A method for purifying lithium bisfluorosulfonylimide and a lithium ion battery

Abstract

The application relates to a purification method of a bifluorosulfonylimide lithium salt, comprising the following steps: modifying a polytetrafluoroethylene film by using plasma and formic acid, immersing the polytetrafluoroethylene film in bifluorosulfonylimide acid after the modification is completed, heating and drying to obtain a modified polytetrafluoroethylene film; dispersing lithium carbonate in dimethyl carbonate, then adding bifluorosulfonylimide acid, obtaining a reaction liquid after the reaction is completed, filtering the reaction liquid to obtain a filtrate, concentrating the filtrate once to obtain a bifluorosulfonylimide lithium electrodialysis crude solution; using the modified polytetrafluoroethylene film to purify the bifluorosulfonylimide lithium electrodialysis crude solution by electrodialysis, then concentrating and crystallizing the obtained electrodialysis liquid to obtain battery-grade bifluorosulfonylimide lithium. The method provided by the application can reduce the purity requirement of raw material lithium carbonate from battery grade to industrial grade, which is beneficial to industrial production.

Description

Technical Field

[0001] This application relates to the field of electrolyte technology, and in particular to a method for purifying lithium bis(fluorosulfonyl)imide and a lithium-ion battery. Background Technology

[0002] Lithium bisfluorosulfonylimide (LiFSI) has the chemical formula F₂NO₄S₂.Li. Due to its excellent high-temperature resistance (stable below 200°C), good low-temperature performance, resistance to hydrolysis, and environmental friendliness, LiFSI is considered a next-generation lithium salt for rechargeable lithium-ion batteries, potentially replacing lithium hexafluorophosphate. Currently, LiFSI is commonly used as an electrolyte additive in lithium-ion batteries, often mixed with lithium hexafluorophosphate, to improve battery capacity and electrochemical performance. Additionally, LiFSI also has important industrial applications as a polymerization catalyst and an antistatic agent.

[0003] The synthesis of lithium bis(fluorosulfonyl)imide generally involves the neutralization of a battery-grade alkaline lithium source and bis(fluorosulfonyl)imide acid, followed by concentration and crystallization purification. The acid-base neutralization process is exothermic, and the use of alkaline lithium sources such as lithium carbonate or lithium hydroxide generates water. Since LiFSI reacts with water to form an oily solvate that is difficult to crystallize, a dehydrating agent is often required before crystallization and purification.

[0004] Patent CN115367718A discloses a method for purifying lithium bis(fluorosulfonyl)imide. Specifically, it discloses adding bismuth trichloride or antimony trichloride as a dehydrating agent to a pretreatment solution containing crude lithium bis(fluorosulfonyl)imide, and carrying out a dehydration reaction at 20℃~40℃. After no acidic gas escapes from the reaction system, the reaction continues for 1h~6h, followed by filtration to obtain a filtrate. The filtrate is then evaporated, concentrated, and recrystallized to obtain high-purity lithium bis(fluorosulfonyl)imide. This method removes most of the water through the pre-addition of antimony trifluoride as a dehydrating agent, reducing the degree of hydrolysis and decreasing the generation of sulfate ions, a decomposition impurity in LiFSI, thus reducing the burden of crystallization purification. However, the addition of antimony trifluoride as a dehydrating agent easily introduces chloride ion impurities, requiring further recrystallization for purification. Patent CN104925765A discloses a method for preparing lithium bis(fluorosulfonyl)imide salt, using thionyl chloride for dehydration to precipitate LiFSI crystals. The reaction generates SO2 and HCl gases, and post-treatment requires a large amount of inert solvent for slurry cleaning, resulting in high acid waste. Patent CN108002355A discloses a method for preparing lithium bis(fluorosulfonyl)imide, which uses thionyl chloride, concentrated sulfuric acid or acid anhydride as dehydrating agents to remove water. After obtaining crude LiFSI, it needs to be recrystallized and purified.

[0005] Based on the above analysis, it is essential to provide a purification method for lithium bis(fluorosulfonyl)imide that does not require the addition of a dehydrating agent, is easy to crystallize, and readily meets the required anion content. Summary of the Invention

[0006] This application provides a purification method for lithium bisfluorosulfonylimide to solve the problems in related technologies where the anion content is difficult to meet the requirements and a dehydrating agent needs to be added before crystallization purification.

[0007] In a first aspect, this application provides a method for purifying lithium bis(fluorosulfonyl)imide, comprising the following steps:

[0008] S101, the polytetrafluoroethylene membrane is modified by plasma and formic acid. After the modification is completed, the polytetrafluoroethylene membrane is immersed in difluorosulfonyl imide acid, heated and dried to obtain the modified polytetrafluoroethylene membrane.

[0009] S102, lithium carbonate is dispersed in dimethyl carbonate, then difluorosulfonyl imide acid is added. After the reaction is completed, a reaction solution is obtained. The reaction solution is filtered to obtain a filtrate. The filtrate is concentrated once to obtain the lithium difluorosulfonyl imide electrodialysis stock solution.

[0010] S103, using a modified polytetrafluoroethylene membrane to purify the lithium difluorosulfonylimide electrodialysis stock solution by electrodialysis, and then concentrating and crystallizing the obtained electrodialysis solution to obtain battery-grade lithium difluorosulfonylimide.

[0011] In some embodiments, the process of modifying the polytetrafluoroethylene (PTFE) membrane using plasma and formic acid is as follows: the PTFE membrane is immersed in formic acid, and plasma is generated above the PTFE membrane using carbon dioxide and air to modify the PTFE membrane.

[0012] In some embodiments, the plasma modification time for the polytetrafluoroethylene (PTFE) membrane is 5–10 minutes. If the plasma modification time is too long, carbonization will occur in the modified PTFE membrane, which is detrimental to the purification of lithium bis(fluorosulfonyl)imide.

[0013] In some embodiments, the volume ratio of carbon dioxide to air is 7:1 to 10:1, and the air contains a small amount of water to provide hydrogen for grafting carboxyl groups.

[0014] In some preferred embodiments, the volume ratio of carbon dioxide to air is 9:1.

[0015] In some embodiments, the process of purifying lithium difluorosulfonylimide electrodialysis stock solution by electrodialysis using modified polytetrafluoroethylene membrane is as follows: multiple modified polytetrafluoroethylene membranes are placed in a polytetrafluoroethylene electrolytic cell, which is divided into multiple electrolysis zones. The modified polytetrafluoroethylene membrane is used to add the lithium difluorosulfonylimide electrodialysis stock solution to the middle electrolysis zone, while anhydrous dimethyl carbonate is added to the remaining electrolysis zones. Electrodialysis is then performed by passing an electric current through the membrane.

[0016] In some embodiments, the voltage for electrodialysis is 10-20V, and the time is 10-20 minutes. If the electrodialysis time is too short, the purified lithium difluorosulfonamide will not meet battery-grade standards.

[0017] In some embodiments, in step S101, the heating temperature is 110–130°C and the time is 15 hours; the drying temperature is 180–220°C.

[0018] In some embodiments, in step S102, lithium carbonate is dispersed in dimethyl carbonate, cooled to 0-5°C, and then difluorosulfonyl imide acid is added.

[0019] In some embodiments, the molar ratio of bis(fluorosulfonyl)imide acid to lithium carbonate is 1:0.525 to 1:0.55.

[0020] In some embodiments, the temperature for electrodialysis purification in step S103 is 10–15°C.

[0021] Secondly, this application also provides a lithium-ion battery, the lithium-ion battery including a lithium-ion battery electrolyte, the lithium-ion battery electrolyte including lithium bis(fluorosulfonyl)imide obtained by the above purification method.

[0022] This application first modifies the surface of a polytetrafluoroethylene (PTFE) membrane using carbon dioxide and air plasma to introduce active groups such as carboxyl groups onto the membrane surface. Then, the membrane is immersed in difluorosulfonyl imide acid to inert the active groups. After drying, a modified PTFE membrane is obtained.

[0023] The beneficial effects of the technical solution provided in this application include:

[0024] 1. The method provided in this application uses a modified polytetrafluoroethylene membrane to perform electrodialysis on lithium difluorosulfonylimide electrodialysis stock solution. Under the action of an electric field, since the modified polytetrafluoroethylene membrane is grafted with difluorosulfonylimide, it has a significant repulsive effect on difluorosulfonylimide ions, greatly hindering the migration of difluorosulfonylimide ions. Other anions, such as fluoride ions, chloride ions, and sulfate ions, migrate directionally under the action of an electric field. Due to different migration rates, they are enriched in different regions. After the electrodialysis is completed, the obtained electrodialysis solution is concentrated and crystallized to obtain battery-grade lithium difluorosulfonylimide with qualified anion impurity content.

[0025] 2. The method provided in this application does not require the addition of a dehydrating agent before crystallization and purification, thus avoiding the introduction of impurities and simplifying the process.

[0026] 3. By using the method provided in this application and reasonably setting the electrodialysis time, the purity requirement of the raw material lithium carbonate can be reduced from battery grade to industrial grade, which is beneficial to industrial production. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 This is a schematic diagram of the reaction mechanism for preparing the modified polytetrafluoroethylene membrane in the embodiments of this application.

[0029] Figure 2 This is a schematic diagram illustrating the electrodialysis process of lithium bisfluorosulfonylimide electrodialysis stock solution according to an embodiment of this application. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0031] This application provides a purification method for lithium bisfluorosulfonylimide, which solves the problem in the prior art that the anion content is not easy to meet the requirements in the purification process of preparing battery-grade lithium bisfluorosulfonylimide.

[0032] The purification method for lithium bis(fluorosulfonyl)imide provided in this application includes the following steps:

[0033] Step S101: The polytetrafluoroethylene (PTFE) membrane is modified using plasma and formic acid. After modification, the PTFE membrane is immersed in difluorosulfonyl imide acid, heated, and dried to obtain the modified PTFE membrane. The reaction mechanism for preparing the modified PTFE membrane is described below. Figure 1 ;

[0034] In step S102, lithium carbonate is dispersed in dimethyl carbonate, and then difluorosulfonyl imide acid is added. After the reaction is completed, a reaction solution is obtained. The reaction solution is filtered to obtain a filtrate. The filtrate is concentrated once to obtain lithium difluorosulfonyl imide electrodialysis stock solution.

[0035] In step S103, multiple modified polytetrafluoroethylene (PTFE) membranes are placed in a PTFE electrolytic cell, which divides the PTFE electrolytic cell into multiple electrolysis zones. The lithium difluorosulfonylimide electrodialysis stock solution is added to the middle electrolysis zone, and anhydrous dimethyl carbonate is added to the other electrolysis zones. Electrodialysis is performed by passing an electric current through the cells. The obtained electrodialysis solution is then concentrated, dichloromethane is added, and crystallization is carried out to obtain battery-grade lithium difluorosulfonylimide.

[0036] In step S103, the electrodialysis process is described below. Figure 2 .

[0037] Example 1

[0038] Example 1 provides a method for purifying lithium bis(fluorosulfonyl)imide, comprising the following steps:

[0039] Step S101: Take a commercially available 0.22μm polytetrafluoroethylene (PTFE) membrane, wipe it clean with acetone and let it dry. Immerse it in a PTFE container filled with formic acid and place it between the upper and lower electrodes. Open the carbon dioxide and compressed air gas passages, control the carbon dioxide:air intake volume ratio to be 9:1, and control the flow rate to be 4L / min. Start the high-voltage power supply and control the voltage to be 8kV to generate a stable atmospheric pressure plasma above the PTFE membrane. Perform plasma modification for 5 minutes. After the modification is completed, remove the membrane and dry it at room temperature. Immerse the PTFE membrane in battery-grade difluorosulfonyl imide acid in a glove box, heat it to 120℃, reflux for 15 hours, remove it, place the membrane on a PTFE plate, and dry it at 200℃ to obtain the modified PTFE membrane.

[0040] In step S102, 7.76g of high-purity lithium carbonate (fluoride ion content 5.2ppm, chloride ion content 2.7ppm, sulfate content 5.2ppm) was added to a 250ml PFA bottle and dispersed in 100g of dimethyl carbonate. The temperature was lowered to 0-5℃, and 36.22g of battery-grade difluorosulfonyl imide acid (fluoride ion content 10.8ppm, chloride ion content 2.3ppm, sulfate content 5.9ppm) was added dropwise over 1-2 hours, with the temperature controlled not exceeding 35℃. The reaction was completed after 2 hours. After the reaction was completed, the reaction solution was obtained. The reaction solution was filtered to remove insoluble matter, and the filtrate was concentrated once to remove most of the water, yielding the lithium difluorosulfonyl imide electrodialysis stock solution.

[0041] Step S103: Install modified PTFE membranes in a PTFE electrolytic cell, and use six modified PTFE membranes to separate seven electrolysis zones numbered 1 to 7. Add the prepared lithium difluorosulfonylimide electrodialysis stock solution to electrolysis zone 4. Fill the remaining zones with anhydrous dimethyl carbonate. Control the temperature at 10-15°C, pass a constant voltage current of 10V for 10 minutes, stop the current, drain the electrodialysis solution from electrolysis zone 4, concentrate it, add dichloromethane, and crystallize to obtain battery-grade lithium difluorosulfonylimide with a yield of 86.5%.

[0042] The battery-grade lithium difluorosulfonylimide obtained was tested and found to have a purity of ≥99.9%; the contents of each impurity were: moisture 18.2 ppm, fluoride ion 9.8 ppm, chloride ion 0.3 ppm, and sulfate ion 20.8 ppm; meeting the battery-grade standard (standard number is "YS / T 1302-2019 Power Battery Electrolyte Lithium Difluorosulfonylimide Salt", the same below).

[0043] Example 2

[0044] The only difference between Example 2 and Example 1 is that the plasma modification time in step S101 is 10 min; the rest of the process is basically the same as in Example 1.

[0045] Example 2 yielded 84.3% of battery-grade lithium bisfluorosulfonylimide. The purity of the obtained battery-grade lithium bisfluorosulfonylimide was ≥99.9%, and the contents of each impurity were: moisture 16.1 ppm, fluoride ion 15.5 ppm, chloride ion 2.8 ppm, and sulfate ion 25.3 ppm; meeting the battery-grade standard.

[0046] Example 3

[0047] The only difference between Example 3 and Example 1 is that Example 3 uses battery-grade lithium carbonate (fluoride ion content 15.7ppm, chloride ion content 20.4ppm, sulfate content 167.2ppm) as feedstock; the rest of the process is basically the same as that of Example 1.

[0048] Testing showed that the battery-grade lithium bis(fluorosulfonyl)imide prepared in Example 3 had a purity of ≥99.9%, with the following impurity contents: moisture 26.0 ppm, fluoride ion 9.2 ppm, chloride ion 3.5 ppm, sulfate ion 30.5 ppm, and a yield of 87.6%, meeting battery-grade standards.

[0049] Example 4

[0050] The only difference between Example 4 and Example 1 is that industrial-grade lithium carbonate (fluoride ion content 30.5ppm, chloride ion content 98.8ppm, sulfate ion content 974.5ppm) is used as feedstock; a constant voltage current of 10V is applied for 20 minutes; the rest of the process is basically the same as in Example 1.

[0051] Testing showed that the battery-grade lithium bis(fluorosulfonyl)imide obtained in Example 4 had a purity of ≥99.9%, with the following impurity contents: moisture 28.3 ppm, fluoride ion 7.3 ppm, chloride ion 6.1 ppm, sulfate 32.8 ppm, and a yield of 85.8%; meeting battery-grade standards.

[0052] Example 5

[0053] The only difference between Example 5 and Example 1 is that: difluorosulfonyl imide acid (fluoride ion content 50.7ppm, chloride ion content 100.1ppm, sulfate ion content 20.1ppm) purified by distillation rather than rectification is used as feed; a constant voltage current of 10V is applied for 20 minutes; the rest of the process is basically the same as in Example 1.

[0054] Testing showed that the battery-grade lithium bis(fluorosulfonyl)imide obtained in Example 5 had a purity of ≥99.9%, with the following impurity contents: moisture 23.9 ppm, fluoride ion 3.3 ppm, chloride ion 8.0 ppm, sulfate ion 5.1 ppm, and a yield of 82.7%; meeting battery-grade standards.

[0055] Example 6

[0056] The only difference between Example 6 and Example 1 is that: industrial-grade lithium carbonate (fluoride ion content 30.5ppm, chloride ion content 98.8ppm, sulfate content 974.5ppm) and difluorosulfonyl imide acid (fluoride ion content 50.7ppm, chloride ion content 100.1ppm, sulfate content 20.1ppm) purified by distillation rather than rectification were used as feedstocks, and a constant voltage current of 10V was applied for 20 minutes; otherwise, it was basically the same as Example 1.

[0057] Testing showed that the battery-grade lithium bis(fluorosulfonyl)imide obtained in Example 6 had a purity of ≥99.9%, with the following impurity contents: moisture 23.0 ppm, fluoride ion 8.2 ppm, chloride ion 10.0 ppm, sulfate ion 39.6 ppm, and a yield of 81.3%; meeting battery-grade standards.

[0058] Comparative Example 1:

[0059] The only difference between Comparative Example 1 and Example 1 is that the plasma modification time is 15 min; otherwise, they are basically the same as Example 1.

[0060] The modified polytetrafluoroethylene membrane prepared in Comparative Example 1 showed obvious carbonization and blackening. The obtained lithium bis(fluorosulfonyl)imide had a purity of ≥99.9%, a moisture content of 37.1 ppm, a fluoride ion content of 43.6 ppm, a chloride ion content of 2.0 ppm, a sulfate content of 187.7 ppm, and a yield of 50.3%; it did not meet the requirements for battery grade.

[0061] The results from Example 1 and Comparative Example 1 show that after the plasma modification time is extended, the modified polytetrafluoroethylene film exhibits carbonization, which is detrimental to the purification of lithium bis(fluorosulfonyl)imide.

[0062] Comparative Example 2:

[0063] The only difference between Comparative Example 2 and Example 1 is that Comparative Example 2 uses industrial-grade lithium carbonate (fluoride ion content 30.5ppm, chloride ion content 98.8ppm, sulfate ion content 974.5ppm) as feed material and is subjected to a constant voltage current of 10V for 5 minutes. The rest is basically the same as Example 1.

[0064] Testing showed that the lithium bis(fluorosulfonyl)imide prepared in Comparative Example 2 had a purity of ≥99.9%, a moisture content of 28.6 ppm, a fluoride ion content of 19.9 ppm, a chloride ion content of 30.2 ppm, a sulfate content of 100.7 ppm, and a yield of 82.1%; it did not meet the battery-grade standard.

[0065] The results of Comparative Example 2 and Example 4 show that if industrial-grade lithium carbonate is used as feedstock and the constant voltage current time during the electrodialysis process is short, not within 10-20 minutes, the purified lithium difluorosulfonylimide cannot meet the battery-grade standard.

[0066] Comparative Example 3:

[0067] The only difference between Comparative Example 3 and Example 1 is that Comparative Example 3 uses distilled rather than purified difluorosulfonyl imide (fluoride ion content 50.7 ppm, chloride ion content 100.1 ppm, sulfate ion content 20.1 ppm) as feedstock; the rest is basically the same as Example 1.

[0068] Testing showed that the lithium bis(fluorosulfonyl)imide prepared in Comparative Example 3 had a purity of ≥99.9%, a moisture content of 33.8 ppm, a fluoride ion content of 14.4 ppm, a chloride ion content of 25.5 ppm, and a sulfate content of 13.0 ppm, with a yield of 80.1%; it did not meet the battery-grade standard.

[0069] The results from Comparative Example 3 and Example 1 show that using bis(fluorosulfonyl)imide acid with a high anion content affects the purification effect of lithium bis(fluorosulfonyl)imide. The results from Comparative Example 3 and Example 5 show that using bis(fluorosulfonyl)imide acid with a high anion content and extending the electrodialysis time can achieve the desired purification effect.

[0070] Comparative Example 4:

[0071] The only difference between Comparative Example 4 and Example 1 is that Comparative Example 4 directly uses a commercially available 0.22μm polytetrafluoroethylene membrane to perform electrodialysis on the prepared lithium difluorosulfonylimide electrodialysis stock solution; the rest of the process is basically the same as that of Example 1.

[0072] Testing showed that the lithium bis(fluorosulfonyl)imide prepared in Comparative Example 4 had a purity of ≥99.9%, a moisture content of 37.4 ppm, a fluoride ion content of 205.9 ppm, a chloride ion content of 3.3 ppm, and a sulfate content of 35.5 ppm, with a yield of 18.4%. This did not meet the battery-grade standard, and the yield was significantly reduced.

[0073] Comparative Example 5:

[0074] The only difference between Comparative Example 5 and Example 1 is that only the commercially available 0.22μm polytetrafluoroethylene membrane in Comparative Example 5 was subjected to plasma modification; the rest of the process was basically the same as in Example 1.

[0075] The plasma modification process of the polytetrafluoroethylene membrane in Comparative Example 5 was as follows: A commercially available 0.22μm polytetrafluoroethylene membrane was taken, wiped clean with acetone and dried, and immersed in a polytetrafluoroethylene container containing formic acid. The membrane was placed between the upper and lower electrodes, and the carbon dioxide and compressed air gas passages were opened. The carbon dioxide:air inlet volume ratio was controlled at 9:1 and the flow rate was controlled at 4L / min. The high-voltage power supply was started and the voltage was controlled at 8kV to generate a stable atmospheric pressure plasma above the polytetrafluoroethylene membrane. The plasma modification was carried out for 5 minutes. After the modification was completed, the membrane was taken out and dried at room temperature to obtain the modified polytetrafluoroethylene membrane.

[0076] Testing revealed that the lithium bis(fluorosulfonyl)imide prepared in Comparative Example 5 had a purity ≥99.9%, a moisture content of 28.5 ppm, a fluoride ion content of 20.1 ppm, a chloride ion content of 4.2 ppm, and a sulfate content of 100.7 ppm, with a yield of 30.2%. This did not meet the battery-grade standard, and the yield was significantly reduced.

[0077] In the description of this specification, the references to terms such as "one embodiment / mode," "some embodiments / modes," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment / mode or example is included in at least one embodiment / mode or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment / mode or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments / modes or examples. Furthermore, without contradiction, those skilled in the art can combine and integrate the different embodiments / modes or examples described in this specification, as well as the features of different embodiments / modes or examples.

[0078] It should be noted that in this application, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element. In this application, "a plurality of" means at least two, such as two, three, etc., unless otherwise expressly specified.

[0079] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for purifying lithium difluorosulfonylimide, characterized in that, Includes the following steps: S101, the polytetrafluoroethylene (PTFE) membrane is modified using plasma and formic acid. After modification, the PTFE membrane is immersed in difluorosulfonyl imide acid, heated, and dried to obtain the modified PTFE membrane. The time for modifying the PTFE membrane using plasma is 5–10 min, the heating temperature is 110–130℃, and the drying temperature is 180–220℃. S102, lithium carbonate is dispersed in dimethyl carbonate, then difluorosulfonyl imide acid is added. After the reaction is completed, a reaction solution is obtained. The reaction solution is filtered to obtain a filtrate. The filtrate is concentrated once to obtain the lithium difluorosulfonyl imide electrodialysis stock solution. S103, using a modified polytetrafluoroethylene membrane to purify the lithium difluorosulfonylimide electrodialysis stock solution by electrodialysis, and then concentrating and crystallizing the obtained electrodialysis solution to obtain battery-grade lithium difluorosulfonylimide.

2. The purification method for lithium bis(fluorosulfonyl)imide according to claim 1, characterized in that, The process of modifying polytetrafluoroethylene (PTFE) membranes using plasma and formic acid is as follows: the PTFE membrane is immersed in formic acid, and plasma is generated above the PTFE membrane using carbon dioxide and air to modify the PTFE membrane.

3. The purification method for lithium bis(fluorosulfonyl)imide according to claim 2, characterized in that, The volume ratio of carbon dioxide to air is 7:1 to 10:

1.

4. The purification method for lithium bis(fluorosulfonyl)imide according to claim 1, characterized in that, The process of purifying lithium difluorosulfonamide electrodialysis stock solution by electrodialysis using modified polytetrafluoroethylene membrane is as follows: multiple modified polytetrafluoroethylene membranes are placed in a polytetrafluoroethylene electrolytic cell, which divides the polytetrafluoroethylene electrolytic cell into multiple electrolysis zones. The lithium difluorosulfonamide electrodialysis stock solution is added to the middle electrolysis zone, and anhydrous dimethyl carbonate is added to the other electrolysis zones. Electrodialysis is then performed by passing an electric current through the membrane.

5. The purification method for lithium bis(fluorosulfonyl)imide according to claim 4, characterized in that, The voltage for electrodialysis is 10-20V, and the time is 10-20 minutes.

6. The purification method for lithium bis(fluorosulfonyl)imide according to claim 1, characterized in that, In step S102, lithium carbonate is dispersed in dimethyl carbonate, cooled to 0-5°C, and then difluorosulfonyl imide acid is added.

7. The purification method for lithium bis(fluorosulfonyl)imide according to claim 1, characterized in that, In step S103, the temperature for electrodialysis purification is 10–15°C.

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