A method for preparing lithium difluorophosphate

Abstract

The patent application discloses a preparation method of lithium difluorophosphate, comprising the following steps: S1, polyphosphoric acid is added into an organic solvent for heating and reflux to obtain a polyphosphoric acid organic solvent suspension, lithium carbonate is added into the suspension, and after stirring for a certain time, water is added, and the stirring reaction is continued to obtain lithium dihydrogen phosphate slurry; S2, the solution obtained in S1 is subjected to vacuum distillation, most of the organic solvent is evaporated, and insoluble impurities are removed by filtration; S3, hydrofluoric acid is added into the filtrate obtained in S2, and the stirring reaction is carried out to obtain a lithium difluorophosphate solution; S4, the solution obtained in S3 is subjected to vacuum distillation, after most of the water is evaporated, an organic solvent is added for stirring and crystallization, and filtration is carried out to obtain lithium difluorophosphate crystals. According to the method, the process conditions of the reaction are optimized, high-purity lithium difluorophosphate products can be prepared from lithium dihydrogen phosphate and hydrofluoric acid at a high conversion rate, and the problems of difficult product extraction, low yield and low product purity in the preparation of lithium difluorophosphate can be solved.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and specifically to a method for preparing lithium difluorophosphate. Background Technology

[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 sophistication of electric vehicle technology has gradually released market demand for large lithium batteries, and lithium batteries are expected to maintain a growth trend in the coming years. Most lithium batteries are rechargeable batteries, and their poor stability, low-temperature performance, and poor cycle characteristics are the main technical challenges hindering their rapid development. Research shows that using lithium difluorophosphate as an electrolyte can effectively improve these shortcomings of lithium batteries.

[0003] Currently, there are many methods for preparing lithium difluorophosphate, such as: 1) methods using Li₂CO₃ and LiPF₆ or P₂O₅ as raw materials. However, the lithium difluorophosphate prepared by this method produces lithium fluoride impurities that are difficult to separate, making product purification difficult. 2) methods using P₄O₂... 10 The preparation methods include: 1) Solid-solid reaction of LiF to synthesize lithium difluorophosphate, but this method is difficult to control during preparation, produces many by-products, and has a low conversion recovery rate. 2) The preparation method using Li3PO4 and PF5 as raw materials to synthesize lithium difluorophosphate requires sophisticated reaction equipment and produces many by-products that are difficult to separate. 3) The preparation method using LiPF6 hydrolysis to prepare lithium difluorophosphate is difficult to control, producing impurities such as Li2PO3F, LiF, and LiPO2F2 that affect battery performance and are difficult to purify, making it hard to obtain high-purity products. 4) The preparation method using organotin fluorides to replace lithium dichlorophosphate in a substitution reaction, but the raw material trimethyltin fluoride is difficult to obtain. 5) The preparation method using lithium dihydrogen phosphate and hydrofluoric acid under catalytic conditions to obtain lithium difluorophosphate. However, this method requires high-purity lithium dihydrogen phosphate beforehand, and after the reaction, the catalyst and lithium difluorophosphate co-crystallize, requiring further purification and separation. The operation is complex and costly, making this process difficult to scale up.

[0004] In summary, existing methods for preparing lithium difluorophosphate have defects. Therefore, this invention proposes a method for preparing lithium difluorophosphate. Summary of the Invention

[0005] In order to overcome the shortcomings of the prior art, the purpose of this invention is to provide a method for preparing lithium difluorophosphate, which addresses the problems of difficult product extraction, low conversion and recovery rate, and numerous and difficult-to-remove impurities that result in low product purity in the current preparation of lithium difluorophosphate.

[0006] The technical solution adopted in this invention is as follows: A method for preparing lithium difluorophosphate includes the following steps: S1. Polyphosphoric acid is added to an organic solvent and heated under reflux to obtain a polyphosphoric acid organic solvent suspension. Lithium carbonate is added to the suspension, and after stirring for a certain time, water is added and the stirring is continued to obtain lithium dihydrogen phosphate slurry. S2. The solution obtained in S1 is subjected to vacuum distillation. After evaporating most of the organic solvent, the solution is filtered to remove insoluble impurities. S3. Add hydrofluoric acid to the filtrate obtained in S2 and stir to react to obtain lithium difluorophosphate solution; S4. The solution obtained in S3 is subjected to vacuum distillation. After most of the water is evaporated, an organic solvent is added, stirred to crystallize, and filtered to obtain lithium difluorophosphate crystals.

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

[0008] In a preferred embodiment of the present invention, in step 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 provides a suitable reaction environment for the present invention.

[0009] In a preferred embodiment of the present invention, the lithium carbonate feeding time in step S1 is 1.5-2.5 hours. The present invention selects a sufficient feeding time, which, with a fixed total feeding amount, reduces the feeding amount per minute, thereby reducing the exothermic phenomenon in the reaction process and allowing for better control of the reaction temperature.

[0010] In a preferred embodiment of the present invention, in step S1, the weight ratio of water 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 sufficient water and ensures adequate reaction time, which helps the materials to react fully and improves the recovery rate.

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

[0012] In a preferred embodiment of the present invention, in step 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] In a preferred embodiment of the present invention, in step 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 and ensures sufficient reaction time, which helps the materials to react fully and improves the recovery rate.

[0014] In a preferred embodiment of the present invention, in step 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, methyl ethyl carbonate, dimethyl carbonate, diethyl carbonate, acetone, methyl tert-butyl ether, and carbon tetrachloride.

[0015] In a preferred embodiment of the present invention, in step S4, the distillation temperature is 75-88℃, the distillation time is 1.0-2.0 hours, the crystallization temperature is 5-16℃, and the crystallization time is 1.5-2.0 hours. The present invention selects a suitable distillation temperature and ensures sufficient distillation time, which helps to reduce the water content of the product and improve the recovery rate. Simultaneously, a suitable crystallization temperature is selected to ensure sufficient crystallization time, which helps the product crystallize completely and further improves the recovery rate.

[0016] Working principle of the invention: The reaction equation for this invention is: 5Li₂CO₃ + 2H₂O(HPO₃)₅ + 20HF → 10LiPO₂F₂ + 5CO₂ + 17H₂O.

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

[0018] The second step of this invention involves evaporating most of the organic solvent by vacuum distillation and then filtering to remove insoluble impurities.

[0019] In the third step of this invention, hydrofluoric acid is added in the presence of a small amount of organic solvent, and dihydrogen phosphate ions are gradually converted into difluorophosphoric acid, which in turn generates lithium difluorophosphate.

[0020] In the final step of this invention, after further vacuum distillation to remove most of the water, an organic solvent is added. Taking advantage of the low solubility of difluorophosphate in the organic solvent, it crystallizes and precipitates. Simultaneously, due to the acidity of the aqueous phase, the byproduct inorganic salt is more readily soluble in the small amount of water remaining in the system, thus achieving the separation of the byproduct from lithium difluorophosphate. After filtration, high-purity lithium difluorophosphate crystals are obtained.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows: The raw materials of this invention are readily available, the operation is convenient, and by optimizing the reaction process conditions, high-purity lithium difluorophosphate can be obtained from lithium dihydrogen phosphate and hydrofluoric acid with a high conversion rate. This method not only offers convenient operation and low requirements for equipment and environmental protection, resulting in less environmental stress, but also features mild reaction conditions, controlled side reactions, easy product separation and extraction, and high product conversion and purity. Compared with existing preparation methods, this invention solves the problems of difficult-to-obtain raw materials, difficult product extraction, low yield, difficult removal of by-products, and low product purity in the preparation of lithium difluorophosphate. Detailed Implementation

[0022] Typical embodiments embodying the features and advantages of the present invention will be specifically described in the following description. It should be understood that the present invention can have various variations in different embodiments without departing from the scope of the present invention, and the description herein is for illustrative purposes only and not intended to limit the present invention.

[0023] The reaction equation used in this invention is: 5Li2CO3+2H2O(HPO3)5+20HF—10LiPO2F2+5CO2+17H2O.

[0024] Example 1 This embodiment discloses a method for preparing lithium difluorophosphate, including the following steps: S1. At room temperature, 836g of 85% polyphosphoric acid (calculated as P2O5) and 3010g of acetonitrile were added to a reactor. The reactor was sealed, and the temperature was slowly raised to 67-71℃. Stirring was then started, and a slight reflux began in the reactor. At this point, 335g of lithium carbonate was slowly added over 1.5 hours. Then, 2950g of water was added, and the temperature was controlled at 67-72℃. The reaction was continued with stirring for 1.0 hour to obtain lithium dihydrogen phosphate slurry.

[0025] S2. After the stirring reaction is complete, turn on the vacuum to reduce the pressure inside the reactor to -0.1 MPa and maintain the temperature at 65-69℃. Then, begin vacuum distillation for 0.5 hours. After most of the organic solvent has evaporated, filter the solution. After removing the filter residue, transfer the filtrate back into the reactor.

[0026] S3. Add 920g of hydrofluoric acid (40% by mass) to the filtrate in the reactor, and heat the reactor to 89-93℃. Continue stirring the reaction for 1.5 hours to obtain a lithium difluorophosphate solution.

[0027] S4. After the reaction is complete, the reactor is cooled to 78-82℃ and the pressure reduced to -0.1MPa. A second vacuum distillation is then initiated for 1.0 hour. After most of the water has evaporated, 3350g of acetone is added and stirring continues. The reactor is then cooled to 5℃, and lithium difluorophosphate begins to crystallize, taking 1.5 hours. After sufficient crystallization, the mixture is filtered. The resulting lithium difluorophosphate filter cake crystals are then slurried with a small amount of acetone. The filter cake is then vacuum dried to obtain 839g of crystalline white lithium difluorophosphate powder, with a yield of 85.8%. Moisture content ≤10ppm, LiF% ≤0.02%, and purity >99.7%.

[0028] Example 2 This embodiment discloses a method for preparing lithium difluorophosphate, including the following steps: S1. At room temperature, 836g of 85% polyphosphoric acid (calculated as P2O5) and 3200g of acetonitrile were added to a reactor. The reactor was sealed, and the temperature was slowly raised to 66-70℃. Stirring was then started, and a slight reflux began in the reactor. At this point, 342g of lithium carbonate was slowly added over a period of 2.2 hours. Then, 3180g of water was added, and the temperature was controlled at 67-71℃. The reaction was continued with stirring for another 2.0 hours to obtain lithium dihydrogen phosphate slurry.

[0029] S2. After the stirring reaction is complete, turn on the vacuum to reduce the pressure inside the reactor to -0.1 MPa and maintain the temperature at 65-69℃. Then, begin vacuum distillation for 0.8 hours. After most of the organic solvent has evaporated, filter the solution and remove the filter residue. Transfer the filtrate back into the reactor.

[0030] S3. Add 960g of 40% hydrofluoric acid to the filtrate in the reactor, and heat the reactor to 89-93℃. Continue stirring the reaction for 3.1 hours to obtain a lithium difluorophosphate solution.

[0031] S4. After the reaction is complete, the reactor is cooled to 78-82℃ and the pressure reduced to -0.1MPa. A second vacuum distillation is then initiated for 1.6 hours. After most of the water has evaporated, 3450g of ethyl acetate is added and stirring continues. The reactor is then cooled to 6℃, and lithium difluorophosphate begins to crystallize, taking 1.5 hours. After complete crystallization, the mixture is filtered. The resulting lithium difluorophosphate filter cake crystals are then slurried with a small amount of ethyl acetate. The filter cake is then vacuum dried to obtain 901g of a white crystalline solid lithium difluorophosphate powder, with a yield of 90.3%. Moisture content is ≤10ppm, LiF% ≤0.02%, and purity is greater than 99.7%.

[0032] Example 3 This embodiment discloses a method for preparing lithium difluorophosphate, including the following steps: S1. At room temperature, 836g of 85% polyphosphoric acid (calculated as P2O5) and 3356g of acetone were added to the reactor, which was then sealed. The temperature was slowly raised to 68-73℃, and stirring was started. A slight reflux began in the reactor. At this point, 355g of lithium carbonate was slowly added over a period of 2.2 hours. Then, 3160g of water was added, and the temperature was controlled at 67-72℃. The reaction was continued with stirring for 1.8 hours to obtain lithium dihydrogen phosphate slurry.

[0033] S2. After the stirring reaction is complete, turn on the vacuum to reduce the pressure inside the reactor to -0.1 MPa and maintain the temperature at 65-69℃. Then, begin vacuum distillation for 1 hour. After most of the organic solvent has evaporated, filter the solution and remove the filter residue. Transfer the filtrate back into the reactor.

[0034] S3. Add 1000g of 40% hydrofluoric acid to the filtrate in the reactor, and heat the reactor to 89-93℃. Continue stirring the reaction for 2.8 hours to obtain a lithium difluorophosphate solution.

[0035] S4. After the reaction is complete, the reactor is cooled to 78-82℃ and the pressure reduced to -0.1MPa. A second vacuum distillation is then initiated for 1.5 hours. After most of the water has evaporated, 4000g of carbon tetrachloride is added and stirring continues. The reactor is then cooled to 10℃, and lithium difluorophosphate begins to crystallize, taking 1.8 hours. After sufficient crystallization, the mixture is filtered. The resulting lithium difluorophosphate filter cake crystals are then slurried with a small amount of carbon tetrachloride. The filter cake is then vacuum dried to obtain 921g of a white crystalline solid lithium difluorophosphate powder, with a yield of 88.9%. Moisture content is ≤10ppm, LiF% ≤0.02%, and purity is greater than 99.7%.

[0036] Example 4 This embodiment discloses a method for preparing lithium difluorophosphate, including the following steps: S1. At room temperature, 836g of 85% polyphosphoric acid (calculated as P2O5) and 3500g of methyl tert-butyl ether were added to a reactor. The reactor was sealed, and the temperature was slowly raised to 67-72℃. Stirring was then started, and a slight reflux began in the reactor. At this point, 370g of lithium carbonate was slowly added over a period of 2.5 hours. Then, 3050g of water was added, and the temperature was controlled at 67-72℃. The reaction was continued with stirring for another 2.0 hours to obtain lithium dihydrogen phosphate slurry.

[0037] S2. After the stirring reaction is complete, turn on the vacuum to reduce the pressure inside the reactor to -0.1 MPa and maintain the temperature at 65-69℃. Then, begin vacuum distillation for 1.3 hours. After most of the organic solvent has evaporated, filter the solution and remove the filter residue. Transfer the filtrate back into the reactor.

[0038] S3. Add 1020g of hydrofluoric acid (40% by mass) to the filtrate in the reactor, and heat the reactor to 89-93℃. Continue stirring the reaction for 3.5 hours to obtain a lithium difluorophosphate solution.

[0039] S4. After the reaction is complete, the reactor is cooled to 78-82℃ and the pressure reduced to -0.1MPa. A second vacuum distillation is then initiated for 2.0 hours. After most of the water has evaporated, 3700g of dimethyl carbonate is added and stirring continues. The reactor is then cooled to 9℃, and lithium difluorophosphate begins to crystallize, taking 1.5 hours. After sufficient crystallization, the mixture is filtered. The resulting lithium difluorophosphate filter cake crystals are then slurried with a small amount of dimethyl carbonate. The filter cake is subsequently vacuum dried to obtain 964g of a white crystalline solid lithium difluorophosphate powder, with a yield of 89.3%. Moisture content is ≤10ppm, LiF% ≤0.02%, and purity is greater than 99.7%.

[0040] Example 5 This embodiment discloses a method for preparing lithium difluorophosphate, including the following steps: S1. At room temperature, 836g of 85% polyphosphoric acid (calculated as P2O5) and 4230g of furan were added to the reactor, which was then sealed. The temperature was slowly raised to 68-73℃, and stirring was started. A slight reflux began in the reactor. At this point, 380g of lithium carbonate was slowly added over a period of 2.5 hours. Then, 3000g of water was added, and the temperature was controlled at 67-73℃. The reaction was continued with stirring for another 2.0 hours to obtain lithium dihydrogen phosphate slurry.

[0041] S2. After the stirring reaction is complete, turn on the vacuum to reduce the pressure inside the reactor to -0.1 MPa and maintain the temperature at 65-69℃. Then, begin vacuum distillation for 1.3 hours. After most of the organic solvent has evaporated, filter the solution and remove the filter residue. Transfer the filtrate back into the reactor.

[0042] S3. Add 1100g of hydrofluoric acid (40% by mass) to the filtrate in the reactor, and heat the reactor to 89-93℃. Continue stirring the reaction for 3.2 hours to obtain a lithium difluorophosphate solution.

[0043] S4. After the reaction is complete, the reactor is cooled to 78-82℃ and the pressure reduced to -0.1MPa. A second vacuum distillation is then initiated for 1.9 hours. After most of the water has evaporated, 3600g of methyl ethyl carbonate is added and stirring continues. The reactor is then cooled to 6℃, and lithium difluorophosphate begins to crystallize, taking 2.0 hours. After sufficient crystallization, the mixture is filtered. The resulting lithium difluorophosphate filter cake crystals are then slurried with a small amount of methyl ethyl carbonate. The filter cake is then vacuum dried to obtain 951g of a white crystalline solid lithium difluorophosphate powder, with a yield of 85.7%. Moisture content is ≤10ppm, LiF% ≤0.02%, and purity is greater than 99.7%.

[0044] Example 6 This embodiment discloses a method for preparing lithium difluorophosphate, including the following steps: S1. At room temperature, 836g of 85% polyphosphoric acid (calculated as P2O5) and 4910g of tetrahydrofuran were added to the reactor, which was then sealed. The temperature was slowly raised to 67-71℃, and stirring was started. A slight reflux began in the reactor. At this point, 401g of lithium carbonate was slowly added over a period of 2.2 hours. Then, 3180g of water was added, and the temperature was controlled at 67-72℃. The reaction was continued with stirring for 2.1 hours to obtain lithium dihydrogen phosphate slurry.

[0045] S2. After the stirring reaction is complete, turn on the vacuum to reduce the pressure inside the reactor to -0.1 MPa and maintain the temperature at 65-69℃. Then, begin vacuum distillation for 1.4 hours. After most of the organic solvent has evaporated, filter the solution and remove the filter residue. Transfer the filtrate back into the reactor.

[0046] S3. Add 1150g of hydrofluoric acid (40% by mass) to the filtrate in the reactor, and heat the reactor to 89-93℃. Continue stirring the reaction for 3.5 hours to obtain a lithium difluorophosphate solution.

[0047] S4. After the reaction is complete, the reactor is cooled to 78-82℃ and the pressure reduced to -0.1MPa. A second vacuum distillation is then initiated for 1.8 hours. After most of the water has evaporated, 3800g of diethyl carbonate is added and stirring continues. The reactor is then cooled to 8℃, and lithium difluorophosphate begins to crystallize, taking 1.5 hours. After sufficient crystallization, the mixture is filtered. The resulting lithium difluorophosphate filter cake crystals are then slurried with a small amount of diethyl carbonate. The filter cake is subsequently vacuum dried to obtain 1029g of crystalline white lithium difluorophosphate powder, with a lithium difluorophosphate yield of 87.9%. Moisture content is ≤10ppm, LiF% ≤0.02%, and purity is greater than 99.7%.

[0048] Example 7 This embodiment discloses a method for preparing lithium difluorophosphate, including the following steps: S1. At room temperature, 836g of 85% polyphosphoric acid (calculated as P2O5) and 6020g of methyl tert-butyl ether were added to a reactor. The reactor was sealed, and the temperature was slowly raised to 67-71℃. Stirring was then started, and a slight reflux began in the reactor. At this point, 418g of lithium carbonate was slowly added over a period of 2.5 hours. Then, 3350g of water was added, and the temperature was controlled at 67-72℃. The reaction was continued with stirring for 2.1 hours to obtain lithium dihydrogen phosphate slurry.

[0049] S2. After the stirring reaction is complete, turn on the vacuum to reduce the pressure inside the reactor to -0.1 MPa and maintain the temperature at 65-69℃. Then, begin vacuum distillation for 1.5 hours. After most of the organic solvent has evaporated, filter the solution and remove the filter residue. Transfer the filtrate back into the reactor.

[0050] S3. Add 1250g of hydrofluoric acid (40% by mass) to the filtrate in the reactor, and heat the reactor to 89-93℃. Continue stirring the reaction for 3.5 hours to obtain a lithium difluorophosphate solution.

[0051] S4. After the reaction is complete, the reactor is cooled to 78-82℃ and the pressure reduced to -0.1MPa. A second vacuum distillation is then initiated for 2.0 hours. After most of the water has evaporated, 5000g of methyl tert-butyl ether is added and stirring continues. The reactor is then cooled to 16℃, and lithium difluorophosphate begins to crystallize, taking 2.0 hours. After sufficient crystallization, the mixture is filtered. The resulting lithium difluorophosphate filter cake crystals are then slurried with a small amount of methyl tert-butyl ether. The filter cake is then vacuum dried to obtain 1039g of crystalline white lithium difluorophosphate powder, with a lithium difluorophosphate yield of 85.2%. Moisture content is ≤10ppm, LiF% ≤0.02%, and purity is greater than 99.7%.

[0052] The above embodiments are merely preferred embodiments of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-substantial changes and substitutions made by those skilled in the art based on the present invention shall fall within the scope of protection claimed by the present invention.

Claims

1. A method for preparing lithium difluorophosphate, characterized in that: Includes the following steps: S1. Polyphosphoric acid is added to an organic solvent and heated under reflux to obtain a polyphosphoric acid organic solvent suspension. Lithium carbonate is added to the suspension, and after stirring for a certain time, water is added and the stirring is continued to obtain lithium dihydrogen phosphate slurry. S2. The solution obtained in S1 is subjected to vacuum distillation. After evaporating most of the organic solvent, the solution is filtered to remove insoluble impurities. S3. Add hydrofluoric acid to the filtrate obtained in S2 and stir to react to obtain lithium difluorophosphate solution; S4. The solution obtained in S3 is subjected to vacuum distillation. After most of the water is evaporated, an organic solvent is added, stirred and crystallized, and filtered to obtain lithium difluorophosphate crystals. In S1, the weight ratio of organic solvent to polyphosphoric acid is 3.6-7.2:1, and the mass ratio of lithium carbonate to polyphosphoric acid is 0.4-0.5:

1. In S1, the reaction temperature is 65-79℃, the mass fraction of polyphosphoric acid (based on P2O5) is ≥85%, and the organic solvent is selected from at least one of acetone, acetonitrile, furan, tetrahydrofuran, and methyl tert-butyl ether.

2. 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.

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

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

5. 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.

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

7. The method for preparing lithium difluorophosphate according to claim 1, characterized in that: In step 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, methyl ethyl carbonate, dimethyl carbonate, diethyl carbonate, acetone, methyl tert-butyl ether, and carbon tetrachloride.

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