Process for purifying lithium salts of bis (fluorosulfonyl) imides

Abstract

The present disclosure relates to a method for purifying a lithium salt of bis (fluorosulfonyl) imide (LiFSI), wherein purified LiFSI is extracted from crude LiFSI by supercritical fluid extraction. The invention also relates to the purified LiFSI obtained therefrom, and to the use of such LiFSI in electrolytes for batteries.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to the prior European patent application No. 22305584.9 filed on April 21, 2022, the entire contents of which are incorporated herein by reference for all purposes. Technical Field

[0003] The present invention relates to a method for purifying lithium salt of bis(fluorosulfonyl)imide (LiFSI) by supercritical fluid extraction, the present invention also relates to purified LiFSI obtained therefrom, and the use of such LiFSI in an electrolyte for a battery. Background Art

[0004] Bis(fluorosulfonyl)imide and its salts, in particular the lithium salt of bis(fluorosulfonyl)imide (LiFSI), are useful compounds in various technical fields, including in battery electrolytes. For these battery applications, the presence of impurities is an important problem.

[0005] In order to suppress the contamination of metal impurities, US2013 / 0331609 (in the name of Nippon Soda Co., Ltd.) proposes a method for producing a fluorosulfonyl imide ammonium salt, which comprises reacting a chlorosulfonyl imide compound with a NH4F(HF) p The fluorinating agent is reacted with a fluorinating agent, wherein p is 0 to 10. The obtained fluorosulfonylimide ammonium salt can then be subjected to a cation exchange reaction to produce another fluorosulfonylimide salt. The process is said to be industrially effective and does not provide metal impurities.

[0006] Similarly, patent documents JP 2016124735 (in the name of Nippon CatalyticChem Ltd.) and JP 2016145147 in the name of Nippon CatalyticChem Ltd.) disclose a method for producing a fluorosulfonyl imide compound, the method comprising reacting a chlorosulfonyl imide compound with NH4F(HF) p reaction, wherein p is 0 to 10. The fluorosulfonyl imide compound may react with an alkali metal compound to produce an alkali metal salt of the fluorosulfonyl imide.

[0007] EP 3381923 (in the name of CLS) discloses a process for producing lithium bis(fluorosulfonyl)imide, which comprises reacting bis(chlorosulfonyl)imide with a fluorinating agent in a solvent, followed by treatment with an alkaline agent to produce ammonium bis(fluorosulfonyl)imide, and then reacting the ammonium bis(fluorosulfonyl)imide with a lithium group to produce lithium bis(fluorosulfonyl)imide.

[0008] WO 2016 / 093399 (in the name of Chun Bo. Co., Ltd.) discloses a method for producing and purifying a lithium salt of a sulfonyl imide. The method comprises reacting chlorosulfonic acid and chlorosulfonyl isocyanate to prepare a chlorosulfonyl imide, then reacting the chlorosulfonyl imide with ammonium fluoride to prepare a fluorosulfonyl imide ammonium salt, then reacting the fluorosulfonyl imide ammonium salt with a lithium compound to obtain a sulfonyl imide lithium salt, and finally purifying the sulfonyl imide lithium salt with the aid of a specific solvent.

[0009] EP 2674395 (in the name of Nippon Soda Co., Ltd.) discloses a method for producing fluorosulfonyl imide ammonium salts, wherein contamination by metal impurities is suppressed to the greatest extent. The method comprises reacting a specific chlorosulfonyl imide ammonium salt with hydrogen fluoride. The fluorosulfonyl imide ammonium salt obtained therefrom is then reacted with an alkali metal compound to obtain a fluorosulfonyl imide alkali metal salt.

[0010] WO 2020 / 099527 (in the name of Solvay SA) discloses a method for producing an alkali metal salt of bis(fluorosulfonyl)imide, which is economically feasible on an industrial scale and provides a high purity product. The method comprises reacting bis(chlorosulfonyl)imide or a salt thereof with ammonium fluoride to form an ammonium salt of bis(fluorosulfonyl)imide, crystallizing and isolating the ammonium salt of bis(fluorosulfonyl)imide by adding at least one precipitation solvent, and reacting the crystallized ammonium salt of bis(fluorosulfonyl)imide with an alkali metal salt to obtain an alkali metal salt of bis(fluorosulfonyl)imide. Summary of the invention

[0011] Although several methods have been disclosed in the art aimed at purifying salts used in the manufacture of electrolytes for battery applications, the Applicant believes that there is still a need to develop methods for purifying LiFSI salts.

[0012] In view of the above technical problems, the present applicant unexpectedly developed a method for manufacturing LiFSI, which is simple to perform in terms of equipment and reaction conditions and is very friendly from an environmental point of view.

[0013] Furthermore, the process of the invention allows obtaining LiFSI in solid form with very high yield and high purity, so that it can then be used in battery electrolyte solutions.

[0014] An advantageous method for preparing LiFSI according to the invention is based on supercritical fluid extraction.

[0015] Therefore, in a first object, the present invention relates to a process for purifying the lithium salt of bis(fluorosulfonyl)imide (LiFSI) which is economically viable on an industrial scale and affords a high-purity product. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 Scheme representing the laboratory setup used in Example 1. DETAILED DESCRIPTION

[0017] In this application:

[0018] - the expression "comprising between ... and ..." is to be understood as including the limit values;

[0019] - any description (even those relating to a specific embodiment) is applicable to and interchangeable with other embodiments of the invention;

[0020] - When an element or component is said to be included in and / or selected from a list of listed elements or components, it should be understood that in the relevant embodiments explicitly considered herein, the element or component may also be any of these listed independent elements or components, or may also be selected from a group consisting of any two or more of the explicitly listed elements or components; any element or component listed in the list of elements or components may be omitted from this list; and

[0021] - Any recitation of numerical ranges by endpoints herein includes all numbers subsumed within the recited range as well as the endpoints of that range and equivalents thereof.

[0022] A first object of the present invention relates to a method for purifying a lithium salt of bis(fluorosulfonyl)imide (LiFSI), the method comprising the following steps:

[0023] a) providing a crude composition comprising LiFSI and at least one other compound [crude LiFSI];

[0024] b) contacting the crude LiFSI with at least one supercritical fluid; and

[0025] c) recovering a composition comprising LiFSI and at least one other compound in an amount lower than that of the crude LiFSI.

[0026] The term "crude composition" (hereinafter referred to as "crude LiFSI") means herein that the composition comprises LiFSI molecules mixed with at least one other compound, which is an undesirable compound that negatively affects the properties of LiFSI when used, for example, as an electrolyte in a battery application. Such at least one other compound may be referred to as an "impurity". The impurities include, for example, ions, one or more solvents, water and / or reaction byproducts. For example, crude LiFSI may comprise 80 to 99 wt.%, preferably 85-98 wt.%, more preferably 90-97 wt.% of LiFSI by weight, the remainder being impurities to be removed by the method of the present invention.

[0027] Advantageously, in the method of the present invention, the crude LiFSI may be in the form of a solid (such as a powder), or in the form of a slurry or a liquid (such as a solution).

[0028] The term "contact" herein means contacting the crude LiFSI with at least one supercritical fluid. For example, such contact is carried out in a container under specific pressure and temperature conditions for a period of time sufficient for the fluid to remove at least a portion of the impurities present in the crude LiFSI, preferably greater than 80.00%, greater than 90.00%, greater than 95.00%, greater than 99.00%, greater than 99.50% or even greater than 99.90%. More preferably, all impurities are removed and the salt exhibits a purity greater than 99.95%, or even greater than 99.99%.

[0029] The term "recovering" herein means removing or extracting purified LiFSI (preferably in solid form) from the vessel in which step b) is carried out.

[0030] The expression "supercritical fluid" means herein a gas (or a mixture of at least two gases) in its supercritical state. Depending on the gas used in step b), the pressure and temperature used in the container in which the contact between the solution and the supercritical fluid occurs are appropriately selected. More specifically, the gas used in step b) is maintained at or above its critical temperature and critical pressure in order to be in a supercritical state.

[0031] According to the method of the present invention, at least one supercritical fluid is used to extract the purified LiFSI salt from the crude LiFSI, which has several advantages. Supercritical fluids, such as sCO2, have many advantages because they are generally readily available, inexpensive, non-toxic, non-explosive, and are not organic solvents. In addition, the method of the present invention operates at moderate temperatures (less than 100°C), which ensures gentle handling of the LiFSI product.

[0032] Preferably, step a) is performed batchwise, semi-continuously or continuously.

[0033] Preferably, the at least one other compound present in the crude LiFSI is selected from the group consisting of, more preferably consisting of: water (H2O), fluoride ions (F - ), chloride ion (Cl - ), sulfate (SO4 2- ), aminosulfonate (NH2SO3 - ) and fluorosulfonate (FSO3 - ).

[0034] For example, the crude LiFSI to be purified may contain at least one of the following impurities: NH4Cl, NH4F, NH4HF2, NH4FSO3, NH4SO3NH2, NH4[N(SO3H)(SO2F)](OFSI), and / or NH4[N(SO3H)2](OSI).

[0035] It will be clear to a person skilled in the art that the parameters of the process according to the present invention can be appropriately selected and optimized based on, for example, the starting materials (particularly based on the amount of one or more other compounds in the crude LiFSI), and the scale on which the process is carried out, for example whether the process is carried out on an industrial scale or a laboratory scale.

[0036] Preferably, step b) is carried out at a pressure P of at least 80 bar.

[0037] Preferably, step b) is carried out at a temperature T between 30°C and 90°C.

[0038] A particular advantage of the process of the present invention is the short contact time in step b). Furthermore, advantageously, the contact time in step b) can be appropriately selected, for example, based on the starting material and the desired yield.

[0039] Preferably, the contact time in step b) varies between a few seconds (e.g. 5 seconds) and 24 hours. More preferably, the contact time in step d) varies between 1 minute and 12 hours, such as between 5 minutes and 10 hours or between 10 minutes and 5 hours.

[0040] In step b), the crude LiFSI is contacted with at least one supercritical fluid.

[0041] Preferably, step b) is performed in a container.

[0042] The term "container" herein means a container that is well suited for the method of the present invention, that is, adapted to withstand the pressures and temperatures used in the method of the present invention, as well as the possible corrosive properties of the reactants and products involved in the method. The container used herein may also be referred to as an extraction vessel or extraction apparatus. Moreover, the container used in this article may be an extraction column (also referred to as a "column") or an autoclave.

[0043] According to a preferred embodiment, step b) comprises contacting the crude LiFSI of step a) with at least one supercritical fluid in a container.

[0044] Preferably, in step b), the crude LiFSI is contacted with a fluid in a supercritical state.

[0045] Preferably, in step b), the crude LiFSI is contacted with two or more fluids in a supercritical state. The two or more fluids may be mixed, or may be contacted with the crude LiFSI in sequence. As an example, the crude LiFSI may be contacted with a mixture of at least two supercritical fluids.

[0046] Furthermore, according to the present invention, at least one other component (also referred to herein as a modifier) ​​may be mixed into the one or more supercritical fluids.

[0047] Advantageously, the at least one other component is chosen from polar solvents having a solubility in the supercritical fluid of less than 10 wt. %, based on the total weight of the supercritical fluid and the one or more other components.

[0048] More preferably, when the at least one other component is used, the amount thereof is in the range of 0.1 to 10 wt.%, such as 0.5 to 8 wt.% or 1 to 6 wt.%, based on the total weight of the supercritical fluid and the one or more other components.

[0049] Preferably, the at least one other component is selected from polar solvents, more preferably, from the group comprising alcohols, toluene, dimethyl sulfoxide (DMSO), acetonitrile, etc. According to a preferred embodiment, the polar solvent is an alcohol. Even more preferably, the alcohol is ethanol.

[0050] According to the invention, step b) can be repeated once or several times.

[0051] For example, the method according to the present invention comprises a first step b) and a second step b'), wherein the same or different supercritical fluids or a mixture of at least two supercritical fluids are used in each of said step b) and said step b').

[0052] In some embodiments, it is preferred that the vessel used to contact the crude LiFSI with at least one supercritical fluid in step b) is at a pressure P of at least 73 bar (7.3 MPa) during extraction.

[0053] In some embodiments, it is preferred that the vessel used to contact the crude LiFSI with the at least one supercritical fluid in step b) is at a temperature T between 30°C and 90°C during the extraction.

[0054] Preferably, the temperature T in the container may vary between 37°C and 75°C, for example between 38°C and 70°C or between 40°C and 65°C.

[0055] Preferably, the pressure P in the container may be at least 80 bar (8.0 MPa), at least 100 bar (10.0 MPa), at least 130 bar (13.0 MPa) or at least 150 bar (15.0 MPa). Very high pressures may be used in the process of the invention, for example, the pressure P in the container may be up to 200 bar (20.0 MPa) or 300 bar (30.0 MPa). The pressure in the container will typically be less than 500 bar (50.0 MPa), for example less than 450 bar (45.0 MPa), less than 400 bar (40.0 MPa), or even less than 350 bar (35.0 MPa).

[0056] According to an embodiment, step b) is performed by injecting crude LiFSI into a pressurized container.

[0057] According to an embodiment, the crude LiFSI is in a solid form, such as in a powder form.

[0058] According to another embodiment, the crude LiFSI is in a liquid form, such as a solution comprising LiFSI and an organic aprotic solvent.

[0059] Preferably, the organic aprotic solvent is selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, γ-valerolactone, dimethoxymethane, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxane, 4-methyl-1,3-dioxolane, methyl formate, methyl acetate, methyl propionate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, sulfolane, 3-methylsulfolane, dimethyl sulfoxide, N,N-dimethylformamide, N-methyloxazolidinone, acetonitrile, valeronitrile, benzonitrile, ethyl acetate, isopropyl acetate, n-butyl acetate, nitromethane and nitrobenzene. More preferably, the solvent is selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate, tetrahydrofuran, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl acetate, isopropyl acetate and n-butyl acetate. Even more preferably, the solvent is selected from dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl acetate, isopropyl acetate and n-butyl acetate. Still more preferably, the solvent is selected from ethyl methyl carbonate and n-butyl acetate.

[0060] The crude LiFSI can be injected into the container, for example, through a syringe or an inlet valve mounted on the container.

[0061] According to another embodiment, step b) comprises:

[0062] b1) introducing crude LiFSI into a container;

[0063] b2) pressurizing the container to a pressure of P;

[0064] b3) heating the container to a temperature T;

[0065] b4) introducing at least one supercritical fluid into the container.

[0066] Preferably, step b2) may be performed before step b3), or step b3) may be performed before step b2), or steps b2) and b3) may be performed concomitantly.

[0067] Preferably, the order of these steps may be as follows: b1), b3), b4) and b2).

[0068] The conditions of temperature, pressure and contact time detailed above for step b) apply to steps b2) and b3) as described above.

[0069] Preferably, step b2) is carried out at a pressure of at least 74 bar (7.4 MPa).

[0070] Preferably, step b3) is carried out at a temperature of at least 30°C.

[0071] The flow rate of the supercritical fluid introduced into the container in step b4) is not particularly limited. A person skilled in the art can determine the flow rate based on the apparatus used and the amount and purity of the crude LiFSi.

[0072] Step b4) can be carried out batchwise, continuously or semi-continuously.

[0073] Preferably, the supercritical fluid used in step b) comprises supercritical carbon dioxide (sCO2). sCO2 is a fluid state of carbon dioxide maintained at or above its critical temperature (31.0°C) and critical pressure (7.3773MPa).

[0074] Advantageously, the supercritical fluid used in step b) may essentially consist of sCO2, or it may comprise sCO2.

[0075] According to an embodiment, sCO2 is mixed with up to 10 wt.% ethanol, for example with 0.1 to 8 wt.% ethanol, the wt.% being based on the total weight of the supercritical fluid and the ethanol.

[0076] The weight ratio of the supercritical fluid to the crude LiFSI used in the method of the present invention may vary between 1 / 1 and 400 / 1. For example, the weight ratio of the supercritical fluid to the crude LiFSI is preferably between 5 / 1 and 350 / 1, such as between 20 / 1 and 300 / 1, between 30 / 1 and 280 / 1, or between 40 / 1 and 250 / 1.

[0077] The process of the present invention can be carried out in batch mode, in continuous or semi-continuous mode.

[0078] Preferably, the process is carried out in a continuous or semi-continuous manner.

[0079] Preferably, the method of the present invention comprises the step of continuously or semi-continuously withdrawing the purified LiFSI salt from the container.

[0080] Preferably, according to the present invention, the crude LiFSI is injected into the container in a continuous or semi-continuous manner. In other words, according to an embodiment, the crude LiFSI is continuously injected into the container, or alternatively, the crude LiFSI is semi-continuously injected into the container. For example, the crude LiFSI can be injected into the container for a certain time (e.g., between 30 seconds and 120 seconds, such as 60 seconds), and then the injection is stopped for another period of time, which can be equal to, shorter than, or longer than the injection time.

[0081] For example, according to the present invention, the supercritical fluid may be introduced into the vessel in a continuous or semi-continuous manner.

[0082] For example, the method of the present invention may include the step of continuously or semi-continuously withdrawing the LiFSI salt from the container.

[0083] The purified LiFSI recovered in step c) is preferably in solid form. The purified LiFSI is preferably recovered in solid form, regardless of whether the starting crude LiFSI is in solid form, in slurry or liquid form. Even more preferably, the purified LiFSI is in the form of a powder.

[0084] In step c), once step b) is completed or while step b) is ongoing, LiFSi in solid form may be recovered.

[0085] According to a specific embodiment of step c), the purified LiFSI flows into a separator together with a supercritical fluid. The pressure is released and the supercritical fluid becomes a gas. This gas is preferably recycled, as described in detail below.

[0086] The method of the present invention may further comprise additional steps, such as preferably at least one step comprising recycling the supercritical fluid.

[0087] For example, the supercritical fluid may be reinjected into the process of the invention as such or after an additional purification step or steps.

[0088] Recirculation of the supercritical fluid can be performed in several ways.

[0089] According to an embodiment, the supercritical fluid may be recirculated in a continuous manner during the method. Preferably, it is recirculated under pressure using a supercritical fluid tube.

[0090] According to another embodiment, the supercritical fluid may be recovered as a liquid phase by releasing the pressure in the container and then repressurizing it in its gaseous form, for example with the aid of a compressor, in order to recirculate it as a supercritical fluid that can be discharged in the container.

[0091] The method of the present invention can be carried out in an apparatus comprising:

[0092] - a container which withstands the pressures and temperatures used, for example a pressure P of at least 73 bar (7.3 MPa) and a temperature P above 30° C.;

[0093] -Solvent trap;

[0094] - Gas tanks and supercritical gas generators;

[0095] - at least one syringe / inlet valve mounted on the container; and

[0096] -Optionally, a separator.

[0097] The container may preferably be made of sapphire, SS316L, glass or graphite filled PTFE.

[0098] The apparatus may include a separator. Different separators may be used in the method of the present invention. In some embodiments, separation may be performed by conventional filtration (also referred to as "dead-end filtration") or cross filtration (also referred to as tangential filtration), as disclosed, for example, in US2007 / 0021570 (in the name of Solvay). Alternatively, a cyclone separator, such as that operating as a gas / solid separator, may be used. A cyclone separator is advantageous because it allows for the recovery of solids that may clog the filter medium. Several mixing devices exist based on this principle. Notably, reference may be made to patent US 7,410,620 (in the name of North Carolina State University).

[0099] Preferably, solid LiFSI is recovered via a frit filter at the end of the process.

[0100] For example, the glass frit filter may be made of stainless steel. Preferably, the glass frit filter has at least one of the following features:

[0101] - a pore size between 1 and 6 μm, preferably 2 to 4 μm;

[0102] - a diameter between 1 and 20 mm, preferably between 5 and 15 mm, more preferably about 10 mm; and / or

[0103] - a thickness of 0.1 to 5 mm, preferably between 0.7 and 3.5 mm, more preferably between 1.5 and 2.5 mm.

[0104] If the device comprises a filter or several filters, the filter or filters may notably be placed at the bottom or at the top of the container.

[0105] A second object of the present invention is the lithium salt of bis(fluorosulfonyl)imide (LiFSI) in solid form obtainable by the process according to the invention.

[0106] Advantageously, such a LiFSI salt is characterized in that it contains LiFSI and:

[0107] -Fluoride ion (F - ), preferably in an amount of less than 100 ppm as measured by ion chromatography (IC); and / or

[0108] -Chloride ion (Cl - ), preferably in an amount of less than 100 ppm as measured by IC; and / or

[0109] -Sulfate (SO4 2- ), preferably in an amount of less than 1,000 ppm as measured by IC; and / or

[0110] -sulfamate (NH2SO3 - ), preferably in an amount of less than 1,000 ppm as measured by IC; and / or

[0111] -Fluorosulfonate (FSO3 - ), preferably in an amount of less than 1,000 ppm as measured by IC; and / or

[0112] - Water, preferably in an amount of less than 50 ppm as measured by KF analysis (oven method).

[0113] Indeed, the object of the present invention is to provide a LiFSI salt with a very low amount of impurities. As detailed below in the experimental section, the inventors have been able to achieve a high level of LiFSI salt purity above 99.90% starting from a crude product with a purity of 98.00%. The LiFSI salt of the present invention preferably has a purity of greater than 99.50%, greater than 99.60%, greater than 99.70%, greater than 99.80%, greater than 99.90% and even greater than 99.95% as measured by Li-NMR.

[0114] The amount of impurities in the LiFSI product recovered at the end of the process according to the invention is preferably selected from:

[0115] -Fluoride ion (F- ), in an amount of up to 50 ppm as measured by ion chromatography (IC), e.g., less than 40 ppm, less than 30 ppm, less than 25 ppm; and / or

[0116] -Chloride ion (Cl - ), in an amount of up to 50 ppm as measured by IC, e.g., less than 40 ppm, less than 30 ppm, less than 20 ppm, less than 10 ppm or even less than 8 ppm; and / or

[0117] -Different from sulfate (SO4 2- ) in an amount up to 100 ppm as measured by IC, e.g., less than 50 ppm, less than 30 ppm, less than 10 ppm, less than 5 ppm; and / or

[0118] - Water in an amount of up to 50 ppm as measured by KF analysis (oven method).

[0119] Preferably, the salt different from sulfate (SO4 2- ) is selected from NH2SO3 - and / or FSO3 - .

[0120] Preferably, the salt different from sulfate (SO4 2- ) is the following amount of acidic substances:

[0121] - less than 50 ppm, e.g., less than 40 ppm, less than 30 ppm, less than 20 ppm, less than 10 ppm or even less than 5 ppm of sulfamate (NH2SO3 as measured by IC - ); and / or

[0122] - less than 50 ppm, such as less than 40 ppm, less than 30 ppm, less than 20 ppm, less than 10 ppm or even less than 5 ppm of fluorosulfonate (FSO3 - ).

[0123] The LiFSI salt of the present invention also preferably comprises at least one of the following chemical entities (as measured by ion chromatography):

[0124] - iron (Fe) in an amount below 1,000 ppm, preferably below 500 ppm, more preferably below 100 ppm;

[0125] - chromium (Cr) in an amount less than 1,000 ppm, preferably less than 500 ppm, more preferably less than 100 ppm;

[0126] - nickel (Ni) in an amount less than 1,000 ppm, preferably less than 500 ppm, more preferably less than 100 ppm;

[0127] - zinc (Zn) in an amount below 1,000 ppm, preferably below 100 ppm, more preferably below 10 ppm;

[0128] - copper (Cu) in an amount below 1,000 ppm, preferably below 100 ppm, more preferably below 10 ppm;

[0129] - bismuth (Bi) in an amount less than 1,000 ppm, preferably less than 100 ppm, more preferably less than 10 ppm;

[0130] -Sodium (Na + ), in an amount lower than 10,000 ppm, preferably lower than 5000 ppm, more preferably lower than 500 ppm or even lower than 100 ppm; and / or

[0131] -Potassium (K + ), the amount is less than 10,000 ppm, preferably less than 5000 ppm, more preferably less than 500 ppm or even less than 100 ppm.

[0132] The purified LiFSI salt of the present invention is characterized in that it contains less than 50 ppm of water as measured by KF analysis (oven method). Preferably, the purified LiFSI salt contains less than 40 ppm, less than 30 ppm, less than 20 ppm, less than 10 ppm or even less than 5 ppm of water.

[0133] A third object of the present invention relates to a solid comprising a lithium salt of bis(fluorosulfonyl)imide (LiFSI) and at least one other substance selected from:

[0134] - water, preferably in an amount of less than 50 ppm as measured by KF analysis via the oven method; and / or

[0135] -Fluoride ion (F - ), preferably in an amount less than 25 ppm; and / or

[0136] -Chloride ion (Cl - ), preferably in an amount less than 8 ppm; and / or

[0137] -Sulfate (SO4 2- ), preferably in an amount less than 20 ppm; and / or

[0138] -Different from sulfate (SO4 2- ) of acidic substances, the amount of which is preferably less than 1 ppm.

[0139] Preferably, the acidic substance is selected from NH2SO3 - and / or FSO3 - .

[0140] A fourth object of the present invention relates to the use of the lithium salt of bis(fluorosulfonyl)imide (LiFSI) according to the invention in solid form in a battery electrolyte solution.

[0141] A fifth object of the present invention is the use of supercritical fluid extraction for purifying a lithium salt of crude bis(fluorosulfonyl)imide (LiFSI) containing LiFSI and impurities.

[0142] Should the disclosure of any patents, patent applications, and publications incorporated by reference into this application conflict with the description of this application to the extent that a term may be unclear, this description shall take precedence.

[0143] The present invention will now be described in more detail with reference to the following examples, which are for illustrative purposes only and are not intended to limit the scope of the present disclosure.

[0144] Examples

[0145] Material:

[0146] Crude LiFSI, purchased from Provisco, has the following characteristics:

[0147] - Water < 10 ppm as determined by KF analysis (oven method) as described in more detail below - F - = 1420 ppm, as determined by IC*

[0148] -Cl - <5ppm as determined by IC*

[0149] -SO4 2- = 11,000 ppm, as determined by IC*

[0150] -NH2SO3 - = 1,190 ppm, as determined by IC*

[0151] -FSO3 - =7,210 ppm, as determined by IC*

[0152] -Purity = 98% as determined by Li-NMR *IC: Ion Chromatography (DIONEX ICS-3000)

[0153] The samples were prepared by a fully automatic oven sample processor (Metrohm) according to KF analysis (oven method); sample weight 0.1 g, carrier gas = N2, oven temperature = 160°C. Titration was performed using a mixture of methanol and NH4F (1:1 v / v). The polarization current for potentiometric determination of the reaction endpoint was 10 μA, and the titration endpoint voltage was 50 mV.

[0154] Example 1 - Preparation of high-purity LiFSI in powder form starting from crude LiFSI

[0155] use Figure 1 The equipment was set up and the experiments were performed using the following procedure.

[0156] 4.524 g of crude LiFSI was introduced into the container. The container was pressurized with sCO2 (temperature 45°C; pressure 200 bar; CO2 / LiFSI solution ratio varied between 30 and 400 during the process). The outlet valve was then opened to the desired level to set the flow rate of CO2 (165 g / min). After about 10 h of contact in the container, the inlet valve between the buffer tank and the container was closed to depressurize the container and the outlet line.

[0157] Obtain the following.

[0158] A mass loss of 0.12 g (2.65 wt.%) was observed after 10 h of extraction. A plateau was observed at CO2 / crude LiFSI mass ratio = 180, corresponding to 5 h of extraction.

[0159] The extracted powder easily flows out of the container under gravity. In contrast, crude LiFSI sticks to the container surface.

[0160] The water content in the sample of the LiFSI solid product was less than 50 ppm as measured according to KF analysis (oven method).

[0161] The main impurity was not detected by ion chromatography (DIONEX ICS-3000): -F - <20ppm

[0162] -Cl - <5ppm

[0163] -SO4 2- <15ppm

[0164] -NH2SO 3- n / d(not detected)

[0165] -FSO3 - n / d(not detected)

[0166] The purity was greater than 99.9% as determined by Li-NMR.

Claims

1. A method for purifying a lithium salt of bis(fluorosulfonyl)imide (LiFSI), the method comprising the following steps: a) providing a crude composition comprising LiFSI and at least one other compound [crude LiFSI]; b) contacting the crude LiFSI with at least one supercritical fluid; and c) recovering a composition comprising LiFSI and at least one other compound in an amount lower than that of the crude LiFSI.

2. The method according to claim 1, wherein: The at least one supercritical fluid in step b) is selected from: one fluid in a supercritical state, or a mixture of at least two fluids in a supercritical state.

3. The method according to any one of claims 1 and 2, wherein: - the crude LiFSI is in the form of a solid, a slurry or a solution; and / or - The purified LiFSI is in the form of a solid.

4. The method according to any one of claims 1 to 3, wherein: Step b) is carried out in a vessel at a pressure P of at least 73 bar (7.3 MPa) and / or at a temperature T between 30°C and 90°C.

5. A method according to any one of the preceding claims, wherein: The supercritical fluid comprises CO2, optionally mixed with at least one polar solvent having a solubility in the supercritical fluid of less than 10 wt.% based on the total weight of the supercritical fluid and the at least one polar solvent.

6. The method according to claim 5, wherein: - the pressure P in the container is comprised between 80 bar (8.0 MPa) and 500 bar (50.0 MPa); and / or - The temperature in the container is comprised between 35°C and 80°C.

7. The method according to any one of claims 1 to 6, wherein: The process is carried out continuously or semi-continuously.

8. A method according to any one of the preceding claims, wherein: The at least one other compound in the crude LiFSI is selected from the group consisting of, preferably consisting of: water (H2O), fluoride ions (F - ), sulfate (SO4 2- ), chloride ion (Cl - ), aminosulfonate (NH2SO3 - ) and fluorosulfonate (FSO3 - ).

9. The method according to any one of claims 1 to 8, further comprising at least one step of recycling the supercritical fluid.

10. The method according to any one of claims 1 to 9, which is carried out in an apparatus comprising: - a container which can withstand a pressure P of at least 80 bar and a temperature P greater than 10° C.; -Solvent trap; - Gas tanks and supercritical gas generators; - at least one syringe / inlet valve mounted on the container; and -Optionally, a separator.

11. A lithium salt of bis(fluorosulfonyl)imide (LiFSI) in solid form obtainable by a process according to any one of claims 1 to 10, characterized in that The salt contains LiFSI and: -Fluoride ion (F - ), preferably in an amount of less than 100 ppm as measured by ion chromatography (IC); and / or -Chloride ion (Cl - ), preferably in an amount of less than 100 ppm as measured by IC; and / or -Sulfate (SO4 2- ), preferably in an amount of less than 1,000 ppm as measured by IC; and / or -sulfamate (NH2SO3 - ), preferably in an amount of less than 1,000 ppm as measured by IC; and / or -Fluorosulfonate (FSO3 - ), preferably in an amount of less than 1,000 ppm as measured by IC; and / or - Water, preferably in an amount of less than 50 ppm as measured by KF analysis (oven method).

12. A solid comprising a lithium salt of bis(fluorosulfonyl)imide (LiFSI) and at least one other substance selected from: - water, preferably in an amount of less than 50 ppm as measured by KF analysis via the oven method; and / or -Fluoride ion (F - ), preferably in an amount less than 25 ppm; and / or -Chloride ion (Cl - ), preferably in an amount less than 8 ppm; and / or -Sulfate (SO4 2- ), preferably in an amount less than 20 ppm; and / or -Different from sulfate (SO4 2- ) of acidic substances, preferably less than 1 ppm.

13. Use of the lithium salt of bis(fluorosulfonyl)imide (LiFSI) according to claim 11 or the solid according to claim 12 in a battery electrolyte solution.

14. Use of supercritical fluid extraction for purifying a crude composition of lithium bis(fluorosulfonyl)imide (crude LiFSI) comprising LiFSI and at least one impurity.

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