Method for purifying sodium difluoro (oxalato) borate

Abstract

The invention discloses a purification method of sodium difluoro (oxalato) borate, which comprises the following steps: S1, regeneration of anion exchange resin: packing the anion exchange resin into a column, regenerating the anion exchange resin by using an alkaline solution, and washing to be neutral; s2, anion exchange resin balancing: balancing the resin column by using a first alkalinity sodium hydroxide solution with the pH value of 8-9; s3, removing sodium tetrafluoroborate: dissolving the sodium difluoro (oxalato) borate crude product, loading a sample, and washing to obtain an intermediate product; s4, elution of an intermediate product: eluting with a second alkalinity sodium hydroxide solution with the pH value of 11-12; s5, primary crystallization: collecting eluted liquid, and crystallizing to obtain primary crystals; and S6, secondary crystallization: dissolving the mixed solid primary crystal in acetonitrile, filtering the acetonitrile solution, and carrying out secondary crystallization on the filtrate to obtain the purified sodium difluoro (oxalato) borate solid. The method has the characteristics of high purity, high product yield and simple process.

Description

Technical Field

[0001] The invention relates to the technical field of sodium ion battery electrolytes, and in particular to a method for purifying sodium difluorooxalatoborate. Background Art

[0002] Sodium-ion battery technology has unique advantages in terms of safety and cost, making it likely to become another secondary battery technology for large-scale commercial applications in addition to lithium-ion batteries.

[0003] The electrolyte of sodium ion battery is generally made of solvent, electrolyte sodium salt, functional additives, etc., which are prepared under certain conditions and in a certain proportion. It is the carrier of ion transmission in the battery. The electrolyte plays the role of conducting ions between the positive and negative electrodes of the sodium battery, which is the guarantee for the sodium ion battery to obtain advantages such as high voltage and high specific energy.

[0004] Sodium salts, as the main component of the electrolyte, play an important role in determining the electrochemical performance of sodium-ion batteries. Sodium salts act as part of the charge carriers that are transported between the two separated electrodes. These charge carriers determine the ionic conductivity of the electrolyte; poor ionic conductivity can degrade many battery parameters. Sodium salts affect the composition of sodium-ion batteries, which may be dissolved and destroyed during cycling, thereby reducing the stability of sodium-ion batteries. Sodium ions produced by sodium salts participate in intercalation (deintercalation) reactions within electrode materials. The solvation structure of sodium ions, including geometric morphology and delocalized electron density, can greatly affect the diffusion of sodium ions, especially through the electrode / electrolyte interface. In addition, the chemical toxicity and corrosiveness of most sodium salts have an important impact on battery safety in practical applications.

[0005] Sodium difluorooxalatoborate (NaDFOB) is a new type of sodium ion battery electrolyte salt with the chemical formula:

[0006]

[0007] Sodium difluorooxalatoborate has half the structure of sodium tetrafluoroborate and half of sodium bis(oxalatoborate) molecules, and has the advantages of both. Sodium difluorooxalatoborate has excellent compatibility with various commonly used solvents, which is in sharp contrast to the solvent-dependent performance of conventional electrolyte salts such as sodium perchlorate (NaClO4) and sodium hexafluorophosphate (NaPF6). When using sodium difluorooxalatoborate organic electrolytes with different solvents (such as EC:DEC, EC:PC, EC:DMC, PC, etc.), Na / Na 0.44The MnO2 half-cells all exhibit good rate performance and cycle life, while traditional organic electrolytes of sodium perchlorate and sodium hexafluorophosphate can only achieve good electrochemical performance in specific solvents. In addition, sodium difluoro(oxalato)borate has good stability and does not produce toxic or dangerous products when exposed to air and water. As a high-performance electrolyte salt for sodium-ion batteries, sodium difluoro(oxalato)borate is widely popular in sodium-ion secondary batteries.

[0008] Currently, the general preparation method is to react boron trifluoride diethyl etherate [BF3O(C2H5)2] with sodium oxalate [Na2C2O4] in an organic solvent. Although sodium tetrafluoroborate (NaBF4) is not used as a raw material, sodium tetrafluoroborate is easily generated during the preparation process. The reaction equation is as follows:

[0009] Na2C2O4 + BF3O(C2H5)2 → C2H5OC2H5 + NaF + NaDFOB

[0010] NaF + BF3O(C2H5)2 → C2H5OC2H5 + NaBF4

[0011] After the reaction, a poor solvent is used for purification by extraction or recrystallization to obtain high-purity sodium difluoro(oxalato)borate. However, the solubility of sodium tetrafluoroborate and sodium difluoro(oxalato)borate in common organic solvents is very similar, and it is difficult to effectively separate them by the recrystallization method. Repeated purification is required, which has become a major problem in the preparation of high-purity sodium difluoro(oxalato)borate.

[0012] In addition, among many preparation process routes of sodium difluoro(oxalato)borate, most of the prepared sodium difluoro(oxalato)borate has the by-product sodium tetrafluoroborate. For example, Chinese Patent Application CN118638139A prepares sodium difluoro(oxalato)borate by reacting anhydrous oxalic acid, a chlorine-containing auxiliary agent and sodium tetrafluoroborate; Chinese Patent Application CN118638139A reacts an aqueous solution of boric acid, sodium hydrogen fluoride and oxalic acid to prepare a crude product; Chinese Patent Application CN105541890A adds sodium fluoride, oxalic acid and hydrogen fluoride to an aqueous solution of boric acid to form a homogeneous mixed aqueous solution, evaporates to dryness to obtain a white precipitate, and then conducts thorough drying to obtain a crude product. The above synthesis methods all generate sodium tetrafluoroborate impurities, which are generally removed by the recrystallization method.

[0013] However, the steps of separating sodium tetrafluoroborate and sodium difluoro(oxalato)borate by the recrystallization method are cumbersome and inefficient. It often requires repeated recrystallization many times to gradually reduce the impurity content. There are problems such as cumbersome operation, high solvent demand and loss, and low product yield of the finally obtained product when used in actual production. Summary of the Invention

[0014] The purpose of the present invention is to provide a purification method of sodium difluoro(oxalato)borate, which has the characteristics of high purity, high product yield and simple process.

[0015] The present invention can be realized by the following technical solutions:

[0016] The present invention discloses a purification method for sodium difluorooxalate borate, comprising the following steps:

[0017] S1. Anion exchange resin regeneration: Pack the anion exchange resin into a column, regenerate the anion exchange resin with an alkaline solution, and wash it with distilled water until the effluent is neutral.

[0018] S2. Anion exchange resin equilibration: Equilibrate the resin column with a sodium hydroxide solution having a first alkalinity with a pH of 8-9.

[0019] S3. Sodium tetrafluoroborate removal: Dissolve and load the crude sodium difluorooxalate borate, and then rinse it with a sodium hydroxide solution having a first alkalinity to remove sodium tetrafluoroborate to obtain an intermediate product.

[0020] S4. Intermediate product elution: Elute the intermediate product in step S3 with a sodium hydroxide solution having a second alkalinity with a pH of 11-12.

[0021] S5. Primary crystallization: Collect the eluted liquid, and after crystallization, obtain a mixed solid primary crystallization of sodium hydroxide and sodium difluorooxalate borate.

[0022] S6. Secondary crystallization: Dissolve the mixed solid primary crystallization in acetonitrile, filter the acetonitrile solution, and perform secondary crystallization on the filtrate to obtain a purified sodium difluorooxalate borate solid.

[0023] Further, in step S1, the mass concentration of the alkaline solution is 3% - 5%, the dosage is 2 - 3 times the volume of the anion resin, and the flow rate ≤ 10 m / h.

[0024] Further, in step S4, the dosage of the second alkaline solution is 3 - 4 times the volume of the resin, and the flow rate is 4 - 6 m / h; if the sodium hydroxide concentration in the eluent is too low, it will cause some ions to be difficult to elute, resulting in a lower yield; if the sodium hydroxide concentration in the eluent is too high, more sodium hydroxide impurities will be introduced.

[0025] Further, in step S1, the type of the anion exchange resin is one or more of D201, D202, D261, D280, 201*7. The factory form of this anion resin is the Cl type, and it needs to be regenerated into the OH type with an alkaline solution before use; among them, the main factors affecting the regeneration effect are the type and concentration of the alkaline solution, the flow rate, the contact time, and the temperature.

[0026] Further, in step S3, the concentration of the solution formed after the crude sodium difluorooxalate borate is dissolved in distilled water is 18 - 20%. Too high a concentration may cause the resin to not fully adsorb the ions in the sample solution, resulting in a decrease in purity.

[0027] Further, in step S3, the amount of the first lye used for rinsing is 1 to 3 times the volume of the resin. When removing sodium tetrafluoroborate by rinsing, if the amount of the rinsing solution is too small, it is not easy to rinse out the unadsorbed sample; if the amount is too large, the well-adsorbed sample will be washed away.

[0028] Further, in step S6, the mass ratio of acetonitrile to sodium difluoro(oxalato)borate is 15 to 20. If the amount of the solvent is small, the mixed salt will not dissolve completely, and more product and impurities will precipitate after crystallization, resulting in high product purity but low yield; if the amount of the solvent is large, the amount of dissolved impurities will also increase, resulting in low product purity.

[0029] Further, in step S6, the pore diameter of the filter paper selected for filtration is 0.3 to 0.5 μm. If the pore diameter of the filter membrane is large, the filtration speed is fast, but it is easy to cause leakage filtration, resulting in less residue and high content of insoluble substances in the filtrate; if the pore diameter of the filter membrane is small, the filtration speed is slow and there is more residue. However, if the pore diameter of the filter membrane is too small, the filtration speed is slow and it is easy to cause crystallization of the reaction solution to block the filter membrane, resulting in filtration failure. Therefore, selecting a filter membrane with a suitable pore diameter can not only filter quickly but also ensure a low content of insoluble substances in the filtrate.

[0030] Further, the crystallization method is evaporation crystallization, which can reduce product loss. The filtrate in S6 can be recovered during evaporation for reuse.

[0031] Further, when loading the sample, an appropriate amount of the above-mentioned crude sodium difluoro(oxalato)borate solution is directly poured into the anion resin column, and the groups on the resin exchange with sodium difluoro(oxalato)borate:

[0032] R-OH + NaDFOB → R-DFOB + NaOH

[0033] Further, the temperature of the operations in the above steps is room temperature, and the liquid flow rate is controlled by a constant flow pump.

[0034] Further, after the anion exchange column is used again, it can be washed with distilled water to balance two column volumes and can be used again.

[0035] The purification method of sodium difluoro(oxalato)borate of the present invention has the following beneficial effects:

[0036] The mixed solution containing difluorooxaloborate is passed through a column in the present invention: Difluorooxaloborate diffuses from the bulk solution to the liquid film (boundary layer) on the surface of the anion exchange resin particles; then it enters the particles and passes through the porous structure inside the resin to reach the region where the functional groups are located; difluorooxaloborate undergoes displacement with the exchangeable anions on the resin functional groups, and difluorooxaloborate is fixed on the resin surface due to Coulomb force. Then, the unfixed impurity ion tetrafluoroborate is washed with a buffer, and then eluted with a high-concentration sodium hydroxide. According to the difference in solubility between sodium hydroxide and sodium difluorooxaloborate in a poor solvent, sodium difluorooxaloborate and sodium hydroxide are separated by the poor solvent recrystallization method, thereby obtaining high-purity sodium difluorooxaloborate.

[0037] The present invention uses an anion exchange resin with high affinity for difluorooxaloborate. By regulating the mass transfer conditions (solution flow rate, temperature, and concentration gradient) of the liquid film boundary layer on the surface of the resin particles, the competitive ion strength, diffusion driving force, resin swelling degree, and functional group activity can be effectively controlled. Among them, the effective control of the competitive ion strength can avoid the interference of impurity ions on the binding process between difluorooxaloborate and the resin, ensuring that difluorooxaloborate can smoothly undergo an exchange reaction with the resin; the optimization of the diffusion driving force can enable ions to reach the resin surface more quickly and participate in the exchange reaction; the reasonable adjustment of the resin swelling degree helps to maintain the structural stability of the resin and provides a good spatial environment for ion exchange; the precise regulation of the functional group activity can significantly improve the adsorption capacity of the resin for difluorooxaloborate. Finally, a process with good purification effect and high yield is obtained.

[0038] The purification method of sodium difluorooxaloborate in the present invention combines the anion exchange resin method with the poor solvent recrystallization method. Compared with the conventional separation method that requires repeated recrystallization, the operation is simple, and the used resin and organic solvent can be reused again. The crude product of sodium difluorooxaloborate can be upgraded to battery grade at one time. Detailed implementation mode

[0039] In order to enable those skilled in the art to better understand the technical solution of the present invention, the products of the present invention will be further described in detail below in conjunction with the embodiments.

[0040] The present invention discloses a purification method of sodium difluorooxaloborate, which includes the following steps:

[0041] S1. Anion exchange resin regeneration: The anion exchange resin is loaded into a column, and the anion exchange resin is regenerated with an alkaline solution and washed with distilled water until the effluent is neutral.

[0042] S2. Anion exchange resin equilibration: The resin column is equilibrated with a sodium hydroxide solution with a first alkalinity of pH 8-9.

[0043] S3. Sodium tetrafluoroborate removal: Dissolve the crude sodium difluoro(oxalato)borate and load the sample, then rinse with a sodium hydroxide solution of the first alkalinity to remove sodium tetrafluoroborate and obtain an intermediate product.

[0044] S4. Intermediate product elution: Elute the intermediate product in step S3 with a sodium hydroxide solution of the second alkalinity with a pH of 11 - 12.

[0045] S5. Primary crystallization: Collect the eluted liquid, and after crystallization, obtain a primary crystallization of a mixed solid of sodium hydroxide and sodium difluoro(oxalato)borate.

[0046] S6. Secondary crystallization: Dissolve the primary crystallization of the mixed solid in acetonitrile, filter the acetonitrile solution, and perform secondary crystallization on the filtrate to obtain a purified sodium difluoro(oxalato)borate solid.

[0047] Furthermore, in step S1, the mass concentration of the alkaline solution is 3% - 5%, the dosage is 2 - 3 times the volume of the anion resin, and the flow rate ≤ 10 m / h.

[0048] Furthermore, in step S4, the dosage of the second alkaline solution is 3 - 4 times the volume of the resin, and the flow rate is 4 - 6 m / h.

[0049] Furthermore, in step S1, the type of the anion exchange resin is one or more of D201, D202, D261, D280, 201*7.

[0050] Furthermore, in step S3, the concentration of the solution formed after the crude sodium difluoro(oxalato)borate is dissolved in distilled water is 18 - 20%.

[0051] Furthermore, in step S3, the dosage of the first alkaline solution for rinsing is 1 - 3 times the volume of the resin.

[0052] Furthermore, in step S6, the mass ratio of acetonitrile to sodium difluoro(oxalato)borate is 15 - 20.

[0053] Furthermore, in step S6, the pore diameter of the filter paper selected for filtration is 0.3 - 0.5 μm.

[0054] Furthermore, the crystallization method is evaporation crystallization in all steps.

[0055] Furthermore, when loading the sample, directly pour an appropriate amount of the above-mentioned crude sodium difluoro(oxalato)borate solution into the anion resin column, and the groups on the resin exchange with sodium difluoro(oxalato)borate:

[0056] R-OH + NaDFOB → R-DFOB + NaOH

[0057] Furthermore, the temperature of the operations in the above steps is room temperature, and the liquid flow rate is controlled by a constant flow pump.

[0058] Further, after the anion exchange column is reused, it can be reused after being washed with distilled water to balance two column volumes for cleaning.

[0059] Example 1

[0060] Step 1: Select a chromatography column with a specification of 14 * 300 ml, pack the column with an appropriate amount of 201 * 7 type anion exchange resin, wash it with 3% sodium hydroxide solution for two column volumes at a flow rate of 5 m / h, and wash it with distilled water until the pH value of the effluent reaches 7.

[0061] Step 2: Balance the column with sodium hydroxide solution at pH 8 until the pH of the effluent is 8.

[0062] Step 3: Dissolve 5 g of crude sodium difluoro(oxalato)borate with a purity of 60.51% in distilled water to prepare a 20% solution, and directly pour the sample into the resin column for loading; balance it with sodium hydroxide at pH = 8 for two column volumes.

[0063] Step 4: Wash and balance with sodium hydroxide at pH = 11 for four column volumes for elution at a flow rate of 4 m / h, and collect the eluted liquid.

[0064] Step 5: Evaporate and crystallize the eluted liquid, add the obtained solid to 55 ml of acetonitrile (the mass ratio of acetonitrile to the theoretically extractable sodium difluoro(oxalato)borate is 55:3) and dissolve it for 5 h.

[0065] Step 6: Filter with a 0.25 μm filter paper, evaporate and crystallize the filtrate to obtain 2.69 g of the required sodium difluoro(oxalato)borate.

[0066] Example 2

[0067] Step 1: Select a chromatography column with a specification of 14 * 300 ml, pack the column with an appropriate amount of 201 * 7 type anion exchange resin, wash it with 3% sodium hydroxide solution for two column volumes at a flow rate of 5 m / h, and wash it with distilled water until the pH value of the effluent reaches 7.

[0068] Step 2: Balance the column with sodium hydroxide solution at pH 8 until the pH of the effluent is 8.

[0069] Step 3: Dissolve 5 g of crude sodium difluoro(oxalato)borate with a purity of 80.22% in distilled water to prepare a 20% solution, and directly pour the sample into the resin column for loading; balance it with sodium hydroxide at pH = 8 for two column volumes.

[0070] Step 4: Wash and balance with sodium hydroxide at pH = 11 for four column volumes for elution at a flow rate of 4 m / h, and collect the eluted liquid.

[0071] Step 5: Evaporate and crystallize the eluted liquid, and add the obtained solid to 74 ml of acetonitrile (the mass ratio of acetonitrile to sodium difluoroborate oxalate that can be theoretically extracted is 74:4) and dissolve for 5 h;

[0072] Step 6: Filter with a 0.25-μm filter paper, evaporate and crystallize the filtrate to obtain 3.61 g of the required sodium difluoroborate oxalate;

[0073] Example 3

[0074] Step 1: Select a chromatography column with a specification of 14 * 300 ml, pack an appropriate amount of 201 * 7 type anion exchange resin, rinse with 3% sodium hydroxide solution for 2 column volumes at a flow rate of 5 m / h, and rinse with distilled water until the pH value of the effluent is 7;

[0075] Step 2: Equilibrate the column with sodium hydroxide solution with a pH of 8 until the pH of the effluent is 8;

[0076] Step 3: Dissolve 5 g of crude sodium difluoroborate oxalate with a purity of 95.14% in distilled water to form a 20% solution, and directly pour the sample into the resin column for loading; equilibrate with sodium hydroxide with a pH = 8 for 2 column volumes;

[0077] Step 4: Rinse and equilibrate with sodium hydroxide with a pH = 11 for 4 column volumes for elution at a flow rate of 4 m / h, and collect the eluted liquid;

[0078] Step 5: Evaporate and crystallize the eluted liquid, and add the obtained solid to 87 ml of acetonitrile (the mass ratio of acetonitrile to sodium difluoroborate oxalate that can be theoretically extracted is 87:4.8) and dissolve for 5 h;

[0079] Step 6: Filter with a 0.25-μm filter paper, evaporate and crystallize the filtrate to obtain 4.29 g of the required sodium difluoroborate oxalate;

[0080] Example 4

[0081] Step 1: Select a chromatography column with a specification of 14 * 300 ml, pack an appropriate amount of 201 * 7 type anion exchange resin, rinse with 3% sodium hydroxide solution for 2 column volumes at a flow rate of 5 m / h, and rinse with distilled water until the pH value of the effluent is 7;

[0082] Step 2: Equilibrate the column with sodium hydroxide solution with a pH of 9 until the pH of the effluent is 9;

[0083] Step 3: Dissolve 5 g of crude sodium difluoroborate oxalate with a purity of 80.22% in distilled water to form a 20% solution, and directly pour the sample into the resin column for loading; equilibrate with sodium hydroxide with a pH = 9 for 2 column volumes;

[0084] Step 4: Rinse and equilibrate with sodium hydroxide at pH = 12 for 4 column volumes, elute at a flow rate of 4 m / h, and collect the eluted liquid;

[0085] Step 5: Evaporate and crystallize the eluted liquid, add the obtained solid to 74 ml of acetonitrile (the mass ratio of acetonitrile to sodium difluoroborate that can be theoretically extracted is 74:4), and dissolve for 5 h;

[0086] Step 6: Filter with a 0.25-μm filter paper, evaporate and crystallize the filtrate to obtain 3.62 g of the required sodium difluoroborate;

[0087] Example 5

[0088] Step 1: Select a chromatography column with a specification of 14 * 300 ml, pack the column with an appropriate amount of 201 * 7 type anion exchange resin, rinse with 3% sodium hydroxide solution for 2 column volumes at a flow rate of 5 m / h, and rinse with distilled water until the pH value of the effluent is 7;

[0089] Step 2: Equilibrate the column with sodium hydroxide solution at pH 8 until the pH of the effluent is 8;

[0090] Step 3: Dissolve 5 g of crude sodium difluoroborate with a purity of 80.22% in distilled water to prepare a 20% solution, and directly pour the sample into the resin column for loading; equilibrate with sodium hydroxide at pH = 8 for 2 column volumes;

[0091] Step 4: Rinse and equilibrate with sodium hydroxide at pH = 11 for 4 column volumes, elute at a flow rate of 4 m / h, and collect the eluted liquid;

[0092] Step 5: Evaporate and crystallize the eluted liquid, add the obtained solid to 74 ml of acetonitrile (the mass ratio of acetonitrile to sodium difluoroborate that can be theoretically extracted is 74:4), and dissolve for 5 h;

[0093] Step 6: Filter with a 0.25-μm filter paper, evaporate and crystallize the filtrate to obtain 3.59 g of the required sodium difluoroborate;

[0094] Example 6

[0095] Step 1: Select a chromatography column with a specification of 14 * 300 ml, pack the column with an appropriate amount of D201 type anion exchange resin, rinse with 3% sodium hydroxide solution for 2 column volumes at a flow rate of 5 m / h, and rinse with distilled water until the pH value of the effluent is 7;

[0096] Step 2: Equilibrate the column with sodium hydroxide solution at pH 8 until the pH of the effluent is 8;

[0097] Step 3: Dissolve 5 g of crude sodium difluoro(oxalato)borate with a purity of 80.22% in distilled water to prepare a 20% solution, and directly pour the sample into the resin column for loading; Equilibrate with sodium hydroxide at pH = 8 for 2 column volumes;

[0098] Step 4: Wash and elute with sodium hydroxide at pH = 11 for 4 column volumes at a flow rate of 4 m / h, and collect the eluted liquid;

[0099] Step 5: Evaporate and crystallize the eluted liquid, and add the obtained solid to 74 ml of acetonitrile (the mass ratio of acetonitrile to the theoretically extractable sodium difluoro(oxalato)borate is 74:4) and dissolve for 5 h;

[0100] Step 6: Filter with a 0.25-μm filter paper, evaporate and crystallize the filtrate to obtain 3.55 g of the required sodium difluoro(oxalato)borate;

[0101] Comparative Example 1

[0102] Step 1: Select a chromatography column with a specification of 14 * 300 ml, pack the column with an appropriate amount of 201*7 type anion exchange resin, wash with 3% sodium hydroxide solution for 2 column volumes at a flow rate of 5 m / h, and wash with distilled water until the pH value of the effluent is 7;

[0103] Step 2: Equilibrate the column with sodium hydroxide solution at pH 8 until the pH of the effluent is 8;

[0104] Step 3: Dissolve 5 g of crude sodium difluoro(oxalato)borate with a purity of 80.22% in distilled water to prepare a 20% solution, and directly pour the sample into the resin column for loading; Equilibrate with sodium hydroxide at pH = 8 for 2 column volumes;

[0105] Step 4: Wash and elute with sodium hydroxide at pH = 10 for 2 column volumes at a flow rate of 8 m / h, and collect the eluted liquid;

[0106] Step 5: Evaporate and crystallize the eluted liquid, and add the obtained solid to 74 ml of acetonitrile (the mass ratio of acetonitrile to the theoretically extractable sodium difluoro(oxalato)borate is 74:4) and dissolve for 5 h;

[0107] Step 6: Filter with a 0.25-μm filter paper, evaporate and crystallize the filtrate to obtain 3.30 g of the required sodium difluoro(oxalato)borate.

[0108] Comparative Example 2

[0109] Step 1: Select a chromatography column with a specification of 14 * 300 ml, pack the column with an appropriate amount of 201*7 type anion exchange resin, wash with 3% sodium hydroxide solution for 2 column volumes at a flow rate of 5 m / h, and wash with distilled water until the pH value of the effluent is 7;

[0110] Step 2: Equilibrate the column with sodium hydroxide solution at pH = 8 until the pH of the effluent is 8;

[0111] Step 3: Dissolve 5 g of crude sodium difluoro(oxalato)borate with a purity of 80.22% in distilled water to prepare a 20% solution, and directly pour the sample into the resin column for loading; Equilibrate with sodium hydroxide at pH = 7 for 1.5 column volumes;

[0112] Step 4: Rinse and equilibrate with sodium hydroxide at pH = 11 for 4 column volumes for elution, with a flow rate of 4 m / h, and collect the eluted liquid;

[0113] Step 5: Evaporate and crystallize the eluted liquid, add the obtained solid to 74 ml of acetonitrile (the mass ratio of acetonitrile to the theoretically extractable sodium difluoro(oxalato)borate is 74:4) and dissolve for 5 h;

[0114] Step 6: Filter with a 0.25-μm filter paper, evaporate and crystallize the filtrate to obtain 3.63 g of the required sodium difluoro(oxalato)borate;

[0115] Testing and Results

[0116] Use a nuclear magnetic resonance spectrometer to test the purity of the product; Yield (%) = actual product mass / theory

[0117] To compare the purification effect of the sodium difluoro(oxalato)borate purification method of the present invention, Table 1 shows the mass fractions of sodium difluoro(oxalato)borate and sodium tetrafluoroborate and the product purity in the crude product before purification and the product obtained after purification in Examples 1-3 and Comparative Examples 1-2. The test results are shown in Table 1:

[0118] Table 1 Performance Test Results

[0119]

[0120] According to the test results of Example 1, Example 2 and Example 3 in Table 1, it can be seen that when the purity of sodium difluoro(oxalato)borate is low (60.51%), medium (80.22%) or high (95.14%), battery-grade products can be obtained by using this method for purification. Relatively speaking, after synthesizing sodium difluoro(oxalato)borate by the conventional synthesis method and filtering to remove insoluble impurities, the content of sodium difluoro(oxalato)borate in the final obtained crude product generally will not be lower than 60%. The method of the present invention has a wide range of applications and can be used for the purification of most reactions that generate sodium tetrafluoroborate impurities.

[0121] According to the test results of Example 2 and Comparative Example 1 in Table 1, it can be seen that the concentration and flow rate of the eluent have a great influence on the yield. Relatively small amounts of low-concentration eluent cannot completely elute the difluoro(oxalato)borate anion at high flow rates. Therefore, the dosage, concentration and flow rate of the eluent need to be strictly controlled within a suitable range.

[0122] As can be seen from the test results of Example 2 and Comparative Example 2 in Table 1, when rinsing to remove sodium tetrafluoroborate, the concentration and dosage of the rinsing solution have a certain impact on the purity, and the concentration and dosage of the rinsing solution need to be controlled within a suitable range.

[0123] The above embodiments are only specific embodiments of the present invention, and the description thereof is relatively specific and detailed, but it should not be construed as limiting the scope of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can be made, and these obvious replacement forms all belong to the protection scope of the present invention.

Claims

1. A purification method of sodium difluorooxalate borate, characterized in that It includes the following steps: S1. Anion exchange resin regeneration: Load the anion exchange resin into a column, regenerate the anion exchange resin with an alkaline solution, and wash it with distilled water until the effluent is neutral; S2. Anion exchange resin equilibration: Equilibrate the resin column with a sodium hydroxide solution having a first alkalinity with a pH of 8-9; S3. Sodium tetrafluoroborate removal: Dissolve and load the crude sodium difluoroborate oxalate, and then rinse it with a sodium hydroxide solution having a first alkalinity to remove sodium tetrafluoroborate to obtain an intermediate product; S4. Intermediate product elution: Elute the intermediate product in step S3 with a sodium hydroxide solution having a second alkalinity with a pH of 11-12; S5. Primary crystallization: Collect the eluted liquid, and after crystallization, obtain a primary crystallization of a mixed solid of sodium hydroxide and sodium difluoroborate oxalate; S6. Secondary crystallization: Dissolve the primary crystallization of the mixed solid in acetonitrile, filter the acetonitrile solution, and perform secondary crystallization on the filtrate to obtain a purified sodium difluoroborate oxalate solid.

2. The purification method of sodium difluoro(oxalato)borate according to claim 1, characterized in that: In step S1, the mass concentration of the alkaline solution is 3%-5%, the dosage is 2-3 times the volume of the anion resin, and the flow rate ≤ 10 m / h.

3. The purification method of sodium difluoro(oxalato)borate according to claim 1, characterized in that: In step S4, the dosage of the second alkaline solution is 3-4 times the volume of the resin, and the flow rate is 4-6 m / h.

4. The purification method of sodium difluoro(oxalato)borate according to claim 1, wherein: In step S1, the model of the anion exchange resin is one or more of D201, D202, D261, D280, 201*7.

5. The purification method of sodium difluoro(oxalato)borate according to claim 1, wherein: In step S3, the concentration of the solution formed after the crude sodium difluoroborate oxalate is dissolved in distilled water is 18-20%.

6. The purification method of sodium difluoro(oxalato)borate according to claim 1, wherein: In step S3, the dosage of the first alkaline solution for rinsing is 1-3 times the volume of the resin.

7. The purification method of sodium difluoro(oxalato)borate according to claim 1, characterized in that: In step S6, the mass ratio of acetonitrile to sodium difluoroborate oxalate is 15-20.

8. The purification method of sodium difluoro(oxalato)borate according to claim 1, wherein: In step S6, the pore size of the filter paper selected for filtration is 0.3-0.5 μm.

9. The purification method of sodium difluoro(oxalato)borate according to claim 1, wherein: The crystallization method is evaporation crystallization in all cases.

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