Preparation method of lithium difluorophosphate

Abstract

The invention discloses a preparation method of lithium difluorophosphate, which comprises the following steps: S1, adding polyphosphoric acid into an organic solvent, heating and refluxing to obtain a polyphosphoric acid organic solvent suspension, adding lithium carbonate into the suspension, stirring and reacting for a certain time, adding water, and continuously stirring and reacting to obtain lithium dihydrogen phosphate slurry; s2, carrying out reduced pressure distillation on the solution obtained in S1, evaporating most of the organic solvent, and filtering to remove insoluble impurities; s3, adding hydrofluoric acid into the filtrate obtained in S2, and stirring for reaction to obtain a lithium difluorophosphate solution; and S4, carrying out reduced pressure distillation on the solution obtained in S3, adding an organic solvent after evaporating most of water, stirring, crystallizing, and filtering to obtain lithium difluorophosphate crystals. By optimizing the process conditions of the reaction, the high-purity lithium difluorophosphate product can be prepared from lithium dihydrogen phosphate and hydrofluoric acid at a relatively high conversion rate, and the problems that the product is difficult to extract, the yield is low and the product purity is low when the lithium difluorophosphate is prepared at present can be solved.

Description

Technical Field

[0001] The invention relates to the technical field of lithium ion batteries, and in particular to a method for preparing lithium difluorophosphate. Background Art

[0002] In recent years, with the rapid development of smart devices such as mobile phones and tablets, the market share of small lithium batteries has grown rapidly. At the same time, the increasing improvement of electric vehicle technology has gradually released the market demand for large lithium batteries, and it is expected that lithium batteries will maintain a growth trend in the next few years. Most lithium batteries are secondary batteries, and their poor stability, poor low temperature resistance, and poor cycle characteristics are the main technical difficulties in their rapid development. Studies have shown that the use of lithium difluorophosphate as an electrolyte can effectively improve these shortcomings of lithium batteries.

[0003] At present, there are many methods for preparing lithium difluorophosphate, such as: 1) 2 CO 3 and LiPF 6 or P 2 O 5 A method for preparing lithium difluorophosphate as raw material. However, the lithium difluorophosphate prepared by this method will produce lithium fluoride impurities that are difficult to separate, and the product is difficult to purify. 4 O 10 The preparation method of synthesizing lithium difluorophosphate by solid-solid reaction with LiF, but the reaction is difficult to control during the preparation, there are many by-products, and the conversion recovery rate is low. 3 PO 4 and PF 5 The preparation method of synthesizing lithium difluorophosphate by raw material reaction has high requirements on reaction equipment and produces a large number of by-products that are difficult to separate. 6 Lithium difluorophosphate is prepared by hydrolysis. This reaction is not easy to control and will produce Li 2 PO 3 F, hoursF, hoursPO 2 F 2 Impurities that affect battery performance are not easy to purify, so it is difficult to obtain high-purity products. 5) Lithium difluorophosphate is prepared by the replacement reaction of organic tin fluoride and lithium dichlorophosphate, but the raw material trimethyltin fluoride is not easy to obtain. 6) Lithium dihydrogen phosphate and hydrofluoric acid are used for catalytic fluorination under catalyst conditions to obtain lithium difluorophosphate. However, this method requires high-purity lithium dihydrogen phosphate to be obtained in advance, and after the reaction, the catalyst and lithium difluorophosphate co-crystallize, which need to be purified and separated again. The operation is complicated and the cost is high, so this process is difficult to carry out large-scale production.

[0004] In summary, the existing preparation method of lithium difluorophosphate has defects, so the present invention provides a preparation method of lithium difluorophosphate. Summary of the invention

[0005] In order to overcome the deficiencies of the prior art, the purpose of the present invention is to provide a method for preparing lithium difluorophosphate, in order to solve the problems that the product extraction of lithium difluorophosphate is difficult, the conversion recovery rate is low, and there are many impurities that are difficult to remove, resulting in low product purity.

[0006] The technical solution adopted by the present invention is as follows: A method for preparing lithium difluorophosphate comprises the following steps: S1, adding polyphosphoric acid to an organic solvent and heating it under reflux to obtain a polyphosphoric acid organic solvent suspension, adding lithium carbonate to the suspension, stirring and reacting for a certain period of time, adding water, and continuing to stir and react to obtain a lithium dihydrogen phosphate slurry; S2, subjecting the solution obtained in S1 to reduced pressure distillation, and after evaporating most of the organic solvent, filtering to remove insoluble impurities; S3, adding hydrofluoric acid to the filtrate obtained in S2, stirring and reacting to obtain a lithium difluorophosphate solution; S4. The solution obtained in S3 is subjected to reduced pressure distillation. After evaporating most of the water, an organic solvent is added to stir and crystallize, and the mixture is filtered to obtain lithium difluorophosphate crystals.

[0007] As a preferred embodiment of the present invention, in S1, the weight ratio of the organic solvent to the polyphosphoric acid is 3.6-7.2:1, and the mass ratio of lithium carbonate to the polyphosphoric acid is 0.4-0.5:1.

[0008] As a preferred embodiment of the present invention, in S1, the reaction temperature is 65-79° C., the polyphosphoric acid concentration is ≥85%, and the organic solvent is selected from at least one of acetone, acetonitrile, furan, tetrahydrofuran, and methyl tert-butyl ether. The preferred organic solvent of the present invention provides a suitable reaction environment for the present invention.

[0009] As a preferred embodiment of the present invention, in S1, the lithium carbonate feeding time is 1.5-2.5 hours. The present invention selects a sufficient feeding time, and when the total dosage is fixed, the dosage per minute can be reduced, thereby reducing the exothermic phenomenon of the reaction process, so as to better control the reaction temperature.

[0010] As a preferred embodiment of the present invention, in S1, the weight ratio of water added to polyphosphoric acid is 3.5-4.0:1, and the reaction time after adding water is 1.0-2.0 hours. The present invention adds enough water and ensures sufficient reaction time, which helps the material to fully react and improves the recovery rate.

[0011] As a preferred embodiment of the present invention, in S2, the distillation temperature is 65-79° C., and the distillation time is 0.5-1.5 hours.

[0012] As a preferred embodiment of the present invention, in S3, the mass fraction of hydrofluoric acid is ≥40%, and the weight ratio of hydrofluoric acid to polyphosphoric acid is 1.1-1.5:1.

[0013] As a preferred embodiment of the present invention, in S3, the reaction temperature is 80-95°C and the reaction time is 1.5-3.5 hours. The present invention selects a suitable reaction temperature to ensure sufficient reaction time, which helps the material to fully react and improves the recovery rate.

[0014] As a preferred embodiment of the present invention, in S4, the weight ratio of the added organic solvent to the polyphosphoric acid is 4.0-6.0:1, and the organic solvent is selected from at least one of ethyl acetate, ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, acetone, methyl tert-butyl ether, and carbon tetrachloride.

[0015] As a preferred embodiment of the present invention, in S4, the distillation temperature is 75-88°C, the distillation time is 1.0-2.0 hours, the crystallization temperature is 5-16°C, and the crystallization time is 1.5-2.0 hours. The present invention selects a suitable distillation temperature to ensure sufficient distillation time, which helps to reduce the water content of the product and improve the recovery rate. At the same time, it also selects a suitable crystallization temperature to ensure sufficient crystallization time, which helps the product to crystallize completely and further improve the recovery rate.

[0016] Working principle of the present invention: The reaction equation of the invention is: 5Li 2 CO 3 +2H 2 O(HPO 3 ) 5 +20HF-10LiPO 2 F 2 +5CO 2 +17H 2 O.

[0017] In the first step of the present invention, polyphosphoric acid and lithium carbonate gradually undergo hydrolysis reaction in the presence of an organic solvent, and the reaction process is mild and controllable, and pure lithium dihydrogen phosphate is finally obtained.

[0018] The second step of the present invention is to evaporate most of the organic solvent by reduced pressure distillation, and then filter to remove insoluble impurities.

[0019] In the third step of the present invention, hydrofluoric acid is added in the presence of a small amount of organic solvent, and dihydrogen phosphate is gradually converted into difluorophosphoric acid to generate lithium difluorophosphate.

[0020] The last step of the present invention is to further evaporate and remove most of the water by further reduced pressure distillation, and then add an organic solvent to make the difluorophosphate crystallize and precipitate by utilizing the characteristic that the difluorophosphate has a low solubility in the organic solvent. At the same time, due to the acidity of the aqueous phase, the byproduct inorganic salt is more easily soluble in a small amount of water remaining in the system, thereby achieving the separation of the byproduct and lithium difluorophosphate, and after filtering, high-purity lithium difluorophosphate crystals can be obtained.

[0021] Compared with the prior art, the present invention has the following beneficial effects: The raw materials of the invention are easy to obtain, and the operation is convenient. Moreover, by optimizing the process conditions of the reaction, lithium dihydrogen phosphate and hydrofluoric acid can be used to prepare a high-purity lithium difluorophosphate product at a higher conversion rate. The preparation method is not only easy to operate in the preparation process, has low requirements on equipment and environmental protection, and has low environmental pressure, but also has mild reaction conditions, controlled side reactions, and easy separation and extraction of products. The conversion rate of the products is high and the purity is high. Compared with the existing preparation method, the problem that the raw materials are not easy to obtain, the product extraction is difficult, the yield is not high, the by-product is not easy to remove, and the product purity is not high when preparing lithium difluorophosphate in the prior art is solved. DETAILED DESCRIPTION

[0022] Typical embodiments that embody the features and advantages of the present invention will be described in detail in the following description. It should be understood that the present invention can have various changes in different embodiments without departing from the scope of the present invention, and the descriptions therein are essentially used as illustrations rather than to limit the present invention.

[0023] The reaction equation used in the present invention is: 5Li 2 CO 3 +2H 2 O(HPO 3 ) 5 +20HF-10LiPO 2 F 2 +5CO 2 +17H 2 O.

[0024] Example 1 This embodiment discloses a method for preparing lithium difluorophosphate, comprising the following steps: S1. At room temperature, 85% polyphosphoric acid (P 2 O 5 836g of sodium bicarbonate (calculated) and 3010g of acetonitrile were added into the reactor respectively, and the reactor was sealed. After slowly heating to 67-71°C, stirring was started, and a weak reflux began in the reactor. At this time, 335g of lithium carbonate was slowly added, and the feeding time was 1.5 hours. Then, 2950g of water was added, and the temperature was controlled at 67-72°C. The stirring reaction was continued for 1.0 hour to obtain lithium dihydrogen phosphate slurry.

[0025] S2. After the stirring reaction is completed, the vacuum is turned on to reduce the pressure in the reactor to -0.1MPa, and the temperature is maintained at 65-69°C, and then the reduced pressure distillation is started, wherein the distillation time is 0.5 hours. After most of the organic solvent is evaporated, the mixture is filtered, and after removing the filter residue, the filtrate is transferred to the reactor.

[0026] S3. Add 920 g of hydrofluoric acid with a mass fraction of 40% to the filtrate in the reactor, heat the reactor to 89-93° C., and continue to stir the reaction. The reaction time is 1.5 hours to obtain a lithium difluorophosphate solution.

[0027] S4. After the reaction is completed, the reactor is cooled to 78-82°C, the pressure is reduced to -0.1MPa, and reduced pressure distillation is started again, and the distillation time is 1.0 hour. After evaporating most of the water, 3350g of acetone is added and continued to stir, and the reactor is cooled to 5°C, and lithium difluorophosphate begins to crystallize, and the crystallization is 1.5 hours. After sufficient crystallization, it is filtered. The lithium difluorophosphate filter cake crystals obtained by filtration are pulped with a small amount of acetone. Subsequently, the filter cake is vacuum dried to obtain 839g of lithium difluorophosphate crystalline white powder solid, wherein the yield of lithium difluorophosphate is 85.8%. Moisture ≤10ppm, LiF%≤0.02%, purity greater than 99.7%.

[0028] Example 2 This embodiment discloses a method for preparing lithium difluorophosphate, comprising the following steps: S1. At room temperature, 85% polyphosphoric acid (P 2 O 5 836g of 1,2-dihydrogen phosphate (DHP) and 3200g of acetonitrile were added to the reactor, which was sealed. After slowly heating to 66-70°C, stirring was started, and a weak reflux began in the reactor. At this time, 342g of lithium carbonate was slowly added, and the feeding time was 2.2 hours. Then, 3180g of water was added, and the temperature was controlled at 67-71°C. The reaction was stirred for 2.0 hours to obtain lithium dihydrogen phosphate slurry.

[0029] S2. After the stirring reaction is completed, the vacuum is turned on to reduce the pressure in the reactor to -0.1MPa, and the temperature is maintained at 65-69°C, and then the reduced pressure distillation is started, wherein the distillation time is 0.8 hours. After most of the organic solvent is evaporated, the mixture is filtered, and after removing the filter residue, the filtrate is transferred to the reactor.

[0030] S3. Add 960 g of hydrofluoric acid with a mass fraction of 40% to the filtrate in the reactor, heat the reactor to 89-93° C., and continue to stir the reaction. The reaction time is 3.1 hours to obtain a lithium difluorophosphate solution.

[0031] S4. After the reaction is completed, the reactor is cooled to 78-82°C, decompressed to -0.1MPa, and reduced pressure distillation is started again, and the distillation time is 1.6 hours. After evaporating most of the water, 3450g of ethyl acetate is added and continued to stir, and the reactor is cooled to 6°C, and lithium difluorophosphate begins to crystallize, and the crystallization is 1.5 hours. After sufficient crystallization, it is filtered. The lithium difluorophosphate filter cake crystals obtained by filtration are pulped with a small amount of ethyl acetate. Subsequently, the filter cake is vacuum dried to obtain 901g of lithium difluorophosphate crystalline white powder solid, wherein the yield of lithium difluorophosphate is 90.3%. Moisture ≤10ppm, LiF% ≤0.02%, and purity is greater than 99.7%.

[0032] Example 3 This embodiment discloses a method for preparing lithium difluorophosphate, comprising the following steps: S1. At room temperature, 85% polyphosphoric acid (P 2 O 5 836g of sodium bicarbonate (calculated) and 3356g of acetone were added into the reactor respectively, and the reactor was sealed. After slowly heating to 68-73°C, stirring was started, and a weak reflux began in the reactor. At this time, 355g of lithium carbonate was slowly added, and the feeding time was 2.2 hours. Then, 3160g of water was added, and the temperature was controlled at 67-72°C. The stirring reaction was continued for 1.8 hours to obtain lithium dihydrogen phosphate slurry.

[0033] S2. After the stirring reaction is completed, the vacuum is turned on to reduce the pressure in the reactor to -0.1MPa, and the temperature is maintained at 65-69°C, and then the reduced pressure distillation is started, wherein the distillation time is 1.0 hour. After most of the organic solvent is evaporated, the mixture is filtered, and after removing the filter residue, the filtrate is transferred to the reactor.

[0034] S3. Add 1000 g of hydrofluoric acid with a mass fraction of 40% to the filtrate in the reactor, heat the reactor to 89-93° C., and continue to stir the reaction. The reaction time is 2.8 hours to obtain a lithium difluorophosphate solution.

[0035] S4. After the reaction is completed, the reactor is cooled to 78-82 ° C, the pressure is reduced to -0.1MPa, and vacuum distillation is started again, and the distillation time is 1.5 hours. After evaporating most of the water, 4000g of carbon tetrachloride is added and continued to stir, and the reactor is cooled to 10 ° C, and lithium difluorophosphate begins to crystallize, and the crystallization is 1.8 hours. After sufficient crystallization, it is filtered. The lithium difluorophosphate filter cake crystals obtained by filtration are pulped with a small amount of carbon tetrachloride. Subsequently, the filter cake is vacuum dried to obtain 921g of lithium difluorophosphate crystalline white powder solid, wherein the yield of lithium difluorophosphate is 88.9%. Moisture ≤10ppm, LiF% ≤0.02%, and purity is greater than 99.7%.

[0036] Example 4 This embodiment discloses a method for preparing lithium difluorophosphate, comprising the following steps: S1. At room temperature, 85% polyphosphoric acid (P 2 O 5 836g of methyl tert-butyl ether (in terms of weight) and 3500g of methyl tert-butyl ether were added to the reactor respectively, and the reactor was sealed. After slowly heating to 67-72°C, stirring was started, and a weak reflux began in the reactor. At this time, 370g of lithium carbonate was slowly added, and the feeding time was 2.5 hours. Then, 3050g of water was added, and the temperature was controlled at 67-72°C. The stirring reaction was continued for 2.0 hours to obtain lithium dihydrogen phosphate slurry.

[0037] S2. After the stirring reaction is completed, the vacuum is turned on to reduce the pressure in the reactor to -0.1MPa, and the temperature is maintained at 65-69°C, and then the reduced pressure distillation is started, wherein the distillation time is 1.3 hours. After most of the organic solvent is evaporated, the mixture is filtered, and after removing the filter residue, the filtrate is transferred to the reactor.

[0038] S3. Add 1020 g of hydrofluoric acid with a mass fraction of 40% to the filtrate in the reactor, heat the reactor to 89-93° C., and continue to stir the reaction for 3.5 hours to obtain a lithium difluorophosphate solution.

[0039] S4. After the reaction is completed, the reactor is cooled to 78-82 ° C, the pressure is reduced to -0.1MPa, and reduced pressure distillation is started again, and the distillation time is 2.0 hours. After evaporating most of the water, 3700g of dimethyl carbonate is added and continued to stir, and the reactor is cooled to 9 ° C, and lithium difluorophosphate begins to crystallize, and the crystallization is 1.5 hours. After sufficient crystallization, it is filtered. The lithium difluorophosphate filter cake crystals obtained by filtration are pulped with a small amount of dimethyl carbonate. Subsequently, the filter cake is vacuum dried to obtain 964g of lithium difluorophosphate crystalline white powder solid, wherein the yield of lithium difluorophosphate is 89.3%. Moisture ≤10ppm, LiF% ≤0.02%, purity greater than 99.7%.

[0040] Example 5 This embodiment discloses a method for preparing lithium difluorophosphate, comprising the following steps: S1. At room temperature, 85% polyphosphoric acid (P 2 O 5836g of sodium bicarbonate (calculated) and 4230g of furan were added into the reactor respectively, and the reactor was sealed. After slowly heating to 68-73°C, stirring was started, and a weak reflux began in the reactor. At this time, 380g of lithium carbonate was slowly added, and the feeding time was 2.5 hours. Then, 3000g of water was added, and the temperature was controlled at 67-73°C. The stirring reaction was continued for 2.0 hours to obtain lithium dihydrogen phosphate slurry.

[0041] S2. After the stirring reaction is completed, the vacuum is turned on to reduce the pressure in the reactor to -0.1MPa, and the temperature is maintained at 65-69°C, and then the reduced pressure distillation is started, wherein the distillation time is 1.3 hours. After most of the organic solvent is evaporated, the mixture is filtered, and after removing the filter residue, the filtrate is transferred to the reactor.

[0042] S3. Add 1100 g of hydrofluoric acid with a mass fraction of 40% to the filtrate in the reactor, heat the reactor to 89-93° C., and continue to stir the reaction. The reaction time is 3.2 hours to obtain a lithium difluorophosphate solution.

[0043] S4. After the reaction is completed, the reactor is cooled to 78-82 ° C, the pressure is reduced to -0.1MPa, and vacuum distillation is started again, and the distillation time is 1.9 hours. After evaporating most of the water, 3600g of ethyl methyl carbonate is added and continued to stir, and the reactor is cooled to 6 ° C, and lithium difluorophosphate begins to crystallize, and the crystallization is 2.0 hours. After sufficient crystallization, it is filtered. The lithium difluorophosphate filter cake crystals obtained by filtration are pulped with a small amount of ethyl methyl carbonate. Subsequently, the filter cake is vacuum dried to obtain 951g of lithium difluorophosphate crystalline white powder solid, wherein the yield of lithium difluorophosphate is 85.7%. Moisture ≤10ppm, LiF% ≤0.02%, and purity is greater than 99.7%.

[0044] Example 6 This embodiment discloses a method for preparing lithium difluorophosphate, comprising the following steps: S1. At room temperature, 85% polyphosphoric acid (P 2 O 5 836g of 1,4-dihydrogen phosphate (60g) and 4910g of tetrahydrofuran were added into the reactor respectively, and the reactor was sealed. After slowly heating to 67-71°C, stirring was started, and a weak reflux began in the reactor. At this time, 401g of lithium carbonate was slowly added, and the feeding time was 2.2 hours. Then, 3180g of water was added, and the temperature was controlled at 67-72°C. The stirring reaction was continued for 2.1 hours to obtain lithium dihydrogen phosphate slurry.

[0045] S2. After the stirring reaction is completed, the vacuum is turned on to reduce the pressure in the reactor to -0.1MPa, and the temperature is maintained at 65-69°C, and then the reduced pressure distillation is started, wherein the distillation time is 1.4 hours. After most of the organic solvent is evaporated, the mixture is filtered, and after removing the filter residue, the filtrate is transferred to the reactor.

[0046] S3. Add 1150 g of hydrofluoric acid with a mass fraction of 40% to the filtrate in the reactor, heat the reactor to 89-93° C., and continue to stir the reaction for 3.5 hours to obtain a lithium difluorophosphate solution.

[0047] S4. After the reaction is completed, the reactor is cooled to 78-82 ° C, the pressure is reduced to -0.1MPa, and vacuum distillation is started again, and the distillation time is 1.8 hours. After evaporating most of the water, 3800g of diethyl carbonate is added and continued to stir, and the reactor is cooled to 8 ° C, and lithium difluorophosphate begins to crystallize, and the crystallization is 1.5 hours. After sufficient crystallization, it is filtered. The lithium difluorophosphate filter cake crystals obtained by filtration are pulped with a small amount of diethyl carbonate. Subsequently, the filter cake is vacuum dried to obtain 1029g of lithium difluorophosphate crystalline white powder solid, wherein the yield of lithium difluorophosphate is 87.9%. Moisture ≤10ppm, LiF%≤0.02%, purity greater than 99.7%.

[0048] Example 7 This embodiment discloses a method for preparing lithium difluorophosphate, comprising the following steps: S1. At room temperature, 85% polyphosphoric acid (P 2 O 5 836g of methyl tert-butyl ether (in terms of weight) and 6020g of methyl tert-butyl ether were added to the reactor respectively, and the reactor was sealed. After slowly heating to 67-71°C, stirring was started, and a weak reflux began in the reactor. At this time, 418g of lithium carbonate was slowly added, and the feeding time was 2.5 hours. Then, 3350g of water was added, and the temperature was controlled at 67-72°C. The stirring reaction was continued for 2.1 hours to obtain lithium dihydrogen phosphate slurry.

[0049] S2. After the stirring reaction is completed, the vacuum is turned on to reduce the pressure in the reactor to -0.1MPa, and the temperature is maintained at 65-69°C, and then the reduced pressure distillation is started, wherein the distillation time is 1.5 hours. After most of the organic solvent is evaporated, the mixture is filtered, and after removing the filter residue, the filtrate is transferred to the reactor.

[0050] S3. Add 1250 g of hydrofluoric acid with a mass fraction of 40% to the filtrate in the reactor, heat the reactor to 89-93° C., and continue to stir the reaction for 3.5 hours to obtain a lithium difluorophosphate solution.

[0051] S4. After the reaction is completed, the reactor is cooled to 78-82 ° C, the pressure is reduced to -0.1MPa, and vacuum distillation is started again, and the distillation time is 2.0 hours. After evaporating most of the water, 5000g of methyl tert-butyl ether is added and continued to stir, and the reactor is cooled to 16 ° C, and lithium difluorophosphate begins to crystallize, and the crystallization is 2.0 hours. After sufficient crystallization, it is filtered. The lithium difluorophosphate filter cake crystals obtained by filtration are pulped with a small amount of methyl tert-butyl ether. Subsequently, the filter cake is vacuum dried to obtain 1039g of lithium difluorophosphate crystalline white powder solid, wherein the yield of lithium difluorophosphate is 85.2%. Moisture ≤10ppm, LiF%≤0.02%, purity greater than 99.7%.

[0052] The above-mentioned embodiments are only preferred embodiments of the present invention and cannot be used to limit the scope of protection of the present invention. Any non-substantial changes and substitutions made by technicians in this field on the basis of the present invention shall fall within the scope of protection required by the present invention.

Claims

1. A method for preparing lithium difluorophosphate, characterized in that: The following steps are involved: S1, adding polyphosphoric acid to an organic solvent and heating it under reflux to obtain a polyphosphoric acid organic solvent suspension, adding lithium carbonate to the suspension, stirring and reacting for a certain period of time, adding water, and continuing to stir and react to obtain a lithium dihydrogen phosphate slurry; S2, subjecting the solution obtained in S1 to reduced pressure distillation, and after evaporating most of the organic solvent, filtering to remove insoluble impurities; S3, adding hydrofluoric acid to the filtrate obtained in S2, stirring and reacting to obtain a lithium difluorophosphate solution; S4. The solution obtained in S3 is subjected to reduced pressure distillation. After evaporating most of the water, an organic solvent is added to stir and crystallize, and the mixture is filtered to obtain lithium difluorophosphate crystals.

2. The method for preparing lithium difluorophosphate according to claim 1, wherein: In the S1, the weight ratio of the organic solvent to the polyphosphoric acid is 3.6-7.2:1, and the mass ratio of lithium carbonate to the polyphosphoric acid is 0.4-0.5:

1.

3. The method for preparing lithium difluorophosphate according to claim 1, characterized in that: In S1, the reaction temperature is 65-79° C., the concentration of polyphosphoric acid is ≥85%, and the organic solvent is selected from at least one of acetone, acetonitrile, furan, tetrahydrofuran, and methyl tert-butyl ether.

4. The method for preparing lithium difluorophosphate according to claim 1, characterized in that: In S1, the lithium carbonate feeding time is 1.5-2.5 hours.

5. The method for preparing lithium difluorophosphate according to claim 1, characterized in that: In S1, the weight ratio of the added water to the polyphosphoric acid is 3.5-4.0:1, and the reaction time after adding water is 1.0-2.0 hours.

6. The method for preparing lithium difluorophosphate according to claim 1, characterized in that: In S2, the distillation temperature is 65-79° C., and the distillation time is 0.5-1.5 hours.

7. The method for preparing lithium difluorophosphate according to claim 1, characterized in that: In S3, the mass fraction of hydrofluoric acid is ≥40%, and the weight ratio of hydrofluoric acid to polyphosphoric acid is 1.1-1.5:

1.

8. The method for preparing lithium difluorophosphate according to claim 1, characterized in that: In S3, the reaction temperature is 80-95° C. and the reaction time is 1.5-3.5 hours.

9. The method for preparing lithium difluorophosphate according to claim 1, characterized in that: In S4, the weight ratio of the added organic solvent to the polyphosphoric acid is 4.0-6.0:1, and the organic solvent is selected from at least one of ethyl acetate, ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, acetone, methyl tert-butyl ether, and carbon tetrachloride.

10. The method for preparing lithium difluorophosphate according to claim 1, characterized in that: In the S4, the distillation temperature is 75-88°C, the distillation time is 1.0-2.0 hours, the crystallization temperature is 5-16°C, and the crystallization time is 1.5-2.0 hours.