Preparation method of lithium difluorophosphate

Abstract

The invention belongs to the field of oxysalts containing halogen and phosphorus, and particularly relates to a preparation method of lithium difluorophosphate. According to the method, phosphorus pentoxide is supplemented for reaction after the first reaction of phosphorus pentoxide and hydrofluoric acid is finished, so that the conversion of monofluorophosphoric acid to difluorophosphoric acid is promoted, the mixed acid with higher content of difluorophosphoric acid is generated, and preparation is made for improving the conversion rate of lithium difluorophosphate; and separating the monofluorophosphate solid in the form of non-lithium monofluorophosphate by using the reaction of the mixed acid and the non-lithium alkali metal source, and finally performing lithiation reaction by using the lithium source to generate the target product lithium difluorophosphate. Compared with the original process, the whole reaction design can improve the conversion rate of the lithium difluorophosphate by more than 15%, improve the utilization rate of the lithium resource by more than 50%, further improve the economy and rationality of the industrial production of the lithium difluorophosphate, and realize the double improvement of resource utilization and benefit.

Description

Technical Field

[0001] The invention belongs to the field of halogen- and phosphorus-containing oxygen-containing acid salts, and specifically relates to a preparation method of lithium difluorophosphate. Background Art

[0002] Lithium difluorophosphate (LiPO2F2) is a lithium salt with great industrial value. When used in lithium-ion batteries, it can not only partially replace the lithium salt in the electrolyte, but also form SEI film at the positive and negative electrodes, thereby improving the high and low temperature performance and cycle stability of the battery.

[0003] The preparation processes of lithium difluorophosphate mainly include: 1) difluorophosphoric anhydride approach, which mainly uses difluorophosphoric anhydride and lithium fluoride as raw materials to prepare LiPO2F2, but difluorophosphoric anhydride is expensive and is not conducive to industrial production. 2) Phosphoric anhydride approach, which mainly uses phosphoric anhydride and lithium fluoride as raw materials to prepare LiPO2F2, but the product of this process is a mixed lithium salt of LiPO2F2 and LiPF6, and the separation and purification of lithium salts are difficult. 3) Lithium hexafluorophosphate approach, for example, Japanese patent JP2005219994A uses lithium hexafluorophosphate and silicon dioxide to react in a carbonate / carboxylate solution to obtain lithium difluorophosphate and by-product silicon tetrafluoride. This approach uses lithium hexafluorophosphate, which is more expensive, as a raw material, which increases the manufacturing cost; at the same time, the separation and purification steps of the product are also relatively complicated, the production efficiency is low, and a large amount of three wastes will be generated, which does not conform to the current green, low-consumption, and efficient production trend.

[0004] The applicant disclosed a combined preparation method of difluorophosphate and monofluorophosphate in the early stage (authorization announcement date 2024.09.13, authorization announcement number CN116101996B), which is to react difluorophosphoric acid and monofluorophosphoric acid (i.e., mixed acid) with phosphorus pentoxide and hydrofluoric acid, and then the mixed acid and alkali metal source are reacted in an organic solvent, and difluorophosphate and monofluorophosphate (mixed salt) are generated after removing the solvent. The mixed salt is dissolved using a good solvent for difluorophosphate, and solid-liquid separation is performed to obtain a difluorophosphate solution and a monofluorophosphate solid. The method is characterized in that the separation and purification operation of the product is very simple, and only involves post-processing operations such as filtration, concentration and drying, and the purity of its difluorophosphate and monofluorophosphate can reach more than 99.9%, and the yield can reach more than 96%, and because the solvent used can be reused, there is no three wastes, and it shows great advantages in industrial production.

[0005] If lithium difluorophosphate is to be produced by the above method, lithium carbonate and other lithium sources are used for reaction, so that lithium difluorophosphate and lithium monofluorophosphate can be easily obtained. The disadvantage is that the reaction process of phosphorus pentoxide and hydrofluoric acid is complicated, and the content of difluorophosphoric acid and monofluorophosphoric acid obtained by conventional reaction is equivalent, and the output of lithium difluorophosphate and lithium monofluorophosphate finally obtained is basically equivalent (even lithium monofluorophosphate is slightly more). Considering the increasing shortage of lithium resources and the extremely high industrial value of lithium difluorophosphate, enterprises hope that more lithium resources can be converted into lithium difluorophosphate, so as to realize the rationalization of resource utilization and maximize production efficiency. Summary of the invention

[0006] The purpose of the present invention is to provide a method for preparing lithium difluorophosphate, so as to solve the problems of low lithium difluorophosphate conversion rate and low lithium resource utilization rate when preparing lithium difluorophosphate using existing methods of difluorophosphate and monofluorophosphate.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is:

[0008] A method for preparing lithium difluorophosphate comprises the following steps:

[0009] (1) phosphorus pentoxide and hydrofluoric acid undergo a first reaction to convert into monofluorophosphoric acid and difluorophosphoric acid; then phosphorus pentoxide is added to undergo a second reaction to promote the conversion of monofluorophosphoric acid into difluorophosphoric acid, thereby obtaining a mixed acid;

[0010] (2) Prepare lithium difluorophosphate using mixed acid as raw material.

[0011] The present invention belongs to an improved invention. After the first reaction of phosphorus pentoxide and hydrofluoric acid is completed, phosphorus pentoxide is added to react to promote the conversion of monofluorophosphoric acid to difluorophosphoric acid, and a mixed acid with a higher difluorophosphoric acid content is generated, so as to prepare for improving the conversion rate of lithium difluorophosphate; the reaction of the mixed acid and a non-lithium alkali metal source is then used to separate the monofluorophosphate solid in the form of non-monofluorolithium phosphate, and finally a lithium source is used to perform a lithiation reaction to generate the target product, lithium difluorophosphate. Compared with the original process, the entire reaction design can increase the conversion rate of lithium difluorophosphate by more than 15%, increase the utilization rate of lithium resources by more than 50%, further improve the economy and rationality of the industrial production of lithium difluorophosphate, and achieve a dual improvement in resource utilization and benefits.

[0012] Preferably, the mass of phosphorus pentoxide added in the second reaction is 1-2% of the mass of phosphorus pentoxide used in the first reaction.

[0013] More preferably, the second reaction time is 0.5 to 1 h.

[0014] Further preferably, the reaction temperature of the first reaction and the second reaction is -20°C to -10°C. More preferably, in the first reaction, the molar ratio of phosphorus pentoxide to hydrofluoric acid is 1:(3.1-4.5), and the reaction time is 3.5-4.5h.

[0015] Preferably, step (2) comprises: S1, the mixed acid and the non-lithium alkali metal source react in a good solvent for difluorophosphate, and separate the solid and liquid to obtain a difluorophosphate solution and a monofluorophosphate solid;

[0016] S2, a difluorophosphate solution and a lithium halide are subjected to a lithiation reaction, and a solid-liquid separation is performed to obtain a lithium difluorophosphate solution, and lithium difluorophosphate is obtained after removing the solvent.

[0017] The above reaction method converts monofluorophosphoric acid into non-lithium monofluorophosphate and difluorophosphoric acid into lithium difluorophosphate through differential conversion of mixed acids, thereby achieving efficient utilization of lithium resources.

[0018] Further preferably, the lithium halide is selected from one or a combination of two or more of lithium chloride, lithium bromide and lithium iodide.

[0019] Preferably, the reaction temperature of the lithiation reaction is 70-90° C., and the reaction time is 6-8 hours.

[0020] Preferably, the good solvent is one or a combination of two or more of ethylene glycol dimethyl ether, ethylene glycol diethyl ether and acetone; the amount of the good solvent used is 2.25 to 3.75 times the mass of the mixed acid.

[0021] Preferably, step (2) comprises: reacting the mixed acid and the lithium source in a good solvent of difluorophosphate, separating the solid and the liquid to obtain a lithium difluorophosphate solution and a lithium monofluorophosphate solid; removing the solvent from the lithium difluorophosphate solution to obtain lithium difluorophosphate. This method directly uses the mixed acid and the lithium source to react to obtain lithium difluorophosphate and lithium monofluorophosphate, and the preparation process is simpler, which is suitable for occasions where there is a production demand for both lithium difluorophosphate and lithium monofluorophosphate. Moreover, since the content of difluorophosphate in the mixed acid is relatively high, the output of lithium difluorophosphate can be relatively increased, thereby meeting the production demand for lithium difluorophosphate. DETAILED DESCRIPTION

[0022] The technical concept of the present invention is to add an appropriate amount of phosphorus pentoxide after the reaction of phosphorus pentoxide and hydrofluoric acid is completed, which can absorb water and hydrofluoric acid, promote the conversion of monofluorophosphoric acid to difluorophosphoric acid, and increase the content of difluorophosphoric acid in the mixed acid. This method can reduce the content of monofluorophosphoric acid and increase the content of difluorophosphoric acid, and finally achieve the concentration of the target product lithium difluorophosphate.

[0023] Furthermore, the mixed acid can convert monofluorophosphoric acid into non-lithium monofluorophosphate and difluorophosphoric acid into lithium difluorophosphate through differential conversion, thereby achieving efficient utilization of lithium resources.

[0024] The above method for preparing lithium difluorophosphate is essentially a method for preparing lithium difluorophosphate and co-producing monofluorophosphate, and the implementation steps are as follows:

[0025] 1. Phosphorus pentoxide and hydrofluoric acid undergo a first reaction to be converted into monofluorophosphoric acid and difluorophosphoric acid; then phosphorus pentoxide is added to undergo a second reaction to promote the conversion of monofluorophosphoric acid into difluorophosphoric acid, thereby obtaining a mixed acid.

[0026] The first reaction above is the prior art. In this step, the hydrofluoric acid is slightly excessive and the phosphorus pentoxide is consumed completely. The hydrofluoric acid is heated and gasified to react with the phosphorus pentoxide. The gasification temperature can be 20-30°C. The molar ratio of phosphorus pentoxide to hydrofluoric acid is 1:(3.1-4.5), and the reaction time is 3.5-4.5h.

[0027] The reaction equation involved in this step is:

[0028] P2O5+3HF→H2PO3F+HPO2F2

[0029] P2O5+4HF→2HPO2F2+H2O

[0030]

[0031] Experiments have shown that adding 1-2% of the mass of phosphorus pentoxide used in the first reaction and reacting for 0.5-1h can effectively promote the conversion of HPO2F2 to H2PO3F and increase the conversion rate by more than 15%.

[0032] 2. The mixed acid and the non-lithium alkali metal source react in a good solvent of difluorophosphate, and the solid and liquid are separated to obtain a difluorophosphate solution and a monofluorophosphate solid.

[0033] This step cleverly utilizes the solubility of difluorophosphate and monofluorophosphate, selects a good solvent for difluorophosphate (also a poor solvent for monofluorophosphate) for reaction, and effectively separates difluorophosphate and monofluorophosphate. In this step, the non-lithium alkali metal source is slightly excessive and insoluble in the reaction solvent, and after the final solid-liquid separation, the non-lithium alkali metal source (if any) enters the monofluorophosphate solid phase, thereby not affecting the purity of the difluorophosphate.

[0034] Specifically, a mixed acid can be added to an organic solvent containing a non-lithium alkali metal source at a temperature of -20 to -10°C, and then the temperature is raised to 30 to 40°C for reaction. The non-lithium alkali metal source can be one or any combination of carbonate, carboxylate, and chloride of a non-lithium alkali metal, wherein the carboxylate is one or both of oxalate and acetate. The amount of the non-lithium alkali metal source is 1.01 to 1.10 times the theoretical molar amount, and the reaction time can be 4 to 6 hours.

[0035] The good solvent is one or a combination of two or more of ethylene glycol dimethyl ether, ethylene glycol diethyl ether and acetone; the amount of the good solvent is 2.25 to 3.75 times the mass of the mixed acid. When the amount of the good solvent is small, the difluorophosphate can also be completely dissolved by heating.

[0036] The reaction equation involved in this step is:

[0037] 2HPO2F2+Me2CO3→2MePO2F2+CO2↑+H2O

[0038] H2PO3F+Me2CO3→Me2PO3F+CO2↑+H2O

[0039] or

[0040] HPO2F2+MeCl→MePO2F2+HCl↑

[0041] H2PO3F+2MeCl→Me2PO3F+2HCl↑

[0042] Wherein Me is a non-lithium alkali metal element, including Na, K, Rb, Cs, etc.

[0043] 3. The difluorophosphate solution and the lithium halide undergo lithiation reaction, and the solid-liquid separation is performed to obtain a lithium difluorophosphate solution, and the lithium difluorophosphate is obtained after removing the solvent.

[0044] Lithium halide can be expressed as LiX, where X is a halogen element other than fluorine, such as Cl, Br, I, At. The reaction temperature of the lithiation reaction is 70 to 90°C, and the reaction time is 6 to 8 hours. The amount of lithium halide used is 1.01 to 1.05 times the theoretical molar amount.

[0045] The reaction equation involved in this step is:

[0046] MePO2F2+LiX→LiPO2F2+MeX

[0047] Wherein X is a halogen element other than fluorine, including Cl, Br, and I. The reason why lithium fluoride is not used is that lithium fluoride has a stronger ionic bond than other halogen element lithium salts, and the reaction is not easy to occur.

[0048] The implementation process of the present invention is further described below in conjunction with specific embodiments.

[0049] 1. Specific embodiment of the preparation method of lithium difluorophosphate of the present invention

[0050] Example 1

[0051] The preparation method of lithium difluorophosphate of this embodiment comprises the following steps:

[0052] (1) First, 426.00 g of phosphorus pentoxide was taken into a polytetrafluoroethylene three-necked flask, and then 200.00 g of anhydrous hydrofluoric acid (equivalent to a molar ratio of phosphorus pentoxide to hydrofluoric acid of 1:3.3) was put into another polytetrafluoroethylene bottle, and after heating at 25° C., the mixture was added into the phosphorus pentoxide through an introduction tube for reaction. During the reaction, the system temperature was maintained at -20° C. to -10° C. by a high-low temperature freezing bath. After reacting for 3.5 h, 5.00 g of phosphorus pentoxide was added, and the reaction was continued for 0.5 h. A mixture of monofluorophosphoric acid and difluorophosphoric acid (608.00 g) was obtained by weighing. After nuclear magnetic resonance spectroscopy detection, the content of monofluorophosphoric acid in the mixture was 40.79%, the content of difluorophosphoric acid was 58.71%, and the content of hydrofluoric acid was 0.50%.

[0053] (2) Add 500.00 g of ethylene glycol dimethyl ether and 154.86 g of Na2CO3 to a 2L four-necked reaction flask with mechanical stirring, a thermometer, and a condenser, then lower the temperature to -15°C, slowly drop 200 g of the product obtained in step (1) into the reaction flask, raise the temperature to 35°C, and start the insulation reaction for 4 hours. After the reaction is completed, 173.26 g of sodium monofluorophosphate ointment is obtained by positive pressure filtration, and 617.32 g of filtrate is obtained. The mass of the ointment after drying is 115.51 g, the purity is 99.00%, and the yield is 97.35%.

[0054] (3) Add 50.44 g of lithium chloride to 617.32 g of the filtrate obtained in step (2) to carry out lithiation reaction at 80° C. The reaction was terminated after 7.5 h, and then positive pressure filtration was performed to obtain 85.97 g of sodium chloride ointment (which can be used to replace sodium carbonate in step (2) to react and prepare a mixed salt) and 581.80 g of filtrate; the filtrate was desolventized and dried to obtain 112.51 g of lithium difluorophosphate, with a purity of 99.96% and a yield of 92.12%.

[0055] Example 2

[0056] The preparation method of lithium difluorophosphate of this embodiment comprises the following steps:

[0057] (1) Same as step (1) in Example 1.

[0058] (2) Add 500.00 g of ethylene glycol dimethyl ether and 273.00 g of KCl to a 2L four-necked reaction bottle with mechanical stirring, a thermometer, and a condenser, then lower the temperature to -15°C, slowly drop 200 g of the product obtained in step (1) into the reaction bottle, and after the addition is completed, heat to 40°C and start heat preservation reaction for 6 hours. After the reaction is completed, 214.67 g of potassium monofluorophosphate ointment is obtained by positive pressure filtration, and 624.20 g of filtrate is obtained. The mass of the ointment after drying is 139.61 g, the purity is 99.11%, and the yield is 96.38%.

[0059] (3) Add 50.48 g of lithium chloride to 624.20 g of the filtrate obtained in step (2) to carry out lithiation reaction at 80° C. The reaction was terminated after 7.5 h, and then positive pressure filtration was performed to obtain 85.97 g of potassium chloride ointment (which can be used for the reaction in step (2) to prepare a mixed salt) and 565.20 g of filtrate; the filtrate was desolventized and dried to obtain 116.52 g of lithium difluorophosphate, with a purity of 99.98% and a yield of 95.37%.

[0060] Example 3

[0061] The preparation method of lithium difluorophosphate of this embodiment comprises the following steps:

[0062] (1) Same as step (1) in Example 1.

[0063] (2) Add 500.00 g of ethylene glycol dimethyl ether and 108.11 g of Li2CO3 to a 2L four-necked reaction bottle with mechanical stirring, a thermometer, and a condenser, then lower the temperature to -15°C, slowly drop 200 g of the product obtained in step (1) into the reaction bottle, raise the temperature to 35°C, and start the insulation reaction for 4 hours. After the reaction is completed, 139.85 g of lithium monofluorophosphate ointment is obtained by positive pressure filtration, and 603.98 g of filtrate is obtained. The mass of the ointment after drying is 89.65 g, the purity is 99.05%, and the yield is 96.37%. After the filtrate is desolventized and dried, the mass of lithium difluorophosphate obtained is 122.02 g, the purity is 99.98%, and the yield is 98.12%.

[0064] Example 4

[0065] The preparation method of lithium difluorophosphate of this embodiment comprises the following steps:

[0066] (1) First, 426.00 g of phosphorus pentoxide was taken into a polytetrafluoroethylene three-necked flask, and then 186.00 g of anhydrous hydrofluoric acid (equivalent to a molar ratio of phosphorus pentoxide to hydrofluoric acid of 1:3.1) was put into another polytetrafluoroethylene bottle, and after heating at 25° C., the mixture was added into the phosphorus pentoxide through an introduction tube for reaction. During the reaction, the system temperature was maintained at -20° C. to -10° C. by a high-low temperature freezing bath. After reacting for 4.0 h, 4.5 g of phosphorus pentoxide was added, and the reaction was continued for 0.5 h. 589.60 g of a mixture of monofluorophosphoric acid and difluorophosphoric acid was obtained by weighing. After nuclear magnetic resonance detection of the fluorine spectrum, the content of monofluorophosphoric acid in the mixture was 41.55%, the content of difluorophosphoric acid was 57.96%, and the content of hydrofluoric acid was 0.49%.

[0067] (2) Add 450.00 g of ethylene glycol dimethyl ether and 152.00 g of Na2CO3 to a 2L four-necked reaction flask with mechanical stirring, a thermometer, and a condenser, then lower the temperature to -15°C, slowly drop 200 g of the product obtained in step (1) into the reaction flask, raise the temperature to 30°C, and start heat preservation reaction for 6.0 hours. After the reaction is completed, 158.09 g of sodium monofluorophosphate ointment is obtained by positive pressure filtration, and 581.11 g of filtrate is obtained. The mass of the ointment after drying is 117.10 g, the purity is 99.08%, and the yield is 96.95%.

[0068] (3) Add 49.79 g of lithium chloride to 581.11 g of the filtrate obtained in step (2) to carry out a lithiation reaction at 70° C. The reaction was terminated after 8.0 h, and then positive pressure filtration was performed to obtain 84.86 g of sodium chloride ointment (which can be used for the reaction in step (2) to prepare a mixed salt) and 546.05 g of filtrate; the filtrate was desolventized and dried to obtain 114.70 g of lithium difluorophosphate, with a purity of 99.94% and a yield of 95.12%.

[0069] Example 5

[0070] The preparation method of lithium difluorophosphate of this embodiment comprises the following steps:

[0071] (1) First, 426.00 g of phosphorus pentoxide was taken into a polytetrafluoroethylene three-necked flask, and then 270.00 g of anhydrous hydrofluoric acid (equivalent to a molar ratio of phosphorus pentoxide to hydrofluoric acid of 1:4.5) was put into another polytetrafluoroethylene bottle, and after heating at 25° C., the mixture was added into the phosphorus pentoxide through an introduction tube for reaction. During the reaction, the system temperature was maintained at -20° C. to -10° C. by a high-low temperature freezing bath. After reacting for 4.5 h, 8.5 g of phosphorus pentoxide was added, and the reaction was continued for 1.0 h. 614.50 g of a mixture of monofluorophosphoric acid and difluorophosphoric acid was obtained by weighing. After nuclear magnetic resonance detection of the fluorine spectrum, the content of monofluorophosphoric acid in the mixture was 37.43%, the content of difluorophosphoric acid was 61.42%, and the content of hydrofluoric acid was 1.15%.

[0072] (2) Add 750.00 g of ethylene glycol dimethyl ether and 161.20 g of NaCl to a 2L four-necked reaction flask with mechanical stirring, a thermometer, and a condenser tube, then lower the temperature to -15°C, slowly drop 200 g of the product obtained in step (1) into the reaction flask, and after the dropwise addition is completed, raise the temperature to 40°C and start the heat preservation reaction for 6 hours. After the reaction is completed, 147.58 g of sodium monofluorophosphate ointment is obtained by positive pressure filtration, and 863.04 g of filtrate is obtained. The mass of the ointment after drying is 106.94 g, the purity is 96.94%, and the yield is 96.17%.

[0073] (3) Add 52.77 g of lithium chloride to 863.04 g of the filtrate obtained in step (2) to carry out a lithiation reaction at 90° C. The reaction was terminated after 7.5 h, and then positive pressure filtration was performed to obtain 91.31 g of sodium chloride ointment (which can be used for the reaction in step (2) to prepare a mixed salt) and 824.50 g of filtrate; the filtrate was desolventized and dried to obtain 122.38 g of lithium difluorophosphate, with a purity of 99.96% and a yield of 95.79%.

[0074] 2. Comparison

[0075] The preparation method of lithium difluorophosphate in this comparative example comprises the following steps:

[0076] (1) First, 426.00 g of phosphorus pentoxide was taken into a polytetrafluoroethylene three-necked flask, and then 200.00 g of anhydrous hydrofluoric acid was put into another polytetrafluoroethylene bottle, and then heated to 25° C. and added to the phosphorus pentoxide through an introduction tube for reaction. During the reaction, the system temperature was maintained at -20° C. to -10° C. by a high-low temperature freezing bath. The reaction was continued for 4 hours. After the reaction was completed, 609.46 g of a mixture of monofluorophosphoric acid and difluorophosphoric acid was weighed to obtain the mixture. After nuclear magnetic resonance detection of the fluorine spectrum, the content of monofluorophosphoric acid in the mixture was 48.98%, the content of difluorophosphoric acid was 50.12%, and the content of hydrofluoric acid was 0.09%.

[0077] (2) Add 1200.00 g of ethylene glycol dimethyl ether and 163.72 g of Na2CO3 to a 2L four-necked reaction bottle with mechanical stirring, a thermometer, and a condenser, then lower the temperature to -15°C, slowly drop 200 g of the product obtained in step (1) into the reaction bottle, raise the temperature to 35°C, and start the insulation reaction for 4 hours. After the reaction is completed, 211.14 g of sodium monofluorophosphate ointment is obtained by positive pressure filtration, and 579.44 g of filtrate is obtained. The mass of the ointment after drying is 137.10 g, the purity is 99.05%, and the yield is 96.37%.

[0078] (3) Add 43.86 g of lithium chloride to 579.44 g of the filtrate obtained in step (2) to carry out lithiation reaction at 80° C. The reaction was terminated after 7.5 h, and then positive pressure filtration was performed to obtain 74.74 g of sodium chloride ointment (which can be used to replace sodium carbonate in step (2) to react and prepare mixed salt) and 548.56 g of filtrate; the filtrate was desolventized and dried to obtain 98.99 g of lithium difluorophosphate, with a purity of 99.96% and a yield of 93.23%.

[0079] 3. Experimental Examples

[0080] The proportions of difluorophosphoric acid, monofluorophosphoric acid and hydrogen fluoride in the mixed acid obtained in each embodiment and comparative example, as well as the conversion rate of lithium resources to lithium difluorophosphate (lithium conversion rate = mass of lithium in lithium difluorophosphate / total amount of lithium added in the reaction) were compared and analyzed. The specific results are shown in Table 1.

[0081] Table 1 Analysis of the mixed acid components and lithium conversion rates of each embodiment and comparative example

[0082]

[0083] The results in Table 1 show that the content of difluorophosphoric acid in the mixed acid obtained by the methods of Examples 1 to 5 is significantly increased, and the quality of the corresponding sodium difluorophosphate obtained is also significantly higher than that of Comparative Example 1, which proves that the method of the present invention can further improve the conversion of lithium difluorophosphate while utilizing the advantages of the co-production process of difluorophosphate and monofluorophosphate, thereby increasing the yield of lithium difluorophosphate.

[0084] In addition, the methods of Example 1, Example 2, Example 4, and Example 5 further realize the utilization rate of lithium resources, and can concentrate lithium resources to produce the target product lithium difluorophosphate, which is convenient for cost control and improves the flexibility of enterprise production.

Claims

1. A method for preparing lithium difluorophosphate, characterized in that: The following steps are involved: (1) phosphorus pentoxide and hydrofluoric acid undergo a first reaction to convert into monofluorophosphoric acid and difluorophosphoric acid; then phosphorus pentoxide is added to undergo a second reaction to promote the conversion of monofluorophosphoric acid into difluorophosphoric acid, thereby obtaining a mixed acid; (2) Prepare lithium difluorophosphate using mixed acid as raw material.

2. The method for preparing lithium difluorophosphate according to claim 1, characterized in that: The mass of phosphorus pentoxide added in the second reaction is 1-2% of the mass of phosphorus pentoxide used in the first reaction.

3. The method for preparing lithium difluorophosphate according to claim 2, characterized in that: The time of the second reaction is 0.5~1h.

4. The method for preparing lithium difluorophosphate according to claim 1, characterized in that: The reaction temperatures of the first reaction and the second reaction are both -20°C to -10°C.

5. The method for preparing lithium difluorophosphate according to claim 4, characterized in that: In the first reaction, the molar ratio of phosphorus pentoxide to hydrofluoric acid is 1:(3.1-4.5), and the reaction time is 3.5-4.5 hours.

6. The method for preparing lithium difluorophosphate according to any one of claims 1 to 5, characterized in that: Step (2) comprises: S1, the mixed acid and the non-lithium alkali metal source react in a good solvent for difluorophosphate, and separate the solid and liquid to obtain a difluorophosphate solution and a monofluorophosphate solid; S2, a difluorophosphate solution and a lithium halide undergo a lithiation reaction, and solid-liquid separation is performed to obtain a lithium difluorophosphate solution, and lithium difluorophosphate is obtained after removing the solvent.

7. The method for preparing lithium difluorophosphate according to claim 6, characterized in that: The lithium halide is selected from one or a combination of two or more of lithium chloride, lithium bromide and lithium iodide.

8. The method for preparing lithium difluorophosphate according to claim 6, characterized in that: The reaction temperature of the lithiation reaction is 70-90° C., and the reaction time is 6-8 hours.

9. The method for preparing lithium difluorophosphate according to claim 6, characterized in that: The good solvent is one or a combination of two or more of ethylene glycol dimethyl ether, ethylene glycol diethyl ether and acetone; the dosage of the good solvent is 2.25 to 3.75 times the mass of the mixed acid.

10. The method for preparing lithium difluorophosphate according to any one of claims 1 to 5, characterized in that: Step (2) comprises: reacting the mixed acid and the lithium source in a good solvent of difluorophosphate, separating the solid from the liquid, and obtaining a lithium difluorophosphate solution and a lithium monofluorophosphate solid; and removing the solvent from the lithium difluorophosphate solution to obtain lithium difluorophosphate.

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