Alkali metal bis(fluorosulfonyl)imide

Abstract

The present invention relates to an alkali metal salt of bis(fluorosulfonyl)imide (MFSI) having decomposition stability against lithium hexafluorophosphate (LiPF6), and a method for producing the same.

Description

[Technical Field]

[0001] The present invention relates to an alkali metal bis(fluorosulfonyl)imide (MFSI) salt having decomposition stability against lithium hexafluorophosphate (LiPF6), and a method for producing the same. [Background technology]

[0002] 2. Description of the Related Art With the popularization of mobile devices, the commercialization of electric vehicles, and the increasing demand for electrical storage devices, secondary batteries with high performance such as high output, high energy density, and high discharge voltage are being developed.

[0003] A lithium secondary battery includes a negative electrode, a positive electrode, a separator, and an electrolyte.

[0004] The electrolyte acts as a medium for the movement of lithium ions between the positive and negative electrodes while improving the thermal, electrical, and physical stability of the battery. It consists of a solvent, salt, and various additives. The salt is typically lithium hexafluorophosphate (LiPF6), which is the most important lithium source and contributes to ionic conductivity, thermal stability, and electrochemical stability. The solvent dissociates the salt, and carbonates and esters are the most commonly used solvents. The type of solvent determines the bulk ionic conductivity, viscosity, density, and wettability. Numerous additives exist, and they are related to the formation of the solid electrolyte interphase (SEI).

[0005] Meanwhile, lithium hexafluorophosphate (LiPF6) is a lithium salt that has excellent performance and is used as an electrolyte in electrolytes due to its relatively low cost, but it has the disadvantage of being decomposed into hydrofluoric acid gas.

[0006] To overcome these shortcomings, LiTFSI (lithium bis(trifluoromethanesulfonyl)imide) and LiFSI (lithium bis(fluorosulfonyl)imide) salts have been developed. These salts exhibit little or no spontaneous decomposition and are more hydrolytically stable than LiPF6.

[0007] Therefore, recently, electrolytes, especially for electric vehicles, have been used in combination with LiPF6, LiFSI, LiPO2F2, lithium difluoro(oxalato)borate (LiDFOP), and lithium bis(oxalato)borate (LiBOB). Summary of the Invention [Problem to be solved by the invention]

[0008] However, the present inventors have found that when LiPF6 is used together with other electrolytes, particularly LiFSI, as an electrolyte for a secondary battery, the hydrolysis of LiPF6 is accelerated.

[0009] Here, the present invention aims to provide a bis(fluorosulfonyl)imide alkali metal salt that does not promote the decomposition of LiPF6. [Means for solving the problem]

[0010] In an effort to achieve the above object, the present invention provides a method for preparing a solution Ai by dissolving 1 g of LiPF6 and 1 g of alkali metal salt of bis(fluorosulfonyl)imide (MFSI) in a mixed solvent of 50 g of water and 50 g of ethylene carbonate (EC); PF6 in solution Ai by ion chromatography ‐ measuring the concentration (Ci) of Stirring the solution Ai at room temperature for 24 hours to obtain a solution At; and PF6 in solution At by ion chromatography ‐ The decomposition stability of LiPF6 is measured by a step of measuring the concentration (Ct) of bis(fluorosulfonyl)imide, and the decomposition stability of LiPF6 calculated by the following formula (1) is 95% or more.

[0011] Formula (1) LiPF6 decomposition stability % = Ct / Ci x 100

[0012] The present invention is characterized in that when the alkali metal bis(fluorosulfonyl)imide salt that enhances the decomposition stability of LiPF6 is dissolved in water at a concentration of 10 wt %, the pH of the solution becomes 6-8. [Effects of the Invention]

[0013] The alkali metal bis(fluorosulfonyl)imide salt of the present invention, which enhances the stability of LiPF6 against decomposition, enhances stability without promoting the decomposition of LiPF6 in the electrolyte. The present invention reduces the generation of HF in the electrolyte, thereby preventing the destruction of the coatings on the positive and negative electrodes. The present invention prevents the decrease in the charge / discharge capacity of the battery, thereby extending the battery life. DETAILED DESCRIPTION OF THE INVENTION

[0014] The present invention will be described in detail below. The terms and words used in the specification and claims should not be construed in a limited manner based on their ordinary or dictionary meanings, but should be construed in a manner that is consistent with the technical concept of the present invention, in accordance with the principle that the inventor can appropriately define the concepts of terms in order to best describe his or her invention.

[0015] The present invention relates to an alkali metal bis(fluorosulfonyl)imide salt that enhances the decomposition stability of LiPF6.

[0016] The present invention strives to provide a bis(fluorosulfonyl)imide alkali metal salt that can enhance the decomposition stability of LiPF6 even when used together with LiPF6 in an electrolyte, and the present invention provides a method for preparing a solution Ai by dissolving 1 g of LiPF6 and 1 g of the bis(fluorosulfonyl)imide alkali metal salt (MFSI) in a mixed solvent of 50 g of water and 50 g of ethylene carbonate (EC), and then isolating PF6 in the solution Ai by ion chromatography. ‐ The step of measuring the concentration (Ci) of solution Ai, the step of stirring solution Ai at room temperature for 24 hours to obtain solution At, and the step of measuring the concentration (Ci) of solution At by ion chromatography. ‐ The present invention has been accomplished by finding that the decomposition stability of LiPF6 can be improved in the case of a bis(fluorosulfonyl)imide alkali metal salt having a decomposition stability of LiPF6 of 95% or more, as measured by measuring the concentration (Ct) of LiPF6 and calculated by the following formula (1):

[0017] Formula (1) LiPF6 decomposition stability % = Ct / Ci x 100

[0018] The alkali metal (M) may be Li, Na, K or Cs, and is preferably Li.

[0019] 1g of bis(fluorosulfonyl)imide alkali metal salt and 1g of LiPF6 were dissolved in a solvent made by mixing water and ethylene carbonate (EC) in a 1:1 weight ratio, and the mixture was stirred at room temperature for 24 hours. ‐ The concentration of the initial PF6 ‐ When the bis(fluorosulfonyl)imide alkali metal salt is 95% or more relative to the concentration, even when the bis(fluorosulfonyl)imide alkali metal salt is used in an electrolyte together with LiPF6, the LiPF6 does not decompose, and the electrolyte is stable, preventing destruction of the coatings on the positive and negative electrodes and preventing a decrease in the charge / discharge capacity of the battery, thereby extending the battery life. On the other hand, when the value is less than 95%, the LiPF6 in the electrolyte may decompose and destroy the coatings on the positive and negative electrodes, thereby shortening the battery life.

[0020] In the ion chromatography method of the present invention, 1 g of a 1 wt % solution of LiiOSO2CF3 (LiOTf) (using a mixed solvent of 50 g of water and 50 g of ethylene carbonate) was mixed with 1 g of the solution Ai as an internal standard substance, and the peak intensity (PF6 ‐ The measurement may be performed by comparing the peak intensity of the OTf / peak intensity of the OTf.

[0021] The alkali metal bis(fluorosulfonyl)imide salt can be prepared by preparing ammonium bis(fluorosulfonyl)imide (NHFSI) using bis(chlorosulfonyl)imide (HCSI) and NH4F, and preparing the alkali metal bis(fluorosulfonyl)imide salt using the ammonium bis(fluorosulfonyl)imide and an alkali metal salt compound.

[0022] However, this method generates a sulfamic acid derivative of the following formula (1) as a by-product, which acts as a cause of deterioration in the quality of the bis(fluorosulfonyl)imide alkali metal salt. The sulfamic acid derivative of formula (1) is expected to be produced by hydrolysis of HCSI.

[0023] [ka]

[0024] When MFSI containing such sulfamic acid derivatives is added to an electrolyte solution together with LiPF6, the sulfamic acid derivative acts as a strong acid, catalyzing the hydrolysis of LiPF6, generating HF within the secondary battery according to the following reaction formula. The generated HF acts on the SEI film on the positive or negative electrode, accelerating its decomposition and ultimately shortening the battery's lifespan. Using LiFSI as an example, this is shown in formula (2) below.

[0025] TIFF0007721804000002.tif73135

[0026] Therefore, the content of sulfamic acid derivatives in MFSI needs to be adjusted.

[0027] The present inventors have endeavored to provide MFSI with a controlled content of sulfamic acid derivatives. They have found that reducing the water content in the solvent used to prepare MFSI or reducing the HF content in the reaction solution can provide MFSI with a reduced content of sulfamic acid derivatives, thereby preventing the decomposition of LiPF in the electrolyte. In particular, when the water content in the reaction solvent for MFSI production is 500 ppm by weight or less, the hydrolysis of HCSI and the like is reduced, reducing the production of sulfamic acid derivatives. As a result, a bis(fluorosulfonyl)imide alkali metal salt can be provided that enhances the decomposition stability of LiPF.

[0028] The present invention is characterized in that the bis(fluorosulfonyl)imide alkali metal salt that enhances the decomposition stability of LiPF6 contains a sulfamic acid derivative at 500 ppm by weight or less. When the sulfamic acid derivative is contained at 500 ppm by weight or less, the decomposition of LiPF6 is not promoted even when the bis(fluorosulfonyl)imide alkali metal salt is added to the electrolyte together with LiPF6.

[0029] The alkali metal bis(fluorosulfonyl)imide salt of the present invention that enhances the decomposition stability of LiPF can be prepared by reacting bis(chlorosulfonyl)imide (HCSI) and NH4F in a solvent 1 while suppressing hydrolysis of the bis(chlorosulfonyl)imide to prepare ammonium bis(fluorosulfonyl)imide (NH4FSI), and preparing the alkali metal bis(fluorosulfonyl)imide salt using the ammonium bis(fluorosulfonyl)imide and an alkali metal salt compound.

[0030] In the present invention, the water content of the solvent 1 can be reduced to 500 ppm by weight or less in order to suppress hydrolysis of the bis(chlorosulfonyl)imide.

[0031] The alkali metal bis(fluorosulfonyl)imide salt of the present invention that enhances the decomposition stability of LiPF6 is (a) reacting bis(chlorosulfonyl)imide with NHF in a solvent 1 under a nitrogen atmosphere while suppressing hydrolysis of the bis(chlorosulfonyl)imide to prepare ammonium bis(fluorosulfonyl)imide; (b) reacting the ammonium bis(fluorosulfonyl)imide obtained in step (a) with an alkali metal salt to prepare a bis(fluorosulfonyl)imide alkali metal salt; (c) adding solvent 2 as an antisolvent to the reaction solution of step (b) to obtain a precipitate; and (d) filtering the solution of step (c) to recover the precipitate, thereby obtaining the alkali metal salt of bis(fluorosulfonyl)imide.

[0032] The reaction in step (a) is as follows:

[0033] TIFF0007721804000003.tif35136

[0034] In the above reaction, 2 to 10 moles, preferably 2.5 to 5 moles, of NHF can be reacted with 1 mole of bis(chlorosulfonyl)imide, in order to sufficiently substitute chlorine atoms in the bis(chlorosulfonyl)imide with fluorine atoms.

[0035] Various acids are produced in the process of formula (3). Among these, HF is also produced. Removing this acid from the reaction solution by bubbling nitrogen gas allows the next step to proceed with a significant amount of HF removed, thereby reducing hydrolysis of the starting material or intermediate product. Nitrogen gas bubbling into the reaction solution can be carried out during the reaction or after the reaction is complete, prior to step (b). Nitrogen gas bubbling can be continued until the pH of the gas emitted from the reaction solution reaches 6-8. It is more preferable to discontinue nitrogen gas bubbling when the pH reaches 6.5-7.5, and even more preferable to discontinue nitrogen gas bubbling when the pH reaches 6.8-7.2.

[0036] The solvent 1 in step (a) is a solvent capable of dissolving the alkali metal salt of bis(fluorosulfonyl)imide, and is preferably at least one selected from the group consisting of diethyl ether, diisopropyl ether, methyl t-butyl ether, methyl acetate, ethyl acetate, propyl acetate, and butyl acetate.

[0037] The present inventors have recognized that if the solvent contains a large amount of water, hydrolysis of the reactants or products may occur, and in particular, if an acid such as HF is present in the reaction solution, the hydrolysis reaction shown in the following formula (4) may be promoted. As a result of this hydrolysis reaction, a sulfamic acid derivative may be produced in the reaction solution.

[0038] TIFF0007721804000004.tif90155

[0039] Therefore, in the present invention, it is necessary to reduce the HF content in the reaction solution to prevent the hydrolysis reaction of the reactants or products, and to keep the water content in the solvent 1 to 500 ppm by weight or less, preferably 200 ppm by weight or less.

[0040] The temperature during the fluorination reaction in step (a) can be adjusted appropriately depending on the progress of the reaction, and is preferably −40° C. to 200° C., more preferably −20° C. to 100° C. The time required for the reaction varies depending on the reaction scale, but is preferably 0.1 to 48 hours, more preferably 0.5 to 24 hours.

[0041] After step (a) and before step (b), a step of cooling the reaction result to room temperature and filtering out insoluble materials such as NH4Cl to obtain a filtrate containing ammonium bis(fluorosulfonyl)imide may be further included.

[0042] Step (b) is a step of reacting the obtained ammonium bis(fluorosulfonyl)imide with an alkali metal salt to produce a bis(fluorosulfonyl)imide alkali metal salt. This reaction can be carried out by recovering the ammonium bis(fluorosulfonyl)imide as a solid and then reacting it with the alkali metal salt. However, if the alkali metal salt is added to the reaction product of step (a) without recovering the ammonium bis(fluorosulfonyl)imide as a solid, a separate ammonium bis(fluorosulfonyl)imide recovery process is not required, which has the advantage of reducing process time and effort and reducing product loss.

[0043] The alkali metal salt may be MF, MOH, M2CO3, etc., preferably MOH, more preferably LiOH.

[0044] The amount of the alkali metal salt used is preferably 1 to 10 moles, more preferably 1 to 5 moles, per mole of ammonium bis(fluorosulfonyl)imide.

[0045] The temperature during the reaction with the alkali metal salt is not particularly limited, but is preferably 0° C. to 200° C., more preferably 10° C. to 100° C. The time required for the reaction varies depending on the reaction scale, but is preferably 0.1 to 48 hours, more preferably 0.5 to 24 hours.

[0046] The reaction can be carried out under normal pressure, but if it is carried out under reduced pressure, the by-product ammonia is removed and the target product is more easily synthesized. When reduced pressure is used, the reaction pressure is not particularly limited, but a pressure of -0.01 torr, which is less than atmospheric pressure, is preferred, and a pressure reduction sufficient to reflux the solvent at 0°C to 100°C is even more preferred.

[0047] The reaction vessel for producing the alkali metal salt of bis(fluorosulfonyl)imide of the present invention is more preferably made of a resin such as a fluororesin or polyethylene.

[0048] After step (b), a step of extracting with water may be included to remove salts and other by-products.

[0049] Solvent 2 in step (c) is an antisolvent that does not dissolve the alkali metal salt of bis(fluorosulfonyl)imide, and may be one or more selected from the group consisting of toluene, hexane, heptane, chloroform, dichloromethane, etc. After step (b), solvent 2 may be added after removing a portion of the solvent from the reaction solution.

[0050] In step (c), the antisolvent may be added in an amount of 50-500 parts by weight, more preferably 80-300 parts by weight, based on 100 parts by weight of the bis(fluorosulfonyl)imide alkali metal salt. If the amount of antisolvent added is less than the above range, the bis(fluorosulfonyl)imide alkali metal salt may not be sufficiently obtained, which is undesirable. If the amount of antisolvent added exceeds the above range, the effect will not increase and other unnecessary by-products may also precipitate, which is undesirable.

[0051] The alkali metal salt of bis(fluorosulfonyl)imide having decomposition stability with LiPF6 prepared by the method of the present invention may have a sulfamic acid content of 500 ppm by weight or less. The alkali metal salt of bis(fluorosulfonyl)imide having decomposition stability with LiPF6 and having a sulfamic acid content of 500 ppm by weight or less is characterized in that when dissolved in water at a concentration of 10 wt%, the solution has a pH of 6-8.

[0052] The alkali metal bis(fluorosulfonyl)imide salt obtained by the production method of the present invention enhances the stability of LiPF6 against decomposition and can be used together with LiPF6 in an electrolyte solution, and can therefore be preferably used as an ionic conductor material in electrochemical devices such as primary batteries, secondary batteries such as lithium ion secondary batteries, electrolytic capacitors, electric double layer capacitors, fuel cells, solar cells, and electrochromic elements.

[0053] (Mode for Carrying Out the Invention) Preferred examples are presented below to aid in understanding the present invention. However, the following examples are merely illustrative of the present invention, and it will be obvious to those skilled in the art that various changes and modifications are possible within the scope of the scope and technical idea of the present invention. It is natural that such changes and modifications also fall within the scope of the claims of the present invention.

[0054] Example 1. Synthesis of bis(fluorosulfonyl)imide lithium salt 1 79.6 g (2.15 moles) of purified ammonium fluoride and 600 g of butyl acetate with a moisture content of 100 ppm by weight were added to a reactor equipped with a stirrer, a condenser, and a thermometer under a nitrogen atmosphere at room temperature. 100 g (0.47 moles) of bis(chlorosulfonyl)imide was slowly added while stirring the mixture, and the temperature was raised to 80°C and the mixture was reacted for 2 hours.

[0055] The reaction mixture was cooled to 60°C without removing the solid phase, and nitrogen gas was bubbled through. The pH of the escaping gas was checked every 5 minutes using pH paper (initial pH: approximately 3), and when the pH reached 7, nitrogen gas bubbling was stopped. The reaction mixture was cooled to below 10°C and filtered.

[0056] The filtered solution containing ammonium bis(fluorosulfonyl)imide was treated with lithium hydroxide monohydrate. and 25.5 g (0.61 mole) of the product was added and stirred. The reaction was then carried out at a temperature below 5° C. for 4 hours, after which the reaction was terminated.

[0057] 75g of distilled water was added and stirred, and the layers were separated and the distilled water layer was removed. 23g of distilled water was added and stirred, and the layers were separated and the distilled water layer was removed. The resulting organic layer was filtered and concentrated, and then 208g of toluene was added and filtered to obtain a solid. This solid was then dried under vacuum to obtain bis(fluorosulfonyl)imide lithium salt, which has decomposition stability of LiPF6. When the bis(fluorosulfonyl)imide lithium salt was dissolved in water at a concentration of 10 wt%, the pH of the solution was 7.4.

[0058] Example 2. Synthesis of bis(fluorosulfonyl)imide lithium salt 2 A bis(fluorosulfonyl)imide lithium salt with LiPF decomposition stability was obtained by the same procedure as in Example 1, except that butyl acetate with a water content of 200 wt ppm was used. When the bis(fluorosulfonyl)imide lithium salt was dissolved in water at a concentration of 10 wt%, the pH of the solution was 7.2.

[0059] Example 3. Synthesis of bis(fluorosulfonyl)imide lithium salt 3 A bis(fluorosulfonyl)imide lithium salt with LiPF decomposition stability was obtained by the same procedure as in Example 1, except that butyl acetate with a water content of 400 wt ppm was used. When the bis(fluorosulfonyl)imide lithium salt was dissolved in water at a concentration of 10 wt%, the pH of the solution was 7.1.

[0060] Comparative Example 1 The same procedure as in Example 1 was repeated to obtain bis(fluorosulfonyl)imide lithium salt, except that gas bubbling was not performed and butyl acetate with a water content of 650 wt ppm was used. When the bis(fluorosulfonyl)imide lithium salt was dissolved in water to a concentration of 10 wt%, the pH of the solution was 5.9.

[0061] Comparative Example 2 Bis(fluorosulfonyl)imide lithium salt was obtained in the same manner as in Example 1, except that gas bubbling was not performed and butyl acetate with a water content of 2,000 wt ppm was used. When the bis(fluorosulfonyl)imide lithium salt was dissolved in water to a concentration of 10 wt%, the pH of the solution was 5.7.

[0062] Experimental Example 1: Decomposition stability test of LiPF6 1 g of the bis(fluorosulfonyl)imide lithium salt prepared in Example 1 and 1 g of LiPF6 were dissolved in a mixed solvent of 50 g of water and 50 g of ethylene carbonate (EC) to prepare a solution Ai, and PF6 in the solution Ai was analyzed by ion chromatography. ‐ The concentration (Ci) of

[0063] The solution Ai was stirred at room temperature for 24 hours to obtain a solution At, and PF6 ‐ The concentration (Ct) of LiPF6 was measured, and the decomposition stability of LiPF6 was calculated using the following formula (1), which is shown in Table 1 below.

[0064] Formula (1) LiPF6 decomposition stability % = Ct / Ci x 100

[0065] The same procedure was carried out using the bis(fluorosulfonyl)imide lithium salts obtained in Examples 2, 3, Comparative Examples 1 and 2, and the decomposition stability of LiPF6 was measured. The results are shown in Table 1 below.

[0066] [Ion chromatography method] The ion chromatography method used in the present invention is as follows.

[0067] When measuring the concentration, a non-hydrolyzable lithium salt was used as an internal standard substance, and 1 g of a 1 wt% solution of LiiOSO2CF3 (LiOTf) (a mixed solvent of 50 g of water and 50 g of ethylene carbonate) was mixed with 1 g of the above solution Ai to measure the peak intensity (PF6 ‐ The peak intensity of the OTf was measured by comparing the peak intensity of the OTf / peak intensity of the OTf.

[0068] A solution prepared by dissolving 0.252 g of sodium bicarbonate and 0.2544 g of sodium carbonate in water to prepare 1 L was used as the developing solvent, and a Thermo Fisher Scientific Dionex IonPac AS23-4 μm IC column was used.

[0069] [Table 1]

[0070] The PF6 decomposition stability measured by the method of the present invention was 95% or more for the bis(fluorosulfonyl)imide lithium salts prepared in Examples 1 to 3, and therefore when these materials are mixed with LiPF6 and used as electrolytes, they can be used efficiently by preventing the decomposition of LiPF6. On the other hand, the bis(fluorosulfonyl)imide lithium salts prepared in the comparative examples showed a stability of 90% or less, and in this case, when these materials are mixed with LiPF6 and used as electrolytes, the decomposition of LiPF6 is promoted. [Industrial Applicability]

[0071] The alkali metal bis(fluorosulfonyl)imide salt obtained by the method of the present invention enhances the stability of LiPF6 against decomposition and can be used together with LiPF6 in an electrolyte solution, and can therefore be used in electrochemical devices such as primary batteries, secondary batteries such as lithium ion secondary batteries, electrolytic capacitors, electric double layer capacitors, fuel cells, solar cells, and electrochromic elements.

Claims

1. LiPF ６ preparing a solution Ai by dissolving 1 g of fluorosulfonyl methylcellulose and 1 g of alkali metal salt of bis(fluorosulfonyl)imide (MFSI) in a mixed solvent of 50 g of water and 50 g of ethylene carbonate (EC); PF in solution Ai by ion chromatography ６ ‐ measuring the concentration (Ci) of Stirring at room temperature for 24 hours to obtain a solution At; and PF in solution At by ion chromatography ６ ‐ The concentration (Ct) of LiPF is measured by the step of measuring the concentration (Ct) of LiPF, which is calculated by the following formula (1): ６ LiPF with decomposition stability of 95% or more ６ It has a decomposition stability of The ion chromatography method is a method of measuring by mixing 1 g of a 1 wt % solution of LiOSO 2 CF 3 (LiOTf) as an internal standard substance with 1 g of the solution Ai and comparing the peak intensity (peak intensity of PF 6 − / peak intensity of OTf), Contains sulfamic acid derivatives at 500 ppm by weight or less, The bis(fluorosulfonyl)imide alkali metal salt is one which, when dissolved in water at a concentration of 10% by weight, gives a solution having a pH of 6-8. Formula (1) LiPF ６ Decomposition stability % = Ct / Ci x 100

2. (a) Bis(chlorosulfonyl)imide and NH under a nitrogen atmosphere ４ reacting F in solvent 1 to prepare ammonium bis(fluorosulfonyl)imide; (b) reacting the ammonium bis(fluorosulfonyl)imide obtained in step (a) with an alkali metal salt to prepare a bis(fluorosulfonyl)imide alkali metal salt; (c) adding solvent 2 as an antisolvent to the reaction solution of step (b) to obtain a precipitate; (d) filtering the solution of step (c) to recover the precipitate, thereby obtaining a bis(fluorosulfonyl)imide alkali metal salt; The solvent 1 contains water at a concentration of 500 ppm by weight or less, (a) during or after the reaction in step (a) and before step (b), nitrogen gas is bubbled through the reaction mixture to remove HF; The solvent 1 is at least one selected from the group consisting of diethyl ether, diisopropyl ether, methyl t-butyl ether, methyl acetate, ethyl acetate, propyl acetate, and butyl acetate; The solvent 2 is at least one selected from the group consisting of toluene, hexane, heptane, chloroform, and dichloromethane; When the alkali metal salt of bis(fluorosulfonyl)imide is dissolved in water at a concentration of 10% by weight, the pH of the solution becomes 6-8. preparing a solution Ai by dissolving 1 g of LiPF 6 and 1 g of the alkali metal salt of bis(fluorosulfonyl)imide in a mixed solvent of 50 g of water and 50 g of ethylene carbonate (EC); measuring the concentration (Ci) of PF 6 − in the solution Ai by ion chromatography; Stirring at room temperature for 24 hours to obtain a solution At; and The decomposition stability of LiPF 6 is 95% or more, as measured by measuring the concentration (Ct) of PF 6 − in the solution At using ion chromatography and calculated by the following formula (1): The ion chromatography method is a method of measuring by mixing 1 g of a 1 wt % solution of LiOSO 2 CF 3 (LiOTf) as an internal standard substance with 1 g of solution Ai and comparing the peak intensity (peak intensity of PF 6 − / peak intensity of OTf). LiPF containing sulfamic acid derivatives in an amount of 500 ppm by weight or less. ６ A method for producing an alkali metal bis(fluorosulfonyl)imide salt having decomposition stability of Formula (1) Decomposition stability % of LiPF 6 = Ct / Ci × 100

3. 3. The LiPF according to claim 2, wherein the step (b) comprises adding an alkali metal salt to the reaction solution of the step (a) without recovering the ammonium bis(fluorosulfonyl)imide as a solid. ６ A method for producing an alkali metal bis(fluorosulfonyl)imide salt having decomposition stability of

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