A method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate

Abstract

The present invention belongs to the technical field of lithium-ion battery materials, and specifically relates to a method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate. The synthesis method comprises the following steps: uniformly mixing lithium tripolyphosphate and phosphorus pentoxide in an ether solvent to obtain a first mixture; introducing a mixed gas of hydrogen fluoride and nitrogen into the first mixture for a mixing reaction to obtain a second mixture; performing solid-liquid separation on the second mixture, and concentrating the filtrate after the solid-liquid separation under reduced pressure, recrystallizing, and drying under reduced pressure to obtain lithium hexafluorophosphate. The present invention uses lithium tripolyphosphate, phosphorus pentoxide, and hydrogen fluoride as raw materials, and adopts a method of uniformly introducing hydrogen fluoride gas into the reaction to directly react and synthesize lithium hexafluorophosphate in an ether solvent. The method has the advantages of mild reaction conditions, high reaction efficiency, high product purity, and high yield. The method solves the problems of high by-products and high costs in the industrial production of lithium hexafluorophosphate.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium ion battery materials, and particularly relates to a method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate. Background Art

[0002] Hexafluorophosphoric acid and its derivatives (such as lithium hexafluorophosphate) have important applications in lithium-ion battery electrolytes and other fields. As a new type of mobile portable power source, lithium-ion batteries offer higher specific capacity and discharge voltage than traditional lead-acid and alkaline batteries, while also being less environmentally friendly. They are widely used in electric vehicles, energy storage systems, computers, communications, and consumer electronics. With the expansion of the new energy vehicle market and the widespread adoption of mobile devices and energy storage systems, demand for lithium-ion batteries and their key material, lithium hexafluorophosphate, is expected to continue to grow.

[0003] In existing technologies, lithium hexafluorophosphate (LFP) is typically prepared using methods such as gas-solid reaction, hydrogen fluoride (HF) solvent, organic solvent, and ion exchange. The gas-solid reaction method primarily involves treating lithium fluoride (LiF) with anhydrous hydrogen fluoride (HF) to form porous LiF. Phosphorus pentafluoride (PF5) gas is then introduced to react with the porous LiF to produce LFP. This method is relatively simple to operate, but requires a dry inert gas atmosphere, requires strict equipment sealing, and the reaction occurs only on the solid surface. This method suffers from drawbacks such as low conversion rate and low product purity. The HF solvent method is the most readily scalable method. It first reacts anhydrous HF with a phosphorus source (such as phosphorus pentoxide) to produce phosphorus pentafluoride. This phosphorus pentafluoride then reacts with an anhydrous HF solution of lithium fluoride to produce lithium hexafluorophosphate. Purification and other steps are then performed to obtain a high-purity product. This method operates in the liquid phase, is easy to control, and is suitable for large-scale production. However, it places high demands on the corrosion resistance and safety of the equipment, consumes a lot of energy, and is difficult to completely remove residual HF in the product, affecting its purity. The organic solvent method uses an organic solvent (such as propylene carbonate or acetonitrile) as the reaction medium, avoiding the use of highly corrosive hydrogen fluoride. However, this method suffers from low reaction efficiency, difficulty achieving high product purity, and potential side reactions between the organic solvent and the reactants. The ion exchange method produces lithium hexafluorophosphate by reacting hexafluorophosphate (such as NaPF6 or KPF6) with a lithium-containing compound (such as LiCl) in an organic solvent. This one-step reaction is simple to operate, but the product purity is low, impurities are easily introduced, and the cost is high. Therefore, developing an efficient, low-cost, and environmentally friendly method for preparing lithium hexafluorophosphate is of great industrial significance. Summary of the Invention

[0004] In response to the shortcomings and deficiencies of the above-mentioned prior art, the present invention aims to provide a method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate. The method of the present invention utilizes lithium tripolyphosphate, phosphorus pentoxide, and hydrogen fluoride gas in an ether solvent to synthesize lithium hexafluorophosphate in a single step, thereby resolving the problems of high byproducts and high costs in the production of lithium hexafluorophosphate.

[0005] The purpose of the present invention is achieved through the following technical solutions:

[0006] A method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate comprises the following steps:

[0007] Mixing lithium tripolyphosphate and phosphorus pentoxide in an ether solvent to obtain a first mixture;

[0008] introducing a mixed gas of hydrogen fluoride and nitrogen into the first mixture for a mixing reaction to obtain a second mixture;

[0009] The second mixture is subjected to solid-liquid separation, and the filtrate after the solid-liquid separation is concentrated under reduced pressure, recrystallized, and dried under reduced pressure to obtain lithium hexafluorophosphate.

[0010] Furthermore, in the above synthesis method, the purity of the lithium tripolyphosphate used is ≥99%, and the purity of the hydrogen fluoride is ≥99%.

[0011] Furthermore, in the above synthesis method, the ether solvent includes at least one of ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, diethylene glycol methyl ethyl ether, and diethylene glycol diethyl ether.

[0012] Furthermore, in the above synthesis method, the mass ratio of the lithium tripolyphosphate to the ether solvent is 1:5~50.

[0013] Furthermore, in the above-mentioned synthesis method, the molar ratio of the lithium tripolyphosphate to the phosphorus pentoxide is 1:1 to 5, more preferably 1:2 to 4. Excess phosphorus pentoxide can promote the conversion of lithium tripolyphosphate, reduce residual difficult-to-separate lithium phosphate in the product, and improve product purity. It can also serve as a dehydrating agent during the reaction, reducing the occurrence of side reactions and the moisture content in the product, thereby improving product yield and quality.

[0014] Furthermore, in the above synthesis method, the volume ratio of the hydrogen fluoride to the nitrogen in the mixed gas is 1:3~6.

[0015] Furthermore, in the above synthesis method, the molar ratio of the amount of hydrogen fluoride introduced to the lithium tripolyphosphate is 30-50:1; more preferably 30-40:1.

[0016] Furthermore, in the above synthesis method, at least one of the following conditions is also satisfied:

[0017] Before uniformly mixing lithium tripolyphosphate and phosphorus pentoxide in the ether solvent, the method further comprises: performing a dehydration pretreatment on the ether solvent;

[0018] The lithium tripolyphosphate and phosphorus pentoxide are uniformly mixed in an ether solvent in an inert gas;

[0019] The temperature of the mixed reaction is 20-90° C., the time for introducing the mixed gas is 0.5-4 h, and the time for the mixed reaction is 0.5-1 h.

[0020] Furthermore, in the above-mentioned synthesis method, the temperature for the reduced pressure concentration and the reduced pressure drying is 40-60°C, and the pressure for the reduced pressure concentration and the reduced pressure drying is -0.095--0.1 MPa; and the recrystallization solvent is an ether solvent. Recrystallization can reduce the moisture content of the lithium hexafluorophosphate product, preventing hydrolysis of the lithium hexafluorophosphate during the subsequent drying process. By using the same ether solvent as the reaction solvent for recrystallization, no other organic phase is introduced, and the recrystallization solvent can be reused as the reaction solvent after a simple dehydration pretreatment.

[0021] Furthermore, in the above synthesis method, the purity of the obtained lithium hexafluorophosphate is ≥99.5%, the moisture content is ≤20 ppm, and the acid value is ≤30 ppm.

[0022] The principle of the present invention is: the reaction equation involved in synthesizing lithium hexafluorophosphate using lithium tripolyphosphate, phosphorus pentoxide and hydrogen fluoride is as follows:

[0023] Li5P3O 10 + 30HF + P2O5→ 5LiPF6+ 15H2O;

[0024] P2O5 + 3H2O→2H3PO4.

[0025] By using ethers with high solubility for the product lithium hexafluorophosphate and inertness to the reaction raw materials as reaction solvents, lithium tripolyphosphate, phosphorus pentoxide, and a mixed gas of hydrogen fluoride and nitrogen are reacted under anhydrous conditions, which can significantly improve the reaction efficiency and reduce the occurrence of side reactions. After the reaction is completed, the unreacted solid raw materials and insoluble by-products are removed by filtration, and then high-purity lithium hexafluorophosphate is obtained through reduced pressure concentration, recrystallization, and reduced pressure drying.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] (1) The present invention uses lithium tripolyphosphate, phosphorus pentoxide and hydrogen fluoride as raw materials to directly react and synthesize lithium hexafluorophosphate in an ether solvent. It has the advantages of simple reaction conditions, high reaction efficiency and high yield, and significantly reduces production costs.

[0028] (2) The present invention uses anhydrous ethers as reaction solvents, which have better solubility for the reaction products. At the same time, the method of uniformly introducing hydrogen fluoride gas into the reaction can significantly reduce the corrosion of reaction equipment and the occurrence of side reactions compared with the existing high-concentration hydrofluoric acid solution reaction system. It has the advantages of mild reaction conditions, high reaction efficiency, high product purity and high yield. DETAILED DESCRIPTION

[0029] The present invention will be further described in detail below with reference to examples, but the embodiments of the present invention are not limited thereto.

[0030] Example 1

[0031] A method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate, comprising the following steps:

[0032] Under inert gas, a reactor was charged with 0.1 mol of lithium tripolyphosphate (purity ≥99%), 0.1 mol of phosphorus pentoxide, and 300 g of dehydrated ethylene glycol dimethyl ether solvent. A 1:3 (volume ratio) mixture of hydrogen fluoride (purity ≥99%) and nitrogen was uniformly introduced through a gas distributor while stirring. The molar ratio of hydrogen fluoride to lithium tripolyphosphate was 30:1. The reaction temperature was maintained between 20 and 30°C, and the aeration time was 2 hours. After the aeration period was complete, the reaction was maintained at this temperature for 0.5 hours. After the reaction was complete, the reaction solution was depressurized to -0.095 MPa, and nitrogen was introduced to remove the gaseous phase. The reaction solution was filtered to remove unreacted solid starting materials and insoluble byproducts. The filtrate was concentrated under reduced pressure (-0.1 MPa, 60°C) to obtain a lithium hexafluorophosphate solution. Ethylene glycol dimethyl ether was then added for recrystallization. The crystalline product was dried under reduced pressure (-0.1 MPa, 60°C) to obtain a solid lithium hexafluorophosphate product.

[0033] The lithium hexafluorophosphate product obtained in this example was tested to have a purity of 99.63% (detected by ion chromatography), a moisture content of 20 ppm (detected by Karl Fischer method), and an acid value of 17 ppm (detected by potentiometric titration); the product yield was 80.30% (actual mass of the lithium hexafluorophosphate product obtained / theoretical mass of the lithium hexafluorophosphate product obtained after complete reaction of lithium tripolyphosphate).

[0034] Example 2

[0035] In this example, the dosage of phosphorus pentoxide was adjusted to 0.05 mol, 0.2 mol, 0.3 mol, 0.4 mol and 0.5 mol respectively, and the other conditions were the same as those in Example 1.

[0036] The test results and yields of the lithium hexafluorophosphate product obtained in this example under different phosphorus pentoxide dosages are shown in Table 1 below.

[0037] Table 1 Detection results and yield results

[0038] Phosphorus pentoxide dosage purity,% Moisture, ppm Acid value, ppm Yield, % 0.05mol 99.05 28 15 67.93 0.2 mol 99.87 19 18 89.14 0.3 mol 99.92 15 23 92.25 0.4 mol 99.96 13 27 94.76 0.5 mol 99.95 12 32 95.13

[0039] The results in Table 1 show that increasing the phosphorus pentoxide dosage can improve product purity and yield while reducing product moisture content. This is because excess phosphorus pentoxide promotes the conversion of lithium tripolyphosphate, reducing the residual, difficult-to-separate lithium phosphate in the product. Unreacted phosphorus pentoxide can be separated by filtration in ether solvents, improving product purity. Furthermore, phosphorus pentoxide acts as a dehydrating agent during the reaction, reducing side reactions and lowering product moisture content, thereby improving product yield and quality. However, excessive phosphorus pentoxide dosage can result in a higher product acid value, likely due to residual phosphoric acid after reaction with water.

[0040] Example 3

[0041] In this embodiment, the molar ratios of the amount of hydrogen fluoride introduced to lithium tripolyphosphate were adjusted to 35:1, 40:1, 45:1 and 50:1, respectively. The other conditions were the same as those in Example 1.

[0042] The test results and yields of the lithium hexafluorophosphate product obtained in this example under different hydrogen fluoride feeding amounts are shown in Table 2 below.

[0043] Table 2 Detection results and yield results

[0044] Hydrogen fluoride intake purity,% Moisture, ppm Acid value, ppm Yield, % 35:1 99.75 21 19 85.22 40:1 99.90 20 26 90.56 45:1 99.89 17 31 91.30 50:1 99.82 16 36 91.32

[0045] The results in Table 2 show that increasing the amount of hydrogen fluoride introduced increases the yield of the product, while the product purity first increases and then decreases. This may be because excessive hydrogen fluoride increases the formation of by-products and also leads to a higher acid value in the product.

[0046] Example 4

[0047] In this embodiment, the reaction temperature is adjusted to 40-50°C, 60-70°C, and 80-90°C, respectively, and the other conditions are the same as those in Example 1.

[0048] The test results and yields of the lithium hexafluorophosphate product obtained at different reaction temperatures in this example are shown in Table 3 below.

[0049] Table 3 Test results and yield results

[0050] Reaction temperature purity,% Moisture, ppm Acid value, ppm Yield, % 40~50℃ 99.70 18 20 82.36 60~70℃ 99.62 17 23 82.88 80~90℃ 99.54 15 28 83.51

[0051] From the results in Table 3, it can be seen that as the reaction temperature increases, the product yield increases slightly, the product purity decreases slightly; the product moisture decreases slightly, and the product acid value increases slightly.

[0052] Example 5

[0053] In this embodiment, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, diethylene glycol methyl ethyl ether, and diethylene glycol diethyl ether are used to replace the ethylene glycol dimethyl ether solvent, and the other conditions are the same as those in Example 1.

[0054] The test results and yields of the lithium hexafluorophosphate product obtained in this example under different reaction solvent conditions are shown in Table 4 below.

[0055] Table 4 Detection results and yield results

[0056] Reaction solvent purity,% Moisture, ppm Acid value, ppm Yield, % Ethylene glycol diethyl ether 99.69 15 16 81.79 Diethylene glycol dimethyl ether 99.72 19 17 80.93 Diethylene glycol methyl ethyl ether 99.80 18 15 81.73 Diethylene glycol diethyl ether 99.75 17 17 81.64

[0057] As can be seen from the results in Table 4, the present invention can achieve good product yield and product quality by using ether solvents that have high solubility for the product lithium hexafluorophosphate and are inert to the reaction raw materials.

[0058] Comparative Example 1

[0059] A method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate, compared with Example 1, uses hydrofluoric acid aqueous solution as the reaction system, and the specific steps are as follows:

[0060] Under inert gas, 0.1 mol of lithium tripolyphosphate (≥99% purity) and 0.2 mol of phosphoric acid were added to a reactor containing a hydrofluoric acid aqueous solution and stirred until uniformly mixed. The molar ratio of hydrogen fluoride to lithium tripolyphosphate in the hydrofluoric acid aqueous solution was 30:1. The reaction temperature was maintained between 20 and 30°C for 2.5 hours. After the reaction was complete, the reaction solution was depressurized to -0.095 MPa, and nitrogen was introduced to remove the gaseous phase. The reaction solution was filtered to remove unreacted solid starting materials and insoluble byproducts. The filtrate was concentrated under reduced pressure (-0.1 MPa, 60°C) to obtain a lithium hexafluorophosphate solution. Ethylene glycol dimethyl ether was then added for recrystallization. The crystalline product was dried under reduced pressure (-0.1 MPa, 60°C) to obtain a solid lithium hexafluorophosphate product.

[0061] The lithium hexafluorophosphate product obtained in this comparative example was tested to have a purity of 99.41%, a moisture content of 27 ppm, and an acid value of 54 ppm; the product yield was 76.26%.

[0062] By comparing the results of this comparative example with those of Example 1, it can be seen that the method of synthesizing lithium hexafluorophosphate by a one-step reaction of lithium tripolyphosphate, phosphorus pentoxide and hydrogen fluoride gas in an ether solvent according to the present invention can significantly improve the reaction efficiency and reduce the occurrence of side reactions compared to the existing method of reacting in a hydrofluoric acid aqueous solution, thereby improving the product yield and purity, and significantly reducing the acid value and residual moisture of the product.

[0063] The above embodiments are preferred implementations of the present invention, but the implementations of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate, characterized in that: The steps include: Mixing lithium tripolyphosphate and phosphorus pentoxide in an ether solvent to obtain a first mixture; introducing a mixed gas of hydrogen fluoride and nitrogen into the first mixture for a mixing reaction to obtain a second mixture; performing solid-liquid separation on the second mixture, and concentrating the filtrate after the solid-liquid separation under reduced pressure, recrystallizing, and drying under reduced pressure to obtain lithium hexafluorophosphate; The molar ratio of the lithium tripolyphosphate to the phosphorus pentoxide is 1:2-4.

2. The method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate according to claim 1, wherein: The purity of the lithium tripolyphosphate is ≥99%, and the purity of the hydrogen fluoride is ≥99%.

3. The method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate according to claim 1, wherein: The ether solvent includes at least one of ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, diethylene glycol methyl ethyl ether, and diethylene glycol diethyl ether; the mass ratio of the lithium tripolyphosphate to the ether solvent is 1:5~50.

4. The method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate according to claim 1, wherein: The volume ratio of the hydrogen fluoride to the nitrogen in the mixed gas is 1:3-6.

5. The method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate according to claim 1, wherein: The molar ratio of the amount of hydrogen fluoride introduced to the lithium tripolyphosphate is 30-50:

1.

6. The method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate according to claim 1, wherein: At least one of the following conditions is also met: Before uniformly mixing lithium tripolyphosphate and phosphorus pentoxide in the ether solvent, the method further comprises: performing a dehydration pretreatment on the ether solvent; The lithium tripolyphosphate and phosphorus pentoxide are uniformly mixed in an ether solvent in an inert gas; The temperature of the mixed reaction is 20-90° C., the time for introducing the mixed gas is 0.5-4 h, and the time for the mixed reaction is 0.5-1 h.

7. The method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate according to claim 1, characterized in that: The temperature of the reduced pressure concentration and the reduced pressure drying is 40-60° C., the pressure of the reduced pressure concentration and the reduced pressure drying is -0.095-0.1 MPa; and the recrystallization solvent is an ether solvent.

8. The method for synthesizing lithium hexafluorophosphate from lithium tripolyphosphate according to claim 1, wherein: The purity of the obtained lithium hexafluorophosphate is ≥99.5%, the moisture content is ≤20ppm, and the acid value is ≤30ppm.