A method for removing acid from lithium difluorophosphate

Abstract

The present invention relates to the technical field of secondary batteries, and in particular to a method for removing acid from lithium difluorophosphate. The method for removing acid from lithium difluorophosphate provided by the present invention comprises the following steps: adding dichlorophosphoric acid and lithium hydride to a lithium difluorophosphate solution under a protective atmosphere to react so as to convert hydrofluoric acid in the lithium difluorophosphate solution into lithium difluorophosphate; the molar ratio of the lithium hydride, the free acid in the lithium difluorophosphate solution, and dichlorophosphoric acid is (1-1.1):2:1. By adding dichlorophosphoric acid and a slightly excess amount of lithium hydride to the lithium difluorophosphate solution, the dichlorophosphoric acid and the slightly excess amount of lithium hydride react with the free acid hydrofluoric acid in the solution, thereby removing hydrofluoric acid from the solution to a great extent and generating the main product lithium difluorophosphate, thereby improving the utilization rate of fluorine resources without introducing other impurities or generating by-product water. The deacidification method is simple, efficient, low-cost, and very suitable for industrial production.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and in particular to a method for removing acid from lithium difluorophosphate. Background Art

[0002] As an effective additive for lithium-ion battery electrolytes, lithium difluorophosphate (LDP) boasts excellent thermal stability, hydrolysis resistance, and electrochemical stability. Its application in lithium-ion batteries can effectively improve the battery's high- and low-temperature performance, making it highly valuable industrially. Currently, the production of LDP inevitably involves a certain amount of HF. The hydrofluoric acid in the electrolyte reacts with the positive and negative electrodes of the secondary battery, damaging the electrodes' electrolyte membranes and impacting the battery's cycle performance and safety. Therefore, deacidification of the free acid in LDP products is necessary.

[0003] The Chinese patent with authorization announcement number CN109941982B, which was authorized on June 3, 2022, discloses a method for purifying lithium difluorophosphate. An alkaline lithium salt is added to the crude solution formed by dissolving crude lithium difluorophosphate in an organic solvent. After heating treatment, the insoluble matter is removed by filtration to obtain a filtrate. After evaporation and concentration, a low-polarity organic solvent is added for crystallization, and then filtered and dried to obtain pure lithium difluorophosphate. Lithium difluorophosphate with a purity of more than 99.9% and an acidity reduced to less than 70 ppm can be obtained to meet the requirements for battery use.

[0004] However, the alkaline lithium salts added in the above-mentioned lithium difluorophosphate purification method, such as lithium carbonate and lithium hydroxide, will react with the free acid in the lithium difluorophosphate to produce water. The water will further react with the lithium difluorophosphate to produce impurities. Even if lithium hydride is used for the reaction, the impurity lithium fluoride will be produced. After the lithium fluoride impurity is removed by post-treatment, it will still cause a huge waste of fluorine resources. Summary of the Invention

[0005] The invention provides a method for deacidifying lithium difluorophosphate, which solves the problems of introducing new reactive impurities and wasting fluorine resources during deacidifying lithium difluorophosphate in the prior art.

[0006] In order to solve the above technical problems, the technical solution of the deacidification method of lithium difluorophosphate of the present invention is as follows:

[0007] A method for deacidifying lithium difluorophosphate comprises the following steps: adding dichlorophosphoric acid and lithium hydride to a lithium difluorophosphate solution under a protective atmosphere to react so as to convert hydrofluoric acid in the lithium difluorophosphate solution into lithium difluorophosphate; the molar ratio of the lithium hydride, the free acid in the lithium difluorophosphate solution and the dichlorophosphoric acid is (1-1.1):2:1.

[0008] The present invention improves the existing technology and provides a lithium difluorophosphate deacidification method. Dichlorophosphoric acid and lithium hydride are added to a lithium difluorophosphate solution, and the dichlorophosphoric acid and lithium hydride react with free acid hydrofluoric acid in the solution. The reaction equation is: LiH+2HF+HPO2Cl2=LiPO2F2+2HCl+H2. The hydrofluoric acid in the solution can be removed to a great extent, achieving an excellent deacidification effect. The main product lithium difluorophosphate is generated, the content of the main product is increased, and the efficient utilization of fluorine resources is achieved. Other impurities are not introduced, and by-product water is not generated. The deacidification method provided by the present invention has a simple process, is highly efficient, and has low cost, and is very suitable for industrial production.

[0009] In order to further improve the reaction efficiency and the acid removal efficiency, preferably, the reaction temperature is 0° C. to 20° C., and the reaction time is 0.5 to 1.0 h.

[0010] In order to further reduce the impurities in lithium difluorophosphate and reduce the impurities and water content, preferably, the purity of the lithium hydride and dichlorophosphoric acid are both ≥99.9%, and the water content is both ≤0.001%.

[0011] In order to further reduce the acidity of the system, preferably, after the reaction is completed, an inert gas is introduced into the system for purging, and then lithium difluorophosphate solid is obtained by crystallization.

[0012] In order to further reduce the content of dissolved hydrogen chloride and hydrogen in the system, preferably, the inert gas purge time is 0.4 to 1.0 h.

[0013] In order to further remove slightly excess lithium hydride and reduce impurities in lithium difluorophosphate, preferably, the inert gas is purged and filtered once, and the crystallization is performed by concentrating and crystallizing the filtrate after the first filtration, and then performing a second filtration to obtain the lithium difluorophosphate solid.

[0014] In order to further improve the crystallization efficiency, preferably, the concentration crystallization temperature is 45-50°C. DETAILED DESCRIPTION

[0015] The technical conception of the acid removal method of lithium difluorophosphate of the present invention is as follows:

[0016] A method for deacidifying lithium difluorophosphate comprises the following steps: adding dichlorophosphoric acid and lithium hydride to a lithium difluorophosphate solution under a protective atmosphere to react so as to convert hydrofluoric acid in the lithium difluorophosphate solution into lithium difluorophosphate; the molar ratio of the lithium hydride, the free acid in the lithium difluorophosphate solution, and the dichlorophosphoric acid is (1-1.1):2:1. The method for deacidifying lithium difluorophosphate provided by the present invention comprises adding dichlorophosphoric acid and lithium hydride to the lithium difluorophosphate solution, wherein the dichlorophosphoric acid and lithium hydride react with the free acid hydrofluoric acid in the solution according to the reaction equation: LiH+2HF+HPO2Cl2=LiPO2F2+2HCl+H2. By controlling the molar ratio of the three, the hydrofluoric acid in the solution can be removed to a great extent without introducing other impurities. The acidity after deacidification provided by the method of the present invention is less than 30 ppm, and the acidity after deacidification is excellent. The main product, lithium difluorophosphate, is generated, the main product content is increased, and the efficient utilization of fluorine resources is achieved. The deacidification method provided by the present invention is simple, efficient, and low-cost, making it very suitable for industrial production.

[0017] In a specific embodiment, the reaction is performed by first adding dichlorophosphoric acid to a lithium difluorophosphate solution, stirring the mixture evenly, and then adding lithium hydride to carry out the reaction under stirring conditions.

[0018] 1. Specific embodiment of the method for removing acid from lithium difluorophosphate of the present invention

[0019] Example 1

[0020] The deacidification method of the lithium difluorophosphate of the present embodiment is as follows:

[0021] Under a nitrogen atmosphere, 5 kg of lithium difluorophosphate solution was taken, and the lithium difluorophosphate solution was obtained by dissolving 1000 g of crude lithium difluorophosphate in 4000 g of ethyl acetate solvent (concentration 20%), dichlorophosphoric acid was added dropwise and stirred evenly, and lithium hydride was added and stirred to react. The molar ratio of lithium hydride, free acid hydrofluoric acid and dichlorophosphoric acid was 1.1:2:1, the reaction temperature was 0°C, the reaction time was 1.0 h, and after the reaction was completed, nitrogen was used for purging, and the purging time was 0.5 h. The product was filtered once, and the filtrate was concentrated and crystallized at 50°C and filtered twice to obtain the lithium difluorophosphate product.

[0022] Example 2

[0023] The deacidification method of lithium difluorophosphate of the present embodiment comprises the following steps:

[0024] Under a nitrogen atmosphere, 5 kg of lithium difluorophosphate solution was taken, and the lithium difluorophosphate solution was obtained by dissolving 1000 g of crude lithium difluorophosphate in 4000 g of ethyl acetate solvent (concentration 20%), dichlorophosphoric acid was added dropwise and stirred evenly, and lithium hydride was added and stirred to react. The molar ratio of lithium hydride, free acid hydrofluoric acid and dichlorophosphoric acid was 1.05:2:1, the reaction temperature was 10°C, the reaction time was 0.8 h, and after the reaction was completed, nitrogen was used for purging, and the purging time was 0.6 h. The product was filtered once, and the filtrate was concentrated and crystallized at 45°C and filtered twice to obtain the lithium difluorophosphate product.

[0025] Example 3

[0026] The deacidification method of lithium difluorophosphate of the present embodiment comprises the following steps:

[0027] Under a nitrogen atmosphere, 5 kg of lithium difluorophosphate solution was taken, and the lithium difluorophosphate solution was obtained by dissolving 1000 g of crude lithium difluorophosphate in 4000 g of ethyl acetate solvent (concentration 20%), dichlorophosphoric acid was added dropwise and stirred evenly, and lithium hydride was added and stirred to react. The molar ratio of lithium hydride, free acid hydrofluoric acid and dichlorophosphoric acid was 1.02:2:1, the reaction temperature was 20°C, the reaction time was 0.5 h, and after the reaction was completed, nitrogen was used for purging, and the purging time was 0.4 h. The reaction was filtered once, and the filtrate was concentrated at 45°C, crystallized, and filtered twice to obtain the lithium difluorophosphate product.

[0028] Example 4

[0029] The deacidification method of lithium difluorophosphate of the present embodiment comprises the following steps:

[0030] Under a nitrogen atmosphere, 5 kg of lithium difluorophosphate solution was taken, and the lithium difluorophosphate solution was obtained by dissolving 1000 g of crude lithium difluorophosphate in 4000 g of ethyl acetate solvent (concentration 20%), dichlorophosphoric acid was added dropwise and stirred evenly, and lithium hydride was added and stirred to react. The molar ratio of lithium hydride, free acid hydrofluoric acid and dichlorophosphoric acid was 1.02:2:1, the reaction temperature was 20°C, the reaction time was 1 h, and after the reaction was completed, nitrogen was used for purging, and the purging time was 0.4 h. The reaction was filtered once, and the filtrate was concentrated at 45°C, crystallized, and filtered twice to obtain the lithium difluorophosphate product.

[0031] Example 5

[0032] The deacidification method of lithium difluorophosphate of the present embodiment comprises the following steps:

[0033] Under a nitrogen atmosphere, 5 kg of lithium difluorophosphate solution was taken, and the lithium difluorophosphate solution was obtained by dissolving 1000 g of crude lithium difluorophosphate in 4000 g of ethyl acetate solvent (concentration 20%), dichlorophosphoric acid was added dropwise and stirred evenly, and lithium hydride was added and stirred to react. The molar ratio of lithium hydride, free acid hydrofluoric acid and dichlorophosphoric acid was 1.02:2:1, the reaction temperature was 20°C, the reaction time was 0.5 h, and after the reaction was completed, nitrogen was used for purging, and the purging time was 1 h. The product was filtered once, and the filtrate was concentrated at 45°C, crystallized, and filtered twice to obtain the lithium difluorophosphate product.

[0034] 2. Experimental Examples

[0035] This experimental example tests the acidity of lithium difluorophosphate by acid-base titration and the chloride ion concentration by silver nitrate titration. A comparative example is set up: Under a nitrogen atmosphere, 5 kg of lithium difluorophosphate solution is added with lithium hydride and stirred for reaction. The molar ratio of lithium hydride to free acid hydrofluoric acid is 1.02:1. The reaction temperature is 20°C and the reaction time is 0.5 h. After the reaction is completed, nitrogen is purged for 0.4 h. The solution is filtered once, and the filtrate is concentrated at 45°C, crystallized, and filtered again to obtain the lithium difluorophosphate product.

[0036] The lithium difluorophosphate products of Examples 1 to 5 and the comparative example were subjected to acidity and chloride ion tests before and after deacidification. The acidity and chloride ion concentration of the lithium difluorophosphate solid before dissolution and deacidification (i.e., before deacidification in Table 1) and the solid concentrated and crystallized after deacidification (i.e., after deacidification in Table 1) were tested respectively. The test results are shown in Table 1.

[0037] Table 1 Changes in product indicators before and after deacidification

[0038]

[0039] It can be seen from the test results in Table 1 that, compared with the comparative example, the deacidification method of lithium difluorophosphate provided by the present invention can effectively reduce the acid concentration in lithium difluorophosphate, and the acidity after deacidification is less than 30 ppm, the deacidification effect is significant, and chloride ions will not be introduced. The operation is simple and suitable for industrial production; in addition, when lithium hydride is used alone for deacidification (comparative example), the following reaction occurs: LiH+HF=LiF+H2. As described in the prior art, it is necessary to select a suitable solvent to remove impurities from LiF, and it is impossible to convert fluoride ions into the main product like the deacidification method of the present application, and to efficiently utilize fluorine resources.

[0040] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for removing acid from lithium difluorophosphate, characterized in that: The method comprises the following steps: adding dichlorophosphoric acid and lithium hydride into a lithium difluorophosphate solution under a protective atmosphere to react so as to convert hydrofluoric acid in the lithium difluorophosphate solution into lithium difluorophosphate; and the molar ratio of the lithium hydride, the free acid in the lithium difluorophosphate solution and the dichlorophosphoric acid is (1-1.1):2:

1.

2. The method for removing acid from lithium difluorophosphate as claimed in claim 1, wherein The reaction temperature is 0°C to 20°C, and the reaction time is 0.5 to 1.0 h.

3. The deacidification method of lithium difluorophosphate as claimed in claim 1, wherein The purity of the lithium hydride and dichlorophosphoric acid is both ≥99.9%, and the water content is both ≤0.001%.

4. The method for removing acid from lithium difluorophosphate as claimed in claim 1, wherein After the reaction is completed, an inert gas is introduced into the system for purging, and then solid lithium difluorophosphate is obtained by crystallization.

5. The method for removing acid from lithium difluorophosphate as claimed in claim 4, wherein: The inert gas purge time is 0.4 to 1.0 h.

6. The method for removing acid from lithium difluorophosphate as claimed in claim 4, wherein: The inert gas purging is followed by a primary filtration, and the crystallization is performed by concentrating and crystallizing the filtrate after the primary filtration, and then performing a secondary filtration to obtain the lithium difluorophosphate solid.

7. The method for removing acid from lithium difluorophosphate as claimed in claim 6, wherein: The concentration and crystallization temperature is 45-50°C.

Images