A method for preparing sodium hexafluorophosphate for sodium batteries from sodium hydroxide

Abstract

The application provides a method for preparing sodium hexafluorophosphate for sodium batteries from sodium bifluoride, which comprises the following steps: taking electronic-grade sodium hydroxide and electronic-grade hydrofluoric acid as raw materials to generate an intermediate product sodium fluoride, and then reacting the intermediate product with phosphorus pentafluoride to obtain high-purity sodium hexafluorophosphate for sodium batteries. The particle size distribution and yield of sodium fluoride crystals are controlled by adjusting the stirring rate and cooling conditions, and then the purity and yield of sodium hexafluorophosphate are controlled, so that the purity of the finally obtained sodium hexafluorophosphate can reach more than 99.8%, the yield can reach more than 85%, the sodium hexafluorophosphate has the advantages of low cost and high yield, and is beneficial to industrialized production.

Description

Technical fields:

[0001] This invention relates to the field of battery materials technology, and more specifically to a method for preparing sodium hexafluorophosphate for sodium batteries using sodium hydroxide. Background technology:

[0002] Lithium hexafluorophosphate (LiPF6) is a commonly used electrolyte material for lithium-ion batteries. The rapid increase in demand for various types of lithium batteries has led to overexploitation of lithium resources and a continuous increase in costs. LiPF6 can be prepared using methods such as gas-solid reaction, anhydrous HF solvent method, organic solvent method, and ion exchange method. Using hydrofluoric acid as a solvent is currently the main process in use.

[0003] Sodium hexafluorophosphate has a similar structure and properties to lithium hexafluorophosphate. Sodium-ion batteries prepared using sodium hexafluorophosphate as an electrolyte can, to some extent, replace lithium-ion batteries. The production process is also similar, and it can be obtained by reacting sodium fluoride (NaF) and phosphorus pentafluoride (PF5) under anhydrous hydrofluoric acid.

[0004] Patent CN1212264C discloses a method for preparing hexafluorophosphate, comprising the following steps: pretreatment of anhydrous hydrogen fluoride, synthesis of hexafluorophosphate, low-temperature solid-liquid separation, and drying. Example 3 describes the synthesis of sodium hexafluorophosphate by mixing dried solid sodium fluoride and pretreated anhydrous hydrogen fluoride, stirring thoroughly to dissolve the dried solid fluoride, then adding phosphorus pentafluoride, and continuing stirring until the synthesis reaction is complete, with a yield of 75% and a purity of 99.5%. Compared with the method used to prepare lithium hexafluorophosphate, the yield and purity are not as high.

[0005] Patent CN 114751431A discloses a method for preparing sodium salts for sodium batteries, which includes the following steps: polyvinyl alcohol and sodium fluoride are mixed uniformly, deionized water is added, and the mixture is dried and calcined to obtain porous sodium fluoride; phosphorus pentafluoride is passed into a mixture of porous sodium fluoride and liquid hydrogen fluoride to react and obtain a sodium hexafluorophosphate solution; the sodium hexafluorophosphate solution is crystallized, filtered, and dried to obtain sodium hexafluorophosphate. Using porous sodium fluoride as the sodium source to prepare sodium hexafluorophosphate can improve the utilization rate of the precursor and effectively increase the yield. However, this method requires the introduction of polyvinyl alcohol as an auxiliary material and high-temperature calcination, resulting in high costs.

[0006] Therefore, there is an urgent need for a method to prepare sodium hexafluorophosphate for sodium batteries that can further improve purity and yield. Summary of the Invention:

[0007] This invention aims to provide a method for preparing sodium hexafluorophosphate for sodium batteries using sodium hydroxide, thereby overcoming the problems of low yield, low purity, and high production cost of sodium hexafluorophosphate produced in the prior art.

[0008] To achieve the above objectives, the technical approach adopted by this invention is as follows:

[0009] Electronic-grade sodium hydroxide and electronic-grade hydrofluoric acid are reacted to produce high-purity electronic-grade sodium fluoride; then, using the obtained high-purity electronic-grade sodium fluoride as a raw material, it is reacted with phosphorus pentafluoride under certain conditions to produce high-purity sodium hexafluorophosphate for sodium batteries.

[0010] The technical solution of the present invention includes the following steps:

[0011] (1) At 30-35℃, prepare an electronic grade sodium hydroxide solution and add it to the polytetrafluoroethylene reactor. Gradually add an electronic grade hydrofluoric acid solution to the reactor. The reaction is carried out at 60-80℃ and 300-500 rpm / min. When the pH is adjusted to 6.5-7, stop adding hydrofluoric acid solution. When the pH is detected to be >7.5, continue adding hydrofluoric acid solution. Repeat the pH adjustment reaction steps to ensure that the pH of the final solution is maintained at 6.5-7.

[0012] (2) The obtained reaction mixture is rapidly cooled to 10-20℃, filtered to obtain wet sodium fluoride, and the wet sodium fluoride is dehydrated and dried at 80-100℃ for 4-6 hours to obtain sodium fluoride crystals.

[0013] (3) Continuously introduce phosphorus pentafluoride, electronic grade hydrofluoric acid and sodium fluoride obtained in step (2) and react them fully at 10-15℃ for 3-5 hours. Add deionized water, stir to dissolve and filter. Stir the filtrate at 0-5℃ for 2-4 hours to crystallize.

[0014] (4) Dry the precipitated crystals at 110-130℃ for 2-4 hours to obtain sodium hexafluorophosphate for sodium batteries.

[0015] Preferably, in step (1), the purity of the electronic-grade sodium hydroxide selected in step (1) is above 99.9%, and the purity of the electronic-grade hydrofluoric acid selected is above 99.95%.

[0016] Preferably, the concentration of the sodium hydroxide aqueous solution in step (1) is 40-100 g / L.

[0017] Preferably, in step (1), the electronic-grade hydrofluoric acid is prepared into a solution with a mass fraction of 30%-50%.

[0018] Preferably, the rapid cooling time in step (2) is 5-20 minutes.

[0019] Preferably, in steps (2) and (3), the filtration uses a polytetrafluoroethylene filter membrane with a pore size of 0.5-1 μm, and the filtration method is negative pressure vacuum filtration with a pressure of -0.1 MPa.

[0020] Preferably, in step (3), the molar ratio of phosphorus pentafluoride to electronic-grade hydrofluoric acid is 1:1-2; and the molar ratio of phosphorus pentafluoride to sodium hydroxide in step (1) is 1.5-2:1.

[0021] In this invention, by controlling the rotation speed, cooling crystallization temperature, and conditions during the synthesis of sodium fluoride, a high yield of sodium fluoride can be ensured, while also allowing the sodium fluoride crystals to have a suitable particle size and distribution, thereby improving the yield and productivity of sodium hexafluorophosphate. This is achieved by controlling the reaction temperature between phosphorus pentafluoride and sodium fluoride.

[0022] After testing, the sodium hexafluorophosphate prepared by this invention has a yield of over 85% and a purity of over 99.8%, both meeting electronic grade standards.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] 1. This invention controls the reaction efficiency and crystallization process by controlling the rotation speed, cooling crystallization temperature and conditions during the synthesis of sodium fluoride, and finally obtains sodium fluoride intermediate product with small and uniform particle size.

[0025] 2. Phosphorus pentafluoride and sodium fluoride react at a lower temperature, keeping the introduced hydrofluoric acid in a liquid state and dissolving some of the sodium fluoride, thereby increasing the reaction area and improving the reaction efficiency. Detailed implementation method:

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention are further described below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely for the purpose of helping to understand the technical content and effects of this invention and should not be considered as limitations on this invention.

[0027] The sources of some components in the following examples and comparative examples are as follows:

[0028] Electronic-grade sodium hydroxide and electronic-grade hydrofluoric acid were purchased from Aladdin Reagent Network.

[0029] Phosphorus pentafluoride was purchased from Shanghai Biyang Industrial Co., Ltd.

[0030] The above description only indicates that the raw materials used in the embodiments or comparative examples of this invention were purchased from mainstream manufacturers in the market, and does not mean that the raw materials must be produced by the aforementioned manufacturers. As long as the raw materials are conventional, they can achieve the expected effect, and there are no special limitations. Where specific conditions or operating methods are not described in this embodiment or comparative example, commonly used conditions or operating methods in the art can be followed.

[0031] Example 1

[0032] (1) At 30℃, prepare 1L of 40g / L electronic grade sodium hydroxide solution and add it to the polytetrafluoroethylene reactor. Gradually add 40% electronic grade hydrofluoric acid solution to the reactor. The reaction is carried out at 60℃ and 300rpm / min. When the pH is adjusted to 7, stop adding hydrofluoric acid solution. When the pH is detected to be >7.5, continue adding hydrofluoric acid solution. Repeat the pH adjustment reaction steps to ensure that the pH of the final solution is maintained at 7.

[0033] (2) The obtained reaction mixture was cooled to 10°C within 20 min and filtered through a 0.5 μm polytetrafluoroethylene filter membrane at -0.1 MPa to obtain sodium fluoride wet material. After dehydrating and drying the sodium fluoride wet material at 80°C for 6 hours, sodium fluoride crystals were obtained.

[0034] (3) Phosphorus pentafluoride, electronic grade hydrofluoric acid and sodium fluoride obtained in step (2) are continuously introduced and reacted at 10°C for 5 hours. The molar ratio of phosphorus pentafluoride to sodium hydroxide in step (1) is 2:1; the molar ratio of phosphorus pentafluoride to electronic grade hydrofluoric acid is 1:1; deionized water is added and stirred to dissolve. The solution is filtered through a 0.5μm polytetrafluoroethylene filter membrane at -0.1MPa. The filtrate is stirred and crystallized at 0°C for 2 hours.

[0035] (4) The precipitated crystals were dried at 110°C for 4 hours to obtain the final sodium hexafluorophosphate product.

[0036] Example 2

[0037] (1) At 35℃, prepare 1L of 100g / L electronic grade sodium hydroxide solution and add it to the polytetrafluoroethylene reactor. Gradually add 50% electronic grade hydrofluoric acid solution to the reactor. React at 80℃ and 500rpm / min. When the pH is adjusted to 6.5, stop adding hydrofluoric acid solution. When the pH is detected to be >7.5, continue adding hydrofluoric acid solution. Repeat the pH adjustment reaction steps to ensure that the pH of the final solution is maintained at 6.5.

[0038] (2) The obtained reaction mixture was cooled to 20°C within 10 min and filtered through a 0.8 μm polytetrafluoroethylene filter membrane at -0.1 MPa to obtain sodium fluoride wet material. After dehydrating and drying the sodium fluoride wet material at 100°C for 4 hours, sodium fluoride crystals were obtained.

[0039] (3) Phosphorus pentafluoride, electronic grade hydrofluoric acid and sodium fluoride obtained in step (2) are continuously introduced and reacted fully at 10°C for 5 hours. The molar ratio of phosphorus pentafluoride to sodium hydroxide in step (1) is 1.5:1; the molar ratio of phosphorus pentafluoride to electronic grade hydrofluoric acid is 1:2; deionized water is added and stirred to dissolve. The solution is filtered through a 0.8μm polytetrafluoroethylene filter membrane at -0.1MPa. The filtrate is stirred and crystallized at 5°C for 4 hours.

[0040] (4) The precipitated crystals were dried at 130°C for 2 hours to obtain the final sodium hexafluorophosphate product.

[0041] Comparative Example 1

[0042] Compared with Example 1, the difference is that the stirring rate in step (1) is adjusted to 100 rpm / min.

[0043] Comparative Example 2

[0044] Compared with Example 1, the only difference is that the reaction temperature in step (3) is 30°C.

[0045] Comparative Example 3

[0046] Compared with Example 1, the only difference is that step (2) is not cooled, but filtered after standing for 20 minutes.

[0047] The sodium hexafluorophosphate prepared in the above examples and comparative examples was weighed, its actual yield was calculated, and its purity was tested. The test results are shown in Table 1 below.

[0048] Table 1. Actual yield and purity test results of the examples and comparative samples

[0049] Actual yield (%) purity(%) Example 1 90.28 99.91 Example 2 89.41 99.85 Comparative Example 1 81.96 99.67 Comparative Example 2 85.07 98.23 Comparative Example 3 84.79 99.13

[0050] As shown in Table 1, the sodium hexafluorophosphate obtained by this invention has a purity of over 99.8% and an actual yield of over 85%.

[0051] Comparing Example 1 and Comparative Example 1, the stirring rate of Comparative Example 1 was lower, which resulted in an incomplete reaction for the synthesis of sodium fluoride, ultimately leading to a lower yield of sodium fluoride and thus affecting the purity and yield of the final product, sodium hexafluorophosphate.

[0052] Comparing Example 1 and Comparative Example 2, the reaction of phosphorus pentafluoride and sodium fluoride at a lower temperature keeps the added electronic-grade hydrofluoric acid in a liquid state, dissolving some of the sodium fluoride. This increases the reaction area and improves the reaction efficiency. Under the conditions of Comparative Example 2, some of the electronic-grade hydrofluoric acid volatilizes, affecting the final reaction efficiency.

[0053] Comparing Example 1 and Comparative Example 3, the rapid cooling step leads to a rapid decrease in supersaturation, resulting in the precipitation of sodium fluoride, an intermediate product with smaller and more uniform particle size, which can further improve the efficiency of synthesizing sodium hexafluorophosphate.

[0054] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A method for preparing sodium hexafluorophosphate from sodium hydroxide for sodium batteries, characterized by, The method comprises the following steps: (1) configuring electronic grade sodium hydroxide solution at 30-35℃ and adding into a polytetrafluoroethylene reactor, gradually adding electronic grade hydrofluoric acid solution into the reactor, and performing reaction at 60-80℃ and 300-500 rpm / min, adjusting PH to 6.5-7, stopping adding hydrofluoric acid solution, and continuing adding hydrofluoric acid solution when PH>7.5 is detected, repeating the PH adjustment reaction step, and ensuring that the PH of the final solution is maintained at 6.5-7; (2) rapidly cooling the obtained reaction mixture to 10-20℃, and filtering to obtain sodium fluoride wet material, and dehydrating and drying the sodium fluoride wet material at 80-100℃ for 4-6 hours to obtain sodium fluoride crystals; (3) continuously introducing phosphorus pentafluoride, electronic grade hydrofluoric acid and sodium fluoride obtained in step (2) into the reactor at 10-15℃ and fully reacting for 3-5 hours, adding deionized water and stirring to dissolve and filter, and stirring the filtrate at 0-5℃ for 2-4 hours to crystallize; the molar ratio of phosphorus pentafluoride to electronic grade hydrofluoric acid is 1:1-2; the molar ratio of phosphorus pentafluoride to sodium hydroxide in step (1) is 1.5-2:1; (4) drying the crystallized product at 110-130℃ for 2-4 hours to obtain sodium battery grade sodium hexafluorophosphate product.

2. The method of claim 1, wherein, The purity of the electronic grade sodium hydroxide selected in step (1) is above 99.9%, and the purity of the electronic grade hydrofluoric acid selected is above 99.95%.

3. The method of claim 1, wherein, The concentration of the electronic grade sodium hydroxide aqueous solution in step (1) is 40-100 g / L.

4. The method of claim 1, wherein, The electronic grade hydrofluoric acid is configured into a solution with a mass fraction of 30%-50% in step (1).

5. The method of claim 1, wherein, The time for rapidly cooling to 10-20℃ in step (2) is 5-20 min.

6. The method of claim 1, wherein, The filtration in steps (2) and (3) both use polytetrafluoroethylene filter membranes with a pore size of 0.5-1 μm, and the filtration mode is negative pressure vacuum filtration with a pressure of-0.1 MPa.