A method for preparing lithium bisfluorosulfonimide

Abstract

The present application relates to a kind of preparation methods of lithium bisfluorosulfonylimide, the preparation method includes the following steps: (1) uniformly mixes p-toluenesulfonyl isocyanate, chlorosulfonic acid and solid acid catalyst, then hydrogen fluoride is introduced to carry out catalytic reaction, the obtained reaction product is separated by solid-liquid, and lithium bisfluorosulfonylimide is obtained;(2) lithium bisfluorosulfonylimide obtained in step (1) is sequentially subjected to cooling crystallization purification, lithiation and post-processing, and the lithium bisfluorosulfonylimide is obtained.The present application uses p-toluenesulfonyl isocyanate as raw material, and chlorosulfonic acid, hydrogen fluoride is synthesized into lithium bisfluorosulfonylimide under the action of solid acid catalyst, avoids the generation of water and the introduction of metal ions and other impurities in the reaction process, the reaction condition is mild and the reaction is complete, simplifies process flow while further improving product purity and production efficiency.

Description

Technical Field

[0001] This invention relates to the field of electrolyte technology for lithium batteries, and specifically to a method for preparing lithium bis(fluorosulfonyl)imide. Background Technology

[0002] Lithium difluorosulfonylimide (LiDIEM) possesses advantages such as high stability, excellent low-temperature performance, good hydrolytic stability, and environmental friendliness. In LiDIEM, the strong electron-attracting ability of fluoride ions reduces the coordination between cations and anions within the lithium salt, thereby enhancing lithium-ion mobility. This material exhibits excellent electrical conductivity, thermal stability, and electrochemical stability, with virtually no side reactions and effective control over battery expansion. Given these properties, LiDIEM is considered a promising advanced electrolyte that could replace lithium hexafluorophosphate, holding immense potential in the development of lithium-ion battery technology.

[0003] Existing methods for preparing lithium bis(fluorosulfonyl)imide mostly use aminosulfonic acid, chlorosulfonyl isocyanate, etc. as raw materials. For example, CN 116040592A discloses a method for preparing lithium bis(fluorosulfonyl)imide, which includes the following steps:

[0004] (1) Chlorosulfonyl isocyanate and fluorosulfonic acid are mixed and stirred while anhydrous hydrogen fluoride gas is continuously introduced. Under the action of a catalyst, the mixture is reacted at 25-130℃ for 4-30 hours to synthesize bisfluorosulfonylimide; (2) Bisfluorosulfonylimide is mixed with an organic solvent, and lithium alloy is added. The mixture is reacted at 15-120℃ for 0.5-15 hours. The product is filtered and subjected to vacuum distillation to obtain lithium bisfluorosulfonylimide. Although this preparation method simplifies the process steps, the use of fluorosulfonic acid and lithium alloy for the reaction not only increases the production cost but may also introduce other metal ions.

[0005] CN 116374965A discloses a method for heterogeneous catalytic synthesis of a key intermediate in lithium bis(fluorosulfonyl)imide electrolytes. The heterogeneous catalytic method includes: reacting chlorosulfonic acid with chlorosulfonyl isocyanate under the action of a heterogeneous catalyst; after the reaction, directly filtering the catalyst and reusing it in the next batch of synthesis; and collecting the target intermediate, bis(fluorosulfonyl)imide, by vacuum distillation of the mother liquor. This method uses a solid heteropolyacid catalyst, which improves the product yield and facilitates separation, but still requires two steps to synthesize the intermediate bis(fluorosulfonyl)imide.

[0006] CN 107055493A discloses a method for preparing lithium bis(fluorosulfonyl)imide, comprising the following steps:

[0007] (1) Fluorination reaction: Dichlorosulfonylimide reacts with hydrogen fluoride under the action of a catalyst to synthesize the intermediate difluorosulfonylimide; (2) The difluorosulfonylimide obtained in step (1) is reacted with lithium carboxylate to obtain lithium difluorosulfonylimide. Although this preparation method ensures the purity of the product, it uses a homogeneous catalyst, perfluorooctyl sulfonyl fluoride, which is a toxic chemical that is strictly restricted in China and has a high subsequent processing cost.

[0008] Therefore, in order to address the shortcomings of existing technologies, there is a need to provide a method for preparing lithium bisfluorosulfonylimide that is simple in process, mild in reaction conditions, low in cost, and can avoid the introduction of impurities. Summary of the Invention

[0009] The purpose of this invention is to provide a method for preparing lithium bisfluorosulfonyl imide, which uses p-toluenesulfonyl isocyanate as raw material and synthesizes bisfluorosulfonyl imide in one step via catalytic synthesis, avoiding the generation of water and the introduction of impurities. After further cooling, crystallization and purification, and lithiation, high-purity lithium bisfluorosulfonyl imide can be obtained. This method has the advantages of simple process, mild reaction conditions and low cost.

[0010] To achieve this objective, the present invention adopts the following technical solution:

[0011] This invention provides a method for preparing lithium bis(fluorosulfonyl)imide, the method comprising the following steps:

[0012] (1) Mix p-toluenesulfonyl isocyanate, chlorosulfonic acid and solid acid catalyst uniformly, and then pass hydrogen fluoride through to carry out catalytic reaction. The reaction product is separated by solid-liquid separation to obtain difluorosulfonyl imide.

[0013] (2) The difluorosulfonyl imide obtained in step (1) is subjected to cooling crystallization purification, lithiation and post-treatment in sequence to obtain the lithium difluorosulfonyl imide.

[0014] The present invention provides a method for preparing lithium bis(fluorosulfonyl)imide, using p-toluenesulfonyl isocyanate, chlorosulfonic acid, and hydrogen fluoride as raw materials. A solid acid catalyst is added to catalyze the one-step synthesis of bis(fluorosulfonyl)imide. After recrystallization and purification, lithiation is performed to obtain lithium bis(fluorosulfonyl)imide. This method avoids the generation of water and the introduction of impurities such as metal ions during the reaction process. Washing with an organic solvent and filtration yields a high-purity product. Furthermore, by using a suitable solid acid catalyst, not only is the interference from impurities that may arise from homogeneous catalysts avoided, but the one-step synthesis of the key intermediate bis(fluorosulfonyl)imide is also promoted. The reaction conditions are milder and the reaction is more complete, effectively reducing the content of chloride ions, sulfate ions, etc., in the product, simplifying the process and further improving product purity. Simultaneously, due to the addition of the solid acid catalyst, only simple solid-liquid separation is needed to obtain high-purity lithium bis(fluorosulfonyl)imide, improving production efficiency. The use of cooling crystallization purification instead of the mainstream distillation process also avoids difficult-to-clean residues in the reaction vessel, thereby reducing production costs.

[0015] Preferably, the molar ratio of p-toluenesulfonyl isocyanate to chlorosulfonic acid in step (1) is 1:(1-4), for example, it can be 1:1, 1:2, 1:3 or 1:4, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0016] Preferably, the amount of solid acid catalyst used in step (1) is 0.5-2% of the amount of p-toluenesulfonyl isocyanate, for example, it can be 0.5%, 1%, 1.5% or 2%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0017] Preferably, the solid acid catalyst in step (1) comprises a single-component oxide or a composite-component oxide.

[0018] Preferably, the single-component oxide includes a single-component metal oxide.

[0019] Preferably, the single-component metal oxide includes SO4. 2- / ZrO2 or SO4 2- / TiO2.

[0020] Preferably, the composite oxide includes spinel-type AB2O4 or perovskite-type ABO3.

[0021] Preferably, the uniform mixing in step (1) is carried out under oil bath heating conditions.

[0022] Preferably, the temperature of the oil bath heating is 80-90℃, for example, it can be 80℃, 82℃, 85℃, 88℃ or 90℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0023] Preferably, the molar ratio of p-toluenesulfonyl isocyanate to hydrogen fluoride in step (1) is 1:(1-7), for example, it can be 1:1, 1:2, 1:3, 1:5 or 1:7, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0024] The hydrogen fluoride is anhydrous hydrogen fluoride gas.

[0025] Preferably, the catalytic reaction time in step (1) is 6-12 hours, for example, 6 hours, 7 hours, 8 hours, 10 hours or 12 hours, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0026] Preferably, the solid acid catalyst obtained after solid-liquid separation in step (1) is recycled.

[0027] Preferably, the cooling crystallization purification in step (2) is carried out in a jacketed reactor.

[0028] Preferably, the cooling rate for cooling crystallization purification in step (2) is 0.9-1.1℃ / 30min, for example, it can be 0.9℃ / 30min, 1℃ / 30min or 1.1℃ / 30min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0029] Preferably, the endpoint temperature for cooling crystallization purification in step (2) is 8-14℃, for example, it can be 8℃, 9℃, 10℃, 12℃ or 14℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0030] Preferably, after cooling and crystallizing to the endpoint temperature in step (2), the temperature is maintained for 1-2 hours, for example, 1 hour, 1.2 hours, 1.5 hours, 1.8 hours or 2 hours, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0031] Preferably, the specific steps of lithiation in step (2) include: mixing the bis(fluorosulfonyl)imide obtained by cooling crystallization purification with a lithium source and heating it in an oil bath to react, and then cooling to precipitate crude bis(fluorosulfonyl)imide lithium.

[0032] The precipitation caused by cooling refers to precipitation caused by natural cooling.

[0033] Preferably, the molar ratio of the bis(fluorosulfonyl)imide to the lithium source is 1:(0.6-1), for example, it can be 1:0.6, 1:0.7, 1:0.8, 1:0.9 or 1:1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0034] Preferably, the lithium source includes lithium fluoride.

[0035] Preferably, the temperature of the oil bath heating is 120-130℃, for example, it can be 120℃, 122℃, 125℃, 128℃ or 130℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0036] Preferably, the reaction time is 1-4 hours, for example, 1 hour, 2 hours, 3 hours, 3.5 hours or 4 hours, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0037] Preferably, the post-processing in step (2) includes washing with organic solvents and filtration.

[0038] Preferably, the organic solvent includes any one of dichloromethane, dichloroethane, or tetrachloroethane.

[0039] Preferably, both steps (1) and (2) are carried out in a nitrogen atmosphere.

[0040] As a preferred embodiment of the preparation method described in this invention, the preparation method includes the following steps:

[0041] (1) p-Toluenesulfonyl isocyanate, chlorosulfonic acid and solid acid catalyst are uniformly mixed under oil bath heating at 80-90℃, and then hydrogen fluoride is introduced to carry out catalytic reaction for 6-12h. The reaction product is separated into solid and liquid to obtain difluorosulfonyl imide and solid acid catalyst. The obtained solid acid catalyst is recovered and recycled. The molar ratio of p-toluenesulfonyl isocyanate to chlorosulfonic acid is 1:(1-4). The amount of solid acid catalyst is 0.5-2% of the amount of p-toluenesulfonyl isocyanate. The molar ratio of p-toluenesulfonyl isocyanate to hydrogen fluoride is 1:(1-7).

[0042] (2) The difluorosulfonyl imide obtained in step (1) is cooled and crystallized in a jacketed reactor at a cooling rate of 0.9-1.1℃ / 30min. After reaching the final temperature of 8-14℃, it is kept at the temperature for 1-2h. Then, the obtained difluorosulfonyl imide is mixed with a lithium source at a molar ratio of 1:(0.6-1) and heated in an oil bath at 120-130℃ for 1-4h. The crude difluorosulfonyl imide is naturally cooled to precipitate. The crude difluorosulfonyl imide is washed and filtered with an organic solvent to obtain the difluorosulfonyl imide lithium.

[0043] Both steps (1) and (2) are carried out in a nitrogen atmosphere.

[0044] Compared with the prior art, the present invention has the following beneficial effects:

[0045] The present invention provides a method for preparing lithium bis(fluorosulfonyl)imide, using p-toluenesulfonyl isocyanate as a raw material and employing a solid acid catalyst to synthesize bis(fluorosulfonyl)imide in one step. After recrystallization and purification, it is then lithiated to obtain lithium bis(fluorosulfonyl)imide. This method avoids the generation of water and the introduction of impurities such as metal ions during the reaction process. A high-purity product is obtained by washing with an organic solvent. The reaction conditions are mild and the reaction is complete, effectively reducing the content of chloride ions, sulfate ions, etc., in the product, simplifying the process while further improving the purity of the product. Due to the addition of the solid acid catalyst, the reaction solution and catalyst can be separated simply by filtration, thus improving production efficiency. The use of a cooling crystallization purification process instead of the mainstream distillation process also avoids difficult-to-clean residues in the reaction vessel, thereby reducing production costs. Detailed Implementation

[0046] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0047] Example 1

[0048] This embodiment provides a method for preparing lithium bis(fluorosulfonyl)imide, the method comprising the following steps:

[0049] (1) Under the condition of oil bath heating at 85℃, p-toluenesulfonyl isocyanate, chlorosulfonic acid and SO4 are uniformly mixed. 2- The catalyst was ZrO2, and then hydrogen fluoride was introduced to carry out the catalytic reaction for 8 hours. The resulting reaction product was filtered to obtain bis(fluorosulfonyl)imide and SO4. 2- / ZrO2 catalyst; the resulting SO4 2- The ZrO2 catalyst is recovered and recycled; the molar ratio of p-toluenesulfonyl isocyanate to chlorosulfonic acid is 1:2; the SO4 2- The amount of ZrO2 catalyst used is 1.5% of the amount of p-toluenesulfonyl isocyanate; the molar ratio of p-toluenesulfonyl isocyanate to hydrogen fluoride is 1:3;

[0050] (2) The difluorosulfonyl imide obtained in step (1) is cooled and crystallized in a jacketed reactor at a cooling rate of 1℃ / 30min. After reaching the final temperature of 10℃, it is kept at the temperature for 1.5h. Then, the obtained difluorosulfonyl imide is mixed with lithium fluoride at a molar ratio of 1:0.8 and heated in an oil bath at 125℃ for 2h. The crude difluorosulfonyl imide is precipitated by natural cooling. The crude difluorosulfonyl imide is washed and filtered with dichloromethane to obtain the difluorosulfonyl imide.

[0051] Both steps (1) and (2) are carried out in a nitrogen atmosphere.

[0052] Example 2

[0053] This embodiment provides a method for preparing lithium bis(fluorosulfonyl)imide, the method comprising the following steps:

[0054] (1) Under the condition of oil bath heating at 80℃, p-toluenesulfonyl isocyanate, chlorosulfonic acid and SO4 are uniformly mixed. 2- The reaction was carried out using a TiO2 catalyst, followed by the introduction of hydrogen fluoride for 12 hours. The resulting reaction products were filtered to obtain bis(fluorosulfonyl)imide and SO4. 2- / TiO2 catalyst; the resulting SO4 2- The TiO2 catalyst is recovered and recycled; the molar ratio of p-toluenesulfonyl isocyanate to chlorosulfonic acid is 1:1; the SO4 2- The amount of TiO2 catalyst used is 0.5% of the amount of p-toluenesulfonyl isocyanate; the molar ratio of p-toluenesulfonyl isocyanate to hydrogen fluoride is 1:1;

[0055] (2) The difluorosulfonyl imide obtained in step (1) was cooled and crystallized in a jacketed reactor at a cooling rate of 0.9℃ / 30min. After reaching the final temperature of 8℃, it was kept at the temperature for 2h. Then, the obtained difluorosulfonyl imide was mixed with lithium fluoride at a molar ratio of 1:0.6 and heated in an oil bath at 120℃ for 4h. The crude difluorosulfonyl imide was naturally cooled to precipitate. The crude difluorosulfonyl imide was washed and filtered with dichloroethane to obtain the difluorosulfonyl imide.

[0056] Both steps (1) and (2) are carried out in a nitrogen atmosphere.

[0057] Example 3

[0058] This embodiment provides a method for preparing lithium bis(fluorosulfonyl)imide, the method comprising the following steps:

[0059] (1) p-Toluenesulfonyl isocyanate, chlorosulfonic acid, and MnCo2O4 catalyst were uniformly mixed under oil bath heating at 90℃, and then hydrogen fluoride was introduced to carry out a catalytic reaction for 6 hours. The resulting reaction product was filtered to obtain difluorosulfonyl imide and MnCo2O4 catalyst. The obtained MnCo2O4 catalyst was recovered and recycled. The molar ratio of p-toluenesulfonyl isocyanate to chlorosulfonic acid was 1:4. The amount of MnCo2O4 catalyst was 2% of the amount of p-toluenesulfonyl isocyanate. The molar ratio of p-toluenesulfonyl isocyanate to hydrogen fluoride was 1:7.

[0060] (2) The difluorosulfonyl imide obtained in step (1) is cooled and crystallized in a jacketed reactor at a cooling rate of 1.1℃ / 30min. After reaching the final temperature of 14℃, it is kept at the temperature for 1h. Then, the obtained difluorosulfonyl imide is mixed with lithium fluoride at a molar ratio of 1:1 and heated in an oil bath at 130℃ for 1h. The crude difluorosulfonyl imide is precipitated by natural cooling. The crude difluorosulfonyl imide is washed and filtered with tetrachloroethane to obtain the difluorosulfonyl imide.

[0061] Both steps (1) and (2) are carried out in a nitrogen atmosphere.

[0062] Example 4

[0063] This embodiment provides a method for preparing lithium bis(fluorosulfonyl)imide, which differs from Example 1 in that, except for the SO4 mentioned in step (1), 2- Except for adjusting the amount of ZrO2 catalyst to 0.2% of the amount of p-toluenesulfonyl isocyanate, all other aspects are the same as in Example 1.

[0064] Example 5

[0065] This embodiment provides a method for preparing lithium bis(fluorosulfonyl)imide, which differs from Example 1 in that, except for the SO4 mentioned in step (1), 2- Except for adjusting the amount of ZrO2 catalyst to 5% of the amount of p-toluenesulfonyl isocyanate, all other aspects are the same as in Example 1.

[0066] Example 6

[0067] This embodiment provides a method for preparing lithium bis(fluorosulfonyl)imide. The difference from Example 1 is that, except for adjusting the oil bath heating temperature in step (1) to 70°C, the rest is the same as in Example 1.

[0068] Example 7

[0069] This embodiment provides a method for preparing lithium bis(fluorosulfonyl)imide. The difference between this embodiment and Embodiment 1 is that, except for adjusting the oil bath heating temperature in step (1) to 95°C, the rest is the same as in Embodiment 1.

[0070] Example 8

[0071] This embodiment provides a method for preparing lithium bis(fluorosulfonyl)imide. The difference from Example 1 is that, except for adjusting the molar ratio of bis(fluorosulfonyl)imide to lithium fluoride in step (2) to 1:0.5, all other aspects are the same as in Example 1.

[0072] Example 9

[0073] This embodiment provides a method for preparing lithium bis(fluorosulfonyl)imide. The difference from Example 1 is that, except for adjusting the molar ratio of bis(fluorosulfonyl)imide to lithium fluoride in step (2) to 1:1.2, all other aspects are the same as in Example 1.

[0074] Comparative Example 1

[0075] This comparative example provides a method for preparing lithium bis(fluorosulfonyl)imide, which differs from Example 1 in that, except for the SO4 described in step (1), 2- Except for replacing the ZrO2 catalyst with an equimolar amount of molybdenum pentachloride, everything else is the same as in Example 1.

[0076] Comparative Example 2

[0077] This comparative example provides a method for preparing lithium bis(fluorosulfonyl)imide. The difference from Example 1 is that the cooling crystallization purification in step (2) is changed to distillation, while the rest is the same as in Example 1.

[0078] Comparative Example 3

[0079] This comparative example provides a method for preparing lithium bis(fluorosulfonyl)imide, which adopts the method for preparing lithium bis(fluorosulfonyl)imide disclosed in CN116040592A.

[0080] In this comparative example, chlorosulfonyl isocyanate, fluorosulfonic acid, and hydrogen fluoride were used as raw materials. A catalytic catalyst was used to synthesize bis(fluorosulfonyl)imide in a single step. A lithium alloy was then added to react the product, which was then filtered and subjected to vacuum distillation to obtain lithium bis(fluorosulfonyl)imide. Although this simplified the process, using fluorosulfonic acid and a lithium alloy not only increased production costs but also introduced other metal ions.

[0081] The lithium bis(fluorosulfonyl)imide prepared in Examples 1-9 and Comparative Examples 1-3 were sampled and analyzed using IC (ion chromatography) and ICP (inductively coupled plasma chromatography). The results are shown in Table 1. The measured metal ions included calcium, magnesium, potassium, iron, aluminum, nickel, chromium, copper, and platinum. If all were qualified, they were marked as "qualified" in the table; if any were unqualified, they were marked as "unqualified".

[0082] Table 1

[0083] Example 1 1.253 2.181 qualified Example 2 1.542 3.243 qualified Example 3 1.447 2.954 qualified Example 4 23.243 18.746 qualified Example 5 3.578 5.435 qualified Example 6 35.654 53.762 qualified Example 7 2.324 2.564 qualified Example 8 1.874 48.573 qualified Example 9 2.124 3.142 qualified Comparative Example 1 3.857 5.097 Unqualified Comparative Example 2 2.073 3.197 qualified Comparative Example 3 3.187 8.958 Unqualified

[0084] As can be seen from Table 1, the preparation method provided by the present invention can yield high-purity lithium difluorosulfonylimide product;

[0085] A comparison of Examples 1 with Examples 4 and 5 shows that insufficient catalyst dosage leads to incomplete reaction and increased chloride ion content, while excessive dosage does not significantly improve product quality. A comparison of Examples 1 with Examples 6 and 7 shows that excessively low oil temperature results in incomplete reaction, while excessively high oil temperature causes a color change in the obtained lithium difluorosulfonylimide. A comparison of Examples 1 with Examples 8 and 9 shows that insufficient lithium fluoride and excess difluorosulfonylimide lead to increased acid and sulfate content in the product; excessive dosage, while ensuring complete reaction of difluorosulfonylimide, results in lithium difluorosulfonylimide coating with excess lithium fluoride, forming hard lumps.

[0086] As can be seen from the comparison between Example 1 and Comparative Examples 1-3, the traditional catalyst requires a two-step reaction to obtain bis(fluorosulfonyl)imide and cannot separate the catalyst from the reaction liquid by filtration alone, resulting in the introduction of impurities such as metal ions; although distillation purification can obtain qualified products, the energy consumption cost is significantly increased in comparison; using lithium alloy as lithium source will introduce other metal ions, resulting in the metal ion content of the obtained lithium(bis(fluorosulfonyl)imide) exceeds the standard.

[0087] In summary, the method for preparing lithium bis(fluorosulfonyl)imide provided by this invention employs a one-step catalytic synthesis of bis(fluorosulfonyl)imide using a solid acid catalyst. After recrystallization and purification, the product is further lithiated to obtain lithium bis(fluorosulfonyl)imide. This method avoids the generation of water and the introduction of impurities such as metal ions during the reaction process. Washing with an organic solvent yields a high-purity product. The one-step catalytic synthesis process features mild reaction conditions and complete reaction, effectively reducing the content of chloride ions, sulfate ions, etc., in the product, simplifying the process while further improving product purity. The addition of a solid acid catalyst simplifies the process, allowing for catalyst separation through simple filtration, thus improving production efficiency. The use of a cooling crystallization purification process instead of the mainstream distillation process avoids difficult-to-clean residues in the reaction vessel and reduces energy costs.

[0088] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the scope of protection and disclosure of the present invention.

Claims

1. A method for preparing lithium bis(fluorosulfonyl)imide, characterized in that, The preparation method includes the following steps: (1) Mix p-toluenesulfonyl isocyanate, chlorosulfonic acid and solid acid catalyst uniformly, and then pass hydrogen fluoride through to carry out catalytic reaction. The reaction product is separated by solid-liquid separation to obtain difluorosulfonyl imide. (2) The difluorosulfonyl imide obtained in step (1) is subjected to cooling crystallization purification, lithiation and post-treatment in sequence to obtain the lithium difluorosulfonyl imide.

2. The preparation method according to claim 1, characterized in that, The molar ratio of p-toluenesulfonyl isocyanate to chlorosulfonic acid in step (1) is 1:(1-4).

3. The preparation method according to claim 1, characterized in that, The amount of solid acid catalyst used in step (1) is 0.5-2% of the amount of p-toluenesulfonyl isocyanate.

4. The preparation method according to claim 1, characterized in that, The solid acid catalyst in step (1) includes a single-component oxide or a composite-component oxide.

5. The preparation method according to claim 4, characterized in that, The single-component oxides include single-component metal oxides.

6. The preparation method according to claim 5, characterized in that, The single-component metal oxide includes SO4. 2- / ZrO2 or SO4 2- / TiO2.

7. The preparation method according to claim 4, characterized in that, The composite component oxides include spinel-type AB2O4 or perovskite-type ABO3.

8. The preparation method according to claim 1, characterized in that, The uniform mixing in step (1) is carried out under the condition of oil bath heating.

9. The preparation method according to claim 8, characterized in that, The oil bath heating temperature is 80-90℃.

10. The preparation method according to claim 1, characterized in that, The molar ratio of p-toluenesulfonyl isocyanate to hydrogen fluoride in step (1) is 1:(1-7).

11. The preparation method according to claim 1, characterized in that, The catalytic reaction in step (1) takes 6-12 hours.

12. The preparation method according to claim 1, characterized in that, The solid acid catalyst obtained after solid-liquid separation in step (1) is recovered and recycled.

13. The preparation method according to claim 1, characterized in that, The cooling crystallization purification step (2) is carried out in a jacketed reactor.

14. The preparation method according to claim 1, characterized in that, The cooling rate for the cooling crystallization purification in step (2) is 0.9-1.1℃ / 30min.

15. The preparation method according to claim 1, characterized in that, The endpoint temperature for cooling crystallization purification in step (2) is 8-14℃.

16. The preparation method according to claim 15, characterized in that, After cooling and crystallizing to the final temperature in step (2), the temperature is maintained for 1-2 hours.

17. The preparation method according to claim 1, characterized in that, The specific steps of lithiation in step (2) include: mixing the bis(fluorosulfonyl)imide obtained by cooling crystallization purification with a lithium source and heating it in an oil bath to react, and then cooling to precipitate crude bis(fluorosulfonyl)imide lithium.

18. The preparation method according to claim 17, characterized in that, The molar ratio of the bis(fluorosulfonyl)imide to the lithium source is 1:(0.6-1).

19. The preparation method according to claim 17, characterized in that, The lithium source includes lithium fluoride.

20. The preparation method according to claim 17, characterized in that, The oil bath heating temperature is 120-130℃.

21. The preparation method according to claim 17, characterized in that, The reaction time is 1-4 hours.

22. The preparation method according to claim 1, characterized in that, The post-processing in step (2) includes organic solvent washing and filtration.

23. The preparation method according to claim 22, characterized in that, The organic solvent includes any one of dichloromethane, dichloroethane, or tetrachloroethane.

24. The preparation method according to claim 1, characterized in that, Both steps (1) and (2) are carried out in a nitrogen atmosphere.

25. The preparation method according to claim 1, characterized in that, The preparation method includes the following steps: (1) Under the condition of oil bath heating at 80-90℃, p-toluenesulfonyl isocyanate, chlorosulfonic acid and solid acid catalyst are uniformly mixed, and then hydrogen fluoride is introduced to carry out catalytic reaction for 6-12h. The reaction product is separated into solid and liquid to obtain difluorosulfonyl imide and solid acid catalyst. The obtained solid acid catalyst is recovered and recycled. The molar ratio of p-toluenesulfonyl isocyanate to chlorosulfonic acid is 1:(1-4). The amount of solid acid catalyst is 0.5-2% of the amount of p-toluenesulfonyl isocyanate. The molar ratio of p-toluenesulfonyl isocyanate to hydrogen fluoride is 1:(1-7). (2) The difluorosulfonyl imide obtained in step (1) is cooled and crystallized in a jacketed reactor at a cooling rate of 0.9-1.1℃ / 30min. After reaching the final temperature of 8-14℃, it is kept at the temperature for 1-2h. Then, the obtained difluorosulfonyl imide is mixed with a lithium source at a molar ratio of 1:(0.6-1) and heated in an oil bath at 120-130℃ for 1-4h. The crude difluorosulfonyl imide is then cooled to precipitate. The crude difluorosulfonyl imide is washed and filtered with an organic solvent to obtain the difluorosulfonyl imide. Both steps (1) and (2) are carried out in a nitrogen atmosphere.