Method for producing difluorophosphate salts

Abstract

To provide a new production method by which a difluorophosphate can be easily produced using a material that is easy to handle without producing strong acidic substances such as hydrochloric acid as a by-product.SOLUTION: A method for producing a difluorophosphate in which a difluorophosphate of a first alkali metal is reacted with a fluoride of a second alkali metal having a larger ionic radius than that of the first alkali metal in a solvent in which the difluorophosphate and the fluoride can be dissolved to obtain the difluorophosphate of the second alkali metal. Preferably, the first alkali metal is at least one selected from lithium and sodium, and the second alkali metal is at least one selected from sodium, potassium, rubidium and cesium.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for producing a difluorophosphate salt used as an additive for batteries such as lithium ion secondary batteries. [Background technology]

[0002] Difluorophosphates of alkali metals other than lithium, such as sodium and cesium, are used as additives for lithium ion secondary batteries and sodium ion batteries (Patent Documents 1 and 2). To date, the following methods have been known for producing difluorophosphates of alkali metals such as sodium and cesium.

[0003] For example, Patent Document 3 describes a method for producing a difluorophosphate, which comprises reacting at least one salt selected from the group consisting of halides, carbonates, borates, phosphates, hydroxides, and oxides of alkali metals, alkaline earth metals, or oniums, one phosphorus compound selected from the group consisting of phosphorus oxychlorides and chlorides, water, and hydrogen fluoride.

[0004] In addition, in Non-Patent Document 1, difluorophosphoric acid is reacted with cesium chloride to produce cesium difluorophosphate. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] US10770749B2 [Patent Document 2] Brochure for WO2014 / 161746 [Patent Document 3] Brochure for WO2016 / 017389 [Non-patent literature]

[0006] [Non-Patent Document 1] Spectrochimica Acta, Vol. 38A, No. 7, pp. 785-790, 1982 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the production method described in Non-Patent Document 1 generates hydrochloric acid, which is a strong acid, as a by-product, and the removal operation is complicated, and difluorophosphoric acid, which is difficult to handle, is used, so there is a demand for a production method of difluorophosphoric acid that does not produce a strong acidic by-product and uses easy-to-handle raw materials. Also, as for Patent Document 3, there is a demand for a new method for producing difluorophosphates of alkali metals other than lithium, which can simply produce difluorophosphates using easy-to-handle raw materials.

[0008] Therefore, an object of the present invention is to solve the above-mentioned problems and to provide a new production method that can simply produce a difluorophosphate using easy-to-handle raw materials without producing a strongly acidic substance such as hydrochloric acid as a by-product. [Means for solving the problem]

[0009] The present inventors have conducted extensive research to solve the above problems. As a result, they have found that the above problems can be solved by reacting a difluorophosphate of a first alkali metal with a fluoride of a second alkali metal having a larger ionic radius than the first alkali metal in a solvent capable of dissolving them, and have completed the present invention. That is, the present invention provides the following.

[0010] [1] A method for producing a difluorophosphate, comprising reacting a difluorophosphate of a first alkali metal with a fluoride of a second alkali metal having an ionic radius larger than that of the first alkali metal in a solvent capable of dissolving the difluorophosphate and the fluoride, to obtain a difluorophosphate of the second alkali metal.

[0011] [2] The first alkali metal is at least one selected from lithium and sodium; The manufacturing method according to [1], wherein the second alkali metal is at least one selected from sodium, potassium, rubidium, and cesium.

[0012] [3] The method according to [1] or [2], wherein a polar solvent in which the solubility of the first alkali metal fluoride is 5 g / 100 g of solvent or less at the reaction temperature is used as the solvent.

[0013] [4] The method according to any one of [1] to [3], wherein a polar solvent in which the solubility of a first alkali metal difluorophosphate is 1 g / 100 g of solvent or more at the reaction temperature and the solubility of a second alkali metal fluoride is 0.1 g / 100 g of solvent or more at the reaction temperature is used as the solvent.

[0014] [5] The method according to any one of [1] to [4], wherein the solvent is at least one polar solvent selected from water, alcohols, and alkylene glycol dialkyl ethers.

[0015] [6] The method according to any one of [1] to [5], wherein the first alkali metal fluoride precipitated in the solvent by the reaction is removed by filtration.

[0016] [7] The method according to any one of [1] to [6], wherein the reaction temperature is 5°C or higher and 80°C or lower. Effect of the Invention

[0017] According to the method for producing a difluorophosphate of the present invention, a difluorophosphate can be simply produced using raw materials that are easy to handle, such as an alkali metal difluorophosphate or an alkali metal fluoride, without producing by-products of strongly acidic substances that are difficult to handle, such as hydrochloric acid. The method of the present invention uses inexpensive raw materials such as lithium or sodium difluorophosphate and alkali metal fluorides, and the by-products are alkali metal fluorides, which can be easily removed by filtration or other procedures, making the purification procedure easy. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The method for producing a difluorophosphate of the present invention will be described in detail below based on preferred embodiments, but the present invention is not limited to these.

[0019] First, the method for producing a difluorophosphate of the present invention will be described. One of the features of the production method of the present invention is that a difluorophosphate of a first alkali metal is reacted with a fluoride of a second alkali metal having a larger ionic radius than that of the first alkali metal in a solvent capable of dissolving the difluorophosphate and the fluoride to obtain a difluorophosphate of the second alkali metal. Since no chlorine compound is used as a raw material in the present invention, the obtained difluorophosphate has an essentially low chlorine impurity content.

[0020] The reaction mechanism in the present invention is a cation exchange reaction represented by the following formula (i). MPO2F2+ AF → APO2F2+ MF (i) (M is a first alkali metal, A is a second alkali metal, and A has a larger ionic radius than M.)

[0021] the first alkali metal M is at least one selected from lithium and sodium, because the reaction proceeds more easily, the by-product fluoride of the first alkali metal M is easily precipitated, and the target difluorophosphate is easily purified; The second alkali metal A is preferably at least one selected from sodium, potassium, rubidium and cesium. However, when the first alkali metal M is sodium, the second alkali metal A is at least one selected from potassium, rubidium, and cesium.

[0022] In view of the fact that the above reaction proceeds more easily, that the by-product fluoride of the first alkali metal M can be efficiently precipitated, and that the target difluorophosphate can be more easily purified, it is preferable that the first alkali metal M and the second alkali metal A have a large difference in ionic radius. For example, when the first alkali metal M is lithium, it is particularly preferable that the second alkali metal A is at least one selected from potassium, rubidium, and cesium, or when the first alkali metal M is sodium, the second alkali metal A is at least one selected from rubidium and cesium.

[0023] The solvent used in the reaction of the present invention is a solvent capable of dissolving the difluorophosphate MPO2F2 of the first alkali metal and the fluoride AF of the second alkali metal having a larger ionic radius than the first alkali metal. As the solvent, it is preferable to use a polar solvent in terms of reaction progress and purification efficiency of the difluorophosphate, which is the target product. In this specification, the polar solvent includes a solvent having a relative dielectric constant of less than 10, which is generally called a low polarity solvent. From the viewpoint of the solubility of the difluorophosphate MPO2F2 of the first alkali metal, the solvent used in the reaction preferably has a relative dielectric constant of 7 or more at 25°C, more preferably 30 or more.

[0024] The solvent used in the present invention preferably has a solubility of the first alkali metal difluorophosphate MPO2F2 at the reaction temperature of 1 g / 100 g of solvent or more, as this has high reactivity and makes it easier to obtain the desired second alkali metal difluorophosphate APO2F2, more preferably 20 g / 100 g of solvent or more, and particularly preferably 40 g / 100 g of solvent or more.

[0025] The solvent used in the present invention preferably has a solubility of the second alkali metal fluoride AF of 0.1 g / 100 g of solvent or more at the reaction temperature, in terms of high reactivity and ease of obtaining the target second alkali metal difluorophosphate APO2F2, more preferably 1 g / 100 g of solvent or more, particularly preferably 10 g / 100 g of solvent or more, and even more preferably 50 g / 100 g of solvent or more.

[0026] In the present invention, the solvent used in the above reaction is preferably one in which the solubility of the first alkali metal fluoride MF at the reaction temperature is lower than that of the second alkali metal fluoride AF, because the by-product of the first alkali metal fluoride can be easily removed from the reaction system by filtration, making purification easy, and the reaction proceeds well. From this viewpoint, the difference (S2-S1) between the solubility S1 (g / 100g of solvent) of the first alkali metal fluoride MF and the solubility S2 (g / 100g of solvent) of the second alkali metal fluoride AF at the reaction temperature is preferably 2g or more, more preferably 4g or more, and particularly preferably 100g or more.

[0027] The solubility of the first alkali metal fluoride at the reaction temperature is, for example, preferably 5 g / 100 g of solvent or less in that the by-product first alkali metal fluoride MF can be easily removed from the reaction system by filtration, more preferably 3 g / 100 g of solvent or less, even more preferably 1 g / 100 g of solvent or less, and particularly preferably 0.5 g / 100 g of solvent or less.

[0028] Particularly preferred solvents for use in the reaction in the production method of the present invention include at least one selected from water, alcohols, and alkylene glycol dialkyl ethers, which are suitable in terms of the progress of the cationic reaction and the ease of purification of the first alkali metal fluoride MF. Examples of alcohols include monohydric aliphatic alcohols having 1 to 4 carbon atoms and polyhydric alcohols having 2 to 6 carbon atoms. Examples of the monohydric aliphatic alcohols include methanol, ethanol, propanol, isopropanol, n-butanol, isobutanol, and the like. Examples of the polyhydric alcohols include ethylene glycol, glycerin, sorbitan, trimethylolpropane, and the like. Furthermore, examples of alkylene glycol dialkyl ethers include those having 4 to 8 carbon atoms, such as 1,3-dimethoxypropane, 1,2-dimethoxyethane, tetraethylene glycol dimethyl ether, and triethylene glycol dimethyl ether. Among them, the solvent used in the reaction in the production method of the present invention is preferably water or an alcohol, in terms of the ease of the cationic reaction proceeding, the ease of precipitation of the fluorinated MF of the first alkali metal, and the ease of removal by filtration, and is more preferably water or a monovalent aliphatic alcohol having 1 to 4 carbon atoms, in terms of the ease of the cationic reaction proceeding, the ease of precipitation of the fluorinated MF of the first alkali metal, the ease of being inexpensive, and the ease of handling. As the monovalent aliphatic alcohol having 1 to 4 carbon atoms, methanol, ethanol, and isopropanol are preferred in terms of industrial applicability.

[0029] The concentration of the first alkali metal difluorophosphate MPO2F2 and the second alkali metal fluoride AF in the solvent is preferably such that the total amount of MPO2F2 and AF is 10% by mass or more relative to the total amount of the solvent, MPO2F2, and AF in terms of reaction progress and productivity, and is preferably such that the total amount of MPO2F2 and AF is 50% by mass or less in terms of industrial applicability. From these viewpoints, the concentration of MPO2F2 and AF in the solvent is preferably such that the total amount of MPO2F2 and AF is 10% by mass or more and 50% by mass or less, more preferably 20% by mass or more and 40% by mass or less, relative to the total amount of the solvent, MPO2F2, and AF.

[0030] The molar ratio of the first alkali metal difluorophosphate MPO2F2 and the second alkali metal fluoride AF used in the reaction is advantageously 1:1 or a range close to it in terms of reaction progress and the small amount of by-products, and for example, AF:MPO2F2=1:0.80 to 1.20, and more preferably 1:0.95 to 1.05.

[0031] In the cationic reaction in the production method of the present invention, the temperature at which the first alkali metal difluorophosphate MPO2F2 and the second alkali metal fluoride AF are reacted is preferably 5° C. or higher in terms of reaction progress, and is preferably 80° C. or lower in terms of suppressing decomposition of difluorophosphate ions. From these points of view, the reaction temperature is more preferably 10° C. or higher and 60° C. or lower, and particularly preferably 20° C. or higher and 40° C. or lower.

[0032] The reaction time in the above reaction is not particularly limited, but is preferably 10 minutes or more from the viewpoint of allowing the reaction to proceed sufficiently, and is preferably 24 hours or less from the viewpoint of a balance between shortening the production time and purity. From these viewpoints, the reaction time is more preferably 20 minutes to 12 hours, and particularly preferably 30 minutes to 6 hours.

[0033] The production method of the present invention may be carried out under an active atmosphere such as air, or under an inert atmosphere such as nitrogen, argon, etc. A nitrogen atmosphere is preferred in that the product is highly stable and even a small amount of decomposition can be suppressed, while an air atmosphere is preferred in that inexpensive production equipment can be used and the production operation is simple.

[0034] In the production method of the present invention, the reaction may be carried out by adding the first alkali metal difluorophosphate MPO2F2 and the second alkali metal fluoride AF to a solvent, or by adding a solvent to MPO2F2 and AF. The addition time of the solvent or the addition time of MPO2F2 and AF can be set arbitrarily. In the production method of the present invention, the reaction may be carried out under atmospheric pressure or under reduced pressure.

[0035] Although a stirring operation may or may not be carried out after the contact of the first alkali metal difluorophosphate MPO2F2 and the second alkali metal fluoride AF, it is preferable to carry out a stirring operation from the viewpoint of productivity.

[0036] After the reaction, if insoluble components are produced in the reaction solution, they can be filtered off by a normal filtration operation. In particular, in the production method of the present invention, the first alkali metal fluoride MF precipitates and can be removed to increase the purification efficiency, and it is believed that the reaction efficiency in the liquid is easily increased by the precipitation of the first alkali metal fluoride MF from the reaction solution.

[0037] In addition, when the first alkali metal fluoride MF is removed by filtration, this includes not only the case where the second alkali metal difluorophosphate APO2F2 is present in the filtrate and APO2F2 and MF are separated by solid-liquid separation, but also the case where the second alkali metal difluorophosphate APO2F2 precipitates in the solvent together with the first alkali metal fluoride MF and both are separated from the reaction solution by solid-liquid separation. In the latter case, it is preferable to filter out the precipitated first alkali metal fluoride MF and the second alkali metal difluorophosphate APO2F2 and then remove them from the reaction solution, and then perform an extraction (filtration) process, which is a purification process described later, on the obtained solid. In other words, it is preferable to dissolve the solid difluorophosphate APO2F2, mix it with a solvent in which the solubility of the first alkali metal fluoride MF is low, such as lithium fluoride, and then filter it to dissolve APO2F2 and obtain a filtrate from which MF has been removed. Suitable types of solvents that dissolve APO2F2 but have low solubility for MF and temperatures thereof include those described below.

[0038] The solvent such as the reaction solvent present in the filtrate in excess can be removed by a general heating operation, a decompression operation, or a crystallization filtration operation. The heating and drying temperature is preferably 30°C to 120°C, more preferably 40°C to 100°C, from the viewpoint of avoiding thermal decomposition and saving energy. When decompression is performed during drying, the pressure (absolute pressure) can be, for example, in the range of 100hPa to 1hPa.

[0039] In the production method of the present invention, the obtained difluorophosphate can be subjected to a further purification step. The purification method is not particularly limited, and known methods such as extraction (filtration), washing, and recrystallization can be used. In addition, the filtrate before drying the solvent can be subjected to an extraction step.

[0040] For extraction (filtration), a solvent is used in which the solid difluorophosphate APO2F2 is dissolved and the fluoride MF of the first alkali metal, such as lithium fluoride, has low solubility. The reactants are dissolved in the solvent, the insoluble matter is separated, and the solvent is removed using an evaporator or the like to obtain a high-purity difluorophosphate.

[0041] For example, a solvent in which the difluorophosphate has low solubility is used for the obtained crude difluorophosphate for washing. By washing with the solvent, impurities can be washed away to obtain a highly pure difluorophosphate.

[0042] For example, the recrystallization is performed by using a solvent that dissolves the difluorophosphate, utilizing the temperature dependency of solubility. The reactant is dissolved in the solvent, and the mixture is heated and cooled to precipitate crystals of the difluorophosphate with high purity.

[0043] When performing solvent extraction on the filtrate before drying, the filtrate is extracted with a solvent in which the solubility of the target difluorophosphate is higher than that of the solvent in the filtrate (hereinafter, sometimes referred to as "filtrate extraction"). A high-purity difluorophosphate can be obtained by removing the extraction solvent with an evaporator or the like.

[0044] The type of solvent used in the above extraction (filtration), washing, recrystallization, and extraction of the filtrate is not particularly limited as long as it does not react with difluorophosphate or cause decomposition or deterioration, and examples thereof include water, carbonates, esters, phosphates, ethers, nitrile compounds, amide compounds, alcohols, alkanes, etc. Examples include, but are not limited to, the following compounds.

[0045] Examples of the carbonate esters include dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, and the like. Of these, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethylene carbonate, and propylene carbonate are preferable. Examples of the esters include methyl acetate, ethyl acetate, etc., and preferably, ethyl acetate and butyl acetate are used. Examples of the phosphate ester include trimethyl phosphate, triethyl phosphate, trimethyl phosphite, and diethylmethyl phosphite. Examples of the ethers include the alkylene glycol dialkyl ethers given above as examples of the reaction solvent, as well as diethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, and the like. An example of the nitrile compound is acetonitrile, etc. An example of the amide compound is dimethylformamide, etc. The alcohols include those exemplified above as the alcohols used as the reaction solvent. Examples of the alkanes include hexane and n-heptane.

[0046] The water or organic solvent may be used alone or in combination of two or more. The solvent may be capable of dissolving the difluorophosphate salt, or may be in an amount that allows stirring even if it cannot.

[0047] For example, the extraction of the filtrate or the extraction (filtration) of the solid difluorophosphate can be performed using the solvents listed above as preferred reaction solvents, which is preferable because it can enhance purification efficiency. The above-listed reaction solvents can be used as examples of the extraction solvent used for the extraction of the filtrate or the extraction (filtration) of the solid difluorophosphate (particularly, a solvent that dissolves the solid difluorophosphate APO2F2 and has low solubility for the fluoride MF of the first alkali metal such as lithium fluoride), but it is preferable to use water and / or alcohols in terms of high purification efficiency. The extraction step using the extraction solvent used for the extraction of the filtrate or the extraction (filtration) of the solid difluorophosphate can be performed at room temperature or at an adjusted temperature, and the temperature is preferably 10°C or higher and 50°C or lower from the viewpoint of suppressing decomposition of the difluorophosphate and enhancing purification efficiency, and is particularly preferably 10°C or higher and 40°C or lower. EXAMPLES

[0048] The present invention will be described in more detail below with reference to examples, but is not limited to these examples.

[0049] [Example 1] <Synthesis of cesium difluorophosphate> A 500 mL perfluoroalkoxyalkane vessel was used as the reaction vessel. The solvent, 38.25 g of lithium difluorophosphate (LiPO2F2), and 51.23 g of cesium fluoride (CsF) were charged into the reaction vessel. 152.47 g of water was used as the solvent. The molar ratio of lithium difluorophosphate (LiPO2F2) and cesium fluoride (CsF) is as shown in Table 1, and the concentration of the raw materials (LiPO2F2 and CsF) in the solvent (i.e., the ratio of the total amount of LiPO2F2 and CsF to the total amount of the solvent, LiPO2F2, and CsF) was the concentration shown in Table 1. The reaction was carried out at 20°C for 30 minutes under stirring in an air atmosphere. The reaction product was filtered to separate the solid and liquid, and the filtrate was concentrated and dried at 40°C under a reduced pressure of 10 hPa absolute pressure to obtain a dry powder. The anion composition of the obtained dry powder was 31 P-NMR and 19When examined by F-NMR, the anion was 99.0 mol % PO2F2, 0.0 mol % PO3F and (PO2F)2O, 0.0 mol % PO4, and 1.0 mol % F. The dry powder was dissolved in water to a concentration of 0.01 mass % to obtain a solution, and the cation composition of the solution was examined by ion chromatography under the following conditions, and the peak area ratio of M(Li):A(Cs) was 5.1:94.9%. In this manufacturing method, LiF, which has low solubility, precipitates in the reaction system and is removed by solid-liquid separation, so almost all of the Cs derived from the raw materials remains in the filtrate. Therefore, based on the results of cation and anion analysis of the dried product obtained by drying the filtrate as is, it is estimated that the ratio of the amount of Cs in the obtained CsPO2F2 to the amount of Cs in the raw material CsF is about 90% or more.

[0050] Ion Chromatography Analysis: Ion chromatography was performed using a Dionex ICS-3000. The eluent was 50 mM MSA (methanesulfonic acid), the column was Cs-16, the column temperature was 30°C, the injection volume was 25 μl, and the retention times (unit μS / min) were Li: 4.49, Na: 5.67, K: 9.30, Rb: 12.2, and Cs: 16.1.

[0051] [Example 2] A 20 mL perfluoroalkoxyalkane vessel was used as the reaction vessel. 1.86 g of lithium difluorophosphate (LiPO2F2) and 2.62 g of cesium fluoride (CsF) were used, with the raw material molar ratios as shown in Table 1. The reaction temperature was as shown in Table 1 below. A dry powder was obtained in the same manner as in Example 1, except for these points. The obtained dry powder was analyzed in the same manner as in Example 1, and the results are shown in Table 1.

[0052] [Example 3] A 20 mL perfluoroalkoxyalkane vessel was used as the reaction vessel. 1.94 g of lithium difluorophosphate (LiPO2F2) and 2.72 g of cesium fluoride (CsF) were used, with the raw material molar ratios as shown in Table 1. The reaction temperature was as shown in Table 1 below. A dry powder was obtained in the same manner as in Example 1, except for these points. The obtained dry powder was analyzed in the same manner as in Example 1, and the results are shown in Table 1.

[0053] [Example 4] A 20 mL perfluoroalkoxyalkane vessel was used as the reaction vessel. 1.28 g of lithium difluorophosphate (LiPO2F2) and 1.76 g of cesium fluoride (CsF) were used, with the raw material molar ratios as shown in Table 1. The reaction temperature, the type of reaction solvent, and the raw material concentration in the solvent were as shown in Table 1 below. A dry powder was obtained in the same manner as in Example 1, except for these points. The obtained dry powder was analyzed in the same manner as in Example 1, and the results are shown in Table 1.

[0054] [Example 5] A 500 mL perfluoroalkoxyalkane vessel was used as the reaction vessel. 16.14 g of lithium difluorophosphate (LiPO2F2) and 22.86 g of cesium fluoride (CsF) were used, and the raw material molar ratio was as shown in Table 1. 1,2-dimethoxyethane (DME) was used as the reaction solvent instead of water, the reaction temperature and the raw material concentration in the solvent were changed as shown in Table 1 below, and the reaction time was set to 24 hours. Other than these points, the reaction was carried out in the same manner as in Example 1, and the reaction liquid was filtered. After filtering, 120 mL of water was added to the obtained residue, and the mixture was shaken at 30°C for 60 minutes and then filtered, and the obtained filtrate was concentrated and dried at 80°C under a reduced pressure of 10 hPa absolute pressure to obtain 30.67 g of dry powder. The obtained dry powder was analyzed in the same manner as in Example 1. The results are shown in Table 1. In this manufacturing method, the solubility of CsPO2F2 produced by the reaction in DME is low, so the precipitated solid was purified. From the amount of the obtained dry powder and the results of cation and anion analysis, the ratio of the amount of Cs in the obtained CsPO2F2 to the amount of Cs in the raw material CsF was calculated to be 80%.

[0055] [Example 6] A 500 mL perfluoroalkoxyalkane vessel was used as the reaction vessel. 21.57 g of lithium difluorophosphate (LiPO2F2) and 8.4 g of sodium fluoride (NaF) were used, and the raw material molar ratio was as shown in Table 1. The raw material concentration in the solvent was changed as shown in Table 1 below. A filtrate was obtained after the reaction was carried out in the same manner as in Example 1 except for these points. The obtained filtrate was concentrated and dried at 40°C under a reduced pressure of 10 hPa absolute pressure to obtain a dry powder. The obtained dry powder was analyzed in the same manner as in Example 1, and the results are shown in Table 1.

[0056] [Example 7] A 20 mL perfluoroalkoxyalkane vessel was used as the reaction vessel. 2.12 g of lithium difluorophosphate (LiPO2F2) and 2.03 g of rubidium fluoride (RbF) were used, and the raw material molar ratio was as shown in Table 1. The raw material concentration in the solvent was changed as shown in Table 1 below. Other than these points, the reaction was carried out in the same manner as in Example 1, and the analysis was carried out in the same manner as in Example 1. The results are shown in Table 1.

[0057] [Example 8] A 20 mL perfluoroalkoxyalkane vessel was used as the reaction vessel. 1.26 g of sodium difluorophosphate (NaPO2F2) and 1.55 g of cesium fluoride (CsF) were used, and the raw material molar ratio was as shown in Table 1. The type of solvent and the raw material concentration in the solvent were changed as shown in Table 1 below. Other than that, the reaction was carried out in the same manner as in Example 1 to obtain a dry powder. 50 mL of methanol was added to the obtained dry powder, and the mixture was shaken at 30°C for 1 hour, and then filtered. The obtained filtrate was concentrated and dried at 40°C under a reduced pressure of 10 hPa absolute pressure to obtain 1.40 g of a dry powder. The obtained dry powder was analyzed in the same manner as in Example 1. The results are shown in Table 1. In this manufacturing method, a purification step was carried out to separate sodium difluorophosphate from the remaining raw materials, by-products, etc. From the amount of the obtained dry powder and the results of cation and anion analyses, the ratio of the amount of Cs in the obtained CsPO2F2 to the amount of Cs in the raw material CsF was calculated to be 56%.

[0058] [Example 9] A 50 mL perfluoroalkoxyalkane vessel was used as the reaction vessel. 5.19 g of lithium difluorophosphate (LiPO2F2) and 2.82 g of potassium fluoride (KF) were used, and the raw material molar ratio was as shown in Table 1. The raw material concentration in the solvent was changed as shown in Table 1 below. Other than these points, the reaction was carried out in the same manner as in Example 1, and the analysis was carried out in the same manner as in Example 1. The results are shown in Table 1.

[0059] [Comparative Example 1] A 250 mL perfluoroalkoxyalkane vessel was used as the reaction vessel. 11.10 g of cesium difluorophosphate (CsPO2F2) and 2.02 g of sodium fluoride (NaF) were used. The raw material concentration in the solvent and the reaction temperature were changed as shown in Table 1 below. Apart from these points, the reaction was carried out in the same manner as in Example 1 to obtain a dry powder. 230 g of methanol was added to the obtained dry powder, and the mixture was shaken at 30°C for 60 minutes and then filtered. The obtained filtrate was concentrated and dried at 30°C under a reduced pressure of 10 hPa absolute pressure to obtain a dry powder. 9.90 g of the obtained dry powder was analyzed in the same manner as in Example 1. The results are shown in Table 1. In this production method, despite the purification, the amount of NaPO2F2 in the dry powder was very small as shown in Table 1. From the amount of the obtained dry powder and the results of the cation analysis and anion analysis, the ratio of the amount of Na in the obtained NaPO2F2 to the amount of Na in the raw material NaF was calculated to be 16%.

[0060] [Comparative Example 2] A 50 mL perfluoroalkoxyalkane vessel was used as the reaction vessel. 1.65 g of lithium difluorophosphate (LiPO2F2) and 2.54 g of cesium chloride (CsCl) were used, the raw material concentrations were changed as shown in Table 1 below, and the reaction time was set to 12 hours. Other than these points, the reaction was carried out in the same manner as in Example 1 to obtain a dry powder. The obtained dry powder was washed with 60 mL of acetone at 30°C, and then dried again at 30°C under a reduced pressure of 10 hPa absolute pressure to obtain a dry powder. The obtained dry powder was analyzed in the same manner as in Example 1. The results are shown in Table 1.

[0061] [Comparative Example 3] A 50 mL perfluoroalkoxyalkane vessel was used as the reaction vessel. 2.09 g of lithium difluorophosphate (LiPO2F2) and 2.64 g of cesium carbonate (Cs2CO3) were used, and the raw material molar ratio was as shown in Table 1. The raw material concentration in the solvent and the type of solvent were changed as shown in Table 1 below. Other than these points, the reaction was carried out in the same manner as in Example 1 to obtain a dry powder. The obtained dry powder was analyzed in the same manner as in Example 1. The results are shown in Table 1.

[0062] [Table 1]

[0063] As shown in Table 1, in each Example in which a difluorophosphate of a first alkali metal was reacted with a fluoride of a second alkali metal having a larger ionic radius than the first alkali metal, the cation exchange reaction of the difluorophosphate proceeded effectively. In Examples 1 to 4, 6, 7, and 9, the dried product of the filtrate showed a high cation exchange rate even though it was not purified. Also, in Examples 5 and 8 in which purification was performed after solid-liquid separation, the target product of relatively high purity was obtained with a yield of 56% or more. On the other hand, in Comparative Example 1 in which the ionic radius of the alkali metal in the raw material fluoride was smaller than that of the alkali metal in the raw material difluorophosphate, it was shown that cation exchange did not progress. Comparative Examples 2 and 3 show cases where an alkali metal chloride or carbonate was used as the cation exchange raw material instead of a fluoride. In these cases, the cation exchange reaction was shown to be inferior to Examples 1 and 4, which were carried out under substantially the same conditions except for this point. In particular, the present inventors have confirmed that when an alkali metal chloride was used as the cation exchange raw material instead of a fluoride, heat was generated during the reaction and the proportion of difluorophosphate ions decomposed was high.

[0064] The solubilities of some of the fluorides used in the examples in the solvents are shown in the table below. The solubility of lithium difluorophosphate in water at 20 to 60°C is 80g / 100g solvent or more, the solubility of methanol at 20°C is 60g / 100g solvent or more, and the solubility of DME at 60°C is 40g / 100g solvent or more. The solubility of sodium difluorophosphate in methanol at 20°C is 10g / 100g solvent or more. The solubility of cesium difluorophosphate in DME at 60°C is about 0.1g / 100g solvent.

[0065] [Table 2]

Claims

1. A method for producing a difluorophosphate, comprising reacting a difluorophosphate of a first alkali metal with a fluoride of a second alkali metal having an ionic radius larger than that of the first alkali metal in a solvent capable of dissolving the difluorophosphate and the fluoride, to obtain a difluorophosphate of the second alkali metal.

2. the first alkali metal is at least one selected from lithium and sodium; 2. The method according to claim 1, wherein the second alkali metal is at least one selected from the group consisting of sodium, potassium, rubidium and cesium.

3. 3. The method according to claim 1, wherein a polar solvent in which the solubility of the first alkali metal fluoride is 5 g / 100 g of solvent or less at the reaction temperature is used as the solvent.

4. The method according to any one of claims 1 to 3, wherein a polar solvent is used as the solvent, in which the solubility of the difluorophosphate of the first alkali metal is 1 g / 100 g of solvent or more at the reaction temperature, and the solubility of the fluoride of the second alkali metal is 0.1 g / 100 g of solvent or more at the reaction temperature.

5. The method according to any one of claims 1 to 4, wherein the solvent is at least one selected from the group consisting of water, alcohols, and alkylene glycol dialkyl ethers.

6. The method according to any one of claims 1 to 5, wherein the first alkali metal fluoride precipitated in the solvent by the reaction is removed by filtration.

7. The method according to any one of claims 1 to 6, wherein the reaction temperature is 5°C or higher and 80°C or lower.

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