to maintain at 40 - 80 °C, and stir and react for 1 hour; 2) Let the reaction product of step 1) stand for layering, discard the lower aqueous phase, and retain the organic phase; use a nitrogen positive pressure filter equipped with a 0.45 μm filter membrane to filter the organic phase to remove trace lithium fluoride and lithium phosphate impurities generated during the reaction, and improve the product purity; 3) Feed the filtrate obtained in step 2) into a crystallization kettle, adjust the temperature to 60 - 90 °C, carry out vacuum distillation at -0.1 MPa for 1.2 h to remove the solvent propylene glycol dimethyl ether, then cool down to 20 °C, add 456 g of dimethyl carbonate, continue to cool down to 4 °C, stir for crystallization for 1 hour, filter to obtain crystals, and carry out vacuum drying at 60 °C and -0.1 MPa for 2 h to obtain 188 g of white solid, which is the prepared lithium difluorophosphate.
[0038] After testing (T / CIESC 0041 - 2022 Lithium Difluorophosphate for Industrial Use), for the prepared lithium difluorophosphate, the yield is 87.1%, the moisture content is ≤10 ppm, the LiF content is ≤0.02 wt.%, and the purity is ≥99.7%.
[0039] Example 2 A preparation method of lithium difluorophosphate, the specific steps are as follows: 1) Add 3500 g of ethyl acetate to a closed reaction kettle, control the temperature at 40 °C, add 304 g of lithium hexafluorophosphate while stirring, then add 439 g of diethylamine and 185 g of water, adjust the temperature to be maintained at 40 - 80 °C, and stir for reaction for 1 hour; 2) Let the reaction product of step 1) stand for liquid separation, discard the lower aqueous phase, and retain the organic phase; filter the organic phase using a nitrogen positive pressure filter equipped with a 0.45 μm filter membrane to remove trace lithium fluoride and lithium phosphate impurities generated during the reaction and improve the product purity; 3) Feed the filtrate obtained in step 2) into a crystallization kettle, adjust the temperature to 60 - 90 °C, carry out vacuum distillation at -0.1 MPa for 1.5 h to remove the solvent ethyl acetate, then cool down to 20 °C, add 480 g of diethyl carbonate, continue to cool down to 4 °C, stir for crystallization for 1 hour, filter to obtain crystals, and carry out vacuum drying at 60 °C and -0.1 MPa for 2 h to obtain 200 g of white solid, which is the prepared lithium difluorophosphate.
[0040] After testing (T / CIESC 0041 - 2022 Lithium Difluorophosphate for Industrial Use), for the prepared lithium difluorophosphate, the yield is 92.6%, the moisture content is ≤10 ppm, the LiF content is ≤0.02 wt.%, and the purity is ≥99.7%.
[0041] Example 3 A preparation method of lithium difluorophosphate, the specific steps are as follows: 1) Add 3200 g of ethylene glycol dimethyl ether to a closed reaction kettle, control the temperature at 40 °C, add 304 g of lithium hexafluorophosphate while stirring, then add 601 g of ethylenediamine and 200 g of water, adjust the temperature to be maintained at 40 - 80 °C, and stir for reaction for 1 hour; 2) Let the reaction product of step 1) stand for phase separation, discard the lower aqueous phase, and retain the organic phase; filter the organic phase using a nitrogen positive pressure filter equipped with a 0.45 μm filter membrane to remove trace lithium fluoride and lithium phosphate impurities generated during the reaction, and improve the product purity; 3) Pump the filtrate obtained in step 2) into a crystallization kettle, adjust the temperature to 60 - 90 °C, carry out vacuum distillation at -0.1 MPa for 1.2 h to remove the solvent ethylene glycol dimethyl ether, then cool down to 20 °C, add 608 g of ethyl methyl carbonate, continue to cool down to 4 °C, stir for crystallization for 1 hour, filter to obtain crystals, and carry out vacuum drying at 60 °C under -0.1 MPa for 2 h to obtain 193 g of white solid, which is the prepared lithium difluorophosphate.
[0042] After testing (T / CIESC 0041 - 2022 Lithium Difluorophosphate for Industrial Use), the prepared lithium difluorophosphate has a yield of 89.4%, water content ≤ 10 ppm, LiF content ≤ 0.02 wt.%, and purity ≥ 99.7%.
[0043] Example 4 A preparation method of lithium difluorophosphate, the specific steps are as follows: 1) Add 3450 g of ethylene glycol diethyl ether to a closed reaction kettle, control the temperature at 40 °C, add 304 g of lithium hexafluorophosphate while stirring, then add 741 g of propylenediamine and 260 g of water, adjust the temperature to be maintained at 40 - 80 °C, and stir for reaction for 1 hour; 2) Let the reaction product of step 1) stand for phase separation, discard the lower aqueous phase, and retain the organic phase; filter the organic phase using a nitrogen positive pressure filter equipped with a 0.45 μm filter membrane to remove trace lithium fluoride and lithium phosphate impurities generated during the reaction, and improve the product purity; 3) Pump the filtrate obtained in step 2) into a crystallization kettle, adjust the temperature to 60 - 90 °C, carry out vacuum distillation at -0.1 MPa for 1.6 h to remove the solvent ethylene glycol diethyl ether, then cool down to 20 °C, add 700 g of dichloromethane, continue to cool down to 4 °C, stir for crystallization for 1 hour, filter to obtain crystals, and carry out vacuum drying at 60 °C under -0.1 MPa for 2 h to obtain 186 g of white solid, which is the prepared lithium difluorophosphate.
[0044] After testing (T / CIESC 0041 - 2022 Lithium Difluorophosphate for Industrial Use), the prepared lithium difluorophosphate has a yield of 86.1%, water content ≤ 10 ppm, LiF content ≤ 0.02 wt.%, and purity ≥ 99.7%.
[0045] Example 5 A preparation method of lithium difluorophosphate, the specific steps are as follows: 1) Add 4800 g of acetonitrile to a sealed reactor, control the temperature at 40 °C, add 304 g of lithium hexafluorophosphate while stirring, then add 439 g of diethylamine and 230 g of water, adjust the temperature to maintain at 40 - 80 °C, and stir and react for 1 hour; 2) Let the reaction product in step 1) stand for liquid separation, discard the lower aqueous phase, and retain the organic phase; filter the organic phase using a nitrogen positive pressure filter equipped with a 0.45 μm filter membrane to remove trace lithium fluoride and lithium phosphate impurities generated during the reaction, and improve the product purity; 3) Pump the filtrate obtained in step 2) into a crystallization kettle, adjust the temperature to 60 - 90 °C, carry out vacuum distillation at -0.1 MPa for 1.5 h to remove the solvent acetonitrile, then cool down to 20 °C, add 900 g of chloroform, continue to cool down to 4 °C, stir and crystallize for 1 hour, filter to obtain crystals, and carry out vacuum drying at 60 °C under -0.1 MPa for 2 h to obtain 196 g of white solid, which is the prepared lithium difluorophosphate.
[0046] After testing (T / CIESC 0041 - 2022 Lithium Difluorophosphate for Industrial Use), the prepared lithium difluorophosphate has a yield of 90.7%, water content ≤ 10 ppm, LiF content ≤ 0.02%, and purity ≥ 99.7%.
[0047] Example 6 A preparation method of lithium difluorophosphate, the specific steps are as follows: 1) Add 4864 g of acetonitrile to a sealed reactor, control the temperature at 40 °C, add 304 g of lithium hexafluorophosphate while stirring, then add 591 g of propylamine and 304 g of water, adjust the temperature to maintain at 40 - 80 °C, and stir and react for 1 hour; 2) Let the reaction product in step 1) stand for liquid separation, discard the lower aqueous phase, and retain the organic phase; filter the organic phase using a nitrogen positive pressure filter equipped with a 0.45 μm filter membrane to remove trace lithium fluoride and lithium phosphate impurities generated during the reaction, and improve the product purity; 3) Pump the filtrate obtained in step 2) into a crystallization kettle, adjust the temperature to 60 - 90 °C, carry out vacuum distillation at -0.1 MPa for 1.6 h to remove the solvent acetonitrile, then cool down to 20 °C, add 912 g of carbon tetrachloride, continue to cool down to 4 °C, stir and crystallize for 1 hour, filter to obtain crystals, and carry out vacuum drying at 60 °C under -0.1 MPa for 2 h to obtain 191 g of white solid, which is the prepared lithium difluorophosphate.
[0048] After testing (T / CIESC 0041 - 2022 Lithium Difluorophosphate for Industrial Use), the prepared lithium difluorophosphate has a yield of 88.4%, water content ≤ 10 ppm, LiF content ≤ 0.02 wt.%, and purity ≥ 99.7%.
[0049] The by-products in the embodiments of the present invention mainly include lithium fluoride and lithium phosphate, which will gradually precipitate during the reaction in step 1), making the reaction solution turbid and reducing the yield.
[0050] By controlling the addition amount of the organic solvent in step 1), the present invention can reduce the occurrence of side reactions.
[0051] By controlling the temperature and reaction time in step 1), the present invention can promote the continuous progress of the reaction process and improve the product yield.
[0052] By nitrogen positive pressure filtration in step 2), the present invention can filter out a small amount of impurities generated and improve the purity of the final product.
[0053] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or perform equivalent replacements for some of the technical features. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.
Claims
1. A preparation method of lithium difluorophosphate, characterized in that, It includes the following steps: 1) Put lithium hexafluorophosphate and organic solvent I into a reaction kettle, stir and mix evenly, then add organic base and water into the reaction kettle, and stir and react for a period of time; 2) Let the reaction product of step 1) stand for liquid separation, discard the lower aqueous phase, retain the upper organic phase, filter the organic phase to obtain a filtrate; 3) Put the filtrate of step 2) into a crystallization kettle, carry out vacuum distillation, then add organic solvent II, stir and crystallize, filter to obtain crystals, and dry to obtain the product.
2. The preparation method according to claim 1, characterized in that, The organic solvent I in step 1) is one of ethyl acetate, acetonitrile, ethyl formate, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, propylene glycol dimethyl ether, or a mixed solvent of two or more in any proportion; the mass ratio of the organic solvent I to lithium hexafluorophosphate is (9~16):
1.
3. The preparation method according to claim 1, characterized in that, The organic base in step 1) is at least one of propylamine, ethylenediamine, propylenediamine, diethylamine, triethylamine; the molar ratio of the organic base to lithium hexafluorophosphate is (1~5):
1.
4. The preparation method according to claim 1, characterized in that, In step 1), the mass ratio of water to lithium hexafluorophosphate is (0.5~1):1; the reaction temperature is 40 - 80 °C; the reaction time is 1 - 4 hours.
5. The preparation method according to claim 1, characterized in that, In step 2), the filtration precision is not greater than 0.5 μm.
6. The preparation method according to claim 1, characterized in that, In step 3), the vacuum distillation temperature is 60 - 90 °C; the distillation pressure is -0.3 ~ -0.1 MPa; the distillation time is 1 - 2 hours.
7. The preparation method according to claim 1, characterized in that, In step 3), the organic solvent II is at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, dichloromethane, chloroform, carbon tetrachloride.
8. The preparation method according to claim 1, characterized in that, In step 3), when adding the organic solvent II, the mass ratio of the organic solvent II to lithium hexafluorophosphate is (1.5~3):
1.
9. The preparation method according to claim 1, characterized in that, In step 3), the crystallization temperature is 0 - 6 °C, and the crystallization time is 1 - 2 hours.
10. The preparation method according to claim 1, characterized in that, The specific steps of step 3) are as follows: put the filtrate obtained in step 2) into a crystallization kettle, carry out vacuum distillation at a temperature of 60 - 90 °C and a pressure of -0.3 ~ -0.1 MPa to remove most of the solvent, then cool down to 15~25 °C, add organic solvent II, continue to cool down to 0~4 °C, stir and crystallize for 1~2 hours, filter to obtain crystals, and dry to obtain the product.