A method for removing water from bis(fluorosulfonyl)imide salt

Abstract

The present invention relates to a method for dehydrating a bisfluorosulfonyl imide salt, belonging to the technical field of preparing low-water-content bisfluorosulfonyl imide salts. The method comprises the following steps: mixing an alkali metal salt and a bisfluorosulfonyl imide acid in a poor solvent for the bisfluorosulfonyl imide salt, heating and refluxing the mixture to generate the bisfluorosulfonyl imide salt; during the heating and refluxing reaction, the poor solvent is subjected to a dehydration treatment before being refluxed into the reaction system. The method for dehydrating the bisfluorosulfonyl imide salt of the present invention utilizes the poor solvent to dynamically remove water during the reaction. This method does not require the addition of other substances for dehydration after the reaction is completed, is simple to operate, and does not introduce impurities that would otherwise be introduced by introducing other chemical reagents for dehydration.

Description

Technical Field

[0001] The invention relates to a method for removing water from a bisfluorosulfonyl imide salt, and belongs to the technical field of preparing low-water-containing bisfluorosulfonyl imide salts. Background Art

[0002] The electrolyte currently used in lithium batteries is primarily lithium hexafluorophosphate (LiPF6), but it easily decomposes in contact with water, affecting battery performance. Lithium bis(fluorosulfonyl)imide (LiFSI) is considered the next-generation lithium-ion battery electrolyte due to its higher pyrolysis temperature, improved electrical conductivity, and improved low-temperature discharge cyclability. However, the production cost of LiFSI is currently higher than that of LiPF6, which limits its application to a certain extent. This high cost is mainly due to the complex preparation process and the subsequent complex water and impurity removal processes.

[0003] Moisture content is one of the important indicators of lithium bis(fluorosulfonyl)imide. Currently, its dehydration methods are mainly divided into two categories, chemical dehydration and physical dehydration. Chemical dehydration: mainly uses dichlorothionyl or organic acid anhydride to react with water in the solution to remove the water in the solution. Its operation is simple, but its disadvantage is that it will introduce impurities and increase the acidity of the solution. Physical dehydration: generally, an organic solvent is used to produce an azeotropic reaction with water to take away the water in the solvent. It will not introduce impurities, but it requires the use of a large amount of solvent, which is costly and complicated to operate. Based on the shortcomings of the existing technology, there is a need for a method that can effectively reduce moisture while introducing as few or no impurities as possible. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for removing water from a bisfluorosulfonyl imide salt, so as to solve the problems in the prior art of complex water removal and easy introduction of impurities in the bisfluorosulfonyl imide salt.

[0005] In order to achieve the above objectives, the technical solution of the present invention is:

[0006] A method for removing water from a bisfluorosulfonyl imide salt comprises the following steps: mixing an alkali metal salt and a bisfluorosulfonyl imide acid in a poor solvent for the bisfluorosulfonyl imide salt, heating and refluxing the mixture to generate the bisfluorosulfonyl imide salt;

[0007] During the heating reflux reaction, the poor solvent is subjected to a water removal treatment before being refluxed into the reaction system.

[0008] The present invention involves reacting an alkali metal salt and a bisfluorosulfonyl imide acid in a poor solvent for the bisfluorosulfonyl imide salt to generate the bisfluorosulfonyl imide salt. Dynamic water removal is performed during the reaction using the poor solvent. This water removal method eliminates the need to add other substances for water removal after the reaction is complete, is simple to operate, and eliminates the introduction of impurities that might otherwise be introduced by introducing other chemical reagents for water removal.

[0009] The dehydration treatment may be to directly dehydrate the poor solvent in a drying device and then reflux it into the reaction system. Preferably, the dehydration treatment is to first flow the refluxed poor solvent into a drying device containing a desiccant, and then flow it into the reaction system from the drying device.

[0010] Preferably, the gas generated by the reaction first enters the drying device and then undergoes condensation and reflux. The gas generated by the reaction includes gas after evaporation of the poor solvent, water vapor, and gas after evaporation of the reactants.

[0011] In order to facilitate evaporation of the poor solvent into gas and further remove water, preferably, the poor solvent is one or both of dichloromethane and an ether compound. The ether compound can be methyl tert-butyl ether. Dichloromethane is more preferably used.

[0012] In order to facilitate the reaction and the removal of water, the reaction temperature is preferably 45 to 50°C, more preferably 45 to 48°C.

[0013] Preferably, the reaction time is 10 to 15 hours, more preferably 10 to 12 hours, as this reaction time allows for a more complete reaction.

[0014] In order to better remove moisture during the reaction, the desiccant is preferably a molecular sieve, more preferably 4A molecular sieve.

[0015] In order to promote the reaction to proceed in the forward direction, preferably, the molar ratio of the alkali metal salt to the bisfluorosulfonyl imide acid is (1.1-1.6):1, more preferably (1.4-1.6):1.

[0016] Preferably, the alkali metal salt is a lithium salt or a sodium salt. The lithium salt or sodium salt is preferably an alkali metal salt that does not produce water when reacting with bis(fluorosulfonyl)imidic acid. Further preferably, the lithium salt is a lithium polyacrylate, lithium polymethacrylate, lithium benzoate, or lithium acetate.

[0017] Lithium polymethacrylate is insoluble in poor solvents for lithium bis(fluorosulfonyl)imide. The lithium bis(fluorosulfonyl)imide obtained after reaction with bis(fluorosulfonyl)imidic acid contains relatively low levels of water, acid, and chlorine impurities and exhibits high purity. The lithium salt is preferably lithium polymethacrylate. The number average molecular weight of the lithium polymethacrylate is 100,000 to 300,000.

[0018] In order to improve the efficiency of the reaction and facilitate the subsequent separation of the bisfluorosulfonyl imide salt, preferably, the molar ratio of the poor solvent to the bisfluorosulfonyl imide acid is (6-10):1. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1This is a diagram of the reaction apparatus involved in the dehydration method of lithium bis(fluorosulfonyl)imide according to Example 1 of the present invention; wherein 1 is a three-necked flask, 2 is a constant pressure dropping funnel, 3 is a thermometer, 4 is a molecular sieve, and 5 is a condenser. DETAILED DESCRIPTION

[0020] The invention allows an alkali metal salt and a bisfluorosulfonyl imide acid to react in a poor solvent for the bisfluorosulfonyl imide salt to generate the bisfluorosulfonyl imide salt, and dynamically removes water with the aid of the poor solvent during the reaction.

[0021] Furthermore, after the reaction, post-treatment is performed to obtain a bisfluorosulfonyl imide salt solid.

[0022] Furthermore, the post-treatment is as follows: after the first solid-liquid separation, the obtained filter cake is dried, then dissolved with a good solvent, solid-liquid separation is performed for the second time, and the filtrate is concentrated, crystallized, filtered, and dried to obtain a bisfluorosulfonyl imide salt solid.

[0023] Furthermore, the good solvent is one of dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0024] Furthermore, bisfluorosulfonyl imide acid is added to the mixed solution of the alkali metal salt and the poor solvent.

[0025] Furthermore, the bis(fluorosulfonyl)imidic acid is added dropwise at a rate of 5 to 10 g / min.

[0026] The reaction apparatus involved in the following embodiments is as follows Figure 1 As shown, the apparatus consists of a three-necked flask 1, a constant-pressure dropping funnel 2, a thermometer 3, a dewatering device, and a condenser 5. The constant-pressure dropping funnel 2 and the thermometer 3 are connected to both sides of the three-necked flask 1. The dewatering device is connected directly above the three-necked flask 1 via a flow guide tube, and the condenser 5 is connected above the dewatering device. The drying device is filled with molecular sieves 4.

[0027] The three-necked flask 1 is used to hold lithium salt and poor solvent and serves as a reactor; the constant pressure dropping funnel 2 is used to dropwise add bisfluorosulfonyl imide acid, so that the bisfluorosulfonyl imide acid is slowly and evenly added to the poor solvent containing lithium salt (i.e., the three-necked flask 1), and then a reaction occurs. The gas generated in the reaction enters the dehydration device through the guide tube, and then enters the condenser 5 located above the dehydration device through the dehydration device, condenses into liquid and then returns to the dehydration device, and is dehydrated by the molecular sieve 4 in the dehydration device. After dehydration, it flows back to the three-necked flask through the reflux bypass and the guide tube connected between the dehydration device and the three-necked flask. The thermometer 3 is used to measure the temperature of the reaction system in the three-necked flask 1.

[0028] The technical solution of the present invention is further described below in conjunction with specific implementation methods.

[0029] 1. The specific embodiment of the water removal method of the bisfluorosulfonyl imide salt of the present invention is as follows:

[0030] Example 1

[0031] The dehydration method of lithium bis(fluorosulfonyl)imide of this embodiment adopts the following steps:

[0032] (1) Weigh 128.8 g of lithium polymethacrylate (number average molecular weight of 100,000 to 300,000) and add it to a three-necked flask. Then add 534 g of DCM (dichloromethane). Stir at 25°C (stirring speed of 300 r / min) and dropwise add 181 g of HFSI (bis(fluorosulfonyl)imide) for 20 min.

[0033] (2) After the addition was completed, the reaction temperature was raised to 45°C and the reaction was heated for 15 hours. During the reaction, the mixed solution of DCM and water was evaporated and first passed through a molecular sieve, then condensed and refluxed and passed through a molecular sieve again to remove water, and finally refluxed into the three-necked flask to continue to serve as a solvent. This cycle was repeated until the reaction was completed (the apparatus involved in the reaction was as follows: Figure 1 shown);

[0034] (3) After the reaction is completed, filter under positive pressure, collect the filter cake, and dry the filter cake at 45°C for 8 hours;

[0035] (4) Dissolving the LiFSI in the filter cake with DMC (dimethyl carbonate), then filtering under positive pressure and collecting the filtrate to separate LiFSI from the solid organic acid (polymethacrylic acid) generated by the reaction;

[0036] (5) The filtrate was subjected to reduced pressure distillation at 60°C using a circulating water vacuum pump to obtain a concentrated solution, which was then filtered under reduced pressure to obtain a filter residue, which was finally dried in a vacuum drying oven at 80°C for 12 h to obtain LiFSI (lithium bis(fluorosulfonyl)imide) solid.

[0037] The water, acid and chlorine contents in the obtained LiFSI solid were measured to be 21 ppm, 10 ppm and 3 ppm, respectively.

[0038] Example 2

[0039] The dehydration method of lithium bis(fluorosulfonyl)imide of this embodiment adopts the following steps:

[0040] (1) Weigh 147.2 g of lithium polymethacrylate (number average molecular weight of 100,000 to 300,000) into a three-necked flask, then add 748 g of DCM. Add 181 g of HFSI dropwise while stirring at 25°C (stirring speed of 300 r / min) for 30 min.

[0041] (2) After the addition is completed, the reaction temperature is raised to 48°C and the reaction is heated for 10 hours. During the reaction, the mixed solution of DCM and water is evaporated and first passed through a molecular sieve, then condensed and refluxed and passed through a molecular sieve again to remove water, and finally refluxed into the three-necked flask to continue as a solvent. This cycle is repeated until the reaction is completed.

[0042] In other embodiments, the reaction temperature can be raised to 50° C. and heated for 10 h. The reaction results are equivalent to those of step (2).

[0043] (3) After the reaction is completed, filter under positive pressure, collect the filter cake, and dry the filter cake at 45°C for 8 hours;

[0044] (4) dissolving the LiFSI in the filter cake with DMC, then filtering under positive pressure and collecting the filtrate to separate LiFSI from the organic acid (polymethacrylic acid) and other impurities generated by the reaction;

[0045] (5) The filtrate was subjected to reduced pressure distillation at 60°C using a circulating water vacuum pump to obtain a concentrated solution, which was then filtered under reduced pressure to obtain a filter residue, which was finally dried in a vacuum drying oven at 80°C for 12 h to obtain LiFSI solid.

[0046] The water, acid and chlorine contents in the obtained LiFSI solid were measured to be 26 ppm, 8 ppm and 4 ppm, respectively.

[0047] Example 3

[0048] The dehydration method of lithium bis(fluorosulfonyl)imide in this embodiment is basically the same as that in Example 1, except that lithium polymethacrylate is replaced with lithium polyacrylate (number average molecular weight of 100,000) in an amount of 109.2 g.

[0049] The water, acid and chlorine contents in the obtained LiFSI solid were measured to be 34 ppm, 400 ppm and 8 ppm, respectively.

[0050] Example 4

[0051] The dehydration method of lithium bis(fluorosulfonyl)imide in this embodiment is substantially the same as that in Example 1, except that lithium polymethacrylate is replaced with lithium benzoate in an amount of 179.2 g.

[0052] The water, acid and chlorine contents in the obtained LiFSI solid were measured to be 30 ppm, 5000 ppm and 6 ppm, respectively.

[0053] Example 5

[0054] The dehydration method of lithium bis(fluorosulfonyl)imide in this embodiment is basically the same as that in Example 1, except that lithium polymethacrylate is replaced with lithium acetate in an amount of 92.4 g.

[0055] The water, acid and chlorine contents in the obtained LiFSI solid were measured to be 33 ppm, 8000 ppm and 5 ppm, respectively.

[0056] 2. Comparative Example

[0057] Comparative Example 1

[0058] The dehydration method of lithium bis(fluorosulfonyl)imide in this comparative example is substantially the same as that in Example 1, except that no molecular sieve is added in step (2) for dehydration.

[0059] The water, acid and chlorine contents in the obtained LiFSI solid were measured to be 599 ppm, 155 ppm and 5 ppm, respectively.

[0060] Comparative Example 2

[0061] The dehydration method of lithium bis(fluorosulfonyl)imide in this comparative example is basically the same as that in Example 2, except that no molecular sieve is added in step (2) for dehydration.

[0062] The water, acid and chlorine contents in the obtained LiFSI solid were measured to be 536 ppm, 148 ppm and 7 ppm, respectively.

Claims

1. A method for removing water from a bis(fluorosulfonyl)imide salt, characterized in that: The following steps are involved: Mixing an alkali metal salt and a bisfluorosulfonyl imide acid in a poor solvent for the bisfluorosulfonyl imide salt, heating and refluxing to generate a bisfluorosulfonyl imide salt; the alkali metal salt is lithium polymethacrylate; During the heating reflux reaction, the poor solvent is subjected to a water removal treatment before being refluxed into the reaction system; The dehydration treatment is to first flow the refluxed poor solvent into a drying device containing a desiccant, and then flow into the reaction system from the drying device; the gas generated by the reaction first enters the drying device, and then is condensed and refluxed.

2. The method for removing water from a bisfluorosulfonyl imide salt according to claim 1, wherein: The poor solvent is one or two of dichloromethane and ether compounds.

3. The method for removing water from a bisfluorosulfonyl imide salt according to claim 1, wherein: The reaction temperature is 45-50° C. and the reaction time is 10-15 hours.

4. The method for removing water from a bisfluorosulfonyl imide salt according to claim 1, wherein: The desiccant is a molecular sieve.

5. The method for removing water from a bisfluorosulfonyl imide salt according to claim 1, wherein: The molar ratio of the alkali metal salt to the bis(fluorosulfonyl)imidic acid is (1.1-1.6):

1.

6. The method for removing water from a bisfluorosulfonyl imide salt according to claim 2, wherein: The molar ratio of the poor solvent to the bis(fluorosulfonyl)imidic acid is (6-10):1.

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