A preparation method of a liquid lithium difluorophosphate solution

Abstract

The present invention provides a method for producing lithium difluorophosphate, comprising the following reaction steps: (1) using a fluorinating agent in a non-aqueous solvent to carry out a fluorination reaction on methyl dichlorophosphate to produce methyl difluorophosphate; (2) filtering the solution to remove chlorides and then carrying out rectification and purification to obtain high-purity methyl difluorophosphate; (3) carrying out a displacement reaction on the obtained methyl difluorophosphate and a lithium halide salt to generate lithium difluorophosphate and a halogenated methane, and the halogenated methane gas escapes from the reaction system and is recovered by low-temperature condensation; (4) the generated lithium difluorophosphate solution is degassed and precisely filtered to obtain a lithium difluorophosphate solution product. The lithium difluorophosphate solution product obtained by the present invention has high purity and low impurity content, can be used as an additive for the production of lithium battery electrolytes, and the reaction raw materials of this method are cheap and easily available, the operation is simple, the production cost is low, the production efficiency is relatively high, the reaction conversion rate is high, the product yield is high, and it is easy to purify. The by-products can be fully recycled and utilized, and it is suitable for large-scale industrial production.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrolytes, and particularly relates to a preparation method of a liquid lithium difluorophosphate solution. Background Art

[0002] Lithium difluorophosphate is mainly used as an additive for lithium-ion battery electrolytes. Research shows that in an electrolyte system with lithium hexafluorophosphate as the electrolyte, adding a certain amount of lithium difluorophosphate can significantly improve the high-temperature cycling performance and high-temperature storage performance of lithium-ion batteries. Adding lithium difluorophosphate to the electrolyte can form a stable solid electrolyte interface film on the positive electrode surface, thereby improving the safety performance, cycling performance, and service life of the battery.

[0003] Currently, the disclosed preparation methods of lithium difluorophosphate (Literatures 1-4) mainly include: a. Using lithium hexafluorophosphate and lithium carbonate as raw materials, and ultrapure water as a catalyst to synthesize lithium difluorophosphate. This reaction has many by-products and it is difficult to purify the product; b. Reacting lithium hexafluorophosphate with silicon dioxide to obtain lithium difluorophosphate; the defect of this method is that the reaction is slow, the cycle is long, and it is difficult to industrialize; c. Reacting lithium hexafluorophosphate with siloxane compounds to obtain lithium difluorophosphate; however, in the existing method, the conversion rate during the reaction process is low.

[0004] In summary, the above main preparation methods of lithium difluorophosphate all use lithium hexafluorophosphate as the main reaction raw material, and obtain the product by adding other different reaction raw materials; and in the above methods, there are also problems such as difficult purification, low conversion rate, low yield, low reaction efficiency, and high production cost.

[0005] Prior Art Documents

[0006]

Patent Document 1

[0007]

Patent Document 2

[0008]

Patent Document 3

[0009]

Patent Document 4

[0010] The object of the present invention is to provide a method for preparing a lithium difluorophosphate solution in liquid state, so as to solve the problems of complex preparation method of lithium difluorophosphate products, difficult impurity removal, long process route, etc. proposed in the above background technology. By using a fluorinating agent in a non-aqueous solvent, methyl dichlorophosphate is subjected to a fluorination reaction to produce methyl difluorophosphate. The reaction solution is filtered to remove the chloride, and then the solution is rectified and purified to obtain high-purity methyl difluorophosphate. Then, the obtained methyl difluorophosphate is subjected to a displacement reaction with a lithium halide salt to generate lithium difluorophosphate and a halogenated methane. The halogenated methane gas escapes from the reaction system and is recovered by low-temperature condensation. The reaction solution of the generated lithium difluorophosphate is degassed and precisely filtered to obtain a lithium difluorophosphate solution product. It avoids the production method that uses expensive lithium hexafluorophosphate as the main reaction raw material and adds other different reaction raw materials, and avoids the problems of difficult purification, low conversion rate, low yield, low reaction efficiency, high comprehensive cost, etc. The yield and main content of lithium difluorophosphate are improved.

[0011] The inventors of the present invention have conducted extensive and in-depth research in view of the above problems, and as a result, it has been found that methyl difluorophosphate is produced by a fluorination reaction of methyl dichlorophosphate using a fluorinating agent in a non-aqueous solvent. After filtering off the chlorides, the solution is subjected to rectification and purification to obtain high-purity methyl difluorophosphate. Then, the obtained methyl difluorophosphate is subjected to a substitution reaction with a lithium halide salt to generate a lithium difluorophosphate reaction solution, which is degassed and precisely filtered to obtain a lithium difluorophosphate solution product, and a high-purity liquid lithium difluorophosphate solution can be easily manufactured, thus completing the present invention. That is, the present invention provides a method for manufacturing a liquid lithium difluorophosphate solution, which is characterized in that: sodium fluoride, potassium fluoride, lithium fluoride, etc. are used as fluorinating agents in a non-aqueous solvent for a fluorination reaction. The fluorinated liquid is filtered to remove chlorides and then subjected to rectification and purification. A lithium halide salt is used to substitute the methyl group of methyl difluorophosphate with lithium, and the generated halogenated methane gas escapes from the reaction system. The resulting reaction solution is further subjected to refining processes such as degassing, concentration, and precision filtration to obtain a high-purity concentrated lithium difluorophosphate solution. The lithium difluorophosphate solution product obtained by the present invention has a high purity and a low impurity content, can be used as an additive for the production of lithium battery electrolytes, and the reaction raw materials of this method are cheap and easily available, the operation is simple, the production cost is low, the production efficiency is high, the reaction conversion rate is high, the product yield is high, and it is easy to purify. The by-products can be fully recycled and it is suitable for large-scale industrial production. Therefore, it is considered that a high-purity liquid lithium difluorophosphate solution can be manufactured. It should be noted that the lower the concentration of acidic impurities contained in the liquid lithium difluorophosphate solution, the more preferable it is. In the solution obtained in the present invention, the concentration of acidic impurities is preferably 50 mass ppm or less, more preferably 30 mass ppm or less. When the concentration of the acidic impurities exceeds the above range, it will have an adverse effect on the characteristics of lithium ion batteries, so it is not preferable. Effects of the Invention According to the present invention, in a non-aqueous organic solvent, methyl dichlorophosphate and a fluorinating agent are reacted, the fluorinated liquid is filtered to remove chlorides and then rectified and purified to obtain high-purity methyl difluorophosphate, and then a lithium halide salt is used to substitute the methyl group of methyl difluorophosphate with lithium, and the generated halogenated methane gas escapes from the reaction system. In the method of further subjecting the resulting reaction solution to refining processes such as degassing, concentration, and precision filtration to obtain a high-purity concentrated lithium difluorophosphate solution, a high-purity liquid lithium difluorophosphate solution can be easily manufactured. A highly purified solution can be obtained by degassing and concentration without further using a purification agent. In particular, a complex device is not required, and it can be manufactured using a single reaction tank, so it is a manufacturing method capable of reducing costs.

[0012] The present invention provides a method for preparing a liquid lithium difluorophosphate solution. The obtained solution can be directly used as a material for lithium battery electrolyte additives. The reaction raw materials of this method are cheap and easily available, the operation is simple, the by-products are few, and it is suitable for industrial production.

[0013] Specifically, the method for preparing the liquid lithium difluorophosphate solution includes the following steps:

[0014] 1) In a non-aqueous solvent, methyl dichlorophosphate is used as a fluorinating agent to carry out a fluorination reaction to produce methyl difluorophosphate.

[0015] 2) After filtering the above reaction solution to remove chlorides, rectification and purification are carried out to obtain high-purity methyl difluorophosphate.

[0016] 3) The obtained methyl difluorophosphate is subjected to a displacement reaction with a lithium halide salt to generate lithium difluorophosphate and a halogenated methane. The halogenated methane gas escapes from the reaction system and is recovered by low-temperature condensation.

[0017] 4) The generated lithium difluorophosphate solution is degassed and precisely filtered to obtain a lithium difluorophosphate solution product.

[0018] Further, the precision filtration is carried out under a filtration pressure of 0.3 to 0.6 MPa using a PP pleated membrane filter element or a modified PP pleated membrane filter element with a filter element accuracy of 0.1 - 0.5 μm.

[0019] Furthermore, the modification method of the modified PP pleated membrane filter element includes the following steps:

[0020] S1: Add 13 - 26 parts of 2-methyl-4-vinylquinoline, 0.005 - 0.2 parts of 2-furanacrylonitrile, 0.01 - 0.5 parts of a photoinitiator, and 200 - 300 parts of DMF into a stirring kettle, stir and react at 30 - 50 °C for 30 - 50 minutes, and then add 40 - 70 parts of a surface-activated PP pleated membrane filter element into the above materials; stir and react at 90 - 110 °C for 2 - 4 h, take out the PP pleated membrane filter element, and dry it;

[0021] S2: Then put it into an irradiation box; irradiate it under 365 nm ultraviolet light for 3 - 30 min to initiate free radical polymerization to form an interpenetrating hydrophilic network structure. After the irradiation is completed, stop the irradiation and cool down to obtain the irradiated modified PP pleated membrane filter element.

[0022] Preparation mechanism of the modified PP pleated membrane filter element:

[0023] Induced by an initiator, 2-methyl-4-vinylquinoline and 2-furanacrylonitrile undergo a free radical polymerization reaction with the surface of the PP pleated membrane filter element, polymerize the monomers onto the membrane surface, and generate a surface-activated PP pleated membrane filter element with complexing lithium ions. It can complex impurities such as lithium fluoride, lithium chloride, and lithium bromide with small molecules, improve the filtration effect, and thus improve the purity of the lithium difluorophosphate solution.

[0024] Further, the photoinitiator is at least one of photoinitiator 1173, photoinitiator 184, photoinitiator 907, photoinitiator 369, photoinitiator 1490, photoinitiator 1700, benzoin dimethyl ether, methyl o-benzoylbenzoate, and photoinitiator TPO.

[0025] Further, the molar ratio of the fluorinating agent to methyl dichlorophosphate is 2 to 2.5:1.

[0026] Further, the water content of the non-aqueous solvent is less than 15 ppm, and the non-aqueous solvent is at least one of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethyl acetate, acetone, diethyl ether, dimethylformamide, ethylene glycol dimethyl ether, acetonitrile, and tetrahydrofuran.

[0027] Further, the reaction temperature in step 1) is 50 to 100 °C. Regarding the temperature for carrying out this reaction, the lower limit is 50 °C, preferably 60 °C, the upper limit is 100 °C, preferably 80 °C. Additionally, when it is higher than 100 °C, it causes coloring and side reactions, so it is not preferred either. The reaction time is 6 to 12 h. The pressure during the above reaction is not particularly limited, and the reaction system is protected by an inert gas.

[0028] Further, the rectification in step 2) is carried out under reduced pressure, the pressure range is 5 to 10 kPa, and the rectification temperature is 60 to 100 °C. Therefore, the upper limit of this temperature is 100 °C, preferably 80 °C. The lower limit of this temperature is 60 °C, preferably 70 °C. The content of methyl difluorophosphate after purification is greater than 99.5%.

[0029] Further, in step 3), the obtained methyl difluorophosphate is subjected to a displacement reaction with a lithium halide salt to generate lithium difluorophosphate and a halogenated methane, and the halogenated methane gas escapes from the reaction system and is recovered by low-temperature condensation. The reaction temperature is 40 to 80 °C, the upper limit of this temperature is 80 °C, preferably 70 °C. The lower limit of this temperature is 40 °C, preferably 50 °C. The reaction time is 8 to 12 h. The pressure during the above reaction is not particularly limited, and the reaction system is protected by an inert gas.

[0030] Further, the lithium difluorophosphate solution generated in step 4) is degassed and precisely filtered to obtain a lithium difluorophosphate solution product. The degassing pressure range is 5 to 10 kPa, and the degassing temperature is 30 to 70 °C. The concentration of lithium difluorophosphate after degassing and filtration is 10%. The upper limit of this degassing temperature is 70 °C, preferably 60 °C. The lower limit of this temperature is 30 °C, preferably 40 °C. The degassing time is 4 to 8 h.

[0031] Further, the reactions in steps 1), 2), 3), and 4) are carried out in an inert gas atmosphere, and the inert gas is at least one of nitrogen, argon, and helium.

[0032] The present invention provides a method for preparing a liquid lithium difluorophosphate solution by reacting with cheap and easily available raw materials under low temperature and conditions. The method includes: using a fluorinating agent and methyl dichlorophosphate in a non-aqueous solvent to carry out a fluorination reaction to produce methyl difluorophosphate. The solution after filtering the reaction solution to remove chlorides is subjected to rectification and purification to obtain high-purity methyl difluorophosphate. Then, the obtained methyl difluorophosphate is subjected to a displacement reaction with a lithium halide salt to generate lithium difluorophosphate and a halogenated methane. The halogenated methane gas escapes from the reaction system and is recovered by low-temperature condensation. The generated lithium difluorophosphate reaction solution is degassed and precisely filtered to obtain a lithium difluorophosphate solution product.

[0033] 1. Preparation of methyl difluorophosphate

[0034] Under an inert gas atmosphere, a fluorinating agent and methyl dichlorophosphate are reacted in a non-aqueous solvent to produce methyl difluorophosphate. Then, the solution after filtering to remove chlorides is subjected to rectification and purification to obtain high-purity methyl difluorophosphate.

[0035] The reaction equation is as follows:

[0036] XF + Cl₂PO₂CH₃ → F₂PO₂CH₃ + XCl

[0037] X is: Na + K + Li + etc.

[0038] In some embodiments, the molar ratio of the fluorinating agent to methyl dichlorophosphate is 2 - 2.5:1. For example: 2:1, 2.1:1, 2.2:1, 2.3:1, 2.4:1, 2.5:1.

[0039] Preferably, the purity of the fluorinating agent is greater than 99.0% and the water content does not exceed 200 ppm.

[0040] In some embodiments, the non-aqueous solvent is at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethyl acetate, dimethylformamide, ethylene glycol dimethyl ether, ether, acetonitrile, and tetrahydrofuran.

[0041] Preferably, the water content of the non-aqueous solvent is less than 15 ppm.

[0042] The reaction temperature is 50 - 100 °C and the reaction time is 6 - 12 h.

[0043] Non-limiting examples of the reaction temperature include: 50 °C, 55 °C, 60 °C, 65 °C, 70 °C, 75 °C, 80 °C, 85 °C, 90 °C, 95 °C, 100 °C, etc.

[0044] Non-limiting examples of the reaction time include: 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, 12 h, and so on.

[0045] 2. Distillation of methyl difluorophosphate

[0046] After filtering the above reaction solution to remove chlorides, high-purity methyl difluorophosphate is obtained by distillation purification.

[0047] The distillation is carried out under reduced pressure, the pressure range is 5-10 kPa, the distillation temperature is 60-100 °C, and the distillation time is 12-18 h.

[0048] Non-limiting examples of the distillation pressure include: 5 kPa, 6 kPa, 6.5 kPa, 7 kPa, 8 kPa, 9 kPa, 10 kPa, and so on.

[0049] Non-limiting examples of the distillation temperature include: 60 °C, 65 °C, 70 °C, 75 °C, 80 °C, 85 °C, 90 °C, 95 °C, 100 °C, and so on.

[0050] Non-limiting examples of the distillation time include: 12 h, 13 h, 14 h, 15 h, 16 h, 17 h, 18 h, and so on.

[0051] 3. Preparation of lithium difluorophosphate solution

[0052] In an inert gas atmosphere, lithium halide and methyl difluorophosphate are reacted in a non-aqueous solvent to produce a lithium difluorophosphate solution.

[0053] The reaction equation is as follows:

[0054] LiY + F2PO2CH3 → F2PO2Li + YCH3

[0055] Y is: F - Cl - Br - I - etc.

[0056] In some embodiments, the molar ratio of the lithium halide to methyl dichlorophosphate is 1-1.1:1. For example: 1:1, 1.01:1, 1.02:1, 1.03:1, 1.04:1, 1.05:1, 1.06:1, 1.07:1, 1.08:1, 1.09:1, 1.1:1.

[0057] Preferably, the purity of the fluorinating agent is greater than 99.0% and the water content does not exceed 50 ppm, the content of methyl dichlorophosphate is greater than 99.5%, and the water content does not exceed 50 ppm.

[0058] In some embodiments, the non-aqueous solvent is at least one of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethyl acetate, dimethylformamide, ethylene glycol dimethyl ether, diethyl ether, acetonitrile, and tetrahydrofuran.

[0059] Preferably, the water content of the non-aqueous solvent is less than 15 ppm.

[0060] The reaction temperature is 40 to 80 °C, and the reaction time is 8 to 12 h.

[0061] Non-limiting examples of the reaction temperature include: 40 °C, 45 °C, 50 °C, 55 °C, 60 °C, 65 °C, 70 °C, 75 °C, 80 °C, and so on.

[0062] Non-limiting examples of the reaction time include: 8 h, 9 h, 10 h, 11 h, 12 h, and so on.

[0063] 4. Refinement of lithium difluorophosphate solution

[0064] The above reaction solution is degassed and precisely filtered to obtain a lithium difluorophosphate solution product.

[0065] The degassing is carried out under reduced pressure, the pressure range is 5 to 10 kPa, the degassing temperature is 40 to 80 °C, and the degassing time is 6 to 10 h.

[0066] Non-limiting examples of the degassing pressure include: 5 kPa, 6 kPa, 6.5 kPa, 7 kPa, 8 kPa, 9 kPa, 10 kPa, and so on.

[0067] Non-limiting examples of the degassing temperature include: 40 °C, 45 °C, 50 °C, 55 °C, 60 °C, 65 °C, 70 °C, 75 °C, 80 °C, and so on.

[0068] Non-limiting examples of the degassing time include: 6 h, 7 h, 8 h, 9 h, 10 h, and so on.

[0069] After degassing, it is precisely filtered and the concentration is adjusted with a non-aqueous solvent to obtain a lithium difluorophosphate solution with a weight concentration of 10%.

[0070] The reaction is carried out in an inert gas atmosphere, and the inert gas is at least one of nitrogen, argon, and helium.

[0071] The chloride ion content in the lithium difluorophosphate solution prepared by the present invention is below 5 ppm; the concentration of free acid in the solution is tested by titration method, and the acidity result is below 50 ppm. Since the chloride and free acid contents in the solution are less, the solution can be directly used as an additive material for lithium battery electrolytes.

[0072] Technical effects:

[0073] A preparation method of a liquid lithium difluorophosphate solution according to the present invention has the following remarkable effects compared with the prior art:

[0074] 1) The preparation method provided by the present invention has mild reaction conditions, high yield of the obtained product, cheap and easily available reaction raw materials, can greatly save costs, and the by-products can be recycled.

[0075] 2) The preparation method provided by the present invention has simple reaction steps, convenient operation, and simple post-treatment of the reaction, increasing the feasibility of industrial production.

[0076] 3) The liquid lithium difluorophosphate solution prepared by the present invention is used as an additive for lithium battery electrolytes, which can improve the working performance of the battery. Specific Embodiments

[0077] The following are the optimized embodiments of the present invention. The present invention is not limited to the following preferred embodiments. It should be noted that for those skilled in the art, based on the inventive concept of this invention, several deformations and improvements made all fall within the protection scope of the present invention.

[0078] Example 1

[0079] In a 500 ml three-necked flask, under nitrogen protection, 300 ml of dimethylformamide and 127.8 g (2.2 mol) of potassium fluoride were added. The temperature was slowly raised with stirring, and 148.9 g (1 mol) of methyl dichlorophosphate was slowly added dropwise at 100 °C over about 2 hours. After completion, the temperature was maintained at the same temperature for 8 hours. After the reaction, it was cooled to below 50 °C, and potassium chloride was removed by filtration. The filtrate was sent to a vacuum distillation system. First, the solvent dimethylformamide was recovered, and then the fore-fraction was separated. After the content of methyl difluorophosphate reached 99%, 112 g of the main fraction of methyl difluorophosphate was collected, and the yield was 95%.

[0080] In a 500 ml three-necked flask, under nitrogen protection, 200 ml of ethyl acetate and 23.3 g (0.55 mol) of lithium chloride were added. The temperature was slowly raised with stirring, and 59 g (0.5 mol) of methyl difluorophosphate was slowly added dropwise at 80 °C. After the addition, the reaction was continued to be kept warm for 8 hours under the same temperature condition. The chloromethane gas generated by the reaction was condensed by deep cooling. After the heat preservation, the vacuum pump was slowly opened to slowly evacuate the reaction flask, which took about 5 hours to complete. Then, the degassed liquid lithium difluorophosphate solution was filtered precisely and the concentration was adjusted with ethyl acetate to obtain a liquid lithium difluorophosphate solution with a weight concentration of 10%.

[0081] The precise filtration is carried out under a filtration pressure of 0.3 MPa using a PP pleated membrane filter element with a filter element accuracy of 0.5 μm.

[0082] Test results of this example: the content of lithium difluorophosphate is 9.53%, free acid (calculated as HCl) is 38 ppm, moisture (Karl Fischer method) is 49 ppm, alkali metal ion content (calculated as K) is 3 ppm, chloride ion content is 12 ppm, heavy metal ion content (calculated as Fe) is 1 ppm, and insoluble matter content is 120 ppm.

[0083] Example 2

[0084] In a 500 ml three-necked flask, under nitrogen protection, add 300 ml of acetonitrile and 84 g (2.2 mol) of sodium fluoride. Stir and slowly heat up, and slowly dropwise add 148.9 g (1 mol) of methyl dichlorophosphate at 80°C over about 2 hours. After completion, maintain the same temperature and keep warm for 9 hours. After the reaction is completed, cool to below 50°C and filter off sodium chloride. The filtrate goes to a vacuum distillation system. First, recover the solvent acetonitrile, then separate the fore-fraction. When the content of methyl difluorophosphate reaches 99%, collect 108 g of the main fraction of methyl difluorophosphate, and the yield is 93%.

[0085] In a 500 ml three-necked flask, under nitrogen protection, add 200 ml of acetonitrile and 14.3 g (0.55 mol) of lithium fluoride. Stir and slowly heat up, and slowly dropwise add 59 g (0.5 mol) of methyl difluorophosphate at 80°C. After the addition is completed, continue to keep warm and react for 8 hours under the same temperature conditions. The fluoromethane gas generated by the reaction is condensed using cryogenic cooling. After the heat preservation is completed, slowly open the vacuum pump to slowly evacuate the reaction flask, which takes about 4 hours to complete. Then, the degassed liquid lithium difluorophosphate solution is precisely filtered and adjusted in concentration with acetonitrile to obtain a liquid lithium difluorophosphate solution with a weight concentration of 10%.

[0086] The said precise filtration is carried out under a filtration pressure of 0.35 MPa using a PP pleated membrane filter element with a filter element accuracy of 0.35 μm.

[0087] Test results of this example: the content of lithium difluorophosphate is 10.2%, free acid (calculated as HCl) is 13 ppm, moisture (Karl Fischer method) is 29 ppm, alkali metal ion content (calculated as Na) is 2 ppm, chloride ion content is 2 ppm, heavy metal ion content (calculated as Fe) is 1 ppm, and insoluble matter content is 97 ppm.

[0088] Example 3

[0089] In a 1000 ml three-necked flask, under nitrogen protection, 500 ml of ethyl methyl carbonate and 57.2 g (2.2 mol) of lithium fluoride were added. The temperature was slowly raised with stirring, and 148.9 g (1 mol) of methyl dichlorophosphate was slowly added dropwise at 105°C over about 2 hours. After completion, the temperature was maintained for 12 hours under the same temperature conditions. After the reaction, it was cooled to below 50°C, and sodium fluoride was removed by filtration. The filtrate was sent to a vacuum distillation system. First, the solvent ethyl methyl carbonate was recovered, and then the fore-fraction was separated. After the content of methyl difluorophosphate reached 99%, 102 g of the main fraction methyl difluorophosphate was collected, with a yield of 90%.

[0090] In a 500 ml three-necked flask, under nitrogen protection, 200 ml of ethyl methyl carbonate and 14.3 g (0.55 mol) of lithium fluoride were added. The temperature was slowly raised with stirring, and 59 g (0.5 mol) of methyl difluorophosphate was slowly added dropwise at 105°C. After the addition was completed, the reaction was continued to be kept warm for 5 hours under the same temperature conditions. The fluoromethane gas generated by the reaction was condensed using cryogenic cooling. After the heat preservation was completed, the vacuum pump was slowly turned on to slowly evacuate the reaction flask, which took about 5 hours to complete. Then, the degassed liquid lithium difluorophosphate solution was filtered precisely and the concentration was adjusted with ethyl methyl carbonate to obtain a liquid lithium difluorophosphate solution with a weight concentration of 10%.

[0091] The precise filtration was carried out using a PP pleated membrane filter element with a filter element accuracy of 0.1 μm under a filtration pressure of 0.4 MPa.

[0092] The test results of this example: the content of lithium difluorophosphate was 9.5%, the free acid (calculated as HCl) was 16 ppm, the moisture (Karl Fischer method) was 38 ppm, the alkali metal ion content (calculated as Na) was 1 ppm, the chloride ion content was 1 ppm, the heavy metal ion content (calculated as Fe) was 1 ppm, and the insoluble content was 93 ppm.

[0093] Example 4

[0094] In a 500 ml three-necked flask, under nitrogen protection, 300 ml of dimethylformamide and 127.8 g (2.2 mol) of potassium fluoride were added. The temperature was slowly raised with stirring, and 148.9 g (1 mol) of methyl dichlorophosphate was slowly added dropwise at 100°C over about 2 hours. After completion, the temperature was maintained for 8 hours under the same temperature conditions. After the reaction, it was cooled to below 50°C, and potassium chloride was removed by filtration. The filtrate was sent to a vacuum distillation system. First, the solvent dimethylformamide was recovered, and then the fore-fraction was separated. After the content of methyl difluorophosphate reached 99%, 112 g of the main fraction methyl difluorophosphate was collected, with a yield of 95%.

[0095] In a 500 ml three-necked flask, under nitrogen protection, 200 ml of ethyl acetate and 23.3 g (0.55 mol) of lithium chloride were added. The temperature was slowly raised with stirring, and 59 g (0.5 mol) of methyl difluorophosphate was slowly added dropwise at 80 °C. After the addition was completed, the reaction was continued to be kept warm at the same temperature for 8 hours. The chloromethane gas generated by the reaction was condensed using cryogenic cooling. After the heat preservation was completed, the vacuum pump was slowly opened to slowly evacuate and degas the reaction flask, which took about 5 hours to complete. Then, the degassed liquid lithium difluorophosphate solution was precisely filtered and the concentration was adjusted with ethyl acetate to obtain a liquid lithium difluorophosphate solution with a weight concentration of 10%.

[0096] The precise filtration is carried out using a modified PP folded membrane filter element with a filter accuracy of 0.5 μm under a filtration pressure of 0.45 MPa.

[0097] The modification method of the modified PP folded membrane filter element includes the following steps:

[0098] S1: Add 13 g of 2-methyl-4-vinylquinoline, 0.05 g of 2-furanacrylonitrile, 0.01 g of photoinitiator 1173, and 200 g of DMF to a stirring kettle, stir and react at 30 °C for 30 minutes, and then add 40 g of a surface-activated PP folded membrane filter element to the above materials; stir and react at 90 °C for 2 h, take out the PP folded membrane filter element, and dry it;

[0099] S2: Then put it into an irradiation box; irradiate it under 365 nm ultraviolet light for 3 min to initiate free radical polymerization to form an interpenetrating hydrophilic network structure. After the irradiation is completed, stop the irradiation and cool down to obtain the irradiated modified PP folded membrane filter element.

[0100] The test results of this example: the content of lithium difluorophosphate is 10.24%, free acid (calculated as HCl) is 12 ppm, moisture (Karl Fischer method) is 36 ppm, alkali metal ion content (calculated as K) is 1 ppm, chloride ion content is 4 ppm, heavy metal ion content (calculated as Fe) is 1 ppm, and insoluble matter content is 55 ppm.

[0101] Example 5

[0102] In a 500 ml three-necked flask, under nitrogen protection, 300 ml of acetonitrile and 84 g (2.2 mol) of sodium fluoride were added. The temperature was slowly raised with stirring, and 148.9 g (1 mol) of methyl dichlorophosphate was slowly added dropwise at 80 °C, and the addition was completed in about 2 hours. After completion, the temperature was maintained at the same temperature for 9 hours. After the reaction was completed, it was cooled to below 50 °C, and sodium chloride was filtered off. The filtrate was sent to a vacuum distillation system. First, the solvent acetonitrile was recovered, and then the fore-fraction was separated. After the content of methyl difluorophosphate reached 99%, 108 g of the main fraction of methyl difluorophosphate was collected, and the yield was 93%.

[0103] In a 500 ml three-necked flask, under nitrogen protection, 200 ml of acetonitrile and 14.3 g (0.55 mol) of lithium fluoride were added. The temperature was slowly raised with stirring, and 59 g (0.5 mol) of methyl difluorophosphate was slowly added dropwise at 80 °C. After the addition was completed, the reaction was continued under the same temperature condition for 8 hours. The fluoromethane gas generated by the reaction was condensed using cryogenic cooling. After the heat preservation was completed, the vacuum pump was slowly turned on to slowly evacuate and degas the reaction flask, which took about 4 hours to complete. Then, the degassed liquid lithium difluorophosphate solution was filtered precisely and the concentration was adjusted with acetonitrile to obtain a liquid lithium difluorophosphate solution with a weight concentration of 10%.

[0104] The precise filtration is carried out using a modified PP pleated membrane filter element with a filter element accuracy of 0.35 μm under a filtration pressure of 0.5 MPa.

[0105] The modification method of the modified PP pleated membrane filter element includes the following steps:

[0106] S1: Add 19 g of 2-methyl-4-vinylquinoline, 0.1 g of 2-furanacrylonitrile, 0.25 g of photoinitiator 184, and 250 g of DMF to a stirring kettle, stir and react at 40 °C for 40 minutes, and then add 55 g of a surface-activated PP pleated membrane filter element to the above materials; stir and react at 100 °C for 3 h, take out the PP pleated membrane filter element and dry it;

[0107] S2: Then put it into an irradiation box; irradiate it under 365 nm ultraviolet light for 18 min to initiate free radical polymerization to form an interpenetrating hydrophilic network structure. After completion, stop irradiation and cool down to obtain the irradiated modified PP pleated membrane filter element.

[0108] The test results of this example: the content of lithium difluorophosphate is 11.02%, free acid (calculated as HCl) is 9 ppm, moisture (Karl Fischer method) is 21 ppm, alkali metal ion content (calculated as Na) is 1 ppm, chloride ion content is 1 ppm, heavy metal ion content (calculated as Fe) is 1 ppm, and insoluble matter content is 37 ppm.

[0109] Example 6

[0110] In a 1000 ml three-necked flask, under nitrogen protection, 500 ml of ethyl methyl carbonate and 57.2 g (2.2 mol) of lithium fluoride were added. The temperature was slowly raised with stirring, and 148.9 g (1 mol) of methyl dichlorophosphate was slowly added dropwise at 105 °C, and the addition was completed in about 2 hours. After completion, the same temperature condition was maintained for heat preservation for 12 hours. After the reaction was completed, it was cooled to below 50 °C, and lithium chloride was removed by filtration. The filtrate was sent to a vacuum distillation system. First, the solvent ethyl methyl carbonate was recovered, and then the fore-fraction was separated. When the content of methyl difluorophosphate reached 99%, 102 g of the main fraction of methyl difluorophosphate was collected, and the yield was 90%.

[0111] In a 500 ml three-necked flask, under nitrogen protection, 200 ml of ethyl methyl carbonate and 14.3 g (0.55 mol) of lithium fluoride were added. The mixture was stirred and slowly heated. At 105 °C, 59 g (0.5 mol) of methyl difluorophosphate was slowly added dropwise. After the addition was completed, the reaction was continued at the same temperature for 5 hours. The fluoromethane gas generated by the reaction was condensed using cryogenic cooling. After the heat preservation was completed, the vacuum pump was slowly turned on to slowly evacuate and degas the reaction flask, which took about 5 hours to complete. Then, the degassed liquid lithium difluorophosphate solution was filtered precisely, and the concentration was adjusted with ethyl methyl carbonate to obtain a liquid lithium difluorophosphate solution with a weight concentration of 10%.

[0112] The precise filtration is carried out using a PP-modified PP pleated membrane filter element with a filter element accuracy of 0.1 μm under a filtration pressure of 0.6 MPa.

[0113] The modification method of the modified PP pleated membrane filter element includes the following steps:

[0114] S1: Add 26 g of 2-methyl-4-vinylquinoline, 0.2 g of 2-furanacrylonitrile, 0.5 g of photoinitiator TPO, and 300 g of DMF to a stirring kettle. Stir and react at 50 °C for 50 minutes. Then add 70 g of a surface-activated PP pleated membrane filter element to the above materials. Stir and react at 110 °C for 4 h, take out the PP pleated membrane filter element, and dry it.

[0115] S2: Then place it in an irradiation chamber; irradiate it under 365 nm ultraviolet light for 30 min to initiate free radical polymerization to form an interpenetrating hydrophilic network structure. After completion, stop irradiation and cool down to obtain the irradiated modified PP pleated membrane filter element.

[0116] The test results of this example: the content of lithium difluorophosphate is 10.45%, free acid (calculated as HCl) is 11 ppm, moisture (Karl Fischer method) is 31 ppm, alkali metal ion content (calculated as Na) is 1 ppm, chloride ion content is 1 ppm, heavy metal ion content (calculated as Fe) is 1 ppm, and insoluble matter content is 38 ppm.

[0117] The present invention is intended to cover all alternatives, modifications, and equivalent technical solutions, which are all included within the scope of the present invention as defined in the claims. Those skilled in the art should recognize that many methods and materials similar or equivalent to those described herein can be used to practice the present invention. The present invention is in no way limited to the methods and materials described herein. In the case where one or more of the incorporated documents, patents, and similar materials are different from or contradictory to the present application (including but not limited to the defined terms, term applications, described technologies, etc.), the present application shall prevail.

[0118] It should be further recognized that certain features of the present invention are described in multiple separate embodiments for clarity, but can also be provided in combination in a single embodiment, and conversely, various features of the present invention are described in a single embodiment for brevity, but can also be provided individually or in any suitable sub-combination.

[0119] Unless otherwise indicated, all scientific and technical terms used in the present invention have the same meaning as commonly understood by those skilled in the art to which the present invention pertains. All patents and published publications referred to in the present invention are incorporated herein by reference in their entirety.

[0120] Unless otherwise indicated, the following definitions shall apply to the terms used in the present invention. For the purposes of the present invention, chemical elements are in accordance with the CAS version of the Periodic Table of the Elements and the 75th Edition (1994) of the Handbook of Chemistry and Physics. The entire content thereof is incorporated into the present invention.

[0121] The term "comprising" or "including" is an open-ended expression, i.e., it includes the content specified in the present invention, but does not exclude other aspects.

Claims

1. A method for preparing a lithium difluorophosphate solution, characterized in that, The preparation method comprises the following steps: The preparation method comprises the following steps: 1) Using a fluorinating agent and methyl dichlorophosphate in a non-aqueous solvent to carry out a fluorination reaction to produce methyl difluorophosphate; 2) After filtering the reaction solution obtained in the above step to remove chlorides, carrying out rectification and purification to obtain high-purity methyl difluorophosphate; 3) Carrying out a displacement reaction between the obtained methyl difluorophosphate and a lithium halide salt to generate lithium difluorophosphate and a halogenated methane, and the halogenated methane gas escapes from the reaction system and is recovered by low-temperature condensation; 4) The generated lithium difluorophosphate solution is degassed and precisely filtered to obtain a lithium difluorophosphate solution product, and the precise filtration is carried out under a filtration pressure of 0.3 to 0.6 MPa using a modified PP pleated membrane filter element with a filter element precision of 0.1 - 0.5 μm; The modification method of the modified PP pleated membrane filter element comprises the following steps: S1: Add 13 - 26 parts of 2-methyl-4-vinylquinoline, 0.005 - 0.2 parts of 2-furanacrylonitrile, 0.01 - 0.5 parts of a photoinitiator, 200 - 300 parts of DMF into a stirring kettle, stir and react at 30 - 50 °C for 30 - 50 minutes, and then add 40 - 70 parts of a surface-activated PP pleated membrane filter element into the above materials; stir and react at 90 - 110 °C for 2 - 4 h, take out the PP pleated membrane filter element, and dry it; The photoinitiator is at least one of photoinitiator 1173, photoinitiator 184, photoinitiator 907, photoinitiator 369, photoinitiator 1490, photoinitiator 1700, benzoin dimethyl ether, methyl o-benzoylbenzoate, photoinitiator TPO; S2: Then put it into an irradiation box; irradiate it under 365 nm ultraviolet light for 3 - 30 min to initiate free radical polymerization to form an interpenetrating hydrophilic network structure, stop irradiation after completion, cool down, and obtain the irradiated modified PP pleated membrane filter element.

2. The preparation method of the lithium difluorophosphate solution according to claim 1, wherein The fluorinating agent is at least one of sodium fluoride, potassium fluoride, and lithium fluoride, and the molar ratio of the fluorinating agent to methyl dichlorophosphate is 2 - 2.5:

1.

3. The preparation method of the lithium difluorophosphate solution according to claim 1, wherein The water content of the non-aqueous solvent is less than 15 ppm, and the non-aqueous solvent is at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethyl acetate, dimethylformamide, ethylene glycol dimethyl ether, acetonitrile, and tetrahydrofuran.

4. The preparation method of the lithium difluorophosphate solution according to claim 1, characterized in that, The reaction temperature in step 1) is 50 - 100 °C, and the reaction time is 12 - 16 h.

5. The preparation method of the lithium difluorophosphate solution according to claim 1, wherein The rectification in step 2) adopts vacuum rectification, the pressure range is 5 - 10 kPa, the rectification temperature is 60 - 100 °C, and the content of methyl difluorophosphate after purification is greater than 99.5%.

6. The preparation method of the lithium difluorophosphate solution according to claim 1, characterized in that, In step 3), carrying out a displacement reaction between methyl difluorophosphate and a lithium halide salt to generate lithium difluorophosphate and a halogenated methane, and the halogenated methane gas escapes from the reaction system and is recovered by low-temperature condensation, the operation temperature is 40 - 80 °C, the reaction time is 8 - 12 h, and a lithium difluorophosphate reaction solution is obtained.

7. The preparation method of the lithium difluorophosphate solution according to claim 1, wherein, In step 4), the generated lithium difluorophosphate solution is degassed and precisely filtered to obtain a lithium difluorophosphate solution product, the degassing pressure range is 5 - 10 kPa, the degassing temperature is 30 - 70 °C, the degassing time is 4 - 8 h, and the lithium difluorophosphate concentration after degassing and filtration is 10%.

8. The preparation method of the lithium difluorophosphate solution according to claim 1, characterized in that, The reactions in the steps 1), 2), 3), and 4) are carried out under an inert gas atmosphere, and the inert gas is at least one of nitrogen, argon, and helium.