Lithium hexafluorophosphate solution and preparation method thereof

Abstract

The invention relates to a lithium hexafluorophosphate solution and a preparation method thereof. The method comprises the following steps: introducing phosphorus pentafluoride into a lithium fluoride suspension dispersed in an organic solvent for reaction to generate a lithium hexafluorophosphate reaction solution; and after the reaction is finished, carrying out gas stripping by using inert gas, adding lithium carbonate powder to further react with residual hydrogen fluoride in the system after the gas stripping is finished, and after the reaction is finished, filtering and deacidifying to obtain a qualified product. The lithium hexafluorophosphate solution provided by the invention has high storage stability, and has no obvious acidity and color number increase after being stored at room temperature for 6 months.

Description

Technical Field

[0001] The invention relates to the field of electrochemical technology, and in particular to a method for preparing a lithium hexafluorophosphate solution with high storage stability. Background Art

[0002] Lithium-ion batteries have the advantages of high energy density, high average output voltage, good cycle performance, and excellent conductivity. They are widely used in electronics, information, computers, military, aerospace, electric vehicles, etc. Lithium hexafluorophosphate is one of the most commonly used electrolytes in lithium-ion batteries.

[0003] Patent CN108640129A reports a method for preparing lithium hexafluorophosphate, wherein phosphorus pentafluoride is introduced into a hydrogen fluoride solution of lithium fluoride to react, thereby obtaining a lithium hexafluorophosphate hydrogen fluoride solution, which is then cooled, crystallized, and dried to obtain solid lithium hexafluorophosphate. This method has the following problems: using anhydrous hydrogen fluoride as a solvent requires a large excess, and the hydrogen fluoride removal process must be performed after the reaction is completed; the product is obtained by crystallization, which is complicated to operate; anhydrous hydrogen fluoride and phosphorus pentafluoride are highly toxic and corrosive, resulting in a large investment.

[0004] Patent CN103069638A reports a method for preparing a lithium hexafluorophosphate solution, in which phosphorus trichloride, chlorine, lithium fluoride and hydrogen fluoride react in an organic solvent to generate lithium hexafluorophosphate. This method is simple to operate, but it still uses anhydrous hydrogen fluoride, and a large amount of hydrogen fluoride and hydrogen chloride will remain in the lithium hexafluorophosphate solution, which are difficult to remove.

[0005] In addition, lithium hexafluorophosphate has poor stability and is easily decomposed by heat. Long-term storage can easily lead to increased acidity and darker color of the solution, which can reduce product quality, cause inconvenience in product transportation, and seriously affect downstream applications.

[0006] Therefore, there is an urgent need to develop a method for preparing a lithium hexafluorophosphate solution with simple process, low cost and excellent product storage stability. Summary of the invention

[0007] In view of the deficiencies of the prior art, the object of the present invention is to provide a lithium hexafluorophosphate solution with high storage stability and a preparation method thereof. The lithium hexafluorophosphate solution prepared by the method has no obvious increase in color number and acidity after being stored at room temperature for 6 months.

[0008] In order to achieve the above purpose, the present invention adopts the following technical solution:

[0009] In a first aspect, the present invention provides a lithium hexafluorophosphate solution with high storage stability, comprising the following components: lithium hexafluorophosphate, an organic solvent, phosphorus oxytrifluoride and lithium monofluorophosphate, wherein the mass ratio of lithium monofluorophosphate to phosphorus oxytrifluoride is 1.5-5:1, preferably 2.0-3.5:1.

[0010] As a preferred solution, in the lithium hexafluorophosphate solution, based on the mass of the lithium hexafluorophosphate solution, the lithium hexafluorophosphate content is 25-35wt%; the phosphorus oxytrifluoride content is 2-30ppm; and the lithium monofluorophosphate content is 3-150ppm.

[0011] The present invention surprisingly found that the lithium monofluorophosphate and phosphorus oxyfluoride contained in the lithium hexafluorophosphate solution are closely related to the storage stability of the prepared lithium hexafluorophosphate solution. When the mass ratio of lithium monofluorophosphate to phosphorus oxyfluoride is between 1.5-5:1, preferably 2.0-3.5:1, the lithium hexafluorophosphate solution has high storage stability.

[0012] The invention controls the mass ratio of lithium monofluorophosphate to phosphorus oxytrifluoride in the lithium hexafluorophosphate solution to be between 1.5 and 5:1, thereby obtaining a lithium hexafluorophosphate solution with good storage stability. After the solution is stored at room temperature for 6 months, the color number and the acidity do not increase significantly.

[0013] In a second aspect, the present invention provides a method for preparing the above-mentioned lithium hexafluorophosphate solution, comprising the following steps:

[0014] S1: introducing phosphorus pentafluoride gas into a suspension of lithium fluoride and an organic solvent to obtain a lithium hexafluorophosphate reaction solution;

[0015] S2: Use inert gas to strip the reaction liquid, and control the acidity of the reaction liquid after stripping to 500-1500ppm;

[0016] S3: adding lithium carbonate to the reaction solution after gas stripping to react with residual hydrogen fluoride, filtering the solid after the reaction is completed to obtain a filtrate, and controlling the acidity of the filtrate to 200-400 ppm;

[0017] S4: The filtrate in S3 is deacidified by resin or molecular sieve to further reduce the acidity to 0.1-50 ppm.

[0018] Preferably, the organic solvent in S1 is one or more of chain esters or cyclic esters.

[0019] Preferably, the chain ester is one or more of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, ethyl fluoroacetate, methyl fluoropropionate, and ethyl fluoropropionate; the cyclic ester is one or more of ethylene carbonate, propylene carbonate, fluoroethylene carbonate, and γ-butyrolactone.

[0020] Preferably, the concentration of lithium fluoride in the suspension in S1 is 2-10wt%, preferably 4-8wt%.

[0021] Preferably, the molar ratio of phosphorus pentafluoride to lithium fluoride is 0.9-1.1:1.

[0022] Preferably, the reaction temperature of the phosphorus pentafluoride and lithium fluoride suspension is 10-50°C, preferably 15-40°C.

[0023] Preferably, the stripping temperature in S2 is 25-50°C, preferably 30-40°C.

[0024] Preferably, the gas stripping time in S2 is 0.5-3 h, preferably 1-2 h.

[0025] Preferably, the acidity of the reaction liquid is controlled to be 800-1200 ppm after gas stripping in S2.

[0026] In S2 described in the present invention, when the gas stripping temperature is low, the solubility of the acidic gas in the reaction solution increases, which is not conducive to improving the gas stripping effect; when the gas stripping temperature is high, lithium hexafluorophosphate is easily decomposed when it is exposed to high temperatures for a long time, so the gas stripping is carried out at 25-50°C, preferably 30-40°C.

[0027] Preferably, the amount of lithium carbonate added in S3 is 0.01-0.5% of the mass of lithium fluoride in S1, preferably 0.1-0.2%.

[0028] Preferably, the lithium carbonate in S3 is added in the form of powder to facilitate the reaction. When the particle size of lithium carbonate is too small, it will cause difficulty in subsequent filtration operation; when the particle size of lithium carbonate is too large, it is not conducive to the reaction with hydrogen fluoride.

[0029] Preferably, the particle size distribution of the lithium carbonate powder in S3 is 20-50 μm, accounting for 70-85%, preferably 80%.

[0030] Preferably, in S3, the reaction time is 0.5-3 h, preferably 1-2 h.

[0031] Preferably, in S3, the reaction temperature is 10-50°C, preferably 15-40°C.

[0032] Preferably, the filtration method in S3 is one or more of pressure filtration, normal pressure filtration, and reduced pressure filtration, preferably reduced pressure filtration.

[0033] In the present invention, after the reaction liquid is stripped, the acidity is controlled between 500-1500ppm, preferably between 800-1200ppm, lithium carbonate is added to the reaction liquid to react with residual hydrogen fluoride to generate lithium fluoride, carbon dioxide and a small amount of water, and the solid mixture of lithium carbonate and lithium fluoride and the filtrate are separated after filtration, and the acidity of the filtrate is controlled to be 200-400ppm. After the solid mixture is washed and dried, the content of lithium carbonate and lithium fluoride is analyzed.

[0034] Preferably, the resin is a basic resin such as DuPont Amberlyst A21, and the molecular sieve is a lithium molecular sieve.

[0035] Preferably, the deacidification flow rate during the deacidification process is 0.5-5 BV / h.

[0036] Preferably, the deacidification process can be a cyclic deacidification or a one-way deacidification, preferably a one-way deacidification.

[0037] Preferably, the acidity of the product after deacidification is controlled to be 0.1-50 ppm, preferably 0.5-40 ppm.

[0038] The technical solution provided by the present invention has the following beneficial effects:

[0039] The invention reacts phosphorus pentafluoride gas with lithium fluoride suspension to obtain lithium hexafluorophosphate solution, and controls the acidity of crude products at different stages through gas stripping, adding lithium carbonate powder, filtering and other steps, and finally obtains qualified products through a deacidification step. The mass ratio of lithium monofluorophosphate to phosphorus oxytrifluoride in the lithium hexafluorophosphate solution is controlled between 1.5 and 5:1, and the color number and acidity of the obtained lithium hexafluorophosphate solution do not increase significantly after being stored at room temperature for 6 months. DETAILED DESCRIPTION

[0040] The present invention is further described below in conjunction with specific examples. These examples are only for illustrating the present invention and are not to be construed as limiting the scope of the present invention or the method for implementing the present invention.

[0041] Lithium monofluorophosphate and phosphorus oxytrifluoride test method:

[0042] The deuterated acetonitrile solution of hexafluorobenzene and the deuterated acetonitrile solution of lithium hexafluorophosphate were added to the inner and outer tubes of the coaxial nuclear magnetic resonance tube respectively for fluorine spectrum detection; the deuterated acetonitrile solution of phosphoric acid and the deuterated acetonitrile solution of lithium hexafluorophosphate were added to the inner and outer tubes respectively for phosphorus spectrum detection. Lithium hexafluorophosphate and deuterated acetonitrile were prepared into a sample with a concentration of 20wt%, and fluorine and phosphorus spectrum detection were performed.

[0043] Acidity test method:

[0044] Weigh 5.0 g of the sample (accurate to 0.0001 g) into 50 mL of acetone, add 3-5 drops of indicator (0.1 g each of methylene blue and neutral red, dissolved in 99.8 g of anhydrous ethanol), and titrate with 0.01 mol / L triethylamine / ethyl methyl carbonate standard solution until the solution turns green, which is the titration end point, and maintain for 3-6 seconds.

[0045] Example 1

[0046] Mix 5 parts by mass of lithium fluoride with 95 parts by mass of ethyl methyl carbonate to obtain a lithium fluoride suspension, pass phosphorus pentafluoride gas into the suspension, the gas flow rate is 8.9 parts by mass / h, control the reaction temperature at 30 ° C, react for 3 hours, and the system is clarified, and the reaction is completed. Use 35 ° C nitrogen gas stripping for 1 hour, and test the acidity of the reaction liquid to 1050ppm. Add 0.1% by mass of lithium carbonate powder (20-50μm particle size distribution accounts for 80%) to it, react at 30 ° C for 1 hour, filter after the reaction to obtain solids and filtrate, and test the acidity of the filtrate to 308ppm. After a single pass of resin deacidification, the deacidification flow rate is 2BV / h, the acidity of the test product is 15ppm, the color number is 20.1Hazen, and the mass ratio of lithium monofluorophosphate to trifluorophosphorus oxyfluoride is 2.11:1. After 6 months of storage at room temperature, the acidity of the test product is 16ppm and the color number is 20.5Hazen.

[0047] Example 2

[0048] Mix 6 parts by mass of lithium fluoride with 94 parts by mass of ethyl methyl carbonate to obtain a lithium fluoride suspension, introduce phosphorus pentafluoride gas into the suspension, the gas flow rate is 7.7 parts by mass / h, control the reaction temperature at 25°C, and the system is clarified after 4 hours of reaction, and the reaction is completed. Use 30°C nitrogen gas stripping for 1.5 hours, and test the acidity of the reaction liquid to be 980ppm. Add 0.2% by mass of lithium carbonate powder (20-50μm particle size distribution accounts for 80%) to it, react at 30°C for 2 hours, filter after the reaction to obtain solids and filtrate, and test the acidity of the filtrate to be 291ppm. After resin single-pass deacidification, the deacidification flow rate is 3BV / h, the test product acidity is 12ppm, the color number is 19.8Hazen, and the mass ratio of lithium monofluorophosphate to trifluorophosphorus oxyfluoride is 2.36:1. After 6 months of storage at room temperature, the acidity of the test product is 14ppm and the color number is 20.7Hazen.

[0049] Example 3

[0050] 7 parts by mass of lithium fluoride and 93 parts by mass of ethyl methyl carbonate were mixed to obtain a lithium fluoride suspension, phosphorus pentafluoride gas was introduced into the suspension, the gas flow rate was 6.1 parts by mass / h, the reaction temperature was controlled at 20°C, the system was clarified after 5 hours of reaction, and the reaction was completed. Use 40°C nitrogen gas stripping for 0.5h, and the acidity of the reaction liquid was tested to be 875ppm. 0.4% by mass of lithium carbonate powder (20-50μm particle size distribution accounted for 75%) was added thereto, reacted at 35°C for 0.5h, and filtered after the reaction to obtain solids and filtrate, and the acidity of the filtrate was tested to be 239ppm. After a single pass of resin deacidification, the deacidification flow rate was 1BV / h, the acidity of the test product was 9ppm, and the color number was 18.1Hazen, where the mass ratio of lithium monofluorophosphate to trifluorophosphorus oxyfluoride was 2.91:1. After storage at room temperature for 6 months, the acidity of the test product was 11ppm and the color number was 18.9Hazen.

[0051] Example 4

[0052] Mix 8 parts by mass of lithium fluoride with 92 parts by mass of ethyl methyl carbonate to obtain a lithium fluoride suspension, introduce phosphorus pentafluoride gas into the suspension, the gas flow rate is 12.3 parts by mass / h, control the reaction temperature at 35°C, and the system is clarified after 3 hours of reaction, and the reaction is completed. Use 25°C nitrogen gas stripping for 2 hours, and test the acidity of the reaction liquid to be 1095ppm. Add 0.3% by mass of lithium carbonate powder (70% of the particle size distribution of 20-50μm) to it, react at 20°C for 3 hours, filter after the reaction to obtain solids and filtrate, and test the acidity of the filtrate to be 323ppm. After resin circulation deacidification, the deacidification flow rate is 4BV / h, the acidity of the test product is 11ppm, and the color number is 19.6Hazen, where the mass ratio of lithium monofluorophosphate to trifluorophosphorus oxyfluoride is 3.12:1. After storage at room temperature for 6 months, the acidity of the test product is 12ppm and the color number is 20.1Hazen.

[0053] Example 5

[0054] Mix 4 parts by mass of lithium fluoride with 96 parts by mass of ethyl methyl carbonate to obtain a lithium fluoride suspension, introduce phosphorus pentafluoride gas into the suspension, the gas flow rate is 6.5 parts by mass / h, control the reaction temperature at 40°C, and the system is clarified after 3 hours of reaction, and the reaction is completed. Use 45°C nitrogen gas stripping for 0.5h, and test the acidity of the reaction liquid to be 817ppm. Add 0.15% by mass of lithium carbonate powder (20-50μm particle size distribution accounts for 85%) to it, react at 40°C for 1.5h, filter after the reaction to obtain solids and filtrate, and test the acidity of the filtrate to be 231ppm. After resin circulation deacidification, the deacidification flow rate is 2.5BV / h, the acidity of the test product is 5ppm, and the color number is 17.3Hazen, where the mass ratio of lithium monofluorophosphate to trifluorophosphorus oxyfluoride is 3.50:1. After 6 months of storage at room temperature, the acidity of the test product is 6ppm, and the color number is 18.1Hazen.

[0055] Comparative Example 1

[0056] Mix 5 parts by mass of lithium fluoride with 95 parts by mass of ethyl methyl carbonate to obtain a lithium fluoride suspension, pass phosphorus pentafluoride gas into the suspension, the gas flow rate is 8.9 parts by mass / h, control the reaction temperature at 30 ° C, react for 3 hours, and the system is clarified, and the reaction is completed. Use 35 ° C nitrogen gas stripping for 1 hour, and test the acidity of the reaction liquid to 1050ppm. Add 0.6% by mass of lithium carbonate powder (20-50μm particle size distribution accounts for 80%) to it, react at 30 ° C for 1 hour, filter after the reaction to obtain solids and filtrate, and test the acidity of the filtrate to be 165ppm. After resin single-pass deacidification, the deacidification flow rate is 2BV / h, the test product acidity is 14ppm, the color number is 19.7Hazen, and the mass ratio of lithium monofluorophosphate to trifluorophosphorus oxyfluoride is 5.50:1. After 6 months of storage at room temperature, the acidity of the test product is 25ppm and the color number is 23.1Hazen.

[0057] Comparative Example 2

[0058] Mix 6 parts by mass of lithium fluoride with 94 parts by mass of ethyl methyl carbonate to obtain a lithium fluoride suspension, introduce phosphorus pentafluoride gas into the suspension, the gas flow rate is 7.7 parts by mass / h, control the reaction temperature at 25°C, and the system is clarified after 4 hours of reaction, and the reaction is completed. Use 10°C nitrogen gas stripping for 1.5 hours, and test the acidity of the reaction liquid to be 1410ppm. Add 0.2% by mass of lithium carbonate powder (80% of the particle size distribution of 20-50μm) to it, react at 30°C for 2 hours, filter after the reaction to obtain solids and filtrate, and test the acidity of the filtrate to be 491ppm. After resin single-pass deacidification, the deacidification flow rate is 3BV / h, the acidity of the test product is 14ppm, the color number is 19.0Hazen, and the mass ratio of lithium monofluorophosphate to trifluorophosphorus oxyfluoride is 0.91:1. After storage at room temperature for 6 months, the acidity of the test product is 20ppm and the color number is 25.0Hazen.

[0059] Comparative Example 3

[0060] Mix 5 parts by mass of lithium fluoride with 95 parts by mass of ethyl methyl carbonate to obtain a lithium fluoride suspension, pass phosphorus pentafluoride gas into the suspension, the gas flow rate is 8.9 parts by mass / h, control the reaction temperature at 30 ° C, react for 3 hours, and the system is clarified, and the reaction is completed. Use 35 ° C nitrogen gas stripping for 1 hour, and test the acidity of the reaction liquid to 1050ppm. Add 0.1% by mass of lithium carbonate powder (20-50μm particle size distribution accounts for 40%), react at 30 ° C for 1 hour, filter after the reaction to obtain solids and filtrate, and test the acidity of the filtrate to 502ppm. After resin single-pass deacidification, the deacidification flow rate is 2BV / h, the test product acidity is 15ppm, the color number is 19.7Hazen, and the mass ratio of lithium monofluorophosphate to trifluorophosphorus oxychloride is 0.93:1. After 6 months of storage at room temperature, the acidity of the test product is 23ppm and the color number is 26.1Hazen.

[0061] It can be seen from the above examples and comparative examples that the lithium hexafluorophosphate solution prepared by the process of the present invention has high stability and has no obvious increase in acidity and color after being stored at room temperature for 6 months.

[0062] Those skilled in the art will appreciate that, based on the teachings of this specification, some modifications or adjustments may be made to the present invention, and these modifications or adjustments should also be within the scope defined by the claims of the present invention.

Claims

1. A lithium hexafluorophosphate solution comprising the following components: lithium hexafluorophosphate, an organic solvent, phosphorus oxytrifluoride and lithium monofluorophosphate, wherein the mass ratio of lithium monofluorophosphate to phosphorus oxytrifluoride is 1.5-5:1, preferably 2.0-3.5:

1.

2. The lithium hexafluorophosphate solution according to claim 1, characterized in that Based on the mass of the lithium hexafluorophosphate solution, the content of lithium hexafluorophosphate is 25-35wt%; the content of phosphorus oxytrifluoride is 2-30ppm, and the content of lithium monofluorophosphate is 3-150ppm.

3. A method for preparing the lithium hexafluorophosphate solution according to claim 1 or 2, comprising the following steps: S1: adding phosphorus pentafluoride into a suspension of lithium fluoride and an organic solvent to obtain a lithium hexafluorophosphate reaction solution; S2: Gas stripping the reaction liquid, controlling the acidity of the reaction liquid after gas stripping to 500-1500ppm; S3: adding lithium carbonate to the reaction solution after gas stripping, reacting, filtering to obtain a filtrate, the acidity of the filtrate is 200-400 ppm; S4: The filtrate in S3 is deacidified by resin or molecular sieve to further reduce the acidity to 0.1-50 ppm.

4. The method according to claim 3, characterized in that The organic solvent in S1 is one or more of chain esters or cyclic esters; preferably, the chain ester is one or more of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, ethyl fluoroacetate, methyl fluoropropionate, and ethyl fluoropropionate; the cyclic ester is one or more of ethylene carbonate, propylene carbonate, fluoroethylene carbonate, and γ-butyrolactone.

5. The method according to claim 3 or 4, characterized in that: The molar ratio of phosphorus pentafluoride to lithium fluoride is 0.9-1.1:

1.

6. The method according to any one of claims 3 to 5, characterized in that: The stripping temperature in S2 is 25-50°C, preferably 30-40°C; the stripping time in S2 is 0.5-3h, preferably 1-2h.

7. The method according to any one of claims 3 to 6, characterized in that: The amount of lithium carbonate added in S3 is 0.01-0.5% of the mass of lithium fluoride in S1, preferably 0.1-0.2%.

8. The method according to any one of claims 3 to 7, characterized in that: The particle size distribution of the lithium carbonate powder in S3 is 20-50 μm, accounting for 70-85%, preferably 80%.

9. The method according to any one of claims 3 to 8, characterized in that: In the step S3, the reaction time is 0.5-3 h, preferably 1-2 h; the reaction temperature is 10-50°C, preferably 15-40°C.

10. The method according to any one of claims 3 to 9, characterized in that: In S4, the resin is a basic resin, preferably DuPont amberlyst A21, and the molecular sieve is a lithium molecular sieve; preferably, the deacidification flow rate during the deacidification process is 0.5-5BV / h.