CN115477297A - Preparation method of lithium difluorophosphate and product obtained by preparation method - Google Patents
Preparation method of lithium difluorophosphate and product obtained by preparation method Download PDFInfo
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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
Technical Field
The invention belongs to the technical field of preparation of lithium battery additives, and particularly relates to a preparation method of lithium difluorophosphate and an obtained product thereof.
Background
Lithium difluorophosphate is initially used for constructing a solid electrolyte interface film on a graphite cathode so as to solve the problem of poor rate capability and poor cycle stability of a high-capacity graphite cathode. Experiments show that a small amount of lithium difluorophosphate can promote the generation of an SEI film of a graphite cathode, the film forming effect is obviously better than that of vinylene carbonate, and meanwhile, when the lithium difluorophosphate is singly added, the content of high-resistance substance lithium fluoride generated by the decomposition of the lithium difluorophosphate in the SEI film is too high, so that the discharge performance of the battery at high magnification is influenced, and the performance is best when the lithium difluorophosphate is used together with the vinylene carbonate. Meanwhile, the low-temperature performance of the ternary/graphite battery can be obviously improved after the lithium difluorophosphate is added, and the cycling stability of the battery added with the lithium difluorophosphate is obviously better. This is because the lithium fluoride component in the SEI film contributes to a reduction in film resistance at low temperatures.
The synthesis of conventional lithium salt additives is generally associated with lithium hexafluorophosphate, and in particular lithium difluorophosphate, is formed by reacting lithium hexafluorophosphate with lithium carbonate or lithium hexafluorophosphate with a siloxane. And because the price of lithium hexafluorophosphate is high, the cost of the lithium salt additive is high. Moreover, the existing preparation method of lithium difluorophosphate generally has the defects of low yield, more byproducts, especially high acid value and high content of lithium phosphate, lithium metaphosphate and lithium monofluorophosphate, which greatly affects the performance of the electrolyte, so that the application range of lithium difluorophosphate is still limited and is difficult to popularize. CN 111717906A put potassium fluoride and phosphorus pentoxide in a solid phase reaction kettle and heat at 150 deg.C for 12h to obtain KPF 2 O 2 Then will react with LiClO 4 Mixing and carrying out a series of extraction reactions to obtain the lithium difluorophosphate. The method has the defects thatThe side reaction in a pressure-resistant solid phase reaction kettle is excessive, phosphorus pentafluoride can be generated, the reaction process is complex, and the requirement on equipment is high.
Disclosure of Invention
The invention provides a preparation method of lithium difluorophosphate and an obtained product thereof.
In order to achieve the above object, the present invention provides a method for preparing lithium difluorophosphate, comprising the steps of:
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; the organic solvent is ethylene glycol dimethyl ether, dimethyl carbonate or ethyl acetate.
Preferably, the N, N-dimethylformamide in the polar aprotic organic system is 4-10% by mass.
Preferably, the molar ratio of the ammonium fluoride to the phosphorus pentoxide in the step 1) is (3.3-3.5): 1; the mass ratio of the ammonium fluoride to the polar aprotic organic system is from 9.
Preferably, in the step 1), the temperature is increased to 80-90 ℃ by adopting a gradient temperature increasing mode for reaction, and the gradient temperature increasing rate is 2.5-3.5 ℃/min; the reaction time is 4-6 h.
Preferably, after the reaction in the step 1) is finished, further purification is carried out; the purification comprises the following steps;
(1) Carrying out suction filtration on a reaction product obtained after the reaction is finished to obtain a filter cake;
(2) Mixing the filter cake with ethanol, and performing suction filtration on the mixture to obtain a filtrate;
(3) And carrying out rotary evaporation on the filtrate to obtain purified ammonium difluorophosphate.
Preferably, the molar ratio of ammonium difluorophosphate to anhydrous lithium hydroxide in step 2) is 1: (1.03-1.08).
Preferably, the reaction temperature in the step 2) is 25-30 ℃ and the reaction time is 1-3 h.
Preferably, in the step 2), stirring is performed during the reaction, and the rotation speed of the stirring is 500 to 1000rpm.
Preferably, after the reaction in the step 2) is finished, the method further comprises the steps of sequentially carrying out suction filtration and rotary evaporation on reaction products, wherein the temperature of the rotary evaporation is 35-40 ℃; the vacuum degree during the rotary steaming is-0.1 to-0.2 Mpa.
The invention provides lithium difluorophosphate prepared by any one of the methods, wherein the acid value of the lithium difluorophosphate is 8-10 ppm, and the purity of the lithium difluorophosphate is more than 99.5%.
Compared with the prior art, the invention has the advantages and positive effects that:
the invention takes ammonium fluoride and phosphorus pentoxide as raw materials, adopts a liquid phase method to synthesize ammonium difluorophosphate first, and then synthesizes lithium difluorophosphate by reacting the ammonium difluorophosphate with lithium hydroxide. The process can synthesize the lithium difluorophosphate without special equipment or higher synthesis temperature, and the reaction raw materials have low cost and are easy to obtain. The synthesized lithium difluorophosphate has high purity, low cost, simple operation and less side reaction.
Drawings
FIG. 1 is an XRD characterization pattern of standard lithium difluorophosphate;
FIG. 2 is an XRD characterization pattern of lithium difluorophosphate of example 1;
FIG. 3 is an XRD characterization pattern of lithium difluorophosphate of comparative example 4;
FIG. 4 is a drawing of lithium difluorophosphate of example 1 19 F, NMR spectrum;
FIG. 5 is a drawing of lithium difluorophosphate of comparative example 4 19 F NMR spectrum.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The invention provides a preparation method of lithium difluorophosphate, which 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; the organic solvent is ethylene glycol dimethyl ether, dimethyl carbonate or ethyl acetate.
The ammonium fluoride, the phosphorus pentoxide and the polar aprotic organic system are mixed, heated to 80-90 ℃ and reacted to obtain the ammonium difluorophosphate. In the invention, the polar aprotic organic system is a mixed solution of N, N-dimethylformamide and acetonitrile; the N, N-dimethylformamide in the polar aprotic organic system is preferably 4 to 10% by mass, and more preferably 5% by mass. In the present invention, the molar ratio of the ammonium fluoride to the phosphorus pentoxide is preferably (3.3 to 3.5): 1; the mass ratio of the ammonium fluoride to the polar aprotic organic system is preferably from 9 to 250, more preferably from 9. In the invention, the polar aprotic solvent can influence solute molecules to generate a solvation effect, and in a weakly acidic environment (phosphorus pentoxide and ammonium fluoride are weakly acidic in a liquid phase mixed system), the aprotic solvent is used for facilitating the monomolecular nucleophilic substitution reaction of the phosphorus pentoxide and the ammonium fluoride, so that the reaction rate is improved, and meanwhile, the N, N-dimethylformamide is used as a connecting bridge between a liquid phase and a solid phase in the system to play a role of a catalyst, so that the continuous reaction is promoted. Compared with the single use of acetonitrile, the solubility of the phosphorus pentoxide and the ammonium fluoride in an aprotic mixed system is higher, and the purity and the yield of the product are improved. It is understood that in the present invention, polar aprotic organic systems formed using N, N-dimethylformamide and acetonitrile are the key operations in the present application, and it is difficult to synthesize the desired ammonium difluorophosphate using conventional single solvents such as N, N-dimethylformamide/ethylene glycol dimethyl ether/dimethyl carbonate/ethyl acetate/methanol, etc.
In the invention, the reaction is carried out by heating to 80-90 ℃ preferably in a gradient heating manner, wherein the gradient heating rate is 2.5-3.5 ℃/min; the reaction time is 4-6 h. Phosphorus pentoxide and ammonium fluoride in the normal-temperature glove box are synthesized into ammonium difluorophosphate by a solid-phase grinding method, but the yield and the purity are very low, a large amount of white smoke is emitted when the phosphorus pentoxide and the ammonium fluoride are in grinding contact in the reaction process, a large amount of heat is emitted, and then the temperature is reduced to room temperature to form hard lumps. The reaction temperature is controlled by adopting a gradient temperature rise mode in the invention, so that other byproducts generated due to local high temperature can be prevented.
After ammonium fluoride salt and phosphorus pentoxide are mixed, when the temperature is raised to start the reaction, the temperature in the system can quickly rise to about 200 ℃, the product ammonium difluorophosphate is decomposed into ammonium phosphate, irreversible damage is generated, and the reaction is uncontrollable. In the invention, ammonium fluoride, phosphorus pentoxide and a polar aprotic organic system are mixed, the whole reaction system exists in a liquid-phase organic solvent, reactants are more fully contacted and collided with each other in a solid-phase method, the reaction is more thorough, and the temperature control under the liquid-phase system is more accurate and uniform, so that corresponding organic byproducts are less, the purity of the intermediate product ammonium difluorophosphate is high, and the purity of the lithium difluorophosphate synthesized by the correlation reaction of the ammonium difluorophosphate and lithium hydroxide is high. Meanwhile, a specific polar aprotic organic system is adopted to promote the forward reaction to improve the yield of ammonium difluorophosphate, so that the yield and the purity of lithium difluorophosphate are further improved.
In the present invention, after completion of the reaction, it is preferable to further perform purification; the purification preferably comprises the following steps;
(1) Carrying out suction filtration on a reaction product obtained after the reaction is finished to obtain a filter cake;
(2) Mixing the filter cake with ethanol, and performing suction filtration on the mixture to obtain a filtrate;
(3) And carrying out rotary evaporation on the filtrate to obtain the purified ammonium difluorophosphate.
In the invention, the temperature of the rotary steaming water bath is preferably controlled to be 35-40 ℃ during the rotary steaming; the vacuum degree during the rotary evaporation is preferably-0.1 to-0.2 Mpa, and more preferably-0.15 Mpa.
After ammonium difluorophosphate is obtained, mixing the ammonium difluorophosphate, anhydrous lithium hydroxide and an organic solvent for reaction to obtain lithium difluorophosphate; the organic solvent is ethylene glycol dimethyl ether, dimethyl carbonate or ethyl acetate, and preferably ethylene glycol dimethyl ether. In the present invention, the molar ratio of ammonium difluorophosphate to anhydrous lithium hydroxide is preferably 1: (1.03-1.08). In the present invention, the reaction temperature is preferably 25 to 30 ℃ and the reaction time is preferably 1 to 3 hours. In the present invention, it is preferable to stir the reaction while the reaction is being carried out, and the rotation speed of the stirring is preferably 500 to 1000rpm. In the present invention, the organic solvent is preferably ethylene glycol dimethyl ether. In the invention, after the reaction is finished, the reaction product is sequentially filtered and evaporated in a rotary manner. In the invention, the temperature of the rotary evaporation is preferably 35-40 ℃; the vacuum degree during the rotary evaporation is preferably-0.1 to-0.2 Mpa, and more preferably-0.15 Mpa. The chemical reaction formula for preparing lithium difluorophosphate in the invention is as follows:
LiOH+NH 4 PO 2 F 2 ＝LiPO 2 F 2 +NH 3 ↑+H 2 O
the invention provides lithium difluorophosphate prepared by any one of the methods, wherein the acid value of the lithium difluorophosphate is 8-10 ppm, and the purity is over 99.5%.
In order to further illustrate the present invention, the following embodiments are described in detail, but they should not be construed as limiting the scope of the present invention.
In the examples, the sources of the raw materials are as follows: the chemical purity grade of the ammonium fluoride is 96 percent of the chemical purity grade of the commercial product of the West Longa, the purity of the phosphorus pentoxide is 99.99 percent of the commercial product of the Meclin, the purity of the N, N-dimethylformamide is 99.8 percent of the commercial product of the Meclin, the purity of the anhydrous acetonitrile is 99.9 percent of the commercial product of the Meclin, and the analytical purity of the ethanol is 99.5 percent of the commercial product of the Allatin.
Example 1
9.17g of ammonium fluoride, 10g of phosphorus pentoxide, and 200g of N, N-dimethylformamide-anhydrous acetonitrile (the mass ratio of N, N-dimethylformamide is 5%, and the balance of 95% is anhydrous acetonitrile) were weighed, and the above-mentioned raw materials were charged into a three-necked flask (in the three-necked flask, mechanical stirring was connected directly above the flask, and a thermometer tube and a condenser tube were connected to the other two ports, respectively). Placing the three-neck flask into an oil bath heating pot, starting gradient temperature rise from normal temperature to 85 ℃, heating rate of 3 ℃/min, and reacting at constant temperature for 5h after reaching 85 ℃.
After the reaction is finished, carrying out suction filtration on the reaction product by using a sand core suction filtration device, and separating filtrate from a filter cake. The filter cake is purified by 300mL of ethanol, and the magnetic stirring speed is 600rpm for 1h. And (3) carrying out suction filtration on the materials by using a sand core funnel, and separating filtrate from a filter cake. The filtrate was transferred to a rotary evaporator bottle for rotary evaporation (the rotary evaporation temperature was 35 ℃ C., and the degree of vacuum was-0.15 MPa), to obtain 8.756g of ammonium difluorophosphate.
In a glove box (without water and oxygen), the ammonium difluorophosphate obtained above and 1.87g of anhydrous lithium hydroxide are reacted in 300mL of glycol dimethyl ether solution (a magnetic stirrer is adopted for stirring in the reaction process, and the rotating speed is 1000 rpm) for 2h. After the reaction, the reaction product was filtered with a sand core funnel, the filtrate was separated from the filter cake, and the filtrate was transferred to a rotary evaporator bottle for rotary evaporation (rotary evaporation temperature 38 ℃ C., vacuum degree-0.15 MPa), to obtain 7.92g of lithium difluorophosphate.
The prepared lithium difluorophosphate is subjected to XRD characterization, specifically shown in figure 2, and compared with a standard lithium difluorophosphate characterization spectrum (specifically shown in figure 1), and 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
8.908g of ammonium fluoride, 10g of phosphorus pentoxide, and 200g of N, N-dimethylformamide-anhydrous acetonitrile (the mass ratio of N, N-dimethylformamide is 5%, and the balance of 95% is anhydrous acetonitrile) were weighed, and the above-mentioned raw materials were put into a three-necked flask (in the three-necked flask, mechanical stirring was connected directly above the flask, and a thermometer tube and a condenser tube were connected to the other two ports, respectively). Placing the three-neck flask into an oil bath heating pot, starting gradient temperature rise from normal temperature to 85 ℃, heating rate of 3 ℃/min, and reacting at constant temperature for 5h after reaching 85 ℃.
After the reaction is finished, carrying out suction filtration on the reaction product by using a sand core suction filtration device, and separating filtrate from a filter cake. The filter cake is purified by 300mL of ethanol, and the magnetic stirring speed is 600rpm for 1h. And (3) carrying out suction filtration on the materials by using a sand core funnel, and separating filtrate from a filter cake. The filtrate was transferred to a rotary evaporator bottle for rotary evaporation (the rotary evaporation temperature was 40 ℃ C., and the degree of vacuum was-0.15 MPa), to obtain 7.863g of ammonium difluorophosphate.
In a glove box (without water and oxygen), the ammonium difluorophosphate obtained above and 1.679g of anhydrous lithium hydroxide are reacted in 300mL of glycol dimethyl ether solution (stirring is carried out by a magnetic stirrer in the reaction process, and the rotating speed is 1000 rpm) for 2h. After the reaction, the reaction product was filtered with a sand core funnel, the filtrate was separated from the filter cake, and the filtrate was transferred to a rotary evaporation bottle for rotary evaporation (rotary evaporation temperature 38 ℃ C., vacuum degree-0.15 MPa), yielding 6.802g of lithium difluorophosphate.
Example 3
8.908g of ammonium fluoride, 10g of phosphorus pentoxide and 240g of N, N-dimethylformamide-anhydrous acetonitrile (the mass ratio of N, N-dimethylformamide is 10%, and the balance is anhydrous acetonitrile) are weighed and added into a three-neck flask (in the three-neck flask, mechanical stirring is connected right above the three-neck flask, and a thermometer sleeve and a condenser tube are respectively connected to the other two necks). The three-neck flask is placed in an oil bath heating pot, the temperature is increased from normal temperature to 80 ℃ in a gradient manner, the heating rate is 3.5 ℃/min, and the constant temperature reaction is carried out for 6h after the temperature reaches 80 ℃.
After the reaction is finished, carrying out suction filtration on the reaction product by using a sand core suction filtration device, and separating filtrate from a filter cake. The filter cake is purified by 300mL of ethanol, and the magnetic stirring speed is 600rpm for 1h. And (3) carrying out suction filtration on the materials by using a sand core funnel, and separating filtrate from a filter cake. The filtrate was transferred to a rotary evaporator bottle to carry out rotary evaporation treatment (the rotary evaporation temperature was 40 ℃ C., and the degree of vacuum was-0.15 MPa), whereby 7.631g of ammonium difluorophosphate was obtained.
The ammonium difluorophosphate obtained above was reacted with 1.679g of anhydrous lithium hydroxide in 300mL of dimethyl carbonate solution in a glove box (anhydrous and oxygen-free) (stirring was carried out by a magnetic stirrer during the reaction at 1000 rpm) for 3 hours. After the reaction, the reaction product was filtered with a sand core funnel, the filtrate was separated from the filter cake, and the filtrate was transferred to a rotary evaporator bottle for rotary evaporation (rotary evaporation temperature 38 ℃ C., vacuum degree-0.15 MPa), to obtain 6.785g of lithium difluorophosphate.
Example 4
8.908g of ammonium fluoride, 10g of phosphorus pentoxide and 200g of N, N-dimethylformamide-anhydrous acetonitrile (the mass percentage of N, N-dimethylformamide is 10%, and the balance is anhydrous acetonitrile) are weighed and added into a three-neck flask (in the three-neck flask, mechanical stirring is connected right above the three-neck flask, and the other two necks are respectively connected with a thermometer sleeve and a condenser tube). Placing the three-neck flask into an oil bath heating pot, starting gradient temperature rise from normal temperature to 90 ℃, heating rate is 2.5 ℃/min, and constant temperature reaction is carried out for 4h after the temperature reaches 90 ℃.
After the reaction is finished, carrying out suction filtration on the reaction product by using a sand core suction filtration device, and separating filtrate from a filter cake. The filter cake is purified by 300mL of ethanol, and the magnetic stirring speed is 600rpm for 1h. And (3) carrying out suction filtration on the materials by using a sand core funnel, and separating filtrate from a filter cake. The filtrate was transferred to a rotary evaporator bottle to carry out rotary evaporation treatment (the rotary evaporation temperature was 40 ℃ C., and the degree of vacuum was-0.15 MPa) to obtain 7.313g of ammonium difluorophosphate.
In a glove box (without water and oxygen), the ammonium difluorophosphate obtained above and 1.679g of anhydrous lithium hydroxide are reacted in 300mL of glycol dimethyl ether solution (stirring is carried out by a magnetic stirrer in the reaction process, and the rotating speed is 1000 rpm) for 3h. After the reaction, the reaction product was filtered with a sand core funnel, the filtrate was separated from the filter cake, and the filtrate was transferred to a rotary evaporator bottle for rotary evaporation (rotary evaporation temperature 38 ℃ C., vacuum degree-0.15 MPa), to obtain 6.565g of lithium difluorophosphate.
Comparative example 1
The difference from example 1 is that only anhydrous acetonitrile was added and N, N-dimethylformamide was not added in the preparation of ammonium difluorophosphate. The other operation steps are completely the same as those of the embodiment 1, and the specific process is as follows:
9.17g of ammonium fluoride, 10g of phosphorus pentoxide and 200g of anhydrous acetonitrile were weighed, and the above-mentioned raw materials were charged into a three-necked flask (in the three-necked flask, mechanical stirring was connected directly above the flask, and a thermo-well tube and a condenser tube were connected to the other two ports, respectively). Placing the three-neck flask into an oil bath heating pot, starting gradient temperature rise from normal temperature to 85 ℃, heating rate of 3 ℃/min, and reacting at constant temperature for 5h after reaching 85 ℃.
After the reaction is finished, carrying out suction filtration on the reaction product by using a sand core suction filtration device, and separating filtrate from a filter cake. The filter cake is purified by 300mL of ethanol, and the magnetic stirring speed is 600rpm for 1h. And (3) carrying out suction filtration on the materials by using a sand core funnel, and separating filtrate from a filter cake. The filtrate was transferred to a rotary evaporator bottle to carry out rotary evaporation treatment (the rotary evaporation temperature was 35 ℃ C., the degree of vacuum was-0.15 MPa) to obtain 5.897g of ammonium difluorophosphate.
In a glove box (without water and oxygen), the ammonium difluorophosphate obtained above and 1.259g of anhydrous lithium hydroxide are reacted in 300mL of glycol dimethyl ether solution (stirring is carried out by a magnetic stirrer during the reaction, and the rotating speed is 1000 rpm) for 2h. After the reaction is finished, the reaction product is filtered by a sand core funnel, the filtrate is separated from the filter cake, and the filtrate is transferred to a rotary evaporation bottle for rotary evaporation (the rotary evaporation temperature is 38 ℃, and the vacuum degree is-0.15 Mpa), so that 4.972g of lithium difluorophosphate is obtained.
Comparative example 2
The difference from example 1 is that only N, N-dimethylformamide was added and anhydrous acetonitrile was not added in the preparation of ammonium difluorophosphate. The other operation steps are completely the same as those in the embodiment 1, and the specific process is as follows:
9.17g of ammonium fluoride, 10g of phosphorus pentoxide and 200g of N, N-dimethylformamide were weighed, and the above-mentioned raw materials were charged into a three-necked flask (in the three-necked flask, mechanical stirring was connected directly above the flask, and a thermo-well tube and a condenser tube were connected to the other two ports, respectively). The three-neck flask is placed in an oil bath heating pot, the temperature is increased from the normal temperature to 85 ℃ in a gradient manner, the heating rate is 3 ℃/min, and the constant temperature reaction is carried out for 5 hours after the temperature reaches 85 ℃.
After the reaction is finished, carrying out suction filtration on the reaction product by using a sand core suction filtration device, and separating filtrate from a filter cake. The filter cake is purified by 300mL of ethanol, and the magnetic stirring speed is 600rpm for 1h. And (3) carrying out suction filtration on the materials by using a sand core funnel, and separating filtrate from a filter cake. Transferring the filtrate to a rotary evaporation bottle for rotary evaporation treatment (the rotary evaporation temperature is 35 ℃, and the vacuum degree is-0.15 Mpa), and obtaining 1.051g of ammonium difluorophosphate.
In a glove box (without water and oxygen), the ammonium difluorophosphate obtained above and anhydrous lithium hydroxide g are reacted in 300mL of glycol dimethyl ether solution (a magnetic stirrer is adopted for stirring in the reaction process, and the rotating speed is 1000 rpm) for 2h. After the reaction, the reaction product was filtered with a sand core funnel, the filtrate was separated from the filter cake, and the filtrate was transferred to a rotary evaporation bottle for rotary evaporation (rotary evaporation temperature 38 ℃ C., vacuum degree-0.15 MPa), to obtain 0.802g of lithium difluorophosphate.
Comparative example 3
The difference from example 1 is that the ratio of N, N-dimethylformamide and anhydrous acetonitrile added in the polar aprotic organic system is different in the production of ammonium difluorophosphate. The other operation steps are completely the same as those of the embodiment 1, and the specific process is as follows:
9.17g of ammonium fluoride, 10g of phosphorus pentoxide, and 200g of N, N-dimethylformamide-anhydrous acetonitrile (the mass ratio of N, N-dimethylformamide is 50%, and the remaining 50% is anhydrous acetonitrile) were weighed, and the above-mentioned raw materials were charged into a three-necked flask (in the three-necked flask, mechanical stirring was connected directly above the flask, and a thermometer tube and a condenser tube were connected to the other two ports, respectively). The three-neck flask is placed in an oil bath heating pot, the temperature is increased from the normal temperature to 85 ℃ in a gradient manner, the heating rate is 3 ℃/min, and the constant temperature reaction is carried out for 5 hours after the temperature reaches 85 ℃.
After the reaction is finished, carrying out suction filtration on the reaction product by using a sand core suction filtration device, and separating filtrate from a filter cake. The filter cake is purified by 300mL of ethanol, and the magnetic stirring speed is 600rpm for 1h. And (3) carrying out suction filtration on the materials by using a sand core funnel, and separating filtrate from a filter cake. The filtrate was transferred to a rotary evaporator to carry out rotary evaporation treatment (the rotary evaporation temperature was 35 ℃ C., the degree of vacuum was-0.15 MPa) to obtain 5.012g of ammonium difluorophosphate.
In a glove box (without water and oxygen), the obtained ammonium difluorophosphate and anhydrous lithium hydroxide g react in 300mL glycol dimethyl ether solution (stirring by a magnetic stirrer in the reaction process, and the rotating speed is 1000 rpm) for 2h. After the reaction is finished, the reaction product is filtered by a sand core funnel, the filtrate is separated from the filter cake, and the filtrate is transferred to a rotary evaporation bottle for rotary evaporation (the rotary evaporation temperature is 38 ℃, and the vacuum degree is-0.15 Mpa), so that 4.227g of lithium difluorophosphate is obtained.
Comparative example 4
Weighing 9.17g of ammonium fluoride and 10g of phosphorus pentoxide, uniformly mixing the materials by using a double-center mixer (the rotating speed is 500rpm, the time is 10 min), repeating the steps for three times, and grinding and crushing the materials by using a copper bowl after each mixing. Transferring the uniformly mixed materials into a polytetrafluoroethylene reaction device, and connecting a corresponding temperature sensor and a tail gas absorption device; the custom reaction device was then transferred to a heating mantle and heated to 200 ℃ for 2h.
After the reaction is finished, carrying out suction filtration on the reaction product by using a sand core suction filtration device, and separating filtrate from a filter cake. The filter cake is purified by 300mL of ethanol, and the magnetic stirring speed is 600rpm for 1h. And (3) carrying out suction filtration on the materials by using a sand core funnel, and separating filtrate from a filter cake. The filtrate was transferred to a rotary evaporator (rotary evaporation temperature 35 ℃ C., vacuum degree-0.15 MPa) to obtain 3.872g of ammonium difluorophosphate.
In a glove box (without water and oxygen), the ammonium difluorophosphate obtained above and 0.766g of anhydrous lithium hydroxide are reacted in 300mL of glycol dimethyl ether solution (a magnetic stirrer is adopted for stirring in the reaction process, and the rotating speed is 1000 rpm) for 2h. After the reaction is finished, the reaction product is filtered by a sand core funnel, filtrate is separated from a filter cake, and the filtrate is transferred to a rotary evaporation bottle for rotary evaporation (the rotary evaporation temperature is 38 ℃, and the vacuum degree is-0.15 Mpa), so that 2.976g of lithium difluorophosphate is obtained.
XRD characterization is carried out on the prepared lithium difluorophosphate, and the specific result is shown in figure 3. As can be seen from FIGS. 1 to 3, although LiPO synthesized in comparative example 4 2 F 2 With LiPO 2 F 2 Standard substance and example 1 Synthesis of LiPO 2 F 2 The main peak position of the characteristic peak of the product is the same, and the peak intensity is basically the same, but three small miscellaneous peaks around the main peak of the lithium difluorophosphate synthesized in the comparative example 4 are observed, which are not the same as the LiPO synthesized in the example 1 2 F 2 High purity and 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 FIGS. 4 and 5, liPO of comparative example 4 2 F 2 The 19F NMR spectrum of (C) has four absorption peaks, which are indicated in LiPO 2 F 2 Fluorine elements in the product structure have two chemical environments, the product type is not single, and other impurities exist; as can be seen from FIG. 4, liPO 2 F 2 The 19F NMR spectrum of (E) has two absorption peaks and no other miscellaneous peaks, which is indicated in LiPO 2 F 2 The fluorine element in the product structure has only one chemical environment and does not contain other impurities. And the corresponding chemical shifts delta are-79.3658 ppm and 81.0441ppm for LiPO 2 F 2 Middle 2 fluorine atoms. Thus, the LiPO prepared in example 1 2 F 2 Has simple structure and high purity.
Comparative example 5
The difference from the embodiment 1 is that only ethylene glycol dimethyl ether is added when ammonium difluorophosphate is prepared, and other operation steps are completely the same as the embodiment 1, and the specific process is as follows:
9.17g of ammonium fluoride, 10g of phosphorus pentoxide and 200g of ethylene glycol dimethyl ether were weighed and added to a three-necked flask (in the three-necked flask, mechanical stirring was connected directly above the flask, and a thermometer tube and a condenser tube were connected to the other two ports, respectively). Placing the three-neck flask into an oil bath heating pot, starting gradient temperature rise from normal temperature to 85 ℃, heating rate of 3 ℃/min, and reacting at constant temperature for 5h after reaching 85 ℃.
After the reaction is finished, carrying out suction filtration on the reaction product by using a sand core suction filtration device, and separating filtrate from a filter cake. The filter cake is purified by 300mL of ethanol, and the magnetic stirring speed is 600rpm for 1h. And (3) carrying out suction filtration on the materials by using a sand core funnel, and separating filtrate from a filter cake. The filtrate was transferred to a rotary evaporator bottle for rotary evaporation (the rotary evaporation temperature was 35 ℃ C., and the degree of vacuum was-0.15 MPa), to obtain 0.526g of ammonium difluorophosphate.
In a glove box (without water and oxygen), the ammonium difluorophosphate obtained above and 0.112g of anhydrous lithium hydroxide are reacted in 300mL of glycol dimethyl ether solution (a magnetic stirrer is adopted for stirring in the reaction process, and the rotating speed is 1000 rpm) for 2h. After the reaction is finished, the reaction product is filtered by a sand core funnel, the filtrate is separated from the filter cake, and the filtrate is transferred to a rotary evaporation bottle for rotary evaporation (the rotary evaporation temperature is 38 ℃, and the vacuum degree is-0.15 Mpa), so that 0.302g of lithium difluorophosphate is obtained.
Performance testing
The performance of the products prepared in examples 1 to 4 and comparative examples 1 to 5 was tested, the specific test indexes were purity, moisture, acid value and yield, the specific results are shown in table 1, and the specific index test methods were as follows:
(1) And (3) purity testing: the experiment adopts an external standard method of ion chromatography to carry out purity test.
(2) And (3) moisture testing: a ammonium difluorophosphate sample is heated to a certain temperature by a cassette furnace, evaporated steam is blown into a Karl Fischer reagent in a reaction cup by dry air, and the measurement is carried out by a coulometric method.
(3) Acid value: the free acid in the sample was titrated with a standard titration solution of sodium hydroxide using a microtiter tube with bromothymol lanviar indicator.
(4) Yield: and (4) carrying out calculation according to the product result in the purity test and the reaction equation.
Table 1 results of performance testing
| 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 invention has the advantages that the yield is greatly improved compared with the method of comparative example 1 by using acetonitrile alone as a solvent, and the yield of ammonium difluorophosphate is extremely low because the comparative example 2 adopts N, N-dimethylformamide and the comparative example 5 adopts glycol dimethyl ether as a solvent, so that the yield of the final lithium difluorophosphate is also low. In comparative example 4, no solvent is added, the ammonium fluoride and the phosphorus pentoxide are directly mixed for reaction, and after white smoke occurs in the process of reaction, the temperature of the system rises to 200 ℃ and is extremely difficult to control, so that along with the decomposition of the ammonium fluoride, a small amount of metaphosphoric acid is firstly generated by the reaction of the phosphorus pentoxide and water in the air at high temperature, the toxicity is extremely high, and then the phosphorus pentoxide is rapidly changed into phosphoric acid, so that the reaction yield is low, and the comprehensive effect is poor. The lithium difluorophosphate prepared by the method provided by the invention has the advantages of high purity, high yield, simple and easy operation, and is suitable for industrial production.
The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.
Claims (10)
1. A preparation method of lithium difluorophosphate is characterized by comprising 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; the organic solvent is ethylene glycol dimethyl ether, dimethyl carbonate or ethyl acetate.
2. The method according to claim 1, wherein the amount of N, N-dimethylformamide in the polar aprotic organic system is 4 to 10% by mass.
3. The method according to claim 1, wherein the molar ratio of ammonium fluoride to phosphorus pentoxide in step 1) is (3.3 to 3.5): 1; the mass ratio of the ammonium fluoride to the polar aprotic organic system is 9.
4. The preparation method according to claim 1, wherein the temperature in step 1) is increased to 80-90 ℃ by gradient temperature increase at a rate of 2.5-3.5 ℃/min; the reaction time is 4-6 h.
5. The method according to claim 1, wherein the step 1) is further purified after completion of the reaction; the purification comprises the following steps;
(1) Filtering a reaction product obtained after the reaction is finished to obtain a filter cake;
(2) Mixing the filter cake with ethanol, and performing suction filtration on the mixture to obtain a filtrate;
(3) And carrying out rotary evaporation on the filtrate to obtain purified ammonium difluorophosphate.
6. The method according to claim 1, wherein the molar ratio of ammonium difluorophosphate to anhydrous lithium hydroxide in step 2) is 1: (1.03-1.08).
7. The preparation method according to claim 1, wherein the reaction temperature in step 2) is 25-30 ℃ and the reaction time is 1-3 h.
8. The method according to claim 1, wherein the stirring is performed during the reaction in step 2), and the rotation speed of the stirring is 500 to 1000rpm.
9. The preparation method according to claim 1, characterized in that after the reaction in step 2) is completed, the method further comprises the steps of sequentially carrying out suction filtration and rotary evaporation on the reaction product, wherein the temperature of the rotary evaporation is 35-45 ℃; the vacuum degree during the rotary steaming is-0.1 to-0.2 Mpa.
10. Lithium difluorophosphate produced by the process according to any one of claims 1 to 9, wherein the lithium difluorophosphate has an acid value of 8 to 10ppm and a purity of 99.5% or more.
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