A method for preparing sodium bisfluorooxalate borate

Abstract

This invention proposes a method for preparing sodium difluorooxalate borate, belonging to the field of lithium battery additive technology. The method includes the following steps: 1) Drying oxalic acid, boric acid, and ammonium bifluoride and then mixing them to obtain a dried mixture; 2) Adding the dried mixture, organic solvent, and catalyst into a reactor for ultrasonication, heating, and stirring to obtain a product containing ammonium difluorooxalate borate; wherein ammonia gas is introduced into the reactor when heating to 70-100℃; 3) Purifying and drying the product containing ammonium difluorooxalate borate sequentially to obtain ammonium difluorooxalate borate; 4) Mixing ammonium difluorooxalate borate and anhydrous sodium carbonate and pressing them into tablets; 5) Calcining the tableted mixture to obtain sodium difluorooxalate borate. The method provided by this invention is simple, cost-effective, and yields a product with high purity.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery additive preparation technology, and particularly relates to a method for preparing sodium difluorooxalate borate. Background Technology

[0002] In the research of sodium-ion battery electrolytes, additives for sodium-ion battery electrolytes have attracted much attention due to their wide variety and the fact that adding small amounts can significantly improve performance. Sodium difluorooxalate borate, possessing characteristics of both sodium tetrafluoroborate and sodium bis(oxalate borate), has seen continuously increasing research interest. However, research on sodium-ion battery electrolyte additives is relatively limited, and synthesis techniques are also scarce. Therefore, studying the synthesis of sodium difluorooxalate borate is of great significance. Studies have shown that sodium difluorooxalate borate is compatible with various solvents and performs well in sodium-ion battery electrolytes, exhibiting a wide electrochemical window, low viscosity, better cycle stability, and the ability to suppress various side reactions in the electrolyte.

[0003] However, due to the later start of research on sodium-ion battery electrolytes, their performance still lags behind that of lithium-ion battery electrolytes. There is limited research on the synthesis of sodium difluorooxalate borate both domestically and internationally, and some processes follow the synthesis route of lithium difluorooxalate borate. However, because sodium and lithium have different Stokes radii, their properties differ significantly. Simply applying the lithium salt method to sodium salts is not advisable; therefore, novel synthesis methods tailored to the properties of sodium ions are needed. Summary of the Invention

[0004] This invention provides a method for preparing sodium difluorooxalate borate, which is simple, economical, and yields a product with high purity.

[0005] To achieve the above objectives, the present invention provides a method for preparing sodium difluorooxalate borate, comprising the following steps:

[0006] 1) Oxalic acid, boric acid and ammonium bifluoride are dried and then mixed to obtain a dried mixture.

[0007] 2) The dried mixture, organic solvent and catalyst are added to the reactor and subjected to ultrasonication, heating and stirring to obtain a product containing ammonium difluorooxalate borate; wherein ammonia gas is introduced into the reactor when the temperature is heated to 70-100°C;

[0008] 3) The product containing ammonium difluorooxalate borate was purified and dried sequentially to obtain ammonium difluorooxalate borate;

[0009] 4) Mix ammonium difluorooxalate borate and anhydrous sodium carbonate and compress into tablets;

[0010] 5) The tableted mixture is calcined to obtain sodium difluorooxalate borate.

[0011] Preferably, in step 1), the molar ratio of oxalic acid to boric acid is 0.8 to 1.5:1; the molar ratio of the mixture of oxalic acid and boric acid to ammonium bifluoride is 1.9 to 2.2:1, wherein the molecular weight of the mixture of oxalic acid and boric acid is the average of the sum of the molecular weights of oxalic acid and boric acid.

[0012] Preferably, in step 1), oxalic acid, boric acid, and ammonium bifluoride are activated before drying, and in step 3) after obtaining difluorooxalic acid borate ammonium; the activation method is ball milling and sieving; the ball milling speed is 80-150 r / min, and the time is 20-40 min; the sieve mesh size is 200-500 mesh.

[0013] Preferably, the drying method in step 1) is vacuum drying; the temperature for vacuum drying of oxalic acid and / or boric acid is 50-90°C and the vacuum degree is 30-100Pa; the temperature for vacuum drying of ammonium bifluoride is 80-100°C and the vacuum degree is 30-100Pa.

[0014] Preferably, the organic solvent in step 2) is one or more of acetonitrile, tetrahydrofuran, and ethyl acetate, and the amount added is 450-550 mL per mole of ammonium bifluoride; the catalyst is antimony pentafluoride, and the amount added is 0.03-0.1 g per mole of ammonium bifluoride.

[0015] Preferably, in step 2), the ultrasonic power is 20-40 kHz, the heating temperature is 70-100°C, the stirring rate is 400-800 r / min, the ammonia gas flow rate is 1-3 L / h, and the reaction time after the ammonia gas is introduced is 3-5 h.

[0016] Preferably, the purification method in step 3) is to filter the product containing ammonium difluorooxalate borate, dissolve the obtained filter cake in methanol and filter it again, and then rotary evaporate the filtrate; the drying method in step 3) is vacuum drying, with a vacuum drying temperature of 80-100°C and a vacuum degree of 30-100 Pa.

[0017] Preferably, in step 4), the molar ratio of ammonium difluorooxalate borate and anhydrous sodium carbonate is 1.8 to 2.0:1.

[0018] Preferably, in step 5), calcination is carried out under nitrogen conditions, with a nitrogen flow rate of 5-10 L / min, a calcination temperature of 140-160°C, and a calcination time of 3-5 h.

[0019] Preferably, after obtaining sodium difluorooxalate borate in step 5), it is further purified and dried. The purification method is to mix sodium difluorooxalate borate with acetonitrile or tetrahydrofuran and filter it, and then rotary evaporate the filtrate. The drying method is vacuum drying at a temperature of 100-120°C, a vacuum degree of 30-100 Pa, and a time of 3-5 hours.

[0020] Compared with the prior art, the advantages and positive effects of the present invention are as follows:

[0021] The method for synthesizing sodium difluorooxalate borate provided by this invention first synthesizes ammonium difluorooxalate borate, and then utilizes the substitution ability of the ammonium salt with the sodium salt to synthesize sodium difluorooxalate borate. Preparing the sodium salt through ammonium salt conversion avoids the problems of incomplete reaction and numerous byproducts encountered in traditional methods using tetrafluoroborate. Furthermore, the use of ammonium salt further enhances the conversion activity and increases the degree of reaction. This method is relatively simple to implement, environmentally friendly, and uses low-cost raw materials, resulting in high economic benefits. Detailed Implementation

[0022] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] This invention provides a method for preparing sodium difluorooxalate borate, comprising the following steps:

[0024] 1) Oxalic acid, boric acid and ammonium bifluoride are dried and then mixed to obtain a dried mixture.

[0025] 2) The dried mixture, organic solvent and catalyst are added to the reactor and subjected to ultrasonication, heating and stirring to obtain a product containing ammonium difluorooxalate borate; wherein ammonia gas is introduced into the reactor when the temperature is heated to 70-100°C;

[0026] 3) The product containing ammonium difluorooxalate borate was purified and dried sequentially to obtain ammonium difluorooxalate borate;

[0027] 4) Mix ammonium difluorooxalate borate and anhydrous sodium carbonate and compress into tablets;

[0028] 5) The tableted mixture is calcined to obtain sodium difluorooxalate borate.

[0029] This invention involves drying oxalic acid, boric acid, and ammonium bifluoride, then mixing them to obtain a dried mixture. In this invention, the oxalic acid, boric acid, and ammonium bifluoride are preferably activated before drying. The activation method is preferably ball milling and sieving; the ball milling speed is preferably 80–150 r / min, and the time is 20–40 min; the sieve mesh size is preferably 200–500 mesh. In this invention, ball milling can obtain raw materials with lower particle sizes, increasing the surface area of ​​ions and enhancing reactivity. In this invention, oxalic acid and boric acid do not react prematurely and are not well soluble in organic solvent systems. Simply relying on mechanical stirring is insufficient to achieve a good mixing effect. Therefore, to further disperse the two powder materials, it is preferable to mix oxalic acid and boric acid together and then ball mill them.

[0030] In this invention, the drying method is preferably vacuum drying. The vacuum drying temperature for oxalic acid and / or boric acid is preferably 50–90°C, and the vacuum degree is preferably 30–100 Pa; the vacuum drying temperature for ammonium bifluoride is preferably 80–100°C, and the vacuum degree is preferably 30–100 Pa. If water is present in the reaction, it can cause partial agglomeration of the materials. Once agglomeration occurs, there is a possibility of concentration imbalance in some materials, hindering the reaction. Furthermore, ammonium bifluoride and water at high temperatures easily generate certain acidic substances, leading to partial decomposition of the materials. In this invention, drying the raw materials avoids the above risks.

[0031] In this invention, the molar ratio of oxalic acid to boric acid is preferably 0.8 to 1.5:1; the molar ratio of the mixture of oxalic acid and boric acid to ammonium bifluoride is preferably 1.9 to 2.2:1, wherein the molecular weight of the mixture of oxalic acid and boric acid is the average of the sum of the molecular weights of oxalic acid and boric acid.

[0032] After obtaining the dried mixture, the present invention adds the dried mixture, organic solvent and catalyst into a reactor for ultrasonication, heating and stirring to obtain a product containing ammonium difluorooxalate borate; wherein when heated to 70-100°C, ammonia gas is introduced into the reactor.

[0033] In this invention, the organic solvent is preferably one or more selected from acetonitrile, tetrahydrofuran, and ethyl acetate, and the addition amount is preferably 450-550 mL per mole of ammonium bifluoride; the catalyst is preferably antimony pentafluoride, and the addition amount is preferably 0.03-0.1 g per mole of ammonium bifluoride. In this invention, the ultrasonic power is preferably 20-40 kHz, the heating temperature is preferably 70-100 °C; the stirring rate is preferably 400-800 r / min; the ammonia gas flow rate is preferably 1-3 L / h, and the reaction time after the ammonia gas is introduced is 3-5 h.

[0034] In this invention, placing the reaction in an ultrasonic bath effectively disperses ions, resulting in more thorough collisions of particles in the heterogeneous reaction. Since a small amount of water is produced in the initial stages of the reaction, a small amount of ammonia is introduced to maintain a slightly alkaline environment, preventing excessive acidity that could lead to product decomposition and improving yield and product purity.

[0035] After obtaining the product containing ammonium difluorooxalate borate, the present invention purifies and dries the product sequentially to obtain ammonium difluorooxalate borate. In the present invention, the preferred purification method is to filter the product containing ammonium difluorooxalate borate, dissolve the resulting filter cake in methanol, filter again, and then rotary evaporate the filtrate. In the present invention, the solubility of methanol in ammonium salts effectively separates byproducts and unreacted raw materials. In the present invention, the rotary evaporation temperature is preferably 40–60°C, and the vacuum degree is 25–35 Pa. In the present invention, the preferred drying method is vacuum drying, with a preferred vacuum drying temperature of 80–100°C and a preferred vacuum degree of 30–100 Pa.

[0036] After obtaining ammonium difluorooxalate borate, this invention mixes it with anhydrous sodium carbonate and compresses it into tablets. In this invention, the ammonium difluorooxalate borate is preferably activated after obtaining it; the activation method is ball milling and sieving; the ball milling speed is preferably 80-150 r / min, and the time is 20-40 min; the sieve mesh size is preferably 200-500 mesh. In this invention, the molar ratio of ammonium difluorooxalate borate to anhydrous sodium carbonate is preferably 1.8-2.0:1. In this invention, the tableting pressure is 15-20 MPa, and the time is 20-30 s.

[0037] After tableting, the mixture is calcined to obtain sodium difluorooxalate borate. Preferably, calcination is carried out under nitrogen atmosphere, with a nitrogen flow rate of 5–10 L / min, a calcination temperature of 140–160 °C, and a calcination time of 3–5 h. This invention employs a solid-phase method for the displacement reaction, avoiding the problems of incomplete reaction and long reaction time associated with the liquid-phase method, which cannot withstand high temperatures.

[0038] Existing technologies use tetrafluoroborate as a raw material to prepare difluorooxalate borate. However, in traditional preparation processes, tetrafluoroborate requires high-temperature and energy-intensive reaction conditions. Furthermore, due to its reactivity, the preparation of difluorooxalate borate often results in incomplete reaction due to uneven mixing. Therefore, in the actual use of sodium tetrafluoroborate, many byproducts are generated, including monofluoro salts and other sodium salts, leading to separation difficulties and wasted purification solvents. This invention uses ammonium salts in the substitution process with sodium metal ions. The high reactivity of ammonium ions allows for a better reaction, increasing the degree of reaction and reducing byproduct formation.

[0039] After obtaining sodium difluorooxalate borate, the present invention further purifies and dries it. In this invention, the preferred purification method is to mix sodium difluorooxalate borate with acetonitrile or tetrahydrofuran and filter the mixture, then rotary evaporate the filtrate; the rotary evaporation temperature is 50–60°C, and the vacuum degree is -0.1 to -0.03 MPa. The drying method is vacuum drying, with a temperature of 100–120°C, a vacuum degree of 30–100 Pa, and a time of 3–5 hours.

[0040] To further illustrate the present invention, the technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0041] Example 1

[0042] First, oxalic acid and boric acid were added to a ball mill at a 1:1 molar ratio, along with oxalic acid beads (the mass ratio of oxalic acid beads to the oxalic acid-boric acid mixture was 1:1.5). The mixture was ball-milled at 100 rpm for 30 minutes. Then, ammonium bifluoride was added to another ball mill, along with oxalic acid beads (the mass ratio of oxalic acid beads to ammonium bifluoride was 1:1.5). The mixture was ball-milled at 100 rpm for 30 minutes. Both products were then sieved through a 500-mesh sieve.

[0043] The sieve underflow of the mixture of oxalic acid and boric acid was vacuum dried (temperature 90°C, vacuum degree 50 Pa) to obtain a dried oxalic acid-boric acid mixture (the mixture of boric acid and oxalic acid was calculated at 76 g per mole). The sieve underflow of ammonium bifluoride was vacuum dried (temperature 80°C, vacuum degree preferably 100 Pa) to obtain dried ammonium bifluoride.

[0044] Example 2

[0045] 500 ml of acetonitrile was transferred into a four-necked flask (a mechanical stirrer was attached to the middle of the four-necked flask, the left port was connected to the ammonia gas line, the right port was connected to the condenser, and the middle port was connected to a thermometer). 2 mol of the dry oxalic acid-boric acid mixture obtained in Example 1 and 1 mol of dry ammonium bifluoride were added to the above four-necked flask, followed by 0.05 g of antimony pentafluoride. The four-necked flask was placed in an ultrasonic dispersion bath. First, the ultrasonic bath was heated to 85°C. After the temperature reached the set value, the ultrasonic generator and stirrer were turned on (ultrasonic power of 35 kHz, mechanical stirring rate of 400 r / min). The ammonia gas flow rate was 1.5 L / h, and the reaction time was 3.5 h. After the reaction was completed, the product was filtered to separate the filtrate and the filter cake. The filter cake was then dissolved in 500 ml of methanol at room temperature using magnetic stirring at 300 r / min for 2 h. The solution was then subjected to rotary evaporation (temperature 50℃, vacuum 30 Pa). The resulting powder was crude ammonium difluorooxalate borate. The crude product was dried at 85℃ and vacuum 30 Pa for 2 h to obtain pure ammonium difluorooxalate borate, with a yield of 91.5% and a purity greater than 99.9%.

[0046] The ammonium difluorooxalate borate prepared in the previous step was crushed using a ball mill at a speed of 100 r / min for 30 min. The mixture was then passed through a 500-mesh sieve, and the undersize was used for the synthesis of sodium difluorooxalate borate in the next step.

[0047] The molar ratio of ammonium difluorooxalate borate undersize to sodium carbonate was 2:1. The two materials were mixed using a planetary mixer, and then the mixture was compressed into tablets using a tablet press at a pressure of 15 MPa. The tablets were placed in a crucible within a tube furnace and heated under nitrogen (155°C, 3.5 h, nitrogen flow rate 5 L / min). The tablets were then dissolved in 1.5 L of acetonitrile, stirred, filtered, and the filtrate was rotary evaporated (55°C, -0.1 MPa). After rotary evaporation, the crude sodium difluorooxalate borate was vacuum dried (110°C, 30 Pa). After 3 h, the product was obtained as sodium difluorooxalate borate. The yield of this step was 93.5%, with a purity greater than 99.9%, and the final yield was 85.6%.

[0048] Purity testing included: Ammonium difluorooxalate borate was tested using NMR, with boron and fluorine spectra measured. Since borate and oxalate ions readily decompose in water, ion chromatography was not feasible for quantitative testing; therefore, NMR was used for semi-quantitative analysis. By combining the two spectra, the presence of a single peak for both F and B, indicating only one chemical environment, confirmed a product purity greater than 99.9%. The second step, sodium difluorooxalate borate, was also tested using the same method, with an additional nitrogen element test to check for the presence of ammonium compounds. The absence of nitrogen, combined with NMR analysis, confirmed a product purity greater than 99.9%.

[0049] Yield testing: The final yield is calculated based on the molar ratio: weight of the feed product * weight of the theoretical product. Final yield = yield of ammonium difluorooxalate borate * yield of sodium difluorooxalate borate.

[0050] Example 3

[0051] 500 ml of acetonitrile was transferred into a four-necked flask (a mechanical stirrer was attached to the middle of the four-necked flask, the left port was connected to the ammonia gas line, the right port was connected to the condenser, and the middle port was connected to a thermometer). 2.1 mol of the dry oxalate-boric acid mixture obtained in Example 1 and 1 mol of dry ammonium bifluoride were added to the above four-necked flask, followed by 0.1 g of antimony pentafluoride. The four-necked flask was placed in an ultrasonic dispersion bath. First, the ultrasonic bath was heated to 100°C. After the temperature reached the set value, the ultrasonic generator and stirrer were turned on (ultrasonic power of 20 kHz, mechanical stirring rate of 800 r / min). The ammonia gas flow rate was 1 L / h, and the reaction time was 5 h. After the reaction was completed, the product was filtered to separate the filtrate and the filter cake. The filter cake was then dissolved in 500 ml of methanol at room temperature using magnetic stirring at 200 r / min for 3 h. The solution was then subjected to rotary evaporation (temperature 50℃, vacuum 30 Pa). The resulting powder was crude ammonium difluorooxalate borate. The crude product was dried at 100℃ and 100 Pa vacuum for 1.5 h to obtain pure ammonium difluorooxalate borate, with a yield of 93.2% and a purity greater than 99.9%.

[0052] The ammonium difluorooxalate borate prepared in the previous step was crushed using a ball mill at a speed of 150 r / min for 20 min. The mixture was then passed through a 500-mesh sieve, and the undersize was used for the synthesis of sodium difluorooxalate borate in the next step.

[0053] The molar ratio of ammonium difluorooxalate borate undersize to sodium carbonate was 1.8:1. The two materials were mixed using a planetary mixer, and then the mixture was compressed into tablets using a tablet press at a pressure of 20 MPa. The tablets were placed in a crucible and heated in a tube furnace under nitrogen (140°C, 5 hours, nitrogen flow rate 5 L / min). The tablets were then dissolved in 1.5 L of tetrahydrofuran, stirred until dissolved, filtered, and the filtrate was rotary evaporated (55°C, -0.1 MPa). After rotary evaporation, the crude sodium difluorooxalate borate was vacuum dried (100°C, 30 Pa) for 4 hours to obtain sodium difluorooxalate borate. The yield of this step was 94.7%, with a purity greater than 99.9%. The final yield was 88.26%.

[0054] Example 4

[0055] 500 ml of acetonitrile was transferred into a four-necked flask (a mechanical stirrer was attached to the middle of the four-necked flask, the left port was connected to the ammonia gas line, the right port was connected to the condenser, and the middle port was connected to a thermometer). 1.9 mol of the dry oxalate-boric acid mixture obtained in Example 1 and 1 mol of dry ammonium bifluoride were added to the above four-necked flask, followed by 0.03 g of antimony pentafluoride. The four-necked flask was placed in an ultrasonic dispersion bath. The ultrasonic bath was first heated to 70°C. After the temperature reached the set value, the ultrasonic generator and stirrer were turned on (ultrasonic power of 40 kHz, mechanical stirring rate of 400 r / min). The ammonia gas flow rate was 3 L / h, and the reaction time was 3 h. After the reaction was completed, the product was filtered to separate the filtrate and the filter cake. The filter cake was then dissolved in 500 ml of methanol at room temperature using magnetic stirring at 300 r / min for 2 h. The solution was then subjected to rotary evaporation (temperature 50℃, vacuum 30 Pa). The resulting powder was crude ammonium difluorooxalate borate. The crude product was dried at 85℃ and vacuum 30 Pa for 2 h to obtain pure ammonium difluorooxalate borate, with a yield of 90.7% and a purity of 99.5%.

[0056] The ammonium difluorooxalate borate prepared in the previous step was crushed using a ball mill at a speed of 150 r / min for 40 min. The mixture was then passed through a 400-mesh sieve, and the undersize was used for the synthesis of sodium difluorooxalate borate in the next step.

[0057] The molar ratio of ammonium difluorooxalate borate undersize to sodium carbonate is 2:1. The two materials are mixed using a planetary mixer, and then the mixture is compressed into tablets using a tablet press at a pressure of 15 MPa. The tablets are placed in a crucible within a tube furnace and heated under nitrogen (160°C, 3 hours, nitrogen flow rate 10 L / min). The tablets are then dissolved in 1.5 L of acetonitrile, stirred, filtered, and the filtrate is rotary evaporated (55°C, -0.1 MPa). After rotary evaporation, the crude sodium difluorooxalate borate is vacuum dried (100°C, 100 Pa) for 3 hours to obtain sodium difluorooxalate borate. The yield of this step is 95.1%, with a purity greater than 99.9%, and the final yield is 86.25%.

[0058] Comparative Example 1

[0059] 500 ml of acetonitrile was transferred into a four-necked flask (a mechanical stirrer was installed at the middle of the four-necked flask, the left port was connected to the ammonia gas line, the right port was connected to the condenser, and the middle port was connected to a thermometer). 2 mol of the dry oxalic acid-boric acid mixture obtained in Example 1 and 1 mol of dry ammonium bifluoride were added to the above four-necked flask, followed by 0.05 g of antimony pentafluoride. The four-necked flask was placed in a water bath and heated to 85°C. After the temperature reached the set value, the stirrer was turned on (mechanical stirring speed of 400 r / min), the ammonia gas flow rate was 1.5 L / h, and the reaction time was 3.5 h. After the reaction was completed, the product was filtered to separate the filtrate and the filter cake. The filter cake was then dissolved in 500 ml of methanol at room temperature using magnetic stirring at 300 r / min for 2 h. The solution was then subjected to rotary evaporation (temperature 50℃, vacuum 30 Pa). The resulting powdery substance was crude ammonium difluorooxalate borate. The crude product was dried at 85℃ and 30 Pa under vacuum for 2 h to obtain pure ammonium difluorooxalate borate, with a yield of 80.4% and a purity of 98.1%.

[0060] Comparative Example 2

[0061] 500 ml of acetonitrile was transferred into a four-necked flask (a mechanical stirrer was attached to the middle of the four-necked flask, the left port was connected to the ammonia gas line, the right port was connected to the condenser, and the middle port was connected to a thermometer). 2 mol of the dry oxalic acid-boric acid mixture obtained in Example 1 and 1 mol of dry ammonium bifluoride were added to the above four-necked flask, followed by 0.05 g of antimony pentafluoride. The four-necked flask was placed in an ultrasonic dispersion bath. First, the ultrasonic bath was heated to 85°C. After the temperature reached the set value, the ultrasonic generator and stirrer were turned on (ultrasonic power of 35 kHz, mechanical stirring speed of 400 r / min). The reaction time was 3.5 h. After the reaction was completed, the product was filtered to separate the filtrate and the filter cake. The filter cake was then dissolved in 500 ml of methanol at room temperature using magnetic stirring at 300 r / min for 2 h. The solution was then subjected to rotary evaporation (temperature 50℃, vacuum 30 Pa). The resulting powder was crude ammonium difluorooxalate borate. The crude product was dried at 85℃ and 30 Pa under vacuum for 2 h to obtain pure ammonium difluorooxalate borate, with a yield of 74.1% and a purity of 85.2%.

[0062] Comparative Example 3

[0063] Transfer 500 ml of acetonitrile into a four-necked flask (with a mechanical stirrer attached to the middle neck, ammonia gas line connected to the left neck, a condenser to the right neck, and a thermometer to the middle neck). Add 2 mol of oxalic acid-boric acid mixture (oxalic acid and boric acid mixed in a 1:1 molar ratio) and 1 mol of ammonium bifluoride to the four-necked flask, then add 0.05 g of antimony pentafluoride. Place the four-necked flask in an ultrasonic dispersion bath. First, heat the ultrasonic bath to 85°C. Once the temperature reaches the set value, turn on the ultrasonic generator and stirrer (ultrasonic power 35 kHz, mechanical stirring speed 400 r / min). The ammonia gas flow rate is 1.5 L / h, and the reaction time is 3.5 h. After the reaction was completed, the product was filtered to separate the filtrate and the filter cake. The filter cake was then dissolved in 500 ml of methanol at room temperature using magnetic stirring at 300 r / min for 2 h. The solution was then subjected to rotary evaporation (temperature 50℃, vacuum 30 Pa). The resulting powder was crude ammonium difluorooxalate borate. The crude product was dried at 85℃ and 30 Pa under vacuum for 2 h to obtain pure ammonium difluorooxalate borate, with a yield of 83.2% and a purity of 91.6%.

[0064] Comparative Example 4

[0065] 500 ml of acetonitrile was transferred into a four-necked flask (a mechanical stirrer was attached to the middle of the four-necked flask, the left port was connected to the ammonia gas line, the right port was connected to the condenser, and the middle port was connected to a thermometer). 2 mol of the dry oxalic acid-boric acid mixture obtained in Example 1 and 1 mol of dry ammonium bifluoride were added to the above four-necked flask, followed by 0.05 g of antimony pentafluoride. The four-necked flask was placed in an ultrasonic dispersion bath. First, the ultrasonic bath was heated to 85°C. After the temperature reached the set value, the ultrasonic generator and stirrer were turned on (ultrasonic power of 35 kHz, mechanical stirring rate of 400 r / min). The ammonia gas flow rate was 1.5 L / h, and the reaction time was 3.5 h. After the reaction was completed, the product was filtered to separate the filtrate and the filter cake. The filter cake was then dissolved in 500 ml of methanol at room temperature using magnetic stirring at 300 r / min for 2 h. The solution was then subjected to rotary evaporation (temperature 50℃, vacuum 30 Pa). The resulting powder was crude ammonium difluorooxalate borate. The crude product was dried at 85℃ and vacuum 30 Pa for 2 h to obtain pure ammonium difluorooxalate borate, with a yield of 91.5% and a purity greater than 99.9%.

[0066] The ammonium difluorooxalate borate prepared in the previous step was crushed using a ball mill at a speed of 100 r / min for 30 min. The mixture was then passed through a 500-mesh sieve, and the undersize was used for the synthesis of sodium difluorooxalate borate in the next step.

[0067] 2 mol of ammonium difluorooxalate borate undersize was mixed with 1 mol of anhydrous sodium carbonate and 500 mL of anhydrous 1,2-dimethoxyethane and stirred at 300 r / min for 2 h. The mixture was then heated to 60 °C and stirred for another 5 h to obtain a mixture containing sodium difluorooxalate borate. The mixture was filtered, and the filter cake was dissolved in 1.5 L of acetonitrile. After stirring and dissolving, the filtrate was evaporated by rotary evaporation (55 °C, -0.1 MPa). After rotary evaporation, the crude sodium difluorooxalate borate was vacuum dried (110 °C, 30 Pa). After 3 h, the product was obtained as sodium difluorooxalate borate. The yield of this step was 90.6%, the purity was 97.8%, and the final yield was 82.9%.

[0068] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing sodium difluorooxalate borate, characterized in that, Includes the following steps: 1) Oxalic acid, boric acid and ammonium bifluoride are dried and then mixed to obtain a dried mixture. 2) The dried mixture, organic solvent and catalyst are added to the reactor and subjected to ultrasonication, heating and stirring to obtain a product containing ammonium difluorooxalate borate; wherein ammonia gas is introduced into the reactor when the temperature is heated to 70~100℃; 3) The product containing ammonium difluorooxalate borate was purified and dried sequentially to obtain ammonium difluorooxalate borate; 4) Mix ammonium difluorooxalate borate and anhydrous sodium carbonate and compress into tablets; 5) The tableted mixture is calcined to obtain sodium difluorooxalate borate; The organic solvent mentioned in step 2) is one or more of acetonitrile, tetrahydrofuran and ethyl acetate, and the amount added is 450~550mL per mole of ammonium bifluoride; the catalyst is antimony pentafluoride, and the amount added is 0.03~0.1g per mole of ammonium bifluoride.

2. The production method according to claim 1, characterized by, In step 1), the molar ratio of oxalic acid to boric acid is 0.8~1.5:1; the molar ratio of the mixture of oxalic acid and boric acid to ammonium bifluoride is 1.9~2.2:1, wherein the molecular weight of the mixture of oxalic acid and boric acid is the average of the sum of the molecular weights of oxalic acid and boric acid.

3. The preparation method according to claim 1, characterized in that, In step 1), oxalic acid, boric acid, and ammonium bifluoride are activated before drying, and in step 3), after obtaining difluorooxalic acid borate ammonium, the activation is performed by ball milling and sieving. The ball milling speed is 80~150 r / min and the time is 20~40 min. The sieve mesh size is 200~500 mesh.

4. The production method according to claim 1, characterized by, The drying method described in step 1) is vacuum drying; the temperature for vacuum drying of oxalic acid and / or boric acid is 50~90℃ and the vacuum degree is 30~100Pa; the temperature for vacuum drying of ammonium bifluoride is 80~100℃ and the vacuum degree is 30~100Pa.

5. The method of claim 1, wherein, In step 2), the ultrasonic power is 20~40KHz, the heating temperature is 70~100℃, the stirring rate is 400~800r / min, the ammonia gas flow rate is 1~3L / h, and the reaction time after the ammonia gas is introduced is 3~5h.

6. The method of claim 1, wherein, The purification method in step 3) is to filter the product containing ammonium difluorooxalate borate, dissolve the obtained filter cake in methanol and filter it again, and then rotary evaporate the filtrate; the drying method in step 3) is vacuum drying, with a temperature of 80~100℃ and a vacuum degree of 30~100Pa.

7. The preparation method according to claim 1, characterized in that, In step 4), the molar ratio of ammonium difluorooxalate borate and anhydrous sodium carbonate is 1.8~2.0:

1.

8. The preparation method according to claim 1, characterized in that, In step 5), calcination is carried out under nitrogen conditions, with a nitrogen flow rate of 5~10 L / min, a calcination temperature of 140~160℃, and a calcination time of 3~5 h.

9. The method of claim 1, wherein, After obtaining sodium difluorooxalate borate in step 5), it is further purified and dried. The purification method is to mix sodium difluorooxalate borate with acetonitrile or tetrahydrofuran and filter it, and then rotary evaporate the filtrate. The drying method is vacuum drying at a temperature of 100~120℃, a vacuum degree of 30~100Pa, and a time of 3~5h.