Preparation method of sodium difluorophosphate

Abstract

The invention relates to a preparation method of sodium difluorophosphate, which comprises the following steps: S1, reacting sodium hexafluorophosphate with cyclic siloxane in an aprotic organic solvent at 50-80 DEG C, and filtering after the reaction to obtain a sodium difluorophosphate crude product and mother liquor; s2, the sodium difluorophosphate crude product is subjected to ultrasonic washing with a washing solvent, a sodium difluorophosphate wet solid is obtained, and the washing solvent is a mixed solvent of tetrahydrofuran and any one of dimethyl carbonate, ethyl acetate and ethylene glycol dimethyl ether; and S3, gradually heating the sodium difluorophosphate wet solid, carrying out vacuum drying, and introducing inert gas at intervals in the drying process for replacement. The method is wide in reaction raw material source, low in price, mild in reaction condition, convenient to operate and easy for industrial production; and the prepared sodium difluorophosphate has high purity and high yield, and can meet the relevant requirements of battery application.

Description

Technical Field

[0001] This invention relates to the field of sodium difluorophosphate technology, and more specifically to a method for preparing sodium difluorophosphate. Background Technology

[0002] Sodium difluorophosphate (NaPO2F2) is a key functional additive in sodium-ion battery electrolytes and can also be used as a main salt. It effectively participates in the formation of the solid electrolyte interphase (SEI) on the electrode surface, generating a stable SEI film mainly composed of inorganic components such as NaF. This significantly inhibits dendrite growth and side reactions in the sodium metal anode, thereby improving the battery's cycle life and safety. Furthermore, its unique chemical structure can optimize the solvation structure of the electrolyte and enhance ionic conductivity. With the rapid development of sodium-ion battery technology, the demand for the industrial production of high-purity sodium difluorophosphate is becoming increasingly urgent.

[0003] Currently, the mainstream synthesis methods for sodium difluorophosphate include:

[0004] Chinese patent CN117228652A discloses a method for reacting sodium fluoride, phosphorus pentoxide, and excess HF under supercritical conditions (10-20 MPa). This process places extremely high demands on the pressure resistance and corrosion resistance of the reaction equipment, incurs huge investment and operating costs, poses significant safety hazards, and is difficult to scale up for industrial production.

[0005] Chinese patent CN116621150A discloses a method that uses phosphorus oxychloride to react with water to produce dichlorophosphoric acid and hydrogen chloride, and then reacts dichlorophosphoric acid with sodium hydrofluoric acid to produce sodium difluorophosphate and hydrogen chloride. However, in this reaction process, the reaction of phosphorus oxychloride with water inevitably produces byproducts such as monochlorophosphoric acid and phosphoric acid, and unreacted phosphorus oxychloride can affect the fluorination reaction, thus affecting the yield of the product.

[0006] Chinese patent CN116715215A discloses a method for preparing sodium difluorophosphate by reacting phosphorus pentachloride, oxalic acid, and sodium hydrofluoric acid in an alcohol-ether solvent. While this method utilizes trace amounts of water as a catalyst to improve the reaction yield and purity, it is highly sensitive to the water content of the solvent; both excessively high and low water content significantly impact the reaction yield. Furthermore, the type of solvent and the ratio of reactants also significantly affect the reaction yield and product purity, making the reaction conditions difficult to control and posing challenges for industrial-scale application.

[0007] Chinese patent CN116692822A provides a relatively mild route for preparing the product by reacting sodium hexafluorophosphate with methylsiloxane in an ester solvent. This method avoids the use of HF and phosphorus pentafluoride, but its main drawback is insufficient product purity (94%–98%), failing to meet the requirements for battery-grade materials. Residual impurities in the product can trigger irreversible side reactions within the battery, severely impairing its overall performance and safety reliability.

[0008] In summary, existing synthetic methods generally suffer from problems such as demanding equipment requirements, difficulty in process control, and insufficient product purity or yield. Therefore, developing a method for preparing high-purity and high-yield battery-grade sodium difluorophosphate with mild reaction conditions suitable for industrial production has become an important research direction in this field. Summary of the Invention

[0009] The purpose of this invention is to provide a method for preparing high-purity and high-yield sodium difluorophosphate with mild reaction conditions, suitable for industrial production.

[0010] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0011] This invention provides a method for preparing sodium difluorophosphate, which includes the following steps:

[0012] S1. Sodium hexafluorophosphate is reacted with cyclic siloxane in an aprotic organic solvent at 50℃~80℃. After the reaction is completed, the mixture is filtered to obtain crude sodium difluorophosphate and mother liquor.

[0013] S2. The crude sodium difluorophosphate is ultrasonically washed with a washing solvent to obtain wet sodium difluorophosphate solid. The washing solvent is any one of dimethyl carbonate, ethyl acetate, ethylene glycol dimethyl ether and a mixture of tetrahydrofuran.

[0014] S3. The wet solid sodium difluorophosphate is gradually heated and vacuum dried, and inert gas is introduced intermittently during the drying process for replacement.

[0015] The reaction equation of this invention is as follows:

[0016] nNaPF6+2[(CH3)2SiO] n →nNaPF2O2+2n(CH3)2SiF2,

[0017] Among them, [(CH3)] 2 SiO] n The cyclic siloxane is defined as n, where n is a positive integer greater than 2, preferably a positive integer between 3 and 6. Specifically, the cyclic siloxane is selected from one or more of hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, and dodecylcyclohexasiloxane.

[0018] This invention prepares sodium difluorophosphate by reacting cyclic siloxanes with sodium hexafluorophosphate. Cyclic siloxanes possess high thermodynamic stability, and their ring strain facilitates ring-opening nucleophilic substitution reactions with sodium hexafluorophosphate. This reaction exhibits good selectivity and effectively suppresses side reactions; simultaneously, the generated byproducts are low-boiling-point difluorosiloxanes, which are easily removed from the reaction system, promoting the forward reaction. However, excessively low reaction temperatures prevent the cyclic siloxanes from ring-opening, halting the reaction; excessively high reaction temperatures lead to ring-opening polymerization of the cyclic siloxanes, affecting product yield and purity. As a further preferred method, the reaction temperature is 60℃~70℃.

[0019] In the purification process, it is difficult to obtain high-purity sodium hexafluorophosphate using conventional washing solvents (such as ethyl acetate). The applicant unexpectedly discovered that using the mixed solvent of this invention combined with ultrasonic washing significantly enhances the dissolution and removal of impurities (such as residual siloxanes and fluorides). Furthermore, by using stepped heating vacuum drying with intermittent introduction of inert gas, the problem of localized overheating and decomposition caused by rapid solvent removal can be effectively alleviated, preventing HF gas generated during drying from re-dissolving into the product, thereby reducing the HF content in the finished product and ensuring product purity.

[0020] In some embodiments, the mass ratio of any one of dimethyl carbonate, ethyl acetate, and ethylene glycol dimethyl ether to tetrahydrofuran in the washing solvent is (0.5–4):1, more preferably (0.5–2):1. A mass ratio that is too high or too low will adversely affect the washing effect; therefore, the above range is preferred.

[0021] In some embodiments, the water content of the washing solvent is controlled to be below 500 ppm, such as below 500 ppm, below 400 ppm, below 300 ppm, below 200 ppm, below 100 ppm, or below 50 ppm. The use of a low-moisture washing solvent can effectively suppress the formation of hydrofluoric acid or phosphoric acid due to product decomposition. Besides controlling the water content of the washing solvent, the water content of other reagents used in the reaction process (such as sodium hexafluorophosphate, cyclic siloxanes, and aprotic organic solvents) is preferably below 500 ppm. The water content of these reagents can be achieved by pretreatment. Specific pretreatment methods can refer to conventional techniques in the art, such as molecular sieve adsorption, heating for dehydration, or azeotropic distillation.

[0022] In some embodiments, the ultrasonic washing is performed two or more times, more preferably two to four times, with the washing solution filtered through a microfiltration membrane after each wash. Ultrasonic-assisted washing significantly improves washing efficiency; furthermore, its cavitation effect effectively desorbs HF and other impurities adsorbed on the surface of solid particles. Filtration after washing prevents these dissolved impurities from re-adsorbing onto the product surface, thereby ensuring product purity.

[0023] In some embodiments, the pore size of the microfiltration membrane is 0.15 μm to 0.3 μm.

[0024] In some embodiments, fresh washing solvent is used for each wash. In this invention, "fresh" means washing solvent that has not been used or reconstituted.

[0025] In some embodiments, the minimum temperature of the vacuum drying is not lower than 65°C and the maximum temperature is not higher than 105°C.

[0026] In some specific embodiments, the gradually increasing temperature vacuum drying includes drying at 65℃~75℃ for 1.5h~2.5h, drying at 85~94℃ for 3h~5h, and finally drying at 95℃~105℃ for 1.5h~2.5h.

[0027] In some embodiments, the mother liquor obtained in step S1 is allowed to stand at 0°C to 5°C, filtered, and the resulting filtrate is used in the next batch of reaction to replace the aprotic organic solvent in step S1. Standing at a lower temperature allows some trace impurities to crystallize out, reducing the accumulation of impurities during recycling and lowering the HF and impurity content of the product from the source.

[0028] In some embodiments, the molar ratio of sodium hexafluorophosphate to cyclic siloxane is 1:(0.2-0.8), such as 1:0.2, 1:0.3, 1:0.4, 1:0.5, 1:0.6, 1:0.7, 1:0.8.

[0029] In some embodiments, the aprotic organic solvent is one or more of dimethyl carbonate, dimethylformamide, diethyl carbonate, acetonitrile, ethyl acetate, and methyl ethyl carbonate.

[0030] In some embodiments, the mass ratio of sodium hexafluorophosphate to the aprotic organic solvent is 1:(3-6), such as 1:3, 1:4, 1:5, 1:6.

[0031] In some implementations, an inert gas is introduced for replacement every 0.5 to 1.5 hours.

[0032] In some embodiments, the inert gas is nitrogen. In other embodiments, the inert gas may be other inert gases conventional in the art.

[0033] Due to the application of the above-mentioned technical solution, the present invention has the following advantages compared with the prior art:

[0034] The reactants of this invention are widely available and inexpensive, the reaction conditions are mild and the operation is convenient, making it easy to industrialize; the sodium difluorophosphate produced has high purity and high yield, which can meet the relevant requirements for battery applications. Attached Figure Description

[0035] Figure 1 The fluorine NMR spectrum of sodium difluorophosphate prepared in Example 2;

[0036] Figure 2 The image shows the phosphorus nuclear magnetic resonance spectrum of sodium difluorophosphate prepared in Example 2. Detailed Implementation

[0037] The present invention will be further described below with reference to embodiments. However, the present invention is not limited to the following embodiments. The implementation conditions used in the embodiments can be further adjusted according to different requirements of specific applications, and the implementation conditions not specified are conventional conditions in the industry. The technical features involved in the various embodiments of the present invention can be combined with each other as long as they do not conflict with each other.

[0038] Unless otherwise specified, the raw materials used in the following examples and comparative examples are all commercially available products, or can be prepared by referring to existing preparation methods.

[0039] Unless otherwise specified, the "ultrasonic washing of crude sodium difluorophosphate with a washing solvent" includes placing the crude sodium difluorophosphate and the washing solvent into a washing container, ensuring the crude product is completely submerged in the washing solvent. Then, the washing container is placed in an ultrasonic cleaning tank, and the ultrasonic process is initiated for 10–15 minutes. The ultrasonic cleaning tank is a commercially available ultrasonic cleaning tank commonly used in this field.

[0040] Example 1: This example provides a method for preparing sodium difluorophosphate, including the following steps:

[0041] (1) All raw materials are pretreated before the reaction to ensure that the moisture content of all raw materials is below 500 ppm.

[0042] (2) Under a nitrogen-protected atmosphere, 2 mol of sodium hexafluorophosphate was dissolved in 1679 g of dimethylformamide and added to a reaction vessel. Then, 1.4 mol of hexamethylcyclotrisiloxane was added dropwise to the reaction vessel over 2 hours. The reaction temperature was controlled at 60°C and refluxed. After no gas was generated, the reaction was continued at this temperature for another 2 hours. The reaction solution was then cooled to room temperature (25°C) and filtered to obtain crude sodium hexafluorophosphate and mother liquor.

[0043] (3) Ethylene glycol dimethyl ether and tetrahydrofuran were mixed at a mass ratio of 2:1 to obtain a washing solvent. The crude sodium difluorophosphate was ultrasonically washed three times with the washing solvent. After each washing, the product was filtered through a 0.22 μm microporous membrane and replaced with a new washing solvent for ultrasonic washing again until the washing was completed, thus obtaining wet solid sodium difluorophosphate.

[0044] (4) Place the wet solid in a vacuum drying oven and dry it by step heating. First, dry it in a vacuum at 70°C for 2 hours, then heat it to 90°C for 4 hours, and finally dry it at 100°C for 2 hours. Nitrogen gas is replaced every hour during the drying process to obtain the final product sodium difluorophosphate.

[0045] Ion chromatography analysis showed a product purity of 99.9% and a yield of 93%; ICP analysis revealed: Fe: 1.9 ppm, Ca: 1.2 ppm, Pb: 0.8 ppm; potentiometric titration showed: Cl... - Not detected; determined by Karl Fischer titration: moisture: 5.3 ppm; determined by acid-base titration: HF: 104.4 ppm.

[0046] This embodiment also provides a method for treating the mother liquor, including: subjecting the mother liquor to low-temperature settling treatment, specifically setting it at 0℃~5℃ for 12 hours, and then filtering to obtain the reaction mother liquor. Testing showed that the sodium hexafluorophosphate content in the reaction mother liquor was 0.06%, and the cyclic siloxane content was 0.1%, making it suitable for continued use.

[0047] Example 2: This example provides a method for preparing sodium difluorophosphate, including the following steps:

[0048] (1) All raw materials are pretreated before the reaction to ensure that the moisture content of all raw materials is below 500 ppm.

[0049] (2) Under a nitrogen-protected atmosphere, 2 mol of sodium hexafluorophosphate was dissolved in 1485 g of the mother liquor from Example 1, followed by the addition of 200 g of dimethylformamide. Then, 1.4 mol of hexamethylcyclotrisiloxane was added dropwise to the reaction vessel over 2 hours. The reaction temperature was controlled at 68°C and refluxed. After no more gas was generated, the reaction was maintained at this temperature for another 2 hours. The reaction solution was then cooled to room temperature (25°C) and filtered to obtain the mother liquor and crude sodium hexafluorophosphate. The mother liquor could be reused after being treated in the same manner as in Example 1.

[0050] (3) Ethylene glycol dimethyl ether and tetrahydrofuran were mixed at a mass ratio of 1:2 to obtain a washing solvent. The crude sodium difluorophosphate was ultrasonically washed three times with the washing solvent. After each washing, the product was filtered through a 0.22 μm microporous membrane and replaced with a new washing solvent for ultrasonic washing again until the washing was completed, thus obtaining wet solid sodium difluorophosphate.

[0051] (4) The wet solid sodium difluorophosphate was dried in the same way as in Example 1.

[0052] Ion chromatography analysis showed a product purity of 99.9% and a yield of 91%; ICP analysis revealed: K: 1.0 ppm, Fe: 1.4 ppm, Ca: 0.8 ppm, Pb: 0.4 ppm; potentiometric titration showed: Cl... - Not detected; water content determined by Karl Fischer titration: 8.6 ppm; HF content determined by acid-base titration: 85.6 ppm. See NMR spectrum. Figure 1 and Figure 2 .

[0053] Example 3: This example provides a method for preparing sodium difluorophosphate, including the following steps:

[0054] (1) All raw materials are pretreated before the reaction to ensure that the moisture content of all raw materials is below 500 ppm.

[0055] (2) Under a nitrogen-protected atmosphere, 2 mol of sodium hexafluorophosphate was dissolved in 1260 g of the reaction mother liquor from Example 2, and then 200 g of dimethylformamide was added. Next, 1.1 mol of octamethylcyclotetrasiloxane was added dropwise to the reaction vessel over 2 hours. The reaction temperature was controlled at 60°C and refluxed. After no more gas was generated, the reaction was continued at this temperature for another 2 hours. The reaction solution was then cooled to room temperature (25°C) and filtered to obtain the mother liquor and crude sodium hexafluorophosphate. The mother liquor could be reused after treatment as described in Example 1.

[0056] (3) Ethyl acetate and tetrahydrofuran were mixed at a mass ratio of 2:1 to obtain a washing solvent. The crude sodium difluorophosphate was ultrasonically washed three times with the washing solvent. After each washing, the product was filtered through a 0.22 μm microporous membrane and replaced with a new washing solvent for ultrasonic washing again until the washing was completed, and the wet solid sodium difluorophosphate was obtained.

[0057] (4) The wet solid sodium difluorophosphate was dried in the same way as in Example 1.

[0058] Ion chromatography analysis showed a product purity of 99.9% and a yield of 93%. ICP analysis revealed the following concentrations: K: 1.3 ppm, Fe: 0.7 ppm, Ca: 0.4 ppm, Pb: 0.5 ppm. Potentiometric titration showed the following concentrations: Cl... - Not detected; determined by Karl Fischer titration: moisture: 5.8 ppm; determined by acid-base titration: HF: 98.4 ppm.

[0059] Example 4: This example provides a method for preparing sodium difluorophosphate, including the following steps:

[0060] (1) All raw materials are pretreated before the reaction to ensure that the moisture content of all raw materials is below 500 ppm.

[0061] (2) Under a nitrogen-protected atmosphere, 2 mol of sodium hexafluorophosphate was dissolved in 1144 g of the mother liquor from Example 3, followed by the addition of 300 g of dimethylformamide. Then, 1.1 mol of octamethylcyclotetrasiloxane was added dropwise to the reaction vessel over 2 hours. The reaction temperature was controlled at 65°C and refluxed. After no more gas was generated, the reaction was maintained at this temperature for another 2 hours. The reaction solution was then cooled to room temperature (25°C) and filtered to obtain the mother liquor and crude sodium hexafluorophosphate. The mother liquor could be reused after being treated in the same manner as in Example 1.

[0062] (3) Ethyl acetate and tetrahydrofuran were mixed at a mass ratio of 1:2 to obtain a washing solvent. The crude sodium difluorophosphate was ultrasonically washed three times with the washing solvent. After each washing, the product was filtered through a 0.22 μm microporous membrane and replaced with a new washing solvent for ultrasonic washing again until the washing was completed, and the wet solid sodium difluorophosphate was obtained.

[0063] (4) The wet solid sodium difluorophosphate was dried in the same way as in Example 1.

[0064] Ion chromatography analysis showed a product purity of 99.9% and a yield of 89%; ICP analysis revealed: K: 1.1 ppm, Fe: 0.8 ppm, Ca: 0.6 ppm, Pb: 0.6 ppm; potentiometric titration showed: Cl... - Not detected; determined by Karl Fischer titration: moisture: 8.7 ppm; determined by acid-base titration: HF: 114.7 ppm.

[0065] Example 5: This example provides a method for preparing sodium difluorophosphate, including the following steps:

[0066] (1) All raw materials are pretreated before the reaction to ensure that the moisture content of all raw materials is below 500 ppm.

[0067] (2) Under a nitrogen-protected atmosphere, 2 mol of sodium hexafluorophosphate was dissolved in 1380 g of methyl ethyl carbonate and then added to a reaction vessel. 0.88 mol of decamethylcyclopentasiloxane was then added dropwise to the reaction vessel over 2 hours. The reaction temperature was controlled at 65°C and refluxed. After no more gas was generated, the reaction was maintained at this temperature for another 2 hours. The reaction solution was then cooled to room temperature (25°C) and filtered to obtain the mother liquor and crude sodium difluorophosphate. The mother liquor could be reused after treatment as described in Example 1.

[0068] (3) Ethylene glycol dimethyl ether and tetrahydrofuran were mixed at a mass ratio of 2:1 to obtain a washing solvent. The crude sodium difluorophosphate was ultrasonically washed three times with the washing solvent. After each washing, the product was filtered through a 0.22 μm microporous membrane and replaced with a new washing solvent for ultrasonic washing again until the washing was completed, thus obtaining wet solid sodium difluorophosphate.

[0069] (4) The wet solid sodium difluorophosphate was dried in the same way as in Example 1.

[0070] Ion chromatography analysis showed a product purity of 99.8% and a yield of 94%; ICP analysis revealed: K: 1.2 ppm, Fe: 0.6 ppm, Ca: 0.6 ppm, Pb: 0.7 ppm; potentiometric titration showed: Cl... - Not detected; determined by Karl Fischer titration: moisture: 5.9 ppm; determined by acid-base titration: HF: 93.8 ppm.

[0071] Example 6: This example provides a method for preparing sodium difluorophosphate, including the following steps:

[0072] (1) All raw materials are pretreated before the reaction to ensure that the moisture content of all raw materials is below 500 ppm.

[0073] (2) Under a nitrogen-protected atmosphere, 2 mol of sodium hexafluorophosphate was dissolved in 1124 g of ethyl acetate and then added to a reaction vessel. 0.74 mol of dodecylcyclohexasiloxane was then added dropwise over 2 hours. The reaction temperature was controlled at 70°C and refluxed. After no more gas was generated, the reaction was maintained at this temperature for another 2 hours. The reaction solution was cooled to room temperature (25°C) and filtered to obtain the mother liquor and crude sodium difluorophosphate. The mother liquor could be reused after treatment as described in Example 1.

[0074] (3) Ethylene glycol dimethyl ether and tetrahydrofuran were mixed at a mass ratio of 2:1 to obtain a washing solvent. The crude sodium difluorophosphate was ultrasonically washed three times with the washing solvent. After each washing, the product was filtered through a 0.22 μm microporous membrane and replaced with a new washing solvent for ultrasonic washing again until the washing was completed, thus obtaining wet solid sodium difluorophosphate.

[0075] (4) The wet solid sodium difluorophosphate was dried in the same way as in Example 1.

[0076] Ion chromatography analysis showed a product purity of 99.9% and a yield of 89%; ICP analysis revealed: K: 0.8 ppm, Fe: 0.3 ppm, Ca: 0.4 ppm, Pb: 0.2 ppm; potentiometric titration showed: Cl... - Not detected; determined by Karl Fischer titration: moisture: 7.3 ppm; determined by acid-base titration: HF: 89.5 ppm.

[0077] Comparative Example 1: This comparative example provides a method for preparing sodium difluorophosphate, comprising the following steps:

[0078] (1) All raw materials are pretreated before the reaction to ensure that the moisture content of all raw materials is below 500 ppm.

[0079] (2) Under a nitrogen-protected atmosphere, 2 mol of sodium hexafluorophosphate was dissolved in 1352 g of methyl ethyl carbonate and then added to a reaction vessel. 1.1 mol of octamethylcyclotetrasiloxane was then added dropwise to the reaction vessel over 2 hours. The reaction temperature was controlled at 65°C and refluxed. After no more gas was produced, the reaction was maintained at this temperature for another 2 hours. The reaction solution was then cooled to room temperature (25°C) and filtered to obtain crude sodium difluorophosphate.

[0080] (3) The crude sodium difluorophosphate was ultrasonically washed three times with ethyl acetate, and filtered through a 0.22 μm microporous membrane after each wash.

[0081] Then, replace the washing solvent and perform ultrasonic washing again until the washing is complete, obtaining wet solid sodium difluorophosphate.

[0082] (4) The wet solid sodium difluorophosphate was dried in the same way as in Example 1.

[0083] Ion chromatography analysis showed a product purity of 96.6% and a yield of 91%. ICP analysis revealed the following concentrations: K: 1.1 ppm, Fe: 0.7 ppm, Ca: 0.5 ppm, Pb: 0.5 ppm. Potentiometric titration showed the following concentrations: Cl... - Not detected; determined by Karl Fischer titration: moisture: 11.2 ppm; determined by acid-base titration: HF: 454.3 ppm.

[0084] Comparative Example 2: This comparative example is basically the same as Example 1, except that the washing solvent is tetrahydrofuran.

[0085] Ion chromatography analysis showed a product purity of 97.4% and a yield of 90%; ICP analysis revealed: K: 1.0 ppm, Fe: 0.4 ppm, Ca: 0.2 ppm, Pb: 0.7 ppm; potentiometric titration showed: Cl... - Not detected; determined by Karl Fischer titration: moisture: 20.4 ppm; determined by acid-base titration: HF: 315.3 ppm.

[0086] Comparative Example 3: This comparative example is basically the same as Example 1, except that the reaction temperature is different, specifically 90°C.

[0087] Ion chromatography analysis showed a product purity of 97.8% and a yield of 86%. ICP analysis revealed the following concentrations: K: 1.5 ppm, Fe: 5.6 ppm, Ca: 1.1 ppm, Pb: 1.4 ppm. Potentiometric titration showed the following concentrations: Cl... - Not detected; determined by Karl Fischer titration: moisture: 7.5 ppm; determined by acid-base titration: HF: 445.8 ppm.

[0088] Comparative Example 4: This comparative example is basically the same as Example 1, except that the drying method of the wet solid sodium difluorophosphate is different. Specifically, the wet solid is dried in a vacuum drying oven at 80°C for 6 hours to obtain sodium difluorophosphate.

[0089] Ion chromatography analysis showed a product purity of 99.1% and a yield of 91%. ICP analysis revealed the following concentrations: K: 1.1 ppm, Fe: 1.6 ppm, Ca: 0.7 ppm, Pb: 1.1 ppm. Potentiometric titration showed the following concentrations: Cl... - Not detected; determined by Karl Fischer titration: moisture: 845 ppm; determined by acid-base titration: HF: 112.8 ppm.

[0090] The present invention has been described in detail above, with the aim of enabling those skilled in the art to understand and implement the invention. However, this description should not be construed as limiting the scope of protection of the invention. All equivalent changes or modifications made in accordance with the spirit and essence of the invention should be included within the scope of protection of the invention.

Claims

1. A method for preparing sodium difluorophosphate, characterized in that, Includes the following steps: S1. Sodium hexafluorophosphate is reacted with cyclic siloxanes in an aprotic organic solvent at 50°C to 80°C. After the reaction is completed, the mixture is filtered to obtain crude sodium difluorophosphate and mother liquor. The cyclic siloxanes are selected from one or more of hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, and dodecylcyclohexasiloxane. S2. The crude sodium difluorophosphate is ultrasonically washed with a washing solvent to obtain wet sodium difluorophosphate solid. The washing solvent is any one of dimethyl carbonate, ethyl acetate, ethylene glycol dimethyl ether and a mixture of tetrahydrofuran. S3. The wet solid sodium difluorophosphate is gradually heated and vacuum dried, and inert gas is introduced intermittently during the drying process for replacement.

2. The method for preparing sodium difluorophosphate according to claim 1, characterized in that, The washing solvent is composed of any one of dimethyl carbonate, ethyl acetate, or ethylene glycol dimethyl ether, and tetrahydrofuran in a mass ratio of (0.5–4):

1.

3. The method for preparing sodium difluorophosphate according to claim 1, characterized in that, The moisture content of the washing solvent is controlled to be below 500 ppm.

4. The method for preparing sodium difluorophosphate according to claim 1, characterized in that, The ultrasonic washing is performed two or more times, and the washing solution is filtered with a microfiltration membrane after each washing.

5. The method for preparing sodium difluorophosphate according to claim 4, characterized in that, The pore size of the microfiltration membrane is 0.15 μm to 0.3 μm; And / or, use fresh washing solvent for each wash.

6. The method for preparing sodium difluorophosphate according to claim 1, characterized in that, The gradual heating vacuum drying process includes drying at 65℃~75℃ for 1.5h~2.5h, drying at 85~94℃ for 3h~5h, and finally drying at 95℃~105℃ for 1.5h~2.5h.

7. The method for preparing sodium difluorophosphate according to claim 1, characterized in that, The mother liquor obtained in step S1 is allowed to stand at 0℃~5℃, filtered, and the resulting filtrate is used in the next batch of reaction to replace the aprotic organic solvent in step S1. The aprotic organic solvent is selectively added to the system of the next batch of reaction.

8. The method for preparing sodium difluorophosphate according to claim 1, characterized in that, The molar ratio of sodium hexafluorophosphate to cyclic siloxane is 1:(0.2-0.8).

9. The method for preparing sodium difluorophosphate according to claim 1, characterized in that, The aprotic organic solvent is one or more of dimethyl carbonate, dimethylformamide, diethyl carbonate, acetonitrile, ethyl acetate, and methyl ethyl carbonate; And / or, the mass ratio of the sodium hexafluorophosphate to the aprotic organic solvent is 1:(3-6).

10. The method for preparing sodium difluorophosphate according to claim 1, characterized in that, Inert gas is introduced every 0.5 to 1.5 hours for replacement; And / or, the inert gas is nitrogen.

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