A method for reducing the content of monosubstituted lithium difluorophosphite trifluoroborate in a mixture

Abstract

This invention discloses a method for reducing the content of monosubstituted difluorophosphate-based lithium trifluoroborate in a mixture. The method includes the following steps: (1) obtaining a concentrated mixture; (2) adding an organic solvent to the concentrated mixture, maintaining the temperature at -60℃ to 20℃ for 0.5h to 144h to precipitate crystals, filtering, and obtaining a solution of disubstituted difluorophosphate-based lithium trifluoroborate; wherein the organic solvent is selected from at least one of dichloromethane, trichloromethane, tetrachloroethane, isopropanol, methyl tert-butyl ether, butyl acetate, and toluene. This invention has the advantages of simple operation and the ability to freely control the content of disubstituted difluorophosphate-based lithium trifluoroborate and monosubstituted difluorophosphate-based lithium trifluoroborate in the product.

Description

Technical Field

[0001] This invention relates to the field of electrolytes, and in particular to a method for reducing the content of monosubstituted difluorophosphate-based lithium trifluoroborate in a mixture. Background Technology

[0002] Lithium salt electrolytes play a crucial role in batteries, directly affecting their electrochemical performance. In particular, lithium salt additives, such as lithium difluorophosphate and lithium difluorooxalate borate, have a modifying effect on the positive and negative electrode interfaces, effectively improving the battery's high-temperature storage and cycle stability. Developing effective novel salt products is an important direction in electrolyte research.

[0003] Patent CN102414902A discloses a novel fluorinated lithium salt compound and an electrolyte containing the novel fluorinated lithium salt. However, as can be seen from paragraphs

[0071] -

[0077] of its specification, this patent can only obtain a mixture of monosubstituted novel fluorinated lithium salt compounds (such as LiBF3(PO2F2), corresponding to the peak at position -85.6ppm (1F,d, J=961.3Hz)) and disubstituted novel fluorinated lithium salt compounds (such as LiBF2(PO2F2)2, corresponding to the peak at position -83.1ppm (1F,d, J=961.3Hz)). It cannot obtain a single substance disubstituted novel fluorinated lithium salt compound, and the patent does not disclose a method for obtaining a single substance disubstituted novel fluorinated lithium salt compound. Indeed, by repeatedly applying the patented method, the final product obtained is a mixture of monosubstituted novel fluorinated lithium salt compounds and disubstituted novel fluorinated lithium salt compounds, and the content of the monosubstituted novel fluorinated lithium salt compounds is much higher than that of the disubstituted novel fluorinated lithium salt compounds.

[0004] In the aforementioned lithium trifluoroborate with difluorophosphate, boron atoms exist in an electron-deficient form to coordinate with the highly electronegative oxygen in the difluorophosphate group, thereby improving solubility. However, boron atoms have poor compatibility with the graphite anode interface. While monosubstituted lithium trifluoroborate with difluorophosphate exhibits superior low impedance performance, its high boron content can lead to problems such as high-temperature storage gas generation and poor long-cycle performance. Therefore, disubstituted lithium trifluoroborate with difluorophosphate has a relatively low boron content, combining excellent low impedance performance, high-temperature storage gas generation, and long-cycle performance.

[0005] Therefore, reducing the content of monosubstituted products in lithium difluorophosphate trifluoroborate has practical application value in order to achieve the best battery performance. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention proposes a simple method for reducing the content of monosubstituted difluorophosphate lithium trifluoroborate in a mixture.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] A method for reducing the content of monosubstituted difluorophosphate-based lithium trifluoroborate in a mixture, the method comprising the following steps:

[0009] (1) Obtain a concentrated solution of the mixture;

[0010] (2) Add organic solvent to the concentrated mixture, keep warm at -60℃ to 20℃ for 0.5h to 144h to precipitate crystals, filter, and obtain a disubstituted difluorophosphate lithium trifluoroborate solution.

[0011] The organic solvent is selected from at least one of dichloromethane, trichloromethane, tetrachloroethane, isopropanol, methyl tert-butyl ether, butyl acetate, and toluene.

[0012] In step (1), the reaction solvent in the mixture is removed by atmospheric distillation or vacuum distillation to obtain a concentrated mixture. Since the concentrated mixture is in a solvated state, it contains the reaction solvent. Specifically, the content of the reaction solvent in the concentrated mixture is at least ≤40 wt%, preferably at least ≤36 wt%, more preferably at least ≤30 wt%, and most preferably at least ≤20 wt%.

[0013] This invention employs a crystallization method to convert monosubstituted difluorophosphate trifluoroborate in a mixture to disubstituted difluorophosphate trifluoroborate, thereby reducing the content of monosubstituted difluorophosphate trifluoroborate in the mixture. During the conversion from monosubstituted difluorophosphate trifluoroborate to disubstituted difluorophosphate trifluoroborate, LiBF4 crystals precipitate.

[0014] The method of the present invention can employ single crystallization or multiple crystallization. The content of monosubstituted difluorophosphate trifluoroborate lithium in the monomer crystallization mixture is higher than that in the mixture after multiple crystallizations. As the number of crystallizations increases, the content of monosubstituted difluorophosphate trifluoroborate lithium decreases and may even disappear.

[0015] Obtaining a disubstituted difluorophosphate-based lithium trifluoroborate solution through steps (1) and (2) of this invention constitutes single crystallization; after obtaining the disubstituted difluorophosphate-based lithium trifluoroborate solution in step (2), repeating steps (1) and (2) once more constitutes secondary crystallization; after obtaining the disubstituted difluorophosphate-based lithium trifluoroborate solution in step (2), repeating steps (1) and (2) two or more times constitutes multiple crystallization.

[0016] In step (2) of the method of the present invention, the organic solvent is preferably at least one of dichloromethane, trichloromethane, and tetrachloroethane, and more preferably dichloromethane.

[0017] In step (2) of the method of the present invention, the volume of the organic solvent is 0.2 to 10 times the volume of the concentrated mixture, preferably 0.5 to 5 times, and more preferably 1 to 2 times.

[0018] In step (2) of the method described in this invention, it is preferable to precipitate crystals at -30 to 0°C for 3 to 36 hours and then filter; more preferably, it is preferable to precipitate crystals at -20 to 0°C for 2 to 24 hours and then filter. Different precipitation times can be used to control the precipitation ratio of LiBF4. As the precipitation time increases, the amount of precipitated LiBF4 increases, and the content of monosubstituted difluorophosphate-based lithium trifluoroborate in the mixture decreases.

[0019] The mixture described in this invention can be a mixture of monosubstituted lithium difluorophosphate trifluoroborate and disubstituted lithium difluorophosphate trifluoroborate prepared from lithium difluorophosphate and boron trifluoride / boron trifluoride complex as raw materials. The reaction formula is as follows:

[0020]

[0021] In this reaction process, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, acetonitrile, and diethyl ether are used as reaction solvents. A certain amount of lithium difluorophosphate (99% purity) is added to the reaction flask, and the system is mixed evenly by stirring. The same molar amount of boron trifluoride gas, boron trifluoride dimethyl carbonate complex, or boron trifluoride diethyl ether complex is introduced into the reaction flask, and the mixture is stirred to react fully.

[0022] In one specific operation, 1 mol of lithium difluorophosphate (99% purity) is added to a reaction flask using dimethyl carbonate as a solvent, and stirring is started to mix the system evenly; then 1 mol of boron trifluoride gas is introduced into the reaction flask, and the reaction is carried out at 25°C for 24 hours to obtain the above mixture.

[0023] The mixture described in this invention can also be a mixture of monosubstituted difluorophosphate-based lithium trifluoroborate and lithium tetrafluoroborate prepared from lithium difluorophosphate and a boron trifluoride / boron trifluoride complex as raw materials and lithium tetrafluoroborate as a reaction promoter. The reaction formula is as follows:

[0024]

[0025] In this reaction process, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, acetonitrile, and diethyl ether are used as reaction solvents. A certain amount of lithium difluorophosphate and lithium tetrafluoroborate are added to the reaction flask. The molar ratio of lithium difluorophosphate to reaction promoter LiBF4 is 1:(0.02-3). Stirring is started to mix the system evenly. Boron trifluoride gas, or boron trifluoride dimethyl carbonate complex, or boron trifluoride diethyl ether complex with the same molar amount as lithium difluorophosphate is introduced into the reaction flask, and the mixture is stirred to react fully.

[0026] In one specific operation, 1 mol of lithium difluorophosphate (99% purity) and 0.5 mol of lithium tetrafluoroborate are added to a reaction flask, with dimethyl carbonate as the solvent. The system is stirred until homogeneous. Then, 1 mol of boron trifluoride gas is introduced into the reaction flask, and the reaction is carried out at 25°C for 24 hours to obtain the above mixture.

[0027] The method for reducing the content of monosubstituted difluorophosphate-based lithium trifluoroborate in a mixture according to the present invention further includes:

[0028] Step (3) remove the organic solvent by evaporation to obtain the disubstituted difluorophosphate lithium trifluoroborate product.

[0029] The crystallization method described in this invention can significantly reduce the content of monosubstituted difluorophosphate-based lithium trifluoroborate in a mixture.

[0030] In one specific embodiment, a single crystallization is employed to generate LiBF4, which precipitates in crystalline form. When the molar ratio of the precipitated LiBF4 to the monosubstituted difluorophosphate trifluoroborate in the concentrated mixture is (0.153–0.256):1, the LiBF4 is removed by filtration, and the organic solvent is then evaporated to obtain a product with a disubstituted difluorophosphate trifluoroborate content of 50–70 wt%. The specific reaction equation is as follows:

[0031]

[0032] In another specific embodiment, multiple crystallization is employed to generate LiBF4, which can precipitate in crystalline form. When the molar ratio of the precipitated LiBF4 to the monosubstituted difluorophosphate trifluoroborate in the concentrated mixture is (0.256-0.5):1, the LiBF4 is removed by filtration, and the organic solvent is then evaporated to obtain a product with a mass content of 70-100 wt% of disubstituted difluorophosphate trifluoroborate.

[0033] The disubstituted difluorophosphate-based lithium trifluoroborate product of this invention contains a mixture of reaction solvents, and the reaction solvents and the disubstituted difluorophosphate-based lithium trifluoroborate exist in a solvated state. Generally, the disubstituted difluorophosphate-based lithium trifluoroborate product contains about 10-40 wt% of the reaction solvent. For ease of calculation, the content of disubstituted difluorophosphate-based lithium trifluoroborate described below in this invention is based on a fluorinated lithium salt content of 100 wt%.

[0034] In a preferred embodiment, through multiple crystallization processes, the disubstituted difluorophosphate-based lithium trifluoroborate product contains at least 60 wt% lithium difluorophosphate-based lithium trifluoroborate. At this point, the product also contains no more than 35 wt% monosubstituted difluorophosphate-based lithium trifluoroborate and no more than 5 wt% trisubstituted difluorophosphate-based lithium trifluoroborate. The disubstituted difluorophosphate-based lithium trifluoroborate product may include 80 wt% lithium difluorophosphate-based lithium trifluoroborate, 90 wt% lithium difluorophosphate-based lithium trifluoroborate, or even more than 99 wt% lithium difluorophosphate-based lithium trifluoroborate.

[0035] The present invention also provides a lithium-ion battery electrolyte, wherein the electrolyte comprises a disubstituted difluorophosphate-based lithium trifluoroborate product obtained by any of the methods described above.

[0036] The present invention also provides a lithium-ion battery, wherein the lithium-ion battery comprises the above-described electrolyte.

[0037] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0038] This invention employs a crystallization method to reduce the content of monosubstituted difluorophosphate-based lithium trifluoroborate in the mixture, significantly increasing the content of disubstituted difluorophosphate-based lithium trifluoroborate. This reduces the proportion of boron in the mixture, improving its compatibility with the graphite anode interface when used in the electrolyte. Simultaneously, it improves the battery's high-temperature storage performance and long-cycle stability, particularly suppressing high-temperature gas generation, improving capacity retention during normal and high-temperature cycles, and reducing the increase in cycle DCIR internal resistance. Detailed Implementation

[0039] The present invention will be further described below with reference to specific embodiments, but the invention is not limited to these specific embodiments. Those skilled in the art should recognize that the present invention covers all alternatives, improvements, and equivalents that may be included within the scope of the claims.

[0040] The mixture used in this embodiment of the invention is prepared from a lithium difluorophosphate and a boron trifluoride complex, yielding monosubstituted lithium difluorophosphate trifluoroborate and disubstituted lithium difluorophosphate trifluoroborate. The specific preparation method includes the following steps: 1 mol of lithium difluorophosphate (99% purity) is added to a reaction flask, using 500 g of dimethyl carbonate as the reaction solvent, and stirring is started to mix the system uniformly; then 1 mol of the boron trifluoride dimethyl carbonate complex is added to the reaction flask, and the reaction is carried out at 25°C for 24 h. A mixture containing monosubstituted lithium difluorophosphate trifluoroborate and disubstituted lithium difluorophosphate trifluoroborate is obtained. The mixture contains 33 wt% disubstituted lithium difluorophosphate trifluoroborate and 67 wt% monosubstituted lithium difluorophosphate trifluoroborate.

[0041] Example 1

[0042] This embodiment provides a method for reducing the content of monosubstituted difluorophosphate-based lithium trifluoroborate in a mixture, specifically including the following steps:

[0043] S1. In a drying room with a dew point of -40°C, the mixture is subjected to rotary evaporation using a rotary evaporator to obtain a concentrated mixture (dimethyl carbonate content of approximately 15 wt%).

[0044] S2. Add dichloromethane to the concentrated mixture at a volume ratio of 1:1, keep warm at -20℃ for 24 hours to precipitate crystals, filter, and obtain a solution of disubstituted difluorophosphate-based lithium trifluoroborate, denoted as composition 1.

[0045] The NMR F-spectrum is shown below:

[0046] Monosubstituted difluorophosphate-based lithium trifluoroborate: δ = -88.87 ppm (d, J = 958.8 Hz), δ = -154.54 ppm (s)

[0047] Disubstituted difluorophosphate-based lithium trifluoroborate: δ = -88.62 ppm (d, J = 963.5 Hz), δ = -147.90 ppm (s)

[0048] The NMR P-spectrum is shown below:

[0049] Monosubstituted difluorophosphate-based lithium trifluoroborate: δ = -29.08 ppm (t, J = 958.8 Hz)

[0050] Disubstituted difluorophosphate-based lithium trifluoroborate: δ = -30.47 ppm (t, J = 963.5 Hz)

[0051] Integrating the area of ​​the P-spectrum NMR peak, composition 1 was found to contain 61.6 wt% disubstituted difluorophosphate lithium trifluoroborate and 38.4 wt% monosubstituted difluorophosphate lithium trifluoroborate. Furthermore, area integration calculations using the F-spectrum showed consistent results and high reliability.

[0052] Example 2

[0053] The operation of this embodiment is the same as that of embodiment 1, except that the mixture is subjected to three crystallizations. Specifically, after obtaining a disubstituted difluorophosphate lithium trifluoroborate solution by crystallization once, steps S1 and S2 are repeated for a second crystallization; after obtaining a disubstituted difluorophosphate lithium trifluoroborate solution by crystallization twice, steps S1 and S2 are repeated for a third crystallization. The obtained disubstituted difluorophosphate lithium trifluoroborate solution is referred to as composition 2.

[0054] The NMR F-spectrum is shown below:

[0055] Monosubstituted difluorophosphate-based lithium trifluoroborate: δ = -88.87 ppm (d, J = 958.8 Hz), δ = -154.54 ppm (s)

[0056] Disubstituted difluorophosphate-based lithium trifluoroborate: δ = -88.62 ppm (d, J = 963.5 Hz), δ = -147.90 ppm (s)

[0057] Trisubstituted difluorophosphate-based lithium trifluoroborate: δ = -88.08 ppm (d, J = 972.8 Hz), δ = -143.46 ppm (s)

[0058] The NMR P-spectrum is shown below:

[0059] Monosubstituted difluorophosphate-based lithium trifluoroborate: δ = -29.37 ppm (t, J = 958.8 Hz)

[0060] Disubstituted difluorophosphate-based lithium trifluoroborate: δ = -30.47 ppm (t, J = 963.5 Hz)

[0061] Trisubstituted difluorophosphate lithium trifluoroborate: δ = -31.83 ppm (t, J = 972.8 Hz)

[0062] Integrating the area of ​​the P-spectrum NMR peak, composition 2 was found to contain 89.2 wt% disubstituted difluorophosphate lithium trifluoroborate, 8.7 wt% monosubstituted difluorophosphate lithium trifluoroborate, and 2.1 wt% trisubstituted difluorophosphate lithium trifluoroborate. Furthermore, area integration calculations using the F-spectrum showed consistent results and high reliability.

[0063] Example 3

[0064] The operation of this embodiment is the same as that of embodiment 1, except that in step S2, the crystallization temperature is increased to 5°C, while other conditions remain unchanged. The resulting disubstituted difluorophosphate-based lithium trifluoroborate solution is referred to as composition 3.

[0065] NMR spectra (P- and F- spectra) were obtained. Integrating the peak areas of the P-spectrum revealed that composition 3 contained 53.8 wt% disubstituted difluorophosphate lithium trifluoroborate and 46.2 wt% monosubstituted difluorophosphate lithium trifluoroborate. Area integration of the F-spectrum also yielded consistent results, indicating high reliability of the calculations.

[0066] Example 4

[0067] The operation of this embodiment is the same as that of embodiment 1, except that in step S2, methyl tert-butyl ether is used instead of dichloromethane, and other conditions remain unchanged. The resulting disubstituted difluorophosphate lithium trifluoroborate solution is referred to as composition 4.

[0068] NMR spectra (P- and F- spectra) were obtained. Integrating the peak areas of the P-spectrum revealed that composition 4 contained 43.8 wt% disubstituted difluorophosphate lithium trifluoroborate and 56.2 wt% monosubstituted difluorophosphate lithium trifluoroborate. Area integration of the F-spectrum showed consistent results and high reliability.

[0069] Example 5

[0070] The operation of this embodiment is the same as that of embodiment 1, except that in step S2, tetrachloroethane is used instead of dichloromethane, and other conditions remain unchanged. The resulting disubstituted difluorophosphate lithium trifluoroborate solution is referred to as composition 5.

[0071] NMR spectra (P- and F- spectra) were obtained. Integrating the peak areas of the P-spectrum revealed that composition 5 contained 58.7 wt% disubstituted difluorophosphate lithium trifluoroborate and 41.3 wt% monosubstituted difluorophosphate lithium trifluoroborate. Area integration of the F-spectrum also yielded consistent results, indicating high reliability of the calculations.

[0072] Example 6

[0073] The operation of this embodiment is the same as that of embodiment 1, except that in step S2, dichloromethane is added to the concentrated mixture at a volume ratio of 1:10, and other conditions remain unchanged. The resulting disubstituted difluorophosphate lithium trifluoroborate solution is referred to as composition 6.

[0074] NMR spectra (P- and F- spectra) were obtained. Integrating the peak areas of the P-spectrum revealed that composition 6 contained 66.4 wt% disubstituted difluorophosphate lithium trifluoroborate and 43.6 wt% monosubstituted difluorophosphate lithium trifluoroborate. Area integration calculations using the F-spectrum showed consistent results and high reliability.

[0075] Example 7

[0076] The operation of this embodiment is the same as that of embodiment 1, except that the mixture is subjected to six crystallizations. Specifically, after obtaining a disubstituted difluorophosphate lithium trifluoroborate solution by one crystallization, steps S1 and S2 are repeated five times, for a total of six crystallization operations. The obtained disubstituted difluorophosphate lithium trifluoroborate solution is referred to as composition 7.

[0077] NMR spectra (P- and F- spectra) were obtained. Integrating the peak areas of the P-spectrum revealed that composition 7 contained 95.6 wt% disubstituted difluorophosphate lithium trifluoroborate, 1.2 wt% monosubstituted difluorophosphate lithium trifluoroborate, and 3.2 wt% trisubstituted difluorophosphate lithium trifluoroborate. Area integration calculations using the F-spectrum showed consistent results and high reliability.

[0078] Comparative Example 1

[0079] This comparative example provides a method for reducing the content of monosubstituted difluorophosphate-based lithium trifluoroborate in a mixture, specifically including the following steps:

[0080] S1. In a drying room with a dew point of -40°C, the mixture is subjected to rotary evaporation using a rotary evaporator to obtain a concentrated mixture (dimethyl carbonate content of approximately 15 wt%).

[0081] S2. Dimethyl carbonate was added to the concentrated mixture at a volume ratio of 1:1, and the mixture was kept at -20°C for 24 hours. No crystals were observed to precipitate, and a solution of disubstituted difluorophosphate lithium trifluoroborate was obtained, which was denoted as composition D1.

[0082] NMR spectra (P- and F- spectra) were obtained. Integrating the peak areas of the P-spectrum revealed that composition D1 contained 33 wt% disubstituted lithium difluorophosphate trifluoroborate and 67 wt% monosubstituted lithium difluorophosphate trifluoroborate. Area integration of the F-spectrum also yielded consistent results, indicating high reliability of the calculations.

[0083] Comparative Example 2

[0084] The operation of this comparative example is the same as that of Example 1, except that in step S2, ethanol is used instead of dichloromethane, and other conditions remain unchanged. The resulting disubstituted difluorophosphate lithium trifluoroborate solution is denoted as composition D2.

[0085] NMR spectra (P- and F- spectra) were obtained. Integrating the peak areas of the P-spectrum revealed that composition D2 contained 35.2 wt% disubstituted difluorophosphate lithium trifluoroborate and 64.8 wt% monosubstituted difluorophosphate lithium trifluoroborate. Area integration calculations using the F-spectrum showed consistent results and high reliability.

[0086] Comparative Example 3

[0087] The operation of this comparative example is the same as that of Example 1, except that in step S2, diethyl ether is used instead of dichloromethane, and other conditions remain unchanged. The resulting disubstituted difluorophosphate lithium trifluoroborate solution is denoted as composition D3.

[0088] NMR spectra (P- and F- spectra) were obtained. Integrating the peak areas of the P-spectrum revealed that composition D3 contained 31.7 wt% disubstituted difluorophosphate lithium trifluoroborate and 68.3 wt% monosubstituted difluorophosphate lithium trifluoroborate. Area integration of the F-spectrum also yielded consistent results, indicating high reliability of the calculations.

[0089] Comparative Example 4

[0090] The operation of this comparative example is the same as that of Example 1, except that in step S2, the crystallization temperature is increased to 25°C, while other conditions remain unchanged. The resulting disubstituted difluorophosphate lithium trifluoroborate solution is referred to as composition D4.

[0091] NMR spectra (P- and F- spectra) were obtained. Integrating the peak areas of the P-spectrum revealed that composition D4 contained 38.8 wt% disubstituted difluorophosphate lithium trifluoroborate and 61.2 wt% monosubstituted difluorophosphate lithium trifluoroborate. Area integration of the F-spectrum also yielded consistent results, indicating high reliability of the calculations.

[0092] Comparative Example 5

[0093] The operation of this comparative example is the same as in Example 1, except that in step S1, the mixture is rotary evaporated using a rotary evaporator to obtain a concentrated mixture (the reaction solvent content is approximately 50%). Other conditions remain unchanged, and the resulting disubstituted difluorophosphate-based lithium trifluoroborate solution is designated as composition D5.

[0094] NMR spectra (P- and F- spectra) were obtained. Integrating the peak areas of the P-spectrum revealed that composition D5 contained 41.3 wt% disubstituted difluorophosphate lithium trifluoroborate and 58.7 wt% monosubstituted difluorophosphate lithium trifluoroborate. Area integration calculations using the F-spectrum showed consistent results and high reliability.

Claims

1. A method for reducing the content of monosubstituted difluorophosphate-based lithium trifluoroborate in a mixture, characterized in that: The method includes the following steps: (1) Obtain a concentrated solution of the mixture; The mixture is a mixture of monosubstituted lithium difluorophosphate trifluoroborate and disubstituted lithium difluorophosphate trifluoroborate prepared from lithium difluorophosphate and boron trifluoride / boron trifluoride complex as raw materials; Alternatively, the mixture is prepared by using lithium difluorophosphate and boron trifluoride / boron trifluoride complex as raw materials and lithium tetrafluoroborate as a reaction promoter to obtain a mixture of monosubstituted difluorophosphate-based lithium trifluoroborate and lithium tetrafluoroborate; The reaction solvent in the mixture is removed by atmospheric distillation or vacuum distillation to obtain a concentrated solution of the mixture. (2) Add organic solvent to the concentrated mixture, keep warm at -60℃ to 20℃ for 0.5h to 144h to precipitate crystals, filter, and obtain a disubstituted difluorophosphate lithium trifluoroborate solution. The organic solvent is selected from at least one of dichloromethane, trichloromethane, tetrachloroethane, isopropanol, methyl tert-butyl ether, butyl acetate, or toluene.

2. The method for reducing the content of monosubstituted difluorophosphate-based lithium trifluoroborate in a mixture according to claim 1, characterized in that: After obtaining the disubstituted difluorophosphate lithium trifluoroborate solution in step (2), repeat steps (1) and (2) once more.

3. The method for reducing the content of monosubstituted difluorophosphate-based lithium trifluoroborate in a mixture according to claim 1, characterized in that: After obtaining the disubstituted difluorophosphate lithium trifluoroborate solution in step (2), continue to repeat steps (1) and (2) two or more times.

4. The method for reducing the content of monosubstituted difluorophosphate-based lithium trifluoroborate in a mixture according to claim 1, characterized in that: The volume of the organic solvent is 0.2 to 10 times the volume of the concentrated mixture.

5. The method for reducing the content of monosubstituted difluorophosphate-based lithium trifluoroborate in a mixture according to claim 1, characterized in that: In step (2), crystals are precipitated by keeping the temperature at -20 to 0℃ for 2 to 24 hours.

6. The method for reducing the content of monosubstituted difluorophosphate-based lithium trifluoroborate in a mixture according to any one of claims 1-5, characterized in that: The method further includes: (3) The organic solvent was evaporated to obtain the disubstituted difluorophosphate lithium trifluoroborate product.

7. The method for reducing the content of monosubstituted difluorophosphate-based lithium trifluoroborate in a mixture according to claim 6, characterized in that: Based on a total fluorinated lithium salt content of 100 wt%, the disubstituted difluorophosphate-based lithium trifluoroborate product contains at least 60 wt% disubstituted difluorophosphate-based lithium trifluoroborate.

8. The method for reducing the content of monosubstituted difluorophosphate-based lithium trifluoroborate in a mixture according to claim 7, characterized in that: Based on a total fluorinated lithium salt content of 100 wt%, the disubstituted difluorophosphate lithium trifluoroborate product contains no more than 35 wt% monosubstituted difluorophosphate lithium trifluoroborate and no more than 5 wt% trisubstituted difluorophosphate lithium trifluoroborate.

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