Process for the purification of a lithium salt of bis (fluorosulfonyl) imide FIELD OF THE INVENTION The present invention relates to a method for purifying 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 . Identifying and quantifying impurities in salts and / or electrolytes, and understanding their impact on battery performance becomes essential. For example, impurities having a mobile proton, due to their interference with electrochemical reactions, lead to lower performance and overall stability of Li-ion batteries. The application of Li-ion batteries requires having products of high purity (minimum of impurities). The existing methods for purifying LiFSI include in particular steps carried out in equipment made of glass, enamelled steel, carbon steel, etc. However, certain metal ions, such as for example sodium ions, can elute from the materials of said equipment and thus contaminate LiFSI. However, the presence of metal ions in 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. Thus, there is a need for a new process for purifying 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 method of purifying a lithium salt of bis (fluorosulfonyl) imide in solution in at least one solvent S1, said method comprising at least one purification step 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 “lithium salt of bis (fluorosulfonyl) imide”, “lithium bis (sulfonyl) imide”, “LiFSI”, “LiN (FS0 2 ) 2” , “ LiN (FS0 2 ) 2” , “ lithium bis (sulfonyl) imide ", or" lithium bis (fluorosulfonyl) imide ". Preferably, the purification step is a step where the lithium salt of bis (fluorosulfonyl) imide is in contact with water. The purification step can be a liquid-liquid extraction step, a concentration step, a decantation step, etc. The equipment can be a reactor, an evaporator, a mixer-settler, a liquid-liquid extraction column, a settling tank, an exchanger. When the purification step is a liquid-liquid extraction, the equipment can be a liquid-liquid extraction column or a mixer-settler. When the purification step is a concentration, the equipment can be an evaporator or an exchanger. When the purification step is a settling, the equipment can be a settling tank. Preferably, the solvent S1 is an organic solvent. According to one embodiment, the organic solvent S1 is chosen from the group consisting of esters, nitriles, ethers, and mixtures thereof. Preferably, the solvent S1 is chosen from ethyl acetate, butyl acetate, tetrahydrofuran, acetonitrile, diethyl ether, and their mixtures, the organic solvent S1 preferably being butyl acetate. According to one embodiment, the purification process according to the invention comprises the following steps: a) 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; a ') optional concentration of said aqueous solution of said salt; b) liquid-liquid extraction of the lithium salt of bis (fluorosulfonyl) imide from said aqueous solution with at least one organic solvent S2; c) concentration of the lithium salt of bis (fluorosulfonyl) imide by evaporation of said organic solvent S2; d) optionally crystallization of the lithium salt of bis (fluorosulfonyl) imide; at least one of steps a), a '), b), or c) being 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 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 of ethylene, and FKM (copolymer of hexafluoropropylene and difluoroethylene). Preferably, the polymeric coating comprises at least one fluoropolymer, and in particular PFA, PTFE or PVDF. 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. Preferably, the method according to the invention is such that: - step a) is carried out in equipment as defined above; and or - step a ′) is carried out in equipment as defined above; and or - step b) is carried out in equipment as defined above; and or - step c) is carried out in equipment as defined above. According to one embodiment, the mass content of LiFSI in the at least solvent S1 is between 5% and 55%, preferably between 5% and 65%, preferably between 10% and 60%, advantageously between 10% and 55% , for example between 10% and 50%, in particular between 15% and 45%, and preferably between 25% and 40% by mass, relative to the total mass of the solution. Step a) can be carried out in equipment chosen from an extraction column, a mixer-settler, and mixtures thereof. According to one embodiment, step a) 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 a) 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 with a polymeric coating preferably as defined above. Mixer-settlers are well known to those skilled in the art. This is typically a single device comprising a mixing chamber and a mixing chamber. settling, the mixing chamber comprising a stirring mobile advantageously allowing the mixing of the two liquid phases. In the settling chamber, the phase separation takes place 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. These can be Rashig rings, Pall 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 their combinations; 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 mixtures thereof. 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, step a) of the process can be repeated at least once, preferably repeated from 1 to 10 times, preferably from 1 to 4 times. When step a) is repeated, it can be carried out in several mixer-settlers in series. Step a) can be carried out continuously or discontinuously, preferably continuously. According to one embodiment, step a), of the purification process according to the invention, comprises the addition of deionized water to the solution of LiFSI in the aforementioned organic solvent S1, for example obtained during synthesis steps earlier, to allow the dissolution of said salt, and the extraction of said salt in water (aqueous phase). In the particular case of a batch process, and during the repetition of step a), 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. According to one embodiment, step a) is such that the mass of deionized water is greater than or equal to one third, preferably greater than or equal to half, of the mass of the initial solution of LiFSI in the organic solvent. S1 (in the case of a single extraction, or for the first extraction only if step a) is repeated at least once). The process according to the invention can comprise the addition of a volume of deionized water in step a) greater than or equal to one third, preferably greater than or equal to half of the volume of solvent S1 of the initial solution. In the event of multiple extractions (repetition of step a)), the extracted aqueous phases are combined together to form a single aqueous solution. Step a) advantageously makes it possible to obtain an aqueous phase and an organic phase, which are separated. Step b) 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 a)). Preferably, in the process according to the invention, the organic phase (s) separated from the aqueous solution extracted in step a) (comprising the organic solvent S1 and LiFSI) is not (are) not reintroduced in subsequent steps b) to d) of the process, in particular it (s) is (are) not subsequently combined with the organic phases extracted during step b) (comprising the organic solvent S2). At the end of step a), 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 a ') The method according to the invention can comprise a concentration step a ′) between step a) and step b), 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 a ′) can be carried out in at least one item of equipment chosen from an evaporator, an exchanger, and their mixtures. According to one embodiment, the concentration step a ′) 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 by a polymeric coating of preferably as defined above or by a coating of silicon carbide. Preferably, step a ′) 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, the purification process according to the invention comprises step a ′). After concentration a ′) of the aqueous solution obtained at the end of step a), a concentrated aqueous LiFSI solution is obtained. Step b) can be carried out on the aqueous solution obtained at the end of step a) or of concentration step a ′) or of a possible other intermediate step. Step b) can be carried out in equipment chosen from an extraction column, a mixer-settler, and mixtures thereof. According to one embodiment, step b) 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 made 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 b) 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 with a polymeric coating preferably as defined above. The extraction column can include: - at least one packing such as, for example, a loose packing and / or a structured packing. These can be Rashig rings, Pall 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 their combinations; 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. 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 mixtures thereof. 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. Step b) of the process according to the invention advantageously makes it possible to recover an organic phase, saturated with water, containing LiFSI (this is a solution of LiFSI in at least organic solvent S2, said solution being saturated with water). The solvent S2 for extracting 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 S2 is chosen from the group consisting of esters, nitriles, ethers, chlorinated solvents, aromatic solvents, and mixtures thereof. Preferably, the solvent S2 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 S2 is chosen from methyl-t-butyl ether, cyclopentylmethyl ether, ethyl acetate, propyl acetate, butyl acetate, and mixtures thereof, According to the invention, step b) of the process can be repeated at least once, preferably repeated from 1 to 10 times, preferably from 1 to 4 times. When step b) is repeated, it can be carried out in several mixer-settlers in series. In the event of multiple extractions (repetition of step b)), the extracted organic phases are combined together to form a single organic solution. Step b) can be carried out continuously or discontinuously, preferably continuously. According to one embodiment, step b) of the purification process according to the invention comprises the addition of at least organic solvent S2 to the aqueous solution of LiFSI, to allow the dissolution of said salt, and the extraction of said salt. salt in the organic phase. In the particular case of a batch process, and during the repetition of step b), the quantity by weight of organic solvent (s) S2 used can vary between 1/6 and 1 times the mass of the aqueous phase. Preferably, the weight ratio of organic solvent (s) S2 / water, during an extraction in 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 b) is between 5% and 35%, preferably between 10% and 25% by mass relative to the total mass of the solution. Step c) can include: - a step c-1) of pre-concentration of the solution obtained in the previous step; and - a step c-2) of concentration of the solution obtained in step c-1). Step c-1) Step c-1) advantageously makes it possible to obtain a solution of LiFSI in the at least organic solvent S2 comprising a LiFSI content by weight 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 c-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 c-1) can be carried out in equipment chosen from an evaporator or an exchanger. According to one embodiment, the pre-concentration step c-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 c-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 c-1) advantageously makes it possible to obtain a solution of LiFSI in the organic solvent S2 comprising a water content by mass of less than or equal to 20,000 ppm. Step c-2) Step c-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 c-2) is carried out in a short path thin film evaporator. Step c-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 with a polymeric coating of preferably as defined above or by a coating of silicon carbide. According to a preferred embodiment, the purification process according to the invention comprises a step c-2) of concentration of the lithium salt of bis (fluorosulfonyl) imide by evaporation of said at least organic solvent S2, in a thin film evaporator at short trip, 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 c-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 c-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 c-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 stated, the term “residence time” is understood to mean the time which elapses between the entry of the solution of bis (fluorosulfonyl) imide lithium salt (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 ° Cpt 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. These 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, wipers, guided wipers. In this case, the rotor can be constituted by a succession of pivot-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 c), 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, the method according to the invention further comprises a step d) of crystallization of the lithium salt of bis (fluorosulfonyl) imide obtained at the end of step c) mentioned above. Preferably, during step d), the LiFSI is cold crystallized, in particular at a temperature less than or equal to 25 ° C. Preferably, the crystallization step d) 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 d) is recovered by filtration. Preparation of LiFSI The initial solution of the lithium salt of bis (fluorosulfonyl) imide in at least one solvent S1 can come from any synthesis of the lithium salt of bis (fluorosulfonyl) imide, comprising in particular the following steps: i) synthesis of bis (chlorosulfonyl) imide; ii) fluorination of bis (chlorosulfonyl) imide to bis (fluorosulfonyl) imide; iii) preparation of an alkali or alkaline earth salt of bis (fluorosulfonyl) imide by neutralization of bis (fluorosulfonyl) imide; iv) optional cation exchange to obtain the lithium salt of bis (fluorosulfonyl) imide. At the end of these steps, the lithium salt of bis (fluorosulfonyl) imide is preferably obtained in solution in an organic solvent (corresponding in particular to the solvent S1), at a mass concentration of between 5% and 50% by mass. relative to the total mass of the solution. Such a method is for example described in document WO 2015/158979. Step iv) Step iv) corresponds to a cation exchange reaction, subsequent to step (iii), comprising the reaction between the alkaline earth salt of bis (fluorosulfonyl) imide and a lithium salt, to obtain the salt of lithium bis (fluorosulfonyl) imide. Step iv) 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 representing a monovalent cation alkali or alkaline earth metal, 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 an evaporator, or by any other device allowing the evaporation of solvent. Filtration can be done using a filter or wringer. Step iv) can be carried out in a reactor based on silicon carbide or based on a fluoropolymer, preferably as defined above; or in a steel reactor 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 silicon carbide. 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 iv) 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 agitator mobiles can be chosen by those 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. Purification process According to a preferred embodiment, the purification process according to the invention comprises the following steps: a) 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; a ') concentration of said aqueous solution of said salt; b) liquid-liquid extraction of the lithium salt of bis (fluorosulfonyl) imide from said aqueous solution with at least one organic solvent S2; c) concentration of the lithium salt of bis (fluorosulfonyl) imide by evaporation of said organic solvent S2, said step c) comprising: - a step c-1) of pre-concentration of the solution obtained in the previous step; and a step c-2) of concentration of the solution obtained in step c-1); d) optionally crystallization of the lithium salt of bis (fluorosulfonyl) imide; at least one of steps a), a '), b), or c-1), preferably all steps a), a'), b) and c-1), being 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, preferably as defined above, or by a coating of silicon carbide. The purification 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 or even zero 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 or even zero content of ions derived from the following metals: Cr, Mn, Fe, Ni, Cu, Al, Zn, Mo, Co, Pb, Na, Si, Mg, Ca. 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