Purification method of bis (fluorosulfonyl) imide salt

Abstract

The invention belongs to the field of preparation of bis (fluorosulfonyl) imide salt, and particularly relates to a purification method of bis (fluorosulfonyl) imide salt. The purification method comprises the following steps: adding a poor solvent of bis (fluorosulfonyl) imide salt into a separator, introducing a liquid salt solution into the poor solvent, stirring and mixing, standing for crystallization, discharging a good solvent after the good solvent in the liquid salt solution is migrated to the position above the poor solvent, and then discharging the poor solvent and crystal salt. The good solvent and the poor solvent are separated through density difference, the poor solvent can be recycled, the good solvent is further subjected to devitrification or distillation separation or continues to prepare a next batch of materials, and the good solvent only occupies a small part of an anti-solvent crystallization method, so that the amount of the solvent needing distillation separation can be effectively reduced, idle running of the poor solvent is avoided, and the cost is reduced. The solvent recovery energy consumption and the production cost are reduced.

Description

Technical Field

[0001] The invention belongs to the field of preparation of bisfluorosulfonyl imide salts, and particularly relates to a method for purifying bisfluorosulfonyl imide salts. Background Art

[0002] Developing new lithium salts as functional additives that can improve the chemical stability of electrolytes and achieve improved high-temperature cycle stability of batteries (such as extending cycle life, improving rate performance and safety, etc.) is one of the technical development directions for optimizing electrolyte performance in recent years. Bis(fluorosulfonyl)imide salts are added to LiPF6 electrolytes as additives. On the one hand, they inhibit the generation of hydrogen fluoride in the electrolyte and block the slow and continuous decomposition of LiPF6, thereby achieving a substantial improvement in the chemical stability of the electrolyte. On the other hand, by improving the conductivity of the electrolyte and exerting its unique SEI film-forming ability, it not only improves the battery cycle capacity, but also effectively improves the low-temperature discharge performance of the battery and the capacity retention rate after high-temperature storage, while also having the effect of inhibiting expansion.

[0003] The bisfluorosulfonyl imide salt added to the electrolyte requires that the purity, moisture, acidity, chloride ions, residual solvent, etc. should all meet certain technical indicators. The preparation of bisfluorosulfonyl imide salt is currently divided into three steps: chlorination, fluorination, and salt formation: the synthesis of bischlorosulfonyl imide, the synthesis of bisfluorosulfonyl imide, and the synthesis of bisfluorosulfonyl imide salt. The synthesis process involves many raw materials and equipment, and there is a lot of heat release during the reaction. Therefore, the bisfluorosulfonyl imide salt synthesized at one time is a crude salt with high water, acid, chloride ions, and residual solvent, and needs to be purified to obtain salt with all indicators qualified.

[0004] A Chinese invention patent with an authorization announcement date of May 26, 2020 and an authorization announcement number of CN 106976849 B discloses a method for purifying lithium bis(fluorosulfonyl)imide, which comprises: (1) dissolving a crude lithium bis(fluorosulfonyl)imide product in an organic solvent (i.e., a good solvent such as dimethyl carbonate), filtering to remove insoluble matter, and obtaining a filtrate; (2) mixing the filtrate and an organic acid anhydride, heating to remove water, and obtaining a mixed solution (i.e., a liquid salt solution); (3) evaporating and concentrating the mixed solution, and then adding a low-polarity organic solvent with a polarity of less than 4 (e.g., a poor solvent such as dichloromethane) for crystallization to obtain a pure lithium bis(fluorosulfonyl)imide product.

[0005] The above purification method is to add a dehydrating agent to remove water after dissolving and filtering the crude salt, and then remove part of the good solvent through evaporation and concentration, and then add a poor solvent for crystallization. The existing problems are: first, the evaporation and concentration process may cause the decomposition of the bis(fluorosulfonyl)imide salt, thereby increasing the insoluble matter of the product (salt decomposes under high temperature conditions to generate LiF, Li2SO4 and other substances that are insoluble in good solvents, called insoluble matter); second, the added dehydrating agent cannot be completely removed from the product, which may cause the acidity of the product to increase; third, the crystallization process of adding a poor solvent is an anti-solvent crystallization. After adding a poor solvent to precipitate the bis(fluorosulfonyl)imide salt, the remaining solvent needs to be recycled. Enterprises often use distillation to separate and reuse the poor solvent and the good solvent, which generates a lot of energy consumption and pushes up the production cost of the bis(fluorosulfonyl)imide salt. Summary of the invention

[0006] The purpose of the present invention is to provide a method for purifying a bisfluorosulfonyl imide salt, so as to solve the problem that a large amount of solvent to be separated by distillation is generated during anti-solvent crystallization in the prior art, resulting in increased production energy consumption and production cost of the bisfluorosulfonyl imide salt.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is:

[0008] A method for purifying a bisfluorosulfonyl imide salt comprises the following steps: adding a poor solvent for the bisfluorosulfonyl imide salt into a separator, introducing a liquid salt solution into the poor solvent, stirring and mixing, standing for crystallization, and after a good solvent in the liquid salt solution migrates to above the poor solvent, first discharging the good solvent, and then discharging the poor solvent and the crystalline salt.

[0009] The present invention belongs to an improved invention. In the anti-solvent crystallization process, the liquid salt solution is promptly dispersed into the poor solvent by stirring and mixing. The static process, on the one hand, promotes the crystal salt to settle to the bottom of the separator, and on the other hand, creates conditions for the good solvent to migrate upward by utilizing the density difference. After that, the good solvent and the poor solvent are discharged separately. The poor solvent can be recycled, and the good solvent is further crystallized or distilled or continued to prepare the next batch of materials. Since the good solvent only accounts for a small part of the anti-solvent crystallization method, the amount of solvent that needs to be distilled and separated can be effectively reduced, the idling of the poor solvent can be avoided, and the energy consumption of solvent recovery and the production cost can be reduced.

[0010] In addition, the use of stirring and mixing and standing crystallization strategy can also avoid the problem of crystals being easily entrained with impurities caused by the continuous stirring crystallization strategy, and is also helpful in reducing the content of insoluble matter, moisture, and free acid. Therefore, this method also further improves the quality of the resulting crystalline salt.

[0011] Preferably, the stirring and mixing time is 30 to 40 minutes; and the standing and crystallizing time is more than 30 minutes.

[0012] Preferably, the separator has multiple stages, the good solvent discharged from the previous stage separator enters the next stage separator, and crystallization is performed again in the same manner, the crystal salt discharged from each stage separator is collected, and the poor solvent discharged from each stage separator is recycled. The multi-stage separation strategy can fully precipitate the bis(fluorosulfonyl)imide salt in the liquid salt solution, thereby improving the product yield.

[0013] Preferably, the mass concentration of the liquid salt solution is above 50%, and the mass ratio of the liquid salt solution to the poor solvent in the separator is 1:(10-20). For example, the mass concentration of the liquid salt solution may be 50-65%.

[0014] Preferably, the poor solvent is precooled, and the temperature of the precooled poor solvent is 0-10°C.

[0015] Preferably, the density of the poor solvent is greater than that of the good solvent; the good solvent is selected from one or a combination of two or more of acetone, ethanol, ether, acetic acid, acetonitrile, isopropanol, methanol, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; the poor solvent is selected from one or two of carbon tetrachloride and dichloromethane.

[0016] Preferably, the liquid salt solution is a crude salt solution of a bisfluorosulfonyl imide salt; the crude salt solution is obtained by removing acid and / or water from a good solvent solution of the crude salt. Further preferably, the preparation of the liquid salt solution comprises: mixing and dissolving the crude salt to be purified, its good solvent, and an acid scavenger, filtering, and obtaining the liquid salt solution; the bisfluorosulfonyl imide salt is an alkali metal salt, the acid scavenger is selected from alkali metal carbonates, and the alkali metal elements in the bisfluorosulfonyl imide salt and the acid scavenger are the same.

[0017] Further preferably, the mass ratio of the crude salt to the good solvent is 1:(0.5-1); the added mass of the alkali metal carbonate is 2‰-5‰ of the mass of the crude salt. By controlling the mass ratio of the crude salt to the good solvent, the present method omits the distillation and concentration step, which can greatly avoid the increase of insoluble matter in the product caused by the decomposition of the bisfluorosulfonyl imide salt. In addition, the addition of a trace amount of alkali metal carbonate can effectively remove the acidity in the crude salt without introducing impurities or increasing the water content.

[0018] More preferably, the temperature of the mixed dissolution is 25±5° C. and the time is 4 to 6 hours. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic diagram of the structure of the separator used in the method of the present invention;

[0020] Figure 2 It is a schematic diagram of the structure of the two-stage separators connected in series in the present invention.

[0021] Among them, 1-separator; 2-discharge port; 3-first valve; 4-first inlet; 5-first outlet; 6-first agitator; 7-second agitator; 8-filter area; 9-solvent temporary storage bin; 10-second outlet; 11-central tube. DETAILED DESCRIPTION

[0022] The technical concept of the present invention is to utilize the density difference between the poor solvent and the good solvent, and simultaneously realize the crystallization of the bisfluorosulfonyl imide salt and the diffusion rearrangement of the poor solvent and the good solvent based on the density difference during the anti-solvent crystallization process. The precipitated crystal salt and the poor solvent with larger density are located at the lower part of the separator, and the good solvent with smaller density is located at the upper part of the poor solvent. After that, the good solvent, the poor solvent, and the crystal salt are discharged successively to realize the classified recovery of various materials. At this time, although the separated good solvent (i.e., mother liquor) still contains a small amount of poor solvent (about 10%), the purpose of recovering the poor solvent has been achieved. Re-crystallization or distillation of this part of the mother liquor to separate the good solvent or continue to prepare the next batch of materials can greatly reduce the distillation energy consumption and reduce production costs.

[0023] The bisfluorosulfonyl imide salt applicable to the present invention is mainly an alkali metal salt, which may be lithium bisfluorosulfonyl imide, sodium bisfluorosulfonyl imide or potassium bisfluorosulfonyl imide.

[0024] The test results of the quality of the crystal salt of this method show that this method also promotes the efficiency of crystallization, and the crystallization time is reduced from about 4 to 5 hours to tens of minutes. The strategy based on stirring, mixing and standing crystallization also brings about the reduction of insoluble matter, moisture and free acid in the crystal salt, and comprehensively improves the quality of the crystal salt. Specifically, the crude salt detection indicators are: moisture: 200-1000ppm, acidity: 100-300ppm, chloride ion: 20-50ppm, and insoluble matter is 1%-3%. After being treated by this method, the purity of the obtained crystal salt reaches more than 99.95%, moisture: <20ppm, acidity: <20ppm, chloride ion: 3-10ppm, and insoluble matter is <200ppm, which fully meets the purity requirements of downstream products.

[0025] In addition, the method does not perform evaporation and concentration during the preparation of the liquid salt solution. Efficient impurity removal is achieved by controlling the ratio of crude salt and good solvent and adding a trace amount of alkali metal carbonate, thereby avoiding the decomposition of the bis(fluorosulfonyl)imide salt and the increase in the insoluble matter content caused by evaporation and concentration.

[0026] Furthermore, if the amount of precipitated crystals is insufficient, two or more stages of separators may be constructed in series to allow the bis(fluorosulfonyl)imide salt in the solution to be fully precipitated, thereby increasing the product yield.

[0027] The structure of the separator used in this method is introduced below.

[0028] like Figure 1As shown, the separator 1 is a cylindrical structure as a whole, including a cylindrical section as a main body and a conical section connected to the lower end of the cylindrical section. A discharge port 2 is provided at the lower end of the conical section, and a first valve 3 is provided on the discharge port 2. A first inlet 4 and a first outlet 5 are provided at the upper part of the cylindrical section, and the position of the first outlet 5 is lower than the first inlet 4. A first agitator 6 and a second agitator 7 are provided on the inner wall of the lower part of the cylindrical section. The first and second agitators are arranged relatively on the same circumference, and the extension direction of the agitator shafts of the first and second agitators is perpendicular to the axis of the separator 1. A filter area 8 is provided on the conical section, and a solvent temporary storage bin 9 surrounding the filter area 8 is provided outside the filter area 8, and a second outlet 10 is provided at the lower end of the solvent temporary storage bin 9. The separator 1 is also connected to a central tube 11 for passing a liquid salt solution, and the lower outlet of the central tube 11 is arranged near the first and second agitators.

[0029] When the separator 1 is working, a poor solvent accounting for about 2 / 3 of the volume of the separator 1 is first introduced from the first inlet 4, and then a certain proportion of liquid salt solution is introduced from the central tube 11. After the liquid salt solution flows out from the central tube 11, it is surrounded by a large amount of poor solvent and is dispersed and diluted under the action of the agitator. Based on the change in solubility, the bisfluorosulfonyl imide salt precipitates from the solution; after the liquid salt solution is introduced and stirred for a period of time to fully mix the good solvent and the poor solvent, it is allowed to stand. At this time, the bisfluorosulfonyl imide salt crystals sink to the bottom of the separation, and the good solvent floats above the poor solvent due to the density difference. The position of the first outlet 5 is adapted to the amount of good solvent introduced into the liquid salt solution, and theoretically, the good solvent in the liquid salt solution is just released. The actual controllable release amount is slightly higher than the theoretical amount, for example, the mass proportion of the poor solvent in the discharged liquid is controlled to be 10-15%, which is convenient for operation and does not affect subsequent use. At this time, the remaining solvent in the separator 1 is basically all poor solvent, and the mother liquor discharged from the first outlet 5 contains only about 10% poor solvent (the rest is good solvent), realizing anti-solvent crystallization and also preliminarily separating the poor solvent and the good solvent.

[0030] Then, the valve of the second outlet 10 is opened, and the bad solvent in the separator 1 enters the solvent temporary storage bin 9 through the filter area 8, and then is discharged from the solvent temporary storage bin 9. The discharged bad solvent can be recycled and reused in the above-mentioned anti-solvent crystallization process or stored in a solvent storage tank. Finally, the valve of the discharge port 2 is opened to discharge the crystal salt at the bottom of the separator 1, and the finished crystal salt is obtained after conventional treatment such as centrifugal separation, nitrogen purging, and drying.

[0031] Furthermore, for the poor solvents that are reused multiple times, their purity decreases. When the purity of the poor solvent decreases to below 95%, the product yield will be affected. At this time, 1-2 level separators can be added to crystallize the mother liquor discharged from the first outlet 5 and then the secondary anti-solvent, and further crystallize the bis(fluorosulfonyl)imide salt in the mother liquor. Figure 2As shown, the first outlet 5 of the upper separator is connected to the central tube of the lower separator, which is used to pass the mother liquid discharged from the first outlet 5 through the central tube, and the second outlet 10 of the upper separator is connected to the first inlet 4 of the lower separator. Then, according to the same operation method, the mother liquid is discharged from the first outlet 5 of the lower separator, the poor solvent is discharged from the second outlet 10 on the solvent temporary storage bin 9 of the lower separator, and the crystal salt is discharged from the discharge port of the lower separator. Whether to add a cooler can be determined based on the temperature of the discharged liquid. When the temperature is low in winter, the liquid temperature can be controlled below 10°C without a cooler; when the temperature is high in summer, a cooler can be further added to the connecting pipeline to pre-cool the temperature of the poor solvent entering the lower separator to 10°C.

[0032] The implementation process of the present invention is described in detail below in conjunction with specific embodiments. Unless otherwise specified in the following embodiments, "%" refers to mass percentage.

[0033] 1. Specific embodiment of the method for purifying the bisfluorosulfonyl imide salt of the present invention

[0034] Example 1

[0035] The purification method of the bisfluorosulfonyl imide salt of this embodiment specifically comprises the following steps:

[0036] 1) Add 50 kg of crude salt of lithium bis(fluorosulfonyl)imide into a dissolving tank (the crude salt has the following test indicators: moisture: 800 ppm, acidity: 250 ppm, chloride ion: 49 ppm, insoluble matter: 1%), then add non-polar organic solvent dimethyl carbonate, the mass ratio of crude salt to dimethyl carbonate is 1:0.6, then add 5‰ of lithium carbonate by mass of crude salt, keep the temperature at 25±5°C, and stir thoroughly to dissolve for 4 hours.

[0037] 2) The fully dissolved mixed material is filtered, and the turbidity of the filtrate is controlled to be ≤10NTU to obtain 79.5kg of a clear and transparent liquid salt solution.

[0038] 3) The polar solvent dichloromethane is pre-cooled to a temperature of 10°C and enters the separator 1 from the first inlet 4. When the container is filled to 2 / 3, the clear and transparent liquid salt solution obtained by filtration in step 2) enters the separator 1 from the central tube 11. The mass ratio of the liquid salt solution to the polar solvent is 1:20. After the liquid salt solution is introduced, the stirring of the side wall of the separator 1 is turned on (the introduction speed of the liquid salt solution is generally 40-50 kg / min). After the introduction is completed, the stirring is continued for 40 minutes, the stirring is stopped, and the mixture is allowed to stand for 30 minutes. The first outlet 5 is opened to discharge the mother liquor (the mass of the discharged mother liquor is 42% of the mass of the introduced liquid salt solution). The non-polar solvent dichloromethane in the mother liquor is detected to account for 10%.

[0039] The second outlet 10 was opened to discharge the dichloromethane, which was cooled by the cooler and then entered the first inlet of the next-stage separator; the precipitated crystalline salt was discharged from the discharge port 2, and after centrifugal separation, nitrogen purging for 10 hours, and drying at 30°C for 10 hours, 48.6 kg of finished crystalline salt was obtained, with a yield of 98.2%.

[0040] Example 2

[0041] The purification method of the bisfluorosulfonyl imide salt of this embodiment specifically comprises the following steps:

[0042] 1) Add 94.4 kg crude salt of sodium bis(fluorosulfonyl)imide into a dissolving tank (the crude salt has the following test indicators: moisture: 1000 ppm, acidity: 100 ppm, chloride ion: 28 ppm, insoluble matter: 1.5%), then add non-polar organic solvent diethyl carbonate, the mass ratio of crude salt to diethyl carbonate is 1:0.7, then add 2‰ of the mass of crude salt of sodium carbonate, keep the temperature at 25±5°C, and stir thoroughly to dissolve for 4 hours.

[0043] 2) The fully dissolved mixed material is filtered, and the turbidity of the filtrate is controlled to be ≤10NTU to obtain 159kg of clear and transparent liquid salt solution.

[0044] 3) The polar solvent dichloromethane is pre-cooled to a temperature of 10°C and enters the separator 1 from the first inlet 4. When the container is filled to 2 / 3, the clear and transparent liquid salt solution obtained by filtration in step 2) enters the separator 1 from the central tube 11. The mass ratio of the liquid salt solution to the polar solvent is 1:10. Then, the stirring of the side wall of the separator 1 is turned on. After the liquid salt solution is introduced, the stirring is continued for 40 minutes. The stirring is stopped, and the mixture is allowed to stand for 30 minutes. The first outlet 5 is opened to discharge the mother liquor (the mass of the discharged mother liquor is 45% of the mass of the introduced liquid salt solution). The non-polar solvent dichloromethane in the mother liquor is detected to account for 8%.

[0045] The second outlet 10 was opened to discharge the dichloromethane, which was cooled by the cooler and then entered the first inlet of the next-stage separator; the precipitated crystalline salt was discharged from the discharge port 2, and after centrifugal separation, nitrogen purging for 2 hours, and drying at 30°C for 2 hours, 91.6 kg of finished crystalline salt was obtained, with a yield of 98.5%.

[0046] Example 3

[0047] The purification method of the bisfluorosulfonyl imide salt of this embodiment specifically comprises the following steps:

[0048] 1) Add 53.4 kg crude salt of potassium bis(fluorosulfonyl)imide into the dissolving tank (the crude salt has the following test indicators: moisture: 400 ppm, acidity: 300 ppm, chloride ion: 21 ppm, insoluble matter: 3%), then add non-polar organic solvent diethyl carbonate, the mass ratio of crude salt to diethyl carbonate is 1:1, then add 5‰ potassium carbonate of the mass of crude salt, keep the temperature at 25±5°C, and stir thoroughly to dissolve for 4 hours.

[0049] 2) The fully dissolved mixed material is filtered, and the turbidity of the filtrate is controlled to be ≤10NTU to obtain 105.2kg of clear and transparent liquid salt solution.

[0050] 3) The polar solvent dichloromethane is pre-cooled to a temperature of 10°C and enters the separator 1 from the first inlet 4. When the container is filled to 2 / 3, the clear and transparent liquid salt solution obtained by filtration in step 2) enters the separator 1 from the central tube. The mass ratio of the liquid salt solution to the polar solvent is 1:15. The stirring of the side wall of the separator 1 is turned on. After the liquid salt solution is completely introduced, the stirring is continued for 40 minutes. The stirring is stopped and the mixture is allowed to stand for 30 minutes. The first outlet 5 is opened to discharge the mother liquor (the mass of the discharged mother liquor is 59% of the mass of the introduced liquid salt solution). The non-polar solvent dichloromethane in the mother liquor is detected to account for 15%.

[0051] The second outlet 10 was opened to discharge the dichloromethane, which was cooled by the cooler and then entered the first inlet of the next-stage separator; the precipitated crystalline salt was discharged from the discharge port 2, and after centrifugal separation, nitrogen purging for 5 hours, and drying at 30°C for 5 hours, 47.5 kg of finished crystalline salt was obtained, with a yield of 97.9%.

[0052] Example 4

[0053] The purification method of the bisfluorosulfonyl imide salt of this embodiment specifically comprises the following steps:

[0054] 1) Add 50 kg of crude salt of lithium bis(fluorosulfonyl)imide into a dissolving tank (the crude salt has the following test indicators: moisture: 800 ppm, acidity: 250 ppm, chloride ion: 49 ppm, insoluble matter: 1%), then add non-polar organic solvent dimethyl carbonate, the mass ratio of crude salt to dimethyl carbonate is 1:0.6, then add 5‰ of lithium carbonate by mass of crude salt, keep the temperature at 25±5°C, and stir thoroughly to dissolve for 4 hours.

[0055] 2) The fully dissolved mixed material is filtered, and the turbidity of the filtrate is controlled to be ≤10NTU to obtain 79.5kg of a clear and transparent liquid salt solution.

[0056] 3) The polar solvent dichloromethane is used as the recovery solvent (the main content is 94.8%, and the remaining content is 5.2% of dimethyl carbonate). After precooling, the temperature is 10°C, and it enters the primary separator 1 from the first inlet 4. When the container is filled to 2 / 3, the clear and transparent liquid salt solution obtained by filtration in step 2) enters the primary separator 1 from the central tube 11, and the mass ratio of the liquid salt solution to the polar solvent is 1:20. Stirring is started while the liquid salt is introduced, and stirring is continued for 30-40 minutes after the introduction is completed, and the stirring is stopped. Let it stand for 30 minutes, and the first outlet 5 of the primary separator is opened. The mass of the discharged mother liquor is 45.67kg (the mass of the discharged mother liquor is 57% of the mass of the liquid salt solution introduced, and the non-polar solvent dichloromethane in the mother liquor is detected to account for 10%), and the separator is carried out from the central tube.

[0057] Open the second outlet 10 of the primary separator, discharge the dichloromethane, cool it in the cooler, and then enter the secondary separator through the first inlet 4 of the secondary separator. If the amount of the poor solvent entering is less than 2 / 3 of the container, it will be supplemented by the first inlet 4 of the secondary separator, and then the operation of the primary separator 1 will be repeated.

[0058] The precipitated crystalline salt was discharged from the discharge port 2 of the separator 1, and after centrifugal separation, nitrogen purging for 10 hours, and drying at 30°C for 10 hours, 37.1 kg of finished crystalline salt was obtained, and the yield was 75%; the crystalline salt precipitated from the secondary separator was discharged from the discharge port 2 of the separator, and after centrifugal separation, nitrogen purging for 10 hours, and drying at 30°C for 10 hours, 9.55 kg of finished crystalline salt was obtained, and the yield was 77%; the comprehensive yield was 94.2%.

[0059] The salt content in the mother liquor discharged from the secondary separator is less than 5kg, so there is no need for tertiary separation and it can be returned to the dissolution tank to continue preparing the next batch of materials.

[0060] 2. Comparison

[0061] The purification method of the bisfluorosulfonyl imide salt of this comparative example specifically comprises the following steps:

[0062] 1) Add 500 kg of crude salt of lithium bis(fluorosulfonyl)imide into a dissolving tank (the crude salt has the following test indicators: moisture: 800 ppm, acidity: 250 ppm, chloride ion: 49 ppm, insoluble matter: 1%), then add non-polar organic solvent dimethyl carbonate, the mass ratio of crude salt to dimethyl carbonate is 1:0.6, then add 5‰ of lithium carbonate by mass of crude salt, keep the temperature at 25±5°C, and stir thoroughly to dissolve for 4 hours.

[0063] 2) The fully dissolved mixed material is filtered, and the turbidity of the filtrate is controlled to be ≤10NTU to obtain 795kg of clear and transparent liquid salt solution.

[0064] 3) The liquid salt solution of step 2) was mixed with pre-cooled dichloromethane (10° C.) at a mixing mass ratio of 1:20, the reactor was stirred for 5 h, centrifuged for 3 h, purged with nitrogen for 10 h, and dried at 30° C. for 10 h to obtain 470.25 kg of finished crystalline salt with a yield of 95%.

[0065] 3. Experimental Examples

[0066] This experimental example tests various indicators of the crystalline salt obtained in each embodiment and comparative example, and refers to the non-ferrous industry standard "YS / T1302-2019 Lithium Bis(Fluorosulfonyl)imide Salt for Power Battery Electrolyte" for analysis. The results are shown in Table 1.

[0067] Table 1 Detection indexes of the crystal salts of various embodiments and comparative examples

[0068]

[0069] It can be seen from the experimental data of Table 1 that the crystalline salt obtained by the method of the embodiment is superior to the comparative example in terms of yield, dimethyl carbonate (DMC) insoluble matter, moisture, and free acid content. The reason may be that the comparative example generally adopts a continuous stirring method to mix a small amount of good solvent and a large amount of poor solvent during the anti-solvent crystallization process, thereby promoting the full precipitation of the bisfluorosulfonyl imide salt, but the crystallization under continuous stirring conditions will cause the crystal nucleus to break and the crystal secondary growth will occur. In the secondary growth process, moisture, acidity, etc. will be mixed, and the increase of moisture and acidity will cause the salt to further decompose in the subsequent drying process, thereby increasing the insoluble matter.

[0070] In terms of recycling of poor solvents, the traditional method requires the overall distillation and separation of the mixed solvent after the anti-solvent crystallization, which consumes a lot of energy for solvent recovery. This method achieves the preliminary separation of the two solvents while the anti-solvent is crystallized, which not only improves the solvent separation efficiency and reduces the energy consumption of solvent recovery, but also promotes the improvement of the quality of the crystal salt to a certain extent.

[0071] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. 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 method for purifying a bisfluorosulfonyl imide salt, characterized in that: The following steps are involved: A poor solvent for bisfluorosulfonyl imide salt is added to a separator, a liquid salt solution is introduced into the poor solvent, the mixture is stirred and allowed to stand for crystallization, and after the good solvent in the liquid salt solution migrates to the top of the poor solvent, the good solvent is discharged first, and then the poor solvent and the crystalline salt are discharged.

2. The method for purifying a bisfluorosulfonyl imide salt according to claim 1, characterized in that: The stirring and mixing time is 30 to 40 minutes; the standing and crystallizing time is more than 30 minutes.

3. The method for purifying a bisfluorosulfonyl imide salt according to claim 1, characterized in that: The separator has multiple stages. The good solvent discharged from the previous stage separator enters the next stage separator and is crystallized again in the same way. The crystal salt discharged from each stage separator is collected, and the poor solvent discharged from each stage separator is recycled.

4. The method for purifying a bisfluorosulfonyl imide salt according to claim 1, characterized in that: The mass concentration of the liquid salt solution is above 50%, and the mass ratio of the liquid salt solution to the poor solvent in the separator is 1:(10-20).

5. The method for purifying a bisfluorosulfonyl imide salt according to claim 1 or 4, characterized in that: The poor solvent is precooled, and the temperature of the precooled poor solvent is 0-10°C.

6. The method for purifying bisfluorosulfonyl imide salt according to claim 1, characterized in that: The density of the poor solvent is greater than that of the good solvent; the good solvent is selected from one or a combination of two or more of acetone, ethanol, ether, acetic acid, acetonitrile, isopropanol, methanol, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; the poor solvent is selected from one or two of carbon tetrachloride and dichloromethane.

7. The method for purifying a bisfluorosulfonyl imide salt according to claim 1, characterized in that: The liquid salt solution is a crude salt solution of bis(fluorosulfonyl)imide salt; the crude salt solution is obtained by removing acid and / or water from a good solvent solution of crude salt.

8. The method for purifying bisfluorosulfonyl imide salt according to claim 7, characterized in that: The preparation of the liquid salt solution comprises: mixing and dissolving the crude salt to be purified, its good solvent and the acid scavenger, filtering to obtain the liquid salt solution; the bisfluorosulfonyl imide salt is an alkali metal salt, the acid scavenger is selected from alkali metal carbonates, and the alkali metal elements in the bisfluorosulfonyl imide salt and the acid scavenger are the same.

9. The method for purifying a bisfluorosulfonyl imide salt according to claim 8, characterized in that: The mass ratio of the crude salt to the good solvent is 1:(0.5-1); the added mass of the alkali metal carbonate is 2‰-5‰ of the mass of the crude salt.

10. The method for purifying bisfluorosulfonyl imide salt according to claim 8, characterized in that: The mixed dissolution is carried out at a temperature of 25±5° C. and for a period of 4 to 6 hours.

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