Preparation method of battery-grade sodium monofluorophosphate

Abstract

In order to overcome the defect of lack of a preparation method of sodium monofluorophosphate applied to a battery, the invention provides a preparation method of battery-grade sodium monofluorophosphate, which comprises the following steps: adding phosphorus pentoxide and sodium oxide into a reactor and uniformly mixing; mixing sodium hexafluorophosphate with a polar solvent to obtain a mixed solution; dropwise adding the mixed solution into a reactor, heating after dropwise adding to obtain a crude product, and purifying the crude product to obtain sodium monofluorophosphate. According to the sodium monofluorophosphate product prepared by the preparation method disclosed by the invention, the main content of sodium monofluorophosphate is greater than 98%, and chloride ions are lt; the turbidity is less than or equal to 1NTU, and the chromaticity is 1t; the sodium monofluorophosphate prepared by the method has the advantages that the content of the free acid is less than 50ppm, the water content is less than 200ppm, and the sodium monofluorophosphate can reach the standard of the sodium monofluorophosphate used in the battery industry.

Description

Technical Field

[0001] The invention belongs to the technical field of sodium monofluorophosphate preparation, and particularly relates to a method for preparing battery-grade sodium monofluorophosphate. Background Art

[0002] Sodium monofluorophosphate is used as an anti-caries agent and tooth desensitizer in toothpaste formulations. Currently, most of the sodium monofluorophosphates used in toothpaste formulations are prepared by a melt-continuous method. The theoretical amount of sodium fluoride and sodium hexametaphosphate are fully mixed and evenly added to a graphite-lined reactor, heated and melted, and the continuously flowing material is cooled to obtain a finished sodium monofluorophosphate product. At present, sodium monofluorophosphates are basically toothpaste-grade products on the market. There is a lack of preparation methods for sodium monofluorophosphates used in sodium ion batteries. Compared with toothpaste-grade products, battery-grade products pay more attention to the control of the main content and the control of impurity ions. There is no national standard for battery-grade sodium monofluorophosphate, so the additives used in the battery industry are defaulted to the ordinary standard as the battery-grade sodium monofluorophosphate standard. Among them, according to the national standard GB24567-2009, the main content of toothpaste-grade sodium monofluorophosphate is 95% purity, which is far lower than the main content required for battery-grade use, and the control of impurity ions does not meet the standards used in secondary batteries. The control of battery-grade product indicators such as turbidity and chromaticity is blank. Therefore, it is necessary to develop new synthesis methods for the preparation of battery-grade sodium monofluorophosphate to meet the use requirements of secondary battery additives. Summary of the invention

[0003] In view of the above problems, the present application provides a method for preparing battery-grade sodium monofluorophosphate. The prepared sodium monofluorophosphate has a main content of >98%, chloride ions <5ppm, turbidity ≤1NTU, chromaticity <10Hazen, etc., and can be used as an additive in secondary batteries.

[0004] The technical solution adopted by the present invention to solve the above technical problems is as follows:

[0005] In one aspect, the present invention provides a method for preparing battery-grade sodium monofluorophosphate, comprising the following steps:

[0006] Add phosphorus pentoxide and sodium oxide into the reactor and mix them evenly;

[0007] mixing sodium hexafluorophosphate with a polar solvent to obtain a mixed solution;

[0008] The mixed solution is added dropwise to a reactor, and after the addition is completed, the mixture is heated to obtain a crude product, and the crude product is purified to obtain sodium monofluorophosphate.

[0009] Preferably, the molar ratio of sodium hexafluorophosphate, phosphorus pentoxide and sodium oxide is 1:(2-5):(5-7).

[0010] Preferably, the mass concentration of sodium hexafluorophosphate in the mixed solution is 5% to 8%.

[0011] Preferably, the polar solvent includes at least one of chain carbonates having 3 to 8 carbon atoms, cyclic carbonates having 3 to 8 carbon atoms, chain aliphatic esters, cyclic aliphatic esters having 3 to 8 carbon atoms, and halogenated hydrocarbons having 1 to 3 carbon atoms.

[0012] Preferably, the halogenated hydrocarbon includes one or more of dichloromethane, chloroform, tetrachloromethane, and 1,2-dichloroethane;

[0013] The chain carbonate includes one or more of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and ethyl propyl carbonate;

[0014] The cyclic carbonate includes one or more of ethylene carbonate, propylene carbonate, vinylene carbonate, fluoroethylene carbonate, catechol carbonate, and vinyl ethylene carbonate;

[0015] The chain fatty ester includes one or more of ethyl acetate, butyl acetate, ethyl butyrate, methyl acetate, ethyl formate, and propyl acetate.

[0016] Preferably, the dropping speed of the mixed solution is ≤0.2 L / min.

[0017] Preferably, the step of heating after the completion of the dropwise addition specifically includes the following steps: under normal pressure, heating step by step after the completion of the dropwise addition, the heating rate is 5°C / h to 15°C / h, heating until the reaction solution refluxes, and the reaction time is 6 to 12 hours.

[0018] Preferably, purifying the crude product comprises the following steps: filtering the crude product for the first time after the reaction is completed, collecting the filter residue and the filtrate, washing the filter residue with a polar solvent and filtering it for the second time, collecting the filtrate, and then combining the two collected filtrates for concentration, crystallization or freeze-drying to obtain sodium monofluorophosphate.

[0019] Preferably, the mass ratio of the filter residue to the polar solvent is 1:(3-10).

[0020] Preferably, the first filtration and the second filtration both use a PVDF filter membrane of 0.1 μm to 2 μm.

[0021] The preparation method of battery-grade sodium monofluorophosphate provided in the present application is prepared by using phosphorus pentoxide, sodium oxide and sodium hexafluorophosphate as reactants. The sodium monofluorophosphate product prepared by the preparation method of the present application is tested to have a main sodium monofluorophosphate content of more than 98%, chloride ions less than 5ppm, turbidity ≤1NTU, chromaticity <10Hazen, free acid less than 50ppm, and moisture less than 200ppm, which can meet the standards of sodium monofluorophosphate used in the battery industry and can be used as an additive in batteries. The sodium monofluorophosphate used can play its due battery additive performance while also avoiding problems such as battery bulging, electrode corrosion, and weakened cycle performance.

[0022] The preparation method of battery-grade sodium monofluorophosphate provided in the present application can meet the battery-grade requirements in various indicators, and the preparation method has low cost and high yield, thereby meeting the requirements of economic benefits and cost control. DETAILED DESCRIPTION

[0023] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0024] The present invention provides a method for preparing battery-grade sodium monofluorophosphate, comprising the following steps:

[0025] Add phosphorus pentoxide and sodium oxide into the reactor and mix them evenly;

[0026] mixing sodium hexafluorophosphate with a polar solvent to obtain a mixed solution;

[0027] The mixed solution is added dropwise to a reactor, and after the addition is completed, the mixture is heated to obtain a crude product, and the crude product is purified to obtain sodium monofluorophosphate.

[0028] The phosphorus pentoxide and sodium oxide in the dropping tank are both powdered at room temperature. Phosphorus pentoxide and sodium oxide are added to the reaction and mixed evenly. The reactor needs to be filled with a protective gas, which includes nitrogen or an inert gas to prevent phosphorus pentoxide from deliquescing and sodium oxide from being oxidized. It is understandable that phosphorus pentoxide and sodium oxide can be mixed first and then added to the reactor; or phosphorus pentoxide and sodium oxide can be added to the reactor first and mixed evenly in the reactor, and the mixing method can be stirring. It should be noted that the mixing of phosphorus pentoxide and sodium oxide needs to be carried out in a protective gas environment, such as mixing in a stirring container or reactor containing an inert gas, or mixing in a closed glove box; this is mainly to prevent oxidation of sodium oxide, deliquesce of phosphorus pentoxide, and reduce the impurity content of the product.

[0029] The materials selected for the reactor include but are not limited to Monel M400, Monel K500, Hastelloy C276, Inconel 600, Hastelloy G3, Hastelloy B3 and other alloys. It is understood that after adding phosphorus pentoxide and sodium oxide into the reactor, the reactor needs to be filled with high-purity anhydrous inert gas to prevent phosphorus pentoxide from deliquescing or sodium oxide from being oxidized.

[0030] Sodium hexafluorophosphate is sensitive to air and carbon dioxide. Sodium hexafluorophosphate is first dissolved in a polar solvent in a glove box and mixed to obtain a mixed solution. Sodium hexafluorophosphate is then mixed with phosphorus pentoxide and sodium oxide by dropwise addition. The reactants are fully contacted, which is beneficial to increase the reaction rate. At the same time, the generated sodium monofluorophosphate can be dissolved in the polar solvent, which is beneficial to the separation of the reactants and the product. It is understandable that the mixed solution can be added to a dropping tank, which should be filled with high-purity anhydrous inert gas to prevent the reactants from being oxidized.

[0031] Compared with the prior art, the preparation method of battery-grade sodium monofluorophosphate provided in the present application uses phosphorus pentoxide, sodium oxide, and sodium hexafluorophosphate as reactants to prepare a sodium monofluorophosphate product. After testing, it is found that the main content of sodium monofluorophosphate is greater than 98%, chloride ions are less than 5ppm, turbidity is ≤1NTU, chromaticity is less than 10Hazen, free acid is less than 50ppm, and moisture is less than 200ppm. It can meet the standards of sodium monofluorophosphate used in the battery industry and can be used as an additive in batteries. The sodium monofluorophosphate used can play its due battery additive performance while also avoiding problems such as battery bulging, electrode corrosion, and weakened cycle performance.

[0032] The preparation method of battery-grade sodium monofluorophosphate provided in the present application can meet the battery-grade requirements in various indicators, and the preparation method has low cost and high yield, thereby meeting the requirements of economic benefits and cost control.

[0033] In some embodiments, the molar ratio of sodium hexafluorophosphate, phosphorus pentoxide, and sodium oxide is 1:(2-5):(5-7).

[0034] The preparation method of battery-grade sodium monofluorophosphate provided in the present application has the following reaction equation:

[0035] 2NaPF 6 +5P 2 O 5 +11Na 2 O=12Na 2 PO 3 F

[0036] According to the reaction equation, the molar ratio of sodium hexafluorophosphate, phosphorus pentoxide, and sodium oxide is 1:(2-5):(5-7). It can be seen that the sodium hexafluorophosphate reacts completely, and the generated sodium monofluorophosphate can be dissolved in a polar solvent, while phosphorus pentoxide and sodium oxide are insoluble in polar solvents, which facilitates the subsequent separation of the generated sodium monofluorophosphate and improves the yield. If the added sodium hexafluorophosphate content is too high, the sodium hexafluorophosphate does not react completely, and then co-dissolves with the sodium monofluorophosphate during the recrystallization process, resulting in a decrease in the main content of the obtained sodium monofluorophosphate, which cannot meet the battery grade requirements. If the phosphorus pentoxide content is too high, more phosphorus pentoxide cannot be completely removed during the recrystallization stage, so the free acid of the obtained sodium monofluorophosphate increases, and when used in batteries, the battery side reactions increase, affecting the battery performance.

[0037] In some embodiments, the mass concentration of sodium hexafluorophosphate in the mixed solution is 5% to 8%.

[0038] Specifically, in the mixed solution, the mass concentration of sodium hexafluorophosphate can be 5%, 5.5%, 5.8%, 6.0%, 6.5%, 7.0%, 7.5%, 8.0%, etc., as long as the mass concentration of sodium hexafluorophosphate in the mixed solution is within the range of 5% to 8%.

[0039] In some preferred embodiments, the mass concentration of sodium hexafluorophosphate in the mixed solution is 5%.

[0040] Sodium hexafluorophosphate and sodium monofluorophosphate can both be dissolved in polar solvents. Adding polar solvents to the reaction not only facilitates full contact of reactants and increases the reaction rate, but also the generated sodium monofluorophosphate is dissolved in the polar solvent, which facilitates the separation of phosphorus pentoxide, sodium oxide and the product after the reaction, thereby increasing the purity of the sodium monofluorophosphate product and improving production efficiency.

[0041] In some embodiments, the polar solvent includes at least one of a chain carbonate having 3 to 8 carbon atoms, a cyclic carbonate having 3 to 8 carbon atoms, a chain aliphatic ester, a cyclic aliphatic ester having 3 to 8 carbon atoms, and a halogenated hydrocarbon having 1 to 3 carbon atoms;

[0042] In some preferred embodiments, the halogenated hydrocarbons include but are not limited to the following types: one or more of dichloromethane, chloroform, tetrachloromethane, and 1,2-dichloroethane;

[0043] The chain carbonate includes but is not limited to the following types: one or more of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and ethyl propyl carbonate;

[0044] The cyclic carbonate includes but is not limited to the following types: one or more of ethylene carbonate, propylene carbonate, vinylene carbonate, fluoroethylene carbonate, and vinyl carbonate;

[0045] The chain fatty esters include, but are not limited to, one or more of ethyl acetate, butyl acetate, ethyl butyrate, methyl acetate, ethyl formate, and propyl acetate.

[0046] In some embodiments, the dropping speed of the mixed solution is ≤0.2 L / min.

[0047] During the dropping process, stirring is performed while dropping, and the dropping speed and stirring speed are controlled within the above ranges, which helps to ensure full contact between sodium hexafluorophosphate, phosphorus pentoxide and sodium oxide, thereby increasing the reaction rate.

[0048] In some embodiments, the step of heating after the completion of the dropwise addition specifically includes the following steps: under normal pressure, heating is performed step by step after the completion of the dropwise addition, the heating rate is 5°C / h to 15°C / h, heating is performed until the reaction solution refluxes, and the reaction time is 6 to 12 hours.

[0049] Specifically, the heating temperature rise rate is 5°C / h to 15°C / h, that is, heating reflux at a rate of 5°C to 15°C per hour, and maintaining reflux for heating reaction. The step-by-step heating method is adopted to increase the temperature evenly and stably, which is conducive to the stability of the reaction and improves the safety performance of the reaction.

[0050] If the heating temperature is too low, the reaction cannot occur; if the heating temperature is too high, the production cost will increase. If the heating time is too short, the reaction will not be complete, and the sodium hexafluorophosphate contained will dissolve in the polar solvent, which will increase the difficulty of purifying sodium monofluorophosphate and the purification cost. If the heating time is too long, the production efficiency will decrease.

[0051] It should be noted that normal pressure is a standard atmospheric pressure, which is 101325Pa.

[0052] In some embodiments, the crude product is purified including the following steps: filtering the crude product for the first time after the reaction is completed, collecting the filter residue and the filtrate, washing the filter residue with a polar solvent and filtering it for the second time, collecting the filtrate, and then combining the two collected filtrates for concentration, crystallization or freeze-drying to obtain the finished sodium monofluorophosphate.

[0053] In some embodiments, the mass ratio of the filter residue to the polar solvent is 1:(3-10).

[0054] The mass of the polar solvent is 3 to 10 times the mass of the filter residue, so that the polar solvent can dissolve the finished sodium monofluorophosphate in the filter residue and improve the yield. If the mass of the polar solvent is too low, the amount of sodium monofluorophosphate dissolved by the polar solvent is reduced, resulting in a reduced yield of the finished sodium monofluorophosphate.

[0055] In some embodiments, the first filtration and the second filtration both use a 0.1 μm to 2 μm PVDF filter membrane.

[0056] The use of the filter membrane is helpful to filter out sodium oxide solid particles, reduce the turbidity and chromaticity of the finished sodium monofluorophosphate, and prepare sodium monofluorophosphate that can be used in batteries.

[0057] The sodium monofluorophosphate prepared in the present application has a main content (purity) of >98%, chloride ion <5ppm, turbidity ≤1NTU, chromaticity <10Hazen, free acid less than 50ppm, and water less than 200ppm, which can meet the standards of additives used in the battery industry and can be used as an additive in batteries. The sodium monofluorophosphate used can play its due battery additive performance while also avoiding problems such as bulging, electrode corrosion, and weakened cycle performance in the battery. The sodium monofluorophosphate provided in the present application is used as an additive in batteries, such as sodium ion batteries, solar cells, and other secondary batteries.

[0058] The present invention is further described below by way of examples.

[0059] Example 1

[0060] This example is used to illustrate a preparation method of battery-grade sodium monofluorophosphate and sodium monofluorophosphate disclosed in the present invention.

[0061] At 25°C, standard atmospheric pressure and inert gas conditions, weigh 213g (1.50mol) of phosphorus pentoxide and 186g (3.00mol) of sodium oxide, mix them evenly, and add them to a Monel M400 reactor. Then weigh 84g (0.50mol) of sodium hexafluorophosphate (wherein the molar ratio of sodium hexafluorophosphate: phosphorus pentoxide: sodium oxide is 1:3:6); then mix the sodium hexafluorophosphate with 1680g of propylene carbonate to prepare a 5% mixed solution, and then place the mixed solution in a dropping tank filled with an inert gas atmosphere. Connect the dropping tank to the reactor, adjust the temperature in the reactor to 15°C, and then start to drop the sodium hexafluorophosphate and propylene carbonate mixed solution at a rate of 0.1L / min, stirring while dropping. After the dropwise addition is completed, heat up step by step, heat at a rate of 10°C per hour, heat to reflux the reaction solution, the reflux temperature is 240°C, and the heating reaction time is 8 hours. After the reaction is completed, a 1 μm PVDF filter membrane is used for the first filtration, and the filter residue is collected. Then, propylene carbonate 3 times the mass of the filter residue is added for dissolution and washing, and then a 1 μm PVDF filter membrane is used for the second filtration, and the filtrate of the second filtration is collected. The filtrates collected twice are combined for room temperature nitrogen blowing crystallization to obtain battery-grade sodium monofluorophosphate.

[0062] Example 2

[0063] The difference between Example 2 and Example 1 is that the amounts of the added reactants sodium hexafluorophosphate, phosphorus pentoxide, and sodium oxide are different, and the molar ratio of the three is different. The rest is the same as Example 1.

[0064] The specific differences are as follows: in Comparative Example 1, 355 g (2.5 mol) of phosphorus pentoxide, 186 g (3 mol) of sodium oxide, and 84 g (0.50 mol) of sodium hexafluorophosphate were added, and the molar ratio of sodium hexafluorophosphate:phosphorus pentoxide:sodium oxide was 1:5:6.

[0065] Example 3

[0066] The difference between Example 3 and Example 1 is that the amounts of the added reactants sodium hexafluorophosphate, phosphorus pentoxide, and sodium oxide are different, and the molar ratio of the three is different. The rest is the same as Example 1.

[0067] The specific differences are as follows: in Comparative Example 1, 355 g (2.5 mol) of phosphorus pentoxide, 217 g (3.5 mol) of sodium oxide, and 84 g (0.50 mol) of sodium hexafluorophosphate were added, and the molar ratio of sodium hexafluorophosphate:phosphorus pentoxide:sodium oxide was 1:5:7.

[0068] Example 4

[0069] The difference between Example 4 and Example 1 is that the amounts of the added reactants sodium hexafluorophosphate, phosphorus pentoxide, and sodium oxide are different, and the molar ratio of the three is different. The rest is the same as Example 1.

[0070] The specific differences are as follows: 210 g (1.48 mol) of phosphorus pentoxide, 140 g (2.26 mol) of sodium oxide, and 84 g (0.50 mol) of sodium hexafluorophosphate were added in Example 4, and the molar ratio of sodium hexafluorophosphate:phosphorus pentoxide:sodium oxide was 1:3:4.5.

[0071] Example 5

[0072] The difference between Example 5 and Example 1 is that the filter membrane used in the second filtration is different, and the rest is the same as Example 1.

[0073] The specific difference is that the second filtration in Example 5 uses medium-speed qualitative filter paper for filtration.

[0074] Example 6

[0075] The difference between Example 6 and Example 1 is that the amounts of the added reactants sodium hexafluorophosphate, phosphorus pentoxide, and sodium oxide are different, and the molar ratio of the three is different. The rest is the same as Example 1.

[0076] The specific differences are as follows: 360 g (2.54 mol) of phosphorus pentoxide, 160 g (2.50 mol) of sodium oxide, and 84 g (0.50 mol) of sodium hexafluorophosphate were added in Example 6, and the molar ratio of sodium hexafluorophosphate:phosphorus pentoxide:sodium oxide was 1:5.1:5.

[0077] Example 7

[0078] The difference between Example 7 and Example 1 is that after the first filtration, the filter residue is collected, and then propylene carbonate 1 times the mass of the filter residue is added for dissolution and washing. The rest is the same as Example 1.

[0079] Example 8

[0080] The difference between Example 8 and Example 1 is that the heating is performed at a rate of 10°C per hour, and the heating temperature is different, and the rest is the same as Example 1. Specifically, in Example 8, the heating is performed at a rate of 10°C per hour, the heating temperature is 45°C, and the heating time is 8h. The heating temperature in Example 8 does not reach the reflux temperature of the reaction solution.

[0081] Test Method

[0082] The sodium monofluorophosphate prepared in the above examples and comparative examples was subjected to the following tests. The test results are shown in Table 1.

[0083] 1) Sodium monofluorophosphate main content test

[0084] The test was performed using an ion chromatograph, with 0.1 g of sample (sodium monofluorophosphate prepared in the above embodiment and comparative example as sample) and ultrapure water to form a 100 mL solution and the test was performed within 15 minutes. After the test was completed, the main content of sodium monofluorophosphate was calculated by spectral integration.

[0085] 2) Yield

[0086] Yield = actual amount of sodium monofluorophosphate obtained / theoretical amount of sodium monofluorophosphate × 100%

[0087] 3) Free acid

[0088] The acid-base titration method uses 0.01 mol / L sodium hydroxide solution to titrate 10 g of sodium monofluorophosphate in ice water, using bromothymol blue as an indicator. The titration endpoint is when the titrated solution changes from yellow to blue. The volume of sodium hydroxide solution consumed is recorded and calculated.

[0089] 4) The test method for moisture is: Coulomb method

[0090] Refer to GB / T6227 for coulometric determination

[0091] 5) Turbidity

[0092] Sodium monofluorophosphate was dissolved in ethyl methyl carbonate to prepare a 3% solution, which was shaken by hand until completely dissolved, and then ultrasonically treated for 10 minutes using an ultrasonic cleaner. The resulting solution was tested using a Hach turbidity meter.

[0093] 6) Chroma

[0094] Use 5 g of sodium monofluorophosphate to prepare a 10% sodium monofluorophosphate / acetonitrile solution, and compare it with a colorimetric tube to see if it is less than 10 Hazen.

[0095] 7) Iron ions

[0096] The test is conducted using the hydroxylamine hydrochloride / o-phenanthroline method and the ascorbic acid-o-phenanthroline method in accordance with the GB / T3049 standard. The description of the iron ion test process in version 1.09 of the WI-22-01-2074 document of the LT97 Raw Material Standard can be used directly.

[0097] 8) Chloride ion

[0098] Refer to GB / T9729 for chloride ion test standard and use turbidimetry for testing. You can directly use the description of chloride ion test process in version 1.09 of WI-22-01-2074 of "LT97 Raw Material Standard".

[0099] Table 1 Data table of various embodiments and comparative examples

[0100]

[0101] As shown in Table 1, compared with Example 4, Example 4 has a lower main content and a lower yield. It is speculated that the added sodium oxide is too low, and the corresponding sodium hexafluorophosphate content in the reactant is too high. It may be due to the incomplete reaction of sodium hexafluorophosphate, and then the co-dissolution phenomenon with sodium monofluorophosphate occurs during the recrystallization process, resulting in a decrease in the main content of the obtained sodium monofluorophosphate, which cannot meet the battery grade requirements. By comparing Example 1 with Example 5, the main content is reduced, the turbidity and chromaticity are high, and the standards for application in batteries cannot be met. It is speculated that the sodium oxide solid particles in the reactant are relatively small, and the use of medium-speed qualitative filter paper may allow the sodium oxide solid to pass through, thereby affecting the main content, turbidity and chromaticity of the product.

[0102] In Example 6, compared with Example 1, the molar ratio of sodium hexafluorophosphate: phosphorus pentoxide: sodium oxide is 1:5.1:5, and the amount of phosphorus pentoxide added is too much, the main content is low, and the yield is reduced. It is speculated that this is because the excess phosphorus pentoxide reacts with water, and the impurities generated after the reaction are difficult to be completely removed by subsequent purification means, resulting in a lower yield of the prepared sodium monofluorophosphate and a lower main content.

[0103] In contrast to Example 7, less polar solvent was added when dissolving and washing the filter residue in Example 1, and the yield in Example 7 was low, indicating that the solubility of sodium monofluorophosphate was poor and more solvent was required to completely dissolve it. Some of the sodium monofluorophosphate remained in the kettle residue during the reaction and was not washed out, resulting in a low yield.

[0104] Comparing Example 1 with Example 8, the heating temperature in Example 8 is lower than the reflux temperature of the reaction solution, the yield is low, the main content is low, and the turbidity and chroma are high, indicating that during the heating process, if the temperature is lower than the reflux temperature of the reaction solution, the reaction solution cannot reflux, affecting the reaction rates of sodium hexafluorophosphate, sodium oxide, and phosphorus pentoxide, increasing the impurities in the product, and the generated sodium monofluorophosphate has a low main content, high turbidity, and high chroma, and cannot be used as an additive in batteries.

[0105] By comparison with Examples 1-3, when the molar ratio of sodium hexafluorophosphate, phosphorus pentoxide and sodium oxide is in the range of 1:(2-5):(5-7), the main content of the prepared sodium monofluorophosphate is greater than 98%, the chloride ion is less than 5ppm, the turbidity is ≤1NTU, the chromaticity is less than 10Hazen, the free acid is less than 50ppm, and the water content is less than 200ppm; the standard of sodium monofluorophosphate used in the battery industry can be met, and the sodium monofluorophosphate can be used as an additive in batteries.

[0106] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing battery-grade sodium monofluorophosphate, characterized in that: The following steps are involved: Add phosphorus pentoxide and sodium oxide into the reactor and mix them evenly; mixing sodium hexafluorophosphate with a polar solvent to obtain a mixed solution; The mixed solution is added dropwise to a reactor, and after the addition is completed, the mixture is heated to obtain a crude product, and the crude product is purified to obtain sodium monofluorophosphate.

2. The method for preparing battery-grade sodium monofluorophosphate according to claim 1, wherein The molar ratio of sodium hexafluorophosphate, phosphorus pentoxide and sodium oxide is 1:(2-5):(5-7).

3. The preparation method of battery-grade sodium monofluorophosphate according to claim 2, characterized in that, The mass concentration of sodium hexafluorophosphate in the mixed solution is 5% to 8%.

4. The method for preparing battery-grade sodium monofluorophosphate according to claim 1, wherein The polar solvent includes at least one of chain carbonates with 3 to 8 carbon atoms, cyclic carbonates with 3 to 8 carbon atoms, chain aliphatic esters, cyclic aliphatic esters with 3 to 8 carbon atoms, and halogenated hydrocarbons with 1 to 3 carbon atoms.

5. The method for preparing battery-grade sodium monofluorophosphate according to claim 4, wherein The halogenated hydrocarbons include one or more of dichloromethane, trichloromethane, tetrachloromethane, and 1,2-dichloroethane; The chain carbonate includes one or more of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and ethyl propyl carbonate; The cyclic carbonate includes one or more of ethylene carbonate, propylene carbonate, vinylene carbonate, fluoroethylene carbonate, and vinyl ethylene carbonate; The chain fatty ester includes one or more of ethyl acetate, butyl acetate, ethyl butyrate, methyl acetate, ethyl formate, and propyl acetate.

6. The method for preparing battery-grade sodium monofluorophosphate according to claim 1, characterized in that: The dropping speed of the mixed solution is ≤0.2 L / min.

7. The method for preparing battery-grade sodium monofluorophosphate according to claim 1, characterized in that: The step of heating after the dropwise addition is specifically comprised of the following steps: at normal pressure, heating is performed step by step after the dropwise addition is completed, the heating rate is 5°C / h to 15°C / h, and the reaction time until the reaction solution refluxes is 6 to 12 hours.

8. The method for preparing battery-grade sodium monofluorophosphate according to claim 1, characterized in that: Purifying the crude product comprises the following steps: filtering the crude product for the first time after the reaction is completed, collecting the filter residue and the filtrate, washing the filter residue with a polar solvent and filtering it for the second time, collecting the filtrate, and then combining the two collected filtrates for concentration, crystallization or freeze-drying to obtain sodium monofluorophosphate.

9. The method for preparing battery-grade sodium monofluorophosphate according to claim 8, characterized in that: The mass ratio of the filter residue to the polar solvent is 1:(3-10).

10. The method for preparing battery-grade sodium monofluorophosphate according to claim 8, characterized in that: The first filtration and the second filtration both use 0.1 μm to 2 μm PVDF filter membranes.

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