Preparation method of high-purity lithium tetrafluoroborate liquid salt

Abstract

The invention relates to a preparation method of a high-purity lithium tetrafluoroborate liquid salt, which comprises the following steps of: reacting lithium metaborate serving as a raw material with calcium fluoride and excessive sulfur trioxide gas for 1-5 hours at the temperature of 60-150 DEG C and the high pressure of 0.2-5 MPa, after the reaction is finished, depressurizing to separate out the sulfur trioxide gas for reuse, dissolving a reaction product in a benign organic solvent under the protection of inert gas, filtering out insoluble substances, and performing vacuum drying to obtain the high-purity lithium tetrafluoroborate liquid salt. And performing resin deacidification and membrane filtration on the filtrate to obtain the product. The method is simple in process, mild in condition and high in product yield, the water content of the prepared high-purity lithium tetrafluoroborate liquid salt is lower than 10 ppm, the free acid content (in terms of HF) is lower than 13 ppm, the Cl content is lower than 3 ppm, the SO4 content is lower than 2-5 ppm, and the single content of various metal impurity ions (such as calcium, iron, magnesium, sodium and potassium) is smaller than or equal to 2 ppm.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery additive technology, specifically relating to a method for preparing high-purity lithium tetrafluoroborate liquid salt. Background Technology

[0002] Lithium tetrafluoroborate (LiBF4), as a key inorganic functional material, is widely used as a core additive in lithium-ion battery electrolytes. Its preparation methods include gas-solid direct reaction, aqueous solution method, and non-aqueous solution method.

[0003] The gas-solid direct reaction method typically uses lithium fluoride (LiF) and boron trifluoride (BF3) gas as raw materials to react and produce lithium tetrafluoroborate under specific conditions. Soviet patent SU1655083A1 prepared LiBF4 by reacting powdered LiF with BF3 gas under high temperature, high pressure, and in the presence of fluorine gas, while SU1785199A1 achieved synthesis by vacuum heating of LiF and continuous injection of BF3 gas. However, such processes place extremely high demands on the corrosion resistance and sealing of the equipment.

[0004] The aqueous solution method typically uses hydrofluoric acid, boric acid, and lithium carbonate as raw materials, and prepares the product through steps such as reaction in aqueous solution, concentration, and crystallization. Japanese Patent JP2002293533 reports a method for obtaining LiBF4 by reacting LiBO2 with an aqueous HF solution at room temperature, followed by concentration and recrystallization, but this method suffers from the problem of difficulty in removing the water of crystallization.

[0005] Non-aqueous solutions often use anhydrous hydrofluoric acid or organic solvents such as carbonates and ethers as reaction media to prepare the product through the reaction of LiF with BF3 complex. Patent CN104291347B describes a mainstream process for preparing high-purity crystalline lithium tetrafluoroborate by dissolving lithium fluoride in anhydrous hydrogen fluoride at room temperature and pressure, reacting it with boron trifluoride gas, and then obtaining the product through cooling crystallization, filtration, and drying. Patent CN103733416B discloses a method of reacting LiF with BF3 in chain carbonates, followed by removal of water and acidic impurities. In addition, existing preparation techniques also include the reaction of boron trifluoride with lithium oxalate to produce a mixture of lithium tetrafluoroborate and lithium difluorooxalate borate. Patent CN111204776B employs a complex process combining silica gel column chromatography and solvent elution to obtain a lithium tetrafluoroborate product with a purity of 99.99%.

[0006] Driven by the demand for low-cost manufacturing of lithium-ion battery electrolytes, there is an urgent need to develop a simple, mild, and cost-effective method for preparing high-purity liquid lithium tetrafluoroborate. Based on this, this application was developed. Summary of the Invention

[0007] The purpose of this invention is to overcome the defects of the prior art and provide a method for preparing high-purity lithium tetrafluoroborate liquid salt with simple process, mild conditions and high product yield.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing high-purity lithium tetrafluoroborate liquid salt involves using high-purity lithium metaborate as raw material, reacting it with high-purity calcium fluoride and excess sulfur trioxide gas at 60–150°C, under high pressure of 0.2–5 MPa and an inert gas atmosphere for 1–5 hours. After the reaction is completed, the sulfur trioxide gas is separated by depressurization and reused. Then, under inert gas protection, the reaction product is dissolved in a good organic solvent, and the insoluble matter is filtered off. The filtrate is then deacidified by resin and filtered through a precision membrane to obtain the final product.

[0009] The chemical reactions involved in the preparation method of high-purity lithium tetrafluoroborate liquid salt of the present invention are as follows: 2CaF2 + LiBO2 + 2SO3 = LiBF4 + 2CaSO4 To improve the conversion rate of solid reactants, facilitate heat transfer and stirring, and prevent agglomeration, a liquid-solid reaction system is selected in the above-mentioned method for preparing high-purity lithium tetrafluoroborate. Preferably, under stirring conditions, the solid raw materials can be uniformly suspended in the liquid medium. Specifically, the high-pressure reaction pressure is 0.2–5 MPa, the temperature is 60–150 °C, and under these conditions, sulfur trioxide is kept in a liquid state; liquid sulfur trioxide is used as the dispersion medium. Furthermore, the reaction time is controlled within 1–5 hours.

[0010] Furthermore, the high-purity lithium metaborate and high-purity calcium fluoride raw materials used are both in powder form, and their purity is ≥99.9%. The sulfur trioxide used has a purity of ≥99.9%.

[0011] To promote efficient and uniform reaction and ensure complete conversion of lithium metaborate to lithium tetrafluoroborate, the molar ratio of the raw materials is n(CaF2):n(LiBO2) = (2-2.1):1, that is, the molar ratio of calcium fluoride to lithium metaborate is (2-2.1):1. The preferred amount of sulfur trioxide is 1.5-3 times the sum of the masses of high-purity lithium metaborate and high-purity calcium fluoride.

[0012] In the method of this invention, after the reaction is completed, the depressurization separation refers to gradually reducing the reaction pressure to room temperature and pressure. During the depressurization process, excess sulfur trioxide is collected in gaseous form and used as raw material for the next batch of reaction.

[0013] The reaction product of this invention contains a large amount of lithium tetrafluoroborate and calcium sulfate, as well as a small amount of unreacted calcium fluoride. High-purity lithium tetrafluoroborate can be obtained by separating the product based on the difference in solubility in organic solvents. Given that the raw materials of this invention are all of high purity and the product contains almost no other solid impurities, it is preferable to dissolve lithium tetrafluoroborate in a good organic solvent, filter out the insoluble matter, and directly obtain the liquid salt.

[0014] Specifically, the benign organic solvent can be esters, ethers, nitriles, alcohols, etc., including but not limited to one or more combinations of dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, methyl acetate, ethyl acetate, propyl propionate, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, tetrahydrofuran, acetonitrile, propionitrile, butyronitrile, adiponitrile, methanol, ethanol, propanol, etc. Further, esters are preferred, including but not limited to one or more combinations of dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, methyl acetate, ethyl acetate, propyl propionate, etc. Calcium sulfate and high-purity calcium fluoride are insoluble in these benign organic solvents, but lithium tetrafluoroborate is readily soluble. Therefore, it is convenient to dissolve the reaction product in an organic solvent and separate and filter out the byproduct calcium sulfate and a small amount of unreacted high-purity calcium fluoride raw material to obtain a liquid lithium tetrafluoroborate solution.

[0015] Furthermore, to avoid side reactions and improve product purity, the benign organic solvent has a main content ≥99.95% and a moisture content not exceeding 50 ppm. Furthermore, the amount of the benign organic solvent used is sufficient to completely dissolve lithium tetrafluoroborate at room temperature.

[0016] In the above preparation process, the filtered byproducts calcium sulfate and a small amount of unreacted high-purity calcium fluoride raw material are used to prepare calcium sulfate whiskers or high-performance anhydrous gypsum after preliminary removal of residual organic solvents. Furthermore, to improve the product yield, the filter cake can be thoroughly washed 2-3 times with a benign organic solvent to recover the small amount of product mixed in the filter cake. The washing liquid is used as a benign solvent for the next batch of reaction.

[0017] Furthermore, the deacidifying resin used can be an ion exchange resin. Preferably, the ion exchange resin is a basic ion exchange resin, used to remove trace amounts of H from liquid lithium tetrafluoroborate. + F - SO4 2- Impurities, etc. Furthermore, the alkaline ion exchange resin is selected from one or more mixed resins selected from styrene-based resins, styrene-divinylbenzene-based resins, acrylic resins, etc.

[0018] Furthermore, the effluent after resin deacidification is filtered by precision membrane filtration to remove trace insoluble substances. Specifically, the pore size of the filter membrane is preferably selected to be 0.2 to 0.5 μm to ensure the high purity of the lithium tetrafluoroborate liquid salt.

[0019] Compared with the prior art, the beneficial effects of the method of the present invention are as follows: This invention overcomes the shortcomings of existing technologies, providing a simple, mild, and high-yield method for preparing high-purity lithium tetrafluoroborate liquid salt. This method does not require complex processes, is easy to operate, and is suitable for large-scale production. The high-purity lithium tetrafluoroborate liquid salt prepared using this method has a water content of less than 10 ppm, a free acid (calculated as HF) of less than 13 ppm, a Cl⁻ content of less than 3 ppm, and a SO₄²⁻ content of less than 10 ppm. 2- The content of each type of metal impurity ion (such as calcium, iron, magnesium, sodium, and potassium) is ≤2 ppm or less than 5 ppm. Detailed Implementation

[0020] The technical solution of the present invention will be further described in detail below with reference to the embodiments, but the scope of protection of the present invention is not limited thereto.

[0021] In the following embodiments, all raw materials used are commercially available products that can be purchased directly, or prepared using conventional techniques in the art. The high-purity lithium metaborate and high-purity calcium fluoride used in this invention are both in powder form, with a purity ≥99.9%; the solid powder raw materials are preferably dried at above 100°C to remove trace moisture. The high-purity sulfur trioxide used has a purity ≥99.9%. The benign organic solvent used has a main content ≥99.95% and a moisture content not exceeding 50 ppm.

[0022] Example 1 A method for preparing high-purity lithium tetrafluoroborate liquid salt, specifically including the following steps: 50g (1mol) of high-purity lithium metaborate powder and 156g (2mol) of high-purity calcium fluoride powder were sequentially added to a reactor purged with high-purity nitrogen. Then, 309g of sulfur trioxide gas was slowly introduced. The reactor pressure was increased to 0.2MPa, and the reaction was carried out at 60℃ with high-pressure stirring for 5 hours. After the reaction was completed, the pressure was slowly reduced to room temperature and pressure, and the excess sulfur trioxide gas was separated for use in the next batch reaction.

[0023] Add 940g of high-purity dimethyl carbonate to a dissolving vessel purged with high-purity nitrogen. Under high-purity nitrogen protection, transfer the product from the reaction vessel to the dissolving vessel, stir and dissolve at room temperature, and filter out insoluble matter (wash the filter cake thoroughly with dimethyl carbonate 2-3 times; the washing liquid is used as a good solvent for the next batch of reaction, and the filter cake is used to prepare calcium sulfate whiskers or high-performance anhydrous gypsum). The filtrate is deacidified by styrene-based ion exchange resin and filtered through a 0.3μm precision membrane to obtain 1032.12g of high-purity lithium tetrafluoroborate liquid salt, with a lithium tetrafluoroborate mass concentration of 8.93%, water content of 10ppm, free acid (calculated as HF) of 13ppm, Cl⁻ of 3ppm, and SO₄²⁻ of 10. 2- The content of each metal impurity ion (such as calcium, iron, magnesium, sodium, and potassium) is ≤2ppm.

[0024] Example 2 A method for preparing high-purity lithium tetrafluoroborate liquid salt, specifically including the following steps: 50 g (1 mol) of high-purity lithium metaborate powder and 159.9 g (2.05 mol) of high-purity calcium fluoride powder were sequentially added to a reactor purged with high-purity nitrogen. Then, 419.8 g of sulfur trioxide gas was slowly introduced. The reactor pressure was increased to 0.5 MPa, and the reaction was carried out at 80°C with high-pressure stirring for 4 hours. After the reaction was completed, the pressure was slowly reduced to room temperature and pressure, and the excess sulfur trioxide gas was separated for use in the next batch reaction.

[0025] 1145.63 g of high-purity diethyl carbonate was added to a dissolving vessel purged with high-purity nitrogen. Under high-purity nitrogen protection, the product in the reaction vessel was transferred to the dissolving vessel and dissolved by stirring at room temperature. Insoluble matter was filtered off (the filter cake was thoroughly washed with diethyl carbonate 2-3 times with stirring; the washing liquid was used as a good solvent for the next batch of reaction, and the filter cake was used to prepare calcium sulfate whiskers or high-performance anhydrous gypsum). The filtrate was deacidified by a styrene-divinylbenzene ion exchange resin and filtered through a 0.3 μm precision membrane to obtain 1237.94 g of high-purity lithium tetrafluoroborate liquid salt. The lithium tetrafluoroborate mass concentration was 7.46%, water content was 9 ppm, free acid (calculated as HF) was 11 ppm, and Cl... - 2ppm, SO4 2- 4ppm, and the content of each of the various metal impurity ions (such as calcium, iron, magnesium, sodium, and potassium) is ≤2ppm.

[0026] Example 3 A method for preparing high-purity lithium tetrafluoroborate liquid salt, specifically including the following steps: 50 g (1 mol) of high-purity lithium metaborate powder and 163.8 g (2.1 mol) of high-purity calcium fluoride powder were sequentially added to a reactor purged with high-purity nitrogen. Then, 534.5 g of sulfur trioxide gas was slowly introduced. The reactor pressure was increased to 0.9 MPa, and the reaction was carried out at 100°C with high-pressure stirring for 3 hours. After the reaction was completed, the pressure was slowly reduced to room temperature and pressure, and the excess sulfur trioxide gas was separated for use in the next batch reaction.

[0027] 1266g of high-purity ethyl methyl carbonate was added to a dissolving vessel purged with high-purity nitrogen. Under high-purity nitrogen protection, the product in the reaction vessel was transferred to the dissolving vessel and dissolved by stirring at room temperature. Insoluble matter was filtered off (the filter cake was thoroughly washed with ethyl methyl carbonate 2-3 times with stirring; the washing liquid was used as a good solvent for the next batch of reaction, and the filter cake was used to prepare calcium sulfate whiskers or high-performance anhydrous gypsum). The filtrate was deacidified by acrylic ion exchange resin and filtered through a 0.2μm precision membrane to obtain 1358.59g of high-purity lithium tetrafluoroborate liquid salt. The lithium tetrafluoroborate mass concentration was 6.82%, water content was 9.3ppm, free acid (as HF) was 12ppm, Cl⁻ was 2.5ppm, and SO₄²⁻ was 0.5ppm. 2- The content of each metal impurity ion (such as calcium, iron, magnesium, sodium, and potassium) is ≤2ppm.

[0028] Example 4 A method for preparing high-purity lithium tetrafluoroborate liquid salt, specifically including the following steps: 50g (1mol) of high-purity lithium metaborate powder and 156g (2mol) of high-purity calcium fluoride powder were sequentially added to a reactor purged with high-purity nitrogen. Then, 618g of sulfur trioxide gas was slowly introduced. The reactor pressure was increased to 1.5MPa, and the reaction was carried out at 120℃ with high-pressure stirring for 2 hours. After the reaction was completed, the pressure was slowly reduced to room temperature and pressure, and the excess sulfur trioxide gas was separated for use in the next batch reaction.

[0029] 215.6 g of high-purity methanol was added to a dissolving vessel purged with high-purity nitrogen. Under high-purity nitrogen protection, the product in the reaction vessel was transferred to the dissolving vessel and dissolved by stirring at room temperature. Insoluble matter was filtered off (the filter cake was thoroughly washed with methanol 2-3 times with stirring; the washing liquid was used as a good solvent for the next batch of reaction, and the filter cake was used to prepare calcium sulfate whiskers or high-performance anhydrous gypsum). The filtrate was deacidified by styrene-based ion exchange resin and filtered through a 0.2 μm precision membrane to obtain 308 g of high-purity lithium tetrafluoroborate liquid salt. The lithium tetrafluoroborate mass concentration was 30%, water content was 9.1 ppm, free acid (calculated as HF) was 11.5 ppm, Cl⁻ was 2.7 ppm, and SO₄²⁻ was 0.5 ppm. 2- 4.1 ppm, with the content of each of the various metal impurity ions (such as calcium, iron, magnesium, sodium, and potassium) ≤2 ppm.

[0030] Example 5 A method for preparing high-purity lithium tetrafluoroborate liquid salt, specifically including the following steps: 50 g (1 mol) of high-purity lithium metaborate powder and 159.9 g (2.05 mol) of high-purity calcium fluoride powder were sequentially added to a reactor purged with high-purity nitrogen. Then, 314.85 g of sulfur trioxide gas was slowly introduced. The reactor pressure was increased to 3 MPa, and the reaction was carried out at 150°C with high-pressure stirring for 1 hour. After the reaction was completed, the pressure was slowly reduced to room temperature and pressure, and the excess sulfur trioxide gas was separated for use in the next batch reaction.

[0031] 277.77 g of high-purity acetonitrile was added to a dissolving vessel purged with high-purity nitrogen. Under high-purity nitrogen protection, the product in the reaction vessel was transferred to the dissolving vessel and dissolved by stirring at room temperature. Insoluble matter was filtered off (the filter cake was thoroughly washed 2-3 times with acetonitrile; the washing liquid was used as a good solvent for the next batch of reaction, and the filter cake was used to prepare calcium sulfate whiskers or high-performance anhydrous gypsum). The filtrate was deacidified by a styrene-divinylbenzene ion exchange resin and filtered through a 0.5 μm precision membrane to obtain 370.36 g of high-purity lithium tetrafluoroborate liquid salt. The lithium tetrafluoroborate mass concentration was 25%, water content was 9.5 ppm, free acid (calculated as HF) was 12 ppm, Cl⁻ was 3 ppm, and SO₄²⁻ was 0.5 ppm. 2- 4.9 ppm, with individual content of various metal impurity ions (such as calcium, iron, magnesium, sodium, and potassium) ≤ 2 ppm.

[0032] Example 6 A method for preparing high-purity lithium tetrafluoroborate liquid salt, specifically including the following steps: 50 g (1 mol) of high-purity lithium metaborate powder and 163.8 g (2.1 mol) of high-purity calcium fluoride powder were sequentially added to a reactor purged with high-purity nitrogen. Then, 427.6 g of sulfur trioxide gas was slowly introduced. The reactor pressure was increased to 5 MPa, and the reaction was carried out at 150°C with high-pressure stirring for 1 hour. After the reaction was completed, the pressure was slowly reduced to room temperature and pressure, and the excess sulfur trioxide gas was separated for use in the next batch of reaction.

[0033] 371.49 g of high-purity ethylene glycol dimethyl ether was added to a dissolving vessel purged with high-purity nitrogen. Under high-purity nitrogen protection, the product in the reaction vessel was transferred to the dissolving vessel and dissolved by stirring at room temperature. Insoluble matter was filtered off (the filter cake was thoroughly washed 2-3 times with ethylene glycol dimethyl ether; the washing liquid was used as a good solvent for the next batch of reaction, and the filter cake was used to prepare calcium sulfate whiskers or high-performance anhydrous gypsum). The filtrate was deacidified by acrylic ion exchange resin and filtered through a 0.5 μm precision membrane to obtain 464.36 g of high-purity lithium tetrafluoroborate liquid salt. The lithium tetrafluoroborate mass concentration was 20%, water content was 9.8 ppm, free acid (as HF) was 11.6 ppm, Cl⁻ was 2.3 ppm, and SO₄²⁻ was 0.5 ppm. 2- The content of each metal impurity ion (such as calcium, iron, magnesium, sodium, and potassium) is ≤2ppm.

Claims

1. A process for the preparation of high purity lithium tetrafluoroborate liquid salt, characterized in that, The method comprises the following steps: The lithium metaborate is used as raw material, and is reacted with calcium fluoride and excessive sulfur trioxide gas at 60-150 ℃, 0.2-5 MPa high pressure and inert gas atmosphere for 1-5 h. After the reaction is completed, the sulfur trioxide gas is separated by decompression and reused, and then the reaction product is dissolved in a benign organic solvent under the protection of inert gas, and the insoluble substances are filtered out. The filtrate is subjected to resin deacidification and membrane filtration to obtain the product.

2. The method for preparing high-purity lithium tetrafluoroborate liquid salt as described in claim 1, characterized in that, The lithium metaborate and calcium fluoride used are powders, and the purity of each is greater than or equal to 99.9%.

3. The method for preparing high-purity lithium tetrafluoroborate liquid salt as described in claim 1, characterized in that, The molar ratio of the calcium fluoride to the lithium metaborate is (2-2.1):

1.

4. The method for preparing high-purity lithium tetrafluoroborate liquid salt as described in claim 1, characterized in that, The amount of the sulfur trioxide used is 1.5-3 times the sum of the mass of the lithium metaborate and the mass of the calcium fluoride.

5. The method for preparing high-purity lithium tetrafluoroborate liquid salt as described in claim 1, characterized in that, The decompression separation refers to gradually reducing the reaction pressure to normal temperature and pressure, and the excessive sulfur trioxide is collected in the form of gas and used as raw material for the next batch of reaction.

6. The method for preparing high-purity lithium tetrafluoroborate liquid salt as described in claim 1, characterized in that, The benign organic solvent includes esters, ethers, nitriles and alcohols, and is one or a combination of two or more of dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, methyl acetate, ethyl acetate, propyl propionate, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, tetrahydrofuran, acetonitrile, propionitrile, butyronitrile, adiponitrile, methanol and ethanol.

7. The method for preparing high-purity lithium tetrafluoroborate liquid salt as described in claim 6, characterized in that, The main content of the benign organic solvent is greater than or equal to 99.95%, and the water content is not more than 50 ppm.

8. The method for preparing high-purity lithium tetrafluoroborate liquid salt as described in claim 1, characterized in that, The deacidification resin used is an ion exchange resin, and the ion exchange resin is selected from one or a combination of two or more of styrene, styrene divinylbenzene and acrylic resin.

9. The method for preparing high-purity lithium tetrafluoroborate liquid salt as described in claim 1, characterized in that, The effluent after resin deacidification is filtered by precision membrane filtration to remove trace insoluble substances, and the pore size of the filter membrane is selected to be 0.2-0.5 μm.