A preparation method of lithium difluorophosphate

Abstract

The present invention relates to a method for preparing lithium difluorophosphate, and belongs to the field of preparation of halogen-containing phosphates. The method for preparing lithium difluorophosphate of the present invention comprises the following steps: introducing ammonia into a mixed solution of lithium hexafluorosilicate and lithium phosphate for reaction; the solvent of the mixed solution is an organic solvent. The method for preparing lithium difluorophosphate of the present invention has cheap and readily available raw materials, mild and easily controllable reaction conditions, and the generated silicon dioxide and lithium fluoride are insoluble in organic solvents and precipitate out, are easy to separate, have low product cost, and have strong market competitiveness.

Description

Technical Field

[0001] The invention relates to a method for preparing lithium difluorophosphate, and belongs to the field of preparation of halogen-containing phosphates. Background Art

[0002] After years of development, lithium-ion power batteries have become 6 ) as the main electrolyte is quite mature, and it is very difficult to further improve the battery cycle life. However, the development of electric vehicles has higher and higher requirements for lithium battery performance, and there is an urgent need to extend battery life to reduce consumer usage costs. The use of additives in electrolytes is one of the important means, among which lithium difluorophosphate (LiPO 2 F 2 ) performs very well. It can form a low-impedance interface film on the positive and negative surfaces of lithium-ion batteries, inhibiting the side reactions between the electrodes and the electrolyte, thereby increasing the cycle life of the battery. With the increasing application of lithium difluorophosphate, its preparation research has also received more and more attention.

[0003] At present, lithium difluorophosphate is mostly prepared with lithium hexafluorophosphate as raw material. For example, in the preparation method of lithium difluorophosphate disclosed in the Chinese invention patent application with application publication number CN113651306A, lithium hexafluorophosphate, phosphorus pentoxide and lithium oxide powder are mixed under anhydrous conditions and heated to react to generate lithium difluorophosphate; in another example, in the preparation method of lithium difluorophosphate disclosed in the Chinese invention patent application with application publication number CN112028046A, lithium hexafluorophosphate and lithium dihydrogen phosphate are mixed under anhydrous conditions, heated to react, and lithium difluorophosphate is obtained; in another example, in the preparation method of lithium difluorophosphate disclosed in the Chinese invention patent application with authorization announcement number CN108706565B, boron trioxide and lithium hexafluorophosphate are added to a non-aqueous solvent under a dry inert gas atmosphere, stirred and mixed, and reacted to generate lithium difluorophosphate; these methods are simple in process and fast in reaction, but the price of the raw material lithium hexafluorophosphate is relatively high, resulting in high cost of lithium difluorophosphate. Other preparation methods of lithium difluorophosphate, such as the preparation method of lithium difluorophosphate disclosed in the Chinese invention patent with authorization announcement number CN113184824B, are to prepare lithium hexafluorosilicate (Li 2 SiF 6 ) and phosphorus pentoxide (P 2 O 5 ) mixed, sintered, then dissolved with an organic solvent, solid-liquid separated, and the resulting clear liquid recrystallized to obtain lithium difluorophosphate. This method uses cheap lithium hexafluorosilicate as raw material and has certain cost advantages, but the preparation process requires high-temperature sintering, and the large number of low-value by-products increases the production cost. Summary of the invention

[0004] The purpose of the present invention is to provide a low-cost method for preparing lithium difluorophosphate.

[0005] In order to achieve the above purpose, the technical solution adopted by the present invention is:

[0006] A method for preparing lithium difluorophosphate comprises the following steps: introducing ammonia into a mixed solution of lithium hexafluorosilicate and lithium phosphate for reaction; the solvent of the mixed solution is an organic solvent.

[0007] The preparation method of lithium difluorophosphate of the present invention has cheap and readily available raw materials, mild and easily controllable reaction conditions, and the generated silicon dioxide and lithium fluoride are insoluble in organic solvents and precipitate, which are easy to separate, and the product cost is low and the market competitiveness is strong. The chemical reaction involved in the preparation method of lithium difluorophosphate of the present invention is:

[0008] Li 2 SiF 6 +Li 3 PO 4 →SiO 2 ↓+LiPO 2 F 2 +4LiF↓.

[0009] The lithium fluorosilicate can be prepared by reacting fluorosilicic acid, a byproduct of wet phosphate fertilizer, with lithium carbonate, and has low cost.

[0010] Ammonia plays a catalytic role in the reaction of lithium hexafluorosilicate and lithium phosphate, catalyzing the cleavage of Si-F bonds and reacting with PO 4 3- O in the 2 , further, the amount of ammonia introduced is 20-40% of the molar amount of lithium phosphate in the mixed solution, for example, 20-30%. During the reaction, ammonia is continuously introduced into the mixed solution, and the flow rate of ammonia is controlled to be 1-20L / min, for example, 1.2-6.2L / min. Further, ammonia is introduced into the mixed solution by bubbling. For example, ammonia is introduced by bubbling from the bottom of the reactor, collected at the top and then introduced from the bottom, and the cycle continues.

[0011] The excess of lithium hexafluorosilicate ensures that the lithium phosphate reacts completely. If the excess is too much, the impurity content of the target product will increase. Furthermore, the molar ratio of lithium hexafluorosilicate to lithium phosphate is (1.02-1.08):1.

[0012] In order to improve production efficiency while avoiding excessive lithium phosphate concentration so that the lithium difluorophosphate generated by the reaction will precipitate, further, in the mixed solution, the mass ratio of lithium phosphate to organic solvent is (3-8):100.

[0013] Furthermore, during the process of introducing ammonia gas, the temperature of the mixed solution is controlled to be not lower than room temperature and not higher than the boiling point of the organic solvent.

[0014] Furthermore, the organic solvent is an alcohol solvent and / or an ether solvent; the temperature of the mixed solution is controlled to be 40-60°C during the introduction of ammonia; the reaction time is 4-8 hours. The alcohol solvent is methanol and / or ethanol, and the ether solvent is ethylene glycol dimethyl ether.

[0015] Furthermore, the preparation method of lithium difluorophosphate further includes the following steps: after the reaction is completed, solid-liquid separation is performed, and then the liquid obtained by solid-liquid separation is concentrated and crystallized; the concentrated crystallization is to evaporate and remove 85-95% of the organic solvent and then cool to 0-5°C to precipitate crystals. In order to further improve the purity of lithium difluorophosphate, further, the preparation method of lithium difluorophosphate further includes the following steps: recrystallize the crystals obtained by concentrated crystallization and then dry them; the recrystallization is to dissolve the product obtained by concentrated crystallization in an organic solvent, remove insoluble matter, evaporate and remove 85-95% of the organic solvent, and then cool to 0-5°C to precipitate crystals. During the recrystallization process, the mass of the organic solvent that dissolves the concentrated crystal product is 10-20 times the mass of the concentrated crystal product.

[0016] Furthermore, during the concentration crystallization and recrystallization process, the evaporation temperature is 65-90°C.

[0017] Furthermore, the drying process is a vacuum drying process; the vacuum degree of the vacuum drying process is -60 to -90 kPa, the temperature is 70-90° C., and the time is 2-4 hours.

[0018] Furthermore, the preparation method of lithium difluorophosphate further comprises the following steps: washing the solid obtained by solid-liquid separation with water to obtain silicon dioxide; removing water from the washing liquid produced by washing to obtain lithium fluoride. The amount of water used to wash the solid obtained by solid-liquid separation is 30 to 50 times the mass of the solid. The temperature of the water used to wash the solid obtained by solid-liquid separation is 30 to 40°C. DETAILED DESCRIPTION

[0019] The technical solution of the present invention is further described below in conjunction with specific implementation methods.

[0020] Example 1

[0021] The preparation method of lithium difluorophosphate of this embodiment comprises the following steps:

[0022] 1) 43.6 g of lithium hexafluorosilicate and 30.0 g of lithium phosphate were dissolved in 1000 g of 50° C. anhydrous ethanol to obtain a mixed solution, and 0.9 g of ammonia was continuously bubbled into the mixed solution at 50° C. (ammonia overflowing from the mixed solution was collected and then introduced again from the bottom of the mixed solution to achieve continuous bubbling of ammonia, and the flow rate of ammonia was controlled to be 1.2 L / min), reacted for 6 h, and filtered to obtain a residue and a filtrate;

[0023] 2) 95% of the solvent was evaporated from the filtrate at 80°C, and then the temperature was lowered to 5°C for crystallization. Lithium difluorophosphate crystals were precipitated to obtain 26.6 g of a crude product. The crude product was then dissolved in 270 g of anhydrous ethanol, and the insoluble matter was removed by filtration. 95% of the solvent was evaporated again at 80°C, and the temperature was lowered to 5°C for crystallization. Lithium difluorophosphate crystals were precipitated, and then vacuum dried at 80°C and a vacuum degree of -80 kPa for 3 h to obtain 23.2 g of high-purity lithium difluorophosphate with a purity of 99.90% and a water content of 9 ppm.

[0024] The filter residue obtained in step 1) is soaked in hot water (40° C.) 30 times its mass and filtered. The filter residue obtained is silicon dioxide, which is sold as white carbon black after drying. The obtained filtrate is concentrated to obtain lithium fluoride crystals.

[0025] Example 2

[0026] The preparation method of lithium difluorophosphate of this embodiment comprises the following steps:

[0027] 1) dissolving 110.0 g of lithium hexafluorosilicate and 80.0 g of lithium phosphate in 1000 g of 60° C. anhydrous ethylene glycol dimethyl ether to obtain a mixed solution, and continuously bubbling 3.5 g of ammonia into the mixed solution at 60° C. (the ammonia overflowing the mixed solution is collected and then introduced again from the bottom of the mixed solution to achieve continuous bubbling of ammonia, and the flow rate of ammonia is controlled to be 6.2 L / min), reacting for 4 h, and filtering to obtain a residue and a filtrate;

[0028] 2) 95% of the solvent was evaporated from the filtrate at 90° C., and then the temperature was lowered to 5° C. for crystallization. Lithium difluorophosphate crystals were precipitated to obtain 71.3 g of a crude product. The crude product was then dissolved in 720 g of anhydrous ethylene glycol dimethyl ether, and insoluble matter was removed by filtration. 95% of the solvent was evaporated again at 90° C., and the temperature was lowered to 5° C. for crystallization. Lithium difluorophosphate crystals were precipitated. The product was then vacuum dried at 90° C. and a vacuum degree of -60 kPa for 2 h to obtain 68 g of high-purity lithium difluorophosphate with a purity of 99.80% and a water content of 12 ppm.

[0029] The filter residue obtained in step 1) is soaked in hot water (30° C.) 50 times its mass and filtered. The filter residue obtained is silicon dioxide, which is sold as white carbon black after drying. The obtained filtrate is concentrated to obtain lithium fluoride crystals.

[0030] Example 3

[0031] The preparation method of lithium difluorophosphate of this embodiment comprises the following steps:

[0032] 1) 70.0 g of lithium hexafluorosilicate and 50.0 g of lithium phosphate were dissolved in 1000 g of 40° C. anhydrous methanol to obtain a mixed solution, and 2.2 g of ammonia was continuously bubbled into the mixed solution at 40° C. (ammonia overflowing from the mixed solution was collected and then introduced again from the bottom of the mixed solution to achieve continuous bubbling of ammonia, and the flow rate of ammonia was controlled to be 2.9 L / min), reacted for 8 h, and filtered to obtain a residue and a filtrate;

[0033] 2) 85% of the solvent was evaporated from the filtrate at 65°C, and then the temperature was lowered to 0°C for crystallization. Lithium difluorophosphate crystals were precipitated to obtain 43.9 g of a crude product. The crude product was then dissolved in 870 g of anhydrous methanol, and the insoluble matter was removed by filtration. 95% of the solvent was evaporated at 65°C, and the temperature was lowered to 0°C for crystallization. Lithium difluorophosphate crystals were precipitated, and then vacuum dried at 65°C and a vacuum degree of -90 kPa for 4 h to obtain 41.1 g of high-purity lithium difluorophosphate with a purity of 99.88% and a water content of 10 ppm.

Claims

1. A method for preparing lithium difluorophosphate, characterized in that: The following steps are involved: Ammonia is introduced into a mixed solution of lithium hexafluorosilicate and lithium phosphate for reaction; the solvent of the mixed solution is an organic solvent; the molar ratio of lithium hexafluorosilicate and lithium phosphate is (1.02-1.08):1, and the mass ratio of lithium phosphate to organic solvent in the mixed solution is (3-8):100; the temperature of the mixed solution is controlled to be 40-60°C during the process of introducing ammonia; and the reaction time is 4-8h.

2. The method for preparing lithium difluorophosphate according to claim 1, wherein: During the reaction, ammonia gas is continuously introduced into the mixed solution; the ammonia gas is introduced into the mixed solution by bubbling.

3. The method for preparing lithium difluorophosphate according to claim 1 or 2, characterized in that: During the process of introducing ammonia gas, the temperature of the mixed solution is controlled to be not lower than room temperature and not higher than the boiling point of the organic solvent.

4. The method for preparing lithium difluorophosphate according to claim 3, characterized in that: The organic solvent is an alcohol solvent and / or an ether solvent.

5. The method for preparing lithium difluorophosphate according to claim 1 or 2, characterized in that: The following steps are also included: After the reaction is completed, solid-liquid separation is performed, and then the liquid obtained by solid-liquid separation is concentrated and crystallized; the concentrated crystallization is to first evaporate and remove 85-95% of the organic solvent and then cool to 0-5°C to precipitate crystals.

6. The method for preparing lithium difluorophosphate according to claim 5, characterized in that: The following steps are also included: The crystals obtained by concentrated crystallization are recrystallized and then dried; the recrystallization is to first dissolve the product obtained by concentrated crystallization in an organic solvent, remove insoluble matter, evaporate 85-95% of the organic solvent, and then cool to 0-5°C to precipitate crystals.

7. The method for preparing lithium difluorophosphate according to claim 6, characterized in that: The drying process is a vacuum drying process; the vacuum degree of the vacuum drying process is -60 to -90 kPa, the temperature is 70-90° C., and the time is 2-4 hours.

8. The method for preparing lithium difluorophosphate according to claim 1 or 2, characterized in that: The following steps are also included: The solid obtained by solid-liquid separation is washed with water to obtain silicon dioxide; and the washing liquid produced by washing is dehydrated to obtain lithium fluoride.