Preparation method of high-purity lithium hexafluorophosphate

Abstract

The invention provides a preparation method of high-purity lithium hexafluorophosphate, which comprises the following steps: preparing silicon dioxide aerogel loaded with lithium fluoride crystals by using lithium hydroxide and sodium fluosilicate in a molar ratio of (7-9): 1 as raw materials, and reacting the silicon dioxide aerogel with phosphorus pentafluoride and anhydrous hydrogen fluoride at low temperature to obtain the high-purity lithium hexafluorophosphate. The purity of the lithium hexafluorophosphate crystal obtained through the method reaches 99.9% or above, the yield reaches 90% or above, and the lithium hexafluorophosphate crystal has the advantages of being high in yield and purity and low in cost and is beneficial to industrial production.

Description

Technical Field:

[0001] The present invention relates to the technical field of battery materials, and more specifically to a method for preparing high-purity lithium hexafluorophosphate. Background Art:

[0002] Lithium hexafluorophosphate is a commonly used electrolyte raw material for lithium-ion batteries. With the rapid growth in the demand for various lithium batteries, it has led to over-exploitation of lithium ore resources and continuous cost increases. The preparation methods of lithium hexafluorophosphate include gas-solid reaction method, anhydrous HF solvent method, organic solvent method, ion exchange method, etc.

[0003] The gas-solid reaction method is a traditional method for preparing lithium hexafluorophosphate, using gaseous phosphorus pentafluoride and solid lithium fluoride as raw materials. The anhydrous HF solvent method is to dissolve lithium fluoride in 20 times the amount of liquid HF at low temperature to form a homogeneous phase to achieve the absorption reaction with phosphorus pentafluoride to generate lithium hexafluorophosphate. It has been found that as long as lithium fluoride is effectively dispersed and then phosphorus pentafluoride is introduced into the dispersion system, lithium hexafluorophosphate with high purity and yield can be obtained.

[0004] Patent CN101209830A discloses a method for preparing lithium hexafluorophosphate, in which solid lithium fluoride and a phosphorus source are contacted and reacted in the presence of a solvent, where the solvent is an organic solvent that is insoluble in solid lithium fluoride and the phosphorus source but soluble in lithium hexafluorophosphate, and the organic solvent is acetonitrile or a carbonate solvent. This method uses an inexpensive organic solvent to replace highly corrosive HF, can carry out the reaction under mild conditions without extreme temperatures, reduces the requirements for the equipment material, reduces the equipment cost, saves energy consumption, greatly reduces the potential hazards in production, and improves safety. However, the overall yield of this method is still not high.

[0005] Patent CN117800367A discloses a method for preparing lithium hexafluorophosphate, including the following steps: after fully mixing lithium fluoride and a fluorine-containing solvent, phosphorus pentafluoride is introduced into the mixed solution to prepare a solution containing lithium hexafluorophosphate. The solvent selected for the process route of this method has low toxicity, can avoid using highly toxic hydrogen fluoride as a solvent in the traditional method, the process is safe and environmentally friendly, and at the same time the solubility of the solvent in lithium hexafluorophosphate is high, which is easy to separate from the product, is conducive to improving the yield of lithium hexafluorophosphate products, and the obtained lithium hexafluorophosphate products have low acidity and are suitable for industrial production. However, the solvents used in this method are fluorine-containing solvents such as methoxyheptafluoropropane and methoxynonafluoropentane, and their prices are relatively high.

[0006] Therefore, there is an urgent need for a method for preparing battery-grade lithium hexafluorophosphate with low cost, high yield, and relatively simple operation. Summary of the Invention:

[0007] The present invention aims to provide a method for producing high-purity lithium hexafluorophosphate to overcome the problems of low yield, low purity, and high production cost of lithium hexafluorophosphate produced in the prior art.

[0008] To achieve the above object, the technical idea adopted by the present invention is as follows:

[0009] Excess high-purity lithium hydroxide is directly reacted with sodium fluorosilicate solution to finally obtain lithium fluoride and silica colloid. At the same time, the silica colloid is further used to form porous silica to load the generated lithium fluoride, thereby improving the reaction efficiency of lithium fluoride and phosphorus pentafluoride and ultimately increasing the yield of lithium hexafluorophosphate.

[0010] The technical solution of the present invention includes the following steps:

[0011] (1) At 30 - 35°C, an aqueous solution of lithium hydroxide is prepared and added to a polytetrafluoroethylene reactor. Sodium fluorosilicate is added to the reactor according to the molar ratio of lithium hydroxide to sodium fluorosilicate of (7 - 9):1, and the temperature is raised to 60 - 80°C for reaction while stirring until no more precipitation occurs;

[0012] (2) The reaction system is filtered to obtain lithium fluoride and silica precipitate;

[0013] (3) The mixture obtained in step (2) is aged, solvent-exchanged, and dried with anhydrous ethanol solvent to prepare silica aerogel loaded with lithium fluoride crystals;

[0014] (4) Phosphorus pentafluoride and anhydrous hydrogen fluoride are continuously introduced and reacted with the silica aerogel loaded with lithium fluoride crystals obtained in step (3) at 5 - 15°C for 2 - 4 hours; after the reaction is completed, the temperature is first raised to 30 - 40°C and allowed to stand for 0.5 - 1 hour, deionized water is added, stirred and dissolved, filtered, and the filtrate is stirred and crystallized at 0 - 10°C for 2 - 4 hours;

[0015] (5) The precipitated crystals are dried at 110 - 130°C to obtain the final lithium hexafluorophosphate product.

[0016] Preferably, the purity of lithium hydroxide in step (1) is above 99.9%, and the purity of lithium fluorosilicate is above 99.5%.

[0017] Preferably, the concentration of the aqueous lithium hydroxide solution in step (1) is 60 - 120 g / L.

[0018] Preferably, the stirring rate in step (1) is 300 - 500 rpm.

[0019] Preferably, the aging temperature in step (3) is 50 - 60°C, and the aging time is 24 - 72 h.

[0020] Preferably, the solvent replacement in step (3) includes ethanol replacement and n-hexane replacement in sequence, and the time for each replacement is 24-48h.

[0021] Preferably, the drying in step (3) includes atmospheric drying; the temperature of the atmospheric drying is 80-100°C, and the time of the atmospheric drying is 10-36h.

[0022] Preferably, the molar ratio of phosphorus pentafluoride in step (4) to sodium fluorosilicate in step (1) is 8-12:1, and the molar ratio of phosphorus pentafluoride to anhydrous hydrogen fluoride is 1:1-2.

[0023] In the present invention, the reaction raw materials selected are lithium hydroxide and sodium fluorosilicate. The reaction formula of lithium hydroxide and sodium fluorosilicate is:

[0024] 4LiOH + Na2SiF6 → 2NaF↓ + 4LiF↓ + SiO2↓ + 2CO2↑

[0025] When lithium hydroxide in the system is in excess, lithium hydroxide and the generated sodium fluoride precipitate will continue to undergo a metathesis reaction to generate lithium fluoride, and its reaction formula is:

[0026] LiOH + NaF → LiF↓ + NaOH

[0027] Therefore, through the technical solution of the present invention, colloidal silica and lithium fluoride crystals can be generated in one reaction. In the form of preparing an aerogel loaded with lithium fluoride crystals, the conversion rate of lithium fluoride crystals to lithium hexafluorophosphate is fully improved.

[0028] Since the requirements for the physical and chemical properties of the aerogel in the present invention are relatively low, only the effect of porous loading needs to be achieved. Therefore, an aerogel loaded with lithium fluoride crystals is prepared by means of aging, solvent replacement, and atmospheric drying. The aerogel prepared by this method can effectively reduce costs.

[0029] After testing, the yield of lithium hexafluorophosphate prepared by the present invention is above 90%, and the purity is above 99.9%, both meeting the standards for the use of battery-grade lithium hexafluorophosphate.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] In the present invention, by reacting excess lithium hydroxide and sodium fluorosilicate at a specific temperature to generate colloidal silica and lithium fluoride crystals, in the form of preparing an aerogel loaded with lithium fluoride crystals, the reaction area is fully increased, and the conversion rate of lithium fluoride crystals to lithium hexafluorophosphate is further improved; at the same time, the present invention directly makes full use of the colloidal silica impurities, which can effectively save production costs. Specific embodiments:

[0032] To make the objectives, technical solutions, and advantages of the implementation of the present invention clearer, the technical solutions of the present invention will be further described below through specific embodiments. Those skilled in the art should be aware that the embodiments are only for helping to understand the technical content and technical effects of the present invention and should not be regarded as a limitation to the present invention.

[0033] The sources of some components in the following embodiments and comparative examples are as described below:

[0034] Lithium hydroxide (purity 99.9%), anhydrous hydrogen fluoride, ethanol, and n-hexane were all purchased from Aladdin. Phosphorus pentafluoride was purchased from Shanghai Biyang Industry Co., Ltd. Sodium fluorosilicate (purity 99.5%) was purchased from Shandong Huian Chemical Co., Ltd.

[0035] The above description only shows that the raw materials used in the embodiments or comparative examples of the present invention are all purchased from mainstream manufacturers in the market, and it does not mean that the raw materials must be produced by the above manufacturers. As long as they are conventional raw materials, they can all play the expected role and there is no special limitation. For those conditions or operation methods not specified in this embodiment or comparative example, the commonly used conditions or operation methods in the art can be followed.

[0036] Example 1

[0037] (1) At 30 °C, 1 L of a 120 g / L lithium hydroxide solution was added to a polytetrafluoroethylene reactor, and 118 g of solid sodium fluorosilicate was gradually added to the reactor while stirring at a stirring speed of 500 rpm. The reaction was carried out at 75 °C until no more precipitation occurred;

[0038] (2) The reaction liquor was directly precipitated at 75 °C to obtain a mixture of silica gel and lithium fluoride crystals;

[0039] (3) The mixture obtained in step (2) was aged, solvent-exchanged, and dried with an anhydrous ethanol solvent to prepare a silica aerogel loaded with lithium fluoride crystals; the aging temperature was 50 °C, and the aging time was 72 h; the solvent exchange included sequentially performing ethanol and n-hexane replacements once, with each replacement time being 24 h; the drying was atmospheric drying; the temperature of the atmospheric drying was 80 °C, and the time of the atmospheric drying was 36 h;

[0040] (4) Phosphorus pentafluoride and anhydrous hydrogen fluoride were continuously introduced and reacted with the silica aerogel loaded with lithium fluoride crystals obtained in step (3) at 15 °C for 3 hours; the molar ratio of phosphorus pentafluoride gas to sodium fluorosilicate in step (1) was 10:1; the molar ratio of introduced phosphorus pentafluoride to anhydrous hydrogen fluoride was 1:2; after the reaction ended, the temperature was first raised to 35 °C and left standing for 1 hour, deionized water was added, stirred and dissolved, filtered, and the filtrate was stirred and crystallized at 0 °C for 3 hours;

[0041] (5) The precipitated crystals are dried at 110 °C for 4 hours to obtain the final lithium hexafluorophosphate product.

[0042] Example 2

[0043] (1) At 30 °C, 1 L of 96 g / L lithium hydroxide solution is added to a polytetrafluoroethylene reactor, and 94 g of sodium fluorosilicate solid is gradually added to the reactor while stirring at a speed of 300 rpm. The reaction is carried out at 65 °C until no more precipitation occurs.

[0044] (2) The reaction liquor is directly precipitated at 65 °C to obtain a mixture of silica gel colloid and lithium fluoride crystals.

[0045] (3) The mixture obtained in step (2) is aged, solvent-exchanged, and dried with anhydrous ethanol solvent to form silica aerogel loaded with lithium fluoride crystals; the aging temperature is 60 °C and the aging time is 36 h; the solvent exchange includes sequentially performing ethanol and n-hexane replacements once, with each replacement time being 36 h; the drying is atmospheric drying; the atmospheric drying temperature is 100 °C and the atmospheric drying time is 24 h.

[0046] (4) Phosphorus pentafluoride is continuously introduced and reacts fully with the silica aerogel loaded with lithium fluoride crystals obtained in step (3) at 10 °C for 5 hours; the molar ratio of phosphorus pentafluoride gas to sodium fluorosilicate in step (1) is 12:1; the molar ratio of introduced phosphorus pentafluoride to anhydrous hydrogen fluoride is 1:1; after the reaction ends, it is first heated to 40 °C and left standing for 1 hour, deionized water is added, stirred and dissolved, filtered, and the filtrate is stirred and crystallized at 5 °C for 4 hours.

[0047] (5) The precipitated crystals are dried at 130 °C for 2 hours to obtain the final lithium hexafluorophosphate product.

[0048] Comparative Example 1

[0049] Compared with Example 1, the only difference is that the reaction temperature in step (1) is 90 °C.

[0050] Comparative Example 2

[0051] Compared with Example 1, the only difference is that the reaction temperature in step (1) is 50 °C.

[0052] Comparative Example 3

[0053] Compared with Example 1, the only difference is that the amount of sodium fluorosilicate used in step (1) is adjusted to 155 g.

[0054] The lithium hexafluorophosphate prepared in the above examples and comparative examples is weighed, its actual yield is calculated, and a purity test is carried out. The test results are shown in Table 1 below.

[0055] Table 1. Actual yield and purity test results of examples and comparative examples

[0056]

[0057]

[0058] It can be seen from Table 1 that the purity of lithium hexafluorophosphate obtained by the present invention reaches more than 99.9%, and the actual yield reaches more than 90%.

[0059] From the comparison between the embodiment and comparative examples 1-2, since the silica colloidal body is used as a carrier of lithium fluoride in the technical solution of the present invention, increasing the reaction temperature of lithium hydroxide and sodium fluorosilicate can promote the reaction between the two, but at high temperature, silica will react with hydroxide, so that part of the added lithium hydroxide will be used to react with silica, and if the temperature is too low, lithium hydroxide and sodium fluoride cannot fully undergo double decomposition reaction, resulting in the generated lithium hexafluorophosphate product containing part of sodium hexafluorophosphate, which reduces the yield and purity of the lithium hexafluorophosphate product.

[0060] From the comparison between the embodiment and comparative example 3, the present invention can convert more sodium fluoride into lithium fluoride through the excess lithium hydroxide, thereby improving the yield of lithium hexafluorophosphate. When the amount of lithium hydroxide is insufficient, the reaction of sodium fluoride and lithium hydroxide to generate lithium fluoride by double decomposition is not sufficient, so that a certain amount of sodium fluoride still remains in the system, resulting in the generated lithium hexafluorophosphate product containing part of sodium hexafluorophosphate, which reduces the yield and purity of the lithium hexafluorophosphate product.

[0061] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A preparation method of high-purity lithium hexafluorophosphate, characterized in that, It includes the following steps: (1) At 30 - 35 °C, prepare an aqueous solution of lithium hydroxide and add it to a polytetrafluoroethylene reactor. Add sodium fluorosilicate to the reactor according to the molar ratio of lithium hydroxide to sodium fluorosilicate of (7 - 9):

1. Heat up to 60 - 80 °C for reaction while stirring until no more precipitation occurs; (2) Filter the reaction system to obtain lithium fluoride and silicon dioxide precipitates; (3) Age, perform solvent replacement, and dry the mixture obtained in step (2) with anhydrous ethanol solvent to prepare a silicon dioxide aerogel loaded with lithium fluoride crystals; (4) Continuously introduce phosphorus pentafluoride and anhydrous hydrogen fluoride and react at 5 - 15 °C for 2 - 4 hours in the presence of the silicon dioxide aerogel loaded with lithium fluoride crystals obtained in step (3); after the reaction ends, first heat up to 30 - 40 °C and let it stand for 0.5 - 1 hour, add deionized water, stir to dissolve, filter, and perform stirring and crystallization on the filtrate at 0 - 10 °C for 2 - 4 hours; (5) Dry the precipitated crystals at 110 - 130 °C to obtain the final lithium hexafluorophosphate product.

2. The method according to claim 1, wherein In step (1), the purity of lithium hydroxide is above 99.9%, and the purity of lithium fluorosilicate is above 99.5%.

3. The method according to claim 1, wherein In step (1), the concentration of the aqueous lithium hydroxide solution is 60 - 120 g / L.

4. The method according to claim 1, characterized in that, In step (1), the stirring rate is 300 - 500 rpm.

5. The method according to claim 1, characterized in that, In step (3), the aging temperature is 50 - 60 °C, and the aging time is 24 - 72 h.

6. The method according to claim 1, wherein In step (3), the solvent replacement includes sequentially performing ethanol and n - hexane replacement, and the time for each replacement is 24 - 48 h.

7. The method according to claim 1, wherein In step (3), the drying includes atmospheric drying; the temperature of the atmospheric drying is 80 - 100 °C, and the time of the atmospheric drying is 10 - 36 h.

8. The method according to claim 1, characterized in that, In step (4), the molar ratio of the amount of phosphorus pentafluoride to sodium fluorosilicate in step (1) is 8 - 12:1, and the molar ratio of phosphorus pentafluoride to anhydrous hydrogen fluoride is 1:1 - 2.

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