Process for the preparation of a lithium salt of bis (fluorosulfonyl) imide FIELD OF THE INVENTION The present invention relates to a method of making a lithium salt of bis (fluorosulfonyl) imide. TECHNICAL BACKGROUND The Li-ion battery market requires the development of higher power batteries. This involves increasing the nominal voltage of Li-ion batteries. To achieve the target voltages, high purity electrolytes are required. Sulfonylimide type anions, due to their very low basicity, are increasingly used in the field of energy storage in the form of inorganic salts in batteries, or organic salts in supercapacitors or in the field of liquids. ionic. In the specific field of Li-ion batteries, the salt currently most used is LiPF 6 . This salt exhibits many disadvantages such as limited thermal stability, sensitivity to hydrolysis and therefore lower battery safety. Recently, new salts possessing the fluorosulfonyl FS0 2 group have been studied and have demonstrated numerous advantages such as better ionic conductivity and resistance to hydrolysis. One of these salts, LiFSI has shown very interesting properties which make it a good candidate to replace LiPF 6 . L’identification et la quantification des impuretés dans les sels et/ou électrolytes, et la compréhension de leurs impacts sur les performances de la batterie deviennent primordiales. Par exemple, les impuretés possédant un proton mobile, en raison de leur interférence avec les réactions électrochimiques, conduisent à des performances et une stabilité globale moindre des batteries Li-ion. L’application des batteries Li-ion nécessite d’avoir des produits de haute pureté (minimum d’impuretés). The existing processes for preparing LiFSI include in particular stages (for example chlorination, fluorination, etc.) using corrosive reagents, and / or the formation of corrosive secondary products, which generate (under the operating conditions) a high corrosion of the material of the equipment used for the reactions. This corrosion induces the contamination of said LiFSI by metal ions originating from said materials. However, the presence of metal ions in the LiFSI in too large a quantity can disturb the operation of the battery and its performance, for example due to the deposition of said metal ions on the electrodes of the battery. In addition, corrosion of the materials of the equipment used compromises the structural integrity of the equipment and shortens its life. Thus, there is a need for a new process for preparing a lithium salt of bis (fluorosulfonyl) imide resulting in a high purity LiFSI having a reduced content of metal ions. DESCRIPTION OF THE INVENTION The present invention relates to a process for preparing a lithium salt of bis (fluorosulfonyl) imide F- (S0 2 ) -NLi- (S0 2 ) -F comprising a step (b) comprising a step of fluorinating bis (chlorosulfonyl). ) imide Cl- (S0 2 ) -NH- (S0 2 ) -Cl with anhydrous HF, optionally in at least one SOI organic solvent, said step (b) being carried out in a reactor made of a corrosion resistant M3 material , or in a reactor containing a base layer made of an M1 material coated with a surface layer made of a corrosion resistant M2 material. In the context of the invention, by “anhydrous HF” is meant THF containing less than 500 ppm of water, preferably less than 300 ppm of water, preferably less than 200 ppm of water. Step (b) Step (b) according to the invention advantageously makes it possible to prepare the bis (fluorosulfonyl) imide F- (S0 2 ) -NH- (S0 2 ) -F. Step (b) of the process is preferably carried out in at least one SOI organic solvent. The SOI organic solvent preferably has a donor number between 1 and 70 and advantageously between 5 and 65. The donor index of a solvent represents the value -DH, DH being the enthalpy of the interaction between the solvent and antimony pentachloride (according to the method described in Journal of Solution Chemistry, vol. 13, no. 9, 1984). As SOI organic solvent, mention may in particular be made of esters, nitriles, dinitriles, ethers, diethers, amines, phosphines, and mixtures thereof. Preferably, the SOI organic solvent is chosen from the group consisting of methyl acetate, ethyl acetate, butyl acetate, acetonitrile, propionitrile, isobutyronitrile, glutaronitrile , dioxane, tetrahydrofuran, triethylamine, tripropylamine, diethylisopropylamine, pyridine, trimethylphosphine, triethylphosphine, diethylisopropylphosphine, and mixtures thereof. In particular, the SOI organic solvent is dioxane. Step (b) can be carried out at a temperature between 0 ° C and the boiling point of the organic solvent SOI (or of the mixture of organic solvents SOI). Preferably, step (b) is carried out at a temperature between 5 ° C and the boiling point of the SOI organic solvent (or of the mixture of SOI organic solvents), preferably between 25 ° C and the boiling point. SOI organic solvent (or mixture of SOI organic solvents). Step (b) can be carried out at a pressure P, preferably between 0 and 16 bar abs. This step (b) is preferably carried out by dissolving the bis (chlorosulfonyl) imide Cl- (S0 2 ) -NH- (S0 2 ) -Cl in the organic solvent SOI, or the mixture of organic solvents SOI, prior to the reaction step with anhydrous HF. The mass ratio between the bis (chlorosulfonyl) imide Cl- (S0 2 ) -NH- (S0 2 ) -Cl and the organic solvent SOI, or the mixture of organic solvents SOI, is preferably between 0.001 and 10, and advantageously between 0.005 and 5. According to one embodiment, anhydrous GI-IF is introduced into the reaction medium in liquid form or in gaseous form, preferably in gaseous form. The molar ratio x between anhydrous HF and the bis (chlorosulfonyl) imide Cl- (S0 2 ) -NH- (S0 2 ) -Cl involved is preferably between 2 and 10, and advantageously between 2 and 5. The reaction step with anhydrous HF can be carried out in a closed medium or in an open medium, preferably step (b) is carried out in an open medium with in particular evolution of HCl in gas form. The surface layer of the reactor of step (b) is the layer capable of being in contact with the reaction medium of fluorination step (b) (for example starting reagents, products generated, etc.), the reaction medium possibly comprising any type of phase: liquid, and / or gas, and / or solid. Preferably, the surface layer of the reactor of step (b) is at least in contact with at least one of the starting reagents, for example bis (chlorosulfonyl) imide. The base layer and the surface layer can be placed against each other by plating. This is for example the case when the material M2 is a nickel-based alloy, as defined below. Preferably, the plating is carried out by welding plating, explosion plating, hot rolling plating, or by cold rolling plating, preferably by explosion plating. According to one embodiment, the surface layer has a thickness of between 0.01 and 20 mm, said thickness of said internal surface layer being less than that of said base layer. Preferably, said internal surface layer has a thickness of between 0.05 and 15 mm, preferably between 0.1 and 10 mm, advantageously between 0.1 and 5 mm. Material M3 The reactor of step (b) can be made of a corrosion resistant M3 material. In particular, the reactor of step (b) is made of a solid material M3 resistant to corrosion. The material M3 can be chosen from pure nickel and an M3 material comprising: - at least 60% by weight of iron, more particularly at least 70% by weight of iron relative to the total weight of material M3; - less than 2% by weight of carbon, advantageously less than 1.5% by weight, preferably less than 1% by weight, more preferably less than 0.75% by weight, in particular less than 0.5% by weight , more particularly less than 0.2% by weight, even more advantageously less than 0.1% by weight relative to the total weight of material M3; and - from 10% to 20% by weight of chromium, advantageously from 15% to 20% by weight, in particular from 16% to 18.5% by weight of chromium relative to the total weight of material M3; and eventually : - less than 15% by weight of nickel, preferably between 10% and 14% by weight of nickel relative to the total weight of material M3; and or - less than 3% by weight of molybdenum, advantageously between 2% and 3% by weight of molybdenum relative to the total weight of material M3; and or - less than 2.5% by weight of manganese, advantageously 2% by weight of manganese relative to the total weight of material M3; and or - less than 2% by weight of silicon, advantageously less than 1.5% by weight, preferably less than 1.25% by weight, more preferably less than 1% by weight of silicon relative to the total weight of material M3. Preferably, the material M3 comprises at least 60% by weight of iron, more particularly at least 70% by weight of iron relative to the total weight of the material M3; and less than 2% by weight of carbon, advantageously less than 1.5% by weight, preferably less than 1% by weight, more preferably less than 0.75% by weight, in particular less than 0.5% by weight , more particularly less than 0.2% by weight, even more advantageously less than 0.1% by weight relative to the total weight of material M3; and from 10% to 20% by weight of chromium, advantageously from 15% to 20% by weight, in particular from 16% to 18.5% by weight of chromium relative to the total weight of material M3; and less than 15% by weight of nickel, preferably between 10% and 14% by weight of nickel relative to the total weight of material M3; and less than 3% by weight of molybdenum, advantageously between 2% and 3% by weight of molybdenum relative to the total weight of material M3; and less than 2.5% by weight of manganese, advantageously 2% by weight of manganese relative to the total weight of material M3; and less than 2% by weight of silicon, advantageously less than 1.5% by weight, preferably less than 1.25% by weight, more preferably less than 1% by weight of silicon relative to the total weight of material M3. In the context of the invention, the term "the material M3 is pure nickel" is understood to mean a material M3 comprising at least 99% by weight of nickel, preferably at least 99.1%, preferably at least 99.2%. , advantageously at least 99.3%, even more advantageously at least 99.4%, for example at least 99.5%, and in particular at least 99.6%, relative to the total weight of said material M3. When the material M3 is pure nickel, it can also further comprise: - iron in a content less than 1% by weight relative to the total weight of material M3, advantageously less than 0.9% by weight, preferably less than 0.8% by weight, more preferably less than 0.7% by weight, in particular less than 0.6% by weight, more particularly less than 0.5% by weight relative to the total weight of the material M3. Preferably, the material M3 comprises between 0.1% and 1% by weight of iron, in particular between 0.3% and 0.8% by weight of iron, more particularly between 0.3% and 0.5% by weight of iron. weight of iron relative to the total weight of material M3; and or - manganese in a content of less than 1% by weight relative to the total weight of material M3, advantageously less than 0.9% by weight, preferably less than 0.8% by weight, more preferably less than 0.7% by weight, in particular less than 0.6% by weight, more particularly less than 0.5% by weight, preferably less than 0.4% by weight, relative to the total weight of material M3; and or - silicon in a content less than 1% by weight relative to the total weight of material M3, advantageously less than 0.9% by weight, preferably less than 0.8% by weight, more preferably less than 0.7% by weight, in particular less than 0.6% by weight, more particularly less than 0.5% by weight relative to the total weight of material M3; and or - copper in a content of less than 1% by weight relative to the total weight of the material M3, advantageously less than 0.9% by weight, preferably less than 0.8% by weight, more preferably less than 0.7% by weight, in particular less than 0.6% by weight, more particularly less than 0.5% by weight, preferably less than 0.4% by weight, particularly preferably less than 0.3% by weight per relative to the total weight of the material M3; and or - carbon in a content of less than 0.1% by weight relative to the total weight of the material M3, advantageously less than 0.09% by weight, preferably less than 0.08% by weight, more preferably less than 0.07% by weight, in particular less than 0.06% by weight, more particularly less than 0.05% by weight, preferably less than 0.04 % by weight, particularly preferably less than 0.03% by weight relative to the total weight of the material M3. By way of example, mention may for example be made of NII201 comprising at least 99% by weight of nickel, at most 0.02% by weight of carbon, at most 0.40% by weight of iron, at most 0.35 wt% manganese, at most 0.35 wt% silicon, and at most 0.25% copper; or Ni200 comprising at least 99% by weight of nickel, at most 0.15% by weight of carbon, at most 0.40% by weight of iron, at most 0.35% by weight of manganese, at most 0, 35% by weight of silicon, and at most 0.25% of copper. According to a preferred embodiment, the corrosion rate of material M3 is less than 100 mGh / year, preferably less than 90 mhi / year, advantageously less than 80 mhi / year, preferably less than 70 mhi / year, even more advantageously less than 60 mhi / year, and in particular less than 50 mhi / year. This rate is measured according to the coupon method ASTM D 2 328-65 T. M1 material According to one embodiment, the material M1 comprises: - i) at least 60% by weight of iron, preferably at least 70% by weight, advantageously at least 75% by weight, even more advantageously at least 80% by weight, more preferably at least 85% by weight, in particular at least 90% by weight, more particularly at least 95% by weight, and even more preferably at least 97% by weight of iron relative to the total weight of material M1; and ü) - less than 2% by weight of carbon, advantageously less than 1.5% by weight, preferably less than 1% by weight, more preferably less than 0.75% by weight, in particular less than 0.5% by weight , more particularly less than 0.2% by weight, preferably less than 0.1% by weight of carbon relative to the total weight of material M1; and or - less than 3% by weight of molybdenum, advantageously less than 2% by weight, preferably less than 1.5% by weight, preferably less than 1.25% by weight, more preferably less than 1% by weight of molybdenum per relative to the total weight of material M1; and or less than 20% by weight of chromium, preferably less than 5% by weight of chromium, advantageously less than 4% by weight, preferably less than 3% by weight, more preferably less than 2% by weight, in particular less than 1 % by weight of chromium relative to the total weight of material M1; and or less than 15% by weight of nickel, preferably less than 5% by weight, advantageously less than 4% by weight, preferably less than 3% by weight, more preferably less than 2% by weight, in particular less than 1% by weight weight of nickel relative to the total weight of material M1; and or - less than 2% by weight of silicon, advantageously less than 1.5% by weight, preferably less than 1.25% by weight, more preferably less than 1% by weight of silicon relative to the total weight of material M1; and or - less than 2.5% by weight of manganese, advantageously less than 2% by weight, preferably less than 1.5% by weight, more preferably less than 1% by weight of manganese relative to the total weight of material M1. According to a preferred embodiment, the material M1 comprises: - i) at least 60% by weight of iron, preferably at least 70% by weight, advantageously at least 75% by weight, even more advantageously at least 80% by weight, more preferably at least 85% by weight, in particular at least 90% by weight, more particularly at least 95% by weight, and even more preferably at least 97% by weight of iron relative to the total weight of material M1; and ü) - less than 2% by weight of carbon, advantageously less than 1.5% by weight, preferably less than 1% by weight, preferably less than 0.75% by weight, more preferably less than 0.5% by weight, more particularly less than 0.2% by weight, and even more advantageously between 0.01% and 0.2% by weight of carbon relative to the total weight of material M1; and or - less than 3% by weight of molybdenum, advantageously less than 2% by weight, preferably less than 1.5% by weight, preferably less than 1.25% by weight, more preferably less than 1% by weight, even more advantageously between 0.1% and 1% by weight of molybdenum relative to the total weight of material M1, and / or less than 5% by weight of chromium, preferably less than 4% by weight, advantageously less than 3% by weight, preferably less than 2% by weight, in particular between 0.5% and 2% by weight of chromium relative to the total weight of material M1; and or - less than 2% by weight of silicon, advantageously less than 1.5% by weight, preferably between 0.1% and 1.5% by weight of silicon relative to the total weight of material M1; and or - less than 2.5% by weight of manganese, advantageously less than 2% by weight, preferably less than 1, 5% by weight, more preferably less than 1% by weight, in particular between 0.1% and 1% by weight of manganese relative to the total weight of material M1. Preferably, the material M1 comprises at least 60% by weight of iron, preferably at least 70% by weight of iron, advantageously at least 75% by weight, even more advantageously at least 80% by weight, more preferably at least 85 % by weight, in particular at least 90% by weight, more particularly at least 95% by weight, and even more preferably at least 97% by weight of iron relative to the total weight of material M1; and less than 2% by weight of carbon, advantageously less than 1.5% by weight, preferably less than 1% by weight, preferably less than 0.75% by weight, more preferably less than 0.5% by weight, more particularly less than 0.2% by weight, and even more advantageously between 0.01% and 0.2% by weight of carbon relative to the total weight of material M1; and less than 3% by weight of molybdenum, According to another preferred embodiment, the material M1 comprises: - at least 60% by weight of iron, more particularly at least 70% by weight of iron relative to the total weight of material M1; - less than 2% by weight of carbon, advantageously less than 1.5% by weight, preferably less than 1% by weight, more preferably less than 0.75% by weight, in particular less than 0.5% by weight , more particularly less than 0.2% by weight, even more advantageously less than 0.1% by weight relative to the total weight of material M1; and - from 10% to 20% by weight of chromium, advantageously from 15% to 20% by weight, in particular from 16% to 18.5% by weight of chromium relative to the total weight of material M1; and eventually : - less than 15% by weight of nickel, preferably between 10% and 14% by weight of nickel relative to the total weight of material M1; and or - less than 3% by weight of molybdenum, advantageously between 2% and 3% by weight of molybdenum relative to the total weight of material M1; and or - less than 2.5% by weight of manganese, advantageously 2% by weight of manganese relative to the total weight of material M1; and or - less than 2% by weight of silicon, advantageously less than 1.5% by weight, preferably less than 1.25% by weight, more preferably less than 1% by weight of silicon relative to the total weight of material M1. Preferably, the material M1 comprises at least 60% by weight of iron, more particularly at least 70% by weight of iron relative to the total weight of the material M1; and less than 2% by weight of carbon, advantageously less than 1.5% by weight, preferably less than 1% by weight, more preferably less than 0.75% by weight, in particular less than 0.5% by weight , more particularly less than 0.2% by weight, even more advantageously less than 0.1% by weight relative to the total weight of material M1; and from 10% to 20% by weight of chromium, advantageously from 15% to 20% by weight, in particular from 16% to 18.5% by weight of chromium relative to the total weight of material M1; and less than 15% by weight of nickel, preferably between 10% and 14% by weight of nickel relative to the total weight of material M1; and less than 3% by weight of molybdenum, advantageously between 2% and 3% by weight of molybdenum relative to the total weight of material M1; and less than 2.5% by weight of manganese, advantageously 2% by weight of manganese relative to the total weight of material M1; and less than 2% by weight of silicon, advantageously less than 1.5% by weight, preferably less than 1.25% by weight, more preferably less than 1% by weight of silicon relative to the total weight of material M1. Material M2 The material M2 can be chosen from the group consisting of enamel, polymers (in particular fluoropolymers), and nickel-based alloys. According to one embodiment, the material M2 is enamel. Typically, the enamel mainly comprises Si0 2, in particular in a mass content greater than 60% by mass, preferably between 60% and 70% by mass. The enamel layer can be obtained by applying a suspension of glass powder in a sufficient thickness on the base layer of the inner wall of the reactor, then heating to ensure the melting of the glass powder, followed by cooling to allow obtaining a layer of enamel. According to a preferred embodiment, the material M2 is chosen from polymers, in particular polyolefins (such as for example polyethylene), and fluoropolymers, such as for example PVDF (polyvinylidene fluoride), PTFE (polytetrafluoroethylene) ), PFAs (copolymers of C 2 F 4 and perfluorinated vinyl ether), FEPs (copolymers of tetrafluoroethylene and perfluoropropene, such as for example copolymer of C 2 F 4 and CsFs), ETFE (copolymer of tetrafluoroethylene and ethylene, and FKM (copolymer of hexafluoropropylene and difluoroethylene), more preferably the material M6 is chosen from PTFE and PFA. According to one embodiment, the material M2 is chosen from nickel-based alloys, in particular from alloys comprising at least 40% by weight of nickel relative to the total weight of material M2. Advantageously, the material M2 is chosen from nickel-based alloys comprising at least 45% by weight of nickel, more preferably at least 50% by weight of nickel, in particular at least 55% by weight of nickel, more particularly at least 60% by weight of nickel, preferably at least 65% by weight of nickel, even more preferably at least 70% by weight of nickel relative to the total weight of material M2. The material M2 can be chosen from nickel-based alloys comprising from 45% to 95% by weight of nickel, preferably from 50% to 90% by weight of nickel relative to the total weight of material M2. The material M2 (nickel-based alloys) can also comprise chromium in a content of less than 35% by weight relative to the total weight of the material M2, advantageously less than 30% by weight, preferably less than 20% by weight, more preferably less than 15% by weight, in particular less than 10% by weight, more particularly less than 5% by weight relative to the total weight of material M2. The material M2 (nickel-based alloys) can also comprise molybdenum in a content of less than 35% by weight relative to the total weight of the material M2, advantageously less than 30% by weight, preferably less than 25% by weight, more preferably less than 20% by weight, in particular less than 15% by weight, more particularly less than 10% by weight relative to the total weight of material M2. Preferably, the material M2 (nickel-based alloys) comprises at least 40% by weight of nickel relative to the total weight of the material M2, preferably at least 45% by weight, more preferably at least 50% by weight, in in particular at least 55% by weight, more particularly at least 60% by weight, preferably at least 65% by weight, more preferably at least 70% by weight of nickel relative to the total weight of material M2; and less than 35% by weight of chromium, advantageously less than 30% by weight, preferably less than 20% by weight, more preferably less than 15% by weight, in particular less than 10% by weight, more particularly less than 5 % by weight of chromium relative to the total weight of material M2; and less than 35% by weight of molybdenum, advantageously less than 30% by weight, particularly less than 5% by weight of molybdenum, relative to the total weight of material M2. The material M2 (nickel-based alloys) can also comprise cobalt in a content of less than 10% by weight relative to the total weight of the material M2, advantageously less than 8% by weight, preferably less than 6% by weight, more preferably less than 4% by weight, in particular less than 3% by weight, more particularly less than 2% by weight relative to the total weight of material M2. The material M2 (nickel-based alloys) can also comprise tungsten in a content of less than 5% by weight relative to the total weight of the material M2, advantageously less than 4% by weight, preferably less than 3% by weight, more preferably less than 2% by weight, in particular less than 1% by weight, relative to the total weight of material M2. The material M2 (nickel-based alloys) can also comprise iron in a content of less than 25% by weight relative to the total weight of the material M2, advantageously less than 20% by weight, preferably less than 15% by weight, more preferably less than 10% by weight, in particular less than 7% by weight, more particularly less than 5% by weight relative to the total weight of material M2. The material M2 (nickel-based alloys) can also comprise manganese in a content of less than 5% by weight relative to the total weight of the alloy, advantageously less than 4% by weight, preferably less than 3% by weight , more preferably less than 2% by weight, in particular less than 1% by weight, more particularly less than 0.5% by weight relative to the total weight of material M2. The material M2 (nickel-based alloys) can also comprise copper in a content of less than 50% by weight, advantageously less than 45% by weight, preferably less than 40% by weight, more preferably less than 35% by weight , in particular less than 30% by weight, more particularly less than 25% by weight of copper relative to the total weight of material M2. Preferably, material M2 (nickel-based alloys) comprises at least 40% by weight of nickel relative to the total weight of material M2, preferably at least 45% by weight of nickel, more preferably at least 50% by weight nickel, in particular at least 55% by weight of nickel, more particularly at least 60% by weight of nickel, preferably at least 65% by weight of nickel, more preferably at least 70% by weight of nickel by relative to the total weight of the material M2; and less than 50% by weight of copper, advantageously less than 45% by weight, preferably less than 40% by weight, more preferably less than 35% by weight, in particular less than 30% by weight, more particularly less than 25 % by weight of copper relative to the total weight of material M2. Preferably, material M2 (nickel-based alloys) comprises at least 40% by weight of nickel relative to the total weight of material M2, preferably at least 45% by weight of nickel, more preferably at least 50% by weight nickel, in particular at least 55% by weight of nickel, more particularly at least 60% by weight of nickel, preferably at least 65% by weight of nickel, more preferably at least 70% by weight of nickel by relative to the total weight of material M2; and less than 35% by weight of chromium, advantageously less than 30% by weight, preferably less than 20% by weight, more preferably less than 15% by weight, in particular less than 10% by weight, more particularly less than 5 % by weight of chromium relative to the total weight of material M2; and less than 25% by weight of iron, advantageously less than 20% by weight, preferably less than 15% by weight, more preferably less than 10% by weight, in particular less than 7% by weight, more particularly less than 5% by weight of iron relative to the total weight M2 material; and optionally less than 35% by weight of molybdenum, advantageously less than 30% by weight, preferably less than 20% by weight, more preferably less than 15% by weight, in particular less than 10% by weight, more particularly less than 5% by weight of molybdenum relative to the total weight of material M2. The material M2 (nickel-based alloys) can comprise less than 4% by weight of titanium relative to the total weight of the material M2, advantageously less than 3% by weight, preferably less than 2% by weight, more preferably less than 1% by weight, in particular less than 0.5% by weight of titanium, more particularly less than 0.05% by weight of titanium relative to the total weight of material M2, preferably the material M2 is devoid of titanium. The material M2 (nickel-based alloys) can comprise less than 6% by weight of niobium relative to the total weight of the material M2, advantageously less than 4% by weight, preferably less than 2% by weight, more preferably less than 1% by weight, in particular less than 0.5% by weight, more particularly less than 0.05% by weight of niobium relative to the total weight of material M2, preferably material M2 is niobium-free. According to one embodiment, the reactor used in step (b) of the method according to the invention comprises a base layer made of a material M1 coated with a surface layer made of a corrosion resistant material M2. , said material M1 comprising: • at least 60% by weight of iron, preferably at least 70% by weight of iron, advantageously at least 75% by weight, even more advantageously at least 80% by weight, more preferably at least 85% by weight, in particular at least 90% by weight, more particularly at least 95% by weight, and even more preferably at least 97% by weight of iron relative to the total weight of material M1; and less than 2% by weight of carbon, advantageously less than 1.5% by weight, preferably less than 1% by weight, preferably less than 0.75% by weight, more preferably less than 0.5% by weight, more particularly less than 0.2% by weight, and even more advantageously between 0.01% and 0.2% by weight of carbon relative to the total weight of material M1; and less than 3% by weight of molybdenum, advantageously less than 2% by weight, preferably less than 1.5% by weight, preferably less than 1.25% by weight, more preferably less than 1% by weight, even more advantageously between 0.1% and 1% by weight of molybdenum relative to the total weight material M1, and / or less than 5% by weight of chromium, preferably less than 4% by weight, advantageously less than 3% by weight, preferably less than 2% by weight, in particular between 0.5% and 2 % by weight of chromium relative to the total weight of material M1; and preferably less than 2% by weight, in particular between 0.5% and 2% by weight of chromium relative to the total weight of material M1; and preferably less than 2% by weight, in particular between 0.5% and 2% by weight of chromium relative to the total weight of material M1; and • the material M2 being chosen from nickel-based alloys, in particular chosen from alloys comprising at least 40% by weight of nickel, advantageously at least 45% by weight, more preferably at least 50% by weight, in particular at less 55% by weight, more particularly at least 60% by weight, preferably at least 65% by weight, even more preferably at least 70% by weight of nickel relative to the total weight of material M2; and / or chromium in a content of less than 35% by weight relative to the total weight of material M2, advantageously less than 30% by weight, preferably less than 20% by weight, more preferably less than 15% by weight, in in particular less than 10% by weight, more particularly less than 5% by weight relative to the total weight of material M2; and / or molybdenum in a content of less than 35% by weight relative to the total weight of material M2, advantageously less than 30% by weight, preferably less than 25% by weight, more preferably less than 20% by weight, in in particular less than 15% by weight, more particularly less than 10% by weight relative to the total weight of material M2; and / or cobalt in a content of less than 10% by weight relative to the total weight of material M2, advantageously less than 8% by weight, preferably less than 6% by weight, more preferably less than 4% by weight, in in particular less than 3% by weight, more particularly less than 2% by weight relative to the total weight of material M2; and / or tungsten in a content of less than 5% by weight relative to the total weight of material M2, advantageously less than 4% by weight, preferably less than 3% by weight, more preferably less than 2% by weight, in particular less than 1% by weight, relative to the total weight of material M2; and / or iron in a content of less than 25% by weight relative to the total weight of material M2, advantageously less than 20% by weight, preferably less than 15% by weight, more preferably less than 10% by weight, in in particular less than 7% by weight, more particularly less than 5% by weight relative to the total weight of material M2; and / or manganese in a content of less than 5% by weight relative to the total weight of the alloy, advantageously less than 4% by weight, preferably less than 3% by weight, more preferably less than 2% by weight, in particular less than 1% by weight, more particularly less than 0.5% by weight relative to the total weight of material M2; and / or copper in a content of less than 50% by weight, advantageously less than 45% by weight, preferably less than 40% by weight, more preferably less than 35% by weight, in particular less than 30% by weight, more particularly less than 25% by weight of copper relative to the total weight of material M2; and / or less than 4% by weight of titanium relative to the total weight of material M2, advantageously less than 3% by weight, preferably less than 2% by weight, more preferably less than 1% by weight, in particular less than 0.5% by weight, more particularly less than 0.05% by weight relative to the total weight of material M2, preferably material M2 being devoid of titanium; According to a preferred embodiment, the reactor used in step (b) of the process according to the invention comprises a base layer made of a material M1 coated with a surface layer made of a material M2 resistant to corrosion, said material M1 comprising: • at least 60% by weight of iron, preferably at least 70% by weight of iron, advantageously at least 75% by weight, even more advantageously at least 80% by weight, more preferably at least 85% by weight, in particular at least 90% by weight, more particularly at least 95% by weight, and even more preferably at least 97% by weight of iron relative to the total weight of material M1; and less than 2% by weight of carbon, advantageously less than 1.5% by weight, preferably less than 1% by weight, preferably less than 0.75% by weight, more preferably less than 0.5% by weight, more particularly less than 0.2% by weight, and even more advantageously between 0.01% and 0.2% by weight of carbon relative to the total weight of material M1; and less than 3% by weight of molybdenum, advantageously less than 2% by weight, preferably less than 1.5% by weight, preferably less than 1.25% by weight, more preferably less than 1% by weight, even more advantageously between 0.1% and 1% by weight of molybdenum relative to the total weight material M1, and / or less than 5% by weight of chromium, preferably less than 4% by weight, advantageously less than 3% by weight, preferably less than 2% by weight, in particular between 0.5% and 2 % by weight of chromium relative to the total weight of material M1; and preferably less than 2% by weight, in particular between 0.5% and 2% by weight of chromium relative to the total weight of material M1; and preferably less than 2% by weight, in particular between 0.5% and 2% by weight of chromium relative to the total weight of material M1; and • the material M2 being chosen from fluoropolymers, and in particular thermoplastic fluoropolymers, such as for example PVDF (polyvinylidene fluoride), PTFE (polytetrafluoroethylene), PFAs (copolymers of C 2 F 4 and of ether perfluorinated vinyl), FEPs (copolymers of tetrafluoroethylene and perfluoropropene, such as for example copolymer of C 2 F 4 and C 3 F 6 ), ETFE (copolymer of tetrafluoroethylene and ethylene, and FKM (copolymer of 'hexafluoropropylene and difluoroethylene). According to another preferred embodiment, the reactor used in step (b) of the process according to the invention comprises a base layer made of a material M1 coated with a surface layer made of a material M2 resistant to corrosion, said material M1 comprising • at least 60% by weight of iron, preferably at least 70% by weight of iron, advantageously at least 75% by weight, even more advantageously at least 80% by weight, more preferably at least 85% by weight, in particular at least 90% by weight, more particularly at least 95% by weight, and even more preferably at least 97% by weight of iron relative to the total weight of material M1; and less than 2% by weight of carbon, advantageously less than 1.5% by weight, preferably less than 1% by weight, preferably less than 0.75% by weight, more preferably less than 0.5% by weight, more particularly less than 0.2% by weight, and even more advantageously between 0.01% and 0.2% by weight of carbon relative to the total weight of material M1; and less than 3% by weight of molybdenum, advantageously less than 2% by weight, preferably less than 1.5% by weight, preferably less than 1.25% by weight, more preferably less than 1% by weight, even more advantageously between 0.1% and 1% by weight of molybdenum relative to the total weight material M1, and / or less than 5% by weight of chromium, preferably less than 4% by weight, advantageously less than 3% by weight, preferably less than 2% by weight, in particular between 0.5% and 2 % by weight of chromium relative to the total weight of material M1; and preferably less than 2% by weight, in particular between 0.5% and 2% by weight of chromium relative to the total weight of material M1; and preferably less than 2% by weight, in particular between 0.5% and 2% by weight of chromium relative to the total weight of material M1; and • the material M2 being enamel. According to another preferred embodiment, the reactor used in step (b) of the process according to the invention is made of a corrosion resistant M3 material, said M3 material comprising at least 60% by weight of iron, plus particularly at least 70% in weight of iron relative to the total weight of material M3; and less than 2% by weight of carbon, advantageously less than 1.5% by weight, preferably less than 1% by weight, more preferably less than 0.75% by weight, in particular less than 0.5% by weight , more particularly less than 0.2% by weight, even more advantageously less than 0.1% by weight relative to the total weight of material M3; and from 10% to 20% by weight of chromium, advantageously from 15% to 20% by weight, in particular from 16% to 18.5% by weight of chromium relative to the total weight of material M3; and less than 15% by weight of nickel, preferably between 10% and 14% by weight of nickel relative to the total weight of material M3; and less than 3% by weight of molybdenum, advantageously between 2% and 3% by weight of molybdenum relative to the total weight of material M3; and less than 2, 5% by weight of manganese, advantageously 2% by weight of manganese relative to the total weight of material M3; and less than 2% by weight of silicon, advantageously less than 1.5% by weight, preferably less than 1.25% by weight, more preferably less than 1% by weight of silicon relative to the total weight of material M3. According to a preferred embodiment, the corrosion rate of material M2 is less than 100 mGh / year, preferably less than 90 mhi / year, advantageously less than 80 mhi / year, preferably less than 70 mhi / year, even more advantageously less than 60 mhi / year, and in particular less than 50 mhi / year. This rate is measured according to the coupon method ASTM D 2 328-65 T. Reactor Preferably, the reactor is supplied with starting reactants by supply lines. The reactor may also include effluent or outlet lines making it possible to evacuate the reaction medium from the reactor. Preferably, the feed or outlet lines of the reactor are made of a specific material capable of also resisting corrosion, for example made of the aforementioned M3 material. The feed lines can be tubular in shape. Alternatively, the feed or outlet lines can be made of a material comprising a base layer made of the aforementioned M1 material coated with a surface layer, capable of being in contact with the reaction medium, made of 'a corrosion resistant M2 material. According to one embodiment, the reactor of step (b) is a stirred reactor provided with stirring mobile (s). Among the stirring wheels, we can for example cite turbines (for example turbines with so-called Rushton straight blades or turbines with curved blades or turbines with curved blades), helical ribbons, propellers (for example propellers with curved blades). profiles), anchors, and their combinations. The stirring mobile (s) can be fixed to a stirring shaft, and can be of the same or different nature. The stirring shaft can be driven by a motor, advantageously located outside the reactor. The design and size of the stirring wheels can be chosen by a person skilled in the art depending on the type of mixture to be produced (mixture of liquids, mixture of liquid and solid, mixture of liquid and gas, mixture of liquid, gas and solid) and desired mixing performance. In particular, the agitation mobile is chosen from the most suitable agitation mobiles to ensure good homogeneity of the reaction medium. Preferably, the agitation mobile (s) is (are) made of a corrosion-resistant material, such as for example made of material M3 as defined above, or may comprise a base layer made of an aforementioned material M1 coated with a surface layer, capable of being in contact with the reaction medium, made of an aforementioned material M2 resistant to corrosion. The reactor of step (b) can include heating means. The reactor of step (b) can be heated by means of a double jacket surrounding the reactor in which a heating fluid, for example steam or water, can circulate. According to one embodiment, step (b) is carried out in a reactor having an overall thermal conductivity greater than or equal to 10 W / m / ° C, preferably greater than or equal to 15 W / m / ° C. When the reactor contains a base layer made of M1 material coated with a surface layer made of corrosion resistant M2 material, the overall thermal conductivity of the reactor composed of M1 and M2 is calculated according to the formula next : li , 2 = (qi + e 2 ) / ((qi / l ΐ ) + (q 2 / l 2 )) with a thickness ei representing the thickness of the material M1, e 2 representing the thickness of the material M2, li representing the thermal conductivity of the material M1 and l 2 representing the thermal conductivity of the material M2. When the reactor is made of an M3 material, the overall thermal conductivity is that of the M3 material. The fluorination reaction typically leads to the formation of HCl, the majority of which can be degassed from the reaction medium (just like the excess HF if the fluorinating agent is HF), for example by stripping of a gas. neutral (such as nitrogen, helium or argon). However, residual HF and / or HCl can be dissolved in the reaction medium. In the case of HCl, the quantities are very low because at the working pressure and temperature, the HCl is mainly in gas form. In particular, the anhydrous HF and HCl mentioned above are corrosive. The same goes for bis (chlorosulfonyl) imide Cl- (S0 2 ) -NH- (S0 2 ) -Cl. The use of the reactor as defined above advantageously makes it possible to resist the corrosivity of the reaction medium (starting reagents, and / or products formed) under the reaction conditions, and thus to avoid contamination of the medium by metal ions. from reactor materials. The method according to the invention may comprise a step (a), prior to step (b), said step (a) comprising a step of chlorinating the sulfamic acid H0- (S0 2 ) -NH 2 to obtain the bis (chlorosulfonyl) imide Cl- (S0 2 ) -NH- (S0 2 ) -Cl, said step (a) preferably being carried out in a reactor made of a corrosion resistant M4 material, or in a reactor containing a base layer made of M5 material coated with a surface layer made of corrosion resistant M6 material. The surface layer of the reactor of step (a) is the layer capable of being in contact with the reaction medium of chlorination step (a) (for example starting reagents, products generated, etc.), the reaction medium possibly comprising any type of phase: liquid, and / or gas, and / or solid. Preferably, the surface layer of the reactor of step (a) is at least in contact with at least one of the starting reagents, such as, for example, sulfamic acid. The base layer and the surface layer can be placed against each other by plating. This is for example the case when the material M6 is a nickel-based alloy, as defined below. Preferably, the plating is carried out by welding plating, explosion plating, hot rolling plating, or by cold rolling plating, preferably by explosion plating. According to one embodiment, the surface layer has a thickness of between 0.01 and 20 mm, said thickness of said internal surface layer being less than that of said base layer. Preferably, said internal surface layer has a thickness of between 0.05 and 15 mm, preferably between 0.1 and 10 mm, advantageously between 0.1 and 5 mm. Material M5 Preferably, the material M5 is the material M1 as defined above. Preferably, the material M5 comprises at least 60% by weight of iron, preferably at least 70% by weight of iron, advantageously at least 75% by weight, even more advantageously at least 80% by weight, more preferably at least 85 % by weight, in particular at least 90% by weight, more particularly at least 95% by weight, and even more preferably at least 97% by weight of iron relative to the total weight of material M5; and less than 2% by weight of carbon, advantageously less than 1.5% by weight, preferably less than 1% by weight, preferably less than 0.75% by weight, more preferably less than 0.5% by weight, more particularly less than 0.2% by weight, and even more advantageously between 0.01% and 0.2% by weight of carbon relative to the total weight of material M5; and less than 3% by weight of molybdenum, Material M6 The material M6 can be chosen from the group consisting of enamel, fluoropolymers, and nickel-based alloys (in particular those described above for material M2). According to one embodiment, the material M6 is enamel. According to one embodiment, the material M6 is chosen from fluoropolymers, and in particular thermoplastic fluoropolymers. Mention may be made, for example, of PVDF (polyvinylidene fluoride), PTFE (polytetrafluoroethylene), PFA (copolymers of C 2 F 4 and of perfluorinated vinyl ether), FEP (copolymers of tetrafluoroethylene and of perfluoropropene, such as for example copolymer of C 2 F 4 and C 3 F 6 ), ETFE (copolymer of tetrafluoroethylene and ethylene, and FKM (copolymer of hexafluoropropylene and difluoroethylene). According to one embodiment, the reactor used in step (a) of the method according to the invention comprises a base layer made of an M5 material coated with an internal surface layer made of an M6 material resistant to corrosion, said material M5 comprising: • at least 60% by weight of iron, preferably at least 70% by weight of iron, advantageously at least 75% by weight, even more advantageously at least 80% by weight, more preferably at least 85% by weight, in particular at least 90% by weight, more particularly at least 95% by weight, and even more preferably at least 97% by weight of iron relative to the total weight of material M5; and less than 2% by weight of carbon, advantageously less than 1.5% by weight, preferably less than 1% by weight, preferably less than 0.75% by weight, more preferably less than 0.5% by weight, more particularly less than 0.2% by weight, and even more advantageously between 0.01% and 0.2% by weight of carbon relative to the total weight of material M5; and less than 3% by weight of molybdenum, preferably less than 2% by weight, preferably less than 1.5% by weight, preferably less than 1.25% by weight, more preferably less than 1% by weight, even more advantageously between 0.1% and 1% by weight of molybdenum relative to the total weight of material M5, and / or less than 5% by weight of chromium, preferentially less than 4% by weight, advantageously less than 3% by weight, preferably less than 2% by weight, in particular between 0.5% and 2% by weight of chromium relative to the total weight of material M5; and and even more advantageously between 0.01% and 0.2% by weight of carbon relative to the total weight of material M5; and less than 3% by weight of molybdenum, preferably less than 2% by weight, preferably less than 1.5% by weight, preferably less than 1.25% by weight, more preferably less than 1% by weight, even more advantageously between 0.1% and 1% by weight of molybdenum relative to the total weight of material M5, and / or less than 5% by weight of chromium, preferentially less than 4% by weight, advantageously less than 3% by weight, preferably less than 2% by weight, in particular between 0.5% and 2% by weight of chromium relative to the total weight of material M5; and and even more advantageously between 0.01% and 0.2% by weight of carbon relative to the total weight of material M5; and less than 3% by weight of molybdenum, preferably less than 2% by weight, preferably less than 1.5% by weight, preferably less than 1.25% by weight, more preferably less than 1% by weight, even more advantageously between 0.1% and 1% by weight of molybdenum relative to the total weight of material M5, and / or less than 5% by weight of chromium, preferentially less than 4% by weight, advantageously less than 3% by weight, preferably less than 2% by weight, in particular between 0.5% and 2% by weight of chromium relative to the total weight of material M5; and preferably less than 1.5% by weight, preferably less than 1.25% by weight, more preferably less than 1% by weight, even more advantageously between 0.1% and 1% by weight of molybdenum relative to the total weight material M5, and / or less than 5% by weight of chromium, preferably less than 4% by weight, advantageously less than 3% by weight, preferably less than 2% by weight, in particular between 0.5% and 2 % by weight of chromium relative to the total weight of material M5; and preferably less than 1.5% by weight, preferably less than 1.25% by weight, more preferably less than 1% by weight, even more advantageously between 0.1% and 1% by weight of molybdenum relative to the total weight material M5, and / or less than 5% by weight of chromium, preferably less than 4% by weight, advantageously less than 3% by weight, preferably less than 2% by weight, in particular between 0.5% and 2 % by weight of chromium relative to the total weight of material M5; and preferably less than 2% by weight, in particular between 0.5% and 2% by weight of chromium relative to the total weight of material M5; and preferably less than 2% by weight, in particular between 0.5% and 2% by weight of chromium relative to the total weight of material M5; and • the material M6 being chosen from nickel-based alloys, in particular from alloys comprising at least 40% by weight of nickel, advantageously at least 45% by weight of nickel, more preferably at least 50% by weight of nickel, in particular at least 55% by weight of nickel, more particularly at least 60% by weight of nickel, preferably at least 65% by weight of nickel, even more preferably at least 70% by weight of nickel relative to the total weight of material M6; and / or chromium in a content of less than 35% by weight relative to the total weight of the material M6, advantageously less than 30% by weight, preferably less than 20% by weight, more preferably less than 15% by weight, in in particular less than 10% by weight, more particularly less than 5% by weight relative to the total weight of material M6; and / or molybdenum in a content of less than 35% by weight relative to the total weight of material M2, advantageously less than 30% by weight, preferably less than 25% by weight, more preferably less than 20% by weight, in in particular less than 15% by weight, more particularly less than 10% by weight relative to the total weight of the material M6; and / or cobalt in a content of less than 10% by weight relative to the total weight of the material M6, advantageously less than 8% by weight, preferably less than 6% by weight, more preferably less than 4% by weight, in in particular less than 3% by weight, more particularly less than 2% by weight relative to the total weight of the material M6; and / or tungsten in a content of less than 5% by weight relative to the total weight of the material M6, advantageously less than 4% by weight, preferably less than 3% by weight, more preferably less than 2% by weight, in in particular less than 1% by weight, relative to the total weight of the material M6; and / or iron in a content of less than 25% by weight relative to the total weight of the material M6, advantageously less than 20% by weight, preferably less than 15% by weight, more preferably less than 10% by weight, in particular less than 7% by weight, more particularly less than 5% by weight relative to the total weight of the material M6; and / or manganese in a content of less than 5% by weight relative to the total weight of the alloy, advantageously less than 4% by weight, preferably less than 3% by weight, more preferably less than 2% by weight, in particular less than 1% by weight, more particularly less than 0.5% by weight relative to the total weight of the material M6; and / or copper in a content of less than 50% by weight, advantageously less than 45% by weight, preferably less than 40% by weight, more preferably less than 35% by weight, in particular less than 30% by weight, more particularly less than 25% by weight of copper relative to the total weight of material M6; and / or less than 4% by weight of titanium relative to the total weight of material M6, advantageously less than 3% by weight, preferably less than 2% by weight, more preferably less than 1% by weight, in particular less than 0.5% by weight of titanium, more particularly less than 0.05% by weight of titanium relative to the total weight of material M6, preferably the material M6 is devoid of titanium; and / or less than 6% by weight of niobium relative to the total weight of material M6, advantageously less than 4% by weight, preferably less than 2% by weight, more preferably less than 1% by weight, in particular less than 0.5% by weight, more particularly less than 0, According to a preferred embodiment, the reactor used in step (a) of the process according to the invention comprises a base layer made of an M5 material coated with a surface layer made of an M6 material resistant to corrosion, said M5 material comprising: • at least 60% by weight of iron, preferably at least 70% by weight of iron, advantageously at least 75% by weight, even more advantageously at least 80% by weight, more preferably at least 85% by weight, in particular at least 90% by weight, more particularly at least 95% by weight, and even more preferably at least 97% by weight of iron relative to the total weight of material M5; and less than 2% by weight of carbon, advantageously less than 1.5% by weight, preferably less than 1% by weight, preferably less than 0.75% by weight, more preferably less than 0.5% by weight, more particularly less than 0.2% by weight, and even more advantageously between 0.01% and 0.2% by weight of carbon relative to the total weight of material M5; and less than 3% by weight of molybdenum, advantageously less than 2% by weight, preferably less than 1.5% by weight, preferably less than 1.25% by weight, more preferably less than 1% by weight, even more advantageously between 0.1% and 1% by weight of molybdenum relative to the total weight material M5, and / or less than 5% by weight of chromium, preferably less than 4% by weight, advantageously less than 3% by weight, preferably less than 2% by weight, in particular between 0.5% and 2 % by weight of chromium relative to the total weight of material M5; and preferably less than 2% by weight, in particular between 0.5% and 2% by weight of chromium relative to the total weight of material M5; and preferably less than 2% by weight, in particular between 0.5% and 2% by weight of chromium relative to the total weight of material M5; and • the material M6 being chosen from fluoropolymers, and in particular thermoplastic fluoropolymers, such as for example PVDF (polyvinylidene fluoride), PTFE (polytetrafluoroethylene), PFA (copolymers of C 2 F 4 and of ether perfluorinated vinyl), FEPs (copolymers of tetrafluoroethylene and perfluoropropene, such as for example copolymer of C 2 F 4 and C 3 F 6 ), ETFE (copolymer of tetrafluoroethylene and ethylene, and FKM (copolymer of 'hexafluoropropylene and difluoroethylene). According to another preferred embodiment, the reactor used in step (a) of the process according to the invention comprises a base layer made of an M5 material coated with a surface layer made of an M6 material resistant to corrosion, said material M5 comprising • at least 60% by weight of iron, preferably at least 70% by weight of iron, advantageously at least 75% by weight, even more advantageously at least 80% by weight, more preferably at least 85% by weight, in particular at least 90% by weight, more particularly at least 95% by weight, and even more preferably at least 97% by weight of iron relative to the total weight of material M5; and less than 2% by weight of carbon, advantageously less than 1.5% by weight, preferably less than 1% by weight, preferably less than 0.75% by weight, more preferably less than 0.5% by weight, more particularly less than 0.2% by weight, and even more advantageously between 0.01% and 0.2% by weight of carbon relative to the total weight of material M5; and less than 3% by weight of molybdenum, advantageously less than 2% by weight, preferably less than 1.5% by weight, preferably less than 1.25% by weight, more preferably less than 1% by weight, even more advantageously between 0.1% and 1% by weight of molybdenum relative to the total weight material M5, and / or less than 5% by weight of chromium, preferably less than 4% by weight, advantageously less than 3% by weight, preferably less than 2% by weight, in particular between 0.5% and 2 % by weight of chromium relative to the total weight of material M5; and preferably less than 2% by weight, in particular between 0.5% and 2% by weight of chromium relative to the total weight of material M5; and preferably less than 2% by weight, in particular between 0.5% and 2% by weight of chromium relative to the total weight of material M5; and • the material M6 being enamel. According to a preferred embodiment, the corrosion rate of material M6 is less than 100 mhi / year, preferably less than 90 mhi / year, advantageously less than 80 mhi / year, preferably less than 70 mhi / year, even more advantageously less than 60 mhi / year, and in particular less than 50 mhi / year. This rate is measured according to the coupon method ASTM D 2 328-65 T. M4 material The reactor of step (a) can be made of a corrosion resistant M4 material. In particular, the reactor is made of a solid M4 corrosion resistant material. Preferably, the material M4 is pure nickel, as defined above. According to a preferred embodiment, the corrosion rate of the material M4 is less than 100 mGh / year, preferably less than 90 mhi / year, advantageously less than 80 mhi / year, preferably less than 70 mhi / year, even more advantageously less than 60 mhi / year, and in particular less than 50 mGh / year. This rate is measured according to the coupon method ASTM D 2 328-65 T. Reactor Preferably, the reactor is supplied with starting reactants by supply lines. The reactor may also include effluent or outlet lines making it possible to evacuate the reaction medium from the reactor. Preferably, the feed or outlet lines of the reactor are made of a specific material capable of also resisting corrosion, for example made of the aforementioned material M4. The feed lines can be tubular in shape. Alternatively, the feed or outlet lines can be made of a material comprising a base layer made of the aforementioned M5 material coated with a surface layer, capable of being in contact with the reaction medium, made of 'an aforementioned M6 material. According to one embodiment, the reactor of step (a) is a stirred reactor provided with stirring mobile (s). Among the stirring wheels, we can for example cite turbines (for example turbines with so-called Rushton straight blades or turbines with curved blades or turbines with curved blades), helical ribbons, propellers (for example propellers with curved blades). profiles), anchors, and their combinations. The stirring mobile (s) can be fixed to a stirring shaft, and can be of the same or different nature. The stirring shaft can be driven by a motor, advantageously located outside the reactor. The design and size of the stirring wheels can be chosen by those skilled in the art according to the type of mixture to be produced (mixture of liquids, mixture of liquid and solid, mixture of liquid and gas, mixture of liquid, gas and solid) and desired mixing performance. In particular, the agitation mobile is chosen from the most suitable agitation mobiles to ensure good homogeneity of the reaction medium. In the particular case of the presence of a medium at least two-phase solid / liquid, or even three-phase solid / liquid / gas, under the reaction conditions implemented in step (a), the stirring unit is advantageously chosen from among the most suitable agitation mobiles to ensure good homogeneity of the reaction medium, Preferably, the stirring mobile (s) is (are) made of a corrosion resistant material, such as for example made of M4 material as defined above, or can comprise a base layer made of the above-mentioned material M5 coated with a surface layer, capable of being in contact with the reaction medium, made of the above-mentioned material M6 resistant to corrosion. The reactor of step (a) can include heating means. The reactor of step (a) can be heated by means of a double jacket surrounding the reactor in which a heating fluid, for example steam or water, can circulate. According to one embodiment, step (a) is carried out in a reactor having an overall thermal conductivity greater than or equal to 10 W / m / ° C, preferably greater than or equal to 15 W / m / ° C. When the reactor contains a base layer made of M5 material coated with a surface layer made of corrosion resistant M6 material, the overall thermal conductivity l 5.6 of the reactor composed of M5 and M6 is calculated according to the following formula: ld, e— (es + qe) / ((s / hs) + (qe / le)) with a thickness e 5 representing the thickness of the material M5, e 6 representing the thickness of the material M6, l 5 representing the thermal conductivity of the material M5 and l 6 representing the thermal conductivity of the material M6. When the reactor is made of an M4 material, the overall thermal conductivity is that of the M4 material. Reaction conditions According to one embodiment, the chlorination step (a) is carried out from sulfamic acid, with at least one sulfur acid and at least one chlorinating agent. Step (a) can be performed: - at a temperature between 30 ° C and 150 ° C, preferably between 30 ° C and 120 ° C and advantageously between 30 ° C and 100 ° C; and or - With a reaction time of between 1 hour and 7 days, preferably between 1 hour and 5 days and advantageously between 1 h and 3 days; and or - at a pressure between 1 bar abs and 7 bar abs, preferably between 1 bar abs and 5 bar abs and advantageously between 1 bar abs and 3 bar abs. According to the invention, the sulfur agent can be chosen from the group consisting of chlorosulfonic acid (CIS0 3 H), sulfuric acid, oleum, and mixtures thereof. Preferably, the sulfur agent is sulfuric acid. According to the invention, the chlorinating agent can be chosen from the group consisting of thionyl chloride (SOCI 2 ), oxalyl chloride (COCI) 2 , phosphorus pentachloride (PCI 5 ), phosphonyl trichloride (PCI 3 ), phosphoryl trichloride (POCI 3 ), and mixtures thereof. Preferably, the chlorinating agent is thionyl chloride. The chlorination step (a) can be carried out in the presence of a catalyst, such as for example chosen from a tertiary amine (such as methylamine, triethylamine, or diethylmethylamine); pyridine; and 2,6-lutidine. The molar ratio between sulfur acid and sulfamic acid can be between 0.7 and 5, preferably between 1 and 5. The molar ratio between the chlorinating agent and the acid can be between 3 and 10, preferably between 2 and 5. In particular, when the sulfur-containing agent is chlorosulfonic acid, the molar ratio between the latter and the sulfamic acid is between 1 and 5, and / or the molar ratio between the chlorinating agent and the sulfamic acid is between 2 and 5. In particular, when the sulfur-containing agent is sulfuric acid (or oleum), the molar ratio between sulfuric acid (or oleum) and sulfamic acid is between 0.7 and 5. In particular, when the sulfur-containing agent is sulfuric acid (or oleum), the molar ratio between sulfuric acid (or oleum) and sulfamic acid is between 1 and 5, and / or the molar ratio between the chlorinating agent and sulfamic acid is between 3 and 10. The aforementioned sulfur-containing and chlorinating agents are in particular corrosive. The same applies to certain products formed such as, for example, bis (chlorosulfonyl) imide Cl- (S0 2 ) -NH- (S0 2 ) -Cl and HCl. The use of the reactor as defined above advantageously makes it possible to resist the corrosivity of the reaction medium (starting reagents, and / or products formed) under the reaction conditions, and thus to avoid contamination of the medium by metal ions. . The process according to the invention may further comprise a step (c), subsequent to step (b), comprising the preparation of an alkali or alkaline earth salt of bis (fluorosulfonyl) imide by neutralization of bis (fluorosulfonyl) imide. Reaction conditions Step (c) of the process according to the invention can be carried out by bringing the bis (fluorosulfonyl) imide into contact with an aqueous solution of a base chosen from alkali or alkaline earth metal carbonates of formula MC0 3 , nH 2 0 or the alkali or alkaline earth metal hydroxides M0H, nH 2 0, with M representing a monovalent cation of alkali or alkaline earth metal and n being able to vary from 0 to 10. Preferably, MOH represents LiOH, NaOH, KOH RbOH, and CsOH. Preferably, MC0 3 represents Na 2 C0 3 , K 2 C0 3 , Rb 2 C0 3 , Cs 2 C0 3 , Li2 C0 3, MC0 3 advantageously representing Na 2 C0 3 , K 2 C0 3 , Rb 2 C0 3 , or Cs 2 C0 3 . Preferably, M does not represent Li + . Preferably, the base used is not a base comprising lithium. Preferably, the base used comprises potassium. Step (c) advantageously allows the preparation of a compound of formula (I) below: F- (S0 2 ) -NM- (S0 2 ) -F (I) in which M is as defined above, M preferably being different from Li + . Step (c) can be carried out for example by adding an aqueous solution of the chosen base. The base / bis (fluorosulfonyl) imide molar ratio F- (S0 2 ) -NH- (S0 2 ) -F can be for example from 1 to 5 when the base is a hydroxide, or from 0.5 to 5 (or from 2 to 10), when the base is a carbonate. The temperature of the reaction of step (c) can for example be between -10 ° C and 40 ° C. The solution obtained at the end of step (c) comprising the alkaline or alkaline-earth salt of bis (fluorosulfonyl) imide, preferably of formula (I), can then be filtered, resulting in a filtrate F and a G. Depending on the nature of the alkali or alkaline earth metal, the desired salt may be present in the filtrate F and / or in the cake G. The alkali metal or alkaline earth metal fluorides are in particular present in the cake G but can also be found in the filtrate F. The filtrate F can be subjected to at least one extraction step with an organic solvent SO2 typically slightly soluble in water, in order to extract the desired salt, preferably of formula (I) mentioned above, in an organic phase. The extraction step typically leads to the separation of an aqueous phase and an organic phase. The above-mentioned organic solvent SO2 is in particular chosen from the following families: esters, nitriles, ethers, chlorinated solvents, aromatic solvents, and mixtures thereof. Preferably, the organic solvent SO 2 is chosen from dichloromethane, ethyl acetate, butyl acetate, tetrahydrofuran, acetonitrile, diethyl ether, and mixtures thereof. In particular, the organic solvent SO2 is butyl acetate. For each extraction, the quantity by mass of organic solvent used can vary between 1/6 and 1 times the mass of the filtrate F. The number of extractions can be between 2 and 10. Preferably, the organic phase, resulting from the extraction (s), has a content by weight of the desired salt, preferably of formula (I), ranging from 5% to 50% by weight. The separated organic phase (obtained at the end of the extraction) can then be concentrated to achieve a desired salt concentration, preferably of formula (I), between 5 and 55%, preferably between 10% and 50% by mass, said concentration possibly being achieved by any means of evaporation known to those skilled in the art. The aforementioned cake G can be washed with an organic solvent SO 3 chosen from the following families: esters, nitriles, ethers, chlorinated solvents, aromatic solvents, and mixtures thereof. Preferably, the organic solvent SO3 is chosen from dichloromethane, ethyl acetate, butyl acetate, tetrahydrofuran, acetonitrile, diethyl ether, and mixtures thereof. In particular, the organic solvent SO3 is butyl acetate. The quantity by mass of organic solvent S03 used can vary between 1 and 10 times the weight of the cake. The total amount of organic solvent SO 3 intended for washing can be used all at once or in several installments with the aim in particular of optimizing the dissolution of the desired salt, preferably of formula (I) mentioned above. Preferably, the organic phase, resulting from the washing (s) of the cake G, has a desired salt content by weight, preferably of formula (I), ranging from 5% to 50% by weight. The separated organic phase resulting from the washing (s) of the cake G can then be concentrated to achieve a desired salt concentration, preferably of formula (I), between 5 and 55%, preferably between 10% and 50. % by mass, said concentration possibly being achieved by any means of evaporation known to those skilled in the art. According to one embodiment, the organic phases resulting from the extraction (s) of the filtrate F and from the washing (s) of the cake G can be combined together, before the possible concentration step. M7, M8 and M9 materials According to a preferred embodiment, step (c) is carried out in a reactor made of a corrosion-resistant material M7, or in a reactor containing a base layer made of an M8 material coated with a layer of surface made of corrosion resistant M9 material. The surface layer of the reactor of step (c) is the layer capable of being in contact with the reaction medium of neutralization step (c) (for example starting reagents, products generated, etc.), the reaction medium possibly comprising any type of phase: liquid, and / or gas, and / or solid. Preferably, the surface layer of the reactor of step (c) is at least in contact with at least one of the starting reagents, for example bis (fluorosulfonyl) imide. The base layer and the surface layer can be placed against each other by plating. This is for example the case when the material M9 is a nickel-based alloy, as defined below. Preferably, the plating is carried out by welding plating, explosion plating, hot rolling plating, or by cold rolling plating, preferably by explosion plating. According to one embodiment, the surface layer has a thickness of between 0.01 and 20 mm, said thickness of said internal surface layer being less than that of said base layer. Preferably, said internal surface layer has a thickness of between 0.05 and 15 mm, preferably between 0.1 and 10 mm, advantageously between 0.1 and 5 mm. Preferably, the material M7 is the material M3 as defined above. More preferably, the material M7 comprises at least 60% by weight of iron, more particularly at least 70% by weight of iron relative to the total weight of the material M7; and less than 2% by weight of carbon, advantageously less than 1.5% by weight, preferably less than 1% by weight, more preferably less than 0.75% by weight, in particular less than 0.5% by weight , more particularly less than 0.2% by weight, even more advantageously less than 0.1% by weight relative to the total weight of material M7; and from 10% to 20% by weight of chromium, advantageously from 15% to 20% by weight, in particular from 16% to 18.5% by weight of chromium relative to the total weight of material M7; and less than 15% by weight of nickel, preferably between 10% and 14% by weight of nickel relative to the total weight of material M7; and less than 3% by weight of molybdenum, advantageously between 2% and 3% by weight of molybdenum relative to the total weight of material M7; and less than 2.5% by weight of manganese, advantageously 2% by weight of manganese relative to the total weight of material M7; and less than 2% by weight of silicon, advantageously less than 1.5% by weight, preferably less than 1.25% by weight, more preferably less than 1% by weight of silicon relative to the total weight of material M7. Preferably, the material M8 is the material M1 as defined above. More preferably, the material M8 comprises at least 60% by weight of iron, preferably at least 70% by weight of iron, advantageously at least 75% by weight, even more advantageously at least 80% by weight, more preferably at least 85 % by weight, in particular at least 90% by weight, more particularly at least 95% by weight, and even more preferably at least 97% by weight of iron relative to the total weight of material M8; and less than 2% by weight of carbon, advantageously less than 1.5% by weight, preferably less than 1% by weight, preferably less than 0.75% by weight, more preferably less than 0.5% by weight, more particularly less than 0.2% by weight, and even more advantageously between 0.01% and 0, 2% by weight of carbon relative to the total weight of material M8; and less than 3% by weight of molybdenum, preferably less than 2% by weight, preferably less than 1.5% by weight, preferably less than 1.25% by weight, more preferably less than 1% by weight, even more advantageously between 0.1% and 1% by weight of molybdenum relative to the total weight of material M8, and / or less than 5% by weight of chromium, preferably less than 4% by weight, advantageously less than 3% by weight, preferably less than 2% by weight, in particular between 0.5% and 2% by weight of chromium relative to the total weight of material M8. Preferably, the material M9 is the material M1 as defined above. More preferably, the material M9 is chosen from polymers, in particular polyolefins (such as for example polyethylene), and fluoropolymers, such as for example PVDF (polyvinylidene fluoride), PTFE (polytetrafluoroethylene), PFAs (copolymers of C 2 F 4 and perfluorinated vinyl ether), FEPs (copolymers of tetrafluoroethylene and perfluoropropene, such as for example copolymer of C 2 F 4 and C 3 F 6), ETFE (copolymer of tetrafluoroethylene and ethylene, and FKM (copolymer of hexafluoropropylene and difluoroethylene), more preferably the material M9 is chosen from PTFE and PFA. According to a preferred embodiment, the reactor used in step (c) of the process according to the invention comprises a base layer made of an M8 material coated with a surface layer made of an M9 material resistant to corrosion, said M8 material comprising: • at least 60% by weight of iron, preferably at least 70% by weight of iron, advantageously at least 75% by weight, even more advantageously at least 80% by weight, more preferably at least 85% by weight, in particular at least 90% by weight, more particularly at least 95% by weight, and even more preferably at least 97% by weight of iron relative to the total weight of material M8 ; and less than 2% by weight of carbon, advantageously less than 1.5% by weight, preferably less than 1% by weight, preferably less than 0.75% by weight, more preferably less than 0.5% by weight, more particularly less than 0.2% by weight, and even more advantageously between 0.01% and 0.2% by weight of carbon relative to the total weight of material M8; and less than 3% by weight of molybdenum, preferably less than 2% by weight, preferably less than 1.5% by weight, preferably less than 1.25% by weight, more preferably less than 1% by weight, even more advantageously between 0, 1% and 1% by weight of molybdenum relative to the total weight of material M8, and / or less than 5% by weight of chromium, preferably less than 4% by weight, advantageously less than 3% by weight, preferably less than 2% by weight, in particular between 0.5% and 2% by weight of chromium relative to the total weight of the material M8; and • the material M9 being chosen from fluoropolymers, and in particular thermoplastic fluoropolymers, such as for example PVDF (polyvinylidene fluoride), PTFE (polytetrafluoroethylene), PFA (copolymers of C 2 F 4 and of ether perfluorinated vinyl), FEPs (copolymers of tetrafluoroethylene and perfluoropropene, such as for example copolymer of C 2 F 4 and C 3 F 6 ), ETFE (copolymer of tetrafluoroethylene and ethylene, and FKM (copolymer of (hexafluoropropylene and difluoroethylene), more preferably the material M9 being chosen from PTFE and PFA. According to another preferred embodiment, the reactor used in step (c) of the process according to the invention is made of a corrosion-resistant M7 material, said M7 material comprising at least 60% by weight of iron, plus particularly at least 70% by weight of iron relative to the total weight of material M7; and less than 2% by weight of carbon, advantageously less than 1.5% by weight, preferably less than 1% by weight, more preferably less than 0.75% by weight, in particular less than 0.5% by weight , more particularly less than 0.2% by weight, even more advantageously less than 0.1% by weight relative to the total weight of material M7; and from 10% to 20% by weight of chromium, advantageously from 15% to 20% by weight, in particular from 16% to 18.5% by weight of chromium relative to the total weight of material M7; and less than 15% by weight of nickel, preferably between 10% and 14% by weight of nickel relative to the total weight of material M7; and less than 3% by weight of molybdenum, advantageously between 2% and 3% by weight of molybdenum relative to the total weight of material M7; and less than 2.5% in weight of manganese, advantageously 2% by weight of manganese relative to the total weight of material M7; and less than 2% by weight of silicon, advantageously less than 1.5% by weight, preferably less than 1.25% by weight, more preferably less than 1% by weight of silicon relative to the total weight of material M7. Reactor Preferably, the reactor of step (c) is supplied with starting reactants by supply lines. The reactor may also include effluent or outlet lines making it possible to evacuate the reaction medium from the reactor. Preferably, the feed or outlet lines of the reactor are made of a specific material capable of also resisting corrosion, for example made of the aforementioned M7 material. The feed lines can be tubular in shape. Alternatively, the feed or outlet lines can be made of a material comprising a base layer made of the aforementioned M8 material coated with a surface layer, capable of being in contact with the reaction medium, made of 'an aforementioned M9 material. According to one embodiment, the reactor of step (c) is a stirred reactor provided with stirring mobile (s). Among the stirring wheels, we can for example cite turbines (for example turbines with so-called Rushton straight blades or turbines with curved blades or turbines with curved blades), helical ribbons, propellers (for example propellers with curved blades). profiles), anchors, and their combinations. The stirring mobile (s) can be fixed to a stirring shaft, and can be of the same or different nature. The stirring shaft can be driven by a motor, advantageously located outside the reactor. The design and size of the stirring wheels can be chosen by those skilled in the art according to the type of mixture to be produced (mixture of liquids, mixture of liquid and solid, mixture of liquid and gas, mixture of liquid, gas and solid) and desired mixing performance. In particular, the agitation mobile is chosen from the most suitable agitation mobiles to ensure good homogeneity of the reaction medium. In the particular case of the presence of a medium at least two-phase solid / liquid, or even three-phase solid / liquid / gas, under the reaction conditions used in step (c), the agitation mobile is advantageously chosen from among the most suitable agitation mobiles to ensure good homogeneity of the reaction medium, Preferably, the stirring wheels are made of a corrosion resistant material, such as for example made of the M7 material as defined above, or can comprise a base layer made of the aforementioned M8 material coated with a surface layer, capable of being in contact with the reaction medium, made of the aforementioned material M9 resistant to corrosion. The reactor of step (c) can include cooling means. The reactor of step (c) can be cooled by means of a double jacket surrounding the reactor in which a cooling fluid, for example water, can circulate. According to one embodiment, step (c) is carried out in a reactor having an overall thermal conductivity greater than or equal to 10 W / m / ° C, preferably greater than or equal to 15 W / m / ° C. When the reactor contains a base layer made of M8 material coated with a surface layer made of corrosion resistant M9 material, the overall thermal conductivity l 8.9 of the reactor composed of M8 and M9 is calculated according to the following formula: ld, 9— (q8 + e 9 ) / ((qb / lb) + (qq / lq)) with a thickness e 8 representing the thickness of the material M8, e 9 representing the thickness of the material M9, l 8 representing the thermal conductivity of the material M8 and l 9 representing the thermal conductivity of the material M9. When the reactor is made of an M7 material, the overall thermal conductivity is that of the M7 material. The neutralization reaction in particular involves compounds which may prove to be corrosive, such as bis (fluorosulfonyl) imide F- (S0 2 ) -NH- (S0 2 ) -F and optionally residual HF. The use of the reactor as defined above advantageously makes it possible to resist the corrosivity of the reaction medium (starting reagents, and / or products formed) under the reaction conditions, and thus to avoid contamination of the medium by metal ions. . Step (d) The process according to the invention can also comprise an optional step (d) of cation exchange, subsequent to step (c), comprising the reaction between the alkaline earth salt of bis (fluorosulfonyl) imide and a salt of lithium, to obtain the lithium salt of bis (fluorosulfonyl) imide. In particular, the process according to the invention comprises this step (d) when the salt obtained in step (c) is not the lithium salt of bis (fluorosulfonyl) imide. Reaction conditions Step (d) is in particular a cation exchange reaction making it possible to convert a compound of the above-mentioned formula (I) F- (S0 2 ) -NM- (S0 2 ) -F (I), M being such that described above, to a lithium salt of bis (fluorosulfonyl) imide. Preferably, the lithium salt is chosen from LiF, LiCl, L i2 C0 3 , LiOH, LiN0 3 , LiBF 4 and their mixtures. The lithium salt can be dissolved in a polar organic solvent chosen from the following families: alcohols, nitriles and carbonates. By way of example, mention may in particular be made of methanol, ethanol, acetonitrile, dimethylcarbonate, ethylmethylcarbonate, and mixtures thereof. The molar ratio of the compound of formula (I) relative to the lithium salt can vary: it can be at least equal to 1 and less than 5. Preferably the molar ratio of the compound of formula (I) / lithium salt is between 1, 2 and 2. The reaction medium can be left with stirring for between 1 to 24 hours, and / or at a temperature of, for example, between 0 ° C and 50 ° C. At the end of the reaction, the reaction medium can be filtered and then optionally be concentrated. The concentration step can optionally be carried out by a thin-film evaporator, by an atomizer, by a rotary evaporator, or by any other device allowing the evaporation of solvent. Filtration can be done using a filter or wringer. The filter or the wringer is preferably made of a material M 'comprising: at least 60% by weight of iron, more particularly at least 70% by weight of iron relative to the total weight of material M ′; - less than 2% by weight of carbon, advantageously less than 1.5% by weight, preferably less than 1% by weight, more preferably less than 0.75% by weight, in particular less than 0.5% by weight , more particularly less than 0.2% by weight, even more advantageously less than 0.1% by weight relative to the total weight of the material M ′; and - from 10% to 20% by weight of chromium, advantageously from 15% to 20% by weight, in particular from 16% to 18.5% by weight of chromium relative to the total weight of the material M ′; and eventually : - less than 15% by weight of nickel, preferably between 10% and 14% by weight of nickel relative to the total weight of material M ′; and or - less than 3% by weight of molybdenum, advantageously between 2% and 3% by weight of molybdenum relative to the total weight of the material M '; and or - less than 2.5% by weight of manganese, advantageously 2% by weight of manganese relative to the total weight of material M '; and or - less than 2% by weight of silicon, advantageously less than 1.5% by weight, preferably less than 1.25% by weight, more preferably less than 1% by weight of silicon relative to the total weight of the material M ' . Preferably, the material M 'comprises at least 60% by weight of iron, more particularly at least 70% by weight of iron relative to the total weight of the material M'; and less than 2% by weight of carbon, advantageously less than 1.5% by weight, preferably less than 1% by weight, more preferably less than 0.75% by weight, in particular less than 0.5% by weight , more particularly less than 0.2% by weight, even more advantageously less than 0.1% by weight relative to the total weight of the material M ′; and from 10% to 20% by weight of chromium, advantageously from 15% to 20% by weight, in particular from 16% to 18, 5% by weight of chromium relative to the total weight of material M '; and less than 15% by weight of nickel, preferably between 10% and 14% by weight of nickel relative to the total weight of material M ′; and less than 3% by weight of molybdenum, advantageously between 2% and 3% by weight of molybdenum relative to the total weight of material M ′; and less than 2.5% by weight of manganese, advantageously 2% by weight of manganese relative to the total weight of material M '; and less than 2% by weight of silicon, advantageously less than 1.5% by weight, preferably less than 1.25% by weight, more preferably less than 1% by weight of silicon relative to the total weight of the material M ' . and less than 3% by weight of molybdenum, advantageously between 2% and 3% by weight of molybdenum relative to the total weight of material M ′; and less than 2.5% by weight of manganese, advantageously 2% by weight of manganese relative to the total weight of material M '; and less than 2% by weight of silicon, advantageously less than 1.5% by weight, preferably less than 1.25% by weight, more preferably less than 1% by weight of silicon relative to the total weight of the material M ' . and less than 3% by weight of molybdenum, advantageously between 2% and 3% by weight of molybdenum relative to the total weight of material M ′; and less than 2.5% by weight of manganese, advantageously 2% by weight of manganese relative to the total weight of material M '; and less than 2% by weight of silicon, advantageously less than 1.5% by weight, preferably less than 1.25% by weight, more preferably less than 1% by weight of silicon relative to the total weight of the material M ' . The filter or wringer preferably comprises a base layer made of material M1 coated with a surface layer made of corrosion resistant material M2, said material M1 comprising: • at least 60% by weight of iron, preferably at least 70% by weight of iron, advantageously at least 75% by weight, even more advantageously at least 80% by weight, more preferably at least 85% by weight, in particular at least 90% by weight, more particularly at least 95% by weight, and even more preferably at least 97% by weight of iron relative to the total weight of material M1; and less than 2% by weight of carbon, advantageously less than 1.5% by weight, preferably less than 1% by weight, preferably less than 0.75% by weight, more preferably less than 0.5% by weight, more particularly less than 0.2% by weight, and even more advantageously between 0.01% and 0.2% by weight of carbon relative to the total weight of material M1; and less than 3% by weight of molybdenum, advantageously less than 2% by weight, 2% by weight, in particular between 0.5% and 2% by weight of chromium relative to the total weight of material M1; and • the material M2 being chosen from nickel-based alloys, in particular chosen from alloys comprising at least 40% by weight of nickel, advantageously at least 45% by weight, more preferably at least 50% by weight, in particular at less 55% by weight, more particularly at least 60% by weight, preferably at least 65% by weight, even more preferably at least 70% by weight of nickel relative to the total weight of material M2; and / or chromium in a content of less than 35% by weight relative to the total weight of material M2, advantageously less than 30% by weight, preferably less than 20% by weight, more preferably less than 15% by weight, in in particular less than 10% by weight, more particularly less than 5% by weight relative to the total weight of material M2; and / or molybdenum in a content of less than 35% by weight relative to the total weight of material M2, advantageously less than 30% by weight, preferably less than 25% by weight, more preferably less than 20% by weight, in in particular less than 15% by weight, more particularly less than 10% by weight relative to the total weight of material M2; and / or cobalt in a content of less than 10% by weight relative to the total weight of material M2, advantageously less than 8% by weight, preferably less than 6% by weight, more preferably less than 4% by weight, in in particular less than 3% by weight, more particularly less than 2% by weight relative to the total weight of material M2; and / or tungsten in a content of less than 5% by weight relative to the total weight of material M2, advantageously less than 4% by weight, preferably less than 3% by weight, more preferably less than 2% by weight, in particular less than 1% by weight, relative to the total weight of material M2; and / or iron in a content of less than 25% by weight relative to the total weight of material M2, advantageously less than 20% by weight, preferably less than 15% by weight, more preferably less than 10% by weight, in in particular less than 7% by weight, more particularly less than 5% by weight relative to the total weight of material M2; and / or manganese in a content of less than 5% by weight relative to the total weight of the alloy, advantageously less than 4% by weight, preferably less than 3% by weight, more preferably less than 2% by weight, in particular less than 1% by weight, more particularly less than 0.5% by weight relative to the total weight of material M2; and / or copper in a content of less than 50% by weight, advantageously less than 45% by weight, preferably less than 40% by weight, more preferably less than 35% by weight, in particular less than 30% by weight, more particularly less than 25% by weight of copper relative to the total weight of material M2; and / or less than 4% by weight more particularly less than 25% by weight of copper relative to the total weight of material M2; and / or less than 4% by weight more particularly less than 25% by weight of copper relative to the total weight of material M2; and / or less than 4% by weight of titanium relative to the total weight of material M2, advantageously less than 3% by weight, preferably less than 2% by weight, more preferably less than 1% by weight, in particular less than 0.5% by weight, more particularly less than 0.05% by weight relative to the total weight of material M2, preferably the material M2 being devoid of titanium; and / or less than 6% by weight of niobium, advantageously less than 4% by weight, preferably less than 2% by weight, more preferably less than 1% by weight, in particular less than 0.5% by weight, more particularly less than 0.05% by weight of niobium relative to the total weight of material M2, preferably material M2 being niobium-free. Reactor Step (d) can be carried out in a reactor based on silicon carbide or based on a fluoropolymer, or in a steel reactor comprising an interior surface, said interior surface capable of being in contact with the salt. lithium bis (fluorosulfonyl) imide being covered by a polymeric coating or by a coating of silicon carbide. The aforementioned fluoropolymer is advantageously chosen from PVDF (polyvinylidene fluoride), PTFE (polytetrafluoroethylene), PFA (copolymers of C 2 F 4 and perfluorinated vinyl ether), and ETFE (copolymer of tetrafluoroethylene and of l ' ethylene). The fluoropolymer is advantageously chosen from PVDF, PFAs, and ETFE. The polymeric coating can be a coating comprising at least one of the following polymers: polyolefins such as for example polyethylene, or fluorinated polymers such as for example PVDF (polyvinylidene fluoride), PTFE (polytetrafluoroethylene), PFAs ( copolymers of C 2 F 4 and perfluorinated vinyl ether), FEPs (copolymers of tetrafluoroethylene and perfluoropropene, such as for example copolymer of C 2 F 4 and C 3 F 6), ETFE (copolymer of tetrafluoroethylene and ethylene, and FKM (copolymer of hexafluoropropylene and difluoroethylene). Preferably, the polymeric coating comprises at least one fluoropolymer, and in particular PFA, PTFE or PVDF. According to one embodiment, the reactor of step (d) is a stirred reactor provided with stirring mobile (s). Among the stirring wheels, we can for example cite turbines (for example turbines with so-called Rushton straight blades or turbines with curved blades or turbines with curved blades), helical ribbons, propellers (for example propellers with curved blades). profiles), anchors, and their combinations. The stirring mobile (s) can be fixed on a central stirring shaft, and can be of the same or different nature. The stirring shaft can be driven by a motor, advantageously located outside the reactor. The design and size of the stirring wheels can be chosen by a person skilled in the art depending on the type of mixture to be produced (mixture of liquids, mixture of liquid and solid, mixture of liquid and gas, mixture of liquid, gas and solid) and desired mixing performance. In particular, the agitation mobile is chosen from the most suitable agitation mobiles to ensure good homogeneity of the reaction medium. Preferably, the stirring mobile (s) is (are) made of a steel material, preferably carbon steel, comprising an outer surface, said outer surface capable of being in contact with the lithium salt of bis (fluorosulfonyl) imide being covered by a polymeric coating, preferably as defined above, or by a coating of silicon carbide. The method according to the invention can also comprise an optional step (e) of purification of the lithium salt of the bis (fluorosulfonyl) imide. Step (e) of purification of the lithium salt of the bis (fluorosulfonyl) imide can be carried out by any known conventional method. It may be, for example, an extraction method, a washing method with solvents, a reprecipitation method, a recrystallization method, or a combination thereof. At the end of step (e) above, the lithium salt of the bis (fluorosulfonyl) imide can be in the form of a solid, or of a composition comprising from 1% to 99.9% by weight lithium salt of bis (fluorosulfonyl) imide. According to a first embodiment, step (e) is a step of crystallization of the LiFSI. Preferably, during step (e), the LiFSI is crystallized in the cold, in particular at a temperature less than or equal to 25 ° C. Preferably, during step (e), the crystallization of the LiFSI is carried out in an organic solvent (“crystallization solvent”) chosen from chlorinated solvents, such as for example dichloromethane, from alkanes such as for example pentane, hexane, cyclohexane and heptane, and from aromatic solvents, such as for example toluene, in particular at a temperature less than or equal to 25 ° C. Preferably, the LiFSI crystallized at the end of step (e) is recovered by filtration. According to a second embodiment, step (e) comprises the following steps: i ′) optionally dissolving the LiFSI in an organic solvent OS1; i) liquid-liquid extracting the lithium salt of bis (fluorosulfonyl) imide with deionized water, and recovering an aqueous solution of said lithium salt of bis (fluorosulfonyl) imide; ii) optional concentration of said aqueous solution of said salt; iii) liquid-liquid extraction of the lithium salt of bis (fluorosulfonyl) imide from said aqueous solution with at least one organic solvent OS2; iv) concentration of the lithium salt of bis (fluorosulfonyl) imide by evaporation of said organic solvent OS2; v) optionally crystallization of the lithium salt of bis (fluorosulfonyl) imide. Preferably, at least one of steps i), ii), iii), or iv) is carried out in: - equipment based on silicon carbide or based on a fluoropolymer; or - a steel equipment, preferably carbon steel, comprising an interior surface, said interior surface capable of being in contact with the lithium salt of bis (fluorosulfonyl) imide being covered by a polymeric coating or by a coating of carbide of silicon. In the context of the invention, the terms “demineralized water” and “deionized water” are used in an equivalent manner. The equipment can be a reactor, an evaporator, a mixer-settler, a liquid-liquid extraction column, a settling tank, an exchanger. The silicon carbide equipment is preferably solid silicon carbide equipment. The equipment based on a fluoropolymer is preferably a solid fluoropolymer equipment. The fluoropolymer is advantageously chosen from PVDF (polyvinylidene fluoride), PTFE (polytetrafluoroethylene), PFA (copolymers of C 2 F 4 and perfluorinated vinyl ether), and ETFE (copolymer of tetrafluoroethylene and ethylene ). The fluoropolymer of the equipment is advantageously chosen from PVDF, PFAs, and ETFE. The polymeric coating can be a coating comprising at least one of the following polymers: polyolefins such as for example polyethylene, or fluorinated polymers such as for example PVDF (polyvinylidene fluoride), PTFE (polytetrafluoroethylene), PFAs ( copolymers of C 2 F 4 and perfluorinated vinyl ether), FEPs (copolymers of tetrafluoroethylene and perfluoropropene, such as for example copolymer of C 2 F 4 and C 3 F 6 ), ETFE (copolymer of tetrafluoroethylene and ethylene, and FKM (copolymer of hexafluoropropylene and difluoroethylene). Preferably, the polymeric coating comprises at least one fluoropolymer, and in particular PFA, PTFE or PVDF. Preferably: - step i) is carried out in equipment as defined above, said equipment preferably being an extraction column or a mixer-settler; and or - step ii) is carried out in equipment as defined above, said equipment preferably being an evaporator or an exchanger; and or - step iii) is carried out in equipment as defined above, said equipment preferably being an extraction column or a mixer-settler; and or - step iv) is carried out in equipment as defined above, said equipment preferably being an evaporator or an exchanger. Step (e) may not include step i ′) mentioned above if the LiFSI obtained in step (d) already comprises an organic solvent. Step i) can be carried out in equipment chosen from an extraction column, a mixer-settler, and mixtures thereof. According to one embodiment, step i) of liquid-liquid extraction is carried out in: an extraction column or a mixer-settler, based on silicon carbide or based on a fluoropolymer, preferably as defined above; or an extraction column or a mixer-settler, made of steel, preferably of carbon steel, said extraction column or said mixer-settler comprising an interior surface, said interior surface capable of being in contact with the lithium salt of bis (fluorosulfonyl) imide being covered with a polymeric coating preferably as defined above or with a coating of silicon carbide. Preferably, step i) of liquid-liquid extraction is carried out in: - an extraction column or a mixer-settler based on a fluoropolymer, such as for example PVDF (polyvinylidene fluoride), or PFA (copolymers of C 2 F 4 and perfluorinated vinyl ether); or an extraction column or a mixer-settler, made of steel, preferably of carbon steel, said extraction column or said mixer-settler comprising an interior surface, said interior surface capable of being in contact with the lithium salt of bis (fluorosulfonyl) imide being covered by a polymeric coating preferably as defined above. Mixer-settlers are well known to those skilled in the art. It is typically a single device comprising a mixing chamber and a settling chamber, the mixing chamber comprising a stirring mobile advantageously allowing the mixing of the two liquid phases. In the settling chamber, the phases are separated by gravity. The feed to the settling chamber from the mixing chamber can be done by overflow, through the bottom of the mixing chamber, or through a perforated wall between the mixing chamber and the settling chamber. The extraction column can include: - at least one packing such as, for example, a loose packing and / or a structured packing. They can be Rashig rings, Pali rings, Saddle rings, Berl saddles, Intalox saddles, or marbles; and or - trays such as for example perforated trays, fixed valve trays, movable valve trays, domed trays, or combinations thereof; and or - devices for atomizing one phase into another, such as for example nozzles, the said packing (s), tray (s), atomization device (s) preferably being made of a polymeric material, the polymeric material possibly comprising at least one polymer chosen from among polyolefins such as for example polyethylene, and fluorinated polymers such as for example PVDF (polyvinylidene fluoride), PTFE (polytetrafluoroethylene), PFA (copolymers of C 2 F 4 and perfluorinated vinyl ether), FEP ( copolymers of tetrafluoroethylene and perfluoropropene, such as, for example, copolymer of C 2 F 4 and C 3 F 6 ), ETFE (copolymer of tetrafluoroethylene and ethylene, and FKM (copolymer of hexafluoropropylene and difluoroethylene). The extraction column can also include baffles integral with the side walls of said column. The baffles advantageously make it possible to limit the phenomenon of axial remixing. In the context of the invention, the term “packing” is understood to mean a solid structure capable of increasing the contact surface between the two liquids brought into contact. The height and / or the diameter of the extraction column typically depends on the nature of the liquids to be separated. The extraction column can be static or agitated. Preferably, the extraction column is stirred, preferably mechanically. It comprises for example one or more stirring wheels fixed on an axial rotary shaft. Among the stirring wheels, we can for example cite turbines (for example turbines with so-called Rushton straight blades or turbines with curved blades or turbines with curved blades), propellers (for example propellers with profiled blades), discs, and their mixtures. Stirring advantageously allows the formation of fine droplets to disperse one liquid phase in the other, and thus increase the interfacial exchange area. Preferably, the stirring speed is chosen so as to maximize the interfacial exchange area. Preferably, the stirring mobile (s) is (are) made of a steel material, preferably carbon steel, comprising an outer surface, said outer surface capable of being in contact with the lithium salt of bis (fluorosulfonyl) imide being covered by a polymeric coating, preferably as defined above, or by a coating of silicon carbide. According to the invention, the aforementioned step i) can be repeated at least once, preferably repeated from 1 to 10 times, preferably from 1 to 4 times. When step i) is repeated, it can be carried out in several mixer-settlers in series. Step i) can be carried out continuously or discontinuously, preferably continuously. According to one embodiment, step i) comprises the addition of deionized water to the solution of LiFSI in the above-mentioned organic solvent OS1, for example obtained during previous synthesis steps, to allow the dissolution of said salt, and extracting said salt in water (aqueous phase). In the particular case of a batch step, and during the repetition of step i), a quantity of deionized water corresponding to at least half of the mass of the initial solution can be added in a first extraction, then an amount greater than or equal to approximately 1/3 of the mass of the initial solution during the second extraction, then an amount greater than or equal to approximately 1/4 of the mass of the initial solution during the third extraction. In the case of multiple extractions (repetition of step i)), the extracted aqueous phases are combined together to form a single aqueous solution. Step i) advantageously makes it possible to obtain an aqueous phase and an organic phase, which are separated. Step ii) is thus advantageously carried out on the aqueous solution extracted in step a) (single aqueous phase or aqueous phases combined in the event of repeating step i)). At the end of step i), an aqueous solution of LiFSI is advantageously obtained. Preferably, the mass content of LiFSI in the aqueous solution is between 5% and 35%, preferably between 10% and 25%, relative to the total mass of the solution. Step (e) can comprise a concentration step ii) between step i) and step iii), preferably to obtain an aqueous LiFSI solution comprising a mass content of LiFSI of between 20% and 80%, in particular between 25% and 80%, preferably between 25% and 70%, and advantageously between 30% and 65% relative to the total mass of the solution. The concentration step can be carried out under reduced pressure, for example at a pressure less than 50 mbar abs (preferably less than 30 mbar abs), and / or at a temperature between 25 ° C and 60 ° C, preferably © this between 25 ° C and 50 ° C, preferably between 25 ° C and 40 ° C. Step ii) can be carried out in at least one item of equipment chosen from an evaporator or an exchanger. According to one embodiment, the concentration step ii) is carried out in: an exchanger or evaporator, based on silicon carbide or based on a fluoropolymer, preferably as defined above; or an exchanger or evaporator, made of steel, preferably of carbon steel, said exchanger or evaporator comprising an interior surface, said interior surface capable of being in contact with the lithium salt of bis (fluorosulfonyl) imide being covered with a polymeric coating of preferably as defined above or by a coating of silicon carbide. Preferably, step ii) is carried out in: - an exchanger or evaporator, based on silicon carbide; or - an exchanger or evaporator, made of steel, preferably of carbon steel, said exchanger or evaporator comprising an interior surface, said interior surface capable of being in contact with the lithium salt of bis (fluorosulfonyl) imide being covered by a coating of carbide of silicon. Preferably, step (e) purification according to the invention comprises step ii). After concentration ii) of the aqueous solution obtained at the end of step a), a concentrated aqueous LiFSI solution is obtained. Step iii) can be carried out on the aqueous solution obtained at the end of step i) or of concentration step ii) or of a possible other intermediate step. Stage iii) can be carried out in equipment chosen from an extraction column, a mixer-settler, and mixtures thereof. According to one embodiment, step iii) of liquid-liquid extraction is carried out in: an extraction column or a mixer-settler, based on silicon carbide or based on a fluoropolymer, preferably as defined above; or an extraction column or a mixer-settler, steel preferably of carbon steel, said extraction column or said mixer-settler comprising an interior surface, said interior surface capable of being in contact with the lithium salt of bis (fluorosulfonyl) imide being covered with a polymeric coating preferably as defined above or with a coating of silicon carbide. Preferably, step iii) of liquid-liquid extraction is carried out in: an extraction column or a mixer-settler based on a fluoropolymer such as, for example, PVDF (polyvinylidene fluoride), or PFA (copolymers of C 2 F 4 and perfluorinated vinyl ether); or an extraction column or a mixer-settler, made of steel, preferably of carbon steel, said extraction column or said mixer-settler comprising an interior surface, said interior surface capable of being in contact with the lithium salt of bis (fluorosulfonyl) imide being covered by a polymeric coating preferably as defined above. The extraction column can be static or agitated. Preferably, the extraction column is stirred, preferably mechanically. It comprises for example one or more stirring wheels fixed on an axial rotary shaft. Among the stirring wheels, we can for example cite turbines (for example turbines with so-called Rushton straight blades or turbines with curved blades or turbines with curved blades), propellers (for example propellers with profiled blades), discs, and their mixtures. Stirring advantageously allows the formation of fine droplets to disperse one liquid phase in the other, and thus increase the interfacial exchange area. Preferably, the stirring speed is chosen so as to maximize the interfacial exchange area. Preferably, the stirring mobile (s) is (are) made of a steel material, preferably carbon steel, comprising an outer surface, said outer surface capable of being in contact with the lithium salt of bis (fluorosulfonyl) imide being covered by a polymeric coating, preferably as defined above, or by a coating of silicon carbide. Stage iii) advantageously makes it possible to recover an organic phase, saturated with water, containing the LiFSI (this is a solution of LiFSI in the at least organic solvent OS2, said solution being saturated with water). The OS2 extraction solvent for the LiFSI salt dissolved in deionized water is advantageously: • a good solvent for the LiFSI salt, that is to say that the LiFSI can have a solubility greater than or equal to 10% by weight relative to the total weight of the sum of LiFSI and solvent; and or • poorly soluble in water, ie it has a solubility less than or equal to 1% by weight relative to the total weight of the sum of solvent and water. According to one embodiment, the organic solvent OS2 is chosen from the group consisting of esters, nitriles, ethers, chlorinated solvents, aromatic solvents, and mixtures thereof. Preferably, the solvent OS2 is chosen from ethers, esters, and mixtures thereof. For example, there may be mentioned diethylcarbonate, dimethylcarbonate, ethylmethyl carbonate, methyl-t-butyl ether, cyclopentylmethyl ether, ethyl acetate, propyl acetate, methyl acetate, acetate butyl, methyl propionate, dichloromethane, tetrahydrofuran, diethyl ether, and mixtures thereof. Preferably, the solvent OS2 is chosen from methyl-t-butyl ether, cyclopentylmethyl ether, ethyl acetate, propyl acetate, butyl acetate, and mixtures thereof, According to the invention, step iii) can be repeated at least once, preferably repeated from 1 to 10 times, preferably from 1 to 4 times. When step iii) is repeated, it can be carried out in several mixer-settlers in series. In the case of multiple extractions (repetition of step iii)), the extracted organic phases are combined together to form a single organic solution. Step iii) can be carried out continuously or discontinuously, preferably continuously. According to one embodiment, step iii) comprises the addition of at least the organic solvent OS2 to the aqueous solution of LiFSI, to allow the dissolution of said salt, and the extraction of said salt in the organic phase. In the particular case of a batch process, and during the repetition of step iii), the quantity by mass of organic solvent (s) OS2 used can vary between 1/6 and 1 times the mass of the aqueous phase. Preferably, the weight ratio organic solvent (s) S2 / water, during an extraction of step b), varies from 1/6 to 1/1, the number of extractions varying in particular from 2 to 10. According to one embodiment, the mass content of LiFSI in solution in the organic phase obtained at the end of step iii) is between 5% and 35%, preferably between 10% and 25% by mass relative to the total mass of the solution. Step iv) can include: a step iv-1) of pre-concentration of the solution obtained in the previous step; and a step iv-2) of concentration of the solution obtained in step iv-1). Step iv-1) advantageously makes it possible to obtain a solution of LiFSI in the at least organic solvent OS2 comprising a weight content of LiFSI of between 20% and 60%, and preferably between 30% and 50% by weight relative to to the total mass of the solution. The pre-concentration step iv-1) can be carried out: - at a temperature ranging from 25 ° C to 60 ° C, preferably from 25 ° C to 50 ° C, and or - under reduced pressure, for example at a pressure below 50 mbar abs, in particular at a pressure below 30 mbar abs. Step iv-1) can be carried out in equipment chosen from an evaporator or an exchanger. According to one embodiment, the pre-concentration step iv-1) is carried out in: an exchanger or an evaporator, based on silicon carbide or based on a fluoropolymer, preferably as defined above; or - an exchanger or an evaporator, made of steel, preferably of carbon steel, said exchanger or evaporator comprising an interior surface, said interior surface capable of being in contact with the lithium salt of bis (fluorosulfonyl) imide being covered by a polymeric coating preferably as defined above or by a coating of silicon carbide. Preferably, step iv-1) is carried out in: - an exchanger or evaporator, based on silicon carbide; or - an exchanger or evaporator, made of steel, preferably of carbon steel, said exchanger or evaporator comprising an interior surface, said interior surface capable of being in contact with the lithium salt of bis (fluorosulfonyl) imide being covered by a coating of carbide of silicon. Step iv-2) can be carried out in equipment chosen from an evaporator, such as for example a thin film evaporator (and preferably a short path thin film evaporator), or an exchanger. Preferably, step iv-2) is carried out in a short path thin film evaporator. Step iv-2) can be carried out in: an exchanger or evaporator, based on silicon carbide or based on a fluoropolymer, preferably as defined above; or - an exchanger or evaporator, made of steel, preferably of carbon steel, said exchanger or evaporator comprising an interior surface, said interior surface capable of being in contact with the lithium salt of bis (fluorosulfonyl) imide being covered by a polymeric coating of preferably as defined above or by a coating of silicon carbide. According to a preferred embodiment, the aforementioned step (e) comprises a step iv-2) of concentration of the lithium salt of bis (fluorosulfonyl) imide by evaporation of said at least organic solvent OS2, in a thin film evaporator in a short time. journey, preferably under the following conditions: - temperature between 30 ° C and 100 ° C; - pressure between 10 -3 mbar abs and 5 mbar abs; - residence time less than or equal to 15 min. According to one embodiment, the concentration step iv-2) is carried out at a pressure between 10 -2 mbar abs and 5 mbar abs, preferably between 5.10 -2 mbar abs and 2 mbar abs, preferably between 5.10 -1 and 2 mbar abs, even more preferably between 0.1 and 1 mbar abs, and in particular between 0.1 and 0.6 mbar abs. According to one embodiment, step iv-2) is carried out at a temperature between 30 ° C and 95 ° C, preferably between 40 ° C and 900, preferably between 40 ° C and 85 ° C, and in particular between 50 ° C and 80 ° C. According to one embodiment, step iv-2) is carried out with a residence time of less than or equal to 10 min, preferably less than or equal to 5 min, and preferably less than or equal to 3 minutes. In the context of the invention, and unless otherwise indicated, the term “residence time” is understood to mean the time which elapses between the entry of the solution of lithium salt of bis (fluorosulfonyl) imide (in particular obtained at the end of step b) above) in the evaporator and the exit of the first drop of the solution. According to a preferred embodiment, the temperature of the condenser of the short path thin film evaporator is between - -55 ° C and 10 ° C, preferably between -50 ° C and 5 ° C, more preferably between - 45 ° C and -10 ° C is advantageously between -40 ° C and -15 ° C. The short path thin film evaporators according to the invention are also known under the name “Wiped film short path” (WFSP). They are typically called so because the vapors generated during evaporation make a “short path” (short distance) before being condensed in the condenser. Among the short path thin film evaporators, mention may in particular be made of the evaporators sold by the companies Buss SMS Ganzler ex Luwa AG, UIC Gmbh or VTA Process. Typically, short path thin film evaporators may have a solvent vapor condenser placed inside the apparatus itself (especially in the center of the apparatus), unlike other types of film evaporators. thin (which are not short-path) in which the condenser is located on the outside of the device. In this type of device, the formation of a thin film of product to be distilled on the internal hot wall of the evaporator can typically be ensured by continuous spreading on the evaporation surface using mechanical means. specified below. The evaporator can in particular be provided at its center with an axial rotor on which the mechanical means which allow the formation of the film on the wall are mounted. They may be rotors equipped with fixed blades: lobed rotors with three or four blades made of flexible or rigid materials, distributed over the entire height of the rotor or else rotors equipped with mobile blades, vanes, wiper brushes, guided wipers. In this case, the rotor can be constituted by a succession of pivotally articulated vanes mounted on a shaft or axis by means of radial supports. Other rotors can be equipped with mobile rollers mounted on secondary axles and said rollers are pressed against the wall by centrifugation. The speed of rotation of the rotor which depends on the size of the device, According to one embodiment, the LiFSI salt solution is introduced into the short path thin film evaporator with a flow rate of between 700 g / h and 1200 g / h, preferably between 900 g / h and 110 g / h for a evaporation surface of 0.04m 2 . According to the invention, at the end of the above-mentioned step iv), the LiFSI can be obtained in solid form, and in particular in crystalline form, or in the form of a concentrated solution, the concentrated solution comprising less than 35% by weight of residual solvent, preferably less than 30% by weight. According to one embodiment, step (e) comprises a step v) of crystallization of the lithium salt of the bis (fluorosulfonyl) imide obtained at the end of step iv) mentioned above. Preferably, during step v), the LiFSI is cold crystallized, in particular at a temperature less than or equal to 25 ° C. Preferably, the crystallization step v) of LiFSI is carried out in an organic solvent S3 (“crystallization solvent”) chosen from chlorinated solvents, such as for example dichloromethane, from alkanes such as for example pentane, l hexane, cyclohexane or heptane, and from aromatic solvents, such as for example toluene, in particular at a temperature less than or equal to 25 ° C. Preferably, the LiFSI crystallized at the end of step v) is recovered by filtration. Process The process according to the invention advantageously leads to a LiFSI, having a high purity, and preferentially to a LiFSI having a high purity and a reduced content of metal ions. By metal ions, is meant in particular ions derived from transition metals (such as for example Cr, Mn, Fe, Ni, Cu), ions derived from poor metals (such as for example Al, Zn and Pb), ions derived from alkali metals (such as for example Na), ions derived from alkaline earth metals (such as for example Mg and Ca), and ions derived from silicon. Thus, the process according to the invention advantageously leads to a LiFSI having a reduced content of ions derived from the following metals: Cr, Mn, Fe, Ni, Cu, Al, Zn, Mo, Co, Pb, Na, Si, Mg, It. In particular, the process according to the invention advantageously results in a composition comprising at least 99.9% by weight of LiFSI, preferably at least 99.95% by weight, preferably at least 99.99% by weight of LiFSI, and said LiFSI optionally comprising at least one of the following impurities in the indicated contents: 0