WO2020099527A1

WO2020099527A1 - Method for producing alkali sulfonyl imide salts

Info

Publication number: WO2020099527A1
Application number: PCT/EP2019/081252
Authority: WO
Inventors: Yeon-Joon Kim; Olivier Buisine
Original assignee: Solvay Sa
Priority date: 2018-11-16
Filing date: 2019-11-14
Publication date: 2020-05-22

Abstract

The invention relates to a new method for producing alkali salt of bis(fluorosulfonyl)imide of high purity, as industrial scale, and with a reasonable cost when compared to the other available methods. Said method comprises the steps of reacting bis(chlorosulfonyl)imide or salts thereof with ammonium fluoride to produce ammonium salt of bis(fluorosulfonyl)imide; crystallizing and separating the ammonium salt of bis(fluorosulfonyl)imide; and reacting the crystallized ammonium salt of bis(fluorosulfonyl)imide with an alkali salt to obtain alkali salt of bis(fluorosulfonyl)imide.

Description

METHOD FOR PRODUCING ALKALI SULFONYL IMIDE SALTS

TECHNICAL FIELD

The present invention relates to a method for producing alkali salts of bis(fluorosulfonyl)imide. More specifically, the invention provides a new method for producing alkali salts of bis(fluorosulfonyl)imide which is economically feasible at industrial scale and which provides a high-purity product.

BACKGROUND ART

Bis(fluorosulfonyl)imide (commonly represented by“FSIH”) and salts thereof, in particular the lithium salt of bis(fluorosulfonyl)imide (commonly represented by“LiFSI”), are useful as intermediate compound or as final compound in a variety of technical field.

The production of bis(fluorosulfonyl)imide and of the lithium salt of bis(fluorosulfonyl)imide is widely described in the literature. Among the various technologies described, the majority use a fluorination reaction either with HF or with metal fluorides, like KF, CsF, AsF3, SbF3, CuF2, ZnF2, SnF2, PbF2, B1F3, etc. Other technologies have been developed, for example using chlorosulfonyl isocyanate in the presence of oleum and of ammonium fluoride or using urea and fluorosulfonic acid.

Bis(fluorosulfonyl)imide and salts thereof are especially useful in battery electrolytes. For this type of use, the presence of impurities is an important issue.

To suppress the contamination of metal impurities, the prior art document US 2013/0331609 suggests a process for producing a fluorosulfonylimide ammonium salt including reacting a chlorosulfonlyimide compound with a fluorinating agent of formula NH4F(HF)_p, wherein p is 0 to 10. The thus obtained fluorosulfonylimide ammonium salt may be subjected to a cation exchange reaction to produce another fluorosulfonylimide salt. This process is said to be industrially efficient and provides no metal impurities.

Similarly, prior art documents JP 2016-124735 and JP 2016-145147 disclose a method for producing a fluorosulfonylimide compound comprising the reaction of a chlorosulfonylimide compound with NH4F(HF)_P, wherein p is 0 to 10. Said fluorosulfonylimide compound may be reacted with an alkali metal compound to produce an alkali metal salt of fluorosulfonylimide. The prior art document EP 3381923 discloses a method for producing lithium bis(fluorosulfonyl)imide in high yield and purity, which is supposed to be simple and cost- effective. Said method consists in reacting bis(chlorosulfonyl)imide with a fluorinating reagent in a solvent, followed by treatment with an alkaline reagent, thereby producing ammonium bis(fluorosulfonyl)imide, and then reacting the ammonium bis(fluorosulfonyl)imide with a lithium base to produce lithium bis(fluorosulfonyl)imide.

The prior art document WO 2016/093399 further disclose a method for producing and purifying lithium salt of sulfonyl imide. Said method consists in reacting chlorosulfonic acid and chlorosulfonyl isocyanate to prepare chlorosulfonyl imide, then reacting said chlorosulfonyl imide with a fluorinated ammonium to prepare a fluorosulfonyl imide ammonium salt, then reacting said fluorosulfonyl imide ammonium salt with a lithium compound to obtain the lithium sulfonyl imide salt, and finally purifying said lithium sulfonyl imide salt with the help of a specific solvent.

Even if these documents claims that the products are obtained with a high purity, we believe that there is still room for improvement for providing a new method for producing bis(fluorosulfonyl)imide and salts thereof which is economically feasible at industrial scale and which provides a high-purity product.

Besides, prior art document WO 2016/093400 discloses a method for preparing lithium bis(fluorosulfonyl)imide salt. Said method consists in reacting chlorosulfonic acid and chlorosulfonyl isocyanate to prepare chlorosulfonyl imide, then reacting said chlorosulfonyl imide with a N-fluoroalkyl ammonium which is described by the following formula:

(with n = 0-10 and Ri is hydrogen, halogen, nitro, fluoroammonia, phenyl, or C1-C6 linear branched or cyclic alkyl or alkoxy)

to obtain an intermediate compound of the following formula:

(with n and Ri as mentioned above). Said intermediate compound is reacted with a salt containing a lithium cation to obtain lithium bis(fluorosulfonyl) imide.

The use of a N-fluoroalkyl ammonium compound as disclosed in WO 2016/093400 as fluorinating agent is unusual and is not compatible with a use of this method at industrial scale. Additionally, the presence of an alkyl group on the intermediate cationic compound may potentially interfere with the solubility of the compounds, and consequently with the recrystallization.

The prior art document EP 2674395 discloses a process for producing a fluorosulfonylimide ammonium salt with good efficiency and maximum suppression of the contamination of metal impurities. Said process consists in reacting a specific chlorosulfonylimide ammonium salt with hydrogen fluoride. Then, the thus obtained fluorosulfonylimide ammonium salt can be reacted with an alkali metal compound to obtain a fluorosulfonylimide alkali metal salt. Compared to other process, this process necessitates an additional step consisting in preparing said chlorosulfonylimide ammonium salt. Additionally, using hydrogen fluoride as fluorinating agent is difficult because HF is a strong acid that is corrosive and highly toxic.

BRIEF DESCRIPTION OF THE INVENTION

The Applicant provides hereafter a new method for producing alkali salt of bis(fluorosulfonyl)imide of high purity, as industrial scale, and with a reasonable cost when compared to the other available methods.

One subject-matter of the invention is a method for producing an alkali salt of bis(fluorosulfonyl)imide, comprising the steps of:

a) reacting bis(chlorosulfonyl)imide or salts thereof with ammonium fluoride to produce ammonium salt of bis(fluorosulfonyl)imide;

b) crystallizing by adding at least one precipitation solvent and separating the ammonium salt of bis(fluorosulfonyl)imide; and

c) reacting the crystallized ammonium salt of bis(fluorosulfonyl)imide with an alkali salt to obtain alkali salt of bis(fluorosulfonyl)imide.

Another object of the present application relates to the intermediate crystallized ammonium salt of bis(fluorosulfonyl)imide obtainable at the end of step (b).

DESCRIPTION OF THE INVENTION

In the present disclosure, the expression“comprised between ... and should be understood has including the limits. The step (a) of the method according to the invention consists in reacting bis(chlorosulfonyl)imide or salts thereof with ammonium fluoride to produce ammonium salt of bis(fluorosulfonyl)imide.

Bis(chlorosulfonyl)imide or salts thereof is used as raw material. It may be represented by the formula:

(CI-SO2-N -SO2-CI) X⁺

wherein X represents one from the group consisting of H, Li, Na, K, Cs and NFL.

According to a preferred embodiment, the raw material is bis(chlorosulfonyl)imide of formula (C1-S0₂)₂-NH (commonly represented by CSIH). CSIH is commercially available, or produced by a known method, for example:

- by reacting chlorosulfonyl isocyanate CISO2NCO with chlorosulfonic acid CISO2OH;

- by reacting cyanogen chloride CNC1 with sulfuric anhydride SO3, and with chlorosulfonic acid CISO2OH;

- by reacting sulfamic acid NH2SO2OH with thionyl chloride SOCI2 and with chlorosulfonic acid CISO2OH.

According to the present invention, the fluorinating agent is ammonium fluoride NH4F. Within the present invention, the expression“ammonium fluoride” also includes HF adducts of ammonium fluoride, for example NFLF(FlF)_n, wherein n is 1 to 10, preferably 1 to 4, more preferably NH4F.HF or NFLF(F1F)2. The fluorinating agent may be commercially available, or produced by a known method.

According to a preferred embodiment, ammonium fluoride is anhydrous. Moisture content may be preferably below 5000 ppm, more preferably below 1000 ppm, even more preferably below 500 ppm.

The amount of ammonium fluoride used is preferably comprised between 1 and 10 equivalents, more preferably between 1 and 7 equivalents, and even more preferably between 2 and 5 equivalents, per 1 mol of the bis(chlorosulfonyl)imide or the salt thereof.

The reaction may be carried out preferably in an organic solvent. Said organic solvent may be selected from the aprotic organic solvents, preferably:

- cyclic and acyclic carbonates, for instance ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, - cyclic and acyclic esters, for instance gamma-butyro lactone, gamma- valero lactone, methyl formate, methyl acetate, methyl propionate, ethyl acetate, ethyl propionate, isopropyl acetate, propyl propionate, butyl acetate,

- cyclic and acyclic ethers, for instance diethylether, diisopropylether, methyl-t- butylether, dimethoxymethane, 1 ,2-dimethoxyethane, tetrahydrofuran, 2- methyltetrahydrofuran, 1,3-dioxane, 4-methyl- 1,3-dioxane, 1,4-dioxane,

- amide compounds, for instance N,N-dimethylformamide, N-methyl oxazolidinone,

- sulfoxide and sulfone compounds, for instance sulfolane, 3-methylsulfolane, dimethylsulfoxide,

- cyano-, nitro-, chloro- or alkyl- substituted alkane or aromatic hydrocarbon, for instance acetonitrile, valeronitrile, adiponitrile, benzonitrile, nitromethane, nitrobenzene.

According to a preferred embodiment, the organic solvent is selected from the group consisting of ethyl acetate, isopropyl acetate, butyl acetate, ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, propylene carbonate, valeronitrile and acetonitrile.

According to a preferred embodiment, the organic solvent is anhydrous. Moisture content may be preferably below 5000 ppm, more preferably below 1000 ppm, more preferably below 500 ppm, more preferably below 100 ppm even more preferably below 50 ppm.

The reaction may be carried out at a temperature of between 0°C and 200°C, preferably, between 30°C and 100°C. Preferably, the reaction is carried out at atmospheric pressure, but it is not excluded to work below or above atmospheric pressure, for instance between 800 mbar and 1.2 bar.

The reaction may be carried out in a batch, semi-batch or continuous mode. According to a preferred embodiment, the ammonium fluoride is first added to the organic solvent. Then, the bis(chlorosulfonyl)imide or a salt thereof may be added to the reaction medium.

By reacting bis(chlorosulfonyl)imide or salts thereof with ammonium fluoride according to the present invention, ammonium salt of bis(fluorosulfonyl)imide can be obtained.

After step (a), but before step (b), the method according to the present invention may comprise a step (a’) which consists in adding a basic compound to the reaction medium. Said basic compound may be a solid, a pure liquid, an aqueous or organic solution or a gas. Said basic compound may be selected from the group consisting of gaseous ammonia, ammonia water, amines, hydroxide, carbonates, phosphates, silicates, borates, formates, acetates, stearates, palmitates, propionates or oxalates of alkali or alkaline-earth metal. Among amines, any type of amines may be convenient, including, aliphatic amines (such as ethylamine, propylamine, butylamine, pentylamine, hexylamine, heptylamine, octylamine, dimethylamine, diethylamine, dipropylamine, dibutylamine, 2-ethylhexylamine, trimethylamine, triethylamine, tripropylamine and tributylamine), alkylenediamines (such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine and pentaethylenehexamine), alkanolamines (such as monoethanolamine, diethanolamine, triethanolamine, monoisopropanolamine, diisopropanolamine and triisopropanolamine), alicyclic amines (such as cyclo hexylamine and dicyclo hexylamine), aromatic amines (such as benzylamine and metaxylenediamine), ethylene oxide adducts of these amines, formamidine, guanidine, amidine, and heterocyclic amines (such as diazabicycloundecene, diazabicyclononene, piperidine, morpholine, piperazine, pyrimidine, pyrrole, imidazole, imidazoline, triazole, thiazole, pyridine and indole). The basic compound used in step (a’) according to the invention is preferably gaseous ammonia or ammonia water.

The amount of basic compound added in step (a’) is preferably of between 0.1 and 10 equivalents, preferably between 0.5 and 5 equivalents, more preferably between 0.5 and 3 equivalents, based on the initial quantity of bis(chlorosulfonyl)imide or salts thereof loaded in step (a) of the method according to the invention.

During step (a’), the temperature is preferably maintained between 0°C and 100°C, more preferably between 15°C and 90°C. Advantageously, this step (a’) may be carried out at the same temperature as the step (a).

Optionally, the method according to the invention may comprise between step (a) and step (a’) an intermediary separation step. This intermediary separation step may be performed by any typical separation means known by the person skilled in the art, for example by filtration (for instance under pressure or under vacuum) or decantation. Alternatively or in addition, such intermediate separation step may be carried out after step (a’) and before step (b). The step (b) of the method according to the invention consists in crystallizing and separating the ammonium salt of bis(fluorosulfonyl)imide.

Before the start of said crystallization at step (b), the concentration of the ammonium salt of bis(fluorosulfonyl)imide within the reaction medium may be comprised between 10% and 95% by weight, preferably between 30% and 80% by weight, and more preferably between 40% and 70% by weight. The reaction medium obtained directly at the end of step (a) may be used as such. Otherwise, the method may comprise a further step consisting in concentrating the ammonium salt of bis(fluorosulfonyl)imide within the reaction medium, typically by evaporating a part of the organic solvent of the reaction medium, by heating, by decreasing the pressure, or both. According to one embodiment, the concentration step may consists in a distillation of the solvent at a temperature comprised between 0°C and 120°C, preferably between 5°C and 80°C, more preferably between 10°C and 70°C. The pressure may be adjusted depending on the nature of the solvent, typically between atmospheric pressure and 10 ² mbar, preferably between 1 mbar and 500 mbar, and more preferably between 5 mbar and 100 mbar. The distillation may be performed by any typical means known by the person skilled in the art on a continuous process mode or on a discontinuous/batch mode, for example a continuous batch mode solvent evaporation, a batch distillation, a continuous flow distillation of a short path, or a thin film evaporator.

Crystallization of the salt is obtained by adding at least one precipitation solvent. At least one precipitation solvent may be added to reaction mixture containing the salt. Said precipitation solvent may preferably be selected among the organic solvent which are highly soluble within the organic solvent of the reaction mixture, and which are bad solvent for the ammonium salt of bis(fluorosulfonyl)imide. Said precipitation solvent may be selected from the group consisting of halogenated solvents like dichloromethane, dichloroethane, chloroform, and carbon tetrachloride; substituted aromatic hydrocarbon solvents like chlorobenzene, dichlorobenzene and toluene; and alkane solvents like cyclohexane, hexane, heptane, and Isopar™. Precipitation solvent may preferably be selected among dichloromethane and dichloroethane. The volume ratio between the precipitation solvent and the organic solvent of the reaction mixture may be comprised between 0.1 and 50, preferably between 0.2 and 20, more preferably between 0.5 and 15, and even more preferably between 1 and 10.

Optionally, water may be added to the reaction mixture before adding the precipitation solvent, at a content which may be of between 0.01% and 20%, preferably between 0.1% and 10%, and more preferably between 1% and 5%, based on the total weight of the reaction mixture.

In addition, crystallization of the salt may be foster by decreasing the temperature of the reaction mixture containing the salt, which may have been optionally previously concentrated. The temperature of the reaction mixture containing the salt may be decreased to a value below the temperature of solubility of the salt. Preferably, the temperature is decreased to a value comprised between the solvent boiling point and -20°C, more preferably between 70°C and -10°C, and even more preferably between 30°C and 0°C. During the reduction of the temperature, the pressure may preferably be kept constant. However, it is not excluded to reduce the pressure simultaneously. It may cause the evaporation of a part of the organic solvent of the reaction mixture. The pressure may be decreased to a value comprised between atmospheric pressure and 10 ² mbar, preferably between 1 mbar and 500 mbar, and more preferably between 5 mbar and 100 mbar.

According to one embodiment of the present invention, the step (b) of the method consists in adding a precipitating solvent without decreasing the temperature of the reaction mixture containing the salt. According to another embodiment, which is preferred, the step (b) of the method consists in adding a precipitating solvent and decreasing the temperature of the reaction mixture containing the salt. The precipitation solvent is preferably added first, and the temperature is decreased afterwards. However, it is not excluded to proceed the other way, or to carry out the two actions simultaneously.

In step (b) according to the invention, the separation of crystalized ammonium salt of bis(fluorosulfonyl)imide may be performed by any typical separation means known by the person skilled in the art, for example by filtration. Filtration may be carried out at atmospheric pressure, under pressure or under vacuum, by any means known by the person skilled in the art. Mesh size of the filtration medium may be preferably of 2 micrometer or below, more preferably of 0.45 micrometer or below, and even more preferably of 0.22 micrometer or below. Separated product may be washed once or several times with appropriate solvent. The crystallization and separation steps may be carried out one time or may be repeated twice or more if necessary to improve the purity of the separated crystallized salt.

Finally, the separated crystallized salt is preferably dried to obtain a pure dry product. Drying step may be carried out by any means known by the person skilled in the art, typically under reduced pressure and/or by heating and/or with an inert gas flow, typically a nitrogen flow.

Advantageously, the crystallized ammonium salt of bis(fluorosulfonyl)imide obtained at the end of step (b) of the method according to the invention has a very high purity. It may show:

- a purity of the salts above 90%, preferably above 95%, more preferably between 99% and 100% (mass percent); and/or - a content of solvent below 20%, preferably below 10%, more preferably between 0% and 1% (mass percent).

One object of the present application relates to the intermediate crystallized ammonium salt of bis(fluorosulfonyl)imide obtained, obtainable or able to be obtained, at the end of step (b).

Preferably, it may show the following contents of anions:

- a chloride (Cl ) content of below 10 000 ppm, preferably below 5 000 ppm, more preferably below 1 000 ppm, more preferably below 500 ppm, more preferably below 100 ppm, more preferably below 50 ppm, more preferably below 20 ppm; and/or

- a fluoride (F ) content of below 10 000 ppm, preferably below 5 000 ppm, more preferably below 1 000 ppm, more preferably below 500 ppm, more preferably below 100 ppm, more preferably below 50 ppm, more preferably below 20 ppm; and/or.

- a sulfate (SO₄ ² ) content of below 30 000 ppm, preferably below 10 000 ppm, more preferably below 5 000 ppm.

Preferably, it may show the following contents of metal elements:

- an iron (Fe) content of below 1 000 ppm, preferably below 800 ppm, more preferably below 500 ppm; and/or

- a chromium (Cr) content of below 1 000 ppm, preferably below 800 ppm, more preferably below 500 ppm; and/or

- a nickel (Ni) content of below 1 000 ppm, preferably below 800 ppm, more preferably below 500 ppm; and/or

- a zinc (Zn) content of below 1 000 ppm, preferably below 100 ppm, more preferably below 10 ppm, and/or

- a copper (Cu) content of below 1 000 ppm, preferably below 100 ppm, more preferably below 10 ppm; and/or

- a bismuth (Bi) content of below 1 000 ppm, preferably below 100 ppm, more preferably below 10 ppm.

Additionally, it may show:

- a sodium (Na) content of below 10 000 ppm, preferably below 5 000 ppm, more preferably below 500 ppm, and/or

- a potassium (K) content of below 10 000 ppm, preferably below 5 000 ppm, more preferably below 500 ppm. The crystallized ammonium salt of bis(fluorosulfonyl)imide is advantageously reengaged into a further reaction. Indeed, the method according to the present invention comprises a further step (c) consisting in reacting the crystallized ammonium salt of bis(fluorosulfonyl)imide with an alkali salt in order to obtain alkali salt of bis(fluorosulfonyl)imide.

The crystallized ammonium salt of bis(fluorosulfonyl)imide may be used as such or solubilized in a solvent, according to the nature of the alkali salt. According to a preferred embodiment, the crystallized ammonium salt of bis(fluorosulfonyl)imide is solubilized in an organic solvent, hereafter called“alkalinization solvent”. The alkalinization solvent may be the same or different from the reaction solvent used in step (a). Advantageously, the alkalinization solvent of step (c) is the same as the reaction solvent of step (a). Said alkalinization solvent may be selected from the aprotic organic solvents, preferably:

- cyclic and acyclic carbonates, for instance ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate,

- cyclic and acyclic esters, for instance gamma-butyro lactone, gamma- valero lactone, methyl formate, methyl acetate, methyl propionate, ethyl acetate, ethyl propionate, isopropyl acetate, propyl propionate, butyl acetate,

- amide compounds, for instance N,N-dimethylformamide, N-methyl oxazolidinone,

According to a preferred embodiment, the alkalinization solvent is selected from the group consisting of ethyl acetate, isopropyl acetate, butyl acetate, ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, propylene carbonate, valeronitrile and acetonitrile.

The alkali salt may be selected from the group consisting of lithium salt, sodium salt and potassium salt. Preferably, the alkali salt is a lithium salt, and the alkali salt of bis(fluorosulfonyl)imide obtained by the method according to the invention is lithium salt of bis(fluorosulfonyl)imide. Examples of alkali salts include alkali hydroxide, alkali hydroxide hydrate, alkali carbonate, alkali hydrogen carbonate, alkali chloride, alkali fluoride, alkoxide compounds, alkyl alkali compounds, alkali acetate, and alkali oxalate. Preferably, alkali hydroxide or alkali hydroxide hydrate may be used in step (c). If the alkali salt is a lithium salt, then the lithium salt may be selected from the group consisting of lithium hydroxide LiOH, lithium hydroxide hydrate LiOfEEEO, lithium carbonate L12CO3, lithium hydrogen carbonate LiHC0₃, lithium chloride LiCl, lithium fluoride LiF, alkoxide compounds such as CEbOLi and EtOLi; alkyl lithium compounds such as EtLi, BuLi and t-BuLi, lithium acetate CEbCOOLi, and lithium oxalate LbCbCb. Preferably, lithium hydroxide LiOH or lithium hydroxide hydrate L1OH.H2O may be used in step (c).

Said alkali salt may be added in step (c) as a solid, as a pure liquid or as an aqueous or organic solution.

The amount of alkali salt used is preferably comprised between 0.5 and 5 mol, more preferably between 0.9 and 2 mol, and even more preferably between 1 and 1.5 mol, per 1 mol of ammonium salt of bis(fluorosulfonyl)imide.

The reaction may be carried out at a temperature of between 0°C and 50°C, more preferably between 15°C and 35°C, and even more preferably at about the room

temperature. Preferably, the reaction is carried out at atmospheric pressure, but it is not excluded to work below or above atmospheric pressure, for instance between 5 mbar and 1.5 bar, preferably between 5 mbar and 100 mbar.

Further treatments may be carried out in order to recover very pure alkali salt of bis(fluorosulfonyl)imide. The reaction medium may be a biphasic (aqueous/organic) solution, especially when the alkali salt used in step (c) is an aqueous solution. In this case, the method may comprise a phase separation step, during which the aqueous phase is removed and the alkali salt of bis(fluorosulfonyl)imide is recovered in the organic phase. Additional steps may comprise filtration, concentration, extraction, recrystallization, purification by chromatography, drying and/or formulation.

Generally speaking, all raw materials used in the method according to the invention, including solvents, reagents, etc., may preferably show very high purity criteria. Preferably, their content of metal components such as Na, K, Ca, Mg, Fe, Cu, Cr, Ni, Zn, is below 10 ppm, more preferably below 2 ppm.

In addition, some of the steps or all steps of the method according to the invention are advantageously carried out in equipment capable of withstanding the corrosion of the reaction medium. For this purpose, materials are selected for the part in contact with the reaction medium that are corrosion-resistant, such as the alloys based on molybdenum, chromium, cobalt, iron, copper, manganese, titanium, zirconium, aluminum, carbon and tungsten, sold under the Hastelloy® brands or the alloys of nickel, chromium, iron and manganese to which copper and/or molybdenum are added, sold under the name Inconel® or Monel™, and more particularly the Hastelloy C276 or Inconel 600, 625 or 718 alloys. Stainless steels may also be selected, such as austenitic steels and more particularly the 304, 304L, 316 or 316L stainless steels. A steel having a nickel content of at most 22% by weight, preferably of between 6% and 20% and more preferentially of between 8% and 14%, is used. The 304 and 304L steels have a nickel content that varies between 8% and 12%, and the 316 and 316L steels have a nickel content that varies between 10% and 14%. More particularly, 316L steels are chosen. Use may also be made of equipment consisting of or coated with a polymeric compound resistant to the corrosion of the reaction medium. Mention may in particular be made of materials such as PTFE (polytetrafluoroethylene or Teflon) or PFA (perfluoroalkyl resins). Glass equipment may also be used. It will not be outside the scope of the invention to use an equivalent material. As other materials capable of being suitable for being in contact with the reaction medium, mention may also be made of graphite derivatives. Materials for filtration have to be compatible with the medium used. Fluorinated polymers (PTFE, PFA), loaded fluorinated polymers (Viton™), as well as polyesters (PET), polyurethanes, polypropylene, polyethylene, cotton, and other compatible materials can be used.

Advantageously, the alkali salt of bis(fluorosulfonyl)imide obtained by the method according to the invention has a very high purity. It may show a purity of salts above 90%, preferably above 95%, more preferably between 99% and 100%.

Preferably, it may show the following contents of anions:

- a fluoride (F ) content of below 10 000 ppm, preferably below 5 000 ppm, more preferably below 1 000 ppm, more preferably below 500 ppm, more preferably below 100 ppm, more preferably below 50 ppm, more preferably below 20 ppm; and/or

- a sulfate (SO4² ) content of below 30 000 ppm, preferably below 10 000 ppm, more preferably below 5 000 ppm. Preferably, it may show the following contents of metal elements:

Additionally, when the alkali salt of bis(fluorosulfonyl)imide is not sodium bis(fluorosulfonyl)imide, it may show:

- a sodium (Na) content of below 10 000 ppm, preferably below 5 000 ppm, more preferably below 500 ppm.

Additionally, when the alkali salt of bis(fluorosulfonyl)imide is not potassium bis(fluorosulfonyl)imide, it may show:

- a potassium (K) content of below 10 000 ppm, preferably below 5 000 ppm, more preferably below 500 ppm.

Thanks to its very high purity, the alkali salt of bis(fluorosulfonyl)imide, and preferably the lithium bis(fluorosulfonyl)imide, obtainable by the method according to the invention, may be advantageously used in electrolyte compositions for batteries.

Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

The invention will now be further described in examples, which are given by way of illustration and which are no intended to limit the specification or the claims in any manner. Example 1 :

The process has been carried out in a 500 mL reactor under N2 with stirring means, a double jacket for thermal regulation, a condenser, a pressure regulator means and a liquid or gas addition means. At room temperature, 400 g of ethyl methyl carbonate were introduced, and 81 g of anhydrous NEFF was suspended. 77 g of molten CSIH was added gradually during 1 hour, and the mixture was heated at 80°C under stirring during 15 hours. It was cooled to room temperature and 25 g of NH₄0H (aq) (ammonia water) was added. The obtained mixture was stirred at room temperature for lh and then filtered.

The obtained product was split in 2 parts: Part I = 305g; Part II = 332g.

Part I was concentrated, further dried under high vacuum (0.3mbar) for 4 days, and cooled down in a refrigerator to turn to 22g of NH₄FSI as a white solid. Yield = 64%

Part II was concentrated to 67g, and transferred to a glass reactor. 123g of dichloromethane were slowly added for 1 hour. Precipitated NH₄FSI was filtered, washed with dichloromethane, and dried in a vacuum oven to afford 27g of NH₄FSI as a white solid. Yield = 73%.

Example 2:

The process has been carried out in a 500 mL reactor under N2 with stirring means, a double jacket for thermal regulation, a condenser, a pressure regulator means and a liquid or gas addition means. At room temperature, 320 g of butyl acetate were introduced, and 62 g of anhydrous NH₄F was suspended. 82 g of molten CSIH was added gradually during 1 hour, and the mixture was heated at 80°C under stirring during 17 hours. It was cooled to room temperature and 26.3 g of NH₄OH (aq) (ammonia water) was added. The obtained mixture was stirred at room temperature for lh and then filtered.

The obtained product was split in 2 parts: Part I = 226g; Part II = 260g.

Part I was concentrated, further dried under high vacuum (0.3mbar) for 4 days, and cooled down in a refrigerator to turn to 20. lg of NH₄FSI as a white solid. Yield = 49%

Part II was concentrated to 70g, and transferred to a glass reactor. 120g of dichloromethane were slowly added for 1 hour. NH₄FSI seed was added. Precipitated NH₄FSI was filtered, washed with dichloromethane, and dried in a vacuum oven to afford 21.4g of NH₄FSI as a white solid. Yield = 61%.

From Example 1 and Example 2, we can see that ammonium bis(fluorosulfonyl)- imide product obtained according to the process of the invention (Parts II) has been recovered in an efficient and inexpensive manner, in a short time, in smooth conditions, using process units which can be implemented at industrial scale. On the contrary, the method according to prior art (Parts I) requires expensive and special technologies. Typically, it requires treatment during a very long time (several days), under high vacuum pressure (0.3mbar), and a final cooling was necessary to obtain a solid product.

Example 3 :

19.8 g of crystallized NH₄FSI obtained form Example 2 was solubilized in 200 g butyl acetate. 4.6 g of a 25wt% aqueous solution of LiOH.EfcO was added. The obtained biphasic mixture was stirred during 1 hour at room temperature, and then decanted. The organic phase was recovered and put into a thin film evaporator at 60°C under reduced pressure (10 ¹ bar). The purity of the obtained LiFSI was above 99.9%, and chlorine and fluorine contents were below 20 ppm. LiFSI total yield was 70%. Examples 4 to 6:

Example 1 - part II has been reproduced, except that a different precipitation solvent was used:

Claims

1. Method for producing an alkali salt of bis(fluorosulfonyl)imide, comprising the steps of:

2. Method according to Claim 1, wherein after step (a), but before step (b), the method further comprises a step (a’) which consists in adding a basic compound to the reaction medium.

3. Method according to Claim 2, wherein said basic compound is selected from the group consisting of gaseous ammonia, ammonia water, amines, hydroxide, carbonates, phosphates, silicates, borates, formates, acetates, stearates, palmitates, propionates or oxalates of alkali or alkaline-earth metal, preferably selected from the group consisting of gaseous ammonia or ammonia water.

4. Method according to any one of Claims 1 to 3, wherein before the start of the crystallization at step (b), the concentration of the ammonium salt of bis(fluorosulfonyl)imide within the reaction medium is comprised between 10% and 95% by weight, preferably between 30% and 80% by weight, and more preferably between 40% and 70% by weight.

5. Method according to any one of Claims 1 to 4, wherein the method comprises a further step consisting in concentrating the ammonium salt of bis(fluorosulfonyl)imide within the reaction medium before the start of the crystallization at step (b).

6. Method according to any one of Claims 1 to 5, wherein the step (b) consists in adding a precipitating solvent and decreasing the temperature of the reaction mixture containing the salt, which may have been optionally previously concentrated, and/or by adding a precipitating solvent.

7. Method according to any one of Claims 1 to 6, wherein the precipitation solvent is selected from the group consisting of halogenated solvents like dichloromethane, dichloroethane, chloroform, and carbon tetrachloride; substituted aromatic hydrocarbon solvents like chlorobenzene and toluene; and alkane solvents like hexane and heptane, preferably selected among dichloromethane and dichloroethane.

8. Method according to any one of Claims 1 to 7, wherein water is added to the precipitation solvent, at a content which is preferably of between 0.01% and 20%, more preferably between 0.1% and 10%, and even more preferably between 1% and 5%, based on the total weight of the precipitation solvent.

9. Method according to any one of Claims 1 to 8, wherein the volume ratio between the precipitation solvent and the organic solvent of the reaction mixture is comprised between 0.1 and 50, preferably between 0.2 and 20, more preferably between 0.5 and 15, and even more preferably between 1 and 10.

10. Method according to any one of Claims 1 to 9, wherein the separated crystallized salt is dried at the end of step (b).

11. Method according to any one of Claims 1 to 10, wherein before step (c), the crystallized ammonium salt of bis(fluorosulfonyl)imide obtained at step (b) is solubilized in an organic alkalinization solvent.

12. Method according to Claim 11, wherein the alkalinization solvent is the same as the reaction solvent used in step (a).

13. The intermediate crystallized ammonium salt of bis(fluorosulfonyl)imide obtainable at the end of step (b) of the method according to any one of Claims 1 to 12.

14. The crystallized ammonium salt of bis(fluorosulfonyl)imide according to Claim 13, showing: - a purity of the salts above 90%, preferably above 95%, more preferably between 99% and 100% (mass percent); and/or

- a content of solvent below 20%, preferably below 10%, more preferably between 0% and 1% (mass percent).

15. The crystallized ammonium salt of bis(fluorosulfonyl)imide according to Claim 13 or Claim 14, showing the following contents of anions:

- a sulfate (SO4² ) content of below 30 000 ppm, preferably below 10 000 ppm, more preferably below 5 000 ppm; and/or

- a bismuth (Bi) content of below 1 000 ppm, preferably below 100 ppm, more preferably below 10 ppm; and/or