Method for preparing sodium hexafluorophosphate, and sodium-ion battery electrolyte

Abstract

Disclosed in the present disclosure are a method for preparing sodium hexafluorophosphate, and a sodium-ion battery electrolyte. The method for preparing sodium hexafluorophosphate comprises: adding sodium fluoride into an organic solvent, and introducing phosphorus pentafluoride to cause the phosphorus pentafluoride to react with the sodium fluoride; filtering the reaction product to obtain a filtrate; evaporating and concentrating the filtrate at a first temperature so that a mass content of the sodium hexafluorophosphate in the filtrate is 20% to 60%; crystallizing and filtering the evaporated and concentrated filtrate at a second temperature to obtain a filter cake and a sodium hexafluorophosphate concentrate product, wherein the second temperature is lower than the first temperature; and drying the filter cake to obtain a sodium hexafluorophosphate solid product.

Description

Methods for preparing sodium hexafluorophosphate and sodium-ion battery electrolyte

[0001] Cross-reference to related applications

[0002] This disclosure claims priority to Chinese patent application No. 202411724532.X, filed on November 28, 2024, the entirety of which is incorporated herein by reference. Technical Field

[0003] This disclosure relates to the field of batteries, and more specifically, to a method for preparing sodium hexafluorophosphate and a sodium-ion battery electrolyte. Background Technology

[0004] Sodium-ion batteries are rechargeable batteries that rely on the reversible migration of sodium ions between the positive and negative electrodes to achieve charging and discharging. Compared to lithium hexafluorophosphate, which is currently widely used in commercial applications, sodium hexafluorophosphate has the following advantages: sodium content in the Earth's crust is about 3%, while lithium content is only 0.065%; in terms of distribution, sodium is more evenly distributed than lithium; in addition, sodium batteries are safer and have a wider standard voltage range.

[0005] The scarcity of lithium resources is likely to become a significant factor limiting the future development of lithium hexafluorophosphate (LiPF6), while sodium-ion batteries require abundant sodium resources and offer high safety performance. Sodium hexafluorophosphate, as an electrolyte material for sodium-ion batteries, has become a hot research topic, and the preparation of high-purity sodium hexafluorophosphate is crucial for the development of sodium-ion batteries.

[0006] However, the current methods for preparing high-purity sodium hexafluorophosphate still need improvement. Summary of the Invention

[0007] This disclosure is intended to at least partially alleviate or resolve at least one of the aforementioned problems.

[0008] In one aspect of this disclosure, a method for preparing sodium hexafluorophosphate is provided. In some embodiments of this disclosure, the method for preparing sodium hexafluorophosphate includes: adding sodium fluoride to an organic solvent, introducing phosphorus pentafluoride to react the phosphorus pentafluoride with the sodium fluoride; filtering the reaction product to obtain a filtrate; evaporating and concentrating the filtrate at a first temperature to obtain a sodium hexafluorophosphate content of 20%–60% by mass; crystallizing the concentrated filtrate at a second temperature, filtering to obtain a filter cake and a concentrated sodium hexafluorophosphate product, wherein the second temperature is lower than the first temperature; and drying the filter cake to obtain a solid sodium hexafluorophosphate product. Therefore, evaporation and concentration can remove some of the solvent and impurities from the reaction raw materials. By evaporation and concentration, the mass content of sodium hexafluorophosphate is maintained within the range of 20% to 60%, which not only helps control the post-processing time but, more importantly, also facilitates the precipitation of some crystals. Subsequent crystallization, filtration, and drying processes result in a high-purity solid product. Meanwhile, the remaining product remains in the remaining solution to form a liquid salt product of suitable concentration. Due to the initial concentration process, the acidity of this liquid salt product also meets the requirements for use. In other words, this application, through the above operations, simultaneously obtains both a solid product and a liquid salt product that meet usage requirements in a relatively short time, which is beneficial for industrial production.

[0009] In another aspect of this disclosure, a sodium-ion battery electrolyte is provided. In some embodiments of this disclosure, the sodium-ion battery electrolyte is formulated using a solid sodium hexafluorophosphate product and / or a concentrated sodium hexafluorophosphate solution product prepared by the methods described above. Therefore, this electrolyte exhibits good electrochemical performance, and its use in sodium-ion batteries is beneficial for improving battery performance. Detailed Implementation

[0010] The embodiments of this disclosure are described in detail below. These embodiments are exemplary and are only used to explain this disclosure, and should not be construed as limiting this disclosure.

[0011] In related technologies, sodium hexafluorophosphate is mostly a solid product. However, the preparation process of sodium hexafluorophosphate often involves solvents, thus requiring a long drying process to obtain a solid product. But if the concentration and drying steps are directly omitted to prepare liquid salt of sodium hexafluorophosphate, problems such as excessively low liquid salt concentration and excessively high acidity will occur.

[0012] In one aspect of this disclosure, a method for preparing sodium hexafluorophosphate is provided. In some embodiments of this disclosure, the method for preparing sodium hexafluorophosphate may include the following steps:

[0013] S100: Sodium fluoride is added to an organic solvent, and phosphorus pentafluoride is introduced to react with sodium fluoride.

[0014] In some embodiments of this disclosure, the organic solvent may be selected from at least one of carbonate solvents and carboxylic acid ester solvents. These solvents do not react with phosphorus pentafluoride, which is beneficial for improving the utilization rate of phosphorus pentafluoride and sodium fluoride; furthermore, these organic solvents can be used in sodium-ion battery electrolytes, and the subsequently obtained sodium hexafluorophosphate concentrate product can be directly used to prepare the electrolyte.

[0015] In some embodiments of this disclosure, the organic solvent is selected from at least one of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, ethyl acetate, and ethyl propionate. These solvents do not react with phosphorus pentafluoride, which facilitates the full reaction between phosphorus pentafluoride and sodium fluoride. These solvents are commonly used electrolyte components in sodium-ion batteries, and the subsequently prepared sodium hexafluorophosphate concentrate can be directly used in the preparation of the electrolyte.

[0016] In some embodiments of this disclosure, the molar ratio of phosphorus pentafluoride to sodium fluoride is 1:1.01 to 1:1.1. For example, the molar ratio of phosphorus pentafluoride to sodium fluoride can be 1:1.01, 1:1.03, 1:1.05, 1:1.08, 1:1.1, etc. Maintaining the ratio of raw materials within the above range is beneficial for a complete reaction and improves the utilization rate of the raw materials. Since the phosphorus pentafluoride raw material may contain a small amount of hydrogen fluoride, a slightly lower amount of phosphorus pentafluoride in the raw material is beneficial for reducing the acidity of the product.

[0017] In some embodiments of this disclosure, the reaction temperature of phosphorus pentafluoride and sodium fluoride can be 0°C to 60°C, and the reaction time can be 0.5h to 5h. In some embodiments, the reaction temperature of phosphorus pentafluoride and sodium fluoride can be 0°C, 10°C, 20°C, 30°C, 40°C, 50°C, 60°C, etc., and the reaction time can be 0.5h, 1h, 2h, 3h, 4h, 5h, etc. This facilitates a complete reaction between phosphorus pentafluoride and sodium fluoride, thereby improving the utilization rate of raw materials and the amount of product.

[0018] In some embodiments of this disclosure, the reaction between phosphorus pentafluoride and sodium fluoride can be carried out under anhydrous and oxygen-free conditions. This avoids the generation of impurities by water and oxygen in the reaction, thus reducing the moisture and impurity content in the product. It should be noted that in actual operation, absolutely anhydrous and oxygen-free conditions are difficult to achieve. Therefore, conditions where H₂O ≤ 0.1 ppm and O₂ ≤ 0.1 ppm can be considered anhydrous and oxygen-free.

[0019] In some embodiments of this disclosure, the sodium fluoride may be electronic-grade sodium fluoride. Electronic-grade sodium fluoride has higher purity, and using it as a raw material helps reduce the content of impurities and moisture in the product.

[0020] In some embodiments of this disclosure, after sodium fluoride is added to the organic solvent, stirring can be started. Under stirring conditions, phosphorus pentafluoride is introduced. During the reaction, stirring can be continued to promote sufficient contact and reaction between phosphorus pentafluoride and sodium fluoride. This disclosure does not specifically limit the stirring rate and time; those skilled in the art can select and set them according to actual needs.

[0021] S200: Filter the reaction product to obtain the filtrate.

[0022] After the reaction is complete, the reaction product is filtered to remove unreacted sodium fluoride and insoluble substances in the reaction product, resulting in a filtrate.

[0023] S300: The filtrate is evaporated and concentrated at the first temperature to make the sodium hexafluorophosphate content in the filtrate 20% to 60% by mass.

[0024] In some embodiments of this disclosure, the mass content of sodium hexafluorophosphate in the filtrate is 20%, 30%, 40%, 50%, 60%, etc., through evaporation and concentration. This allows for the removal of some impurities (such as acidic impurities like hydrogen fluoride), thereby improving the purity of the product; it also shortens the evaporation and concentration process time, thus reducing the preparation time of sodium hexafluorophosphate; and when the mass content of sodium hexafluorophosphate is within the above-mentioned range, some solid product will precipitate, promoting crystallization.

[0025] In some embodiments of this disclosure, the first temperature is 20°C to 100°C, and the evaporation and concentration pressure is -0.095 MPa to -0.09 MPa. For example, the first temperature can be 20°C, 40°C, 50°C, 70°C, 90°C, 100°C, etc., and the evaporation and concentration pressure can be -0.095 MPa, -0.094 MPa, -0.093 MPa, -0.092 MPa, -0.09 MPa, etc. Evaporation and concentration at the above temperatures and pressures can promote the evaporation of organic solvents, which is beneficial for the rapid removal of some solvents and the precipitation of solid products. The evaporated organic solvents can also carry away unreacted phosphorus pentafluoride and acidic impurities (such as hydrogen fluoride) in phosphorus pentafluoride, thereby helping to reduce the acidity of the product. In addition, the boiling point of the organic solvent is lowered under negative pressure, and the organic solvent can be evaporated at a lower temperature, reducing the risk of thermal decomposition of the product.

[0026] S400: The filtrate after evaporation and concentration is crystallized at a second temperature, filtered, and the filter cake and sodium hexafluorophosphate concentrate product are obtained.

[0027] The second temperature is lower than the first temperature. After evaporation and concentration, the temperature is lowered to promote the precipitation of sodium hexafluorophosphate solid.

[0028] In some embodiments of this application, the second temperature is lower than the first temperature. The second temperature can be between -10°C and 25°C, for example, -10°C, -5°C, 0°C, 10°C, 15°C, 25°C, etc. At the above temperatures, the solubility of sodium hexafluorophosphate in organic solvents decreases, which is beneficial for the precipitation of sodium hexafluorophosphate solid.

[0029] After crystallization, filtration is performed to separate the solid and liquid, yielding a filter cake and a concentrated sodium hexafluorophosphate solution. The concentrated sodium hexafluorophosphate solution can be directly used to prepare the electrolyte, while the filter cake undergoes further drying to obtain the solid product.

[0030] In some embodiments, the sodium hexafluorophosphate concentrate product may contain 15% to 40% sodium hexafluorophosphate by mass, for example, 15%, 20%, 25%, 30%, 35%, 40%, etc. The solubility of sodium hexafluorophosphate in organic solvents (e.g., carbonate solvents or carboxylic acid ester solvents) varies depending on the specific composition of the solvent. The sodium hexafluorophosphate content in the liquid product can be adjusted according to the solvent composition. Liquid products with the above-mentioned content can be directly used to prepare sodium-ion battery electrolytes.

[0031] In some embodiments of this application, the concentrated sodium hexafluorophosphate solution has an acidity ≤100ppm, moisture ≤50ppm, color ≤45Hazen, and insoluble matter ≤200ppm. Therefore, the liquid product contains few impurities, and all indicators meet the requirements for electrolyte components, making it suitable for direct use in electrolyte preparation.

[0032] S500: The filter cake is dried to obtain sodium hexafluorophosphate solid product.

[0033] After drying the filter cake, the resulting solid product has high purity and can be directly used in the preparation of electrolytes for sodium-ion batteries.

[0034] In some embodiments of this application, the drying temperature can be 40℃ to 100℃, the drying pressure can be -0.095MPa to -0.09MPa, and the drying time can be 2h to 6h. For example, the drying temperature can be 40℃, 50℃, 70℃, 90℃, 100℃, etc., the drying pressure can be -0.095MPa, -0.094MPa, -0.093MPa, -0.092MPa, -0.091MPa, -0.09MPa, etc., and the drying time can be 2h, 3h, 4h, 5h, 6h, etc. This is beneficial for removing solvents and avoiding thermal decomposition of solvents, which is beneficial for improving the purity of solid products; performing the drying process in a shorter time is beneficial for saving energy and reducing production costs.

[0035] In some embodiments of this application, the sodium hexafluorophosphate solid product has an acidity ≤100ppm, moisture ≤50ppm, insoluble matter ≤200ppm, and solvent residue ≤300ppm. Therefore, the sodium hexafluorophosphate solid product has high purity, and using it to formulate sodium-ion battery electrolytes is beneficial for improving the electrochemical performance of the electrolyte.

[0036] In summary, the method proposed in this disclosure for preparing sodium hexafluorophosphate can simultaneously yield both solid sodium hexafluorophosphate and concentrated sodium hexafluorophosphate solution. Both the solid and liquid products have high purity and low impurity content, and can be used to prepare electrolytes. Furthermore, the method proposed in this disclosure does not completely evaporate the solvent during the evaporation and concentration steps, which shortens the evaporation time, avoids the waste of organic solvents, and significantly improves the utilization rate of raw materials.

[0037] In another aspect of this application, a sodium-ion battery electrolyte is proposed. In some embodiments of this application, the sodium-ion battery electrolyte is formulated using a solid sodium hexafluorophosphate product and / or a concentrated sodium hexafluorophosphate solution product prepared by the methods described above. Therefore, this sodium-ion battery electrolyte exhibits excellent electrochemical performance, and its application in sodium-ion batteries is beneficial for improving the overall performance of the battery.

[0038] The present disclosure will be described below through specific embodiments. Those skilled in the art will understand that the specific embodiments below are for illustrative purposes only and do not limit the scope of the disclosure in any way. Furthermore, in the following embodiments, unless otherwise specified, the materials and equipment used are commercially available. If specific processing conditions and methods are not explicitly described in the later embodiments, conditions and methods known in the art can be used for processing.

[0039] Example 1

[0040] (1) Under anhydrous and oxygen-free conditions, add electronic grade sodium fluoride to the organic solvent dimethyl carbonate, start stirring, and introduce phosphorus pentafluoride. The molar ratio of phosphorus pentafluoride to sodium fluoride is 1:1.05. Control the reaction temperature at 40℃ and the reaction time at 2.5h.

[0041] (2) After the reaction is complete, filter out the unreacted sodium fluoride to control the insoluble matter in the product;

[0042] (3) The filtrate was evaporated and concentrated at 40℃ and -0.095MPa until the mass content of sodium hexafluorophosphate in the filtrate was 40%.

[0043] (4) After evaporation and concentration, the temperature is lowered to 10°C to crystallize, and then filtered to obtain filter cake and sodium hexafluorophosphate concentrate products.

[0044] (5) The filter cake was dried at 90℃, -0.095MPa for 5h to obtain sodium hexafluorophosphate solid product.

[0045] Sodium hexafluorophosphate was prepared in Examples 2 to 18 using the same method as in Example 1. The differences are recorded in Table 1. The preparation parameters of Comparative Examples 1 to 3 are also recorded in Table 1.

[0046] Comparative Example 1

[0047] Steps (1) and (2) are the same as in Example 1; in step (3), the filtrate is concentrated by evaporation at 40°C and -0.095 MPa, and the solvent is evaporated to dryness to obtain sodium hexafluorophosphate solid.

[0048] Comparative Example 2

[0049] Steps (1) and (2) are the same as in Example 1; in step (3), the filtrate is evaporated and concentrated at 40°C and -0.095 MPa until the mass content of sodium hexafluorophosphate in the filtrate is 15%, and a liquid product is obtained.

[0050] Comparative Example 3

[0051] The difference from Example 1 is that in step (3), the sodium hexafluorophosphate content in the filtrate is 80% by mass after evaporation and concentration. The remaining parameters and steps are the same as in Example 1.

[0052] Table 1

[0053] The parameters of the products prepared in Examples 1 to 18 and Comparative Examples 1 to 3 are recorded in Table 2. Acidity was determined by point titration, moisture was determined by Karl Fischer moisture analyzer, insoluble matter was determined by membrane filtration, solvent residue was determined by gas phase headspace analysis, sodium hexafluorophosphate content was determined by ion chromatography, and color was determined by platinum-cobalt colorimetry.

[0054] Table 2

[0055] As shown in Tables 1 and 2, sodium hexafluorophosphate prepared according to the method proposed in this disclosure can yield both solid and liquid products with high purity. In Comparative Example 2, the NaPF6 content after evaporation and concentration was low, and the resulting liquid product had high acidity, high water content, and a large amount of insoluble matter. This indicates that the evaporation and concentration in Comparative Example 2 was insufficient, failing to remove a significant amount of acidic substances and water, resulting in low product purity. The evaporation and concentration in Comparative Examples 1 and 3 were time-consuming and energy-intensive. Using the method proposed in this disclosure, the sodium hexafluorophosphate is prepared by evaporation and concentration until the filtrate contains 20%–60% sodium hexafluorophosphate. Subsequent crystallization, filtration, and drying processes yield both solid and liquid products with high purity, and significantly shorten the preparation time and reduce energy consumption. Compared to Examples 1–3, the molar ratio of PF5 to NaF in Example 4 is higher, resulting in a concentrated product with slightly higher acidity than the concentrated products in Examples 1–3, but it can still be directly used to prepare the electrolyte. Compared with Examples 1-3, the molar ratio of PF5 to NaF in Example 5 is lower, resulting in lower acidity in both the solid product and the concentrated liquid product, but this will increase the cost to some extent.

[0056] In the description of this specification, references to terms such as "some embodiments," "other embodiments," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment is included in at least one embodiment of this disclosure. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples, without contradiction. Additionally, it should be noted that in this specification, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features.

[0057] Although embodiments of the present disclosure have been shown and described above, it is to be understood that the above embodiments are exemplary and should not be construed as limiting the present disclosure. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present disclosure.

Claims

1. A method for preparing sodium hexafluorophosphate, wherein, include: Sodium fluoride is added to an organic solvent, and phosphorus pentafluoride is passed through it to react with sodium fluoride. The reaction product was filtered to obtain the filtrate; The filtrate is evaporated and concentrated at a first temperature to obtain a sodium hexafluorophosphate content of 20% to 60% by mass. The filtrate after evaporation and concentration is crystallized at a second temperature, filtered, and a filter cake and sodium hexafluorophosphate concentrate product are obtained, wherein the second temperature is lower than the first temperature; The filter cake is dried to obtain sodium hexafluorophosphate solid product.

2. The method according to claim 1, wherein, The organic solvent is selected from at least one of carbonate solvents and carboxylic acid ester solvents.

3. The method according to claim 1 or 2, wherein, The organic solvent is selected from at least one of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, ethyl acetate, and ethyl propionate.

4. The method according to any one of claims 1 to 3, wherein, The molar ratio of phosphorus pentafluoride to sodium fluoride is 1:1.01 to 1:1.

1.

5. The method according to any one of claims 1 to 4, wherein, The reaction temperature of phosphorus pentafluoride with sodium fluoride is 0℃~60℃, and the reaction time is 0.5h~5h.

6. The method according to any one of claims 1 to 5, wherein, The first temperature is 20℃~100℃, and the pressure of the evaporation and concentration is -0.095MPa~-0.09MPa.

7. The method according to any one of claims 1 to 6, wherein, The second temperature is -10℃ to 25℃.

8. The method according to any one of claims 1 to 7, wherein, The drying process is carried out at a temperature of 40℃ to 100℃, at a pressure of -0.095MPa to -0.09MPa, and for a time of 2h to 6h.

9. The method according to any one of claims 1 to 8, wherein, The sodium hexafluorophosphate solid product has an acidity of ≤100ppm, a moisture content of ≤50ppm, an insoluble content of ≤200ppm, and a solvent residue of ≤300ppm.

10. The method according to any one of claims 1 to 9, wherein, The sodium hexafluorophosphate concentrate product contains 15% to 40% sodium hexafluorophosphate by mass.

11. The method according to any one of claims 1 to 10, wherein, The sodium hexafluorophosphate concentrate product has an acidity ≤100ppm, moisture ≤50ppm, color ≤45Hazen, and insoluble matter ≤200ppm.

12. A sodium-ion battery electrolyte, wherein, It is formulated from sodium hexafluorophosphate solid product and / or sodium hexafluorophosphate concentrate product prepared by any one of claims 1 to 11.

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