Method for manufacturing bis(fluorosulfonyl)imide salts

Abstract

The present invention relates to a method for preparing a salt of bis(fluorosulfonyl)imide, preferably lithium bis(fluorosulfonyl)imide (LiFSI).

Description

DescriptionMethod for manufacturing bis(fluorosulfonyl)imide saltsCross reference to related patent application

[0001] The present invention claims priority from patent application filed in Europe on 7 August 2023, no. 23306344.5, the whole content of this application being incorporated herein for all purposes.Technical field

[0002] The present invention relates to a method for preparing a salt of bis(fluorosulfonyl)imide, preferably 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.

[0004] Several methods and processes for the manufacture of alkali metal salts of bis(fluorosulfonyl)imide are currently under development and have been described in literature and patent documents.

[0005] Among the various technologies described, the direct lithiation of HFSI to produce high purity LiFSI uses a variety of solvents and lithiation agents, and is followed by post-purification treatments to recover solid LiFSI.

[0006] US 2019 / 0292054 (in the name of Nippon Shokubai Co., Ltd.) discloses a method for producing an alkali metal salt of bis(fluorosulfonyl)imide comprising reacting bis(fluorosulfonyl)imide with an alkali metal compound in a reaction solution containing an organic solvent selected from carbonate solvents, cyclic ether solvents, linear ether solvents having two or more oxygen atoms in the molecule, cyclic ester solvents, sulfolane solvents, N,N-dimethyl formamide, dimethyl sulfoxide and N-methyl oxazolidinone. Among the alkali metal compounds listed in the general description, LiCI and LiF are disclosed as the most preferred. The patent application clearly discloses that when these compounds are used, the purification of the bis(fluorosulfonyl)imide alkali metal salt becomes easy because the boiling point of HCI and HF generated as by / product in the reaction are low which enables to remove the by product by a volatilization operation. On the contrary, hydroxides, carbonates and bicarbonates are known to generate water, which is preferably avoided because it has harmful influence on the battery. The examples in this patent application are performed with LiF only.

[0007] EP 3825278 (in the name of Shanghai Rolechem Co., Ltd.) discloses a method for preparing a bis(fluorosulfonyl)imide salt starting from bis(fluorosulfonyl)imide and M+nXn" wherein M is selected from Li, Na, K, Rb and Cs; and X is an anion includingat least one element of B, O, N, P and Si, and n being equal to or higher than 2; wherein the two ingredients are mixed into a non-aqueous solvent, reacted and a post-treatment (including for example filtration, vacuum concentration and recrystallization in a poor solvent) is then carried out to obtain the final product. The M+nXn“ compound is used in molar excess to the bis(fluorosulfonyl)imide, the ratio being preferably 4:0.5. However, such a large excess of the M+nXn“ compound makes the process very expensive. Also this patent application discloses that when alkali hydroxides are used, water is generated in the process and a side reaction of hydrolysis of the FSI salt occurs, generating impurity ions.

[0008] US 10,505,228 (in the name of Synthio Chemicals, LLC.) discloses a method for removing water from a liquid solution comprising a non-aqueous solvent, a hygroscopic metal salt and water. More in particular, an embodiment of the method includes mixing the following ingredients: (i) HFSI, (ii) an aprotic solvent and (iii) a lithium base, to produce a solution mixture comprising LiFSI and a vapor comprising water. Such step is followed by the removal of said vapor from the solution to produce the aprotic electrolyte solution. Under the process disclosed in this patent, the neutralization and the distillation or drying steps are disclosed as subsequent steps. As a consequence, the water content in the reaction medium during the process can be relatively high and remain high at the end of the neutralization step, which might generate FSI side-products. In addition, the overall process requires a long time.

[0009] CN 113 800485 (in the name of DO FLUORIDE CHEMICALS CO., LTD.) discloses a preparation method of lithium bis(fluorosulfonyl)imide, which comprises the steps of : reacting bis(fluorosulfonyl)imide with lithium phosphate in a non-aqueous solvent to obtain a reaction solution containing lithium bis(fluorosulfonyl)imide, and performing post-treatment to obtain lithium bis(fluorosulfonyl)imide. Among the other, Embodiment 4 discloses a method wherein lithium phosphate was added to dimethyl carbonate at 60°C. HFSI was added for 1.5 hour in a molar ratio 2:1.1 (HFSI to Li- phosphate, wherein HFSI is in molar excess). The reaction solution was filtered, concentrated under reduced pressure at 50°C and LiFSI crystals were obtained with a purity of 96.6%.

[0010] US 10,829,377 (in the name of NIPPON CATALYTIC CHEM IND) discloses a method for producing a bis(fluorosulfonyl)imide alkali metal salt, wherein the alkali metal compound is preferably selected from LiCI, LiF and U2CO3. However, the latter is known for generating water on the reaction with bis(fluorosulfonyl)imide and the generation of water is undesired because it would have a harmful influence on the battery. For this reason, LiCI and LiF are most preferred. Indeed, the use of these two reactants is said to be advantageous because the purification would be easier because the boiling point of HCI and HF generated as by-product is low, which enables to remove the by-product by a volatilization operation. Also, LiF is particularly preferred because it has small influence on the final product battery. The hydroxide and the alkoxide - due to their high basisicty - may cause side reactions with the organic solvent contained in the reaction solution.Summary of the invention

[0011] The Applicant is aware that when solid LiFSi is obtained, handling such a hygroscopic product in perfectly anhydrous conditions to formulate electrolytes is very laborious and associated with safety risks (including among the others manual operations and operator exposure), especially at industrial scale.

[0012] Hence, the Applicant faced the problem of providing a stable LiFSi solution, in a suitable solvent, which can be directly used in the manufacture of an electrolyte formulation.

[0013] More in particular, the Applicant faced the problem of developing a method for the manufacture of a solution of a high purity salt of bis(fluorosulfonyl)imide wherein the formation of FSI side-products is limited or even avoided, such that a high purity salt of bis(fluorosulfonyl)imide is obtained and the need for post-purification step(s) is limited.

[0014] Surprisingly, the Applicant developed a method for the manufacture of a solution of a high purity salt of bis(fluorosulfonyl)imide wherein certain FSI side-products are in the solid form, such that they can be easily removed via filtration or any other solid / liquid separation method.

[0015] Also, the method developed by the Applicant is characterized by a low energy consumption, which makes it suitable for industrial application.

[0016] Further to the above, the inventive method according to the present invention advantageously provides for a solution in a solvent suitable for non-aqueous electrolyte formulations, whose handling is much easier than the solid form.

[0017] Thus, a first aspect of the present invention relates to a method for manufacturing a solution comprising at least one organic aprotic solvent and at least one salt of bis(fluorosulfonyl)imide.Detailed description of the invention

[0018] In the present description and in the following claims:- the numerical ranges include the limits, unless otherwise specified;- 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 listed elements or components; any element or component recited in a list of elements or components may be omitted from such list;- any recitation herein of numerical ranges by endpoints includes all numbers subsumed within the recited ranges as well as the endpoints of the range and equivalents;- the terms “heating” ot “to heat” are intended to indicate the process of providingheat to the reactants until the desired temperature is reached or the process of controlling the temperature of the reactants.

[0019] In a first embodiment, the present invention relates to a method for manufacturing a solution [solution (S1)] comprising at least one organic aprotic solvent and at least one bis(fluorosulfonyl)imide salt (salt-FSI) represented by the following formula (I):whereinMn+represents a metal cation and n is an integer from 1 to 4, preferably 1 or 2; said method comprising the following steps:(A) contacting hydrogen bis(fluorosulfonyl)imide (HFSI), at least one aprotic organic solvent [solvent (S)] and at least one alkali metal compound [compound (AM)], so as to provide a reaction mixture [mixture (M1)],(B) heating said mixture (M1) at a temperature of at least 30°C,(C) letting said mixture (M1) to react, thus obtaining a solution (S) comprising at least one aprotic solvent, at least one salt-FSI of formula (I) as defined above and at least one by-product in the form of solid particles, and(D) separating at least a part of said at least one by-product in the form of solid particles, thus obtaining said solution (S1).

[0020] Surprisingly, the Applicant found that controlling the temperature of mixture (M1), in particular heating at a temperature of at least 30°C, allows to hydrolyse certain byproducts as insoluble solid particles, which can be easily removed via step (D), while the salt-FSI is stable and does not degrade into additional impurities.

[0021] Without being bound by any theory, the Applicant noted that in the method of the present invention, water can derive from several sources. For example, watere can be present in traces in the aprotic solvent (S), or as residual moisture or stoichiometric water in compound (AM) (for example when such compound is an hydrate form, such as UHO.H2O), or water is formed by the reaction of HFSI with the compound (AM) (for example when such compound is a hydroxide or a carbonate). Such water can reacts with certain species, for example with FSCL anions according to the following reaction:FSO3- + H2O F’ + SO42’ + 2H+

[0022] Advantageously, the F- and SC>42’ anions form with lithium cations insoluble salts, which precipitates as Li F and U2SO4 and can subsequently removed via a solid / liquid separation. Thus, advantageously, the method according to the present invention allows to avoid a step of removal of water, such as for example drying or azeotropic drying, while steps (B) and (C) proceed, or at the end of step (C), or after step (C) and before step (D). In other words, in the method according to the presentinvention, no step of water removal is performed after step (B) and / or after step (C) and / or before step (D).

[0023] Preferably, said metal cation Mn+is an alkali metal cation, more preferably selected from Na, Li, K, Rb, and Cs. Among these, Li, Na and K are more preferred.

[0024] Preferably, said compound (AM) is selected from inorganic compounds, and more preferably from the group comprising, even more preferably consisting of: LiOH, NaOH, KOH, RbOH, CsOH, LiOH.H2O, NaOH.H2O, KOH.H2O, RbOH.H2O, CSOH. H2O, Li2CO3, Na2CO3, K2CO3, Rb2CO3, Cs2CO3, LiHCO3, NaHCO3, KHCO3, RbHCO3and CsHCO3. More preferably, said compound (AM) is selected from the group consisting of LiOH.H2O, NaOH.H2O, KOH.H2O, RbOH.H2O, CsOH.H2O, Li2CO3, Na2CO3, K2CO3, Rb2CO3and Cs2CO3. Even more preferably, said compound (AM) is selected from the group consisting of: LiOH.H2O and Li2CO3.

[0025] Preferably, the amount of said compound (AM) is from about 1.0 mol to about 2.0 mol, more preferably of from 1 .0 mol to 1.5 mol, even more preferably of from 1.0 mol to 1.4 mol, and still more preferably from 1.0 mol to 1.3 mol, per 1.0 mol of HFSI. Even more preferably, the amount of said compound (AM) is from about 1.00 to 1.10 per mol of HFSI.

[0026] Preferably, said solvent (S) is selected in the group comprising ethylene carbonate, propylene carbonate, butylene carbonate, y-butyrolactone, y-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 (S) 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. Even more preferably, said solvent (S) is selected from ethyl methyl carbonate and n-butyl acetate.

[0027] According to a preferred aspect, said solvent (S) has a water content of 500 ppm or less, preferably of 250 ppm or less, more preferably of 100 ppm or less, and even more preferably of 50 ppm or less.

[0028] Preferably, step (A) is performed at room temperature, which is a temperature below 30°C, such as between 20°C and 27°C.

[0029] Preferably, step (B) is performed at a temperature of at least 30°C, more preferably at a temperature of at least 32°C, and even more preferably at a temperature of at least 35°C.

[0030] Preferably, step (B) is performed at a temperature lower than 120°C, more preferably lower than 100°C, even more preferably lower than 80°C.

[0031] Advantageously, step (B) is performed at a temperature between 35°C and 60°C, more preferably between 35°C and 55°C.

[0032] Preferably, step (A) and step (B) are performed simultaneously.

[0033] According to this embodiment, step (A) is performed by contacting said compound (AM), said solvent (S1) and the HFSI, while heating.

[0034] Preferably, step (A), step (B) and step (C) are performed simultaneously.

[0035] Preferably, step (A), step (B) and step (C) are performed at the same pressure.

[0036] Preferably, each of step (A), step (B) and step (C) is performed at a pressure from 1 mbar to 1 bar. More preferably at a pressure of about 1 bar.

[0037] The reaction time required for step (C) is not limited. Preferably, the reaction time for step (C) is from about 10 minutes to 48 hours, more preferably between 30 minutes and 24 hours and even more preferably between 1 hour and 10 hours.

[0038] Preferably, step (A) comprises:(Ai) providing a mixture (M-i) comprising said compound (AM) and said solvent (S), and(Aii) contacting said HFSI with said mixture (M-i), so as to provide mixture (M1).

[0039] According to an embodiment, the method of the present invention comprises:(Ai) providing a mixture (M-i) comprising said compound (AM) and said solvent (S), (Aii) contacting said HFSI with said mixture (M-i), so as to provide mixture (M1);(B) heating said mixture (M1) at a temperature of at least 30°C,(C) letting said mixture (M1) to react, thus obtaining a solution (S) comprising at least one aprotic solvent, at least one salt-FSI of formula (I) as defined above and at least one by-product in the form of solid particles, and(D) separating at least a part of said at least one by-product in the form of solid particles, thus obtaining said solution (S1).

[0040] Preferably, step (Ai) is performed at room temperature, which is below 30°C, preferably between 20°C and 27°C.

[0041] Preferably, step (Aii) and step (B) are performed simultaneously, after step (Ai). Thus, step (Aii) is performed under heating at a temperature of at least 30°C, preferably of at least 32°C and even more preferably at a temperature of at least 35°C.

[0042] Preferably, step (Aii) is performed at a temperature lower than 120°C, more preferably lower than 100°C, even more preferably lower than 80°C.

[0043] According to another embodiment, the method of the present invention comprises: (Ai) providing a mixture (M-i) comprising said compound (AM) and said solvent (S), (B*) heating said compound (AM) and said solvent (S) at a temperature of at last 30°C,(Aii) contacting said HFSI with said mixture (M-i), so as to provide mixture (M1),(C) letting said mixture (M1) to react, thus obtaining a solution (S) comprising at least one aprotic solvent, at least one salt-FSI of formula (I) as defined above and at least one by-product in the form of solid particles, and(D) separating at least a part of said at least one by-product in the form of solid particles, thus obtaining said solution (S1).

[0044] According to this embodiment, step (B*) corresponds to step (B) disclosed above.

[0045] Preferably, step (Ai) and step (B*) are performed simultaneously, followed by step (Aii).

[0046] Preferably, step (Ai) is performed at a temperature of at least 30°C, preferably of at least 32°C and even more preferably at a temperature of at least 35°C.

[0047] Preferably, step (Ai) is performed at a temperature lower than 120°C, more preferably lower than 100°C, even more preferably lower than 80°C.

[0048] According to this embodiment, step (Aii) is performed at the same temperature of step (Ai).

[0049] Advantageously, solution (S1) obtained at the end of step (C) comprises between 5 and 70 wt.% of said salt of formula (I), based on the total weight of the solution.

[0050] Preferably, said solution (S1) comprises between 2 and 60 wt.% of said salt of formula (I), more preferably between 5 and 50 wt.% and even more preferably between 10 and 40 wt.%.

[0051] Preferably, step (D) is performed by filtration or centrifugation.

[0052] Preferably, the filtration is performed with a polymeric filter, such as polytetrafluoroethylene (PTFE) filter.

[0053] Step (D) can be performed via other solid / liquid separation methods, depending on the circumstances.

[0054] Advantageously, the at least one by-product in the form of solid particles contains sulfate anions and / or fluoride anions.

[0055] Without being bound by any theory, the Applicant surprisingly found that by properly controlling the temperature under step (B), the FSCh' anions decompose into insoluble species, notably sulfate anions and / or fluoride anions, which are easily separated from composition (S) under step (D).

[0056] The method of the present invention is advantageously performed in a reaction vessel, which is preferably made from a material selected from glass, a fluororesin or a polyethylene resin.

[0057] The reaction vessel is preferably suitable for being heated at the temperature selected under step (B).

[0058] The method of the present invention may be carried out in a continuous or semi- continuous mode.

[0059] According to a preferred embodiment, the present invention relates to a method for the manufacture of a solution [solution (S1A)] comprising at least one organic aprotic solvent and lithium bis(fluorosulfonyl)imide (LiFSI), said method comprising the steps of:(AA) contacting HFSI, at least one aprotic organic solvent [solvent (S)] and at least one lithium compound [compound (AM-L)], so as to provide a reaction mixture [mixture (M1A)],(BA) heating said mixture (M1A) at a temperature of at least 30°C, and(CA) letting said mixture (M1A) to react, thus providing said solution (SA) comprising at least one aprotic solvent, LiFSI and at least one by-product in the form of solid particles, and(DA) separating at least a part of said at least one by-product in the form of solid particles, thus obtaining said solution (S1A).

[0060] Preferably, said compound (AM-L) is selected from the group comprising, more preferably consisting of: LiOH, LiOH.H2O, U2CO3, UHCO3. More preferably, said compound (AM-L) is selected from the group consisting of LiOH.H2O and U2CO3.

[0061] All the conditions described above for each of step (A), step (B), step (C) and / or step (D), fully apply to step (AA), step (BA), step (CA) and step (DA), respectively.

[0062] Advantageously, the ratio between the amount of FSCh' in solution (S1) or solution (S1A) obtained according to the present invention and the amount of FSOs' in the HFSI is preferably 0.85:1 or less, the amount of FSOs' being measured by ion chromatography (IC).

[0063] A further object of the present invention is the use of said solution (S1) or of said solution (S1A) in a non-aqueous battery electrolyte solution.

[0064] Alternatively, if required by the final use or other circumstances, solid LiFSi can be obtained by properly processing said solution (S1) or (S1A).

[0065] Preferably, said processing is performed via known methods, such as concentration, precipitation, washing and drying.

[0066] 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.

[0067] The disclosure 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.

[0068] Experimental Section

[0069] Example 1 - Preparation of LiFSi solution

[0070] A glass flask equipped with a mechanical stirring, a thermostated bath, a temperature probe, a syringe pump and a condenser was placed inside a glovebag, fed with dry argon flow.

[0071] The flask was loaded with ethyl methyl carbonate (EMC) and LiOH.H2O (LiOH.H2O:HFSI 1.05:1 mol). The temperature setpoint of the condenser was set at 40°C, and the reaction medium at 40°C.

[0072] HFSI was added to the reaction mixture over 1 hour.The HFSI contained the following impurities (ppm): F = 132.0 Cl = 37.5 SO42' = 197.7 NH2SO3- = 676.5 FSO3- = 80.4

[0073] The LiFSi concentration in the mixture at the end of the 1 hour addition was in the range 20-40 wt%. The reaction medium was maintained for 3 hours at the same temperature.

[0074] The slurry thus obtained was analyzed by ion chromatography to determine the anionic species. The results are in Table 1 below.

[0075] The setup was then filtered on a 0.22 micron PTFE membrane obtaining a filtrate of EMC containing LiFSI, which was analyzed by ion chromatography. The results are in Table 1 below.Table 1

[0076] Example 2 - Preparation of LiFSI solution

[0077] This example was performed following the procedure described in Example 1 above, but the starting HFSI contained the following impurities (ppm): F’ = 100.2 Ch = 43.5 SO42’ = 105.0 NH2SO3- = 221.7 FSO3’ = 112.2.

[0078] The slurry obtained was analyzed by ion chromatography to determine anionic impurities. The results are in Table 2 below.

[0079] The setup was then filtered on a 0.22 micron PTFE membrane obtaining a filtrate of EMC containing LiFSI, which was analyzed by ion chromatography. The results are in Table 2 below.Table 2

[0080] Example 3 - Preparation of LiFSI solution

[0081] This example was performed following the procedure described in Example 1 above, but the temperature setpoint of the condenser was set at 50°C, and the reaction medium at 50°C.

[0082] The starting HFSI contained the following impurities (ppm): F = 100.2 Cl = 43.5 SO42’ = 105.0 NH2SO3- = 221.7 FSO3- = 112.2.

[0083] The slurry obtained was analyzed by ion chromatography to determine anionic impurities. The results are in Table 3 below.The setup was then filtered on a 0.22 micron PTFE membrane obtaining a filtrate of EMC containing LiFSI, which was analyzed by ion chromatography. The results are in Table 3 below.Table 3

[0084] Comparative Example 1 - Preparation of LiFSI solution

[0085] This example was performed following the procedure described in Example 1 above, but the temperature setpoint of the condenser was set at 0°C, and the reaction medium at 0°C.

[0086] The HFSI contained the following impurities (ppm): F’ = 132.0 Ch = 37.5 SO42- = 197.7NH2SO3’ = 676.5 FSO3- = 80.4

[0087] The slurry thus obtained was analyzed by ion chromatography to determine anionic impurities and the following results were obtained: F’ = 59.0 Ch = 53.0 SO ’ = 117.0 NH2SO3- = 319.0 FSO3’ = 136.0

[0088] Comparative Example 2 - Preparation of LiFSI solution

[0089] This example was performed following the procedure described in Example 1 above, but the temperature setpoint of the condenser was set at 20°C, and the reaction medium at 20°C.

[0090] The HFSI contained the following impurities (ppm): F’ = 132.0 Ch = 37.5 SO42- = 197.7 NH2SO3- = 676.5 FSO3- = 80.4

[0091] The slurry thus obtained was analyzed by ion chromatography to determine anionic impurities and the following results were obtained: F = 44.5 Cl = 39.0 SO42’ = 93.5.0 NH2SO3- = 400.0 FSO3- = 120.0.

[0092] The residual acidity at the end of the 3 h maturation time was evaluated on the filtrate obtained according to the present invention and on the slurries of Comparative Examples 1 and 2.

[0093] The residual acidity was measured as follows: weightier : Mettler XPE105DR 10 1003 17 potentiometer : Titrando 808 (04 5001 05) titrating: aqueous NaOH, 0.1 N solvent : water electrode : combination electrode, reference Ag / AgCI ; KCI 3M test sample : about 2 g

[0094] The results are summarized in the following table 4.Table 4

[0095] The above results showed that the method according to the present invention promoted the hydrolysis of FSC impurity in insoluble side products, which were easily removed by filtration, thus obtaining compositions characterized by a higher percentage of neutralized HFSI, which translate into a higher purity.

Claims

Claims1. A method for manufacturing a solution [solution (S1)] comprising at least one organic aprotic solvent and at least one bis(fluorosulfonyl)imide salt (salt-FSI) represented by thwhereinMn+represents a metal cation and n is an integer from 1 to 4, preferably 1 or 2; said method comprising the following steps:(A) contacting hydrogen bis(fluorosulfonyl)imide (HFSI), at least one aprotic organic solvent [solvent (S)] and at least one alkali metal compound [compound (AM)], so as to provide a reaction mixture [mixture (M1)],(B) heating said mixture (M1) at a temperature of at least 30°C,(C) letting said mixture (M1) to react, thus obtaining a solution (S) comprising at least one aprotic solvent, at least one salt-FSI of formula (I) as defined above and at least one by-product in the form of solid particles, and(D) separating at least a part of said at least one by-product in the form of solid particles, thus obtaining said solution (S1).

2. The method according to Claim 1, wherein the amount of said compound (AM) is from about 1.0 mol to about 2.0 mol per 1.0 mol of HFSI.

3. The method according to Claim 1 or 2, wherein said compound (AM) is selected from inorganic compounds.

4. The method according to any one of Claims 1 to 3, wherein said metal cation Mn+is an alkali metal cation, preferably selected from Na, Li, K, Rb, and Cs.

5. The method according to any one of the preceding Claims, wherein said compound (AM) is selected from the group comprising, preferably consisting of: LiOH, NaOH, KOH, RbOH, CsOH, LiOH.H2O, NaOH.H2O, KOH.H2O, RbOH.H2O, CsOH.H2O, Li2CO3, Na2CO3, K2CO3, Rb2CO3, Cs2CO3, LiHCO3, NaHCO3, KHCO3, RbHCO3and CSHCO3.

6. The method according to any one of the preceding Claims, wherein said solvent (S) is selected in the group comprising ethylene carbonate, propylene carbonate, butylene carbonate, y-butyrolactone, y-valerolactone, dimethoxymethane, 1 ,2-dimethoxy ethane, 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, dimethyl sulfoxide, N,N-dimethylformamide, N-methyl oxazolidinone, acetonitrile, valeronitrile, benzonitrile, ethyl acetate, isopropyl acetate, n-butyl acetate, nitromethane and nitrobenzene.

7. The method according to any one of the preceding Claims, wherein step (B) is performed at a temperature of at least 32°C, and / or at a temperature lower than 120°C, more preferably lower than 100°C, even more preferably lower than 80°C.

8. The method according to any one of the preceding Claims, wherein step (A) comprises:(Ai) providing a mixture (M-i) comprising said compound (AM) and said solvent (S), and(Aii) contacting said HFSI with said mixture (M-i), so as to provide mixture (M1).

9. The method according to any one of the preceding Claims, wherein step (Aii) and step (B) are performed simultaneously after step (Ai).

10. The method according to any one of the preceding Claims, wherein step (B) is performed after step (Ai) and before step (Aii).

11. The method according to any one of the preceding Claims, wherein step (D) is performed by filtration or centrifugation.

12. The method according to any one of the preceding Claims, wherein: said at least one bis(fluorosulfonyl)imide salt (salt-FSI) is lithium bis(fluorosulfonyl)imide; and said compound (AM) is selected from the group comprising, more preferably consisting of: LiOH, LiOH.H2O, U2CO3, LiHCCh; and / or said solvent (S) is selected from ethyl methyl carbonate and n-butyl acetate.

13. The method according to any one of Claims 1 to 12, wherein said at least one byproduct in the form of solid particles contains sulfate anions and / or fluoride anions.

14. The method according to any one of the preceding Claims, wherein the ratio between the amount of FSCh' in solution (S1) and the amount of FSOs' in HFSI is 0.85:1 or less, the amount of FSOa" being measured by ion chromatography (IC).

15. A method for the manufacture of a solution [solution (S1A)] comprising at least one organic aprotic solvent and lithium bis(fluorosulfonyl)imide (LiFSI), said method comprising the steps of:(AA) contacting HFSI, at least one aprotic organic solvent [solvent (S)] and at least one lithium compound [compound (AM-L)], so as to provide a reaction mixture [mixture (M1A)],(BA) heating said mixture (M1A) at a temperature of at least 30°C, and(CA) letting said mixture (M1A) to react, thus providing said solution (SA) comprising at least one aprotic solvent, LiFSI and at least one by-product in the form of solid particles, and(DA) separating at least a part of said at least one by-product in the form of solid particles, thus obtaining said solution (S1A).

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