CN115477297A - A kind of preparation method of lithium difluorophosphate and the product thereof - Google Patents

Abstract

The invention provides a preparation method of lithium difluorophosphate and an obtained product thereof, belonging to the technical field of preparation of lithium battery additives. The preparation method of the lithium difluorophosphate comprises the following steps: 1) Mixing ammonium fluoride, phosphorus pentoxide and a polar aprotic organic system, heating to 80-90 ℃, and reacting to obtain ammonium difluorophosphate; the polar aprotic organic system is a mixed solution of N, N-dimethylformamide and acetonitrile; 2) Mixing the ammonium difluorophosphate and anhydrous lithium hydroxide with an organic solvent, and reacting to obtain lithium difluorophosphate; and the organic solvent is ethylene glycol dimethyl ether, dimethyl carbonate or ethyl acetate for reaction to obtain the lithium difluorophosphate. The preparation method of lithium difluorophosphate provided by the invention has the advantages of high product purity, high yield and stable reaction.

Description

A kind of preparation method of lithium difluorophosphate and the product thereof

technical field

The invention belongs to the technical field of lithium battery additive preparation, and in particular relates to a preparation method of lithium difluorophosphate and a product obtained therefrom.

Background technique

Lithium difluorophosphate was originally used in graphite anodes to construct solid electrolyte interface membranes to solve the problems of poor rate performance and cycle stability of high-capacity graphite anodes. Experiments have shown that a small amount of lithium difluorophosphate can promote the formation of graphite negative electrode SEI film, and the film-forming effect is significantly better than that of vinylene carbonate. At the same time, when lithium difluorophosphate is added alone, the SEI film will produce The content of lithium fluoride, a high-resistance substance, is too high, which affects the discharge performance of the battery at a high rate, so it has the best performance when used together with vinylene carbonate. At the same time, adding lithium difluorophosphate to the ternary/graphite battery can significantly improve the low-temperature performance of the battery, and the cycle stability of the battery added with lithium difluorophosphate is significantly better. This is because the lithium fluoride component in the SEI film contributes to the reduction of film resistance at low temperatures.

The synthesis of traditional lithium salt additives is usually related to lithium hexafluorophosphate, especially the synthesis of lithium difluorophosphate, which is formed by the reaction of lithium hexafluorophosphate and lithium carbonate or lithium hexafluorophosphate and siloxane. However, due to the high price of lithium hexafluorophosphate, the cost of lithium salt additives remains high. Moreover, the existing preparation methods of lithium difluorophosphate generally have low yield and many by-products, especially the acid value and the high content of lithium phosphate, lithium metaphosphate and lithium monofluorophosphate, which have a great impact on the performance of the electrolyte , leading to the application range of lithium difluorophosphate is still limited, it is difficult to popularize. Patent CN 111717906 A puts potassium fluoride and phosphorus pentoxide in a solid-phase reactor and heats it at a constant temperature of 150°C for 12 hours to obtain KPF ₂ O ₂ , which is then mixed with LiClO ₄ to undergo a series of extraction reactions to obtain lithium difluorophosphate. The disadvantage of this method is that there are too many side reactions in the pressure-resistant solid-phase reactor, and phosphorus pentafluoride may be generated. The reaction process is relatively complicated, and the requirements for equipment are relatively high.

Contents of the invention

The present invention provides a preparation method of lithium difluorophosphate and the product thereof. The preparation method of lithium difluorophosphate provided by the present invention is simple, the reaction is stable, the cost of reaction raw materials is low, easy to obtain, and the prepared product has high purity, The yield is high.

In order to achieve the above object, the invention provides a kind of preparation method of lithium difluorophosphate, comprises the steps:

1) Mix ammonium fluoride, phosphorus pentoxide and a polar aprotic organic system, heat up to 80-90°C, and react to obtain ammonium difluorophosphate;

The polar aprotic organic system is a mixed solution of N,N-dimethylformamide and acetonitrile;

2) Mix the ammonium difluorophosphate and anhydrous lithium hydroxide with an organic solvent to react to obtain lithium difluorophosphate; the organic solvent is ethylene glycol dimethyl ether, dimethyl carbonate or ethyl acetate.

Preferably, in terms of mass percentage, N,N-dimethylformamide in the polar aprotic organic system is 4%-10%.

Preferably, the molar ratio of ammonium fluoride to phosphorus pentoxide in step 1) is (3.3-3.5):1; the mass ratio of ammonium fluoride to polar aprotic organic system is 9:150-250.

Preferably, in step 1), the temperature is raised to 80-90° C. for reaction in step 1), and the rate of the gradient temperature rise is 2.5-3.5° C./min; the reaction time is 4-6 hours.

Preferably, after the reaction in step 1) is completed, purification is also carried out; the purification includes the following steps;

(1) The reaction product obtained after the reaction is completed is subjected to suction filtration to obtain a filter cake;

(2) The filter cake is mixed with ethanol, and the mixture is suction filtered to obtain a filtrate;

(3) Rotating the filtrate to obtain purified ammonium difluorophosphate.

Preferably, the molar ratio of ammonium difluorophosphate and anhydrous lithium hydroxide in step 2) is 1: (1.03-1.08).

Preferably, the reaction temperature in step 2) is 25-30° C., and the time is 1-3 hours.

Preferably, stirring is carried out during the reaction in step 2), and the rotation speed of the stirring is 500-1000 rpm.

Preferably, after the reaction in step 2) is completed, the reaction product is also subjected to suction filtration and rotary evaporation in sequence, the temperature of the rotary evaporation is 35-40°C; the vacuum degree during the rotary evaporation is -0.1--0.2 MPa.

The present invention provides the lithium difluorophosphate prepared by any one of the above methods, the acid value of the lithium difluorophosphate is 8-10ppm, and the purity is above 99.5%.

Compared with prior art, advantage and positive effect of the present invention are:

The invention uses ammonium fluoride and phosphorus pentoxide as raw materials, adopts a liquid phase method to first synthesize ammonium difluorophosphate, and then uses ammonium difluorophosphate to react with lithium hydroxide to synthesize lithium difluorophosphate. The process can synthesize lithium difluorophosphate without special equipment or high synthesis temperature, and the cost of reaction raw materials is low and easy to obtain. The synthesized lithium difluorophosphate has high purity, low cost, simple operation and less side reactions.

Description of drawings

Fig. 1 is the XRD characterization pattern of standard lithium difluorophosphate;

Fig. 2 is the XRD characterization pattern of the lithium difluorophosphate of embodiment 1;

Fig. 3 is the XRD characterization pattern of the lithium difluorophosphate of comparative example 4;

Fig. 4 is the ¹⁹ F NMR spectrogram of the lithium difluorophosphate of embodiment 1;

FIG. 5 is the ¹⁹ F NMR spectrum of lithium difluorophosphate in Comparative Example 4.

detailed description

The technical solutions in the embodiments of the present invention will be clearly and completely described below, obviously, the described embodiments are only some of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

The invention provides a kind of preparation method of lithium difluorophosphate, comprises the steps:

1) Mix ammonium fluoride, phosphorus pentoxide and a polar aprotic organic system, heat up to 80-90°C, and react to obtain ammonium difluorophosphate;

The polar aprotic organic system is a mixed solution of N,N-dimethylformamide and acetonitrile;

2) Mix the ammonium difluorophosphate and anhydrous lithium hydroxide with an organic solvent to react to obtain lithium difluorophosphate; the organic solvent is ethylene glycol dimethyl ether, dimethyl carbonate or ethyl acetate.

The invention mixes ammonium fluoride, phosphorus pentoxide and polar aprotic organic system, heats up to 80-90 DEG C, and reacts to obtain ammonium difluorophosphate. In the present invention, the polar aprotic organic system is a mixed solution of N,N-dimethylformamide and acetonitrile; Methylformamide is preferably 4% to 10%, more preferably 5%. In the present invention, the molar ratio of the ammonium fluoride to phosphorus pentoxide is preferably (3.3-3.5): 1; the mass ratio of the ammonium fluoride to the polar aprotic organic system is preferably 9:150- 250, more preferably 9:200. In the present invention, polar aprotic solvent can have influence on solute molecule, produces solvation effect, in weakly acidic environment (phosphorus pentoxide and ammonium fluoride present weakly acidic in liquid phase mixing system), use non The protic solvent is conducive to the unimolecular nucleophilic substitution reaction between phosphorus pentoxide and ammonium fluoride, which increases the reaction rate. ", thereby promoting the continuation of the reaction. Compared with using acetonitrile alone, the solubility of phosphorus pentoxide and ammonium fluoride in the aprotic mixed system is greater, and the product purity and yield will be improved. It can be understood that, in the present invention, the polar aprotic organic system formed by N,N-dimethylformamide and acetonitrile is the key operation of this application, and a conventional single solvent such as N,N-dimethyl Formamide/ethylene glycol dimethyl ether/dimethyl carbonate/ethyl acetate/methanol are difficult to synthesize the required ammonium difluorophosphate.

In the present invention, it is preferred to use a gradient temperature rise method to raise the temperature to 80-90° C. for the reaction, the rate of the gradient temperature rise is 2.5-3.5° C./min; the reaction time is 4-6 hours. Phosphorus pentoxide and ammonium fluoride in a normal temperature glove box synthesized ammonium difluorophosphate through solid phase grinding, but the yield and purity were very low, and during the reaction process phosphorus pentoxide and ammonium fluoride came into contact with grinding. A large amount of white smoke emits a large amount of heat, and then the temperature drops to room temperature to form a hard agglomerate. Explain that under anhydrous and oxygen-free environment, when phosphorus pentoxide reacts with ammonium fluoride, the activity is stronger and the reaction is violent. In the present invention, the mode of gradient heating is adopted to control the reaction temperature, which can prevent other by-products from being produced due to local high temperature.

After ammonium fluoride and phosphorus pentoxide are mixed, when the temperature rises to start the reaction, the temperature in the system will rapidly rise to about 200°C, and the product ammonium difluorophosphate will be decomposed and decomposed into ammonium phosphate, causing irreversible damage. Can not control. In the present invention, ammonium fluoride, phosphorus pentoxide and polar aprotic organic system are mixed, and the whole reaction system exists in the liquid phase organic solvent, and the contact and collision between the reactants will be more sufficient than that of the solid phase method. , the reaction is more thorough, and the temperature control in the liquid phase system is more accurate and uniform, so the corresponding organic by-products should be less, and the intermediate product ammonium difluorophosphate has high purity, which is conducive to the synthesis of lithium difluorophosphate by the associated reaction with lithium hydroxide High purity. At the same time, the use of a specific polar aprotic organic system can promote the positive reaction to increase the yield of ammonium difluorophosphate, thereby further improving the yield and purity of lithium difluorophosphate.

In the present invention, after the reaction is completed, purification is preferably carried out; the purification preferably includes the following steps;

(1) The reaction product obtained after the reaction is completed is subjected to suction filtration to obtain a filter cake;

(2) The filter cake is mixed with ethanol, and the mixture is suction filtered to obtain a filtrate;

(3) Rotating the filtrate to obtain purified ammonium difluorophosphate.

In the present invention, the temperature of the rotary steaming water bath is preferably controlled at 35-40°C during the rotary evaporation; the vacuum degree during the rotary evaporation is preferably -0.1-0.2Mpa, more preferably -0.15Mpa.

After obtaining ammonium difluorophosphate, the present invention mixes the ammonium difluorophosphate and anhydrous lithium hydroxide with an organic solvent to react to obtain lithium difluorophosphate; the organic solvent is ethylene glycol dimethyl ether, dicarbonate Methyl ester or ethyl acetate, preferably ethylene glycol dimethyl ether. In the present invention, the molar ratio of the ammonium difluorophosphate to anhydrous lithium hydroxide is preferably 1:(1.03-1.08). In the present invention, the temperature of the reaction is preferably 25-30° C., and the reaction time is preferably 1-3 hours. In the present invention, stirring is preferably performed during the reaction, and the rotation speed of the stirring is preferably 500-1000 rpm. In the present invention, the organic solvent is preferably ethylene glycol dimethyl ether. In the present invention, after the reaction is completed, suction filtration and rotary evaporation of the reaction product are also included in sequence. In the present invention, the temperature of the rotary evaporation is preferably 35-40°C; the vacuum degree during the rotary evaporation is preferably -0.1-0.2Mpa, more preferably -0.15Mpa. The chemical reaction formula of preparing lithium difluorophosphate among the present invention is as follows:

LiOH+NH ₄ PO ₂ F ₂ ＝LiPO ₂ F ₂ +NH ₃ ↑+H ₂ O

The present invention provides the lithium difluorophosphate prepared by any one of the above methods, the acid value of the lithium difluorophosphate is 8-10ppm, and the purity is above 99.5%.

In order to further illustrate the present invention, the technical solutions provided by the present invention will be described in detail below in conjunction with examples, but they should not be construed as limiting the protection scope of the present invention.

The sources of raw materials in the examples are as follows: ammonium fluoride is a commercially available product from Xilong Chemical with a purity of 96%, phosphorus pentoxide is a commercially available product from McLean with a purity of 99.99%, and N,N-dimethylformamide is a The commercially available product McLean has a purity of 99.8%, anhydrous acetonitrile is a commercially available product with a purity of 99.9%, and ethanol is a commercially available product Aladdin with an analytical purity of 99.5%.

Example 1

Weigh 9.17g of ammonium fluoride, 10g of phosphorus pentoxide, 200g of N,N-dimethylformamide-anhydrous acetonitrile (the mass of N,N-dimethylformamide accounts for 5%, and the remaining 95% is anhydrous water acetonitrile), the above-mentioned raw materials are added in the three-necked flask (in the three-necked flask, a mechanical stirrer is connected directly above, and the other two are respectively connected with a thermometer sleeve and a condenser). The three-neck flask was placed in an oil bath heating pot, and the temperature was gradually raised from normal temperature to 85°C at a heating rate of 3°C/min, and the reaction was performed at a constant temperature for 5 hours after reaching 85°C.

After the reaction is finished, the reaction product is suction-filtered with a sand core suction filter device, and the filtrate is separated from the filter cake. The filter cake was purified with 300 mL of ethanol, and the magnetic stirring speed was 600 rpm for 1 h. The above materials are suction filtered with a sand core funnel, and the filtrate is separated from the filter cake. The filtrate was transferred to a rotary evaporator bottle for rotary evaporation treatment (rotary evaporation temperature was 35°C, vacuum degree was -0.15Mpa), and 8.756 g of ammonium difluorophosphate was obtained.

In the glove box (anhydrous and oxygen-free), the ammonium difluorophosphate obtained above was reacted with anhydrous lithium hydroxide 1.87g in 300mL ethylene glycol dimethyl ether solution (using a magnetic stirrer to stir during the reaction, Speed 1000rpm) 2h. After the reaction was finished, the reaction product was suction-filtered with a sand core funnel, the filtrate was separated from the filter cake, and the filtrate was transferred to a rotary evaporator for rotary evaporation (the rotary evaporation temperature was 38°C and the vacuum degree was -0.15Mpa) to obtain two Lithium fluorophosphate 7.92g.

The prepared lithium difluorophosphate is characterized by XRD, as shown in Figure 2, and compared with the standard lithium difluorophosphate characterization spectrum (as shown in Figure 1), it can be known that the prepared product is lithium difluorophosphate .

The prepared lithium difluorophosphate was subjected to nuclear magnetic characterization (BRUKER AVANCE III 400 superconducting nuclear magnetic resonance spectrometer (NMR)), as shown in FIG. 4 .

Example 2

Weigh 8.908g of ammonium fluoride, 10g of phosphorus pentoxide, 200g of N,N-dimethylformamide-anhydrous acetonitrile (the mass of N,N-dimethylformamide accounts for 5%, and the remaining 95% is anhydrous water acetonitrile), the above-mentioned raw materials are added in the three-necked flask (in the three-necked flask, a mechanical stirrer is connected directly above, and the other two are respectively connected with a thermometer sleeve and a condenser). The three-neck flask was placed in an oil bath heating pot, and the temperature was gradually raised from normal temperature to 85°C at a heating rate of 3°C/min, and the reaction was performed at a constant temperature for 5 hours after reaching 85°C.

After the reaction is finished, the reaction product is suction-filtered with a sand core suction filter device, and the filtrate is separated from the filter cake. The filter cake was purified with 300 mL of ethanol, and the magnetic stirring speed was 600 rpm for 1 h. The above materials are suction filtered with a sand core funnel, and the filtrate is separated from the filter cake. The filtrate was transferred to a rotary evaporator bottle for rotary evaporation treatment (the rotary evaporation temperature was 40° C., and the vacuum degree was −0.15 Mpa), to obtain 7.863 g of ammonium difluorophosphate.

In the glove box (anhydrous and oxygen-free), the ammonium difluorophosphate obtained above was reacted with anhydrous lithium hydroxide 1.679g in 300mL ethylene glycol dimethyl ether solution (using a magnetic stirrer to stir during the reaction, Speed 1000rpm) 2h. After the reaction was finished, the reaction product was suction-filtered with a sand core funnel, the filtrate was separated from the filter cake, and the filtrate was transferred to a rotary evaporator for rotary evaporation (rotary evaporation temperature was 38°C; vacuum degree was -0.15Mpa) to obtain two Lithium fluorophosphate 6.802g.

Example 3

Weigh 8.908g of ammonium fluoride, 10g of phosphorus pentoxide, 240g of N,N-dimethylformamide-anhydrous acetonitrile (the mass of N,N-dimethylformamide accounts for 10%, and the remaining 90% is anhydrous water acetonitrile), the above-mentioned raw materials are added in the three-necked flask (in the three-necked flask, a mechanical stirrer is connected directly above, and the other two are respectively connected with a thermometer sleeve and a condenser). The three-neck flask was placed in an oil bath heating pot, and the temperature was gradually raised from normal temperature to 80°C at a heating rate of 3.5°C/min, and the reaction was carried out at a constant temperature for 6 hours after reaching 80°C.

After the reaction is finished, the reaction product is suction-filtered with a sand core suction filter device, and the filtrate is separated from the filter cake. The filter cake was purified with 300 mL of ethanol, and the magnetic stirring speed was 600 rpm for 1 h. The above materials are suction filtered with a sand core funnel, and the filtrate is separated from the filter cake. The filtrate was transferred to a rotary evaporator for rotary evaporation treatment (the rotary evaporation temperature was 40° C., and the vacuum degree was -0.15 Mpa), to obtain 7.631 g of ammonium difluorophosphate.

In a glove box (anhydrous and oxygen-free), react the ammonium difluorophosphate obtained above with 1.679 g of anhydrous lithium hydroxide in 300 mL of dimethyl carbonate solution (a magnetic stirrer is used to stir during the reaction, and the rotating speed is 1000 rpm ) 3h. After the reaction was finished, the reaction product was suction-filtered with a sand core funnel, the filtrate was separated from the filter cake, and the filtrate was transferred to a rotary evaporator for rotary evaporation (rotary evaporation temperature was 38°C; vacuum degree was -0.15Mpa) to obtain two Lithium fluorophosphate 6.785g.

Example 4

Weigh 8.908g of ammonium fluoride, 10g of phosphorus pentoxide, 200g of N,N-dimethylformamide-anhydrous acetonitrile (the mass of N,N-dimethylformamide accounts for 10%, and the remaining 90% is anhydrous water acetonitrile), the above-mentioned raw materials are added in the three-necked flask (in the three-necked flask, a mechanical stirrer is connected directly above, and the other two are respectively connected with a thermometer sleeve and a condenser). Put the three-neck flask in the oil bath heating pot, start from normal temperature to 90°C gradiently, the heating rate is 2.5°C/min, after reaching 90°C, keep the constant temperature for 4 hours.

After the reaction is finished, the reaction product is suction-filtered with a sand core suction filter device, and the filtrate is separated from the filter cake. The filter cake was purified with 300 mL of ethanol, and the magnetic stirring speed was 600 rpm for 1 h. The above materials are suction filtered with a sand core funnel, and the filtrate is separated from the filter cake. The filtrate was transferred to a rotary evaporator for rotary evaporation treatment (the rotary evaporation temperature was 40° C., and the vacuum degree was −0.15 Mpa), to obtain 7.313 g of ammonium difluorophosphate.

In the glove box (anhydrous and oxygen-free), the ammonium difluorophosphate obtained above was reacted with anhydrous lithium hydroxide 1.679g in 300mL ethylene glycol dimethyl ether solution (using a magnetic stirrer to stir during the reaction, Speed 1000rpm) 3h. After the reaction was finished, the reaction product was suction-filtered with a sand core funnel, the filtrate was separated from the filter cake, and the filtrate was transferred to a rotary evaporator for rotary evaporation (rotary evaporation temperature was 38°C; vacuum degree was -0.15Mpa) to obtain two Lithium fluorophosphate 6.565g.

Comparative example 1

The difference from Example 1 is that only anhydrous acetonitrile is added, and N,N-dimethylformamide is not added during the preparation of ammonium difluorophosphate. Other operation steps are exactly the same as embodiment 1, and concrete process is as follows:

Weigh 9.17g of ammonium fluoride, 10g of phosphorus pentoxide, and 200g of anhydrous acetonitrile, and add the above-mentioned raw materials into a three-necked flask (in the three-necked flask, a mechanical stirrer is directly connected to the top, and the other two ports are respectively connected to a thermometer sleeve and a condenser) . The three-neck flask was placed in an oil bath heating pot, and the temperature was gradually raised from normal temperature to 85°C at a heating rate of 3°C/min, and the reaction was performed at a constant temperature for 5 hours after reaching 85°C.

After the reaction is finished, the reaction product is suction-filtered with a sand core suction filter device, and the filtrate is separated from the filter cake. The filter cake was purified with 300 mL of ethanol, and the magnetic stirring speed was 600 rpm for 1 h. The above materials are suction filtered with a sand core funnel, and the filtrate is separated from the filter cake. The filtrate was transferred to a rotary evaporator bottle for rotary evaporation treatment (rotary evaporation temperature was 35°C, vacuum degree was -0.15Mpa), and 5.897g of ammonium difluorophosphate was obtained.

In the glove box (anhydrous and oxygen-free), the ammonium difluorophosphate obtained above was reacted with 1.259 g of anhydrous lithium hydroxide in 300 mL of ethylene glycol dimethyl ether solution (using a magnetic stirrer to stir during the reaction, Speed 1000rpm) 2h. After the reaction was finished, the reaction product was suction-filtered with a sand core funnel, the filtrate was separated from the filter cake, and the filtrate was transferred to a rotary evaporator for rotary evaporation (the rotary evaporation temperature was 38°C and the vacuum degree was -0.15Mpa) to obtain two Lithium fluorophosphate 4.972g.

Comparative example 2

The difference from Example 1 is that only N,N-dimethylformamide is added and no anhydrous acetonitrile is added during the preparation of ammonium difluorophosphate. Other operation steps are exactly the same as embodiment 1, and concrete process is as follows:

Weigh 9.17g of ammonium fluoride, 10g of phosphorus pentoxide, and 200g of N,N-dimethylformamide, and add the above-mentioned raw materials into a three-necked flask (in the three-necked flask, a mechanical stirrer is directly connected to the top, and the other two are respectively connected to a thermometer. casing and condenser). The three-neck flask was placed in an oil bath heating pot, and the temperature was gradually raised from normal temperature to 85°C at a heating rate of 3°C/min, and the reaction was performed at a constant temperature for 5 hours after reaching 85°C.

After the reaction is finished, the reaction product is suction-filtered with a sand core suction filter device, and the filtrate is separated from the filter cake. The filter cake was purified with 300 mL of ethanol, and the magnetic stirring speed was 600 rpm for 1 h. The above materials are suction filtered with a sand core funnel, and the filtrate is separated from the filter cake. The filtrate was transferred to a rotary evaporator for rotary evaporation treatment (rotary evaporation temperature was 35°C, vacuum degree was -0.15Mpa), and 1.051 g of ammonium difluorophosphate was obtained.

In a glove box (anhydrous and oxygen-free), react the above-mentioned ammonium difluorophosphate with anhydrous lithium hydroxide g in 300 mL of ethylene glycol dimethyl ether solution (use a magnetic stirrer to stir during the reaction, and the rotation speed 1000rpm) 2h. After the reaction was finished, the reaction product was suction-filtered with a sand core funnel, the filtrate was separated from the filter cake, and the filtrate was transferred to a rotary evaporator for rotary evaporation (the rotary evaporation temperature was 38°C and the vacuum degree was -0.15Mpa) to obtain two Lithium fluorophosphate 0.802g.

Comparative example 3

The difference from Example 1 is that when preparing ammonium difluorophosphate, the addition ratios of N,N-dimethylformamide and anhydrous acetonitrile in the polar aprotic organic system are different. Other operation steps are exactly the same as embodiment 1, and concrete process is as follows:

Weigh 9.17g of ammonium fluoride, 10g of phosphorus pentoxide, 200g of N,N-dimethylformamide-anhydrous acetonitrile (the mass of N,N-dimethylformamide accounts for 50%, and the remaining 50% is anhydrous water acetonitrile), the above-mentioned raw materials are added in the three-necked flask (in the three-necked flask, a mechanical stirrer is connected directly above, and the other two are respectively connected with a thermometer sleeve and a condenser). The three-neck flask was placed in an oil bath heating pot, and the temperature was gradually raised from normal temperature to 85°C at a heating rate of 3°C/min, and the reaction was performed at a constant temperature for 5 hours after reaching 85°C.

After the reaction is finished, the reaction product is suction-filtered with a sand core suction filter device, and the filtrate is separated from the filter cake. The filter cake was purified with 300 mL of ethanol, and the magnetic stirring speed was 600 rpm for 1 h. The above materials are suction filtered with a sand core funnel, and the filtrate is separated from the filter cake. The filtrate was transferred to a rotary evaporator for rotary evaporation treatment (rotary evaporation temperature was 35°C, vacuum degree was -0.15Mpa), and 5.012g of ammonium difluorophosphate was obtained.

In a glove box (anhydrous and oxygen-free), react the above-mentioned ammonium difluorophosphate with anhydrous lithium hydroxide g in 300 mL of ethylene glycol dimethyl ether solution (use a magnetic stirrer to stir during the reaction, and the rotation speed 1000rpm) 2h. After the reaction was finished, the reaction product was suction-filtered with a sand core funnel, the filtrate was separated from the filter cake, and the filtrate was transferred to a rotary evaporator for rotary evaporation (the rotary evaporation temperature was 38°C and the vacuum degree was -0.15Mpa) to obtain two Lithium fluorophosphate 4.227g.

Comparative example 4

Weigh 9.17g of ammonium fluoride and 10g of phosphorus pentoxide, and uniformly mix them with a double-center mixer (500rpm rotation speed, 10min time), repeat three times, and grind and pulverize the materials with a copper bowl after each mixing. Transfer the uniformly mixed material to the polytetrafluoroethylene reaction device, connect the corresponding temperature sensor and the exhaust gas absorption device; then transfer the customized reaction device to the heating mantle, heat at 200°C, and keep the temperature constant for 2h.

After the reaction is finished, the reaction product is suction-filtered with a sand core suction filter device, and the filtrate is separated from the filter cake. The filter cake was purified with 300 mL of ethanol, and the magnetic stirring speed was 600 rpm for 1 h. The above materials are suction filtered with a sand core funnel, and the filtrate is separated from the filter cake. The filtrate was transferred to a rotary evaporation bottle for rotary evaporation treatment (rotary evaporation temperature was 35° C., vacuum degree was -0.15 Mpa), and 3.872 g of ammonium difluorophosphate was obtained.

In a glove box (anhydrous and oxygen-free), react the above-mentioned ammonium difluorophosphate with 0.766 g of anhydrous lithium hydroxide in 300 mL of ethylene glycol dimethyl ether solution (use a magnetic stirrer to stir during the reaction, Speed 1000rpm) 2h. After the reaction was finished, the reaction product was suction-filtered with a sand core funnel, the filtrate was separated from the filter cake, and the filtrate was transferred to a rotary evaporator for rotary evaporation (the rotary evaporation temperature was 38°C and the vacuum degree was -0.15Mpa) to obtain two Lithium fluorophosphate 2.976g.

The prepared lithium difluorophosphate was characterized by XRD, as shown in FIG. 3 . As can be seen from Figures 1 to 3, although the LiPO ₂ F ₂ synthesized in Comparative Example 4 and the LiPO ₂ F ₂ standard substance and the LiPO ₂ F ₂ product characteristic peaks synthesized in Example 1 have the same main peak position and the peak intensity is basically the same, but for There are three small miscellaneous peaks around the main peak of lithium difluorophosphate synthesized in ratio 4, which shows that the LiPO ₂ F ₂ synthesized in Example 1 is not as pure as LiPO 2 F 2 , and has a single structure.

The prepared lithium difluorophosphate was subjected to nuclear magnetic characterization (BRUKER AVANCE III 400 superconducting nuclear magnetic resonance spectrometer (NMR)), as shown in FIG. 5 . As can be seen from Fig. 4 and Fig. 5, there are four absorption peaks in the 19F NMR spectrogram of LiPO ₂ F ₂ of comparative example 4, explaining that the fluorine element in the LiPO ₂ F ₂ product structure has two chemical environments, product category It is not single, there are other impurities; as can be seen from Figure 4, there are two absorption peaks in the 19F NMR spectrum of LiPO ₂ F ₂ , and no other miscellaneous peaks appear, indicating that the fluorine element in the LiPO ₂ F ₂ product structure has only one chemical environment without other impurities. And the corresponding chemical shift δ is -79.3658ppm and 81.0441ppm corresponding to 2 fluorine atoms in LiPO ₂ F ₂ . Therefore, the LiPO ₂ F ₂ prepared in Example 1 has a single structure and high purity.

Comparative example 5

The difference from Example 1 is that only ethylene glycol dimethyl ether is added when preparing ammonium difluorophosphate, and other operating steps are exactly the same as in Example 1, and the specific process is as follows:

Take by weighing 9.17g of ammonium fluoride, 10g of phosphorus pentoxide, and 200g of ethylene glycol dimethyl ether, and add the above-mentioned raw materials in a three-necked flask (in the three-necked flask, a mechanical stirrer is directly connected to the top, and the other two ports are respectively connected to a thermometer sleeve and a three-necked flask. condenser tube). The three-neck flask was placed in an oil bath heating pot, and the temperature was gradually raised from normal temperature to 85°C at a heating rate of 3°C/min, and the reaction was performed at a constant temperature for 5 hours after reaching 85°C.

After the reaction is finished, the reaction product is suction-filtered with a sand core suction filter device, and the filtrate is separated from the filter cake. The filter cake was purified with 300 mL of ethanol, and the magnetic stirring speed was 600 rpm for 1 h. The above materials are suction filtered with a sand core funnel, and the filtrate is separated from the filter cake. The filtrate was transferred to a rotary evaporation bottle for rotary evaporation treatment (rotary evaporation temperature was 35° C., vacuum degree was -0.15 Mpa), and 0.526 g of ammonium difluorophosphate was obtained.

In the glove box (anhydrous and oxygen-free), the ammonium difluorophosphate obtained above was reacted with anhydrous lithium hydroxide 0.112g in 300mL ethylene glycol dimethyl ether solution (using a magnetic stirrer to stir during the reaction, Speed 1000rpm) 2h. After the reaction was finished, the reaction product was suction-filtered with a sand core funnel, the filtrate was separated from the filter cake, and the filtrate was transferred to a rotary evaporator for rotary evaporation (the rotary evaporation temperature was 38°C and the vacuum degree was -0.15Mpa) to obtain two Lithium fluorophosphate 0.302g.

Performance Testing

The performance of the products prepared in Examples 1 to 4 and Comparative Examples 1 to 5 is tested. The specific test indicators are purity, moisture, acid value and productivity. The specific results are as shown in Table 1. The specific index test methods are as follows:

(1) Purity test: This experiment uses the external standard method of ion chromatography to carry out the purity test.

(2) Moisture test: Heat the ammonium difluorophosphate sample to a certain temperature with a cassette furnace, blow the evaporated water vapor into the Karl Fischer reagent in the reaction cup with dry air, and measure it by the coulometric method.

(3) Acid value: Use a microburette to titrate the free acid in the sample with bromothymol cyanide indicator and sodium hydroxide standard titration solution.

(4) Yield: According to the product result in the purity test, bring it into the reaction equation to calculate.

Table 1 performance test results

purity% Moisture PPM acid value Yield% Example 1 99.927 132 9.1 86.769 Example 2 99.881 121 9.2 74.926 Example 3 99.601 176 9.8 72.952 Example 4 99.512 199 9.9 71.693 Comparative example 1 93.323 264 10.4 51.208 Comparative example 2 41.231 254 178.2 3.649 Comparative example 3 72.800 221 32.1 33.962 Comparative example 4 62.651 2243 3225 20.577 Comparative example 5 43.323 195 10.4 1.444

As can be seen from Table 1, the method provided by the present invention uses acetonitrile as a solvent alone in Comparative Example 1, and the yield is greatly improved, while Comparative Example 2 adopts N,N-dimethylformamide and Comparative Example 5 adopts ethylene glycol Dimethyl ether is used as solvent, and the productive rate of ammonium difluorophosphate is extremely low, thereby causes the productive rate of final lithium difluorophosphate to be also lower. In Comparative Example 4, no solvent was added, and ammonium fluoride and phosphorus pentoxide were directly mixed for reaction. After white smoke occurred in the middle of the reaction, the temperature of the system soared to 200°C, which was extremely difficult to control. Decomposition, further phosphorus pentoxide reacts with moisture in the air at high temperature to generate a small amount of metaphosphoric acid, which is extremely toxic, and then quickly becomes phosphoric acid, so the reaction yield is low and the overall effect is poor. It can be concluded that the lithium difluorophosphate prepared by the method provided by the present invention has high purity and high yield, and is easy to operate and is suitable for industrial production.

The above is only a preferred embodiment of the present invention, and it should be pointed out that for those of ordinary skill in the art, some improvements and modifications can be made without departing from the principle of the present invention. It should be regarded as the protection scope of the present invention.

Claims (10)

1. a preparation method of lithium difluorophosphate, is characterized in that, comprises the steps: 1) Mix ammonium fluoride, phosphorus pentoxide and a polar aprotic organic system, heat up to 80-90°C, and react to obtain ammonium difluorophosphate; The polar aprotic organic system is a mixed solution of N,N-dimethylformamide and acetonitrile; 2) Mix the ammonium difluorophosphate and anhydrous lithium hydroxide with an organic solvent to react to obtain lithium difluorophosphate; the organic solvent is ethylene glycol dimethyl ether, dimethyl carbonate or ethyl acetate. 2. The preparation method according to claim 1, characterized in that, in terms of mass percentage, the N,N-dimethylformamide in the polar aprotic organic system is 4% to 10%. 3. The preparation method according to claim 1, characterized in that, the molar ratio of ammonium fluoride to phosphorus pentoxide in step 1) is (3.3～3.5): 1; ammonium fluoride and polar aprotic organic The mass ratio of the system ammonium fluoride to the polar aprotic organic system is 9:150-200. 4. The preparation method according to claim 1, characterized in that, in step 1), the temperature is raised to 80-90° C. for reaction by means of gradient heating, and the rate of the gradient temperature rise is 2.5-3.5° C./min; The reaction time is 4~6h. 5. The preparation method according to claim 1, characterized in that, after the reaction in step 1) is completed, purification is also carried out; said purification comprises the following steps; (1) The reaction product obtained after the reaction is completed is subjected to suction filtration to obtain a filter cake; (2) The filter cake is mixed with ethanol, and the mixture is suction filtered to obtain a filtrate; (3) Rotating the filtrate to obtain purified ammonium difluorophosphate. 6. The preparation method according to claim 1, characterized in that the molar ratio of ammonium difluorophosphate and anhydrous lithium hydroxide in step 2) is 1:(1.03-1.08). 7. The preparation method according to claim 1, characterized in that the reaction temperature in step 2) is 25-30° C., and the time is 1-3 hours. 8. The preparation method according to claim 1, characterized in that, in step 2), stirring is carried out during the reaction, and the stirring speed is 500-1000 rpm. 9. The preparation method according to claim 1, characterized in that, after the reaction in step 2) is completed, further comprising suction filtration and rotary steaming of the reaction product successively, the temperature of the rotary steaming is 35-45° C.; The vacuum degree during the rotary evaporation is -0.1～-0.2Mpa. 10. The lithium difluorophosphate prepared by the method according to any one of claims 1-9, characterized in that the lithium difluorophosphate has an acid value of 8-10 ppm and a purity of more than 99.5%.

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