Sodium difluoro (oxalato) borate ultramicro crystal as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of sodium ion batteries, and relates to a sodium difluoro (oxalato) borate ultramicro crystal and a preparation method and application thereof, the preparation method comprises the following steps: (1) mixing oxalic acid, boric acid and deionized water, adding ammonium fluoride and hydrofluoric acid aqueous solution into the obtained slurry at low temperature, and then carrying out heating reaction; (2) after the reaction is finished, carrying out solid-liquid separation, and carrying out cooling crystallization on the obtained filtrate to obtain ammonium difluoro (oxalato) borate; (3) dissolving ammonium difluoro (oxalato) borate in deionized water, adding sodium carbonate for reaction, and then evaporating and concentrating until crystals are separated out to obtain a concentrated solution; and (4) carrying out supercritical fluid anti-solvent crystallization on the concentrated solution to obtain the sodium difluoro (oxalato) borate ultrafine crystal. The preparation method provided by the invention effectively overcomes the bottlenecks of the traditional process in the aspects of crystal granularity, solubility and safety, and is beneficial to large-scale popularization and application.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery technology, and relates to a sodium difluorooxalate borate ultracrystal, and more particularly to a sodium difluorooxalate borate ultracrystal, its preparation method and application. Background Technology

[0002] With the global energy structure transformation and the rapid development of renewable energy, sodium-ion batteries, with their advantages of low cost, abundant resources, and environmental friendliness, are gradually becoming an important supplement to lithium-ion batteries, showing broad application prospects in areas such as grid energy storage, low-speed electric vehicles, and large-scale energy storage. However, improving the performance of sodium-ion batteries still faces many challenges, among which electrolyte optimization is particularly crucial. As the core component of the electrolyte, the electrolyte salt directly affects the battery's ionic conductivity, electrochemical stability, and cycle life.

[0003] While widely used sodium salts such as sodium hexafluorophosphate (NaPF6) possess high ionic conductivity, their thermal stability and hydrolysis resistance are poor, making them prone to decomposition under high temperature or high voltage conditions, thus affecting battery performance. Furthermore, the complex and costly preparation process of NaPF6 further limits its large-scale application. Therefore, developing novel, efficient, stable, and lower-cost sodium-ion battery electrolyte salts has become a current research hotspot.

[0004] Sodium difluorooxalate borate (NaDFOB), as an emerging electrolyte salt, has attracted widespread attention due to its unique chemical structure and excellent performance. NaDFOB possesses the following advantages: high ionic conductivity, maintaining stable transport performance over a wide temperature range; excellent thermal stability, contributing to improved battery safety in high-temperature environments; good electrochemical stability, suitable for high-voltage sodium-ion battery systems; and environmental friendliness, with widely available raw materials and a more environmentally friendly preparation process. However, traditional NaDFOB synthesis methods still face challenges such as harsh reaction conditions, low product purity, and numerous side reactions, hindering its industrial-scale promotion and application.

[0005] CN117304217A discloses a method for preparing sodium difluorooxalate borate. This method involves reacting ammonium bifluoride with oxalic acid and boric acid in an organic solvent to prepare ammonium difluorooxalate borate. Subsequently, the ammonium cations in the ammonium difluorooxalate borate are replaced with sodium ions by pressing and calcining to obtain sodium difluorooxalate borate. This method involves organic solvent reactions and the heating and drying of ammonium bifluoride, which can easily release highly toxic HF gas into the air, posing a significant hazard to equipment and operators. Furthermore, the calcination step is prone to problems such as NaDFOB decomposition, thus affecting the purity and yield of the product.

[0006] CN105541890A discloses a method for synthesizing sodium difluorooxalate borate. First, sodium fluoride, oxalic acid, and hydrogen fluoride are added to a boric acid solution to form a homogeneous mixed aqueous solution. After evaporation to dryness, a white precipitate is obtained. The precipitate is then thoroughly dried to obtain a crude product, which is then extracted in acetonitrile or diethyl ether solvent. Finally, the extract is evaporated and crystallized to obtain the NaDFOB product. However, this method is not only cumbersome in its synthesis and purification steps and has a low product yield, but also results in NaDFOB with low purity and high impurity content, greatly limiting its industrial application.

[0007] Furthermore, existing sodium difluorooxalate borate (NaDFOB) preparation processes all employ traditional phase-change-dependent crystallization methods such as evaporation crystallization and cooling crystallization. These methods lack precise control over crystal growth during crystallization, resulting in NaDFOB products with excessively large crystal particles and uneven particle size distribution. This physical defect further affects its solubility in downstream electrolytes: not only is the dissolution rate slow, increasing energy and time costs, but incomplete dissolution also easily leads to decreased electrolyte homogeneity. Ultimately, this phenomenon limits sodium ion transport efficiency, degrades battery rate performance, and accelerates battery capacity decay, thus adversely affecting the overall battery performance.

[0008] Therefore, it is evident that providing a method for preparing sodium difluorooxalate borate that overcomes the bottlenecks of traditional processes in terms of crystal size, solubility, and safety has become an urgent problem for those skilled in the art. Summary of the Invention

[0009] To address the shortcomings of existing technologies, the present invention aims to provide sodium difluorooxalate borate ultracrystals, their preparation method, and applications, overcoming the bottlenecks of traditional processes in terms of crystal particle size, solubility, and safety, and improving industrial feasibility.

[0010] To achieve this objective, the present invention adopts the following technical solution:

[0011] In a first aspect, the present invention provides a method for preparing sodium difluorooxalate borate ultracrystals, comprising the following steps:

[0012] (1) Mix oxalic acid, boric acid and deionized water, add ammonium fluoride and hydrofluoric acid aqueous solution to the resulting slurry under low temperature conditions, and then carry out the reaction by raising the temperature;

[0013] (2) After the reaction is completed, solid-liquid separation is performed, and the obtained filtrate is cooled and crystallized to obtain ammonium difluorooxalate borate;

[0014] (3) Dissolve the ammonium difluorooxalate borate in deionized water, add sodium carbonate to react, and then evaporate and concentrate until crystals precipitate to obtain a concentrated solution;

[0015] (4) The concentrated solution is subjected to supercritical fluid antisolvent crystallization to obtain sodium difluorooxalate borate ultracrystals.

[0016] The preparation method provided by this invention uses ammonium fluoride as the fluorine source and hydrofluoric acid aqueous solution as the hydrogen source, and mixes them under low temperature conditions. The reaction is relatively mild, effectively avoiding the risk of violent reaction and large-scale emission of hydrogen fluoride gas under high temperature conditions. It is particularly suitable for industrial production. At the same time, this invention does not require the use of organic solvents, making it safer and more environmentally friendly. It also eliminates the need for high-temperature drying of hydrogen fluoride-containing raw materials, thereby preventing the decomposition of raw materials at high temperatures and the generation of hydrogen fluoride gas, thus improving operational safety.

[0017] Furthermore, compared to traditional crystallization methods that rely on phase change, such as evaporation crystallization or cooling crystallization, the supercritical fluid antisolvent crystallization process used in this invention produces sodium difluorooxalate borate ultracrystals with uniform particle size distribution and controllable particle morphology, which facilitates subsequent purification and full dissolution in the electrolyte.

[0018] Preferably, the molar ratio of oxalic acid, boric acid, ammonium fluoride and hydrofluoric acid in step (1) is (1-1.2):(1-1.2):(1-1.2):1, and more preferably 1.1:1:1:1.

[0019] Preferably, the concentration of the hydrofluoric acid aqueous solution in step (1) is 30-50 wt%.

[0020] Preferably, the temperature of the low-temperature condition in step (1) is ≤20℃, and more preferably 0-10℃.

[0021] Preferably, the target temperature for the heating reaction in step (1) is 40-50℃, and the reaction time is 3-6h.

[0022] Preferably, the reactor material used in the heating reaction in step (1) includes at least one of polytetrafluoroethylene, perfluoroethylene propylene, Monel alloy, Hastelloy alloy or Inconel alloy.

[0023] Preferably, the solid-liquid separation method in step (2) includes filtration.

[0024] Preferably, the cooling crystallization temperature in step (2) is -10℃ to 10℃, and the crystallization time is 30-40 min.

[0025] Preferably, the cooling and crystallization process in step (2) is accompanied by stirring, and the stirring rate is 400-600 rpm.

[0026] Preferably, the mass ratio of ammonium difluorooxalate borate and sodium carbonate in step (3) is (2.95-3.10):1, and more preferably 3:1.

[0027] Preferably, the reaction in step (3) is carried out at a temperature of 50-60°C for 2-3 hours.

[0028] Preferably, the evaporation and concentration temperature in step (3) is 100-120°C.

[0029] Preferably, the supercritical fluid antisolvent crystallization in step (4) includes sequential reaction vessel pre-equilibration and antisolvent crystallization.

[0030] Preferably, the pre-equilibration of the reactor includes: preheating the reactor and then injecting carbon dioxide until the pressure stabilizes.

[0031] Preferably, the preheating temperature of the reactor is 30-50°C.

[0032] Preferably, the target value for pressure stabilization is 10-15 MPa, and the stabilization time is 20-30 min.

[0033] Preferably, the antisolvent crystallization includes: continuously injecting carbon dioxide into the reaction vessel while simultaneously atomizing and spraying concentrated liquid into the reaction vessel to obtain sodium difluorooxalate borate ultracrystals.

[0034] Preferably, the carbon dioxide injection flow rate is 20-30 g / min.

[0035] Preferably, the injection flow rate of the concentrate is 3-5 mL / min.

[0036] Preferably, the atomization pressure of the atomization is 16-18 MPa.

[0037] In a second aspect, the present invention provides sodium difluorooxalate borate ultracrystals, which are prepared by the preparation method described in the first aspect.

[0038] The D50 particle size of the sodium difluorooxalate borate ultracrystals is ≤5.5μm.

[0039] Thirdly, the present invention provides an electrolyte containing at least sodium difluorooxalate borate ultracrystals as described in the second aspect.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] (1) The preparation method provided by the present invention uses ammonium fluoride as a fluorine source and hydrofluoric acid aqueous solution as a hydrogen source, and mixes them under low temperature conditions. The reaction is relatively mild and effectively avoids the risk of violent reaction and large amount of hydrogen fluoride gas escape under high temperature conditions. It is particularly suitable for industrial production. At the same time, the present invention does not require the use of organic solvents, which is safer and more environmentally friendly. It also does not require high temperature drying of raw materials containing hydrogen fluoride, thereby preventing the raw materials from decomposing at high temperature to produce hydrogen fluoride gas and improving operational safety.

[0042] (2) Compared with traditional crystallization methods that rely on phase change, such as evaporation crystallization or cooling crystallization, the supercritical fluid antisolvent crystallization process used in this invention produces sodium difluorooxalate borate ultracrystals with uniform particle size distribution and controllable particle morphology, which facilitates subsequent purification and full dissolution in electrolyte. Detailed Implementation

[0043] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0044] One embodiment of the present invention provides a method for preparing sodium difluorooxalate borate ultracrystals, comprising the following steps:

[0045] (1) Mix oxalic acid, boric acid and deionized water, add ammonium fluoride and hydrofluoric acid aqueous solution to the resulting slurry under low temperature conditions, and then carry out the reaction by raising the temperature;

[0046] (2) After the reaction is completed, solid-liquid separation is performed, and the obtained filtrate is cooled and crystallized to obtain ammonium difluorooxalate borate;

[0047] (3) Dissolve the ammonium difluorooxalate borate in deionized water, add sodium carbonate to react, and then evaporate and concentrate until crystals precipitate to obtain a concentrated solution;

[0048] (4) The concentrated solution is subjected to supercritical fluid antisolvent crystallization to obtain sodium difluorooxalate borate ultracrystals.

[0049] The preparation method provided by this invention uses ammonium fluoride as the fluorine source and hydrofluoric acid aqueous solution as the hydrogen source, and mixes them under low temperature conditions. The reaction is relatively mild, effectively avoiding the risk of violent reaction and large-scale emission of hydrogen fluoride gas under high temperature conditions. It is particularly suitable for industrial production. At the same time, this invention does not require the use of organic solvents, making it safer and more environmentally friendly. It also eliminates the need for high-temperature drying of hydrogen fluoride-containing raw materials, thereby preventing the decomposition of raw materials at high temperatures and the generation of hydrogen fluoride gas, thus improving operational safety.

[0050] Furthermore, compared to traditional crystallization methods that rely on phase change, such as evaporation crystallization or cooling crystallization, the supercritical fluid antisolvent crystallization process used in this invention produces sodium difluorooxalate borate ultracrystals with uniform particle size distribution and controllable particle morphology, which facilitates subsequent purification and full dissolution in the electrolyte.

[0051] In some embodiments, the molar ratio of oxalic acid, boric acid, ammonium fluoride and hydrofluoric acid in step (1) is (1-1.2):(1-1.2):(1-1.2):1, for example, it can be 1:1:1:1, 1:1:1.1:1, 1:1:1.2:1, 1.1:1.1:1:1, 1.1:1.1:1.1:1, 1.1:1.1:1.2:1, 1.2:1.2:1:1, 1.2:1.2:1.1:1 or 1.2:1.2:1.2:1, more preferably 1.1:1:1:1, but it is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0052] In this invention, the preferred molar ratio of oxalic acid, boric acid, ammonium fluoride, and hydrofluoric acid in step (1) is 1.1:1:1:1. The core consideration is to precisely control the reaction equilibrium and product purity. This ratio ensures the complete reaction of boric acid. Equal amounts of ammonium fluoride and hydrofluoric acid can effectively provide the required two fluorine atoms and maintain suitable acidity. The oxalic acid is deliberately added in excess by 10% to strongly suppress the formation of the byproduct tetrafluoroborate ion by competing for fluoride ions, while driving the complexation reaction to proceed more thoroughly, thereby ensuring high purity of the final product.

[0053] In some embodiments, the concentration of the hydrofluoric acid aqueous solution in step (1) is 30-50 wt%, for example, it can be 30 wt%, 32 wt%, 34 wt%, 36 wt%, 38 wt%, 40 wt%, 42 wt%, 44 wt%, 46 wt%, 48 wt%, or 50 wt%, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0054] In some embodiments, the temperature of the low-temperature condition in step (1) is ≤20°C, for example, it can be 0°C, 2°C, 4°C, 6°C, 8°C, 10°C, 12°C, 13°C, 14°C, 16°C, 18°C ​​or 20°C, and more preferably 0-10°C, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0055] This invention fundamentally prevents the volatilization of highly toxic hydrogen fluoride gas by controlling the mixing process under low-temperature conditions, ensuring operational safety, effectively suppressing side reactions, and ultimately guaranteeing product purity.

[0056] In some embodiments, the target temperature for the heating reaction in step (1) is 40-50°C, for example, it can be 40°C, 41°C, 42°C, 43°C, 44°C, 45°C, 46°C, 47°C, 48°C, 49°C or 50°C, and the reaction time is 3-6h, for example, it can be 3h, 3.5h, 4h, 4.5h, 5h, 5.5h or 6h, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0057] In some embodiments, the reactor material used for the heating reaction in step (1) includes at least one of polytetrafluoroethylene, perfluoroethylene propylene, Monel alloy, Hastelloy alloy, or Inconel alloy.

[0058] In this invention, the reactor made of the above-mentioned material fundamentally ensures production safety and prevents the risk of leakage caused by equipment corrosion and perforation due to hydrofluoric acid. It is an important factor in ensuring stable process operation and obtaining high-purity products.

[0059] In some embodiments, the solid-liquid separation method in step (2) includes filtration.

[0060] In some embodiments, the cooling crystallization temperature in step (2) is from -10°C to 10°C, for example, it can be -10°C, -8°C, -6°C, -4°C, -2°C, 0°C, 2°C, 4°C, 6°C, 8°C or 10°C, and the crystallization time is 30-40 min, for example, it can be 30 min, 31 min, 32 min, 33 min, 34 min, 35 min, 36 min, 37 min, 38 min, 39 min or 40 min, but it is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0061] This invention limits the temperature range and crystallization time for cooling and crystallization, ensuring that crystallization occurs within the optimal process window. This synergistically improves product performance and process efficiency, resulting in a significant cost-performance advantage.

[0062] In some embodiments, the cooling and crystallization process in step (2) is accompanied by stirring at a speed of 400-600 rpm, such as 400 rpm, 420 rpm, 440 rpm, 460 rpm, 480 rpm, 500 rpm, 520 rpm, 540 rpm, 560 rpm, 580 rpm or 600 rpm, but not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0063] In some embodiments, the mixing mass ratio of ammonium difluorooxalate borate and sodium carbonate in step (3) is (2.95-3.10):1, for example, it can be 2.95:1, 2.96:1, 2.97:1, 2.98:1, 2.99:1, 3.00:1, 3.01:1, 3.02:1, 3.03:1, 3.04:1, 3.05:1, 3.06:1, 3.07:1, 3.08:1, 3.09:1 or 3.10:1, and is more preferably 3:1, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0064] In some embodiments, the reaction temperature in step (3) is 50-60°C, for example, it can be 50°C, 51°C, 52°C, 53°C, 54°C, 55°C, 56°C, 57°C, 58°C, 59°C or 60°C, and the time is 2-3 hours, for example, it can be 2 hours, 2.1 hours, 2.2 hours, 2.3 hours, 2.4 hours, 2.5 hours, 2.6 hours, 2.7 hours, 2.8 hours, 2.9 hours or 3 hours, but it is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0065] In this invention, ammonium difluorooxalate borate is dissolved in deionized water and then reacted with sodium carbonate. By heating at 50-60°C, ammonium ions and sodium ions are completely exchanged, and the reaction is driven to completion by the release of ammonia and carbon dioxide generated.

[0066] In some embodiments, the evaporation and concentration temperature in step (3) is 100-120°C, for example, it can be 100°C, 102°C, 104°C, 106°C, 108°C, 110°C, 112°C, 114°C, 116°C, 118°C or 120°C, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0067] In this invention, the evaporation and concentration temperature needs to be limited to a reasonable range. When the evaporation and concentration temperature is higher than 120°C, the product is prone to decomposition; when the evaporation and concentration temperature is lower than 100°C, water cannot be fully evaporated. Furthermore, the evaporation and concentration in this invention continues until crystals precipitate in order to improve the subsequent crystallization efficiency.

[0068] In some embodiments, step (4) of supercritical fluid antisolvent crystallization includes sequential reaction vessel pre-equilibration and antisolvent crystallization.

[0069] In some embodiments, the reactor pre-equilibration includes: preheating the reactor and then injecting carbon dioxide until the pressure stabilizes.

[0070] In some embodiments, the preheating temperature of the reactor is 30-50°C, for example, it can be 30°C, 32°C, 34°C, 36°C, 38°C, 40°C, 42°C, 44°C, 46°C, 48°C or 50°C, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0071] In some embodiments, the target value for pressure stabilization is 10-15 MPa, for example, it can be 10 MPa, 11 MPa, 12 MPa, 13 MPa, 14 MPa or 15 MPa, and the stabilization time is 20-30 min, for example, it can be 20 min, 21 min, 22 min, 23 min, 24 min, 25 min, 26 min, 27 min, 28 min, 29 min or 30 min, but it is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0072] In some embodiments, the antisolvent crystallization includes: continuously injecting carbon dioxide into a reaction vessel while simultaneously atomizing and spraying a concentrated solution into the reaction vessel to obtain sodium difluorooxalate borate ultracrystals.

[0073] In some embodiments, the carbon dioxide injection flow rate is 20-30 g / min, for example, it can be 20 g / min, 21 g / min, 22 g / min, 23 g / min, 24 g / min, 25 g / min, 26 g / min, 27 g / min, 28 g / min, 29 g / min or 30 g / min, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0074] In some embodiments, the injection flow rate of the concentrate is 3-5 mL / min, for example, it can be 3 mL / min, 3.2 mL / min, 3.4 mL / min, 3.6 mL / min, 3.8 mL / min, 4 mL / min, 4.2 mL / min, 4.4 mL / min, 4.6 mL / min, 4.8 mL / min or 5 mL / min, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0075] In some embodiments, the atomization pressure of the atomization is 16-18 MPa, for example, it can be 16 MPa, 16.5 MPa, 17 MPa, 17.5 MPa or 18 MPa, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0076] One embodiment of the present invention also provides sodium difluorooxalate borate ultracrystals, which are prepared by the preparation method described in any of the above embodiments.

[0077] The D50 particle size of the sodium difluorooxalate borate ultracrystals is ≤5.5μm, for example, it can be 4.0μm, 4.1μm, 4.2μm, 4.3μm, 4.4μm, 4.5μm, 4.6μm, 4.7μm, 4.8μm, 4.9μm, 5.0μm, 5.1μm, 5.2μm, 5.3μm, 5.4μm or 5.5μm, but is not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0078] One embodiment of the present invention also provides an electrolyte containing at least the sodium difluorooxalate borate ultracrystals described in any of the above embodiments.

[0079] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0080] Example 1

[0081] This embodiment provides sodium difluorooxalate borate ultracrystals and their preparation method, specifically including the following steps:

[0082] (1) Add oxalic acid and boric acid in a molar ratio of 1.1:1 to a polytetrafluoroethylene reactor containing an appropriate amount of deionized water, cool the mixture and maintain it at 0°C; then slowly add ammonium fluoride, the amount of which is in a molar ratio of 1:1.1 to oxalic acid; then slowly add a 40wt% hydrofluoric acid aqueous solution, wherein the molar ratio of hydrofluoric acid to oxalic acid is 1:1.1, while the system temperature does not exceed 20°C; after all materials have been added, raise the temperature of the reaction system to 40°C and keep it at this temperature for 6 hours.

[0083] (2) After the reaction is complete, the reaction solution is filtered; then the filtrate is placed in a -10℃ environment and stirred at 600 rpm for 30 min to crystallize, and then filtered to obtain ammonium difluorooxalate borate.

[0084] (3) Dissolve ammonium difluorooxalate borate in deionized water, then add anhydrous sodium carbonate at a mass ratio of ammonium difluorooxalate borate to sodium carbonate of 3:1. After reacting at 50°C for 3 hours, heat the solution to 100°C for evaporation and crystallization until the solution is concentrated until crystals begin to precipitate, and obtain concentrated solution.

[0085] (4) Preheat the reactor to 30°C and inject CO2 to make the pressure inside the reactor reach 12MPa. Maintain this pressure for 20 minutes. Under the condition of maintaining the set temperature and pressure, continue to inject CO2 at a flow rate of 25g / min. At the same time, atomize the concentrated liquid into the reactor at a flow rate of 5mL / min and an atomization pressure of 18MPa to obtain sodium difluorooxalate borate ultracrystals.

[0086] Example 2

[0087] This embodiment provides sodium difluorooxalate borate ultracrystals and their preparation method, specifically including the following steps:

[0088] (1) Add oxalic acid and boric acid in a molar ratio of 1.1:1 to a polytetrafluoroethylene propylene reactor containing an appropriate amount of deionized water. Cool the mixture and maintain it at 10°C. Then slowly add ammonium fluoride in a molar ratio of 1:1.1 to oxalic acid. Then, under the condition that the system temperature does not exceed 20°C, slowly add a 40wt% aqueous solution of hydrofluoric acid, in which the molar ratio of hydrofluoric acid to oxalic acid is 1:1.1. After all materials have been added, raise the temperature of the reaction system to 50°C and keep it at this temperature for 3 hours.

[0089] (2) After the reaction is complete, the reaction solution is filtered; then the filtrate is placed at 10°C and stirred at 400 rpm for 40 min to crystallize, and then filtered to obtain ammonium difluorooxalate borate.

[0090] (3) Dissolve ammonium difluorooxalate borate in deionized water, then add anhydrous sodium carbonate at a mass ratio of ammonium difluorooxalate borate to sodium carbonate of 3:1. After reacting at 60°C for 2 hours, heat the solution to 100°C for evaporation and crystallization until the solution is concentrated until crystals begin to precipitate, and obtain concentrated solution.

[0091] (4) Preheat the reactor to 50°C and inject CO2 to make the pressure inside the reactor reach 12MPa. Maintain this pressure for 30 minutes. Under the condition of maintaining the set temperature and pressure, continue to inject CO2 at a flow rate of 25g / min. At the same time, atomize the concentrated liquid into the reactor at a flow rate of 3mL / min and an atomization pressure of 18MPa to obtain sodium difluorooxalate borate ultracrystals.

[0092] Example 3

[0093] This embodiment provides sodium difluorooxalate borate ultracrystals and their preparation method, specifically including the following steps:

[0094] (1) Oxalic acid and boric acid with a molar ratio of 1.1:1 were added to a Monel alloy reactor containing an appropriate amount of deionized water. The mixture was cooled and maintained at 3°C. Then ammonium fluoride was slowly added with a molar ratio of 1:1.1 to oxalic acid. Subsequently, under the condition that the system temperature does not exceed 20°C, a 40wt% aqueous solution of hydrofluoric acid was slowly added dropwise, with a molar ratio of 1:1.1 between hydrofluoric acid and oxalic acid. After all materials were added, the reaction system was heated to 42°C and kept at this temperature for 4 hours.

[0095] (2) After the reaction is complete, the reaction solution is filtered; then the filtrate is placed at 7°C and stirred at 500 rpm for 35 min to crystallize, and then filtered to obtain ammonium difluorooxalate borate.

[0096] (3) Dissolve ammonium difluorooxalate borate in deionized water, then add anhydrous sodium carbonate at a mass ratio of ammonium difluorooxalate borate to sodium carbonate of 3:1. After reacting at 58°C for 2.5 h, heat the solution to 110°C for evaporation and crystallization until the solution is concentrated until crystals begin to precipitate, and obtain concentrated solution.

[0097] (4) Preheat the reactor to 40°C and inject CO2 to make the pressure inside the reactor reach 12MPa. Maintain this pressure for 25 minutes. Under the condition of maintaining the set temperature and pressure, continue to inject CO2 at a flow rate of 25g / min. At the same time, spray the concentrated liquid into the reactor at a flow rate of 3.8mL / min and an atomization pressure of 18MPa to obtain sodium difluorooxalate borate ultracrystals.

[0098] Example 4

[0099] This embodiment provides sodium difluorooxalate borate ultracrystals and their preparation method, specifically including the following steps:

[0100] (1) Add oxalic acid and boric acid in a molar ratio of 1.1:1 to a polytetrafluoroethylene reactor containing an appropriate amount of deionized water, cool the mixture and maintain it at 7°C; then slowly add ammonium fluoride in a molar ratio of 1:1.1 to oxalic acid; then slowly add a 40wt% hydrofluoric acid aqueous solution at a system temperature not exceeding 20°C, wherein the molar ratio of hydrofluoric acid to oxalic acid is 1:1.1; after all materials have been added, raise the temperature of the reaction system to 45°C and keep it at this temperature for 6 hours.

[0101] (2) After the reaction is complete, the reaction solution is filtered; then the filtrate is placed in a -5℃ environment and stirred at 500 rpm for 35 min to crystallize, and then filtered to obtain ammonium difluorooxalate borate.

[0102] (3) Dissolve ammonium difluorooxalate borate in deionized water, then add anhydrous sodium carbonate at a mass ratio of ammonium difluorooxalate borate to sodium carbonate of 3:1. After reacting at 53°C for 2.5 h, heat the solution to 105°C for evaporation and crystallization until the solution is concentrated until crystals begin to precipitate, and obtain concentrated solution.

[0103] (4) Preheat the reactor to 30°C and inject CO2 to make the pressure inside the reactor reach 12MPa. Maintain this pressure for 20 minutes. Under the condition of maintaining the set temperature and pressure, continue to inject CO2 at a flow rate of 25g / min. At the same time, atomize the concentrated liquid into the reactor at a flow rate of 4.5mL / min and an atomization pressure of 18MPa to obtain sodium difluorooxalate borate ultracrystals.

[0104] Example 5

[0105] This embodiment provides sodium difluorooxalate borate ultracrystals and their preparation method, specifically including the following steps:

[0106] (1) Add oxalic acid and boric acid in a molar ratio of 1.1:1 to a polytetrafluoroethylene reactor containing an appropriate amount of deionized water, cool the mixture and maintain it at 9°C; then slowly add ammonium fluoride in a molar ratio of 1:1.1 to oxalic acid; then slowly add a 40wt% aqueous solution of hydrofluoric acid in a molar ratio of 1:1.1 to oxalic acid while keeping the system temperature below 20°C; after all materials have been added, raise the temperature of the reaction system to 48°C and keep it at this temperature for 5 hours.

[0107] (2) After the reaction is complete, the reaction solution is filtered; then the filtrate is placed in a 0°C environment and stirred at 450 rpm for 33 min to crystallize. The filtrate is then filtered to obtain ammonium difluorooxalate borate.

[0108] (3) Dissolve ammonium difluorooxalate borate in deionized water, then add anhydrous sodium carbonate at a mass ratio of ammonium difluorooxalate borate to sodium carbonate of 3:1. After reacting at 55°C for 2.2 h, heat the solution to 120°C for evaporation and crystallization until the solution is concentrated until crystals begin to precipitate, and obtain concentrated solution.

[0109] (4) Preheat the reactor to 35°C and inject CO2 to make the pressure inside the reactor reach 12MPa. Maintain this pressure for 28 minutes. Under the condition of maintaining the set temperature and pressure, continue to inject CO2 at a flow rate of 25g / min. At the same time, atomize the concentrated liquid into the reactor at a flow rate of 4mL / min and an atomization pressure of 18MPa to obtain sodium difluorooxalate borate ultracrystals.

[0110] The yield, purity, and D50 particle size of the sodium difluorooxalate borate ultracrystals obtained in Examples 1-5 are shown in Table 1 below.

[0111] Table 1

[0112]

[0113] As shown in Table 1, the sodium difluorooxalate borate ultracrystals prepared by this invention have a high yield (>95%), high purity (>80%), and small particle size, making them particularly suitable for industrial production.

[0114] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing sodium difluorooxalate borate ultracrystals, characterized in that, The preparation method includes the following steps: (1) Mix oxalic acid, boric acid and deionized water, add ammonium fluoride and hydrofluoric acid aqueous solution to the resulting slurry under low temperature conditions, and then carry out the reaction by raising the temperature; (2) After the reaction is completed, solid-liquid separation is performed, and the obtained filtrate is cooled and crystallized to obtain ammonium difluorooxalate borate; (3) Dissolve the ammonium difluorooxalate borate in deionized water, add sodium carbonate to react, and then evaporate and concentrate until crystals precipitate to obtain a concentrated solution; (4) The concentrated solution is subjected to supercritical fluid antisolvent crystallization to obtain sodium difluorooxalate borate ultracrystals.

2. The method for preparing sodium difluorooxalate borate ultracrystals according to claim 1, characterized in that, The molar ratio of oxalic acid, boric acid, ammonium fluoride and hydrofluoric acid in step (1) is (1-1.2):(1-1.2):(1-1.2):1, and more preferably 1.1:1:1:1; And / or, the concentration of the hydrofluoric acid aqueous solution in step (1) is 30-50 wt%.

3. The method for preparing sodium difluorooxalate borate ultracrystals according to claim 1 or 2, characterized in that, The temperature of the low-temperature condition in step (1) is ≤20℃, and more preferably 0-10℃; And / or, the target temperature for the heating reaction in step (1) is 40-50℃, and the reaction time is 3-6h; And / or, the reactor material used for the heating reaction in step (1) includes at least one of polytetrafluoroethylene, perfluoroethylene propylene, Monel alloy, Hastelloy alloy or Inconel alloy.

4. The method for preparing sodium difluorooxalate borate ultracrystals according to claim 1, characterized in that, The solid-liquid separation method described in step (2) includes filtration; And / or, the cooling crystallization temperature in step (2) is -10°C to 10°C, and the crystallization time is 30-40 min; And / or, the cooling crystallization process in step (2) is accompanied by stirring at a rate of 400-600 rpm.

5. The method for preparing sodium difluorooxalate borate ultracrystals according to claim 1, characterized in that, In step (3), the mixing mass ratio of ammonium difluorooxalate borate and sodium carbonate is (2.95-3.10):1, more preferably 3:1; And / or, the reaction in step (3) is carried out at a temperature of 50-60°C for 2-3 hours; And / or, the temperature of the evaporation and concentration in step (3) is 100-120°C.

6. The method for preparing sodium difluorooxalate borate ultracrystals according to claim 1, characterized in that, Step (4) supercritical fluid antisolvent crystallization includes sequential reaction vessel pre-equilibration and antisolvent crystallization.

7. The method for preparing sodium difluorooxalate borate ultracrystals according to claim 6, characterized in that, The pre-equilibration of the reactor includes: preheating the reactor and then injecting carbon dioxide until the pressure stabilizes; The preheating temperature of the reactor is 30-50℃; And / or, the target value for pressure stabilization is 10-15 MPa, and the stabilization time is 20-30 min.

8. The method for preparing sodium difluorooxalate borate ultracrystals according to claim 6 or 7, characterized in that, The antisolvent crystallization process includes: continuously injecting carbon dioxide into the reaction vessel while simultaneously atomizing and spraying the concentrated liquid into the reaction vessel to obtain sodium difluorooxalate borate ultracrystals. The injection flow rate of the carbon dioxide is 20-30 g / min; And / or, the injection flow rate of the concentrate is 3-5 mL / min; And / or, the atomization pressure of the atomization is 16-18 MPa.

9. A sodium difluorooxalate borate ultracrystal, characterized in that, The sodium difluorooxalate borate ultracrystals were prepared by the preparation method described in any one of claims 1-8; The D50 particle size of the sodium difluorooxalate borate ultracrystals is ≤5.5μm.

10. An electrolyte, characterized in that, The electrolyte contains at least the sodium difluorooxalate borate ultracrystals as described in claim 9.

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