A method and system for preparing sodium difluorophosphate

Abstract

The application discloses a preparation method and system of sodium difluorophosphate. The preparation method of sodium difluorophosphate comprises the following steps: S100, in a water-free or low-water environment, a solid-phase reaction is generated between diphosphorus pentoxide and ammonium fluoride to obtain an ammonium difluorophosphate intermediate, and volatile substances generated in the reaction process are removed and recovered; S200, in a water-free organic solvent, the ammonium difluorophosphate intermediate obtained in the step S100 is reacted with a sodium salt to obtain sodium difluorophosphate. The preparation method of sodium difluorophosphate provided by the application uses diphosphorus pentoxide and ammonium fluoride as reaction raw materials to obtain an ammonium difluorophosphate intermediate, and then reacts to obtain sodium difluorophosphate. The obtained sodium difluorophosphate has high purity and few impurities. The preparation method of sodium difluorophosphate has the advantages of cheap and easily obtained raw materials, stable chemical properties, few by-products generated in the reaction, and easy recovery, and is a green and safe preparation method of sodium difluorophosphate.

Description

Technical Field

[0001] This invention relates to the field of electrolyte additive preparation technology, and in particular to a method and system for preparing sodium difluorophosphate. Background Technology

[0002] Sodium difluorophosphate, with the molecular formula NaPO2F2, is a white powdery solid used in sodium-ion batteries. As an electrolyte additive, it can reduce impedance and improve the battery's cycle performance and high-temperature storage performance.

[0003] Currently, there are two main methods for preparing sodium difluorophosphate. One is the solid-phase method, which involves heating sodium fluoride and phosphorus pentoxide solid under sealed conditions to react. This method has simple reaction components, but the yield of the target product, sodium difluorophosphate, is low, only 10-20%. Moreover, the reaction conditions are difficult to control, and it is easy to generate POF3 (phosphoryl fluoride), which is highly corrosive to equipment. Corrosion-resistant equipment is required, and byproducts such as Na3PO4 are generated. It also has high energy consumption and complex post-processing. The other method is the direct reaction method, which involves directly reacting difluorophosphoric acid with oxygen-containing sodium salts to produce sodium difluorophosphate. This method has a higher yield of sodium difluorophosphate than the solid-phase method, but difluorophosphoric acid has poor stability, is highly toxic and corrosive, and requires high-quality production equipment. It also generates a lot of industrial wastewater, which is not conducive to environmental protection and large-scale production. Existing technologies use sodium hexafluorophosphate hydrolysis to synthesize sodium difluorophosphate. This method uses sodium hexafluorophosphate as raw material, which is costly and leaves a lot of residual free acid after hydrolysis, resulting in unstable production quality and making it difficult to meet the stringent requirements of battery-grade applications for moisture, acidic impurities, and metallic impurities.

[0004] Therefore, how to prepare sodium difluorophosphate in a low-cost, high-efficiency, low-waste, and easily scalable manner remains a technical problem that urgently needs to be solved in the industry. Summary of the Invention

[0005] The purpose of this invention is to provide a method and system for preparing sodium difluorophosphate, which aims to solve the problems of high cost, high energy consumption, difficulty in controlling impurities, and large amount of waste in the preparation of sodium difluorophosphate in the prior art.

[0006] This invention proposes a method for preparing sodium difluorophosphate, comprising the following steps: S100. In an anhydrous or low-water environment, phosphorus pentoxide and ammonium fluoride undergo a solid-phase reaction to obtain ammonium difluorophosphate intermediate. During the reaction, volatile substances generated in the reaction are removed and recovered. S200. In an anhydrous organic solvent, the ammonium difluorophosphate intermediate obtained in step S100 is reacted with sodium salt to obtain sodium difluorophosphate.

[0007] In one embodiment, in step S100, the phosphorus pentoxide and the ammonium fluoride react after being mixed and activated in a mechanochemical device.

[0008] In one embodiment, the mechanochemical apparatus is a ball mill or a twin-screw extruder.

[0009] In one embodiment, the reaction temperature in step S100 is 60°C-110°C, and the reaction temperature in step S200 is 20°C-60°C.

[0010] In one embodiment, the reaction temperature in step S100 is 80°C-100°C.

[0011] In one embodiment, the molar ratio of phosphorus pentoxide to ammonium fluoride in step S100 is 1:(4-8).

[0012] In one embodiment, the molar ratio of phosphorus pentoxide to ammonium fluoride in step S100 is preferably 1:(5-6).

[0013] In one embodiment, in steps S100 and S200, the moisture content of the reaction system is ≤500ppm.

[0014] In one embodiment, the sodium salt in step S200 is selected from one or both of NaOCH3 and NaOCH2CH3.

[0015] In one embodiment, in step S200, the anhydrous organic solvent is selected from ether solvents or carbonate solvents.

[0016] In one embodiment, in step S200, the anhydrous organic solvent is selected from anhydrous dimethyl ether or anhydrous dimethyl carbonate.

[0017] In one embodiment, the preparation method further includes a recovery step of recovering the ammonia gas generated in steps S100 and S200: S300, the sodium difluorophosphate obtained in step S200 is purified to obtain the sodium difluorophosphate product.

[0018] A second aspect of this application proposes a system for preparing sodium difluorophosphate, comprising a gas recovery unit and a mechanochemical reaction unit, a displacement reaction unit, and a purification unit connected in sequence. The mechanochemical reaction unit is used to react phosphorus pentoxide with ammonium fluoride in a solid phase to obtain ammonium difluorophosphate intermediate; The displacement reaction unit is used to react the ammonium difluorophosphate intermediate with the sodium salt to obtain sodium difluorophosphate. The refining unit is used to refine the sodium difluorophosphate obtained from the displacement reaction unit to obtain the sodium difluorophosphate product. The gas recovery unit is connected to the mechanochemical reaction unit, the displacement reaction unit, and the purification unit via a closed pipeline and is used to recover volatile substances.

[0019] In one embodiment, a one-way valve, a vacuum pump, or a gas purging device is provided between the mechanochemical reaction unit and the gas recovery unit to continuously remove volatile substances generated by the reaction.

[0020] The embodiments of the present invention have the following beneficial effects: The method for preparing sodium difluorophosphate proposed in this invention involves reacting phosphorus pentoxide and ammonium fluoride as reactants to obtain ammonium difluorophosphate intermediate, which is then reacted to obtain sodium difluorophosphate. This method uses readily available and chemically stable raw materials for phosphorus pentoxide preparation, produces few byproducts that are easily separated and recovered, making it a green and safe method for preparing sodium difluorophosphate. This method helps reduce waste discharge and post-treatment burden; it avoids the use of highly corrosive raw materials such as HF and POCl3, significantly improving process safety; it employs an intermediate conversion pathway, resulting in high reaction selectivity and few byproducts; the reaction is carried out in an anhydrous system, effectively inhibiting hydrolysis and improving product purity; during the reaction, gaseous byproducts and moisture generated are removed through vacuuming, inert gas purging, or negative pressure operation, reducing moisture retention and the possibility of hydrolysis side reactions. Simultaneously, it reduces the presence of ammonia in the reaction system, improving reaction safety and keeping the reaction in a non-equilibrium state, which is conducive to promoting the forward reaction and increasing the reaction rate and extent. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] in: Figure 1 This is a schematic diagram of the preparation method of sodium difluorophosphate in one embodiment of the present invention; Figure 2 This is a schematic diagram of a sodium difluorophosphate preparation system in one embodiment of the present invention; Figure 3 This is the nuclear magnetic resonance spectrum of sodium difluorophosphate in Example 1 of the present invention. Detailed Implementation

[0023] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] Furthermore, the use of terms such as "first" and "second" in this invention is for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, features defined with "first" and "second" may explicitly or implicitly include at least one of the stated features. Additionally, the technical solutions of the various embodiments can be combined with each other, but only on the basis of being achievable by those skilled in the art. When the combination of technical solutions is contradictory or impossible to implement, such a combination of technical solutions should be considered non-existent and not within the scope of protection claimed by this invention.

[0025] Reference Figure 1 This invention provides a method for preparing sodium difluorophosphate, which includes the following steps: S100, in an anhydrous or low-water environment, phosphorus pentoxide and ammonium fluoride undergo a solid-phase reaction to obtain an ammonium difluorophosphate intermediate, and volatile substances generated during the reaction are removed and recovered; S200, in an anhydrous organic solvent, the ammonium difluorophosphate intermediate obtained in step S100 is reacted with a sodium salt to obtain sodium difluorophosphate.

[0026] The method for preparing sodium difluorophosphate proposed in this invention involves reacting phosphorus pentoxide and ammonium fluoride as reactants to obtain ammonium difluorophosphate intermediate, which is then reacted to obtain sodium difluorophosphate. This method uses readily available and chemically stable raw materials for phosphorus pentoxide preparation, produces few byproducts that are easily separated and recovered, making it a green and safe method for preparing sodium difluorophosphate. This method helps reduce waste discharge and post-treatment burden; it avoids the use of highly corrosive raw materials such as HF and POCl3, significantly improving process safety; it employs an intermediate conversion pathway, resulting in high reaction selectivity and few byproducts; the reaction is carried out in an anhydrous system, effectively inhibiting hydrolysis and improving product purity; during the reaction, gaseous byproducts and moisture generated are removed through vacuuming, inert gas purging, or negative pressure operation, reducing moisture retention and the possibility of hydrolysis side reactions. Simultaneously, it reduces the presence of ammonia in the reaction system, improving reaction safety and keeping the reaction in a non-equilibrium state, which is conducive to promoting the forward reaction and increasing the reaction rate and extent.

[0027] Specifically, the reaction equation for step S100 is shown in Equation 1: P4O 10+8NH4F→4NH4PO2F2+4NH3↑+2H2O, Equation 1; The reaction equation for step (2) is shown in Equation 2: NH4PO2F2 + NaOCH3 → NaPO2F2 + NH3↑ + HOCH3, Equation 2.

[0028] It should be noted that although a certain amount of water will be generated during the reaction process according to the reaction mechanism, the reaction system is in a dynamic non-equilibrium state due to the mechanochemical strengthening conditions adopted in this invention. Under the effects of continuous mixing, local heating and volatile product extraction, the generated water is not likely to stay at the reaction interface for a long time and accumulate continuously, thereby reducing its impact on the stability of ammonium difluorophosphate, inhibiting the occurrence of hydrolysis side reactions, and improving the reaction selectivity and subsequent conversion efficiency.

[0029] In one specific embodiment, the preparation method further includes a recovery step of recovering the ammonia gas generated in steps S100 and S200.

[0030] Specifically, the ammonia gas generated in steps S100 and S200 is absorbed and treated to obtain recovered NH3 or ammonium hydroxide. In the preparation process of this invention, ammonia gas is generated when phosphorus pentoxide reacts with ammonium fluoride to produce ammonium difluorophosphate in step S100; ammonia gas is also generated when ammonium difluorophosphate undergoes a cation exchange reaction with sodium salt in step S200. Recovering the ammonia gas allows for separation and purification to prepare reactants, reducing waste gas emissions and achieving material recycling, which is beneficial for energy conservation and emission reduction.

[0031] In one embodiment, in step S100, the phosphorus pentoxide and the ammonium fluoride are mixed in a mechanochemical device and then activated and reacted.

[0032] Phosphorus pentoxide and ammonium fluoride are mixed in a mechanochemical apparatus, where solid-phase activation and transformation are achieved through shearing, impact, localized temperature rise, and continuous renewal of the reaction interface. Compared to traditional production routes, this process avoids the use of highly corrosive raw materials such as HF and POCl3, which improves production safety. Furthermore, compared to traditional liquid-phase reactions, the solid-phase pathway takes place in a low-free-liquid phase or near-anhydrous environment, which helps reduce side reactions, generates less waste liquid, and reduces the burden of post-treatment, making it more suitable for industrial implementation.

[0033] In one embodiment, the mechatronic device is a ball mill or a twin-screw extruder, preferably a twin-screw extruder. Twin-screw extrusion enables continuous production and facilitates online control, making it suitable for industrial scale-up.

[0034] Specifically, the ball mill can be a planetary ball mill or a vibratory ball mill.

[0035] In one embodiment, in step S100, the reaction temperature range between phosphorus pentoxide and ammonium fluoride is 60℃-110℃. The solid-phase reaction conditions are mild and energy consumption is low, making the production process energy-saving and environmentally friendly, and significantly reducing production costs.

[0036] Specifically, the reaction temperature in step S100 can be selected as 60℃, 70℃, 80℃, 90℃, 100℃, or 110℃.

[0037] In a preferred embodiment, in step S100, the reaction temperature range of phosphorus pentoxide and ammonium fluoride is 80℃-110℃. Within this temperature range, both lower reaction energy consumption costs and milder production conditions are considered, while ensuring the production efficiency of low-temperature solid-phase reaction.

[0038] In one embodiment, in step S200, the reaction temperature range between the sodium difluorophosphate and the sodium salt is 20°C-60°C. A lower reaction temperature helps reduce energy consumption and operational risks, and to some extent improves displacement selectivity, thereby increasing the yield of sodium difluorophosphate. Since step S200 is a heterogeneous reaction, reducing the particle size of the sodium oxide particles helps increase the interfacial contact area and reduce mass transfer resistance; therefore, fine-particle-size sodium oxide powder is preferred.

[0039] Specifically, the reaction temperature in step S200 can be selected as 20℃, 25℃, 30℃, 37℃, 40℃, 50℃, 55℃ or 60℃.

[0040] Preferably, the average particle size of the sodium oxide is less than 10 μm, more preferably less than 1 μm, in order to improve the reaction rate and conversion efficiency.

[0041] In one embodiment, in step S100, the molar ratio of phosphorus pentoxide to ammonium fluoride is 1:(4-8). Excess phosphorus pentoxide increases the contact between ammonium fluoride and phosphorus pentoxide, which is beneficial for improving solid-phase conversion efficiency, ensuring complete consumption of ammonium fluoride during the reaction, improving production process safety, and reducing the risk of production accidents. Furthermore, after the reaction is complete, the absence or minimal presence of ammonium fluoride in the equipment reduces the emission of hazardous chemicals, lowering negative environmental impacts and waste disposal costs.

[0042] Specifically, the molar ratio of phosphorus pentoxide to ammonium fluoride is selected from 1:4, 1:5, 1:6, 1:7 or 1:8.

[0043] In a preferred embodiment, in step S100, the molar ratio of phosphorus pentoxide to ammonium fluoride is 1:(5-6). Limiting the molar ratio of phosphorus pentoxide to ammonium fluoride to this ratio balances the safety and economy of the reaction, improves the output efficiency per unit time, reduces time costs, and ensures the safety of the reaction.

[0044] Specifically, the molar ratio of phosphorus pentoxide to ammonium fluoride is selected from 1:5, 1:5.5 or 1:6.

[0045] In one embodiment, the sodium salt is selected from one or both of NaOCH3 and NaOCH2CH3.

[0046] In one embodiment, in step S200, the anhydrous organic solvent is selected from ether solvents or carbonate solvents.

[0047] In one embodiment, in step S200, the anhydrous organic solvent is selected from anhydrous dimethyl ether or anhydrous dimethyl carbonate.

[0048] like Figure 1 As shown, in one embodiment, after step S200, the method further includes: step S300, refining the sodium difluorophosphate obtained in S200 to obtain the sodium difluorophosphate product.

[0049] In one embodiment, in step S300, the refining process involves crystallizing the sodium difluorophosphate using an antisolvent, followed by separation, washing, and drying to obtain the sodium difluorophosphate product.

[0050] Specifically, the antisolvent is an acetone or carbonate system.

[0051] Specifically, after refining, the purity of the obtained sodium difluorophosphate product is ≥99.0%, and the moisture content is ≤100ppm and the metal content is ≤10ppm, in order to maintain the structural stability of difluorophosphate and inhibit its hydrolysis side reactions, thereby improving the product selectivity and purity.

[0052] Furthermore, the antisolvent crystallization can be continuous antisolvent crystallization, and the mother liquor of crystallization can be recovered and reused after dehydration treatment, thereby improving solvent utilization and reducing emissions, increasing solvent utilization, reducing production costs, stabilizing product quality, and being suitable for large-scale production.

[0053] like Figure 2 As shown, according to a second aspect of this application, a system for preparing sodium difluorophosphate is proposed, comprising a gas recovery unit and a mechanochemical reaction unit, a displacement reaction unit, and a purification unit connected in sequence. The mechanochemical reaction unit is used to cause a solid-phase reaction between phosphorus pentoxide and ammonium fluoride to generate ammonium difluorophosphate intermediate; The displacement reaction unit is used to cause a cation displacement reaction between the ammonium difluorophosphate intermediate and the sodium salt to generate sodium difluorophosphate. The gas recovery unit is used to recover the waste gas generated in the mechanochemical reaction unit and the displacement reaction unit; The mechanochemical reaction unit, displacement reaction unit, gas recovery unit, and refining unit are connected by sealed pipelines and maintain a low-moisture environment.

[0054] Specifically, the displacement reaction unit includes a stirred tank; the refining unit includes a crystallization device, a solid-liquid separation device, a washing device, and a drying device.

[0055] Furthermore, the solid-liquid separation equipment can be selected from a filter or a centrifuge; the drying equipment can be selected from a vacuum drying oven or a drying furnace.

[0056] Furthermore, the gas recovery unit includes a condensation device and an absorption device. Ammonia gas, after cooling, enters the absorption device and is absorbed in water or an acidic solution to form ammonia water or ammonium salt solution, thereby achieving centralized recovery and resource utilization of ammonia gas. By uniformly recovering the ammonia gas generated in the two-step reaction, not only can waste gas emissions be reduced, but the recovered ammonia gas or ammonia water can also be further used in the preparation of ammonium fluoride or other processes, thereby reducing raw material consumption and improving the overall economic efficiency and environmental friendliness of the process. In addition, the recovery of ammonia gas and water can reduce the retention of water in the reaction system, thus helping to maintain a low-moisture reaction environment and improving reaction stability and product quality.

[0057] Furthermore, the absorption device is selected from an alkaline washing tower or an acid washing tower.

[0058] More specific implementation examples will be described in detail below.

[0059] Example 1 This embodiment is for pilot production: (1) Accurately weigh 1.00 g of phosphorus pentoxide and 2.12 g of ammonium fluoride and place them into a ball mill jar. Seal and fill with N2. Run intermittently for 3 hours at 400 rpm (run for 10 min / stop for 5 min). Monitor the temperature during the reaction process and ensure it does not exceed 100℃. The reaction yields ammonium difluorophosphate, and a small amount of NH3 is guided to volatilize into the condenser through the exhaust port. (2) The obtained solid was transferred to a stirred tank under an inert atmosphere, 20 mL of anhydrous dimethyl ether was added, the reaction temperature was controlled at 40 °C, and 0.84 g of sodium ethoxide powder (particle size less than 1 micrometer) was slowly added under stirring. The stirring was continued for 40 min, and the temperature was raised to 40 °C and maintained for 1 h to allow ammonium difluorophosphate to react with sodium salt to generate sodium difluorophosphate. NH3 was recovered into NH4OH solution under N2 atmosphere using a condenser. (3) After the reaction is completed, the reaction solution is cooled to 20°C, filtered to remove byproducts and unreacted solids, and the filtrate containing sodium difluorophosphate is collected. Acetone is slowly added dropwise to the sodium difluorophosphate solution as an antisolvent until it is supersaturated and crystallized. The solid is collected by centrifugation, washed twice with cold acetone at 5°C, and vacuum dried at 80°C for 12 hours under <0.1mbar conditions to obtain NaPO2F2 powder. During the reaction, the crystallization mother liquor is recovered and dehydrated before being returned to steps (2) and (3) for reuse to reduce solvent consumption and emissions.

[0060] The NaPO2F2 powder obtained in this embodiment has a purity of 99.3%, a moisture content of 37 ppm, and a NaPO2F2 yield of 80.5%.

[0061] The product¹ 9 F NMR spectrum as shown Figure 3 This shows that, at δ≈ A characteristic bimodal signal appears at 83 ppm, with a phosphorus-fluorine coupling constant J(PF) of approximately 900-1000 Hz, indicating the presence of typical PO2F2 in the system. - Structural unit; at the same time, 31 P NMR spectrum at δ≈ A triplet signal is observed at 13 ppm, and the coupling constant is... 19 The consistent NMR results further confirm that the phosphorus atom is coupled with two equivalent fluorine atoms. The above NMR results indicate that the product prepared in this invention is sodium difluorophosphate with a well-defined structure.

[0062] Example 2 This example is a pilot production: (1) Phosphorus pentoxide is fed at a rate of 1.52 kg / h and NH4F is fed at a rate of 3.9 kg / h. The reaction is carried out in a twin-screw extruder. The operating parameters of the twin-screw extruder are set as follows: Zone 1 50℃, Zone 2 80℃, Zone 3 100℃, local temperature not exceeding 110℃, residence time 10 min, and ammonium difluorophosphate generation efficiency ≥85%. Vacuum extraction is set at -0.05~-0.08 MPa in Zones 2 and 3 to discharge a small amount of volatiles and moisture. The extracted gas is treated by a condenser and an alkaline washing tower to recover NH3.

[0063] (2) Ammonium difluorophosphate is discharged into a continuous displacement reactor, and sodium methoxide powder is added to anhydrous dimethyl carbonate solvent at a rate of 1.19 kg / h. The ammonium difluorophosphate is reacted to obtain sodium difluorophosphate; the reaction is carried out at 50°C for 120 min; ammonia is absorbed by a weak acid to obtain NH4OH, with a recovery rate ≥75%. (3) Filter to separate solid impurities, slowly add carbonate antisolvent to sodium difluorophosphate solution until supersaturation and crystallization, collect solid by centrifugation, wash twice with cold carbonate twice the volume of solid weight, and vacuum dry at 80°C for 12 h under <0.1 mbar conditions to obtain NaPO2F2 powder; during the reaction, the mother liquor of crystallization is recovered and dehydrated and returned to steps (2) and (3) for reuse to reduce solvent consumption and emissions.

[0064] In this example, the sodium difluorophosphate contained ≤5ppm of ICP (metal) and the total yield of sodium difluorophosphate was 82.5%.

[0065] Example 3 The difference between this embodiment and embodiment 2 is that in step (2), the ammonium difluorophosphate obtained in step (1) is continuously fed into the reaction unit, anhydrous diethyl carbonate is used as an organic solvent, sodium ethoxide submicron powder is added to the reaction system at 30°C, and sodium difluorophosphate is obtained by reacting under anhydrous conditions; the ammonia gas generated during the reaction is recovered by a condenser and further recovered as NH4OH solution by an absorption tower, with a recovery rate ≥75%.

[0066] The NaPO2F2 powder product obtained in this embodiment has a purity of 99.3%, a moisture content of ≤410 ppm, and a total sodium difluorophosphate yield of 84.3%.

[0067] Comparative Example 1 This example is a pilot production: The difference between this example and Example 3 is that in step (2), the organic solvent used in the metathesis reaction of ammonium difluorophosphate and sodium salt was not dehydrated, and the anhydrous conditions of the metathesis reaction system were not strictly controlled.

[0068] The NaPO2F2 powder obtained in this example has a purity of 81.3%, contains a large amount of sodium fluoride byproducts, and has a moisture content of 420 ppm, with a yield of 71.2%.

[0069] Compared to the examples, the comparative example, under uncontrolled moisture conditions, showed increased water retention in the reaction system, leading to partial hydrolysis of the difluorophosphate structure, thereby reducing product purity and increasing byproduct formation. This invention, through a combination of mechanochemical enhancement and low-moisture control, reduces the impact of small amounts of water generated during the reaction on intermediate stability through volatile matter extraction and system regulation, thereby suppressing hydrolysis side reactions and improving product purity and overall yield.

[0070] The above description discloses only preferred embodiments of the present invention and should not be construed as limiting the scope of the present invention. Therefore, equivalent variations made in accordance with the claims of the present invention are still within the scope of the present invention.

Claims

1. A method for preparing sodium difluorophosphate, characterized in that, Includes the following steps: S100. In an anhydrous or low-water environment, phosphorus pentoxide and ammonium fluoride undergo a solid-phase reaction to obtain ammonium difluorophosphate intermediate. During the reaction, volatile substances generated in the reaction are removed and recovered. S200. In an anhydrous organic solvent, the ammonium difluorophosphate intermediate obtained in step S100 is reacted with a sodium salt to obtain sodium difluorophosphate.

2. The method for preparing sodium difluorophosphate according to claim 1, characterized in that, In step S100, the phosphorus pentoxide and the ammonium fluoride are mixed in a mechanochemical device and then activated and reacted. Preferably, the mechanochemical apparatus is a ball mill or a twin-screw extruder.

3. The method for preparing sodium difluorophosphate according to claim 1, characterized in that, In step S100, the reaction temperature is 60-110℃; in step S200, the reaction temperature is 20℃-60℃. Preferably, in step S100, the reaction temperature is 80-100℃.

4. The method for preparing sodium difluorophosphate according to claim 1, characterized in that, In step S100, the molar ratio of phosphorus pentoxide to ammonium fluoride is 1:(4-8).

5. The method for preparing sodium difluorophosphate according to claim 1, characterized in that, In steps S100 and S200, the moisture content of the reaction system is ≤500ppm.

6. The method for preparing sodium difluorophosphate according to claim 1, characterized in that, In step S200, the sodium salt is selected from one or both of NaOCH3 and NaOCH2CH3.

7. The method for preparing sodium difluorophosphate according to claim 1, characterized in that, In step S200, the anhydrous organic solvent is an ether solvent or a carbonate solvent; Preferably, the anhydrous organic solvent is anhydrous dimethyl ether or anhydrous dimethyl carbonate.

8. The method for preparing sodium difluorophosphate according to claim 1, characterized in that, The preparation method further includes a recovery step for recovering the ammonia gas generated in steps S100 and S200. S300: The sodium difluorophosphate obtained in step S200 is purified to obtain the sodium difluorophosphate product.

9. A system for preparing sodium difluorophosphate, characterized in that, It includes a gas recovery unit and a mechanochemical reaction unit, a displacement reaction unit, and a purification unit connected in sequence; The mechanochemical reaction unit is used to react phosphorus pentoxide with ammonium fluoride in a solid phase to obtain ammonium difluorophosphate intermediate; The displacement reaction unit is used to react the ammonium difluorophosphate intermediate with the sodium salt to obtain sodium difluorophosphate. The refining unit is used to refine the sodium difluorophosphate obtained from the displacement reaction unit to obtain the sodium difluorophosphate product. The gas recovery unit is connected to the mechanochemical reaction unit, the displacement reaction unit, and the purification unit via a closed pipeline and is used to recover volatile substances.

10. The system for preparing sodium difluorophosphate according to claim 9, characterized in that, A one-way valve, a vacuum pump, or a gas purging device is provided between the mechanochemical reaction unit and the gas recovery unit to continuously remove volatile substances generated by the reaction.

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