A method for producing a lithium fluorosulfonate concentrated solution

Abstract

The present application relates to the field of lithium ion battery electrolyte additives, and particularly relates to a manufacturing method of lithium fluorosulfonate concentrate, comprising the following steps: (1) reacting chlorosulfonic acid with lithium fluoride in a non-aqueous solvent to generate lithium fluorosulfonate, and hydrogen chloride gas is generated in the reaction; (2) performing vacuum degassing on the reaction solution to remove hydrogen chloride and other gases remaining in the solution, and then filtering to remove the insoluble substances such as lithium fluoride contained therein. The present inventors use chlorosulfonic acid and lithium fluoride to prepare lithium fluorosulfonate, and chlorosulfonic acid belongs to a bulk raw material, which is easy to obtain as a raw material. In addition, there is no other side reaction problem in the reaction process, and the generated product is single. The purity and yield of the obtained product are very high, and it is very suitable for large-scale production in the industry of preparing electrolyte of lithium batteries.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of lithium ion battery electrolyte additives, in particular to a preparation method of lithium fluorosulfonate concentrate. BACKGROUND

[0002] Lithium fluorosulfonate is a new type of lithium salt, which can be used as an additive for lithium ion battery electrolyte. When lithium fluorosulfonate is used as an additive for lithium ion batteries, the high-temperature charge-discharge cycling characteristics of lithium batteries can be greatly improved, and the high battery capacity retention rate can be maintained.

[0003] At present, in the known preparation method of lithium fluorosulfonate, fluorosulfonic acid is mostly reacted with lithium salt, and there are many side reactions and by-products in the reaction, which brings many impurities, affects the yield, and at the same time, fluorosulfonic acid used as raw material is difficult to obtain, and is easy to react with moisture in the air to generate hydrogen fluoride, which is difficult to handle. The production and use of lithium fluorosulfonate are severely limited.

[0004] As a synthesis method of lithium fluorosulfonate, it is known that lithium fluorosulfonate is synthesized by the reaction of fluorosulfonic acid with various lithium salts (lithium carboxylate or lithium chloride, etc.). For example, patent document 1 discloses a method of reacting fluorosulfonic acid or sulfur trioxide with lithium halide in anhydrous hydrofluoric acid, patent documents 2-4 disclose a method of reacting fluorosulfonic acid with lithium carboxylate, and patent documents 3-5 disclose a method of reacting fluorosulfonic acid with lithium halide.

[0005] Prior art documents

[0006] Patent documents

[0007] Patent document 1: Japanese patent publication No. 2016-8145

[0008] Patent document 2: Japanese patent publication No. 2012-232888

[0009] Patent document 3: International publication No. 2012 / 141180

[0010] Patent document 4: US publication No. 2014 / 38062

[0011] Patent document 5: Japanese patent publication No. 2012-218985 SUMMARY

[0012] In view of the defects of the prior art, such as many by-products, low yield, and complex operation process, the present application provides a new preparation method of lithium fluorosulfonate concentrate, which has simple raw materials, few by-products, less impurities, high yield, environmental friendliness, and easy industrialization.

[0013] To solve the above prior art problems, the present application relates to a method for producing lithium fluorosulfonate concentrate, comprising the following steps:

[0014] (1) reacting chlorosulfonic acid with lithium fluoride in a non-aqueous solvent to produce lithium fluorosulfonate, and generating hydrogen chloride gas in the reaction;

[0015] (2) degassing the reaction solution under reduced pressure to remove residual hydrogen chloride gas and other gases in the solution, and then removing the insoluble lithium fluoride contained therein by filtration to obtain lithium fluorosulfonate concentrate.

[0016] As an embodiment, in step (1), the molar ratio of chlorosulfonic acid to lithium fluoride is 1.0: (1.0-1.05).

[0017] As an embodiment, in step (1), the non-aqueous solvent has a water content of 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.

[0018] As an embodiment, the reaction temperature in step (1) is 0-60°C, and preferably the reaction temperature is 40-60°C, and the reaction time is 6-12h.

[0019] As an embodiment, the degassing in step (2) is performed under reduced pressure, the pressure range is 0.50-10kpa, the degassing temperature is 0-60°C, and preferably the temperature is 40-60°C, and the degassing time is 5-10h.

[0020] As an embodiment, steps (1) and (2) are both performed under an inert atmosphere of nitrogen, argon, or helium.

[0021] As an embodiment, after the lithium fluorosulfonate in step (2) is degassed and precision filtered, high-quality lithium fluorosulfonate concentrate is obtained, and the filter membrane in the precision filter is a PTFE flat membrane.

[0022] As an embodiment, the PTFE flat membrane is modified to improve the filtration efficiency, and the modification method is as follows:

[0023] The PTFE flat membrane is subjected to plasma pretreatment in a nitrogen atmosphere, the plasma pretreatment power is 70-500W, and the pretreatment time is 25-200s, then the pretreated PTFE microfiltration membrane is immersed in an immersion liquid, the reaction temperature is 60-80°C, and the reaction time is 2-5h, and finally the PTFE membrane is washed with deionized water, dried, and a modified PTFE flat membrane is obtained.

[0024] As an implementation form, the preparation method of the impregnation solution is: 0.05-0.6 parts of lithium acrylate is added into 10-15 parts of mercaptoethanolamine, 2-5 parts of an organic amine, 100-200 parts of DMF, and stirring reaction is carried out at a temperature of 50-58 DEG C for 30-60 min, then 19-30 parts of diallylamine hydrochloride is added, and stirring reaction is carried out at a temperature of 50-58 DEG C for 60-100 min, so that the impregnation solution is prepared.

[0025] As an implementation form, the organic amine is at least one selected from tripropyl phosphine, dimethyl phenyl phosphine, methyl diphenyl phosphine and triphenyl phosphine.

[0026] The lithium fluorosulfonate concentrated solution prepared by the method is applied to a lithium ion battery, and when the concentration of the lithium fluorosulfonate concentrated solution is 40±5%, the lithium fluorosulfonate concentrated solution can be directly used to configure an electrolyte.

[0027] Technical effects:

[0028] Compared with the prior art, the manufacturing method of the lithium fluorosulfonate concentrated solution has the following remarkable effects:

[0029] 1) The preparation method provided by the application has mild reaction conditions, high product yield, and cheap and easily available raw materials, so that the cost can be greatly saved.

[0030] 2) The preparation method provided by the application has simple reaction, no side reaction, simple operation, no impurities after reaction, simple post-treatment, and increased feasibility of industrial production.

[0031] 3) The lithium fluorosulfonate concentrated solution prepared by the application is used as an additive for a lithium battery, and can ensure the working performance of the battery.

[0032] 4) The lithium acrylate and the mercaptoethanolamine first perform a mercapto-alkene addition reaction, then perform a mercapto-alkene addition reaction with diallylamine hydrochloride, and then are subjected to irradiation crosslinking, so that the PTFE flat film has lithium ions, the compatibility with the lithium fluorosulfonate concentrated solution is improved, the ethanolamine functional group has the performance of capturing hydrogen chloride, and trace hydrogen chloride can be removed, so that the purity of the product is improved.

[0033] Term definition

[0034] The present application is intended to encompass all alternatives, modifications and equivalents thereof which are included within the scope of the present application as defined by the claims. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments described herein. The present application is not intended to be limited to the embodiments described herein which are presented as examples only but include all alternatives, modifications and equivalents falling within the scope of the present application as defined by the following claims.

[0035] It should be further appreciated that certain features of the application, which are, for clarity, described in the context of separate embodiments, can also be provided in combination in a single embodiment, and conversely that various features of the application, which are, for brevity, described in the context of a single embodiment, can also be provided separately or in any suitable subcombination.

[0036] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as those commonly understood by one of ordinary skill in the art to which this application belongs. All patents and publications referred to in this application are incorporated herein by reference in their entirety.

[0037] Unless defined otherwise, all terms of art used herein have the same meaning as commonly understood by one of ordinary skill in the art in the field of the application. The following definitions, however, are provided for the following terms used herein:

[0038] The terms "comprising" or "comprise" or "including" or "including" are open-ended, that is, they mean including, but not limited to, that which is specifically recited. DETAILED DESCRIPTION

[0039] The present application provides a method for preparing lithium fluorosulfonate concentrated solution by using cheap and readily available raw materials at lower temperature and conditions. The method comprises: reacting chlorosulfonic acid with lithium fluoride suspended in a non-aqueous solvent in an inert gas atmosphere to obtain a lithium fluorosulfonate solution, and then degassing and precision filtering the obtained lithium fluorosulfonate concentrated solution to obtain a high-quality lithium fluorosulfonate concentrated solution.

[0040] The reaction equation of the method is as follows:

[0041] HSO3Cl + LiF = LiSO3F + HCl

[0042] In some embodiments, the molar ratio of chlorosulfonic acid to lithium fluoride is 1:1 to 1.05, for example: 1:1, 1:1.01, 1:1.02, 1:1.03, 1:1.04, 1:1.05.

[0043] Preferably, the water content of the lithium fluoride is not more than 50 ppm.

[0044] In some embodiments, the non-aqueous solvent is at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethyl acetate, dimethyl formamide, ethylene glycol dimethyl ether, acetonitrile, and tetrahydrofuran.

[0045] Preferably, the water content of the non-aqueous solvent is less than 15 ppm.

[0046] The reaction temperature is 0-60℃, and the reaction time is 6-12h.

[0047] Non-limiting examples of the reaction temperature include: 0℃, 5℃, 10℃, 15℃, 20℃, 25℃, 30℃, 35℃, 40℃, 45℃, 50℃, 55℃, 60℃, etc.

[0048] Preferably, the reaction temperature is 40-60℃.

[0049] Non-limiting examples of the reaction time include: 6h, 7h, 8h, 9h, 10h, 11h, 12h, etc.

[0050] Preferably, the reaction time is 8-10h.

[0051] The degassing is performed under reduced pressure, the pressure range is 0.5-10kpa, the degassing temperature is 0-60℃, and the degassing time is 5-10h.

[0052] Non-limiting examples of the degassing pressure include: 0.5kpa, 1kpa, 1.5kpa, 2kpa, 2.5kpa, 3kpa, 3.5kpa, 4kpa, 4.5kpa, 5kpa, 5.5kpa, 6kpa, 6.5kpa, 7kpa, 7.5kpa, 8kpa, 8.5kpa, 9kpa, 9.5kpa, 10kpa, etc.

[0053] Preferably, the degassing pressure is 2-6kpa.

[0054] Non-limiting examples of the degassing temperature include: 0℃, 5℃, 10℃, 15℃, 20℃, 25℃, 30℃, 35℃, 40℃, 45℃, 50℃, 55℃, 60℃, etc.

[0055] Preferably, the degassing temperature is 40-60℃.

[0056] Non-limiting examples of the degassing time include: 4h, 4.5h, 5h, 5.5h, 6h, 6.5h, 7h, 7.5h, 8h, 8.5h, 9h, 9.5h, 10h, etc.

[0057] Preferably, the degassing time is 6-8h.

[0058] The entire process is performed under an inert gas atmosphere, and the inert gas is at least one of nitrogen, argon, and helium.

[0059] The content of chloride ion in the lithium fluorosulfonate concentrate prepared by the method is less than 5 ppm; the concentration of acid in the solution is less than 100 ppm by titration method. The solution can be directly used as an additive material for lithium batteries due to the low content of chlorides and free acid in the solution.

[0060] The following is an optimized embodiment of the present application, and the present application is not limited to the following preferred embodiment. It should be pointed out that, on the basis of the inventive concept, several modifications and improvements made by those skilled in the art are within the scope of the present application.

[0061] Example 1

[0062] In a 500 ml three-necked flask, 200 ml of dimethyl carbonate and 25.94 g (1 mol) of lithium fluoride were added under nitrogen protection, and the mixture was slowly heated under stirring. Then, 116.5 g (1 mol) of chlorosulfonic acid was slowly added at 60°C for about 2 hours. The reaction was continued at the same temperature for 6 hours. After the reaction was completed, the flask was slowly degassed by vacuum pumping at the same temperature for about 4 hours. Then, the degassed lithium fluorosulfonate concentrate was filtered, and the concentration was adjusted with dimethyl carbonate to obtain a lithium fluorosulfonate concentrate with a weight concentration of 40%.

[0063] The lithium fluorosulfonate concentrate obtained by degassing and precision filtering has high quality. The filter membrane in the precision filter is a PTFE flat membrane.

[0064] The PTFE flat membrane is modified to improve the filtering efficiency. The modification method is as follows: the PTFE flat membrane is pretreated by plasma in a nitrogen atmosphere, the plasma pretreatment power is 120 W, and the pretreatment time is 200 s. Then, the pretreated PTFE microfiltration membrane is immersed in an immersion liquid, the reaction temperature is 60°C, and the reaction time is 5 h. Finally, the PTFE membrane is washed with deionized water and dried to obtain a modified PTFE flat membrane.

[0065] The preparation method of the immersion liquid is as follows: 0.2 g of lithium acrylate is added to 10 g of mercaptoethanolamine, 2 g of tripropyl phosphine, 100 g of DMF, and stirred at a temperature of 50°C for 60 min. Then, 19 g of diallylamine hydrochloride is added, and stirred at a temperature of 50°C for 100 min to obtain the immersion liquid.

[0066] The test results are as follows: the purity of the lithium fluorosulfonate concentrate is 99.88%, the free acid (calculated as HCl) is 85 ppm, the moisture (Karl Fischer method) is 180 ppm, the content of alkali metal ions (calculated as K and Na) is 2 ppm, the content of chloride ions is 9 ppm, the content of heavy metal ions (calculated as Fe) is 1 ppm, and the content of insoluble substances is 107 ppm.

[0067] Example 2

[0068] In a three-necked flask, 200 ml of dimethyl carbonate was added under nitrogen protection, 28.5 g (1.1 mol) of lithium fluoride was slowly heated under stirring, 116.5 g (1 mol) of chlorosulfonic acid was slowly added at 40°C for about 2 hours, and the reaction was continued for 5 hours at the same temperature. After the reaction was completed, the lithium fluorosulfonate concentrated solution was degassed by vacuum pumping for about 4 hours at the same temperature. Then the degassed lithium fluorosulfonate concentrated solution was filtered precisely, and the concentration was adjusted with dimethyl carbonate to obtain a lithium fluorosulfonate concentrated solution with a weight concentration of 40%.

[0069] The lithium fluorosulfonate concentrated solution obtained after degassing and precise filtration of the lithium fluorosulfonate has high quality, and the filter membrane in the precise filter is a PTFE flat membrane.

[0070] The PTFE flat membrane is modified to improve the filtration efficiency, and the modification method is as follows: the PTFE flat membrane is pretreated by plasma in a nitrogen atmosphere, the plasma pretreatment power is 350 W, and the pretreatment time is 100 s, then the pretreated PTFE microfiltration membrane is immersed in an immersion liquid, the reaction temperature is 70°C, and the reaction time is 3 h, finally the PTFE membrane is washed with deionized water and dried to obtain a modified PTFE flat membrane.

[0071] The preparation method of the immersion liquid is as follows: 0.4 g of lithium acrylate is added to 12 g of mercaptoethanolamine, 3 g of dimethylphenylphosphine, 150 g of DMF, and stirred at a temperature of 54°C for 45 min, then 25 g of diallylamine hydrochloride is added, and stirred at a temperature of 54°C for 80 min to obtain the immersion liquid.

[0072] The detection results are as follows: the purity of the lithium fluorosulfonate concentrated solution is 99.93%, the free acid (calculated as HCl) is 125 ppm, the moisture (Karl Fischer method) is 105 ppm, the alkali metal ion content (calculated as K and Na) is 1 ppm, the chloride ion content is 4 ppm, the heavy metal ion content (calculated as Fe) is 1 ppm, and the insoluble content is 120 ppm.

[0073] Example 3

[0074] In a 2000ml three-necked flask, under nitrogen protection, 1000ml dimethyl carbonate, 136.2g (5.05mol) lithium fluoride, slowly heated under stirring, 582.5g (5mol) chlorosulfonic acid was slowly added dropwise at 50°C, about 3 hours for dropwise addition. Maintaining the same temperature, continue to keep for 6 hours. After the reaction, maintain the same temperature, open the vacuum pump to slowly vacuum the reaction bottle, about 4 hours to complete. Then the degassed lithium fluorosulfonate concentrate was concentrated and adjusted to a concentration of 40% by weight with dimethyl carbonate.

[0075] The lithium fluorosulfonate concentrate obtained after degassing and precision filtration has high quality, and the filter membrane in the precision filter is a PTFE flat membrane.

[0076] The PTFE flat membrane is modified to improve the filtration efficiency, and the modification method is as follows: the PTFE flat membrane is pretreated by plasma in a nitrogen atmosphere, the plasma pretreatment power is 500W, and the pretreatment time is 50s, then the pretreated PTFE microfiltration membrane is immersed in the immersion liquid, the reaction temperature is 80°C, and the reaction time is 2h, finally the PTFE membrane is washed with deionized water and dried to obtain a modified PTFE flat membrane.

[0077] The preparation method of the immersion liquid is as follows: 0.6g lithium acrylate is added to 15g mercaptoacetic acid ethanolamine, 5g triphenylphosphine, 200g DMF, and stirred at a temperature of 58°C for 30min, then 30g of diallylamine hydrochloride is added, and stirred at a temperature of 58°C for 60min to obtain the immersion liquid.

[0078] The test results are as follows: the purity of lithium fluorosulfonate concentrate is 99.95%, the free acid (calculated as HCl) is 96ppm, the moisture (Karl Fischer method) is 64ppm, the alkali metal ion content (calculated as K, Na) is 1ppm, the chloride ion content is 3ppm, the heavy metal ion content (calculated as Fe) is 1ppm, and the insoluble content is 112ppm.

[0079] Comparative Example 1

[0080] In a 500ml three-necked flask, under nitrogen protection, 200ml dimethyl carbonate, 25.94g (1mol) lithium fluoride, slowly heated under stirring, 116.5g (1mol) chlorosulfonic acid was slowly added dropwise at 60°C, about 2 hours for dropwise addition. Maintaining the same temperature, continue to keep for 6 hours. After the reaction, maintain the same temperature, open the vacuum pump to slowly vacuum the reaction bottle, about 4 hours to complete. Then the degassed lithium fluorosulfonate concentrate was concentrated and adjusted to a concentration of 40% by weight with dimethyl carbonate.

[0081] The detection results are as follows: purity of lithium fluorosulfonate concentrate 96.78%, free acid (calculated by HCl) 313 ppm, moisture (Karl Fischer method) 230 ppm, alkali metal ion content (calculated by K and Na) 4 ppm, chloride ion content 13 ppm, heavy metal ion content (calculated by Fe) 2 ppm, and insoluble content 143 ppm.

[0082] Comparative Example 2

[0083] In a 500 ml three-necked flask, 200 ml of dimethyl carbonate and 25.94 g (1 mol) of lithium fluoride were added under nitrogen protection, and the mixture was slowly heated under stirring. Then, 116.5 g (1 mol) of chlorosulfonic acid was slowly added dropwise at 60°C for about 2 hours. The reaction was continued for 6 hours at the same temperature. After the reaction was completed, the lithium fluorosulfonate concentrate was degassed by slowly vacuumizing the flask under the same temperature condition for about 4 hours. Then, the degassed lithium fluorosulfonate concentrate was filtered precisely, and the concentration was adjusted with dimethyl carbonate to obtain lithium fluorosulfonate concentrate with a weight concentration of 40%.

[0084] The lithium fluorosulfonate concentrate obtained after degassing and precise filtration of the lithium fluorosulfonate has high quality, and the filter membrane in the precise filter is a PTFE flat membrane.

[0085] The PTFE flat membrane is modified to improve the filtration efficiency, and the modification method is as follows: the PTFE flat membrane is pretreated by plasma in a nitrogen atmosphere, the plasma pretreatment power is 120 W, and the pretreatment time is 200 s. Then, the pretreated PTFE microfiltration membrane is immersed in an immersion liquid, the reaction temperature is 60°C, and the reaction time is 5 hours. Finally, the PTFE membrane is washed with deionized water and dried to obtain a modified PTFE flat membrane.

[0086] The preparation method of the immersion liquid is as follows: 0.2 g of lithium acrylate, 2 g of tripropyl phosphine, and 100 g of DMF are stirred and reacted at a temperature of 50°C for 60 min. Then, 19 g of diallylamine hydrochloride is added, and the mixture is stirred and reacted at a temperature of 50°C for 100 min to obtain the immersion liquid.

[0087] The detection results are as follows: purity of lithium fluorosulfonate concentrate 98.45%, free acid (calculated by HCl) 275 ppm, moisture (Karl Fischer method) 198 ppm, alkali metal ion content (calculated by K and Na) 3 ppm, chloride ion content 11 ppm, heavy metal ion content (calculated by Fe) 1 ppm, and insoluble content 131 ppm.

[0088] Comparative Example 3

[0089] In a 500ml three-necked flask, under nitrogen protection, 200ml dimethyl carbonate, 25.94g (1mol) lithium fluoride, slowly heated to 60°C under stirring, slowly drop 116.5g (1mol) chlorosulfonic acid, about 2 hours drop completed. Continue to maintain the same temperature for 6 hours. After the reaction is completed, maintain the same temperature, open the vacuum pump to slowly vacuum the reaction bottle, about 4 hours to complete. Then the degassed lithium fluorosulfonate concentrate is concentrated and filtered, and the concentration is adjusted with dimethyl carbonate to obtain a lithium fluorosulfonate concentrate with a weight concentration of 40%.

[0090] The lithium fluorosulfonate concentrate obtained after degassing and precision filtering has high quality, and the filter membrane in the precision filter is a PTFE flat membrane.

[0091] The PTFE flat membrane is modified to improve the filtering efficiency, and the modification method is as follows: the PTFE flat membrane is pretreated by plasma in a nitrogen atmosphere, the plasma pretreatment power is 120W, and the pretreatment time is 200s, then the pretreated PTFE microfiltration membrane is immersed in the immersion liquid, the reaction temperature is 60°C, and the reaction time is 5h, finally the PTFE membrane is washed with deionized water and dried to obtain a modified PTFE flat membrane.

[0092] The preparation method of the immersion liquid is as follows: 0.2g lithium acrylate is added to 10g mercaptoacetic acid ethanolamine, 2g tripropyl phosphine, 100g DMF, and stirred at 50°C for 160min to obtain the immersion liquid.

[0093] The detection results are as follows: the purity of lithium fluorosulfonate concentrate is 97.98%, the free acid (calculated as HCl) is 301ppm, the moisture (Karl Fischer method) is 215ppm, the alkali metal ion content (calculated as K, Na) is 3ppm, the chloride ion content is 12ppm, the heavy metal ion content (calculated as Fe) is 1ppm, and the insoluble content is 140ppm.

[0094] Although the present application has been described in detail above with general description, specific embodiments and examples, certain supplements and optimizations can be made to the discrimination model of the method based on the present application. Therefore, these modifications or improvements made without deviating from the spirit of the present application are within the scope of the present application.

Claims

1. A method for preparing a concentrated lithium fluorosulfonate solution, comprising the following steps: (1) reacting chlorosulfonic acid with lithium fluoride in a non-aqueous solvent to produce lithium fluorosulfonate, and generating hydrogen chloride gas in the reaction; (2) performing vacuum degassing on the reaction solution obtained in step (1) to remove the residual hydrogen chloride gas in the solution, and then performing precision filtration on the solution through a precision filter to remove the insoluble lithium fluoride contained in the solution, thereby obtaining the concentrated lithium fluorosulfonate solution; in step (1), the molar ratio of the chlorosulfonic acid to the lithium fluoride is 1.0: (1.0-1.05); in step (1), the non-aqueous solvent has a water content of 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; the pressure range for the vacuum degassing in step (2) is 0.50-10 kPa, and the temperature is 40-60℃; the filter membrane in the precision filter is a PTFE modified flat membrane; the PTFE modified flat membrane is prepared by the following method: pre-treating the PTFE flat membrane in a nitrogen atmosphere by plasma, the plasma pre-treatment power is 70-500 W, and the pre-treatment time is 25-200 s, then immersing the pre-treated PTFE flat membrane in an immersion liquid, the reaction temperature is 60-80℃, and the reaction time is 2-5 h, and finally washing the PTFE flat membrane with deionized water, drying, and obtaining the PTFE modified flat membrane; the immersion liquid is prepared by the following method: adding 0.05-0.6 parts of lithium acrylate to 10-15 parts of mercaptoacetic acid ethanolamine, 2-5 parts of an organic amine, 100-200 parts of DMF, stirring and reacting at a temperature of 50-58℃ for 30-60 min, then adding 19-30 parts of diallylamine hydrochloride, stirring and reacting at a temperature of 50-58℃ for 60-100 min, and obtaining the immersion liquid. the organic amine is at least one of tripropylphosphine, dimethylphenylphosphine, methyldiphenylphosphine, and triphenylphosphine.

2. The method for producing a lithium fluorosulfonate concentrated solution according to claim 1, characterized by, the reaction temperature in step (1) is 40-60℃, and the reaction time is 6-12 h.

3. The method for producing a lithium fluorosulfonate concentrated solution according to claim 1, characterized by, steps (1) and (2) are both performed in an inert atmosphere of nitrogen, argon, or helium.