Preparation method of lithium difluorobis(oxalate)phosphate

Abstract

The invention provides a method for preparing lithium difluorobisoxalate phosphate. Lithium hexafluoroborate and oxalic acid are reacted under the action of silicon tetrachloride. A series of process parameters such as solvent, material ratio, reaction time, purification process, etc. are controlled to obtain lithium difluorobisoxalate phosphate with high yield and high purity, low water content, acidity, chloride ion concentration and cation concentration, and the prepared lithium difluorobisoxalate phosphate has high yield and high purity, and is suitable for use as an electrolyte additive for lithium ion batteries.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium ion batteries and relates to an electrolyte additive, lithium difluorobisoxalate phosphate, and specifically to a preparation method of lithium difluorobisoxalate phosphate. Background Art

[0002] Lithium-ion batteries have the advantages of high operating voltage, long cycle life, low self-discharge, and green environmental protection. They have become an important new energy source and are widely used in 3C products, power vehicles, energy storage and other fields. Due to the different working environments of batteries and the differences in the types of electrical appliances, lithium-ion batteries are diverse in shape, form, performance and other aspects. For example, the batteries used in small electrical appliances commonly seen in life are button-type, square-type, cylindrical and other different types. However, no matter what kind of lithium-ion battery it is, its basic composition is composed of four parts: positive electrode (Cathode), negative electrode (Anode), separator (Separator) and electrolyte (Electrolyte). Each part exists independently and affects each other, and together determines the overall performance of the lithium-ion battery.

[0003] Among them, the electrolyte plays an important role as a "transporter" of lithium ions in lithium-ion batteries. Therefore, as a carrier for lithium ion transportation in the internal circuit, the electrolyte needs to have the ability to efficiently conduct lithium and electronic insulation within a certain temperature range. At the same time, since the electrolyte is in direct contact with the positive and negative electrodes, its electrochemical window, chemical stability, and the interface characteristics of the positive and negative electrodes, diaphragms, etc. must also meet the requirements of use. In addition, since the operation of the battery is accompanied by redox reactions, in order to ensure battery safety and work efficiency, the electrolyte must also be inert in the electrochemical reaction, and the electrolyte needs to resist thermal, electrical, and mechanical shocks to a certain extent. Therefore, whenever the electrode material is improved, the adjustment and optimization of the electrolyte often reflects its importance, and even its irreplaceability.

[0004] At present, due to the potential difference between the positive and negative electrodes that far exceeds the water decomposition voltage, the mainstream technical route for electrolytes is a comprehensive system consisting of suitable organic solvents and lithium salts. For example, a typical lithium-ion electrolyte system used commercially is generally composed of main materials such as high-purity organic solvents, electrolyte lithium salts and necessary additives, which are configured in a specific proportion under certain conditions.

[0005] The differences of existing lithium-ion battery electrolyte products are mainly reflected in the formulas adjusted according to customer needs and the additives that improve performance. In particular, with the further improvement of performance requirements such as battery safety, charge and discharge rate, cycle life, and high voltage characteristics, the required formula complexity and the diversity of additives adapted to it will gradually increase. In particular, under the current trend of high nickel and high voltage, the research and development of additives generally lags behind the demand for electrolytes, and niche additives are not only protected by patents, but also have complex ingredients, high purity requirements, and high production difficulties, making additives one of the main technical barriers of current electrolytes. At present, the cost of additives in LFP electrolytes and ternary electrolytes accounts for 10% and 20-25% respectively. Among them, the cost of additives in ternary electrolytes accounts for a higher proportion than that of iron-lithium products. This is mainly because the commonly used ternary cathode materials in commercial applications still have some problems: such as (1) commercial high-voltage carbonate electrolytes are very prone to severe oxidative decomposition reactions on lithium metal (greater than 4.5V vs. Li / Li + ), while ether electrolytes that are stable and compatible with lithium metal negative electrodes are difficult to meet the requirements of high-voltage positive electrodes; (2) Although high-concentration electrolytes inhibit the growth of lithium dendrites, the lack of compatibility between ether-based electrolytes and high-voltage positive electrodes will still shorten the cycle life of lithium-ion batteries; (3) Ternary positive electrode particles will produce microcracks during multiple cycles (especially during deep delithiation). Such microcracks lead to the destruction of the internal electrical connection of the positive electrode material and the intensification of the electrode-electrolyte reaction, ultimately reducing the battery capacity / effective energy density.

[0006] Therefore, how to optimize electrolyte additives to optimize the composition of the electrolyte and thus improve the interfacial chemical conditions of the electrode (such as optimizing the composition and other physicochemical characteristics of the solid electrolyte interface (SEI film) at the electrode interface, increasing the electrode reversible capacity, extending the cycle life, reducing the internal resistance, and improving the conductivity of the electrolyte, etc.) has become a major challenge currently faced.

[0007] At present, lithium salts are mainly used as electrolyte additives. As electrolyte additives, lithium salts can form sediments by decomposition and deposit on the surface of positive electrode materials to form a good SEI film, inhibiting the interfacial reaction of highly oxidizing transition metal ions in contact with electrolytes and the decomposition of electrolytes under high voltages, reducing the dissolution of transition metals such as nickel and manganese, and improving the structural stability of materials. At the same time, lithium salts as additives will not only not consume the active lithium in the battery during the formation of SEI films, but also reduce the loss of corresponding capacity and improve battery performance. They are very potential lithium ion electrolyte additives. At present, some lithium salt additives containing phosphorus or boron, such as lithium difluorophosphate (LiPO2F2), lithium bis(oxalatoborate) (LiBOB), lithium difluorobis(oxalatoborate) (LiDFOP), etc., have attracted much attention because they can form a stable SEI film on the surface of high-nickel positive electrodes, making the charging voltage reach 4.3V. Since the highest occupied molecular orbital (HOMO) energy of these oxalate ion additives is usually higher than the HOMO energy of the electrolyte solvent, they are preferentially oxidized and lose electrons on the positive electrode surface, which is beneficial to improve the cycle and rate performance of the battery under high voltage. Among them, lithium difluorobis(oxalatoborate) has good thermal stability and film-forming properties, high conductivity and electrochemical window, but it is difficult to dissolve in some solvents with low dielectric constants, thus affecting the specific energy and utilization efficiency of the battery. While LiPO2F2 additives have been widely used in high energy density lithium-ion power batteries, they cannot fully meet the requirements of long cycle, high safety and low cost of high energy density lithium-ion batteries. Therefore, lithium difluorobis(oxalatoborate) has been studied as a new electrolyte lithium salt additive for use in lithium-ion batteries. This is mainly because in addition to the fact that the oxalate ions in the additive can form a stable SEI film on the surface of the high-nickel positive electrode, the F element in them has a very strong electronegativity, which enhances the antioxidant properties of the additive and weakens its anti-reduction properties. Therefore, it will undergo reduction decomposition on the negative electrode surface before other solvents. This characteristic helps to form a stable SEI interface film on the negative electrode surface and improve the cycle stability of lithium-ion batteries. At the same time, the flash point of the F-containing additive is relatively high, which can improve the flame retardant properties of the electrolyte and improve the thermal stability of lithium-ion batteries. Therefore, with the acceleration of my country's new energy vehicle development policy, the development of high-performance lithium difluorobisoxalatoborate additives has important economic and social benefits.

[0008] When lithium difluorobis(oxalato)phosphate is used in lithium-ion batteries, its purity requirements are very high, especially the chloride ion concentration, cation concentration and acidity. In order to reduce the chloride ion concentration in lithium difluorobis(oxalato)phosphate, or avoid introducing it into the raw materials, or require complex post-processing steps.

[0009] The preparation route of lithium difluorobisoxalate phosphate includes:

[0010] (1) Reaction of lithium hexafluorophosphate and oxalic acid

[0011] Chinese patent CN102216311A discloses a method for preparing LiDFBP from lithium hexafluorophosphate, silicon tetrachloride and oxalic acid. Silicon tetrachloride is introduced into the raw materials of this method, which not only increases the difficulty of post-reaction treatment, but also limits the reaction temperature due to the low boiling point of silicon tetrachloride, resulting in incomplete reaction conversion and low product purity. Chinese patent CN104334564A also discloses a method for preparing LiDFBP from lithium hexafluorophosphate, silicon tetrachloride and oxalic acid. In addition to using silicon tetrachloride, thionyl chloride is also used in the post-reaction treatment process to remove unreacted oxalic acid by chlorination and decomposition, resulting in excessive chloride ion content in the final product. Chinese patent CN109742447A discloses a method of first reacting dichlorodimethylsilane with oxalic acid to generate dimethylsilyl oxalate, and then reacting with lithium hexafluorophosphate to obtain difluorobisoxalate lithium phosphate solution in two steps; Chinese patent CN109850926A discloses a method of synthesizing LiDFBP and LiTFOP using trimethylchlorosilane, oxalic acid, lithium hexafluorophosphate and adding organic bases such as pyridine; the above methods all have the problems of relatively complex reaction steps, long reaction time, low yield, residual chloride ions, etc. Chinese patent CN108910919A discloses a method of preparing difluorobisoxalate lithium phosphate using hexamethyldisilazane, oxalic acid, and lithium hexafluorophosphate as raw materials, which will generate one equivalent of ammonia, which will further attack the product and cause side reactions, so the method is not very practical. Korean patent KR101395663B1 discloses a method for preparing high-purity LiDFBP or a mixture thereof with LiTFOP by reacting lithium hexafluorophosphate with (CH3)3SiOCOCOOSi(CH3)3 compounds (bis(trialkylsilyl) oxalate). The method has mild reaction conditions, no chloride ions are introduced, and the product is easy to purify. However, the method has poor atom economy, the raw materials are not easy to obtain and have low activity, the reaction time is long, and the result is a mixture of lithium difluorobis(oxalate)phosphate and lithium tetrafluorooxalatephosphate.

[0012] (2) Reaction of lithium hexafluorophosphate and ammonium oxalate

[0013] Chinese patent CN110845539A discloses a method for preparing a mixed solution of lithium hexafluorophosphate and ammonium oxalate using lithium hexafluorophosphate and ammonium oxalate as raw materials, and then reacting to obtain a lithium difluorobis(oxalyl)phosphate solution without introducing an initiator. This method also has the problem of side reactions between ammonium substances and raw materials and products, and the purity of the product is not high.

[0014] (3) Reaction of lithium hexafluorophosphate and oxalyl chloride

[0015] Chinese patent CN110204576A discloses a method for preparing lithium difluorobis(oxalyl)phosphate using lithium hexafluorophosphate, oxalyl chloride and N-hydroxysuccinimide as raw materials. This method still does not avoid using chlorine-containing raw materials, and is also prone to causing the problem of high chloride ion content in the product.

[0016] (4) Reaction of phosphorus pentafluoride and lithium oxalate

[0017] Chinese patent CN109956976A discloses a method for preparing lithium difluorobis(oxaloyl)phosphate by reacting phosphorus pentafluoride and lithium oxalate. The method needs to react in supercritical carbon dioxide, has a high reaction pressure, and has high equipment requirements.

[0018] (5) Reaction of phosphorus pentafluoride and anhydrous aluminum oxalate

[0019] Chinese patent CN202010506865.0A discloses a method for preparing lithium difluorobisoxalate phosphate, comprising: in an aprotic solvent, lithium hexafluorophosphate and anhydrous aluminum oxalate react to obtain lithium difluorobisoxalate phosphate; the moisture content of anhydrous aluminum oxalate is less than 500ppm. The method has high product selectivity, high product yield and good product quality. However, when lithium difluorobisoxalate phosphate is prepared by this method, the content of metal cations in the obtained lithium difluorobisoxalate phosphate is relatively high, such as the content of aluminum ions is about 14-25ppm. Summary of the invention

[0020] In view of this, the object of the present invention is to provide a method for preparing lithium difluorobis(oxalyl)phosphate.

[0021] Unless otherwise specified, the parts described in the present invention are parts by weight and the percentages described are mass percentages.

[0022] To achieve the above object, the technical scheme of the present invention is: a preparation method of lithium difluorobisoxalate phosphate, comprising the following steps: dissolving lithium hexafluoroborate with a solvent, adding oxalic acid, stirring continuously at 52-60°C at a speed of 300-500 rpm, dropping silicon tetrachloride under nitrogen protection, stirring continuously for 40-50 minutes after the dropping is completed, recovering silicon tetrafluoride gas and hydrogen chloride gas generated during the reaction by soda lime, concentrating and removing the solvent after the reaction is completed, and obtaining lithium difluorobisoxalate phosphate. The solvent is at least one of dimethyl carbonate, ethyl methyl carbonate, ethyl acetate, and diethyl carbonate.

[0023] During the experiment, it was found that the yield of using ethyl acetate as a reaction medium (solvent) was much lower than that of using dimethyl carbonate, ethyl methyl carbonate or dimethyl carbonate as a reaction medium. From the perspective of economic cost, using ethyl acetate as a reaction medium is undoubtedly the most economical when achieving the same effect. In order to reduce costs as much as possible, a mixed solution of ethyl acetate and other organic solvents is considered as a reaction medium; however, the experimental results show that once the control is not good, not only the problem of low yield will occur, but also a large amount of foam will be generated in the reaction vessel, which will not only affect the reaction, but also bring greater trouble to the separation of the product; at the same time, it will affect the color of the product. After a large number of experiments and repeated studies, the inventor found that the mass ratio of ethyl acetate to dimethyl carbonate is 2:2-3, which can effectively avoid the generation of foam, and industrial production is smoother.

[0024] Furthermore, the molar ratio of lithium hexafluorophosphate to anhydrous oxalic acid is 1:1.5-2.5.

[0025] Furthermore, the molar ratio of lithium hexafluorophosphate to silicon tetrachloride is 1:1-1.5.

[0026] A method for preparing lithium difluorobisoxalate phosphate comprises the following steps:

[0027] Lithium hexafluoroborate is dissolved in a solvent, oxalic acid is added, and the mixture is continuously stirred at a speed of 300-500 rpm at 52-60° C. Silicon tetrachloride is added dropwise under nitrogen protection, and stirring is continued for 40-50 minutes after the addition is completed. Silicon tetrafluoride gas and hydrogen chloride gas generated during the reaction are recovered by soda lime, and the solvent is concentrated and removed after the reaction is completed to obtain lithium difluorobisoxalate phosphate; the solvent is a mixed solvent with a mass ratio of ethyl acetate to dimethyl carbonate of 2:2-3; the molar ratio of lithium hexafluorophosphate to anhydrous oxalic acid is 1:1.5-2.5; and the molar ratio of lithium hexafluorophosphate to silicon tetrachloride is 1:1-1.5.

[0028] Furthermore, the method further comprises a purification step of the crude lithium difluorobisoxalate phosphate. The purification step comprises mixing the crude lithium difluorobisoxalate phosphate with an organic solvent, heating the mixture to 40-50° C. in a water bath, stirring and filtering the mixture at a speed of 300-500 rpm, and cooling and crystallizing the filtrate to obtain the lithium difluorobisoxalate phosphate; the organic solvent is a mixed solvent of acetonitrile and ethyl methyl carbonate in a mass ratio of 1:2-3.

[0029] Furthermore, in the above purification step, the weight mass ratio of the crude lithium difluorobis(oxalyl)phosphate to the organic solvent is 1:5-15, preferably 1:8-12.

[0030] Furthermore, the temperature of cooling crystallization in the above purification step is selected from 3 to 10°C, more preferably 4 to 8°C, and most preferably 4 to 6°C. The applicant has found that within the most preferred range, the chloride ion content can be significantly reduced.

[0031] A method for preparing lithium difluorobisoxalate phosphate comprises the following steps:

[0032] (1) dissolving lithium hexafluoroborate in a solvent, adding oxalic acid, stirring continuously at 52-60° C. at a speed of 300-500 rpm, adding silicon tetrachloride dropwise under nitrogen protection, and continuing stirring for 40-50 minutes after the addition is completed. Recovering silicon tetrafluoride gas and hydrogen chloride gas generated during the reaction by soda lime, and concentrating and removing the solvent after the reaction is completed to obtain lithium difluorobis(oxalyl)phosphate; the solvent is a mixed solvent of ethyl acetate and dimethyl carbonate in a mass ratio of 2:2-3; the molar ratio of lithium hexafluorophosphate to anhydrous oxalic acid is 1:1.5-2.5; the molar ratio of lithium hexafluorophosphate to silicon tetrachloride is 1:1-1.5;

[0033] (2) mixing the lithium difluorobisoxalate phosphate prepared in step (1) with an organic solvent, heating the mixture in a water bath to 40-50° C., stirring and filtering the mixture at a speed of 300-500 rpm, and cooling the filtrate to crystallize the lithium difluorobisoxalate phosphate; the organic solvent is a mixed solvent of acetonitrile and ethyl methyl carbonate in a mass ratio of 1:2-3; the weight mass ratio of the crude lithium difluorobisoxalate phosphate to the organic solvent is 1:8-12; and the cooling crystallization temperature is 4-6° C.

[0034] The beneficial effects of the present invention are as follows: the present invention provides a method for preparing lithium difluorobisoxalate phosphate, wherein lithium hexafluoroborate and oxalic acid are reacted under the action of silicon tetrachloride, a composite solvent is formed by a specific ratio of ethyl acetate and dimethyl carbonate, and a specific material ratio, reaction time and purification process are combined to prepare lithium difluorobisoxalate phosphate, which has high yield, simple post-processing process, effectively avoids the generation of foam, and makes industrial production smoother. The lithium difluorobisoxalate phosphate prepared by the present invention has high purity, low moisture, acidity, chloride ion concentration and cation concentration, and is suitable for use as an electrolyte additive for lithium ion batteries. DETAILED DESCRIPTION

[0035] The examples are provided to better illustrate the content of the present invention, but the content of the present invention is not limited to the examples. Therefore, those skilled in the art can make non-essential improvements and adjustments to the implementation scheme according to the above content of the invention, which still fall within the protection scope of the present invention.

[0036] Example 1

[0037] Add 100g of dimethyl carbonate to the reaction bottle, then add 15g of lithium hexafluorophosphate to dissolve, then add 100g of ethyl acetate, add anhydrous oxalic acid, the molar ratio of lithium hexafluorophosphate to oxalic acid is 1:2. Stir continuously at 300-400 rpm at 52-55°C, add silicon tetrachloride dropwise, the molar ratio of lithium hexafluorophosphate to silicon tetrachloride is 1:1.1, and add completely in 50 minutes. At the beginning of the dropwise addition, SiF4 and HCl gases are generated, and soda lime is used to absorb SiF4 and HCl gases. After the silicon tetrachloride is added dropwise, continue stirring for 45 minutes to terminate the reaction. Concentrate and remove the solvent to obtain lithium difluorobisoxalate phosphate.

[0038] LiDFOP was dissolved in a mixed solvent (a mixed solvent of acetonitrile and ethyl methyl carbonate in a mass ratio of 1:2), the weight mass ratio of lithium difluorobisoxalate phosphate to the organic solvent was 1:10, heated to 45°C in a water bath, stirred and filtered, and the filtrate was cooled at 4-6°C to crystallize to obtain pure lithium difluorobisoxalate phosphate, with a yield of 89.2%.

[0039] Example 2

[0040] Add 100g of dimethyl carbonate to the reaction bottle, then add 15g of lithium hexafluorophosphate to dissolve, then add 100g of ethyl acetate, add anhydrous oxalic acid, the molar ratio of lithium hexafluorophosphate to oxalic acid is 1:1.8. Stir continuously at 55-58°C at a speed of 400-500 rpm, add silicon tetrachloride dropwise, the molar ratio of lithium hexafluorophosphate to silicon tetrachloride is 1:1.2, and add completely in 45 minutes. At the beginning of the dropwise addition, SiF4 and HCl gases are generated, and soda lime is used to absorb SiF4 and HCl gases. After the silicon tetrachloride is added dropwise, continue stirring for 40 minutes to terminate the reaction. Concentrate and remove the solvent to obtain a crude product of lithium difluorobisoxalate phosphate.

[0041] The crude product of lithium difluorobisoxalate phosphate (LiDFOP) was dissolved in a mixed solvent (a mixed solvent of acetonitrile and ethyl methyl carbonate in a mass ratio of 1:3), the weight mass ratio of the crude product of lithium difluorobisoxalate phosphate to the organic solvent was 1:8, heated to 40°C in a water bath, stirred and filtered, and the filtrate was cooled and crystallized at 4-6°C to obtain the final product of lithium difluorobisoxalate phosphate, with a yield of 88.5%.

[0042] Example 3

[0043] Add 100g of dimethyl carbonate to the reaction bottle, then add 15g of lithium hexafluorophosphate to dissolve, then add 100g of ethyl acetate, add anhydrous oxalic acid, the molar ratio of lithium hexafluorophosphate to oxalic acid is 1:2.2. Stir continuously at 52-55°C at a speed of 300-400 rpm, add silicon tetrachloride dropwise, the molar ratio of lithium hexafluorophosphate to silicon tetrachloride is 1:1.3, and add completely in 40 minutes. At the beginning of the dropwise addition, SiF4 and HCl gases are generated, and soda lime is used to absorb SiF4 and HCl gases. After the silicon tetrachloride is added dropwise, continue stirring for 50 minutes to terminate the reaction. Concentrate and remove the solvent to obtain a crude lithium difluorobisoxalate phosphate.

[0044] The crude product of lithium difluorobisoxalate phosphate (LiDFOP) was dissolved in a mixed solvent (a mixed solvent of acetonitrile and ethyl methyl carbonate in a mass ratio of 1:2), the weight mass ratio of the crude product of lithium difluorobisoxalate phosphate to the organic solvent was 1:12, heated to 40°C in a water bath, stirred and filtered, and the filtrate was cooled and crystallized at 4-6°C to obtain the final product of lithium difluorobisoxalate phosphate, with a yield of 86.7%.

[0045] Example 4

[0046] Add 200g of ethyl acetate to the reaction bottle, then add 15g of lithium hexafluorophosphate to dissolve, then add 100g of ethyl acetate, add anhydrous oxalic acid, the molar ratio of lithium hexafluorophosphate to oxalic acid is 1:2.1. Stir continuously at 300-400 rpm at 52-55°C, add silicon tetrachloride dropwise, the molar ratio of lithium hexafluorophosphate to silicon tetrachloride is 1:1.1, and add dropwise in 50 minutes. SiF4 and HCl gases are generated at the beginning of the dropwise addition, and the SiF4 and HCl gases are absorbed by soda lime. After the addition of silicon tetrachloride is completed, continue stirring for 50 minutes to terminate the reaction. Concentrate and remove the solvent to obtain crude lithium difluorobisoxalate phosphate.

[0047] The crude product of lithium difluorobisoxalate phosphate (LiDFOP) was dissolved in a mixed solvent (a mixed solvent of acetonitrile and ethyl methyl carbonate in a mass ratio of 1:2), the weight mass ratio of the crude product of lithium difluorobisoxalate phosphate to the organic solvent was 1:10, heated to 45°C in a water bath, stirred and filtered, and the filtrate was cooled and crystallized at 9-10°C to obtain the final product of lithium difluorobisoxalate phosphate, with a yield of 85.1%.

[0048] Example 5

[0049] Add 100g of dimethyl carbonate to the reaction bottle, then add 15g of lithium hexafluorophosphate to dissolve, then add 100g of ethyl acetate, add anhydrous oxalic acid, the molar ratio of lithium hexafluorophosphate to oxalic acid is 1:2.1. Stir continuously at 52-55°C at a speed of 300-400 rpm, add silicon tetrachloride dropwise, the molar ratio of lithium hexafluorophosphate to silicon tetrachloride is 1:1.1, and add completely in 50 minutes. SiF4 and HCl gases are generated at the beginning of the addition, and the SiF4 and HCl gases are absorbed by soda lime. After the addition of silicon tetrachloride is completed, continue stirring for 50 minutes to terminate the reaction. Concentrate and remove the solvent to obtain lithium difluorobisoxalate phosphate with a yield of 96.3%.

[0050] Example 6

[0051] Add 200g of ethyl acetate to the reaction bottle, then add 15g of lithium hexafluorophosphate to dissolve, add anhydrous oxalic acid, the molar ratio of lithium hexafluorophosphate to oxalic acid is 1:2.1. Stir continuously at 52-55°C at a speed of 300-400 rpm, add silicon tetrachloride dropwise, the molar ratio of lithium hexafluorophosphate to silicon tetrachloride is 1:1.1, and add completely in 50 minutes. SiF4 and HCl gases are generated at the beginning of the addition, and the SiF4 and HCl gases are absorbed by soda lime. After the addition of silicon tetrachloride is completed, continue stirring for 50 minutes to terminate the reaction. Concentrate and remove the solvent to obtain lithium difluorobisoxalate phosphate with a yield of 57.4%.

[0052] Index detection

[0053] The purity, moisture content, acidity, Cl of lithium difluorobis(oxalate)phosphate prepared in Examples 1-6 were determined using the methods specified in national standard GB / T 19282-2014 and industry standard T / SHXCL-0011-2021. - , the total content of sodium ions and potassium ions (which may be introduced in the raw materials), and the content of the concentrations of other metal cations. The results are shown in Table 1.

[0054] Table 1 Lithium difluorobis(oxalyl)phosphate test results

[0055] Example purity Moisture acidity <![CDATA[Cl - ]]> <![CDATA[Na + 、K + ]]> Other metal cations Example 1 99.92％ 30ppm 120ppm ≤5ppm ≤10ppm ≤5ppm Example 2 99.91％ 38ppm 146ppm ≤5ppm ≤10ppm ≤5ppm Example 3 99.90％ 40ppm 153ppm ≤5ppm ≤10ppm ≤5ppm Example 4 98.75％ 42ppm 210ppm 39ppm 27ppm 12ppm Example 5 95.17％ 77ppm 288ppm 104ppm 27ppm 24ppm Example 6 90.16％ 83ppm 314ppm 125ppm 31ppm 46ppm

Claims

1. A method for preparing lithium difluorobis(oxalate)phosphate, comprising the following steps: (1) lithium hexafluorophosphate is dissolved in a solvent, oxalic acid is added, and the mixture is continuously stirred at 52-60° C. at a speed of 300-500 rpm. silicon tetrachloride is added dropwise under nitrogen protection, and stirring is continued for 40-50 minutes after the addition is completed. Silicon tetrafluoride gas and hydrogen chloride gas generated during the reaction are recovered by soda lime. After the reaction is completed, the solvent is concentrated and removed to obtain lithium difluorobis(oxalyl)phosphate; the solvent is a mixed solvent of ethyl acetate and dimethyl carbonate in a mass ratio of 2:2-3; the molar ratio of lithium hexafluorophosphate to anhydrous oxalic acid is 1:1.5-2.5; the molar ratio of lithium hexafluorophosphate to silicon tetrachloride is 1:1-1.5; (2) mixing the lithium difluorobisoxalate phosphate prepared in step (1) with an organic solvent, heating the mixture in a water bath to 40-50° C., stirring and filtering the mixture at a speed of 300-500 rpm, and cooling the filtrate to crystallize the lithium difluorobisoxalate phosphate; the organic solvent is a mixed solvent of acetonitrile and ethyl methyl carbonate in a mass ratio of 1:2-3; the mass ratio of the crude lithium difluorobisoxalate phosphate to the organic solvent is 1:8-12; and the cooling and crystallization temperature is 4-6° C.