A preparation method of tetrafluoroborate

Abstract

This application relates to the technical field of battery electrolytes. An embodiment of this application discloses a preparation method of tetrafluoroborate, and the method includes: introducing excessive boron trifluoride into a lithium fluoride solution to react to generate lithium tetrafluoroborate, and obtaining a first reaction solution; adding sodium fluoride to the first reaction solution to obtain a second reaction solution containing sodium tetrafluoroborate; filtering the second reaction solution to obtain sodium tetrafluoroborate solid and filtrate; obtaining lithium tetrafluoroborate solid based on the filtrate; using excessive boron trifluoride to react with lithium fluoride can avoid the difficulty in filtering residual fine powder of lithium fluoride, resulting in the residual of lithium fluoride and causing the fluoride ion in the product to exceed the standard. After the reaction of excessive boron trifluoride and lithium fluoride, adding sodium fluoride to react with the excessive remaining boron trifluoride can avoid the acid value and moisture of the product from exceeding the standard, and sodium fluoride can be easily removed by filtration subsequently. Finally, high-purity lithium tetrafluoroborate is obtained and sodium tetrafluoroborate is obtained at the same time.

Description

Technical Field

[0001] This application relates to the technical field of battery electrolytes, and particularly relates to a preparation method of tetrafluoroborate. Background Art

[0002] High-purity tetrafluoroborate has the advantages of low moisture, low temperature sensitivity, and good safety performance in the electrolyte applications of lithium batteries or sodium batteries. Currently, the preparation processes of tetrafluoroborate include aqueous solution method, gas-solid reaction method, and non-aqueous solution method. Since tetrafluoroborate is extremely easy to absorb water, for example, during the preparation of lithium tetrafluoroborate by the aqueous solution method, lithium tetrafluoroborate will appear in the form of monohydrate, and dehydration is required at about 200°C. Heating at about 200°C will cause the decomposition of lithium tetrafluoroborate salt, resulting in a decrease in the battery cycle performance when used as an electrolyte material. Therefore, the aqueous solution method is not suitable for preparing battery-grade products. The gas-solid reaction method such as the solid-phase-gas-phase contact method has relatively high requirements for equipment and the preparation process. The generated tetrafluoroborate is wrapped on the surface of the solid raw material fluoride salt, which is not conducive to the deep progress of the reaction, the reaction is uneven, and the production efficiency is low.

[0003] The non-aqueous solution method usually uses high-purity fluoride salt and boron trifluoride as raw materials to react in an organic solution. It has low requirements for production operations and equipment, high yield, and is suitable for large-scale production. For example, CN110272056A, CN105293512A, etc. mix lithium fluoride and boron trifluoride in an organic solvent to react to generate lithium tetrafluoroborate, and then obtain lithium tetrafluoroborate crystals through steps such as concentration and crystallization. However, lithium fluoride has low solubility in organic solvents, and organic solvents are prone to form suspensions (slurries), and micro-powdery lithium fluoride is difficult to filter, which makes the organic solvent containing lithium fluoride difficult to recycle, and the production process is difficult to be continuous. Moreover, even trace residues of lithium fluoride will cause the fluoride ions in the product to exceed the standard, and it is difficult to obtain a high-purity product. CN102803142A uses an excessive amount of boron trifluoride gas to make the tetrafluoroborate solution circulate in the production system. However, an excessive amount of boron trifluoride gas often causes the acid value, moisture, etc. of the product to exceed the standard, still not meeting the purity requirements of high-quality tetrafluoroborate. Summary of the Invention

[0004] This application aims to solve at least one of the technical problems existing in the prior art. For this reason, this application proposes a preparation method of tetrafluoroborate to solve the problems that the acid value and moisture of the product exceed the standard when preparing tetrafluoroborate by the current non-aqueous solution method, and the fluoride ions in the product exceed the standard due to the residue of lithium fluoride.

[0005] Specifically, this application provides a preparation method of tetrafluoroborate, and the method includes:

[0006] Reacting an excessive amount of boron trifluoride with a lithium fluoride solution to generate lithium tetrafluoroborate, and obtaining a first reaction solution;

[0007] Sodium fluoride is added to the first reaction solution to obtain a second reaction solution containing sodium tetrafluoroborate;

[0008] The second reaction solution is filtered to obtain sodium tetrafluoroborate solid and filtrate;

[0009] Lithium tetrafluoroborate solid is obtained based on the filtrate.

[0010] In the embodiment of the above method for preparing a tetrafluoroborate, obtaining lithium tetrafluoroborate solid based on the filtrate includes:

[0011] The filtrate is concentrated and then a poor solvent is added to obtain lithium tetrafluoroborate solid.

[0012] In the embodiment of the above method for preparing a tetrafluoroborate, the poor solvent is selected from at least one of toluene, dichloromethane, dichloroethane, trichloropropane, tetrachloroethane, xylene, and isopropyl ether.

[0013] In the embodiment of the above method for preparing a tetrafluoroborate, after adding the poor solvent to the second filtrate, the temperature is lowered to 0 - 10 °C and then filtered, and dried under vacuum to obtain lithium tetrafluoroborate solid.

[0014] In the embodiment of the above method for preparing a tetrafluoroborate, the solvent in the lithium fluoride solution is selected from at least one of ethylene glycol dimethyl ether, acetonitrile, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate.

[0015] In the embodiment of the above method for preparing a tetrafluoroborate, the molar ratio of lithium fluoride to boron trifluoride is 1:(1.1 - 2).

[0016] In the embodiment of the above method for preparing a tetrafluoroborate, the ratio of the difference in molar amount between the boron trifluoride and the lithium fluoride to the molar amount of the sodium fluoride is 1:(1 - 2), preferably 1:(1 - 1.2), and more preferably 1:(1 - 1.1).

[0017] In the embodiment of the above method for preparing a tetrafluoroborate, the reaction temperature of the excessive boron trifluoride with the lithium fluoride solution is 20 - 60 °C.

[0018] In the embodiment of the above method for preparing a tetrafluoroborate, after reacting the excessive boron trifluoride with the lithium fluoride solution to generate lithium tetrafluoroborate, the method further includes: keeping warm for 1 - 6 h.

[0019] In the embodiment of the above method for preparing a tetrafluoroborate, after adding sodium fluoride to the first reaction solution, keep warm and react for 0.5 - 6 h, then lower the temperature to 20 - 25 °C and filter to obtain the filtrate.

[0020] One or more of the above embodiments of the present application have at least one or more of the following beneficial effects: Reacting an excessive amount of boron trifluoride with lithium fluoride can avoid problems such as the difficulty in filtering the remaining fine powder of lithium fluoride and the exceeding of fluoride ions in the product due to the residue of lithium fluoride; adding sodium fluoride to react with the excessive remaining boron trifluoride after the reaction of excessive boron trifluoride and lithium fluoride can avoid the exceeding of the acid value and moisture of the product, and the excessive sodium fluoride can be easily removed by filtration subsequently, and finally lithium tetrafluoroborate with high purity is obtained while sodium tetrafluoroborate is obtained.

[0021] Additional aspects and advantages of the present application will be given in part in the following description, become apparent in part from the following description, or be understood through the practice of the present application. Detailed implementation manners

[0022] Some embodiments of the present application are described below. Those skilled in the art should understand that these embodiments are only used to explain the technical principle of the present application and are not intended to limit the protection scope of the present application.

[0023] As described in the background art, currently, the preparation methods of tetrafluoroborates include aqueous solution method, gas-solid reaction method and non-aqueous solution method. Among them, the gas-solid reaction method has high requirements for equipment and preparation operations and is difficult to achieve large-scale production. In the preparation process of the aqueous solution method, due to the extremely water-absorbing characteristics of tetrafluoroborates, it is difficult to remove water by drying. After being applied to batteries, the performance of the batteries is not good and the yield is not high. Therefore, the non-aqueous solution method with low requirements for production operations and equipment and high yield is widely used at present. In the process of preparing tetrafluoroborates by the current non-aqueous solution method, in order to ensure the full reaction of raw materials and improve the yield, the dosage of a certain raw material is often increased. For example, when preparing lithium tetrafluoroborate, an excessive amount of boron trifluoride gas is usually introduced into an organic solvent containing lithium fluoride. However, the excessive boron trifluoride gas will cause problems such as the exceeding of the acid value and moisture of the product, and the fine powder of lithium fluoride is difficult to filter, and the excessive lithium fluoride will also cause the exceeding of fluoride ions in the product.

[0024] To solve the above problems, the present application creatively proposes a preparation method of tetrafluoroborates. First, reacting an excessive amount of boron trifluoride with lithium fluoride can avoid the problems that the remaining fine powder of lithium fluoride is difficult to filter and the fluoride ions in the product exceed the standard due to the residue of lithium fluoride. Then, adding sodium fluoride to react with the excessive remaining boron trifluoride after the reaction of boron trifluoride and lithium fluoride can avoid the exceeding of the acid value and moisture of the product. The sodium fluoride can be easily removed by filtration subsequently, and finally lithium tetrafluoroborate with high purity is obtained while sodium tetrafluoroborate is obtained.

[0025] The present application will be specifically elaborated below through specific embodiments.

[0026] Specifically, the embodiments of the present application provide a preparation method of tetrafluoroborates, and the method includes:

[0027] S1. Pass an excessive amount of boron trifluoride into a lithium fluoride solution to react and form lithium tetrafluoroborate, obtaining a first reaction solution.

[0028] S2. Add sodium fluoride to the first reaction solution to obtain a second reaction solution containing sodium tetrafluoroborate.

[0029] S3. Filter the second reaction solution to obtain sodium tetrafluoroborate solid and a filtrate.

[0030] S4. Obtain lithium tetrafluoroborate solid based on the filtrate.

[0031] Reacting an excessive amount of boron trifluoride with lithium fluoride can avoid problems such as the difficulty in filtering the remaining fine powdered lithium fluoride and the excessive fluoride ions in the product due to the residue of lithium fluoride. After the reaction of boron trifluoride and lithium fluoride, add sodium fluoride to react with the excessive remaining boron trifluoride to avoid the over-standard of the acid value and moisture of the product. Finally, high-purity lithium tetrafluoroborate is obtained, and at the same time, sodium tetrafluoroborate is obtained. The purification of sodium tetrafluoroborate can be achieved by dissolving, filtering to remove sodium fluoride, and then recrystallizing.

[0032] In step S2 of the present application, by adding sodium fluoride, the excessive boron trifluoride remaining after reacting with lithium fluoride is consumed, avoiding the over-standard of the acid value, moisture, etc. of the product caused by the excess of boron trifluoride. In some embodiments of the present application, step S4 includes:

[0033] After concentrating the filtrate, add a poor solvent to obtain lithium tetrafluoroborate solid.

[0034] The term "poor solvent" refers to a solvent with a weak dissolving ability for the solute. The solubility of lithium tetrafluoroborate in the poor solvent is low, and the intermolecular interaction with the solvent is weak. Therefore, lithium tetrafluoroborate can crystallize out. By selecting a suitable poor solvent, good lithium tetrafluoroborate crystals can be obtained, and at the same time, the impurities are placed in the solution.

[0035] In some embodiments of the present application, the poor solvent is selected from at least one of toluene, dichloromethane, dichloroethane, trichloropropane, tetrachloroethane, xylene, and isopropyl ether. The solubility of lithium tetrafluoroborate in toluene, dichloromethane, dichloroethane, trichloropropane, tetrachloroethane, xylene, and isopropyl ether is low.

[0036] In some embodiments of the present application, after adding the poor solvent to the second filtrate, cool it to 0 - 10 °C and filter, and then dry it under vacuum to obtain lithium tetrafluoroborate solid. Optionally, cool it to 0 °C, 1 °C, 2 °C, 3 °C, 4 °C, 5 °C, 6 °C, 7 °C, 8 °C, 9 °C, 10 °C, or any temperature value within the above temperature range.

[0037] In some embodiments of the present application, the solvent in the lithium fluoride solution is selected from at least one of ethylene glycol dimethyl ether, acetonitrile, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate.

[0038] The solubility of sodium tetrafluoroborate and lithium tetrafluoroborate in different solvents varies greatly. The solubility of lithium tetrafluoroborate in ethylene glycol dimethyl ether, acetonitrile, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate is very good. Subsequent crystallization requires the addition of poor solvents such as toluene, dichloromethane, dichloroethane, trichloropropane, tetrachloroethane, xylene, and isopropyl ether to crystallize out. Sodium tetrafluoroborate is insoluble in ethylene glycol dimethyl ether, acetonitrile, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate. The reaction solvent for the reaction of lithium fluoride and boron trifluoride is selected from at least one of ethylene glycol dimethyl ether, acetonitrile, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate. After adding sodium fluoride to react to form sodium tetrafluoroborate, sodium tetrafluoroborate can be precipitated first, and then at least one of poor solvents such as toluene, dichloromethane, dichloroethane, trichloropropane, tetrachloroethane, xylene, and isopropyl ether is added to precipitate lithium tetrafluoroborate, and high-purity lithium tetrafluoroborate crystals can be obtained.

[0039] In some embodiments of the present application, the molar ratio of lithium fluoride to boron trifluoride is 1:(1.1 - 2).

[0040] Boron trifluoride is in excess to maximize the reaction to consume lithium fluoride and reduce the residue of fine-powdered and difficult-to-filter lithium fluoride.

[0041] In some embodiments of the present application, the added sodium fluoride is used to consume the excess boron trifluoride. The molar amount of sodium fluoride should be greater than or equal to the difference in the molar amounts of boron trifluoride and lithium fluoride. The difference in the molar amounts of boron trifluoride and lithium fluoride refers to the value obtained by subtracting the molar amount of lithium fluoride from the molar amount of boron trifluoride. Specifically, the ratio of the value obtained by subtracting the molar amount of lithium fluoride from the molar amount of boron trifluoride to the molar amount of sodium fluoride is 1:(1 - 2). However, to reduce unnecessary waste and at the same time obtain a better yield of lithium tetrafluoroborate, the input amount of sodium fluoride should not be too large. The above ratio range is preferably 1:(1 - 1.2), more preferably 1:(1 - 1.1). For example, it can be 1:1.01, 1:1.02, 1:1.03, 1:1.04, 1:1.05, 1:1.06, 1:1.07, 1:1.08, 1:1.09, 1:1, 1:1.1, 1:1.12, 1:1.13, 1:1.14, 1:1.15, 1:1.16, 1:1.17, 1:1.18, 1:1.19, 1:1.2, or any value within the above range.

[0042] In some embodiments of the present application, the reaction temperature of excessive boron trifluoride and lithium fluoride solution is 20~60°C. Optionally, the reaction temperature of excessive boron trifluoride and lithium fluoride solution can be 60°C, 58°C, 55°C, 50°C, 45°C, 40°C, 38°C, 35°C, 30°C, 28°C, 25°C, 20°C, or any temperature value within the above temperature range.

[0043] In some embodiments of the present application, after excessive boron trifluoride is introduced into the lithium fluoride solution to react to form lithium tetrafluoroborate, keep warm for 1~6h. Optionally, keep warm for 1h, 1.5h, 2h, 3h, 3h, 4h, 5h, 6h, or any duration value within the above duration range.

[0044] In some embodiments of the present application, after adding sodium fluoride to the first reaction solution, the method further includes:

[0045] Keep warm and react for 0.5~6h, and cool down to 20-25°C.

[0046] Optionally, keep warm and react for 0.5h, 1.5h, 2h, 3h, 4h, 5h, 6h, or any duration value within the above duration range. Cool down to 20°C, 21°C, 22°C, 23°C, 24°C, 25°C, or any temperature value within the above temperature range.

[0047] The present application will be further described in detail below with reference to embodiments. It should be noted that the following embodiments are only used to illustrate the present application and are not used to limit the scope of the claims of the present application. It cannot be understood as a limitation of the protection scope of the present application. Some non-essential improvements and adjustments made by those skilled in the art based on the content of the present application above still fall within the protection scope of the present application.

[0048] In the following embodiments, the reagents, materials and instruments used, if not otherwise specified, are all commercially available products and can be obtained by purchase.

[0049] Example 1: Add 25.94 g (1.0 mol) of lithium fluoride to 500 g of acetonitrile solution, control the temperature at 50~60°C, introduce 74.59 g (1.1 mol) of boron trifluoride, and finish introducing it in about 1 hour. After finishing, keep warm at 50°C~60°C for 1 hour, add 4.20 g of sodium fluoride (0.1 mol), keep warm and react for half an hour, then cool down to 20-25°C and filter. The filter cake is rinsed with acetonitrile and then dried in vacuum to obtain 10.87 g of sodium tetrafluoroborate. The filtrate is concentrated under reduced pressure to about 180 g, 300 g of poor solvent toluene is added dropwise, and the temperature is cooled down to 0-5°C and filtered. The filter cake is dried in vacuum to obtain 90.1 g of lithium tetrafluoroborate.

[0050] Example 2: In 500 g of dimethyl carbonate solution, 25.94 g (1.0 mol) of lithium fluoride was added. The temperature was controlled at 40 - 50 °C, and 101.71 g (1.5 mol) of boron trifluoride was introduced. It was introduced completely in about 2 hours. After introduction, it was kept warm at 40 - 50 °C for 1.5 hours. Then 21.41 g (0.51 mol) of sodium fluoride was added. After reacting under warm conditions for 1 hour, the temperature was lowered to 20 - 25 °C and filtered. The filter cake was rinsed with dimethyl carbonate and then dried in vacuum to obtain 55.22 g of sodium tetrafluoroborate. The filtrate was concentrated to about 180 g, 300 g of poor solvent o - xylene was added dropwise, and the temperature was lowered to 5 - 10 °C and filtered. The filter cake was dried in vacuum to obtain 89 g of lithium tetrafluoroborate.

[0051] Example 3: In 500 g of diethyl carbonate solution, 25.94 g (1.0 mol) of lithium fluoride was added. The temperature was controlled at 20 - 30 °C, and 135.62 g (2.0 mol) of boron trifluoride was introduced. It was introduced completely in about 3 hours. After introduction, it was kept warm at 20 - 30 °C for 3 hours. Then 42.83 g (1.02 mol) of sodium fluoride was added. After reacting under warm conditions for 3 hours, the temperature was lowered to 20 - 25 °C and filtered. The filter cake was rinsed with diethyl carbonate and then dried in vacuum to obtain 110.38 g of sodium tetrafluoroborate. The filtrate was concentrated to about 180 g, 300 g of poor solvent dichloromethane was added dropwise, and the temperature was lowered to 5 - 10 °C and filtered. The filter cake was dried in vacuum to obtain 90.5 g of lithium tetrafluoroborate.

[0052] Example 4: In 500 g of ethylene glycol dimethyl ether solution, 25.94 g (1.0 mol) of lithium fluoride was added. The temperature was controlled at 50 - 60 °C, and 81.37 g (1.2 mol) of boron trifluoride was introduced. It was introduced completely in about 1 hour. After introduction, it was kept warm at 50 - 60 °C for 2 hours. Then 10.08 g (0.24 mol) of sodium fluoride was added. After reacting under warm conditions for 1 hour, the temperature was lowered to 20 - 25 °C and filtered. The filter cake was rinsed with ethylene glycol dimethyl ether and then dried in vacuum to obtain 23.52 g of sodium tetrafluoroborate. The filtrate was concentrated under reduced pressure to about 180 g, 300 g of poor solvent tetrachloroethane was added dropwise, and the temperature was lowered to 0 - 5 °C and filtered. The filter cake was dried in vacuum to obtain 88 g of lithium tetrafluoroborate.

[0053] Example 5: In 500 g of ethyl methyl carbonate solution, 25.94 g (1.0 mol) of lithium fluoride was added. The temperature was controlled at 25 - 35 °C, and 88.15 g (1.3 mol) of boron trifluoride was introduced. It was introduced completely in about 4 hours. After introduction, it was kept warm at 25 - 35 °C for 4 hours. Then 15.12 g (0.36 mol) of sodium fluoride was added. After reacting under warm conditions for 2 hours, the temperature was lowered to 20 - 25 °C and filtered. The filter cake was rinsed with ethyl methyl carbonate and then dried in vacuum to obtain 35.28 g of sodium tetrafluoroborate. The filtrate was concentrated under reduced pressure to about 180 g, 300 g of poor solvent dichloroethane was added dropwise, and the temperature was lowered to 0 - 5 °C and filtered. The filter cake was dried in vacuum to obtain 88.5 g of lithium tetrafluoroborate.

[0054] Example 6: 25.94 g (1.0 mol) of lithium fluoride was added to 500 g of dimethyl carbonate solution. The temperature was controlled at 25 - 35 °C, and 128.83 g (1.9 mol) of boron trifluoride was introduced, which was completed in about 4 hours. After completion, it was kept warm at 25 °C - 35 °C for 4 hours. Then 41.99 g (1.0 mol) of sodium fluoride was added. After reacting with heat preservation for 4 hours, the temperature was lowered to 20 - 25 °C and filtered. The filter cake was rinsed with dimethyl carbonate and dried in vacuum to obtain 102.71 g of sodium tetrafluoroborate. The filtrate was concentrated to about 180 g, 300 g of poor solvent trichloropropane was added dropwise, and the temperature was lowered to 0 - 5 °C and filtered. The filter cake was dried in vacuum to obtain 91.5 g of lithium tetrafluoroborate.

[0055] Comparative Example 1: 25.94 g (1.0 mol) of lithium fluoride was added to 500 g of acetonitrile solution. The temperature was controlled at 50 - 60 °C, and 74.59 g (1.1 mol) of boron trifluoride was introduced, which was completed in about 1 hour. After completion, it was kept warm at 50 °C - 60 °C for 1 hour. After filtration, the filtrate was concentrated under reduced pressure to about 180 g, 300 g of poor solvent toluene was added dropwise, and the temperature was lowered to 0 - 5 °C and filtered. The filter cake was dried in vacuum to obtain 82.5 g of lithium tetrafluoroborate.

[0056] Comparative Example 2: 25.94 g (1.0 mol) of lithium fluoride was added to 500 g of dimethyl carbonate solution. The temperature was controlled at 40 - 50 °C, and 67.81 g (1.0 mol) of boron trifluoride was introduced, which was completed in about 1 hour. After completion, it was kept warm at 40 °C - 50 °C for 2 hours. After filtration, the filtrate was concentrated under reduced pressure to about 180 g, 300 g of poor solvent o - xylene was added dropwise, and the temperature was lowered to 5 - 10 °C and filtered. The filter cake was dried in vacuum to obtain 79.7 g of lithium tetrafluoroborate.

[0057] Comparative Example 3: 25.94 g (1.0 mol) of lithium fluoride was added to 500 g of diethyl carbonate solution. The temperature was controlled at 50 - 60 °C, and 61.71 g (0.91 mol) of boron trifluoride was introduced. After completion, it was kept warm for 1 hour. After filtration, the filtrate was concentrated under reduced pressure to about 180 g, 300 g of poor solvent dichloromethane was added dropwise, and the temperature was lowered to 5 - 10 °C and filtered. The filter cake was dried in vacuum to obtain 74.0 g of lithium tetrafluoroborate.

[0058] The yields of lithium tetrafluoroborate in the above - mentioned examples and comparative examples are shown in Table 1:

[0059] Table 1

[0060] Boron trifluoride Lithium fluoride Sodium fluoride Sodium tetrafluoroborate Lithium tetrafluoroborate Yield of lithium tetrafluoroborate Example 1 74.59g 25.94g 4.2g 10.87g 90.1g 96.11% (calculated based on lithium fluoride) Example 2 101.71g 25.94g 21.41g 55.22g 89g 94.93% (calculated based on lithium fluoride) Example 3 135.62g 25.94g 42.83g 110.38g 90.5g 96.53% (calculated based on lithium fluoride) Example 4 81.37g 25.94g 10.08g 23.52g 88g 93.87% (calculated based on lithium fluoride) Example 5 88.15g 25.94g 15.12g 35.28g 88.5g 94.4% (calculated based on lithium fluoride) Example 6 128.83g 25.94g 41.99g 102.71g 91.5g 97.6% (calculated based on lithium fluoride) Comparative Example 1 74.59g 25.94g 0 0 82.5g 88% (calculated based on lithium fluoride) Comparative Example 2 67.81g 25.94g 0 0 79.7g 85% (calculated based on lithium fluoride) Comparative Example 3 61.71g 25.94g 0 0 74.0g 78.93% (calculated based on lithium fluoride)

[0061] The product qualities of lithium tetrafluoroborate and sodium tetrafluoroborate in the above - mentioned examples and comparative examples are shown in Table 2:

[0062] Table 2

[0063] Fluoride ion in lithium tetrafluoroborate Purity of lithium tetrafluoroborate Moisture in lithium tetrafluoroborate Acid value of lithium tetrafluoroborate Fluoride ion in sodium tetrafluoroborate Purity of sodium tetrafluoroborate Moisture in sodium tetrafluoroborate Acid value of sodium tetrafluoroborate Example 1 2 ppm 99.99% 25 ppm 33 ppm 4 ppm 99.99% 36 ppm 31 ppm Example 2 4 ppm 99.99% 24 ppm 19 ppm 0.79% 99.20% 25 ppm 10 ppm Example 3 3 ppm 99.99% 17 ppm 20 ppm 0.76% 99.23% 30 ppm 12 ppm Example 4 1 ppm 99.99% 15 ppm 17 ppm 6.61% 93.38% 29 ppm 5 ppm Example 5 2 ppm 99.99% 11 ppm 12 ppm 6.68% 93.31% 35 ppm 8 ppm Example 6 3 ppm 99.99% 16 ppm 19 ppm 4.09% 95.90% 21 ppm 16 ppm Comparative Example 1 12 ppm 99.1% 120 ppm 950 ppm / / / / Comparative Example 2 540 ppm 99.3% 110 ppm 254 ppm / / / / Comparative Example 3 1100 ppm 99.2% 15 ppm 14 ppm / / / /

[0064] As can be seen from Table 1 and Table 2: When an excessive amount of boron trifluoride reacts with lithium fluoride and then sodium fluoride is added, the yield of lithium tetrafluoroborate can be significantly increased, the fluoride ion impurities and moisture in the lithium tetrafluoroborate product can be significantly reduced, the acid value of the lithium tetrafluoroborate product can be significantly decreased, and the purity of the lithium tetrafluoroborate product can be increased. As can be seen by referring to Comparative Example 1, Comparative Example 2, and Comparative Example 3: When the molar ratio of boron trifluoride to lithium fluoride is less than or equal to 1, the yield of lithium tetrafluoroborate significantly decreases, the fluoride ion impurities significantly increase, and the acid value significantly rises. In addition, as shown in Table 2, the lithium tetrafluoroborate prepared in Examples 1 - 6 has less fluoride ion impurities and moisture, high purity, and a low acid value.

[0065] In summary, the method for preparing tetrafluoroborate provided in the embodiments of the present application can prepare high-purity lithium tetrafluoroborate and sodium tetrafluoroborate products. The prepared sodium tetrafluoroborate and lithium tetrafluoroborate have a low acid value, and the fluoride ion and moisture content are extremely low, which is suitable for use as an electrolyte in batteries.

[0066] In the description of this specification, the description with reference to terms such as "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0067] In addition, the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first" and "second" can explicitly or implicitly include at least one of the features. In the description of the present application, "a plurality" means at least two, such as two, three, etc., unless otherwise specifically defined.

[0068] Although the embodiments of the present application have been shown and described above, it can be understood that the above embodiments are exemplary and cannot be construed as limiting the present application. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present application.

Claims

1. A method for preparing tetrafluoroborate, characterized in that: The method comprises: An excess of boron trifluoride is introduced into a lithium fluoride solution to react and generate lithium tetrafluoroborate, thereby obtaining a first reaction solution; the solvent in the lithium fluoride solution is selected from at least one of ethylene glycol dimethyl ether, acetonitrile, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; adding sodium fluoride to the first reaction solution to obtain a second reaction solution containing sodium tetrafluoroborate; Filtering the second reaction liquid to obtain sodium tetrafluoroborate solid and filtrate; The method comprises obtaining lithium tetrafluoroborate solid based on the filtrate, comprising: After concentrating the filtrate, a poor solvent is added to obtain lithium tetrafluoroborate solid; The poor solvent is selected from at least one of toluene, dichloromethane, dichloroethane, trichloropropane, tetrachloroethane, xylene and isopropyl ether.

2. The method for preparing a tetrafluoroborate according to claim 1, characterized in that: After adding the poor solvent, the temperature is lowered to 0-10° C., filtered, and vacuum dried to obtain lithium tetrafluoroborate solid.

3. The method for preparing a tetrafluoroborate according to claim 1, characterized in that: The molar ratio of the lithium fluoride to the boron trifluoride is 1:(1.1-2).

4. The method for preparing a tetrafluoroborate according to claim 1, characterized in that: The ratio of the molar difference between the boron trifluoride and the lithium fluoride to the molar amount of the sodium fluoride is 1:(1-2).

5. The method for preparing a tetrafluoroborate according to claim 1, characterized in that: The reaction temperature of the excess boron trifluoride and the lithium fluoride solution is 20-60°C.

6. The method for preparing a tetrafluoroborate according to claim 5, characterized in that: After an excess amount of boron trifluoride is introduced into the lithium fluoride solution to react and generate lithium tetrafluoroborate, the solution is kept warm for 1 to 6 hours.

7. The method for preparing a tetrafluoroborate according to claim 1, characterized in that: After adding sodium fluoride to the first reaction solution, the reaction is kept warm for 0.5-6 hours, and the temperature is lowered to 20-25° C. and filtered to obtain a filtrate.