Lithium difluorophosphate and preparation method thereof

Abstract

The invention belongs to the technical field of lithium ion battery electrolyte materials, and particularly relates to lithium difluorophosphate and a preparation method thereof. The method comprises the following steps: firstly, introducing trifluoromethyl hypofluoride gas into dichlorophosphoric acid, and carrying out fluorination reaction to generate difluorophosphoric acid; reacting the difluorophosphoric acid with a carbonic ester organic solvent containing lithium chloride to obtain a lithium difluorophosphate synthetic solution, and filtering and drying the lithium difluorophosphate synthetic solution to obtain a lithium difluorophosphate crude product; and dissolving the lithium difluorophosphate crude product in ethylene glycol dimethyl ether, adding lithium carbonate for deacidification, filtering, concentrating, crystallizing and drying to obtain the lithium difluorophosphate. According to the method, trifluoromethyl hypofluoride is selected as a fluorination reagent to fluorinate dichlorophosphoric acid, and a traditional alkali metal fluorination reagent is not selected, so that introduction of impurity metal ions is avoided, and the purity of lithium difluorophosphate is improved; lithium chloride is selected to replace conventional lithium carbonate to react with difluorophosphoric acid, so that the difluorophosphoric acid is prevented from being changed into monofluorophosphoric acid, and the yield of the lithium difluorophosphate is improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion battery electrolyte materials, and particularly relates to lithium difluorophosphate and a preparation method thereof. BACKGROUND

[0002] With the increase of the demand for lithium ion batteries in the market, the performance requirements for lithium ion batteries are becoming more and more strict. The electrolyte is an important component in the lithium ion battery and is an important factor determining the performance of the lithium ion battery. Lithium difluorophosphate is a widely used electrolyte additive and plays a very important role in improving the performance of the battery. Research shows that a small amount of lithium difluorophosphate added in a lithium hexafluorophosphate electrolyte system can significantly improve the high and low temperature cycle performance of the battery.

[0003] At present, the most common method for preparing lithium difluorophosphate is the lithium hexafluorophosphate method. For example, patent document CN112320783A discloses a catalytic preparation method of lithium difluorophosphate. Lithium hexafluorophosphate and lithium carbonate are reacted in an organic solvent to generate lithium difluorophosphate, lithium fluoride and carbon dioxide. The method generates a large amount of lithium fluoride while preparing lithium difluorophosphate, and the mass of the generated lithium fluoride is basically the same as that of lithium difluorophosphate, so the efficiency of converting lithium elements into lithium difluorophosphate is low. Patent document CN116022764A discloses a method for preparing lithium difluorophosphate by reacting lithium hexafluorophosphate and organosiloxane. The method generates a large amount of fluorosilane gas while preparing lithium difluorophosphate, and the utilization rate of fluorine elements is only one-third. Patent document CN108640096A discloses a preparation method of lithium difluorophosphate and lithium difluorophosphate. First, dichlorophosphoric acid and a fluorination reagent are subjected to a fluorination reaction to prepare lithium difluorophosphate, and then the lithium difluorophosphate and a lithium source substance are reacted in a non-aqueous solvent to prepare lithium difluorophosphate. The fluorination reagent includes MFx, wherein x=1, 2 or 3, and M is hydrogen, an alkali metal, an alkaline earth metal or a transition metal. The use of these fluorination reagents has the risk of introducing impurity metal ions.

[0004] Therefore, it is currently an urgent problem to be solved to develop a method for efficiently preparing lithium difluorophosphate with high purity and high yield. SUMMARY

[0005] Therefore, the purpose of the present application is to provide a preparation method of lithium difluorophosphate. The method uses trifluoromethyl hypofluorous acid as a fluorination reagent to fluorinate dichlorophosphoric acid, instead of traditional alkali metal fluorination reagents, thereby avoiding the introduction of impurity metal ions and improving the purity of lithium difluorophosphate. Lithium chloride is used instead of conventional lithium carbonate to react with lithium difluorophosphate, thereby avoiding the conversion of lithium difluorophosphate into monofluorophosphoric acid and improving the yield of lithium difluorophosphate.

[0006] To achieve the above-mentioned purpose, the present application adopts the following technical solutions: The application provides a preparation method of lithium difluorophosphate, which comprises the following steps: S1, trifluoromethyl hypofluorous acid gas is introduced into dichlorophosphoric acid to generate difluorophosphoric acid through fluorination reaction; S2, lithium chloride is added into a carbonate organic solvent to uniformly form a mixed solution; S3, the difluorophosphoric acid in S1 is added into the mixed solution in S2 to perform reaction, so as to obtain a lithium difluorophosphate synthesis solution; S4, the lithium difluorophosphate synthesis solution in S3 is filtered and dried to obtain a lithium difluorophosphate crude product; S5, the lithium difluorophosphate crude product in S4 is dissolved in a lithium difluorophosphate good solvent, and then an alkaline substance is added to reduce the acid, and then the filtrate is obtained through filtration; the filtrate is concentrated, crystallized and dried to obtain lithium difluorophosphate.

[0007] The reaction equation in S1 is HPO2Cl2+2CF4O=HPO2F2+2CF3OCl; The reaction equation in S3 is HPO2F2+LiCl=LiPO2F2+HCl; The reaction equation in S5 is 2HPO2F2+Li2CO3=2LiPO2F2+CO2+H2O Further, the molar ratio of the dichlorophosphoric acid to trifluoromethyl hypofluorous acid in S1 is 1:2.0-2.2, and preferably 1:2.1-2.2.

[0008] Further, the residual amount of the dichlorophosphoric acid in S1 is monitored by nuclear magnetic resonance, and the reaction is stopped when the content of the dichlorophosphoric acid in the reaction solution is 0.

[0009] Further, the dichlorophosphoric acid in S1 is heated to 40±2 ℃, and then the trifluoromethyl hypofluorous acid gas is introduced, and the reaction temperature is controlled to be 40-60 ℃; tail gas in the reaction process is absorbed into an alkali liquor, and the alkali liquor comprises a sodium hydroxide aqueous solution.

[0010] Further, the mass ratio of the lithium chloride to the carbonate organic solvent in S2 is 9-11:1; the carbonate organic solvent comprises dimethyl carbonate, diethyl carbonate or methyl ethyl carbonate.

[0011] Further, the molar ratio of the difluorophosphoric acid to lithium chloride in S3 is 1:0.95-0.99, and preferably 1:0.98-0.99; the reaction temperature is 30-40 ℃, 95%-98% of the difluorophosphoric acid is reacted in the reaction process, tail gas in the reaction process is absorbed into an alkali liquor until the absorption liquid does not bubble any more, and then nitrogen is introduced to remove the acid after the reaction is completed; the alkali liquor comprises a sodium hydroxide aqueous solution.

[0012] Further, the lithium difluorophosphate synthesis solution in S4 is subjected to solid-liquid separation to obtain a lithium difluorophosphate ointment; the lithium difluorophosphate ointment is dried at 100-140 ℃ under nitrogen protection to obtain a lithium difluorophosphate crude product. The high temperature in the drying process can effectively remove residual hydrogen chloride gas and carbonic acid ester solvent.

[0013] Further, the lithium difluorophosphate good solvent in S5 includes ethylene glycol dimethyl ether (DME), and the lithium difluorophosphate crude product has good solubility in the ethylene glycol dimethyl ether (DME); the basic substance includes lithium carbonate, and the lithium carbonate can react with residual lithium difluorophosphate at 40±2 ℃ to achieve the purpose of acid removal.

[0014] Further, the filtrate in S5 is subjected to distillation concentration to remove the solvent therein, until the residual solvent is 2-3 times the mass of the lithium difluorophosphate, and the concentration is stopped; then, a poor solvent of lithium difluorophosphate is added, and after the lithium difluorophosphate solid is precipitated, solid-liquid separation is performed, and the filter cake ointment is dried at 100-140 ℃ for 6-10 h to obtain a lithium difluorophosphate powder; the addition amount of the poor solvent is 3-4 times the mass of the filtrate after concentration, and the poor solvent includes dichloromethane (DCM).

[0015] The application further improves the lithium difluorophosphate prepared by the preparation method of the lithium difluorophosphate.

[0016] The application has the following beneficial effects: (1) The preparation method provided by the application selects trifluoromethyl hypofluorous acid ester as a fluorinating agent for fluorination of dichlorophosphoric acid, instead of a traditional alkali metal fluorinating agent, thereby avoiding the introduction of impurity metal ions and obtaining a product with high purity and excellent quality.

[0017] (2) The preparation method provided by the application selects lithium chloride to react with lithium difluorophosphate, instead of lithium carbonate; because lithium difluorophosphate is unstable and will deteriorate when coming into contact with water to generate monofluorophosphoric acid, the reaction equation is: HPO2F2+H2O=H2PO3F+HF, and the generated monofluorophosphoric acid will also react with lithium carbonate, thereby reducing the utilization rate of lithium carbonate. The raw material selection of the application is reasonable, the risk of the deterioration of lithium difluorophosphate into monofluorophosphoric acid is avoided, and the yield of lithium difluorophosphate is significantly improved. BRIEF DESCRIPTION OF DRAWINGS

[0018] In order to more clearly illustrate the technical solutions of the application, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description are only some embodiments of the application, and other drawings can also be obtained by those skilled in the art without any creative effort.

[0019] ATTACHMENT Figure 1The lithium difluorophosphate prepared in Example 1 of this invention 19 F-NMR spectrum; Appendix Figure 2 The lithium difluorophosphate prepared in Example 1 of this invention 31 P-NMR spectrum. Detailed Implementation

[0020] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. All mentioned embodiments are implemented based on the technical solutions of the present invention, and detailed implementation processes are given. However, it should be stated that the scope of protection of the present invention is not limited to the following embodiments.

[0021] The following embodiments provide detailed implementation procedures for the technical solutions of the present invention. Unless otherwise specified, the experimental methods used in the following experimental examples are conventional methods; unless otherwise specified, the materials and reagents used are commercially available.

[0022] Example 1 67.45 g of dichlorophosphoric acid was added to a tetrafluoromethane reaction flask, and the temperature was raised to 40 °C. 114 g of trifluoromethyl hypofluoroester was slowly introduced to initiate the reaction, maintaining the reaction temperature between 40 and 60 °C. The tail gas was treated with sodium hydroxide aqueous solution for absorption. The remaining amount of dichlorophosphoric acid was monitored by NMR throughout the reaction process. The reaction was stopped when the dichlorophosphoric acid content in the reaction solution reached zero. The weight of the dichlorophosphoric acid was 50.99 g.

[0023] 200g of diethyl carbonate and 20.78g of anhydrous lithium chloride were added to a 500ml three-necked flask and stirred. The resulting difluorophosphate was then added dropwise to the mixture through a constant-pressure dropping funnel. The reaction was carried out at 40℃ for approximately 4 hours. During the reaction, the tail gas was absorbed with sodium hydroxide aqueous solution until the absorbent stopped bubbling. After the reaction was complete, nitrogen gas was introduced to purge the acid and remove the hydrogen chloride gas dissolved in the diethyl carbonate. After purging the acid, the lithium difluorophosphate synthesis solution was filtered under positive pressure to obtain lithium difluorophosphate ointment. Under nitrogen protection, the lithium difluorophosphate ointment was dried at 120℃ for 6 hours to obtain crude lithium difluorophosphate.

[0024] The obtained crude lithium difluorophosphate was added into 200 g of DME and stirred to dissolve, then 0.8 g of lithium carbonate was added, and the reaction was stirred at 40°C. After 4 h of reaction, the reaction mixture was filtered. The obtained filtrate was concentrated by vacuum distillation, and 70-100 g of DME was distilled out. The concentration was stopped, and 450 g of dichloromethane was added to the concentrated solution to stir and crystallize. After crystallization, solid-liquid separation was performed to obtain a lithium difluorophosphate ointment. The lithium difluorophosphate ointment was dried at 110°C for 6 h to obtain 53.43 g of lithium difluorophosphate product with a purity of 99.95%, a moisture content of 75 ppm, an acidity (calculated as hydrogen fluoride) of 53 ppm, and a yield of 99.03%. The nuclear magnetic resonance fluorine spectrum and the nuclear magnetic resonance phosphorus spectrum of the prepared lithium difluorophosphate product are shown in FIGS. Figure 1 and Figure 2 As can be seen from the figures, the characteristic peak of the fluorine atom of lithium difluorophosphate has a chemical shift of about -70 to -80 ppm, and appears as a doublet due to coupling with ³¹P. In the nuclear magnetic resonance phosphorus spectrum, the characteristic peak of the phosphorus atom of lithium difluorophosphate has a chemical shift of usually -15 to -30 ppm, and is split into a triplet due to coupling with two equivalent F atoms.

[0025] Example 2 A four-fluorine reaction bottle was charged with 67.45 g of phosphorus oxychloride, and the temperature was raised to 40°C. Then, 114 g of trifluoromethyl hypofluorite was slowly introduced into the reaction bottle to control the reaction temperature at 40-60°C. The tail gas was absorbed by an aqueous sodium hydroxide solution. The entire reaction process was monitored by nuclear magnetic resonance to determine the residual amount of phosphorus oxychloride. When the content of phosphorus oxychloride in the reaction solution was 0, the reaction was stopped. The weight of lithium difluorophosphate was 50.98 g.

[0026] A 500 ml three-necked flask was charged with 200 g of dimethyl carbonate and 20.78 g of anhydrous lithium chloride, which were stirred and mixed. Then, the obtained lithium difluorophosphate was added into the mixture through a constant-pressure dropping funnel at 40°C for about 4 h of reaction. During the reaction, the tail gas was absorbed by an aqueous sodium hydroxide solution until the absorption solution stopped bubbling. After the reaction was completed, nitrogen was introduced to remove the acid, and the hydrogen chloride gas dissolved in the dimethyl carbonate was removed. After the acid removal, the lithium difluorophosphate synthesis solution was filtered under positive pressure to obtain a lithium difluorophosphate ointment. Under the protection of nitrogen, the lithium difluorophosphate ointment was dried at 120°C for 6 h to obtain a crude lithium difluorophosphate product.

[0027] The obtained crude lithium difluorophosphate was added into 200 g of DME and stirred to dissolve, then 0.7 g of lithium carbonate was added, and the reaction was stirred at 40°C. After 4 h of reaction, filtration was performed. The obtained filtrate was concentrated by vacuum distillation, and 70-100 g of DME was distilled off. The concentration was stopped, and 450 g of dichloromethane was added to the concentrated solution to stir and crystallize. After crystallization, solid-liquid separation was performed to obtain a lithium difluorophosphate paste. The lithium difluorophosphate paste was dried at 110°C for 6 h to obtain 53.4 g of lithium difluorophosphate product, with a purity of 99.95%, a moisture content of 67 ppm, an acidity (calculated as hydrogen fluoride) of 52 ppm, and a yield of 98.97%.

[0028] Example 3 A four-fluorine reaction bottle was charged with 134.89 g of phosphorus oxychloride, and the temperature was raised to 40°C. Then 208 g of trifluoromethyl hypofluorite was slowly introduced to perform the reaction, and the reaction temperature was controlled at 40-60°C. The tail gas was absorbed by sodium hydroxide aqueous solution. The whole reaction process was monitored by nuclear magnetic resonance to determine the residual amount of phosphorus oxychloride. When the content of phosphorus oxychloride in the reaction solution was 0, the reaction was stopped. The weight of the obtained lithium difluorophosphate was 101.98 g.

[0029] A 1000 ml three-necked flask was charged with 400 g of diethyl carbonate and 41.56 g of anhydrous lithium chloride, which were stirred and mixed. Then the obtained lithium difluorophosphate was added into the mixture through a constant-pressure dropping funnel, and the reaction was performed at 40°C for about 4 h. During the reaction, the tail gas was absorbed by sodium hydroxide aqueous solution until the absorption solution stopped bubbling. After the reaction was completed, nitrogen was introduced to remove the acid, and the hydrogen chloride gas dissolved in the dimethyl carbonate was removed. After the acid removal, the lithium difluorophosphate synthesis solution was filtered under positive pressure to obtain a lithium difluorophosphate paste. Under the protection of nitrogen, the lithium difluorophosphate paste was dried at 120°C for 6 h to obtain the crude lithium difluorophosphate.

[0030] The obtained crude lithium difluorophosphate was added into 400 g of DME and stirred to dissolve, then 1.6 g of lithium carbonate was added, and the reaction was stirred at 40°C. After 4 h of reaction, filtration was performed. The obtained filtrate was concentrated by vacuum distillation, and 140-200 g of DME was distilled off. The concentration was stopped, and 950 g of dichloromethane was added to the concentrated solution to stir and crystallize. After crystallization, solid-liquid separation was performed to obtain a lithium difluorophosphate paste. The lithium difluorophosphate paste was dried at 110°C for 6 h to obtain 106.6 g of lithium difluorophosphate product, with a purity of 99.95%, a moisture content of 83 ppm, an acidity (calculated as hydrogen fluoride) of 45 ppm, and a yield of 98.79%.

[0031] Comparative Example The preparation method of lithium difluorophosphate of the present comparative example is different from that of Example 1 in that 20.78 g of anhydrous lithium chloride in the preparation of crude lithium difluorophosphate is replaced by 18.5 g of lithium carbonate; and 0.8 g of lithium carbonate added in the purification of crude lithium difluorophosphate is replaced by 3 g of lithium carbonate. The method finally obtains 26.8 g of lithium difluorophosphate product, with a yield of 49.67%.

[0032] The above description of disclosed embodiments enables one skilled in the art to make or use the application. Numerous modifications to these embodiments will be apparent to those skilled in the art, and the generic principles defined herein can be applied to other embodiments without departing from the spirit or scope of the application. Therefore, the present application is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing lithium difluorophosphate, characterized in that, Includes the following steps: S1, trifluoromethyl hypofluoroester gas is introduced into dichlorophosphoric acid, and difluorophosphoric acid is generated through a fluorination reaction; S2, Lithium chloride is added to a carbonate organic solvent and mixed to form a mixed solution; S3, add the difluorophosphoric acid described in S1 to the mixed solution described in S2 to react and obtain lithium difluorophosphate synthesis solution; S4, the lithium difluorophosphate synthesis solution described in S3 is filtered and dried to obtain crude lithium difluorophosphate; S5, dissolve the crude lithium difluorophosphate from S4 in a good solvent for lithium difluorophosphate, add an alkaline substance to reduce the acidity, and then filter to obtain a filtrate; the filtrate is concentrated, crystallized, and dried to obtain lithium difluorophosphate.

2. The method for preparing lithium difluorophosphate as described in claim 1, characterized in that, The molar ratio of dichlorophosphoric acid to trifluoromethyl hypofluoroester in S1 is 1:2.0 to 2.

2.

3. The method for preparing lithium difluorophosphate as described in claim 1, characterized in that, In S1, the remaining amount of dichlorophosphoric acid is monitored using nuclear magnetic resonance. The reaction is stopped when the content of dichlorophosphoric acid in the reaction solution is 0.

4. The method for preparing lithium difluorophosphate as described in claim 1, characterized in that, In step S1, the dichlorophosphoric acid is heated to 40±2℃ and then the trifluoromethyl hypofluoroester gas is introduced to control the reaction temperature at 40~60℃; the tail gas during the reaction process is absorbed in the alkaline solution.

5. The method for preparing lithium difluorophosphate as described in claim 1, characterized in that, The mass ratio of lithium chloride to carbonate organic solvent in S2 is 9-11:1; the carbonate organic solvent includes dimethyl carbonate, diethyl carbonate, or methyl ethyl carbonate.

6. The method for preparing lithium difluorophosphate as described in claim 1, characterized in that, The molar ratio of difluorophosphoric acid to lithium chloride in S3 is 1:0.95 to 0.99; the reaction temperature is 30-40℃; the tail gas during the reaction is absorbed in the alkaline solution; after the reaction is completed, nitrogen gas is introduced to remove the acid.

7. The method for preparing lithium difluorophosphate as described in claim 1, characterized in that, The lithium difluorophosphate synthesis solution described in S4 is subjected to solid-liquid separation to obtain lithium difluorophosphate ointment; the lithium difluorophosphate ointment is dried under nitrogen protection at 100-140℃ to obtain crude lithium difluorophosphate.

8. The method for preparing lithium difluorophosphate as described in claim 1, characterized in that, The good solvent for lithium difluorophosphate mentioned in S5 includes ethylene glycol dimethyl ether; the alkaline substance includes lithium carbonate.

9. The method for preparing lithium difluorophosphate as described in claim 1, characterized in that, The filtrate described in S5 is concentrated by distillation to remove the solvent until the remaining solvent is 2 to 3 times the mass of lithium difluorophosphate, at which point the concentration is stopped. Then, a poor solvent for lithium difluorophosphate is added, and after the lithium difluorophosphate solid precipitates, solid-liquid separation is performed to obtain a filter cake ointment. The filter cake ointment is dried at 100-140℃ for 6 to 10 hours to obtain lithium difluorophosphate. The amount of the poor solvent added is 3 to 4 times the mass of the concentrated filtrate, and the poor solvent includes dichloromethane.

10. A lithium difluorophosphate prepared by the method of any one of claims 1-9.

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