Method for improving synthesis reaction rate of lithium difluorophosphate

Abstract

The invention discloses a method for improving the synthesis reaction rate of lithium difluorophosphate, which comprises the following steps: in an organic solvent, taking lithium hexafluorophosphate, chain siloxane and / or cyclic siloxane as raw materials, and adding water accounting for 0.1-5.0% of the molar weight of the lithium hexafluorophosphate to prepare the lithium difluorophosphate. The method disclosed by the invention is simple in process and mild in reaction condition, and the reaction yield can be increased while the reaction time is greatly shortened.

Description

Technical Field

[0001] The invention relates to the field of lithium ion battery electrolyte, in particular to a preparation process of lithium difluorophosphate, and more particularly to a method for improving the synthesis reaction rate of lithium difluorophosphate. Background Art

[0002] Lithium difluorophosphate is a commonly used additive in lithium-ion battery electrolytes, which can improve the battery's high and low temperature resistance, cycle stability and rate performance.

[0003] The early preparation methods of lithium difluorophosphate mainly include: direct hydrolysis of lithium hexafluorophosphate, difluorophosphoric anhydride lithiation, and difluorophosphoric anhydride lithiation. However, the yield of lithium hexafluorophosphate hydrolysis is very low, and the product is not single; the preparation process of difluorophosphoric anhydride lithiation is complicated and its stability is poor, and it is impossible to successfully obtain a product that can be used in lithium battery electrolyte.

[0004] After that, new synthesis processes for lithium difluorophosphate appeared one after another, such as the method for preparing lithium difluorophosphate by reacting lithium hexafluorophosphate with lithium metaphosphate disclosed in patent CN108862232A; the method for preparing lithium difluorophosphate by using an organic tin reagent method disclosed in patent CN103259040A; and the method for preparing lithium difluorophosphate by reacting a mixed gas obtained by reacting pyrophosphate with fluorine gas and then passing it into an anhydrous hydrogen fluoride solution of lithium fluoride to react. However, the above methods either have low product purity or are prone to introduce impurities, and are not suitable for industrial applications.

[0005] At present, there are two main industrial processes for lithium difluorophosphate:

[0006] 1) The reaction process of lithium hexafluorophosphate and lithium carbonate has the following disadvantages: a large amount of by-product lithium fluoride is generated at the same time. Although it can be removed through complex separation steps, the content of insoluble matter in the product will inevitably increase. In addition, due to purity problems, the by-product lithium fluoride cannot be directly utilized, resulting in a waste of resources and high overall production costs.

[0007] 2) The reaction process of lithium hexafluorophosphate and siloxane has high yield and purity, but the reaction speed is slow, especially when using low-cost cyclic siloxane, the reaction time can even reach 30 hours, and the production efficiency is low.

[0008] Patent CN101507041A discloses a method for reacting lithium hexafluorophosphate with a compound containing a Si-O-Si structure to generate lithium difluorophosphate. When chain siloxane is used as a raw material and the reaction temperature is 60°C, the reaction time is 6 to 15 hours, the yield is 92.8 to 95%, and the purity is 97.9 to 99.1%; when cyclic siloxane is used as a raw material, the reaction time is 30 hours, the yield is 93.9 to 96.5%, and the purity is 98.2 to 99.1%.

[0009] Patent CN1146341170A also discloses a method for obtaining lithium difluorophosphate by reacting lithium hexafluorophosphate and siloxane, but does not disclose the reaction time. The yield is about 95% and the purity is about 98-99%.

[0010] Patent CN110127647AA discloses a method for preparing lithium difluorophosphate by reacting lithium hexafluorophosphate and hexamethyldisiloxane (chain siloxane). Although the reaction time is shortened to 2 to 4 hours, it is necessary to add a chemical equivalent of concentrated sulfuric acid, which has a huge adverse effect on product purity, post-processing, and three waste treatment.

[0011] Patent CN107244663A discloses a method for generating lithium difluorophosphate by reacting lithium hexafluorophosphate with a compound containing a Si-OC structure. When the reaction temperature is 60°C and the reaction time is 12 hours, the yield is also relatively low, only 88.5-93.7%, and the purity is 98.9-99.5%. Summary of the invention

[0012] In order to solve the above technical problems, the present invention proposes a method for improving the synthesis reaction rate of lithium difluorophosphate. The method still uses lithium hexafluorophosphate and siloxane as reaction raw materials. On the basis of ensuring excellent product purity, it can greatly reduce the reaction time, appropriately lower the reaction temperature, and at the same time improve the reaction yield, which is very suitable for industrial application.

[0013] The objective of the present invention is achieved through the following technical solutions:

[0014] A method for improving the reaction rate of lithium difluorophosphate synthesis, the method comprising: using lithium hexafluorophosphate and siloxane of the structure represented by the following formula (I) and / or (II) as raw materials in an organic solvent, adding water with a molar amount of 0.1 to 5.0% of the lithium hexafluorophosphate to prepare lithium difluorophosphate;

[0015]

[0016] In the formula, R1, R2, R3, R4, R5, R6, R7, and R8 are independently selected from C1-C4 alkyl groups, and n is selected from a positive integer of 3 to 6; preferably, R1, R2, R3, R4, R5, R6, R7, and R8 are independently selected from methyl, ethyl, n-propyl, or isopropyl groups, and n is 3 or 4; more preferably, the siloxane is selected from at least one of hexamethyldisiloxane, hexaethyldisiloxane, octamethyltrisiloxane, hexamethylcyclotrisiloxane, and octamethylcyclotetrasiloxane.

[0017] The present invention can greatly promote the reaction and improve the reaction efficiency by adding water to the reaction system. It is speculated that the reason may be that water can make the reaction raw materials generate acyl fluoride intermediates, which can activate the silicon-oxygen bond and increase the reaction activity, while reducing the yield decrease caused by side reactions such as self-decomposition of lithium hexafluorophosphate.

[0018] Especially when the siloxane is a cyclic siloxane, the addition of water can significantly increase the reaction rate, which can be increased by at least 4 times or even 10 times (i.e., the reaction time is shortened to 1 / 4 or even 1 / 10 of the original time).

[0019] Preferably, 0.5-2.0% of the molar amount of water of the lithium hexafluorophosphate is added to prepare lithium difluorophosphate.

[0020] The water may be purified water, distilled water or ultrapure water, and may even be achieved by adding an aqueous organic solvent.

[0021] The water is added after lithium hexafluorophosphate and siloxane are added to the reaction system. The water is added in a manner including, but not limited to: adding water to the reaction system after lithium hexafluorophosphate and siloxane are all added to the reaction system; or, adding water to the reaction system after lithium hexafluorophosphate and part of the siloxane are added to the reaction system; or, first adding lithium hexafluorophosphate to the reaction system, and then adding the siloxane mixed with water to the reaction system. Generally, it is sufficient to ensure that lithium hexafluorophosphate does not react with water alone for a long time before the siloxane is added, so as to avoid excessive hydrolysis of lithium hexafluorophosphate.

[0022] Preferably, the method comprises the following steps:

[0023] (1) lithium hexafluorophosphate is dissolved in an organic solvent;

[0024] (2) adding part of siloxane to the reaction system;

[0025] (3) adding water to the reaction system;

[0026] (4) Finally, the remaining siloxane is added to the reaction system.

[0027] Furthermore, in step (3), after water is added to the reaction system, the reaction temperature is raised to 35-85°C, and the reaction time is 0.2-8h. Preferably, after water is added to the reaction system, the reaction temperature is raised to 40-60°C, and the reaction time is 0.5-4h, and more preferably the reaction time is 0.5-2h.

[0028] The reaction pressure is not particularly limited, and the reaction may be carried out under normal pressure, reduced pressure, or sealed pressurized conditions.

[0029] In step (4), the remaining siloxane can be added all at once or in batches or dropwise. However, in order to prevent excessive generation of fluorosilane gas per unit time, the addition rate can be reasonably selected according to the exhaust gas treatment capacity of the equipment system.

[0030] In the method, the organic solvent is selected from at least one of C1-C6 carbonate, C1-C6 carboxylate, ether solvent or acetonitrile.

[0031] The C1-C6 carbonate is selected from at least one of dimethyl carbonate, diethyl carbonate or ethyl methyl carbonate; the C1-C6 carboxylate is selected from at least one of ethyl acetate, butyl acetate, propyl acetate, methyl acetate, methyl propionate, ethyl propionate and propyl propionate; the ether solvent is selected from at least one of methyl ethyl ether, ethyl ether, isopropyl ether, butyl ether and methyl tert-butyl ether.

[0032] In the method, the molar ratio of lithium hexafluorophosphate to oxygen atoms in siloxane is 1:(2.0-3.0), preferably 1:(2.0-2.4).

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] The present invention greatly promotes the improvement of reaction efficiency by adding an appropriate amount of water to the reaction system of lithium hexafluorophosphate and siloxane. While lowering the reaction temperature, the reaction time can also be greatly reduced and the reaction yield can be improved. The method of the present invention is particularly suitable for industrial application and can increase production efficiency by multiples. DETAILED DESCRIPTION

[0035] The present invention is further described below in conjunction with specific embodiments, but the present invention is not limited to these specific embodiments. Those skilled in the art should recognize that the present invention covers all possible alternatives, improvements and equivalents within the scope of the claims.

[0036] Example 1

[0037] This embodiment provides a method for preparing lithium difluorophosphate, which specifically comprises the following steps:

[0038] 1.52 g (10 mmol) of lithium hexafluorophosphate and 3 mL of dimethyl carbonate (electronic grade purity) were added to a reaction bottle, and after stirring to dissolve, 1.63 g (5.5 mmol) of octamethylcyclotetrasiloxane was added first, and then 0.0036 g (0.2 mmol) of pure water was added using a microinjector. The temperature was then raised to 50°C to start the reaction. The reaction was completed after about 0.5 h. At this time, the reaction solution was detected using nuclear magnetic resonance fluorine spectrum to show that the lithium hexafluorophosphate had been completely converted.

[0039] After cooling to room temperature, the final product was filtered and separated, and after drying, 1.05 g of lithium difluorophosphate was obtained, with a yield of 97.2% and a purity of 99.0%.

[0040] Example 2

[0041] The operation of this embodiment is the same as that of embodiment 1, except that the reaction temperature is lowered to 35°C, and the other operations remain unchanged. It can be seen from the detection that the raw material is completely converted after about 8 hours of reaction. Finally, 1.05 g of lithium difluorophosphate is obtained, with a yield of 97.2% and a purity of 98.2%.

[0042] Example 3

[0043] The operation of this embodiment is the same as that of embodiment 1, except that the reaction temperature is raised to 85°C, and the other operations remain unchanged. It can be seen from the detection that the raw material has been completely converted after about 0.3h of reaction. Finally, 0.98g of lithium difluorophosphate is obtained, with a yield of 90.7% and a purity of 97.8%.

[0044] Example 4

[0045] 152g (1mol) of lithium hexafluorophosphate and 300mL of dimethyl carbonate (electronic grade purity) were added to a reaction bottle, and after stirring and dissolving, 3g of octamethylcyclotetrasiloxane was first added, and then 0.36g (0.02mol) of pure water was added. Then, the temperature was raised to 50°C and 160g of octamethylcyclotetrasiloxane (0.55mol in total was added twice) was added dropwise. The addition time was about 4h, and then the temperature was kept warm for about 0.5h until no tail gas was generated. At this time, the reaction solution was detected by nuclear magnetic resonance fluorine spectrum to show that the lithium hexafluorophosphate had been completely converted. After the reaction was completed, it was cooled to room temperature, and the difluorodimethylsilane gas remaining in the solvent was evacuated by a vacuum pump, and then filtered and dried to obtain 105.6g of lithium difluorophosphate with a yield of 97.8% and a purity of 99.3%.

[0046] Example 5

[0047] The operation of this embodiment is the same as that of embodiment 4, except that the reaction temperature is lowered to 40°C, and the other operations remain unchanged. It can be seen from the detection that the raw material is completely converted after about 2 hours of heat preservation reaction. Finally, 105.5g of lithium difluorophosphate is obtained, with a yield of 97.7% and a purity of 99.1%.

[0048] Example 6

[0049] The operation of this embodiment is the same as that of embodiment 4, except that the reaction temperature is raised to 60°C, and the other operations remain unchanged. It can be seen from the detection that the raw material is completely converted after about 0.4h of heat preservation reaction. Finally, 104.8g of lithium difluorophosphate is obtained, with a yield of 97.0% and a purity of 98.9%.

[0050] Example 7

[0051] The operation of this embodiment is the same as that of embodiment 4, except that the amount of pure water is reduced to 0.18 g (0.01 mol), and the other operations remain unchanged. It can be seen from the detection that the raw material is completely converted after about 1.5 hours of heat preservation reaction. Finally, 103.6 g of lithium difluorophosphate is obtained, with a yield of 95.9% and a purity of 99.0%.

[0052] Example 8

[0053] The operation of this embodiment is the same as that of embodiment 4, except that the amount of pure water is reduced to 0.09 g (0.005 mol), and the other operations remain unchanged. It can be seen from the detection that the raw material is completely converted after about 4 hours of heat preservation reaction. Finally, 102.8 g of lithium difluorophosphate is obtained, with a yield of 95.2% and a purity of 99.4%.

[0054] Example 9

[0055] 152g (1mol) of lithium hexafluorophosphate and 300mL of ethyl methyl carbonate (electronic grade purity) were added to a reaction flask, and after stirring and dissolving, 10g of hexamethylcyclotrisiloxane was added first, and then 0.36g (0.02mol) of pure water was added. Then, the temperature was raised to 55°C and 167.6g of hexamethylcyclotrisiloxane (a total of 0.8mol) was added in batches. The addition was completed in 8 times within about 2h, and then the temperature was kept warm for about 0.5h until no tail gas was generated. At this time, the reaction solution was detected by nuclear magnetic resonance fluorine spectrum to show that the lithium hexafluorophosphate had been completely converted. After the reaction was completed, it was cooled to room temperature, and the difluorodimethylsilane gas remaining in the solvent was evacuated by a vacuum pump, and then filtered and dried to obtain 104.5g of lithium difluorophosphate with a yield of 96.7% and a purity of 98.5%.

[0056] Example 10

[0057] The operation of this embodiment is the same as that of embodiment 9, except that the type of siloxane is changed, decamethylcyclopentasiloxane is used instead of hexamethylcyclotrisiloxane, and the total amount of decamethylcyclopentasiloxane is changed to 155.4 g (0.42 mol), and the other operations remain unchanged. It can be seen from the detection that the raw material conversion is complete after about 0.5 h of heat preservation reaction. Finally, 106.0 g of lithium difluorophosphate is obtained, with a yield of 98.1% and a purity of 98.0%.

[0058] Embodiment 11

[0059] 76 g (0.5 mol) of lithium hexafluorophosphate and 150 mL of diethyl carbonate (electronic grade purity) were added to a reaction flask, and after stirring and dissolving, 3.2 g of hexamethyldisiloxane was first added, and then 0.009 g (0.0005 mol) of pure water was added. Then, the temperature was raised to 55° C. and 175 g of hexamethyldisiloxane (1.1 mol in total was added twice) was added dropwise. The addition time was about 3 h, and then the temperature was kept warm for about 1 h until no tail gas was generated. At this time, the reaction solution was detected by nuclear magnetic resonance fluorine spectrum to show that the lithium hexafluorophosphate had been completely converted. After the reaction was completed, it was cooled to room temperature, and the trimethylfluorosilane gas remaining in the solvent was evacuated by a vacuum pump, and then filtered and dried to obtain 100.3 g of lithium difluorophosphate with a yield of 92.8% and a purity of 98.9%.

[0060] Example 12

[0061] 76 g (0.5 mol) of lithium hexafluorophosphate and 150 mL of propyl propionate (electronic grade purity) were added to a reaction flask, the temperature was raised to 50°C, 0.018 g (0.001 mol) of pure water was added after stirring and dissolving, and then 117 g of methyltris(trimethylsilyloxy)silane (0.375 mol) was immediately added dropwise for about 2 h, and then the mixture was kept warm for about 0.5 h until no tail gas was generated. At this time, the reaction solution was detected by nuclear magnetic resonance fluorine spectrum to show that the lithium hexafluorophosphate had been completely converted. After the reaction was completed, the mixture was cooled to room temperature, and the fluorosilane gas remaining in the solvent was extracted by a vacuum pump, and then filtered and dried to obtain 103.1 g of lithium difluorophosphate with a yield of 95.4% and a purity of 98.8%.

[0062] Example 13

[0063] 76 g (0.5 mol) of lithium hexafluorophosphate and 150 mL of ethylene glycol dimethyl ether (electronic grade purity) were added to a reaction flask, 50 g of hexaethyldisiloxane was added after stirring to dissolve, and then 0.45 g (0.025 mol) of pure water was added, and then the temperature was raised to 60° C., and 220 g of hexaethyldisiloxane (a total of 1.1 mol) was added dropwise for about 1 hour, and then the temperature was kept warm for about 0.2 hours. At this time, the reaction solution was detected by nuclear magnetic resonance fluorine spectrum to show that the lithium hexafluorophosphate had been completely converted. After the reaction was completed, the reaction solution was subjected to reduced pressure distillation to remove most of the ethylene glycol dimethyl ether and triethylfluorosilane. After adding about 200 mL of dichloromethane, the mixture was stirred, filtered, and dried to obtain 90.3 g of lithium difluorophosphate with a yield of 83.6% and a purity of 95.5%.

[0064] Embodiment 14

[0065] 15.2 g (100 mmol) of lithium hexafluorophosphate and 30 mL of dimethyl carbonate with a water content of 600 ppm (a total of 0.018 g of water, 1 mmol) were added to a reaction bottle, and 35.7 g (220 mmol) of hexamethyldisiloxane was immediately added. The temperature was then raised to 45°C to start the reaction. The reaction was completed after about 2 hours. At this time, the reaction solution was detected using nuclear magnetic resonance fluorine spectrum to show that the lithium hexafluorophosphate had been completely converted.

[0066] After cooling to room temperature, the final product was filtered and separated, and after drying, 10.1 g of lithium difluorophosphate was obtained with a yield of 93.5% and a purity of 98.9%.

[0067] Comparative Example 1

[0068] 15.2 g (100 mmol) of lithium hexafluorophosphate and 30 mL of dimethyl carbonate (electronic grade purity) were added to a reaction bottle, and 16.3 g (55 mmol) of octamethylcyclotetrasiloxane was added after stirring to dissolve. The temperature was then raised to 60°C to start the reaction. After about 0.5 h, only a small amount of white turbidity was produced in the solution. At this time, the reaction solution was detected by nuclear magnetic resonance fluorine spectrum to show that the lithium hexafluorophosphate was rarely converted. The reaction was continued for about 5 h, and the white precipitate in the solution began to increase significantly. At the same time, a large amount of tail gas began to be generated. Using nuclear magnetic resonance to track the reaction, it was found that the reaction was not yet completed at this time. The reaction lasted for about 24 h, and the reaction was basically converted. After the reaction was carried out for a total of 30 h, the temperature was cooled to room temperature, filtered, and dried to obtain 10.2 g of lithium difluorophosphate with a yield of 94.4% and a purity of 98.1%.

[0069] Comparative Example 2

[0070] 152 g (1 mol) of lithium hexafluorophosphate and 30 mL of dimethyl carbonate (electronic grade purity) were added to a reaction bottle, stirred and dissolved, and 357 g (220 mmol) of hexamethyldisiloxane was added dropwise at 45° C. The addition time was about 3 h. After the addition was completed, the reaction was continued to be kept warm for about 8 h. The tail gas generation began to decrease. The reaction liquid was detected by nuclear magnetic resonance and it was found that a small amount of lithium hexafluorophosphate was not converted. The reaction was kept warm for about 18 h. The reaction was completely converted, cooled to room temperature, filtered, and dried to obtain 100.1 g of lithium difluorophosphate with a yield of 92.7% and a purity of 98.7%.

[0071] Comparative Example 3

[0072] 1.52 g (10 mmol) of lithium hexafluorophosphate and 3 mL of dimethyl carbonate (electronic grade purity) were added to a reaction bottle, and after stirring to dissolve, 1.63 g (5.5 mmol) of octamethylcyclotetrasiloxane was added. The temperature was then raised to 35°C to start the reaction. After about 48 hours, no precipitate was generated and the solution was basically clear. The reaction solution was detected using nuclear magnetic resonance fluorine spectrum, showing that the lithium hexafluorophosphate was basically not converted.

Claims

1. A method for increasing the reaction rate of lithium difluorophosphate synthesis, characterized in that: In an organic solvent, lithium hexafluorophosphate and siloxane of the structure represented by the following formula (I) and / or (II) are used as raw materials, and water with a molar amount of 0.1 to 5.0% of the lithium hexafluorophosphate is added to prepare lithium difluorophosphate; In the formula, R1, R2, R3, R4, R5, R6, R7, and R8 are independently selected from C1-C4 alkyl groups, and n is selected from a positive integer of 3 to 6.

2. The method for improving the synthesis reaction rate of lithium difluorophosphate according to claim 1, characterized in that: Adding water in an amount of 0.5 to 2.0% of the molar amount of the lithium hexafluorophosphate to prepare lithium difluorophosphate.

3. The method for improving the synthesis reaction rate of lithium difluorophosphate according to claim 1 or 2, characterized in that: The water is added after lithium hexafluorophosphate and siloxane are added into the reaction system.

4. The method for improving the synthesis reaction rate of lithium difluorophosphate according to claim 3, characterized in that: The method comprises the following steps: (1) lithium hexafluorophosphate is dissolved in an organic solvent; (2) adding part of siloxane to the reaction system; (3) adding water to the reaction system; (4) Finally, the remaining siloxane is added to the reaction system.

5. The method for improving the synthesis reaction rate of lithium difluorophosphate according to claim 4, characterized in that: After adding water to the reaction system, the reaction temperature is raised to 35-85°C and the reaction time is 0.2-8h.

6. The method for increasing the reaction rate of lithium difluorophosphate synthesis according to claim 5, characterized in that: After adding water to the reaction system, the reaction temperature is raised to 40-60°C and the reaction time is 0.5-4h.

7. The method for increasing the reaction rate of lithium difluorophosphate synthesis according to claim 1, characterized in that: R1, R2, R3, R4, R5, R6, R7, R8 are independently selected from methyl, ethyl, n-propyl or isopropyl, and n is 3 or 4.

8. The method for increasing the reaction rate of lithium difluorophosphate synthesis according to claim 7, characterized in that: The siloxane is selected from at least one of hexamethyldisiloxane, hexaethyldisiloxane, octamethyltrisiloxane, hexamethylcyclotrisiloxane or octamethylcyclotetrasiloxane.

9. The method for improving the reaction rate of lithium difluorophosphate synthesis according to claim 1, characterized in that: The organic solvent is selected from at least one of C1-C6 carbonate, C1-C6 carboxylate, ether solvent or acetonitrile.

10. The method for increasing the reaction rate of lithium difluorophosphate synthesis according to claim 9, characterized in that: The C1-C6 carbonate is selected from at least one of dimethyl carbonate, diethyl carbonate or ethyl methyl carbonate; the C1-C6 carboxylate is selected from at least one of ethyl acetate, butyl acetate, propyl acetate, methyl acetate, methyl propionate, ethyl propionate and propyl propionate; the ether solvent is selected from at least one of methyl ethyl ether, ethyl ether, isopropyl ether, butyl ether and methyl tert-butyl ether.

11. The method for increasing the reaction rate of lithium difluorophosphate synthesis according to claim 1, characterized in that: The molar ratio of lithium hexafluorophosphate to oxygen atoms in siloxane is 1:(2.0 to 3.0).

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