A preparation method of sodium difluorophosphate

Abstract

The present invention belongs to the technical field of sodium ion battery electrolytes, and in particular to a method for preparing sodium difluorophosphate. The method for preparing sodium difluorophosphate of the present invention comprises the following steps: reacting an oxygen-containing sodium salt and PF5 in a supercritical carbon dioxide fluid, generating NaF that is insoluble in supercritical carbon dioxide fluid, and dissolving sodium difluorophosphate in a supercritical carbon dioxide fluid. After the reaction, a supercritical carbon dioxide fluid containing sodium difluorophosphate is obtained, and then sodium difluorophosphate is obtained by separation. The present invention adopts supercritical carbon dioxide (SCO2) as a solvent. In SCO2, the solubility of the oxygen-containing sodium salt is greatly improved, and the contact area between PF5 and the oxygen-containing sodium salt is increased, thereby improving reaction efficiency. The whole step is simple, the reaction is easily controlled, and the yield is high; the sodium difluorophosphate prepared by the reaction is then extracted and separated by SCO2, and the process is pure and pollution-free, and the product purity is high.

Description

Technical Field

[0001] The invention belongs to the technical field of sodium ion battery electrolytes, and particularly relates to a method for preparing sodium difluorophosphate. Background Art

[0002] Sodium-ion batteries, with their significant advantages of abundant raw material resources and low cost, have become the next generation of energy storage systems for large-scale applications, such as in smart grids and low-speed electric vehicles. A key component of sodium-ion batteries is a stable electrolyte solution containing one or more chemical additives that can limit the growth of electrochemical impedance, reduce the self-discharge rate during storage, and extend battery life. The development of new electrolyte additives will support improvements in the energy density, cost, and life of sodium-ion batteries, and is an important research direction for improving sodium battery performance.

[0003] Common electrolyte additives mainly include cyclic carbonates, organic sulfates and sulfonates, Lewis adducts, alkyl-substituted phosphites, and phosphates. 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 cycling performance and high-temperature storage performance. However, since the current mainstream research focuses on lithium-ion batteries, the corresponding electrolyte additive is lithium difluorophosphate. There are currently many patent reports related to lithium difluorophosphate, such as Chinese invention patents CN112537763A and CN103052592B, both of which disclose methods for preparing lithium difluorophosphate. However, there are currently few special studies on sodium salts. The main reason is that the solubility and reactivity of sodium phosphate are different from those of lithium phosphate, and the relevant yields reported in the patents cannot be achieved. This is also the reason why sodium phosphate salts have similar properties to lithium phosphate salts, but commercial suppliers cannot be found.

[0004] Currently, methods for preparing difluorophosphates are mainly divided into two categories. One is the solid-phase method, which involves mixing sodium fluoride and phosphorus pentoxide solids in a sealed tube and heating them. However, the yield of sodium difluorophosphate prepared by this method is only 10-20%, and the energy consumption is high, and the reaction is difficult to control. The other is the direct reaction method, which involves directly reacting difluorophosphoric acid with sodium carbonate to produce sodium difluorophosphate. However, this method has disadvantages such as the instability, high toxicity, high price of difluorophosphoric acid, and the difficulty in obtaining high-purity raw materials, making it unfavorable for industrial production.

[0005] In summary, the current preparation technology of sodium difluorophosphate is not yet mature, and the preparation process still has many problems, which prevents industrial promotion. Therefore, it is urgent to develop a preparation method of sodium difluorophosphate with simple process, efficient reaction and high product purity. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for preparing sodium difluorophosphate, so as to solve the problems of difficult reaction control, low yield and low purity in the preparation of sodium difluorophosphate in the prior art.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is:

[0008] A preparation method of sodium difluorophosphate comprises the following steps: reacting an oxygen-containing sodium salt and PF5 in a supercritical carbon dioxide fluid, wherein the generated NaF is insoluble in the supercritical carbon dioxide fluid and the sodium difluorophosphate is soluble in the supercritical carbon dioxide fluid; after the reaction, supercritical carbon dioxide fluid containing the sodium difluorophosphate is obtained; and then separation is performed to obtain the sodium difluorophosphate.

[0009] The present invention has conducted in-depth research for the above-mentioned purpose, and found that oxygen-containing sodium salts have poor solubility in organic solvents, especially ester solvents, resulting in gas-solid-liquid three-phase mixing, resulting in low sodium difluorophosphate yield, long reaction time, waste of raw materials and high energy consumption; in order to improve the solubility of oxygen-containing sodium salts, the present invention adopts supercritical carbon dioxide (SCO2) as a solvent. In SCO2, the solubility of oxygen-containing sodium salts is greatly improved. PF5 is introduced into the reaction, which increases the contact area between PF5 and the oxygen-containing sodium salt and improves the reaction efficiency. The present invention uses SCO2 to replace the reaction solvent, has simple steps, easy reaction control, and high yield; the sodium difluorophosphate prepared by the reaction of the present invention is then extracted and separated by SCO2, the process is pure and pollution-free, and the product purity is high.

[0010] Preferably, the separation is to convert carbon dioxide from a supercritical state into a liquid or gaseous state to cause crystallization of sodium difluorophosphate. Further preferably, carbon dioxide is converted from a supercritical state into a liquid state by cooling or a combination of cooling and pressure reduction. Carbon dioxide can be converted from a supercritical state into a liquid state to cause crystallization of sodium difluorophosphate by progressively lowering the temperature or by progressively lowering the temperature and pressure simultaneously.

[0011] Preferably, the temperature is lowered to -30°C to -10°C. Lowering the temperature to -30°C to -10°C while maintaining a pressure that keeps the carbon dioxide in a liquid state, such as 6 to 8 MPa, can further improve crystallization efficiency. The crystallization time is generally 4 to 6 hours.

[0012] In terms of raw material cost and reaction effect, preferably, the oxygen-containing sodium salt is any one of sodium carbonate, sodium phosphate, sodium hydroxide, sodium oxide, sodium peroxide, sodium dihydrogen phosphate and disodium hydrogen phosphate.

[0013] Those skilled in the art will appreciate that when using SCO2 as a solvent, the required reaction conditions are: the reaction environment is maintained above the critical temperature (35°C) and critical pressure (8 MPa) of CO2. Preferably, the reaction temperature is 35-70°C and the pressure is 8-30 MPa. Furthermore, the reactor temperature is 50-60°C and the pressure is 15-30 MPa.

[0014] Preferably, the reaction involves placing an oxygenated sodium salt in a reactor, introducing PF5 and carbon dioxide gas, and setting the reactor temperature and pressure to maintain a supercritical carbon dioxide state. The reactor is connected to an external circulation line, and the reaction circulates within the reactor-external circulation line, maintaining a supercritical carbon dioxide state during the reaction. This method achieves an internal circulation reaction, with PF5 circulating within the supercritical equipment, enabling full utilization of the PF5. Filters are installed at both the inlet and outlet of the reactor to block the generated NaF, ensuring that the NaF remains within the reactor.

[0015] Preferably, after the reaction, the reactor is connected to a separation kettle, and the supercritical carbon dioxide fluid containing sodium difluorophosphate is converted into a liquid state in the separation kettle and crystallized to precipitate sodium difluorophosphate. The recrystallization method is used to purify sodium difluorophosphate, which is environmentally friendly, low-cost, and has a good purification effect. The reaction and post-processing for preparing sodium difluorophosphate in the present invention are integrated into one, forming a full-process clean production process for sodium difluorophosphate. The entire production process is environmentally friendly and has low waste emissions.

[0016] To further ensure the yield of sodium difluorophosphate, preferably, during the reaction, PF5 is in excess relative to the oxygen-containing sodium salt. PF5 is 1.05 times or more of the theoretical amount, for example, 1.05 to 1.2 times.

[0017] The beneficial effects of the present invention are:

[0018] (1) The preparation method of the present invention uses phosphorus pentafluoride and oxygen-containing sodium salt as raw materials, SCO2 as solvent, and undergoes a circulation reaction in a supercritical device. The steps are simple, the reaction is easy to control, and the yield is high;

[0019] (2) The present invention integrates the reaction and post-treatment for preparing sodium difluorophosphate to form a clean production process for the entire process of sodium difluorophosphate, with good purification effect and high product purity. The entire production process is green and environmentally friendly, and no three wastes are generated. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic diagram of the supercritical carbon dioxide equipment used in the preparation of sodium difluorophosphate in the present invention. DETAILED DESCRIPTION

[0021] The technical solutions of the present invention are further described below with reference to specific embodiments, but the present invention is not limited to these embodiments.

[0022] 1. The specific embodiment of the preparation method of sodium difluorophosphate of the present invention is as follows:

[0023] Example 1

[0024] The preparation method of the sodium difluorophosphate of the present embodiment comprises the following steps:

[0025] The oxygen-containing sodium salt in this embodiment is sodium carbonate, and the reaction formula of sodium carbonate and phosphorus pentafluoride is shown in formula (1):

[0026] PF5+2Na2CO3→NaPO2F2+3NaF+2CO2↑ (1).

[0027] Schematic diagram of supercritical carbon dioxide equipment Figure 1 As shown, the inlet and outlet of the reactor are both equipped with filter screens.

[0028] (1) CO2 and PF5 (2 L, 0.09 mol) were mixed in a mixer by a pump and then introduced into a reactor containing sodium carbonate (18 g, 0.17 mol). V1 and V2 were opened, V3, V4, V5, and V6 were closed, and CO2 was continuously introduced. The CO2 flow rate was adjusted so that the reactor reached the set reaction pressure of 8 MPa. The temperature of the reactor was adjusted to 35°C. At this time, the carbon dioxide was in a supercritical state, so that the solid sodium carbonate in the reactor was dissolved and fully contacted with PF5. The reaction circulated in the reactor-mixer under the action of the pump. The carbon dioxide remained in a supercritical state during the reaction. Since the inlet and outlet of the reactor were both equipped with filter screens, the NaF generated by the reaction was insoluble in SCO2 and was blocked by the filter screen, so that the NaF was always in the reactor.

[0029] (2) After the reaction for 1 hour, close V2, open V3, and adjust valves V4, V5, and V6. The regulating valve V4 can make the separation kettle reach the set separation pressure. At this time, the reaction circulates in the reactor-separation kettle-mixer; the sodium difluorophosphate generated by the reaction is dissolved in SCO2, and the SCO2 containing sodium difluorophosphate enters the separation kettle. The temperature of the separation kettle is gradually lowered to -10°C, and the carbon dioxide is converted from a supercritical state to a liquid state. At the same time, sodium difluorophosphate is crystallized and precipitated. The crystallization time is 5 hours. The sodium difluorophosphate crystals are filtered and washed twice with a liquid carbon dioxide solvent. Then, after removing the carbon dioxide, 8.4 g of purified sodium difluorophosphate product is obtained with a yield of 80%.

[0030] In other embodiments, the oxygen-containing sodium salt can be adjusted to sodium phosphate, sodium oxide, or sodium peroxide.

[0031] Example 2

[0032] The preparation method of the sodium difluorophosphate of the present embodiment comprises the following steps:

[0033] The oxygen-containing sodium salt in this embodiment is sodium carbonate, and the schematic diagram of the supercritical carbon dioxide equipment is as follows: Figure 1 As shown, the inlet and outlet of the reactor are both equipped with filter screens.

[0034] (1) CO2 and PF5 (4 L, 0.18 mol) were mixed in a mixer by a pump and then introduced into a reactor containing sodium carbonate (32 g, 0.3 mol). V1 and V2 were opened, and V3, V4, V5, and V6 were closed. The CO2 flow rate was adjusted so that the reactor reached the set reaction pressure of 15 MPa. The temperature of the reactor was adjusted to 50°C. At this time, the carbon dioxide was in a supercritical state, so that the solid sodium carbonate in the reactor was dissolved and fully contacted with PF5. The reaction circulated in the reactor-mixer under the action of the pump. The carbon dioxide remained in a supercritical state during the reaction. Since the inlet and outlet of the reactor were both equipped with filters, the NaF generated by the reaction was insoluble in SCO2 and was blocked by the filter, so that the NaF remained in the reactor.

[0035] (2) After the reaction for 1 hour, close V2, open V3, and adjust valves V4, V5, and V6. The regulating valve V4 can make the separation kettle reach the set separation pressure. At this time, the reaction circulates in the reactor-separation kettle-mixer; the sodium difluorophosphate generated by the reaction is dissolved in SCO2, and the SCO2 containing sodium difluorophosphate enters the separation kettle. By gradually lowering the temperature of the separation kettle to -20°C and gradually lowering the pressure to 6MPa, the carbon dioxide is converted from a supercritical state to a liquid state, and sodium difluorophosphate is crystallized and precipitated. The crystallization time is 4 hours. After crystallization, the pressure is reduced to 5MPa, and sodium difluorophosphate crystals are obtained by filtration. The obtained crystals are washed twice with liquid carbon dioxide solvent, and then the carbon dioxide is removed to obtain 16g of purified sodium difluorophosphate product with a yield of 86%.

[0036] Example 3

[0037] The preparation method of the sodium difluorophosphate of the present embodiment comprises the following steps:

[0038] 1) CO2 and PF5 (4 L, 0.18 mol) were mixed in a mixer by a pump and then introduced into a reactor containing sodium carbonate (32 g, 0.3 mol). V1 and V2 were opened, and V3, V4, V5, and V6 were closed. The CO2 flow rate was adjusted to make the reactor reach the set reaction pressure of 30 MPa, and the temperature of the reactor was adjusted to 70°C. At this time, the carbon dioxide was in a supercritical state, so that the solid sodium carbonate in the reactor was dissolved and fully contacted with PF5. The reaction circulated in the reactor-mixer under the action of the pump, and the carbon dioxide remained in a supercritical state during the reaction process. Since the inlet and outlet of the reactor were both provided with filter screens, the NaF generated by the reaction was insoluble in SCO2 and blocked by the filter screen, so that the NaF was always in the reactor.

[0039] 2) After the reaction for 1 h, close V2, open V3, and adjust valves V4, V5, and V6, wherein the regulating valve V4 can make the separation kettle reach the set separation pressure. At this time, the reaction is circulated in the reactor-separation kettle-mixer; the sodium difluorophosphate generated by the reaction is dissolved in SCO2, and the SCO2 containing sodium difluorophosphate enters the separation kettle. The temperature of the separation kettle is gradually reduced to -15°C and the pressure is gradually reduced to 6 MPa, and the carbon dioxide is converted from a supercritical state to a liquid state. At the same time, sodium difluorophosphate is crystallized and precipitated. The crystallization time is 6 hours. After crystallization, the pressure is reduced to 5 MPa, and sodium difluorophosphate crystals are obtained by filtration. The obtained crystals are washed twice with a liquid carbon dioxide solvent, and then the carbon dioxide is removed to obtain 17.3 g of purified sodium difluorophosphate product with a yield of 93%.

[0040] Example 4

[0041] The preparation method of the sodium difluorophosphate of the present embodiment comprises the following steps:

[0042] The oxygen-containing sodium salt in this embodiment is sodium phosphate, and the reaction formula of sodium phosphate and phosphorus pentafluoride is shown in formula (2):

[0043] PF5+Na3PO4→2NaPO2F2+NaF (2).

[0044] In this example, the oxygen-containing sodium salt was adjusted to sodium phosphate (24 g, 0.15 mol) and PF5 (4 L, 0.18 mol). The remaining steps were similar to those in Example 3 to obtain 32 g of purified sodium difluorophosphate product with a yield of 86%.

[0045] Example 5

[0046] The preparation method of the sodium difluorophosphate of the present embodiment comprises the following steps:

[0047] The oxygen-containing sodium salt in this embodiment is sodium oxide, and the reaction formula of sodium oxide and phosphorus pentafluoride is shown in formula (3):

[0048] PF5+2Na2O→NaPO2F2+3NaF (3).

[0049] In this example, the oxygen-containing sodium salt was adjusted to sodium oxide (18 g, 0.3 mol) and PF5 (4 L, 0.18 mol). The remaining steps were similar to those in Example 3 to obtain 14.9 g of purified sodium difluorophosphate product with a yield of 80%.

[0050] 2. Experimental Examples

[0051] This experimental example tested the sodium difluorophosphate products prepared in Examples 1 to 5, and the specific detection methods are shown in Table 1.

[0052] Table 1 Test items, methods and instruments

[0053] project Detection method Testing instruments Main content Ion chromatography test Ion Chromatograph Moisture Loss on drying method GBT6284-2006 Moisture meter Free acid Acid-base titration Potentiometric titrator <![CDATA[Chloride ion (Cl - )]]> Ion chromatography test Ion Chromatograph / <![CDATA[Sulfate (SO4 2- )]]> Ion chromatography test Ion Chromatograph Metal ions Ion chromatography test Ion Chromatograph

[0054] The product test results of each embodiment are shown in Table 2.

[0055] Table 2 Test results of sodium difluorophosphate products obtained in Examples 1 to 3

[0056]

[0057]

[0058] The test results in Table 2 show that the sodium difluorophosphate product prepared by the method of the present invention has high purity and low impurity content, and can meet the application requirements of sodium battery electrolyte salt or additive.

[0059] The above describes the embodiments of the present invention in detail, but the present invention is not limited to the above embodiments. Various changes can be made within the knowledge of ordinary technicians in the relevant technical field without departing from the scope of the present invention.

Claims

1. A method for preparing sodium difluorophosphate, characterized in that, The following steps are involved: The method comprises placing an oxygen-containing sodium salt in a reactor, introducing PF5 and carbon dioxide gas, setting the temperature and pressure of the reactor so that the carbon dioxide is in a supercritical state, generating NaF that is insoluble in the supercritical carbon dioxide fluid, and dissolving sodium difluorophosphate in the supercritical carbon dioxide fluid, and obtaining a supercritical carbon dioxide fluid containing sodium difluorophosphate after the reaction; connecting the reactor to a separation kettle, and converting the supercritical carbon dioxide fluid containing sodium difluorophosphate into a liquid state in the separation kettle and crystallizing to precipitate sodium difluorophosphate; and the oxygen-containing sodium salt is any one of sodium carbonate, sodium phosphate, sodium hydroxide, sodium oxide, sodium peroxide, sodium dihydrogen phosphate and disodium hydrogen phosphate.

2. The preparation method of sodium difluorophosphate according to claim 1, wherein During the reaction, PF5 is in excess relative to the oxygen-containing sodium salt.

3. The preparation method of sodium difluorophosphate according to claim 1, wherein The carbon dioxide is converted from a supercritical state to a liquid state by cooling or by combining cooling and pressure reduction.

4. The preparation method of sodium difluorophosphate according to claim 3, wherein The cooling is to -30°C to -10°C.

5. The preparation method of the sodium difluorophosphate according to any one of claims 1 to 4, wherein The reaction temperature is 35-70° C. and the pressure is 8-30 MPa.

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