WO2025032030A1 - High purity ammonium bis(fluorosulfonyl)imide and method for its manufacturing - Google Patents

Abstract

The present invention relates to a high purity ammonium salt of bis(fluoro sulfonyl)imide (NH₄FSI), to a process for its manufacturing and to its use as an intermediate to produce high purity lithium bis(fluorosulfonyl)imide (LiFSI).

Description

1 SPOP 2023/006 Description High purity ammonium bis(fluorosulfonyl)imide and method for its manufacturing Cross reference to related patent application [0001] The present invention claims priority filed in Europe on 07 Aug 2023 under No.23306342.9, the whole content of this application being incorporated herein by reference for all purposes. Technical field [0002] The present invention relates to a high purity ammonium salt of bis(fluoro sulfonyl)imide (NH4FSI), to a process for its manufacturing and to its use as an intermediate to produce high purity lithium bis(fluorosulfonyl)imide (LiFSI). Background [0003] Bis(fluorosulfonyl)imide and salts thereof, in particular the lithium salt of bis(fluorosulfonyl)imide (LiFSI), are useful compounds in a variety of technical fields, including in battery electrolytes. For these battery applications, the presence of impurities is an important issue, both in the final compound (e.g. LiFSI) and in the intermediate compounds (e.g. NH4FSI). [0004] To suppress the contamination of metal impurities, US 2013/0331609 (in the name of Nippon Soda Co., Ltd.) suggests a process for producing a fluorosulfonyl imide ammonium salt including reacting a chlorosulfonlyimide compound with a fluorinating agent of formula NH4F(HF)p, wherein p is 0 to 10. The obtained fluorosulfonyl imide ammonium salt may then be subjected to a cation exchange reaction to produce another fluorosulfonyl imide salt. 2 SPOP 2023/006 This process is said to be industrially efficient and to provide no metal impurities. [0005] Similarly, patent documents JP 2016124735 (in the name of Nippon Catalytic Chem Ltd.) and JP 2016145147 (in the name of Nippon Catalytic Chem Ltd.) disclose a method for producing a fluorosulfonyl imide compound comprising the reaction of a chlorosulfonyl imide compound with NH₄F(HF)_p, wherein p is 0 to 10. The fluorosulfonyl imide compound may be reacted with an alkali metal compound to produce an alkali metal salt of fluorosulfonyl imide. [0006] EP 3381923 (in the name of CLS) discloses a method for producing lithium bis(fluorosulfonyl)imide, which 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. [0007] WO 2016/093399 (in the name of Chun Bo. Ltd.) discloses a method for producing and purifying lithium salt of sulfonyl imide. This 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. [0008] EP 2674395 (in the name of Nippon Soda Co., Ltd.) discloses a process for producing a fluorosulfonyl imide ammonium salt with maximum suppression of the contamination of metal impurities. This process consists in reacting a specific chlorosulfonyl imide ammonium salt with hydrogen fluoride. The fluorosulfonyl imide ammonium salt obtained therefrom is then reacted with an alkali metal compound to obtain a fluorosulfonylimide alkali metal salt. 3 SPOP 2023/006 [0009] WO 2020/099527 (in the name of Solvay SA) discloses a process for producing an alkali salt of bis(fluorosulfonyl)imide economically feasible at industrial scale and which provides a high-purity product. Said process consists in reacting bis(chlorosulfonyl)imide or salts thereof with ammonium fluoride to produce ammonium salt of bis(fluorosulfonyl)imide, crystallizing by adding at least one precipitation solvent 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. [0010] KR 2020/0061058 discloses a method for purifying ammonium bis(fluorosulfonyl)imide via the reaction between HCSI and NH4-F, which is then neutralized with a neutralization agent (eg. NR4OHH) followed by re- crystallization in an organic solvent, thus reaching a purity of 99.50%. [0011] WO 2021/082450 discloses a method for purifying HFSI by using sCO2, wherein the method requires the addition of a strong acid to the HFSI composition. At the end of such method, the sCO2 extracts the final product and not the impurities. Summary of the invention [0012] Although several methods have been disclosed in the art aimed at purifying the salts used in the manufacturing of electrolytes for battery applications, the Applicant perceived that there is still the need of developing methods for preparing NH4FSI salt having high purity, such that it can be used as an intermediate for the manufacture of high purity LiFSI. [0013] Facing the above technical problem, the Applicant unexpectedly developed a process for the manufacture of NH4FSI, which is simple to perform in terms of apparatus and reaction conditions and very friendly from an environmental 4 SPOP 2023/006 perspective. The advantageous process for preparing NH4FSI according to the present invention is based on supercritical fluid extraction. [0014] Thus, an object of the present invention relates to a process for purifying an ammonium salt of bis(fluorosulfonyl)imide (NH₄FSI), which is economically feasible at industrial scale and which provides a high-purity product. [0015] Further to the above, the Applicant surprisingly manufactured a composition comprising ammonium salt of bis(fluorosulfonyl)imide, such composition being characterized by the presence of a specific compound in a determined amount. To the best of the Applicant knowledge, such compound has never been disclosed in the art as an ingredient in a composition comprising ammonium bis(fluorosulfonyl)imide. Drawings [0016] Figure 1 represents a scheme of the laboratory setup used in Example 1. Disclosure of the invention [0017] In the present application: - any description, even though described in relation to a specific embodiment, is applicable to and interchangeable with other embodiments of the present invention; - where an element or component is said to be included in and/or selected from a list of recited elements or components, it should be understood that in related embodiments explicitly contemplated here, the element or component can also be any one of the individual recited elements or components, or can also be selected from a group consisting of any two or more of the explicitly 5 SPOP 2023/006 listed elements or components; any element or component recited in a list of elements or components may be omitted from such list; and - any recitation of numerical ranges by endpoints includes all numbers subsumed within the recited ranges as well as the endpoints of the range and equivalents, unless otherwise specified. [0018] In a first aspect, the present invention relates to a composition [composition (COMP)] comprising: - an ammonium salt of bis(fluoro sulfonyl)imide [NH4FSI]; and - at least one compound of formula (I) as represented below or an ammonium salt thereof (I)

in an amount up to 200 ppm, as measured by ion chromatography. [0019] Preferably, composition (COMP) comprises said compound of formula (I) or an ammonium salt thereof in an amount below 100 ppm, more preferably below 75 ppm, even more preferably below 50 ppm and still more preferably below 25 or below 15 ppm or even below 5 ppm. [0020] Preferably, composition (COMP) comprises said compound of formula (I) or an ammonium salt thereof in an amount of at least 0.1 ppm, more preferably of at least 0.5 ppm, even more preferably of at least 1.0 ppm. [0021] Optionally, composition (COMP) can comprise further compounds and/or ingredients. 6 SPOP 2023/006 [0022] For example, composition (COMP) can comprise at least one salt of FSO3- in an amount up to 100 ppm, as measured by ion chromatography. [0023] Preferably, said composition (COMP) comprises said at least one salt of FSO₃- in an amount from 0.1 ppm to 100 ppm, preferably from 0.5 ppm to 50 ppm, more preferable from 0.5 ppm to 20 ppm and even more preferably from 0.5 ppm to 5 ppm. [0024] In alternative or at the same time, composition (COMP) can comprise at least one compound of formula (II) or a salt thereof: (II) . [0025] Preferably, said compound of formula (II) is in an amount up to 100 ppm as measured by ion chromatography. [0026] Preferably, composition (COMP) comprises said compound (II) in an amount up to 50 ppm and more preferably up to 10 ppm, as measured by ion chromatography. [0027] For sake of clarity, it should be understood that compounds (I) and (II) exist as represented above or in their deprotonated forms. [0028] For example, compound (I) exists as follows:

7 SPOP 2023/006 .

[0029] (II) exists as follows:

.

[0030] Advantageously, composition (COMP) comprises NH4FSI salt in an amount above 99.50%, preferably more than 99.60%, more preferably more than 99.80%, and even more preferably more than 99.95%, as measured by ¹⁹F- NMR. [0031] Even more preferably, the composition according to the present invention further comprises: - fluoride (F-), in an amount up to 50 ppm, for example less than 40 ppm, less than 30 ppm, less than 25 ppm and even more preferably less than 10 ppm as measured by Ion Chromatography (IC); and/or - chloride (Cl-), in an amount up to 50 ppm, for example less than 40 ppm, less than 30 ppm, less than 20 ppm, less than 10 ppm or even less than 5 ppm as measured by IC; and/or - sulfate (SO₄ ^2-), in an amount up to 50 ppm, for example less than 40 ppm, less than 30 ppm, less than 25 ppm and even more preferably less than 10 ppm as measured by IC; and/or 8 SPOP 2023/006 - acid substances different from sulfate (SO4^2-), in an amount up to 100 ppm, for example less than 50 ppm, less than 30 ppm, less than 10 ppm as measured by IC; and/or - water, in an amount up to 50 ppm, as measured by the KF analysis (oven method). [0032] Preferably, said acid substances different from sulfate (SO₄ ^2-) are selected from NH₂SO₃- and/or FSO₃-. [0033] Preferably, said acid substances different from sulfate (SO4^2-) are in the following amounts: - less than 50 ppm of sulfamate (NH₂SO₃-), for example less than 40 ppm, less than 30 ppm, less than 20 ppm, less than 10 ppm or even less than 5 ppm as measured by IC; and/or - less than 50 ppm of fluorosulfonate (FSO₃-), for example less than 40 ppm, less than 30 ppm, less than 20 ppm, less than 10 ppm or even less than 5 ppm as measured by IC. [0034] Composition (COMP) is further characterized in that it contains less than 50 ppm of water, as measured by the KF analysis (oven method), preferably, less than 40 ppm, less than 30 ppm, less than 20 ppm, less than 10 ppm or even less than 5 ppm of water. [0035] Preferably, said composition (COMP) is a solid composition. [0036] However, composition (COMP) can be provided as a liquid composition. [0037] According to this embodiment, composition (COMP) further comprises at least one solvent [solvent (S1)]. [0038] Preferably, said at least one solvent (S1) is selected in the group comprising, more preferably consisting of: ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, γ-valerolactone, dimethoxymethane, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxane, 9 SPOP 2023/006 4-methyl-1,3- dioxolane, methyl formate, methyl acetate, methyl propionate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, sulfolane, 3- methylsulfolane, dimethylsulfoxide, N,N-dimethylformamide, N-methyl oxazolidinone, acetonitrile, valeronitrile, benzonitrile, ethyl acetate, isopropyl acetate, n-butyl acetate, nitromethane and nitrobenzene. [0039] Even more preferably, said solvent (S1) is selected from ethylene carbonate, propylene carbonate, butylene carbonate, tetrahydrofuran, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl acetate, isopropyl acetate and n-butyl acetate, even more preferred solvents include dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl acetate, isopropyl acetate and n-butyl acetate. Advantageously, said solvent (S1) is selected from ethyl methyl carbonate and n-butyl acetate. [0040] A second object of the present invention relates to a method for purifying an ammonium salt of bis(fluorosulfonyl)imide (NH₄FSI), said method comprising the steps of: (a) providing a crude composition [crude NH₄FSI] comprising NH₄FSI and at least one other compound; (b) contacting said crude NH4FSI with at least one supercritical fluid; and (c) recovering a composition comprising NH4FSI and at least one other compound in an amount lower than the crude NH₄FSI. [0041] The term “crude composition” (hereinafter referred to as “crude NH4FSI”) hereby means that the composition comprises NH4FSI molecules in admixture with at least one other compound, which is an undesired compound negatively affecting the properties of the NH₄FSI when used - for example - as an intermediate for the production of high purity LiFSI electrolyte for battery applications. Such at least one other compound can be referred to as an “impurity”. Said impurity comprises for example ions, solvent(s), water and/or reaction by-products. For example, the crude 10 SPOP 2023/006 NH4FSI may comprise from 80 to 99 wt.% of NH4FSI, preferably 85-98 wt.%, more preferably 90-97 wt.% by weight, the rest being impurities to be removed through the process of the present invention. [0042] Advantageously, in the process of the present invention, the crude NH₄FSI can be in the form of a solid, such as powder, or of a slurry or of a liquid, such as a solution. [0043] The term “contacting” hereby means that the crude NH₄FSI is put in contact with the at least one supercritical fluid. For example, such contacting takes place in a vessel, under specific conditions of pressure and temperature, for a period of time sufficient for the fluid to remove at least part of the impurities present in the crude NH4FSI, preferably more than 80.00%, more than 90.00%, more than 95.00%, more than 99.00%, more than 99.50% or even more than 99.90%. More preferentially, all the impurities are removed and the NH₄FSI salt presents a purity above 99.95%, or even above 99.99%. [0044] The term “recovering” hereby means that the purified NH4FSI, preferably in solid form, is removed or extracted from the vessel in which step b) is carried out. [0045] The expression “supercritical fluid” hereby means a gas (or a mixture of at least two gasses) in its supercritical state. Depending on the gas employed in step b), the pressures and temperatures to be used in the vessel in which the contact between the solution and the supercritical fluid takes place are properly selected. More precisely, in order to be in a supercritical state, the gas employed in step b) is held at or above its critical temperature and critical pressure. [0046] According to the process of the present invention, at least one supercritical fluid is used to extract a purified NH₄FSI salt from the crude NH₄FSI, with several advantages. Supercritical fluids, such as sCO₂, offer many advantages, as they are usually easily available, inexpensive, non-toxic, non- 11 SPOP 2023/006 explosive, and not organic solvents. Additionally, the process of the present invention operates at a moderate temperature (below 100ºC), which ensures a gentle treatment of the NH₄FSI product. [0047] Preferably, step (a) is carried out in batch, semi-continuously or continuously. [0048] Preferably, said at least one other compound present in the crude NH4FSI is selected from the group comprising, more preferably consisting of: water (H₂O), fluoride (F-), chloride (Cl-), sulfate (SO₄ ^2-), sulfamate (NH₂SO₃-), flurosulfonate (FSO3-), [NH(SO2F)(SO2NH2)] or an ammonium salt thereof, and/or [NH(SO3H)(SO2F)] or an ammonium salt thereof. [0049] For example, the crude NH₄FSI to be purified may contain at least one of the following impurities: NH4Cl, NH4F, NH4HF2, NH4FSO3, NH4SO3NH2, [NH(SO2F)(SO2NH2)] or an ammonium salt thereof, and/or [NH(SO₃H)(SO₂F)] or an ammonium salt thereof. [0050] The parameters of the process according to the present invention can be properly selected and optimized based for example on the starting material (in particular, on the amount of the other compound(s) in the crude NH₄FSI) and on the scale at which the process is performed, for example is the process is performed at industrial scale or at laboratory scale. [0051] Preferably, step (b) is carried out at a pressure (p) of at least 80 bars. [0052] Preferably, step (b) is carried out at a temperature (T) between 30ºC and 90ºC. [0053] A particular advantage of the process of the present invention is that the contacting time under step (b) is short. Also, advantageously, the contacting time under step (b) can be properly selected for example on the basis of the starting material and the desired yield. [0054] Preferably, the contacting time under step (b) varies between a few seconds, for example 5 seconds, and 24 hours. More preferably, the contacting time 12 SPOP 2023/006 under step d) varies between 1 minute and 12 hours, for example between 5 minutes and 10 hours or between 10 minutes and 5 hours or between 20 minutes and 3 hours. [0055] In step (b), the crude NH₄FSI is contacted with at least one supercritical fluid. [0056] Preferably, step (b) takes place in a vessel. [0057] The term “vessel” hereby means a container which is well-suited for the process of the present invention, that-is-to-say which is adapted to withstand the pressures and temperatures used in the process of the present invention, as well as to the possible corrosive character of the reactants and products involved in this process. The vessel used herein may also be called an extraction vessel or an extraction device. Also, the vessel used herein can be an extraction column (also referred to as “column”) or an autoclave. [0058] According to a preferred embodiment, step (b) consists in contacting the crude NH₄FSI of step (a) with at least one supercritical fluid in a vessel. [0059] Preferably, under step (b), the crude NH4FSI is contacted with one fluid in a supercritical state. [0060] Preferably, under step (b), the crude NH₄FSI is contacted with two or more fluids in a supercritical state. Said two or more fluids may be mixed or may be contacted with the crude NH4FSI sequentially. As an example, the crude NH₄FSI may be contacted with a mixture of at least two supercritical fluids. [0061] Additionally, according to the present invention, at least one other component, also called herein modifier, may be mixed to the supercritical fluid(s). [0062] Advantageously, said at least one other component is selected from polar solvents having a solubility in the supercritical fluid below 10 wt.% based on the total weight of the supercritical fluids and the other component(s). 13 SPOP 2023/006 [0063] More preferably, when used, said at least one other component is in an amount ranging from 0.1 to 10 wt.%, for example from 0.5 to 8 wt.% or from 1 to 6 wt.%, based on the total weight of the supercritical fluids and the other component(s). [0064] Preferably, said at least one other component is selected from polar solvents, more preferably, in the group comprising: alcohol, toluene, dimethyl sulfoxide (DMSO), acetonitrile, and the like. According to a preferred embodiment, said polar solvent is alcohol. Even more preferably, said alcohol is ethanol. [0065] According to the present invention, step (b) may be repeated one or several times. [0066] For example, the process according to the present invention comprises a first step (b) and a second step (b’), wherein the same or different supercritical fluid(s), or a mixture of at least two supercritical fluids, are used in each of said step (b) and said step (b’). [0067] In some embodiments, the vessel which is preferably used to contact the crude NH₄FSI with the at least one supercritical fluid under step (b) is at a pressure (p) of at least 73 bars (7.3 MPa) during the extraction. [0068] In some embodiments, the vessel which is preferably used to contact the crude NH4FSI with the at least one supercritical fluid under step (b) is at a temperature (T) between 30 ºC and 90 ºC during the extraction. [0069] Preferably, the temperature (T) in the vessel may vary between 37 ºC and 75 ºC, for example between 38 ºC and 70 ºC or between 40 ºC and 65 ºC. [0070] Preferably, the pressure (p) in the vessel may be at least 80 bars (8.0 MPa), at least 100 bars (10.0 MPa), at least 130 bars (13.0 MPa), at least 150 bars (15.0 MPa) or at least 200 bars (20.0 MPa). A very high pressure can be used in the process of the present invention, for example, the pressure (p) in the vessel may be up to 300 bars (30.0 MPa). The pressure in the vessel will 14 SPOP 2023/006 usually be less than 500 bars (50.0 MPa), for example less than 450 bars (45.0 MPa), less than 400 bars (40.0 MPa), or even less than 350 bars (35.0 MPa). [0071] According to an embodiment, step (b) is carried out by injection of the crude NH4FSI in the vessel, which is already pressurized. [0072] According to a preferred embodiment, said crude NH₄FSI is in a solid form, preferably in the form of powder. [0073] According to another embodiment, said crude NH4FSI is in a liquid form, such as a solution comprising NH4FSI and an organic aprotic solvent. [0074] Preferably, said organic aprotic solvent is selected in the group comprising ethylene carbonate, propylene carbonate, butylene carbonate, γ- butyrolactone, γ-valerolactone, dimethoxymethane, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxane, 4-methyl-1,3 - dioxolane, methyl formate, methyl acetate, methyl propionate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, sulfolane, 3- methylsulfolane, dimethylsulfoxide, N,N-dimethylformamide, N-methyl oxazolidinone, acetonitrile, valeronitrile, benzonitrile, ethyl acetate, isopropyl acetate, n-butyl acetate, nitromethane and nitrobenzene. More preferably, said solvent is selected from ethylene carbonate, propylene carbonate, butylene carbonate, tetrahydrofuran, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl acetate, isopropyl acetate and n-butyl acetate. Even more preferably, said solvent is selected from dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl acetate, isopropyl acetate and n-butyl acetate. Still more preferably, said solvent is selected from ethyl methyl carbonate and n-butyl acetate. [0075] The crude NH₄FSI may for example be injected in the vessel through an injector or an entry valve which is mounted on the vessel. 15 SPOP 2023/006 [0076] According to another embodiment, step (b) comprises: (b1) introducing the crude NH4FSI in the vessel; (b2) pressurizing the vessel to a pressure (p); (b3) heating the vessel to a temperature (T); (b4) introducing the at least one supercritical fluid in the vessel. [0077] Preferably, step (b2) may be performed before step (b3), or step (b3) may be performed before step (b2), or step (b2) and step (b3) may be performed concomitantly. [0078] Preferably, the sequence of the steps might be as follows: (b1), (b3), (b4) and (b2). [0079] The conditions of temperature, pressure and contacting time detailed above with regard to step (b), apply to step (b2) and (b3) as described above. [0080] Preferably, step (b2) is performed at a pressure of at least 74 bars (7.4 MPa). [0081] Preferably, step (b3) is performed at a temperature of at least 30°C. [0082] The flow rate for introducing the supercritical fluid in the vessel under step (b4) is not particularly limited. Advantageously, the flow rate can be selected based on the apparatus used and the amount and purity of the crude NH4FSI. [0083] Step (b4) can be performed in batch, continuously or semi-continuously. [0084] Preferably, the supercritical fluid used in step (b) comprises supercritical carbon dioxide (sCO2). sCO2 is a fluid state of carbon dioxide that is held at or above its critical temperature (31.0 ºC) and critical pressure (7.3773 MPa). [0085] Advantageously, the supercritical fluid used in step (b) may consist essentially in sCO₂, or it may consist in sCO₂. [0086] According to an embodiment, the sCO2 is mixed with up to 10 wt.% of ethanol, for example with 0.1 to 8 wt.% of ethanol, the wt.% being based on the total weight of the supercritical fluid and the ethanol. 16 SPOP 2023/006 [0087] The weight ratio of the supercritical fluid to the crude NH4FSI used in the process of the present invention may vary between 1/1 and 400/1. For example, the weight ratio of the supercritical fluid to the crude NH₄FSI preferably varies between 5/1 and 350/1, for example between 20/1 and 300/1, between 30/1 and 250/1 or between 40/1 and 200/1. [0088] The process of the present invention may be carried out in a batch mode, in a continuous or semi-continuous mode. [0089] Preferably, the process is carried out in a continuous or semi-continuous manner. [0090] Preferably, the process of the present invention comprises a step of continuously or semi-continuously withdrawing the purified salt of NH4FSI from the vessel. [0091] Preferably, according to the present invention, the injection of the crude NH₄FSI in the vessel is performed in a continuous or semi-continuous manner. In other words, according to an embodiment, the crude NH4FSI is continuously injected in the vessel or alternatively the crude NH₄FSI is semi- continuously injected in the vessel. For example, the crude NH₄FSI may be injected in the vessel for a certain time (as an example between 30 and 120 sec, for example 60 sec) and then the injection is stopped for another period of time, which can be equal to, shorter or longer than the injection time. [0092] For example, according to the present invention, the supercritical fluid can be introduced in the vessel in a continuous or semi-continuous manner. [0093] For example, the process of the present invention may comprise a step of continuously or semi-continuously withdrawing the salt of NH₄FSI from the vessel. [0094] The purified NH₄FSI which is recovered in step (c) is preferably in solid form. The purified NH₄FSI is preferably recovered in the solid form, regardless if 17 SPOP 2023/006 the starting crude NH4FSI is in solid form, in the form of a slurry or a liquid. Even more preferably, said purified NH4FSI is in the form of a powder. [0095] Under step (c), the NH₄FSI in solid form can be recovered once step (b) is finished or while step b) is proceeding. [0096] According to a specific embodiment of step (c), the purified NH4FSI flows into a separator with the supercritical fluid. The pressure is released and the supercritical fluid becomes a gas. Such gas is preferably recycled, as detailed below. [0097] The process of the present invention may further comprise additional steps, such as preferably at least one step consisting in recycling the supercritical fluid. [0098] For example, the supercritical fluid may be reinjected in the process of the present invention as such or after additional step(s) of purification. [0099] The recycling of the supercritical fluid may be performed in several ways. [00100] According to an embodiment, the supercritical fluid may be recycled in a continuous way during the process. Preferably, it is recycled using a supercritical fluid pipe under pressure. [00101] According to another embodiment, the supercritical fluid may be recovered as a liquid phase by releasing the pressure in the vessel, and then repressurizing it in its gas form, for example by means of a compressor, in order to recycle it as a supercritical fluid which can be rejected in the vessel. [00102] The process of the present invention may be carried out in an equipment comprising: - a vessel, which withstands the pressure and temperature used, for example a pressure (p) of at least 73 bars (7.3 MPa) and a temperature (T) above 30ºC; - a solvent trap; 18 SPOP 2023/006 - a gas tank and a supercritical gas generator; - at least one injector/entry valve mounted on the vessel; and - optionally a separator. [00103] The vessel may preferably be made of sapphire, SS316L, glass or graphite filled PTFE. [00104] The equipment may include a separator. Different separators may be used in the process of the present invention. In some embodiments, the separation may be carried out through traditional filtration (also referred to as "dead end filtration") or cross filtration, which is also called tangential filtration, as disclosed for example in US 2007/0021570 (in the name of Solvay SA.). Alternatively, cyclonic separators may be used, for example those which operate as gas/solid separators. The cyclonic separators are advantageous as solids which could plug the filter media are recovered. Several hybrid devices exist based on this principle. Reference can notably be made to patent US 7,410,620 (in the name of North Carolina State University). [00105] Preferably, the solid NH₄FSI is recovered at the end of the process via a frit filter. [00106] For example, said frit filter can be made of stainless steel. Preferably, said frit filter has at least one of the following characteristics: pore size between 1 and 6 µm, preferably from 2 to 4 µm; diameter between 1 and 20 mm, preferably between 5 and 15 mm, more preferably about 10 mm; and/or a thickness from 0.1 to 5 mm, preferably between 0.7 and 3.5 mm, more preferably between 1.5 and 2.5 mm. [00107] If the equipment comprises a filter, or several filters, the filter(s) may notably be positioned at the bottom or at the top of the vessel. [00108] Advantageously, composition (COMP) as defined above is obtained at the end of step (c) of the method according to the present invention. 19 SPOP 2023/006 [00109] Another object of the present invention relates to the use of ammonium salt of bis(fluorosulfonyl)imide (NH4FSI) in the solid form according to the present invention, as an intermediate in the manufacture of LiFSI, a battery electrolyte salt. [00110] Another object of the present invention is the use of supercritical fluid extraction for purifying a crude ammonium salt of bis(fluorosulfonyl)imide (NH₄FSI) comprising NH₄FSI and at least one impurity. [00111] 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. [00112] The present invention will be now described in more detail with reference to the following examples, whose purpose is merely illustrative and not intended to limit the scope of the disclosure. [00113] EXAMPLES [00114] Materials: [00115] Crude NH4FSI, manufactured by the Applicant, with the following characterization: - water <50 ppm, as determined by the KF analysis (oven method) described below in more details - F- = <500 ppm, as determined by IC* - Cl- = 20 ppm, as determined by IC* - SO₄ ^2- = <300 ppm, as determined by IC* - NH2SO3- = <200 ppm, as determined by IC* - FSO₃- = <10 ppm, as determined by IC* 20 SPOP 2023/006 - NFSI = <500 ppm, as determined by IC* - Purity = 92%, as determined by ¹⁹F-NMR *IC: Ion Chromatography (DIONEX ICS-3000) [00116] According to the KF analysis (oven method), the sample was prepared by a fully automated oven sample processor (Metrohm); sample weight as 0.1 g, carrier gas = N₂, oven temperature = 160ºC. The titration was performed using a mixture of methanol and NH₄F (1:1 v/v). The polarization stream for potentiometric determination of reaction endpoint was 10 µA and titration endpoint voltage was 50 mV. [00117] Example 1 – Preparation of high purity NH4FSI in powder form starting from crude NH4FSI [00118] The equipment setup of Figure 1 was used and the experiment was performed with the following procedure. [00119] 10.668 g of crude NH4FSI was introduced into a vessel. The vessel was pressurized with sCO₂ (Temperature 45ºC; Pressure 200 bars; CO₂/NH₄FSI solution ratio varying between 30 and 400 along the process). The exit valve was then opened at the desired level to set the flow rate of CO2 (165 g/h). After a contacting time in the vessel of about 2.8 h, the entry valve, between the buffer tank and the vessel, was closed so that the vessel and the exit line were depressurized. [00120] The following were obtained. [00121] A mass loss of 0.07 g (0.6 wt.%) was observed after 2.8 h of extraction. A plateau was observed at CO₂/crude NH₄FSI mass ratio = 100, corresponding to 2.8 h of extraction. [00122] The extracted powder flowed easily off the vessel with gravity. In comparison, the crude NH₄FSI was sticky on the vessel surface. 21 SPOP 2023/006 [00123] The water content in a sample of the NH4FSI solid product was lower than 50 ppm, as measured according to the KF analysis (oven method). [00124] No main impurities were detected by Ion Chromatography (DIONEX ICS- 3000): - F- = < 10 ppm - Cl- = < 5 ppm - SO₄ ^2- = < 10 ppm - NH2SO3- = <10 ppm - FSO3- = <10 ppm - [N(SO₂NH₂)(SO₂F)]- = <10 ppm - [N(SO3H)(SO2F)]- = <10 ppm [00125] A purity of more than 99.9 % was determined by ¹⁹F-NMR.

Claims

22 SPOP 2023/006 Claims 1. A composition [composition (COMP)] comprising: - an ammonium salt of bis(fluoro sulfonyl)imide [NH₄FSI salt]; and - at least one compound of formula (I) as represented below or an ammonium salt thereof (I)

in an amount up to 200 ppm, as measured by ion chromatography. 2. Composition (COMP) according to Claim 1, said composition (COMP) comprising said compound of formula (I) or an ammonium salt thereof: - in an amount below 100 ppm, more preferably below 75 ppm, even more preferably below 50 ppm and still more preferably below 25 or below 15 ppm. 3. Composition (COMP) according to Claim 1 or 2, said composition (COMP) comprising at least one salt of FSO₃- in an amount up to 100 ppm, as measured by ion chromatography. 4. Composition (COMP) according to any one of Claims 1 to 3, said composition (COMP) comprising at least one compound of formula (II) or a salt thereof: (II) ,

23 SPOP 2023/006 preferably in an amount up to 100 ppm as measured by ion chromatography. 5. Composition (COMP) according to any one of the preceding Claims, said composition (COMP) comprising NH₄FSI salt in an amount above 99.50%, preferably more than 99.60%, more preferably more than 99.80%, and even more preferably more than 99.95%, as measured by ¹⁹F-NMR. 6. Composition (COMP) according to any one of the preceding Claims, said composition (COMP) comprising: - fluoride (F-), in an amount up to 50 ppm, for example less than 40 ppm, less than 30 ppm, less than 25 ppm and even more preferably less than 10 ppm as measured by Ion Chromatography (IC); and/or - chloride (Cl-), in an amount up to 50 ppm, for example less than 40 ppm, less than 30 ppm, less than 20 ppm, less than 10 ppm or even less than 5 ppm as measured by IC; and/or - sulfate (SO₄ ^2-), in an amount up to 50 ppm, for example less than 40 ppm, less than 30 ppm, less than 25 ppm and even more preferably less than 10 ppm as measured by IC; and/or - acid substances different from sulfate (SO4^2-), in an amount up to 100 ppm, for example less than 50 ppm, less than 30 ppm, less than 10 ppm as measured by IC; and/or - water, in an amount up to 50 ppm, as measured by the KF analysis (oven method). 7. A method for purifying an ammonium salt of bis(fluorosulfonyl)imide (NH₄FSI), said method comprising the steps of: (a) providing a crude composition [crude NH₄FSI] comprising NH₄FSI and at least one other compound; (b) contacting said crude NH4FSI with at least one supercritical fluid; and 24 SPOP 2023/006 (c) recovering a composition comprising NH4FSI and at least one other compound in an amount lower than the crude NH4FSI. 8. The method according to Claim 7, wherein said at least one supercritical fluid in step (b) is selected from: one fluid in a supercritical state, or a mixture of at least two fluids in supercritical state. 9. The method according to any one of Claims 7 or 8, wherein step (b) is carried out in a vessel at a pressure (p) of at least 73 bars (7.3 MPa) and/or a temperature (T) between 30 ºC and 90 ºC. 10. The method according to any one of Claims 7 to 9, wherein said supercritical fluid(s) comprises CO2, optionally in admixture with at least one polar solvent having a solubility in the supercritical fluid below 10 wt.% based on the total weight of said supercritical fluid and said at least one polar solvent. 11. The method according to any one of Claims 7 to 10, wherein step (b) comprises: (b1) introducing the crude NH₄FSI in the vessel; (b2) pressurizing the vessel to a pressure (p); (b3) heating the vessel to a temperature (T); (b4) introducing the at least one supercritical fluid in the vessel. 12. The method according to Claim 11, wherein step (b2) and (b3) are performed concomitantly. 13. The method according to any one of Claims 7 to 12, wherein said at least one other compound is selected from the group comprising, preferably consisting of: water (H₂O), fluoride (F-), chloride (Cl-), sulfate (SO₄ ^2-), sulfamate (NH₂SO₃-), flurosulfonate (FSO₃-), [NH(SO2NH2)(SO2F)] or an ammonium salt thereof, and/or [NH(SO3H)(SO2F)] or an 25 SPOP 2023/006 ammonium salt thereof. 14. The method according to any one of Claims 7 to 13, further comprising at least one step of recycling the supercritical fluid. 15. Composition (COMP) according to any one of Claims 1 to 6, which is obtainable by the method according to any one of Claims 7 to 14. 16. Use of composition (COMP) according to any one of Claims 1 to 6, as an intermediate in the manufacture of lithium bis(fluorosulfonyl)imide. 17. Use of supercritical fluid extraction for purifying a crude composition of ammonium bis(fluorosulfonyl)imide (crude NH₄FSI) comprising NH₄FSI and at least one impurity, preferably selected from the group comprising, preferably consisting of: water (H₂O), fluoride (F-), chloride (Cl-), sulfate (SO4^2-), sulfamate (NH2SO3-), flurosulfonate (FSO3-), NH₄[N(SO₂NH₂)(SO₂F)] , and/or NH₄[N(SO₃H)(SO₂F)].