Combined preparation method of lithium difluorophosphate and lithium tetrafluoroborate

Abstract

The invention relates to the technical field of secondary batteries, in particular to a combined preparation method of lithium difluorophosphate and lithium tetrafluoroborate. The combined preparation method of the lithium difluorophosphate and the lithium tetrafluoroborate comprises the following steps: reacting lithium phosphate with phosphorus pentafluoride and boron trifluoride in a supercritical carbon dioxide fluid, and separating to obtain the lithium tetrafluoroborate and the lithium difluorophosphate. Supercritical carbon dioxide is adopted as a reaction solvent, lithium phosphate reacts with BF3 and PF5 to generate lithium difluorophosphate and lithium tetrafluoroborate, the reaction equation is PF5 + Li3PO4 + BF3 = 2LiPO2F2 + LiBF4, the problem that the reaction solvent and reaction gas are prone to complexation is avoided, the activity of PF5 and BF3 is high, the reaction efficiency is high, and the product yield can be increased.

Description

Technical Field

[0001] The invention relates to the technical field of secondary batteries, and in particular to a method for jointly preparing lithium difluorophosphate and lithium tetrafluoroborate. Background Art

[0002] Lithium difluorophosphate is a lithium battery electrolyte additive that can form a protective film on the positive and negative electrodes of the battery, significantly improving the high and low temperature cycle performance of the battery; lithium tetrafluoroborate is insensitive to moisture in the environment, has good chemical stability and thermal stability, and can broaden the operating temperature range of lithium-ion batteries; both are good lithium battery additives that can improve battery performance, extend service life, and are widely used.

[0003] The main preparation method of lithium difluorophosphate is to react lithium hexafluorophosphate and lithium carbonate in an organic solvent to generate lithium difluorophosphate. However, this method has high raw material costs and requires filtering by-products and recycling. The preparation method of lithium tetrafluoroborate is mainly to react lithium fluoride with boron trifluoride in anhydrous HF or an organic solvent. The overall cost of this process is relatively high and requires crystallization and purification.

[0004] The Chinese patent with authorization announcement number CN107226463B, which was authorized on September 3, 2019, discloses a method for the joint preparation of lithium difluorophosphate and lithium tetrafluoroborate, wherein lithium hexafluorophosphate, lithium carbonate, and boron trifluoride are reacted at 60-120°C for 6-10 hours in the presence of a reaction solvent, an appropriate amount of solvent is removed, and the solid-liquid separation is performed. The solid phase is the lithium difluorophosphate product (LiPO2F2), and the liquid phase is further desolventized to obtain lithium tetrafluoroborate (LiBF4). Through one-step reaction and simple separation and purification, two high-quality lithium salt products can be obtained. The yield of LiPO2F2 is 84.9-91.9%, and the purity is 99.3%. The yield of LiBF4 is 83.9-86.7%, and the purity is 98.6%. The method is simple, efficient, and has a high conversion rate. The reaction solvent can be properly recovered and can also be applied. It is environmentally friendly and suitable for industrial production.

[0005] However, in the above-mentioned combined preparation method, the reaction gas is easy to complex with the organic solvent to produce by-products, the yield is low, and two concentration separation and purification are required, the process is complicated and the energy consumption is high. Summary of the invention

[0006] The invention provides a combined preparation method of lithium difluorophosphate and lithium tetrafluoroborate, which solves the problem of low yield in the combined preparation of lithium difluorophosphate and lithium tetrafluoroborate in the prior art.

[0007] In order to solve the above technical problems, the technical solution of the combined preparation method of lithium difluorophosphate and lithium tetrafluoroborate of the present invention is: A combined preparation method of lithium difluorophosphate and lithium tetrafluoroborate comprises the following steps: reacting lithium phosphate with phosphorus pentafluoride and boron trifluoride in a supercritical carbon dioxide fluid to separate lithium tetrafluoroborate and lithium difluorophosphate.

[0008] The present invention provides a pioneering method for the combined preparation of lithium difluorophosphate and lithium tetrafluoroborate. The reaction principle is: PF5+Li3PO4+BF3=2LiPO2F2+LiBF4. Lithium difluorophosphate and lithium tetrafluoroborate are synthesized by a one-step method. The process is simple, and the reactants phosphorus pentafluoride and boron trifluoride have high activities, the reaction time is shorter, the reaction efficiency is high, and the yield is high. Supercritical carbon dioxide fluid is used as a reaction solvent. Compared with the existing process using hydrogen fluoride, esters, ethers and other solvents, there is no problem of complexation between the solvent and the reaction gas, and the product purity is high.

[0009] In order to further complete the reaction of lithium phosphate, preferably, the molar ratio of lithium phosphate, phosphorus pentafluoride and boron trifluoride is 1: (1-1.4): (1-1.4). More preferably, the molar ratio of lithium phosphate, phosphorus pentafluoride and boron trifluoride is 1: (1-1.3): (1-1.3).

[0010] In order to further improve the reaction efficiency, preferably, the temperature of the reaction is 30-80°C, the pressure of the reaction is 7.5-20MPa, and the reaction time is 0.5-3h. It is understood that during the reaction, carbon dioxide gas is introduced to increase the temperature and pressure to a supercritical state, the temperature of the temperature is 30-80°C, the pressure of the pressure is 7.5-20MPa, and the reaction is carried out at this temperature and pressure. More preferably, the temperature of the reaction is 40-70°C, the pressure of the reaction is 10-20MPa, and the reaction time is 1-1.5h.

[0011] The separation can adopt a conventional separation method: after the reaction is completed, the solvent carbon dioxide is first cooled and depressurized to obtain a solid mixture of lithium difluorophosphate and lithium tetrafluoroborate, and an organic solvent (dimethyl carbonate) is added to dissolve and then solid-liquid separation is performed, and the obtained solid phase is lithium difluorophosphate, and the liquid phase is further concentrated to obtain lithium tetrafluoroborate. In order to further improve the purity and yield of the product and reduce the energy consumption of post-processing, preferably, the separation is firstly carried out by using an entrainer for extraction separation to obtain lithium tetrafluoroborate, and then the temperature and pressure are further reduced to separate and obtain lithium difluorophosphate. The separation of the two products can be achieved by extraction separation and decompression and temperature separation, and the obtained product has high purity and high yield, and can reduce the energy consumption of post-processing, reduce costs, and be green and environmentally friendly.

[0012] In order to further improve the purity and yield of the product, preferably, the entrainer is dimethyl carbonate, and the mass ratio of the entrainer to the theoretically obtained lithium tetrafluoroborate is (1-1.6):1.

[0013] In order to further improve the purity and yield of lithium difluorophosphate products, preferably, the extraction time during extraction separation is 1 to 2 h.

[0014] In order to further improve the purity and yield of lithium tetrafluoroborate products, preferably, the temperature during cooling and pressure reduction separation is 15 to 30 °C, and the pressure is normal pressure.

[0015] In order to further improve the extraction efficiency and avoid introducing impurities, preferably, the purity of the dimethyl carbonate is ≥99.95%, and the water content is ≤10 ppm. Detailed implementation manners

[0016] The technical concept of the combined preparation method of lithium difluorophosphate and lithium tetrafluoroborate of the present invention is as follows: In the prior art, lithium difluorophosphate and lithium tetrafluoroborate are obtained by reacting lithium hexafluorophosphate, lithium carbonate, and boron trifluoride in a reaction solvent (carbonate solvent, carboxylate solvent, nitrile solvent, ketone solvent, or ether solvent). The reaction equation is: LiPF6 + 2Li2CO3 + 4BF3 → LiPO2F2 + 4LiBF4 + 2CO2↑. However, in this method, the BF3 gas used is likely to complex with the organic solvent to generate by-products, resulting in low product yield, low reaction efficiency, and long reaction time.

[0017] In the present invention, supercritical carbon dioxide fluid is used as the reaction solvent, and lithium phosphate reacts with PF5 and BF3 to generate lithium difluorophosphate and lithium tetrafluoroborate. The reaction equation is: PF5 + Li3PO4 + BF3 = 2LiPO2F2 + LiBF4, avoiding the problem that the reaction solvent is easily complexed with the reaction gas. Moreover, PF5 and BF3 have high activity, shorter reaction time, high reaction efficiency, and can improve the product yield. The yield of LiPO2F2 obtained by the combined preparation method provided by the present invention is 95.7 to 96.1%, and the purity is 99.85 to 99.88%. The yield of LiBF4 is 93.6 to 95.8%, and the purity is 99.69 to 99.85%.

[0018] The combined preparation method of lithium difluorophosphate and lithium tetrafluoroborate of the present invention includes the following steps: in supercritical carbon dioxide fluid, lithium phosphate, phosphorus pentafluoride, and boron trifluoride with a molar ratio of 1:(1 to 1.4):(1 to 1.4) are reacted at 30 to 80 °C for 0.5 to 3 h; after the reaction is completed, extraction separation is carried out for 1 to 2 h using the entrainer dimethyl carbonate to obtain a lithium tetrafluoroborate system and a lithium difluorophosphate system. The mass ratio of the entrainer to the theoretically obtained lithium tetrafluoroborate is (1 to 1.6):1. The lithium tetrafluoroborate system is cooled and depressurized to obtain a lithium tetrafluoroborate solution. The temperature during cooling and pressure reduction is 15 to 30 °C, and the pressure is normal pressure; the lithium difluorophosphate system is continuously cooled and depressurized to separate out lithium difluorophosphate solid.

[0019] It is understandable that after extraction and separation using the entrainer dimethyl carbonate, a dimethyl carbonate solution containing lithium tetrafluoroborate and a small amount of carbon dioxide fluid are obtained. By cooling and depressurizing this system, the carbon dioxide is converted into a gas, and a lithium tetrafluoroborate solution (with dimethyl carbonate as the solvent) is obtained. The temperature during the cooling and depressurizing process is 15 - 30 °C, and the pressure is normal pressure. Post-treatment operations such as concentration can be further performed on the obtained lithium tetrafluoroborate solution to further improve the purity. After extraction and separation, the remaining carbon dioxide fluid of lithium difluorophosphate in the reaction system is further cooled and depressurized to convert the carbon dioxide into a gas, obtaining solid lithium difluorophosphate.

[0020] In a specific embodiment, the reaction is carried out in a reaction kettle. The preparation method is to first add lithium phosphate into the reaction kettle, then introduce carbon dioxide gas, heat up to 30 - 80 °C and increase the pressure to 7.5 - 20 MPa to make the carbon dioxide reach the supercritical state, and stir for dispersion; then introduce phosphorus pentafluoride gas and boron trifluoride gas, and carry out a heat-insulating and pressure-holding reaction for 0.5 - 3 h under stirring conditions.

[0021] The present invention will be described in detail below with reference to specific examples. The raw materials used in the following examples are all conventional commercially available products and are well-known to those skilled in the art of this technology.

[0022] I. Specific examples of the combined preparation method of lithium difluorophosphate and lithium tetrafluoroborate of the present invention Example 1 The combined preparation method of lithium difluorophosphate and lithium tetrafluoroborate in this example is as follows: Add 11.6 g (0.1 mol) of lithium phosphate into the reaction kettle, introduce carbon dioxide, heat up (to 40 °C) and increase the pressure (to 10 MPa) to the supercritical state, and stir for dispersion; introduce 13.86 g (0.11 mol) of phosphorus pentafluoride and 7.48 g (0.11 mol) of boron trifluoride into the reaction kettle, and carry out a heat-insulating and pressure-holding reaction for 1 h under stirring conditions; after the reaction is completed, pump 12 g of the entrainer diethyl carbonate into this system through a metering pump, extract for 1 h, then separate the extract and pump it into a separation kettle. Cool (to 20 °C) and depressurize (to normal pressure) the separation kettle to obtain 20.83 g of a lithium tetrafluoroborate solution with a lithium tetrafluoroborate purity of 99.69% and a yield of 93.6%; the remaining solution system in the reaction kettle is further cooled (to 20 °C) and depressurized (to normal pressure) to obtain 20.7 g of lithium difluorophosphate with a purity of 99.85% and a yield of 95.7%.

[0023] Example 2 The combined preparation method of lithium difluorophosphate and lithium tetrafluoroborate in this example is as follows: Add 11.6 g (0.1 mol) of lithium phosphate into the reaction kettle, introduce carbon dioxide, heat up to 70 °C and increase the pressure to 20 MPa until it reaches the supercritical state, and stir for dispersion; introduce 16.38 g (0.13 mol) of phosphorus pentafluoride and 8.84 g (0.13 mol) of boron trifluoride into the reaction kettle, and keep the temperature and pressure constant under stirring for 1.5 h; after the reaction is completed, pump 12 g of the entrainer diethyl carbonate into the system, extract for 1 h, then separate the extract and pump it into the separation kettle. Cool the separation kettle to 20 °C and reduce the pressure to atmospheric pressure to obtain 21.02 g of lithium tetrafluoroborate solution with a purity of 99.85% and a yield of 95.8%; cool the remaining solution system in the reaction kettle to 20 °C and reduce the pressure to atmospheric pressure to obtain 20.78 g of lithium difluorophosphate with a purity of 99.88% and a yield of 96.1%.

[0024] II. Comparative Example Referring to the method for jointly preparing lithium difluorophosphate salt and lithium tetrafluoroborate salt disclosed in the invention patent CN107226463B, lithium hexafluorophosphate, lithium carbonate, and boron trifluoride are reacted at 60 - 120 °C for 6 - 10 h in the presence of a reaction solvent. The yield of LiPO2F2 prepared is 84.9 - 91.9%, the purity is 99.3%, the yield of LiBF4 is 83.9 - 86.7%, and the purity is 98.6%.

[0025] Compared with the comparative example, the yields and purities of LiPO2F2 and LiBF4 prepared by the present invention are higher, and the reaction time is shorter (0.5 - 3 h), which reflects a higher reaction efficiency.

[0026] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for the combined preparation of lithium difluorophosphate and lithium tetrafluoroborate, characterized in that: The method comprises the following steps: reacting lithium phosphate with phosphorus pentafluoride and boron trifluoride in supercritical carbon dioxide fluid to separate lithium tetrafluoroborate and lithium difluorophosphate.

2. The combined preparation method of lithium difluorophosphate and lithium tetrafluoroborate as claimed in claim 1, characterized in that: The molar ratio of the lithium phosphate, phosphorus pentafluoride and boron trifluoride is 1:(1-1.4):(1-1.4).

3. The combined preparation method of lithium difluorophosphate and lithium tetrafluoroborate as claimed in claim 1, characterized in that: The reaction temperature is 30-80° C., the reaction pressure is 7.5-20 MPa, and the reaction time is 0.5-3 h.

4. The combined preparation method of lithium difluorophosphate and lithium tetrafluoroborate according to claim 1, characterized in that: The separation is to first use an entrainer to perform extraction separation to obtain lithium tetrafluoroborate, and then continue to reduce the temperature and pressure to separate to obtain lithium difluorophosphate.

5. The combined preparation method of lithium difluorophosphate and lithium tetrafluoroborate as claimed in claim 4, characterized in that: The entrainer is dimethyl carbonate, and the mass ratio of the entrainer to the theoretically obtained lithium tetrafluoroborate is (1-1.6):

1.

6. The combined preparation method of lithium difluorophosphate and lithium tetrafluoroborate as claimed in claim 4, characterized in that: The extraction time during extraction separation is 1~2h.

7. The combined preparation method of lithium difluorophosphate and lithium tetrafluoroborate as claimed in claim 4, characterized in that: The temperature during temperature and pressure reduction separation is 15~30℃ and the pressure is normal pressure.

8. The combined preparation method of lithium difluorophosphate and lithium tetrafluoroborate as claimed in claim 5, characterized in that: The purity of the dimethyl carbonate is ≥99.95%, and the moisture content is ≤10ppm.