A method for preparing high-purity lithium hexafluorophosphate

Abstract

The invention relates to the technical field of lithium ion batteries, and discloses a method for preparing high-purity lithium hexafluorophosphate: 1) reacting phosphorus pentachloride with anhydrous hydrofluoric acid or hydrogen fluoride gas to prepare phosphorus pentafluoride; 2) dissolving LiF in an anhydrous HF solvent, and then introducing the obtained phosphorus pentafluoride therein to react and obtain an anhydrous HF solution of lithium hexafluorophosphate; 3) distilling the obtained anhydrous HF solution of lithium hexafluorophosphate to remove hydrofluoric acid, and obtain solid lithium hexafluorophosphate; 4) using supercritical carbon dioxide or liquid carbon dioxide as a solvent, recrystallizing the lithium hexafluorophosphate solid to obtain purified lithium hexafluorophosphate. The present invention uses carbon dioxide for recrystallization, which is green and environmentally friendly, low in cost, safe in the production process, and good in purification effect; the present invention collects and separates the reaction gas in step 2) and then reuses it to form a full-process clean production process of lithium hexafluorophosphate, and the entire production process is green and environmentally friendly, with less waste discharge.

Description

Technical Field

[0001] The invention relates to the technical field of lithium ion batteries, and in particular to a method for preparing high-purity lithium hexafluorophosphate. Background Art

[0002] Lithium hexafluorophosphate is the most important component (electrolyte) in lithium-ion battery electrolyte. It has good ion migration number and dissociation constant, high electrical conductivity and electrochemical stability, as well as good antioxidant properties and aluminum foil passivation ability, and can be matched with various positive and negative electrode materials.

[0003] The production of lithium hexafluorophosphate using anhydrous hydrogen fluoride as a solvent is easy to carry out and easy to industrialize, which is currently a more mature lithium hexafluorophosphate production process route. The raw material phosphorus pentafluoride is a colorless gas, easy to hydrolyze, and has strong moisture absorption. Since high-purity phosphorus pentafluoride is expensive, the manufacturing cost of lithium hexafluorophosphate is very high. The prior art generally uses a method of reacting phosphorus pentachloride with anhydrous hydrogen fluoride to prepare phosphorus pentafluoride. The crude phosphorus pentafluoride can be directly reacted with LiF to prepare lithium hexafluorophosphate. For example, CN1210810A discloses a method for preparing lithium hexafluorophosphate, which discloses that a gaseous mixture of gaseous phosphorus pentafluoride and hydrochloric acid is contacted with a hydrofluoric acid solution of lithium fluoride in a tower to prepare lithium hexafluorophosphate; CN104555959A discloses a method for rapidly preparing lithium hexafluorophosphate, which includes: reacting hydrogen fluoride liquid with phosphorus pentachloride to obtain a mixed gas of phosphorus pentafluoride and hydrogen chloride, and passing the mixed gas of phosphorus pentafluoride and hydrogen chloride into hydrogen fluoride and lithium fluoride to obtain a lithium hexafluorophosphate solution. Directly using crude phosphorus pentafluoride to prepare lithium hexafluorophosphate involves the problem of gas separation after polymerization. The gas discharged after polymerization mainly includes phosphorus pentafluoride, hydrogen fluoride and hydrogen chloride, and there is little research on this in the prior art.

[0004] The use of anhydrous hydrogen fluoride as a solvent to produce lithium hexafluorophosphate is easy to carry out and easy to industrialize, and is currently a more mature lithium hexafluorophosphate production process route.

[0005] The prior art generally uses an organic solvent crystallization method to purify lithium hexafluorophosphate. CN103539168A discloses a method for purifying lithium hexafluorophosphate, including: 1) dissolving crude lithium hexafluorophosphate in a carbonate and / or ether solvent at a temperature of 0 to 40°C, and filtering the dissolved solution to obtain a transparent liquid; 2) adding 0.1 to 10 volumes of alkane and / or ether solvents of the volume of the transparent liquid to the transparent liquid, mixing and stirring, so that lithium hexafluorophosphate nucleates and crystallizes in the mixed solvent; 3) aging for 0.5 to 24 hours, filtering the mixed solution to obtain lithium hexafluorophosphate crystals; 4) repeating steps 1) to 3) at least once; 5) vacuuming and drying at a low temperature of -40 to 90°C to obtain high-purity lithium hexafluorophosphate. However, this scheme requires at least three recrystallizations during implementation to obtain high-purity lithium hexafluorophosphate. Summary of the invention

[0006] In view of the above-mentioned deficiencies in the prior art, the object of the present invention is to provide a method for preparing high-purity lithium hexafluorophosphate.

[0007] To achieve the above objectives, the present invention adopts the following technical solutions:

[0008] A method for preparing high-purity lithium hexafluorophosphate comprises the following steps:

[0009] 1) Reacting phosphorus pentachloride with anhydrous hydrofluoric acid or hydrogen fluoride gas to prepare phosphorus pentafluoride;

[0010] 2) dissolving LiF in an anhydrous HF solvent, and then introducing the phosphorus pentafluoride obtained in step 1) into the solvent, so that the phosphorus pentafluoride reacts with LiF to obtain an anhydrous HF solution of lithium hexafluorophosphate;

[0011] 3) distilling the obtained anhydrous HF solution of lithium hexafluorophosphate to remove hydrofluoric acid to obtain solid lithium hexafluorophosphate;

[0012] 4) Using supercritical carbon dioxide as a solvent, the lithium hexafluorophosphate solid obtained in step 3) is recrystallized to obtain purified lithium hexafluorophosphate.

[0013] Furthermore, in the step 4), the carbon dioxide is first controlled to be in a supercritical state to dissolve the lithium hexafluorophosphate to form a supercritical carbon dioxide solution of lithium hexafluorophosphate, and then the carbon dioxide is converted from the supercritical state to a liquid state by gradually reducing the temperature or gradually reducing the temperature and pressure at the same time, and the lithium hexafluorophosphate is crystallized from the solution to form a suspension, and the suspension is filtered to obtain lithium hexafluorophosphate crystals, and the obtained crystals are washed twice with a liquid carbon dioxide solvent, and the obtained crystals are freed from carbon dioxide to obtain a purified lithium hexafluorophosphate product.

[0014] Preferably, the temperature is lowered to -30°C to -10°C, and the pressure is maintained at a pressure that allows the carbon dioxide to remain in a liquid state.

[0015] Preferably, the crystallization time is 4 to 6 hours.

[0016] Furthermore, the gas discharged during the reaction of step 2) is collected, and the hydrogen fluoride is first pressurized and condensed; and the remaining gas is then separated by activated carbon pressure and temperature swing adsorption.

[0017] Furthermore, the pressurized condensation has a temperature of 15°C to -98°C and a pressure of 0.1MPa to 5MPa.

[0018] Preferably, in the activated carbon temperature and pressure swing adsorption process: the adsorption pressure is 0.5~0.8MPa, and the temperature in the adsorption stage is 25~50°C; the desorption pressure is -0.05~-0.1MPa, and the desorption temperature is 80~120°C.

[0019] Preferably, the distillation temperature in step 3) is 30-50°C.

[0020] Preferably, in step 2), the molar ratio of phosphorus pentafluoride to LiF is about 1.1-1.2:1.

[0021] Furthermore, in step 1), phosphorus pentachloride is prepared by reacting phosphorus trichloride with chlorine.

[0022] Furthermore, phosphorus trichloride, a raw material for preparing phosphorus pentachloride, is prepared by reacting yellow phosphorus with chlorine.

[0023] Furthermore, the hydrogen chloride gas obtained after the activated carbon adsorption separation is subjected to a catalytic oxidation reaction, and the mixed gas obtained by the reaction is separated to obtain chlorine gas, and the obtained chlorine gas is introduced as a raw material into the preparation step of phosphorus pentachloride for reuse and / or the obtained chlorine gas is introduced as a raw material into the preparation step of phosphorus trichloride for reuse.

[0024] Furthermore, the gas desorbed after the activated carbon adsorption separation can be recycled to step 2) for reuse.

[0025] Furthermore, in the step 3), the distilled hydrogen fluoride is condensed in a condenser and then recycled to the step 2) for reuse.

[0026] Those skilled in the art will understand the required conditions when using supercritical carbon dioxide as a solvent. For example, when using supercritical carbon dioxide fluid as a reaction solvent, the reaction environment is maintained above the critical temperature and critical pressure of CO2, and the reaction temperature is preferably 35~100°C; the pressure is preferably 8~50MPa.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] (1) The present invention directly reacts crude phosphorus pentafluoride with lithium fluoride, and the resulting crude lithium hexafluorophosphate is recrystallized using carbon dioxide, which is green and environmentally friendly, low in cost, safe in the production process, and good in purification effect.

[0029] (2) The present invention can form a full-process clean production process for lithium hexafluorophosphate, and the entire production process is green and environmentally friendly with less waste emissions. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0031] Figure 1 This is a process flow chart of the preparation method of high-purity lithium hexafluorophosphate according to an embodiment of the present invention. DETAILED DESCRIPTION

[0032] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several variations and improvements may be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0033] Example 1

[0034] like Figure 1 As shown, a method for preparing high-purity lithium hexafluorophosphate comprises the following steps:

[0035] 1) Phosphorus pentachloride is reacted with anhydrous hydrofluoric acid or hydrogen fluoride gas to prepare phosphorus pentafluoride; the phosphorus pentachloride is prepared by reacting phosphorus trichloride with chlorine; the phosphorus trichloride used as the raw material for preparing the phosphorus pentachloride is prepared by reacting yellow phosphorus with chlorine;

[0036] 2) dissolving LiF in an anhydrous HF solvent, and then introducing the phosphorus pentafluoride obtained in step 1) into the solvent, wherein the phosphorus pentafluoride reacts with LiF to obtain an anhydrous HF solution of lithium hexafluorophosphate, wherein the molar ratio of phosphorus pentafluoride to LiF is about 1.1-1.2:1;

[0037] 3) distilling the obtained anhydrous HF solution of lithium hexafluorophosphate to remove hydrofluoric acid at a distillation temperature of 30-50° C. to obtain solid lithium hexafluorophosphate; the distilled hydrogen fluoride is condensed in a condenser and recycled to step 2) for reuse;

[0038] 4) Using supercritical carbon dioxide as a solvent, the lithium hexafluorophosphate solid obtained in step 3) is recrystallized: the lithium hexafluorophosphate solid obtained in step 3) is added to a crystallization kettle, and liquid carbon dioxide is added, and the temperature is maintained at 35° C. and the pressure is 8 MPa, so that the liquid carbon dioxide is converted into a supercritical state, and the lithium hexafluorophosphate solid in the crystallization kettle is dissolved to form a supercritical carbon dioxide solution of lithium hexafluorophosphate, and then the temperature is gradually reduced to -10° C. to convert the carbon dioxide from the supercritical state to a liquid state, and at the same time, the lithium hexafluorophosphate is crystallized from the solution to form a suspension, and the crystallization time is 5 hours. After the crystallization, the pressure is reduced to about 6 MPa, and the suspension is filtered to obtain lithium hexafluorophosphate crystals. The obtained crystals are washed twice with liquid carbon dioxide solvent, and the mother liquor is reused. After the obtained crystals are freed from carbon dioxide, the purified lithium hexafluorophosphate product is obtained.

[0039] Collect the gas discharged during the reaction of step 2), firstly pressurize and condense at a temperature of 15°C to -98°C and a pressure of 0.1MPa to 5MPa to convert hydrogen fluoride into liquid; then separate the remaining gas through activated carbon pressure and temperature swing adsorption to obtain hydrogen chloride gas, carry out catalytic oxidation reaction, and separate the mixed gas obtained by the reaction to obtain chlorine gas, and introduce the obtained chlorine gas as a raw material into the preparation step of phosphorus pentachloride for reuse and / or introduce the obtained chlorine gas as a raw material into the preparation step of phosphorus trichloride for reuse. The gas desorbed after activated carbon adsorption separation is mainly phosphorus pentafluoride, which can be recycled to step 2) for reuse; in step 3), the distilled hydrogen fluoride can be recycled to step 2) for reuse after condensation in a condenser.

[0040] During the pressure and temperature swing adsorption process of activated carbon: the adsorption pressure is 0.5~0.8MPa, and the temperature in the adsorption stage is 25~50℃; the desorption pressure is -0.05~-0.1MPa, and the desorption temperature is 80~120℃.

[0041] In this embodiment, the activated carbon is prepared by the following method, which specifically comprises the following steps:

[0042] Ⅰ) Crush the pine branches to 40-60 mesh;

[0043] II) impregnating the pine branch powder obtained in step 1) with a zinc chloride solution having a mass concentration of 30% to 60% for 2 to 3 hours;

[0044] III) After drying the pine branches impregnated in step II), place them in a tubular furnace, introduce nitrogen at a rate of 40-60 mL / min, maintain the temperature in the furnace at 400-600° C., and keep the temperature for 2-3 hours.

[0045] The purity of the purified lithium hexafluorophosphate product obtained by crystallization in this example is 99.99%, the water content is less than 1 ppm, and the free acid content (calculated as HF) is less than 5 ppm.

[0046] Example 2

[0047] A method for preparing high-purity lithium hexafluorophosphate comprises the following steps:

[0048] 1) Phosphorus pentachloride is reacted with anhydrous hydrofluoric acid or hydrogen fluoride gas to prepare phosphorus pentafluoride; the phosphorus pentachloride is prepared by reacting phosphorus trichloride with chlorine; the phosphorus trichloride used as the raw material for preparing the phosphorus pentachloride is prepared by reacting yellow phosphorus with chlorine;

[0049] 2) dissolving LiF in an anhydrous HF solvent, and then introducing the phosphorus pentafluoride obtained in step 1) into the solvent, wherein the phosphorus pentafluoride reacts with LiF to obtain an anhydrous HF solution of lithium hexafluorophosphate, wherein the molar ratio of phosphorus pentafluoride to LiF is about 1.1-1.2:1;

[0050] 3) distilling the obtained anhydrous HF solution of lithium hexafluorophosphate to remove hydrofluoric acid at a distillation temperature of 30-50° C. to obtain solid lithium hexafluorophosphate; the distilled hydrogen fluoride is condensed in a condenser and recycled to step 2) for reuse;

[0051] 4) Using supercritical carbon dioxide as a solvent, the lithium hexafluorophosphate solid obtained in step 3) is recrystallized: in a crystallization kettle, the lithium hexafluorophosphate solid obtained in step 3) is added, and liquid carbon dioxide is added, and the temperature is maintained at 50° C. and the pressure is 8 MPa, so that the liquid carbon dioxide is converted into a supercritical state, and the lithium hexafluorophosphate solid in the crystallization kettle is dissolved to form a supercritical carbon dioxide solution of lithium hexafluorophosphate, and then the temperature is gradually reduced to -20° C. and the pressure is gradually reduced to 6 MPa to perform crystallization, and the crystallization time is 4 hours. After crystallization, the pressure is reduced to about 5 MPa, and the suspension is filtered to obtain lithium hexafluorophosphate crystals. The obtained crystals are washed twice with liquid carbon dioxide solvent, and the mother liquor is reused. After removing carbon dioxide from the obtained crystals, the purified lithium hexafluorophosphate product is obtained.

[0052] Collect the gas discharged during the reaction of step 2), firstly pressurize and condense at a temperature of 15°C to -98°C and a pressure of 0.1MPa to 5MPa to convert hydrogen fluoride into liquid; then separate the remaining gas through activated carbon pressure and temperature swing adsorption to obtain hydrogen chloride gas, carry out catalytic oxidation reaction, and separate the mixed gas obtained by the reaction to obtain chlorine gas, and introduce the obtained chlorine gas as a raw material into the preparation step of phosphorus pentachloride for reuse and / or introduce the obtained chlorine gas as a raw material into the preparation step of phosphorus trichloride for reuse. The gas desorbed after activated carbon adsorption separation is mainly phosphorus pentafluoride, which can be recycled to step 2) for reuse; in step 3), the distilled hydrogen fluoride can be recycled to step 2) for reuse after condensation in a condenser.

[0053] During the activated carbon pressure and temperature swing adsorption process: the adsorption pressure is 0.5-0.8 MPa, the temperature in the adsorption stage is 25-50° C.; the desorption pressure is -0.05--0.1 MPa, and the desorption temperature is 80-120° C. The activated carbon preparation source is the same as in Example 1.

[0054] The purity of the purified lithium hexafluorophosphate product obtained by crystallization in this example is 99.99%, the water content is less than 1 ppm, and the free acid content (calculated as HF) is less than 7 ppm.

[0055] Example 3

[0056] A method for preparing high-purity lithium hexafluorophosphate comprises the following steps:

[0057] 1) Phosphorus pentachloride is reacted with anhydrous hydrofluoric acid or hydrogen fluoride gas to prepare phosphorus pentafluoride; the phosphorus pentachloride is prepared by reacting phosphorus trichloride with chlorine; the phosphorus trichloride used as the raw material for preparing the phosphorus pentachloride is prepared by reacting yellow phosphorus with chlorine;

[0058] 2) dissolving LiF in an anhydrous HF solvent, and then introducing the phosphorus pentafluoride obtained in step 1) into the solvent, wherein the phosphorus pentafluoride reacts with LiF to obtain an anhydrous HF solution of lithium hexafluorophosphate, wherein the molar ratio of phosphorus pentafluoride to LiF is about 1.1-1.2:1;

[0059] 3) distilling the obtained anhydrous HF solution of lithium hexafluorophosphate to remove hydrofluoric acid at a distillation temperature of 30-50° C. to obtain solid lithium hexafluorophosphate; the distilled hydrogen fluoride is condensed in a condenser and recycled to step 2) for reuse;

[0060] 4) Using supercritical carbon dioxide as a solvent, recrystallizing the lithium hexafluorophosphate solid obtained in step 3): adding the lithium hexafluorophosphate solid obtained in step 3) to a crystallization kettle, adding liquid carbon dioxide, maintaining the temperature at 40° C. and the pressure at 10 MPa, converting the liquid carbon dioxide into a supercritical state, dissolving the lithium hexafluorophosphate solid in the crystallization kettle, forming a supercritical carbon dioxide solution of lithium hexafluorophosphate, and then gradually reducing the temperature to -15° C. and the pressure to 6 MPa to perform crystallization for 6 hours. After crystallization, the pressure is reduced to about 5 MPa, and lithium hexafluorophosphate crystals are obtained by filtration. The obtained crystals are washed twice with liquid carbon dioxide solvent, and the mother liquor is reused. After removing carbon dioxide from the obtained crystals, purified lithium hexafluorophosphate products are obtained.

[0061] Collect the gas discharged during the reaction of step 2), firstly pressurize and condense at a temperature of 15°C to -98°C and a pressure of 0.1MPa to 5MPa to convert hydrogen fluoride into liquid; then separate the remaining gas through activated carbon pressure and temperature swing adsorption to obtain hydrogen chloride gas, carry out catalytic oxidation reaction, and separate the mixed gas obtained by the reaction to obtain chlorine gas, and introduce the obtained chlorine gas as a raw material into the preparation step of phosphorus pentachloride for reuse and / or introduce the obtained chlorine gas as a raw material into the preparation step of phosphorus trichloride for reuse. The gas desorbed after activated carbon adsorption separation is mainly phosphorus pentafluoride, which can be recycled to step 2) for reuse; in step 3), the distilled hydrogen fluoride can be recycled to step 2) for reuse after condensation in a condenser.

[0062] During the activated carbon pressure and temperature swing adsorption process: the adsorption pressure is 0.5-0.8 MPa, the temperature in the adsorption stage is 25-50° C.; the desorption pressure is -0.05-0.1 MPa, and the desorption temperature is 80-120° C. The activated carbon source is the same as in Example 1.

[0063] The purity of the purified lithium hexafluorophosphate product obtained by crystallization in this example is 99.99%, the water content is less than 2 ppm, and the free acid content (calculated as HF) is less than 6 ppm.

[0064] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A method for preparing high-purity lithium hexafluorophosphate, characterized in that: The following steps are involved: 1) Reacting phosphorus pentachloride with anhydrous hydrofluoric acid or hydrogen fluoride gas to prepare phosphorus pentafluoride; 2) dissolving LiF in an anhydrous HF solvent, and then introducing the phosphorus pentafluoride obtained in step 1) into the solvent, so that the phosphorus pentafluoride reacts with LiF to obtain an anhydrous HF solution of lithium hexafluorophosphate; 3) distilling the obtained anhydrous HF solution of lithium hexafluorophosphate to remove hydrofluoric acid to obtain solid lithium hexafluorophosphate; 4) using supercritical carbon dioxide as a solvent to recrystallize the lithium hexafluorophosphate solid obtained in step 3) to obtain purified lithium hexafluorophosphate; In the step 4), the carbon dioxide is first controlled to be in a supercritical state to dissolve the lithium hexafluorophosphate to form a supercritical carbon dioxide solution of lithium hexafluorophosphate, and then the carbon dioxide is converted from the supercritical state to a liquid state by gradually reducing the temperature to -30°C to -10°C or gradually reducing the temperature and pressure at the same time, and the pressure is maintained to keep the carbon dioxide in a liquid state, and the lithium hexafluorophosphate is crystallized from the solution to form a suspension, and the suspension is filtered to obtain lithium hexafluorophosphate crystals, and the obtained crystals are washed twice with a liquid carbon dioxide solvent, and the obtained crystals are freed of carbon dioxide to obtain a purified lithium hexafluorophosphate product.

2. The method for preparing high-purity lithium hexafluorophosphate according to claim 1, characterized in that: The crystallization time is 4 to 6 hours.

3. The method for preparing high-purity lithium hexafluorophosphate according to claim 1, characterized in that: In step 2), the gas discharged during the reaction is first pressurized and condensed into hydrogen fluoride; and the remaining gas is then separated by activated carbon pressure and temperature swing adsorption.

4. The method for preparing high-purity lithium hexafluorophosphate according to claim 3, characterized in that: The pressurized condensation has a temperature of 15°C to -98°C and a pressure of 0.1MPa to 5MPa.

5. The method for preparing high-purity lithium hexafluorophosphate according to claim 3, characterized in that: During the pressure and temperature swing adsorption process of activated carbon: the adsorption pressure is 0.5~0.8MPa, and the temperature in the adsorption stage is 25~50℃; the desorption pressure is -0.05~-0.1MPa, and the desorption temperature is 80~120℃.

6. The method for preparing high-purity lithium hexafluorophosphate according to claim 1, characterized in that: The distillation temperature in step 3) is 25-40°C.

7. The method for preparing high-purity lithium hexafluorophosphate according to claim 1, characterized in that: In the step 2), the molar ratio of phosphorus pentafluoride to LiF is 1.1-1.2:

1.

8. The method for preparing high-purity lithium hexafluorophosphate according to claim 3, characterized in that: In the step 1), phosphorus pentachloride is prepared by reacting phosphorus trichloride with chlorine. The phosphorus trichloride, a raw material for preparing phosphorus pentachloride, is prepared by reacting yellow phosphorus with chlorine. The hydrogen chloride gas obtained after activated carbon adsorption separation is collected, subjected to catalytic oxidation reaction, and chlorine is obtained after separation of the mixed gas obtained by the reaction. The obtained chlorine is introduced as a raw material into the phosphorus pentachloride preparation step for reuse and / or the obtained chlorine is introduced as a raw material into the phosphorus trichloride preparation step for reuse. The hydrofluoric acid obtained after pressurized condensation, the hydrofluoric acid removed by distillation in step 3), and the gas obtained by desorption after activated carbon adsorption separation are all circulated to step 2) for reuse.

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