Method for the preparation of lithium bis(fluorosulfonyl)imide

Abstract

The present application provides a method for preparing lithium bis(fluorosulfonyl)imide, comprising the following steps: (a) a synthesis step; (b) an evaporation step; (c) an extraction step; (d) an alkalization step; (e) a dehydration step; (f) a desolventization step; (g) a crystallization step; and (h) a drying step, and comprises a lithium bis(fluorosulfonyl)imide prepared by means of the method, an electrolyte comprising the lithium bis(fluorosulfonyl)imide, and a secondary battery comprising said electrolyte.

Description

Method for the preparation of lithium bis(fluorosulfonyl)imide Technical field This application relates to a method for the preparation of lithium bis(fluorosulfonyl)imide, as specified in any of claims 1 to 12. Background Due to the special molecular structure of lithium bis(fluorosulfonyl)imide (LiFSI), increased conductivity can be achieved by adding LiFSI to an electrolyte solution. Furthermore, LiFSI exhibits high thermal stability, a wide electrochemical range, and a low corrosion rate, which improves the cyclic performance and high-discharge-rate performance of power batteries, making it an excellent choice as an electrolyte lithium salt for lithium-ion batteries. Patent CN 111620315A relates to a method for preparing lithium bis(fluorosulfonyl)imide. Patent CN 112919435A relates to a method for preparing high-purity bis(fluorosulfonyl)imide and an alkali metal salt. Patent CN 102378755A relates to a method for producing a salt or complex comprising an imide and an organic base. In the prior art, the synthesis and purification of LiFSI present numerous problems in large-scale industrial production. The synthesis process is cumbersome, time-consuming, has a low product conversion rate, high consumption of raw materials and auxiliaries, and is difficult to recycle, making it uneconomical. The present invention aims to solve at least some of these problems and proposes a new continuous method for the production of LiFSI, such that the LiFSI meets battery-level standards for purity and water content, has a low production cost, generates less waste, and is suitable for industrial production. Summary The present application is made in view of the aforementioned problems, and its purpose is to provide a method for the preparation of lithium bis(fluorosulfonyl)imide in order to solve the problems of low product purity, high moisture, high production cost and large amount of waste in the preparation of LiFSI in the prior art. To achieve the above purpose, the present application provides a method for the preparation of lithium bis(fluorosulfonyl)imide as specified in any one of claims 1 to 12. A first aspect of this application provides a method for the preparation of lithium bis(fluorosulfonyl)imide as specified in any one of claims 1 to 12, which includes the following steps: (a) a synthesis step: subjecting sulfuryl fluoride, ammonia gas and triethylamine to reaction in a reactor in the presence of a solvent to obtain a stream 1 containing (SO2F-NH-SO2F) ·Et3N, a hydrogen fluoride salt of triethylamine and triethylamine; (b) an evaporation step: perform evaporation of stream 1 to obtain a stream 2 containing (SO2F-NH-SO2F)·Et3N and the hydrogen fluoride salt of triethylamine, perform post-treatment of the evaporated solvent and optionally the triethylamine, and then return to step (a); (c) an extraction step: washing the stream 2 obtained in step (b) with water in an extraction tower or static mixer to obtain an oil phase 3 containing (SO2F-NH-SO2F) ·Et3N and an aqueous phase water containing the hydrogen fluoride salt of triethylamine, and separating the oil phase 3; (d) an alkalization step: introducing the oily phase 3 obtained in step (c) into an evaporator to mix it with an aqueous solution of lithium hydroxide and obtain a mixture stream β1-1, and then performing a reduced pressure evaporation on the β1-1 stream to obtain a β1-2 stream containing lithium bis(fluorosulfonyl)imide; (e) Dehydration step: adding the β1-2 stream containing lithium bis(fluorosulfonyl)imide and an ester solvent to an evaporator for evaporation, thereby obtaining a β2 stream containing lithium bis(fluorosulfonyl)imide; 1. (f) Desolventization step: adding the β2 stream obtained in step (e) and an ester solvent to an evaporator for evaporation, yielding crude lithium bis(fluorosulfonyl)imide β3; and 2. (g) Crystallization step: pump the crude lithium bis(fluorosulfonyl)imide β3 obtained in step (f) into a crystallization reactor and add dichloromethane to precipitate the lithium bis(fluorosulfonyl)imide crystals. The preparation method described in this application yields high-purity, low-moisture LiFSI. Furthermore, the method is suitable for continuous production with low production costs and minimal waste, and has significant industrial application potential. In any embodiment, optionally, the method further includes the following step: (h) a drying step: purging the lithium bis(fluorosulfonyl)imide crystals precipitated in step (g) with inert gas in a dryer, to obtain powdered lithium bis(fluorosulfonyl)imide crystals with a water content of less than 50 ppm. In any embodiment, optionally, in step (a), the molar ratio of sulfuryl fluoride, ammonia gas and triethylamine is (1, 5-3, 5) :1: (1-6) , optionally (2-3) :1: (1-5) . In any embodiment, optionally, in step (a), the solvent is selected from acetonitrile, propionitrile, isopropionitrile, diethyl ether, propyl ether, isopropyl ether, tetrahydrofuran, acetone, butanone, methyl isobutyl ketone, methylpyrrolidone or a mixture of two or more of these, preferably acetonitrile. In any embodiment, optionally, in step (a), the reaction temperature is not greater than 25 °C, optionally between 3 °C and 20 °C; and / or the reaction pressure is not greater than 0.4 MPa, optionally not greater than 0.25 MPa. In any embodiment, optionally, between step (a) and step (b), the method further includes a filtering step of stream 1 to remove a solid sulfonamide byproduct (NH2-SO2-NH2), for example, using a tetrafluorocarbon filter bag with a pore diameter of 5 µm to 20 µm. In any embodiment, optionally, in step (b), stream 1 is evaporated by means of a falling film evaporator. In any embodiment, optionally, in step (c), the aqueous phase containing the triethylamine hydrogen fluoride salt is delivered for recycling treatment, and the triethylamine obtained after subjecting the aqueous phase to an alkalization and purification treatment is recyclable. In any embodiment, optionally, in step (d), the oily phase 3 is mixed and stirred with the aqueous lithium hydroxide solution to react for 0.5-3 hours, or optionally 1-2 hours. In any embodiment, optionally, in step (d), the condensate obtained by evaporation of stream β1-1 is subjected to separation by decantation. The liquid in the upper layer is an aqueous triethylamine solution, which is delivered for recycling treatment; and the liquid in the lower layer is condensate, which is recycled to prepare the aqueous lithium hydroxide solution required for the alkalization step. In any embodiment, optionally, in step (d), the volumetric ratio of stream 3 with respect to the aqueous lithium hydroxide solution is optionally (0.8-5) :1, or (1-1.2) :1. In any embodiment, optionally, in steps (e) and (f), an aqueous solution of the ester solvent obtained by condensation is supplied for recycling treatment. In any embodiment, optionally, in step (e), the resulting β2 stream contains between 0.1% and 2% by volume of water and between 20% and 40% by volume of the ester solvent. In any embodiment, optionally, the crude lithium bis(fluorosulfonyl)imide β3 obtained in step (f) has a water content of between 2000 ppm and 4000 ppm, preferably 3000 ppm. In any embodiment, optionally, steps (e) and (f) are carried out in the same evaporator, or in different evaporators. In any embodiment, optionally, during steps (d) and (e), the following side reaction takes place: (SO2F-N-SO2F) Li++4LiOHNH2SO3Li+Li2SO4+2LiF+H2O, and a lithium compound by-product is removed by centrifugation and filtration prior to the desolventization stage (f), optionally by a scraping centrifuge or a disc centrifuge. In any embodiment, optionally, in steps (e) and (f), the ester solvent is independently selected from an organic solvent with a boiling point above 70 °C, preferably above 80 °C, and preferably between 100 °C and 130 °C, and insoluble in water. Optionally, the ester solvent contains a carbonate solvent, including ethylene carbonate, ethylmethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylenepropyl carbonate, butylene carbonate, fluoroethylene carbonate, or a mixture of two or more of these. Optionally, the ester solvent may contain a carboxylate solvent such as propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, or a mixture of two or more of these. Additionally, optionally, the ester solvent can be ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate. In any embodiment, optionally, the evaporation temperature in the alkalization stage (d) is controlled between 30 °C and 40 °C, or between 30 °C and 35 °C; and / or the evaporation temperature in the dehydration stage (e) is controlled between 40 °C and 55 °C, or between 45 °C and 50 °C; and / or the evaporation temperature in the desolventization stage (f) is controlled between 60 °C and 80 °C, or between 65 °C and 75 °C. In any embodiment, optionally, in stages (d), (e), and (f), the aqueous lithium hydroxide solution is added so that the pH of the mixture in the evaporator is maintained in the range of 7 to 9, preferably 8 to 9. In any embodiment, optionally, in steps (d), (e) and (f), the concentration of the aqueous lithium hydroxide solution is from 1 mol / L to 15 mol / L, or from 2 mol / L to 10 mol / L. In any embodiment, between steps (g) and (h), the method includes pumping a stream containing the ester solvent, dichloromethane, and lithium bis(fluorosulfonyl)imide crystals obtained in crystallization step (g) to a two-in-one device with filtration and washing functions, where a crystallization liquid containing diethyl carbonate and dichloromethane is supplied for recovery treatment, and the remaining lithium bis(fluorosulfonyl)imide crystals fall by gravity into a drying vessel located in the lower layer of the two-in-one device. In any embodiment, optionally, in step (h), a liquid phase obtained after drying and condensation comprises dichloromethane and water, and the liquid phase is recycled and delivered to the crystallization step (g). In any embodiment, optionally, the water content of the dichloromethane used in the crystallization step (g) is 200 ppm. In any embodiment, optionally, the method further comprises (i) a canning step after the drying step (h), wherein, after dissolution with a solvent and optionally acid removal, water removal and filtration, the powdered lithium bis(fluorosulfonyl)imide crystals obtained in step (h) meet the following criteria: HF 50 µg / g, and water content 20 µg / g. In any embodiment, optionally, the solvent used in the dissolution of step (i) is the same as the ester solvents used in steps (e) and (f). In an unclaimed aspect, the present application further provides lithium bis(fluorosulfonyl)imide prepared by the method described in the first aspect of the present application. In an unclaimed aspect, this application further provides an electrolyte solution comprising lithium bis(fluorosulfonyl)imide prepared according to the method described in the first aspect of this application. In an unclaimed aspect, this application further provides a secondary battery comprising the electrolyte solution described in the third aspect of this application. The electrolyte solution or secondary battery of the non-claimed aspect of this application includes lithium bis(fluorosulfonyl)imide prepared according to the first aspect of this application and, therefore, has at least the same advantages as the method described in the first aspect of this application. Brief description of the drawings Figure 1 shows a schematic flow diagram of a one-stage process (synthesis-evaporation-extraction), in which an extraction tower is used. Figure 2 shows a schematic flow diagram of a one-stage process (synthesis-evaporation-extraction), in which a static mixer is used. Figure 3 shows a schematic flow diagram of a β-stage process (alkalinization-dehydration-desolventization). Figure 4 is a schematic flow diagram of a crystallization and drying process. Figure 5 is a schematic flow diagram of a canning process. Description of the achievements The embodiments disclosing a method for preparing lithium bis(fluorosulfonyl)imide of this application are described in detail below, with reference to the drawings as appropriate. However, in some cases, unnecessarily detailed descriptions may be omitted. For example, the detailed description of a known subject and the repeated description of a virtually identical structure are omitted. This is done to avoid unnecessary redundancy and to facilitate understanding by those skilled in the art. Furthermore, the drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter described in the claims. The "range" disclosed here is defined by a lower and an upper limit. A given range is defined by selecting a lower and an upper limit, and these limits define the boundaries of that range. The range defined in this way may or may not include extreme values ​​and can be combined arbitrarily; that is, any lower limit can be combined with any upper limit to form a range. For example, if the ranges 60–120 and 80–110 are specified for a given parameter, it is understood that the ranges 60–110 and 80–120 are also provided for. Furthermore, if the minimum values ​​of the range are 1 and 2, and the maximum values ​​are 3, 4, and 5, the following ranges can be provided: 1–3, 1–4, 1–5, 2–3, 2–4, and 2–5.In this application, unless otherwise specified, the numeric range "ab" is a shorthand representation of any combination of real numbers between a and b, where both a and b are real numbers. For example, the numeric range "0-5" means that all real numbers between "0-5" have been listed, and "0-5" is simply a shorthand representation of a combination of these numeric values. Furthermore, when a parameter is expressed as an integer 2, it is equivalent to indicating that the parameter is, for example, an integer 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or similar. Unless otherwise specified, all embodiments and optional embodiments of this application may be combined to form a new technical solution. Unless otherwise specified, all technical and optional features of this application may be combined to form a new technical solution. Unless otherwise specified, all steps in this application may be executed sequentially or randomly, but preferably sequentially. For example, a method includes steps (a) and (b), meaning that the method may include steps (a) and (b) executed sequentially, or steps (b) and (a) executed sequentially. For example, the aforementioned method may also include step (c), meaning that step (c) may be added to the method in any order; for example, the method may include steps (a), (b), and (c), steps (a), (c), and (b), steps (c), (a), and (b), or similar. Unless otherwise specified, the words "comprising" and "containing" mentioned in this application are either open or closed. For example, the words "comprising" and "containing" may mean that other components not listed may also be included or contained, or that only the listed components may be included or contained. The terms "more than" and "less than" used in this application include the number at the end; for example, "more than one" means one or more, and "more than one of A and B" means "A", "B" or "A and B". In this application, unless otherwise specified, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, a condition "A or B" is met if any of the following situations occur: A is true (or present) and B is false (or not present); A is false (or not present) and B is true (or present); or both A and B are true (or present). Unless otherwise specified, both the content and the percentage in the context of the present invention are based on mass. The process route of the present invention is: synthesis evaporation extraction alkalization dehydration desolventization crystallization drying packaging (packaging is an optional process). Each step will be described in more detail below. [Synthesis stage] In the synthesis step (a) of the present invention, sulfuryl fluoride, ammonia gas, and triethylamine react in a reactor in the presence of a solvent to obtain a stream 1 containing (SOF-NH-SOF)·EtN, a hydrogen fluoride salt of triethylamine and triethylamine. In some embodiments, in step (a), the molar ratio of sulfuryl fluoride, ammonia gas, and triethylamine is (1.5-3.5) : 1: (1-6), and optionally (2-3) : 1: (1-5). In the present application, an excess of ammonia gas can cause a side reaction that clouds the reaction mixture and hinders its subsequent filtration. Furthermore, to clarify the color of the materials, prolonged contact between sulfuryl fluoride and triethylamine must be avoided. Therefore, preferably, in step (a), acetonitrile and triethylamine are first added to the reaction device, then a portion of ammonia gas is introduced (preferably 2% to 15% of the total amount of ammonia gas, preferably 3% to 10%), and finally, ammonia gas and sulfuryl fluoride are introduced simultaneously for the reaction. In some embodiments, in step (a), the solvent is selected from acetonitrile, propionitrile, isopropionitrile, diethyl ether, propyl ether, isopropyl ether, tetrahydrofuran, acetone, butanone, methyl isobutyl ketone, methylpyrrolidone or a mixture of two or more of these, preferably acetonitrile. In general, the reaction in (a) can be carried out in any suitable manner. For example, the reaction can be carried out in a batch reactor or in at least a semi-batch reactor or at least a continuous reactor. In some embodiments, in step (a), the reaction temperature does not exceed 25 °C, and optionally is between 3 °C and 20 °C; and / or the reaction pressure does not exceed 0.4 MPa, and optionally does not exceed 0.25 MPa. Excessive temperature and pressure will cause more side reactions. Furthermore, if the temperature is too high, the solvent will readily vaporize, generating excessive pressure in the reactor and affecting production safety. The inventors of the present invention have discovered that when the molar ratio of sulfuryl fluoride to ammonia gas in step (a) is (1.5-4) :1, preferably (2-3) : 1, particularly good yields and color qualities are obtained. In some embodiments, in step (a), at the end of the reaction, stream 1 is obtained containing between 30% and 50% by weight of (SO2F-NH-SO2F)·Et3N, between 15% and 30% by weight of the triethylamine hydrogen fluoride salt, and between 0% and 10% by weight of triethylamine. In some embodiments, stream 1 can be filtered more than once, preferably 2 to 5 times, optionally 2 times, and the pore diameter of the filter bag used for downstream filtration is smaller than that of the upstream filter bag. [Evaporation stage] In the evaporation step (b) of the present invention, stream 1 is evaporated to obtain a stream 2 containing (SO2F-NH-SO2F)·Et3N and the hydrogen fluoride salt of triethylamine. The evaporated solvent and, optionally, the triethylamine are post-treated, and then the method returns to step (a). In some embodiments, a stream 1 filtration stage is included between stages (a) and (b) to remove a solid sulfonamide byproduct (NH2-SO2-NH2), for example, using a tetrafluorocarbon filter bag with a pore diameter of 5 µm to 20 µm, or 6 µm to 15 µm. In some embodiments, in step (b), stream 1 is evaporated by a falling film evaporator. The heat exchange medium used in the falling film evaporator can be hot water, and the temperature of the hot water reservoir ranges from 50 °C to 100 °C, preferably from 60 °C to 80 °C. In some embodiments, in step (b), evaporation can be carried out using one, two or more evaporators connected in series, preferably a falling film evaporator. In some embodiments, in step (b), the gas stream evaporated by the evaporator is condensed by a condenser, preferably one comprising a pre-condenser and a post-condenser. The pre-condenser condenses with water at 0°C, and the post-condenser with water at -15°C. Optionally, the internal pressure of the condenser is from -0.01 MPa to -0.1 MPa. Those skilled in the art will understand that the water used in the condenser contains between 30% and 50% by weight of antifreeze, which may be ethylene glycol or glycerin. In some embodiments, the evaporation of step (b) may be carried out in one or more evaporators. In some embodiments, in step (b), stream 2 includes between 60% and 80% by weight of (SO2F-NH-SO2F)·Et3N and between 20% and 40% by weight of the triethylamine hydrogen fluoride salt. [Extraction stage] In the extraction step (c) of the present invention, the stream 2 obtained in step (b) is washed with water in an extraction tower or a static mixer to obtain an oily phase 3 containing (SO2F-NH-SO2F) ·Et3N and an aqueous phase water containing the triethylamine hydrogen fluoride salt, and the oily phase 3 is separated. In some embodiments, in step (c), the mass ratio between the water and stream 2 entering the extraction tower or static mixer is 1:(1-2), optionally 1:(1, 1-1, 5). In some embodiments, in step (c), the oil phase 3 also includes between 5% and 15% by weight of water. In some embodiments, in step (c), the water includes between 75% and 80% by weight of water and between 20% and 25% by weight of the triethylamine hydrogen fluoride salt. In some embodiments, in step (c), the aqueous phase containing the triethylamine hydrogen fluoride salt is delivered for recycling treatment, and the triethylamine obtained after the aqueous phase undergoes alkalization and purification treatment is recyclable. In addition to the triethylamine hydrogen fluoride salt, the aqueous phase also contains an impurity ion (such as F₂, SO₂, FSO₄, or Cl₂). After alkalization and purification treatment in a recycling section, the triethylamine is recyclable, and additional economic benefits can be generated by selling the KF. In the present application, the extraction tower may be an extraction tower conventionally used in the art, such as a packed extraction tower, a perforated tray extraction tower, a rotating disc extraction tower, a vibrating perforated tray tower, a multi-stage centrifugal extraction tower, preferably a rotating disc extraction tower. In some embodiments, the extraction stage is carried out in an extraction tower (agitation frequency of 15 ± 1 Hz; weight ratio of deionized water to 2 is 1:(1-1.6), preferably 1:(1-1.4)). A light (low-density) phase enters the bottom of the extraction tower and is discharged from the top; a heavy phase enters the top and is discharged from the bottom. The intermediate phase is spirally agitated (which improves the agitation effect and optimizes washing and separation conditions). The inventors found that using the extraction tower allows for better separation of impurity ions (such as F-). After treatment with the static mixer, if the F- ion content in 3 is 1000 ppm, a higher consumption of LiOH is required in the subsequent alkalization process. However, after treatment in the extraction tower, the F ion content in 3 is 100 ppm. In the present application, the measurement of the F ion content and water content in a stream is a well-known method in the art, for example, the measurement of F ion content by alizarin complex colorimetry and the measurement of water content by the Karl Fischer method. In some embodiments, in step (c), the stream 2 obtained in step (b) is washed with water in the extraction tower. In this application, the static mixer may be a conventional static mixer, such as a pipe static mixer. If a static mixer is used, the resulting mixture must be pumped to a stratification tank for separation by sedimentation, which typically takes 1 to 10 hours, preferably 2 to 6 hours. In this application, unless otherwise specified, the water used in the text is always deionized water, which allows minimizing the type and amount of impurities present in the final product. [Alkalinization stage] In the alkalization step (d) of the present invention, the oily phase 3 obtained in step (c) is introduced into an evaporator to mix it with an aqueous solution of lithium hydroxide to obtain a mixture stream β1-1. Subsequently, this β1-1 stream is evaporated at reduced pressure to obtain a β1-2 stream containing lithium bis(fluorosulfonyl)imide. In some embodiments, the alkalinization of step (d) is carried out according to the following reaction formula: (SO2F-NH-SO2F) ·Et3N+LiOH (SO2F-N-SO2F) -Li+ (LiFSI) +Et3N+H2O. The reaction principle involves a strong base replacing a weak base, and the alkalinity of LiOH is greater than that of triethylamine in (SO2F-NH-SO2F)·Et3N, so the triethylamine is replaced and removed by falling-film evaporation. Simultaneously, the LiOH reacts with (SO2F-NH-SO2F)·Et3N to generate lithium bis(fluorosulfonyl)imide (abbreviated as LiFSI). In some embodiments, in step (d), before evaporating the β1-1 stream, the oil phase 3 is mixed and stirred with the aqueous lithium hydroxide solution to react for 0.5-3 hours, or even 1-2 hours. In some embodiments, in step (d), the concentration of the aqueous lithium hydroxide solution is from 1 mol / L to 15 mol / L, optionally from 2 mol / L to 10 mol / L. In some embodiments, in step (d), the condensate obtained by evaporation of stream β1-1 is subjected to separation by decantation. The liquid in the upper layer is an aqueous triethylamine solution, which is recycled; and the liquid in the lower layer is condensate, which is recycled to prepare the aqueous lithium hydroxide solution required for the alkalization step. In some embodiments, in step (d), the volumetric ratio between the oily phase 3 and the aqueous lithium hydroxide solution is (0.8-5) :1, and optionally (1-1.2) :1. In some embodiments, in step (d), the aqueous lithium hydroxide solution is added so that the pH of the mixture in the evaporator is maintained in a range of 7 to 9, preferably 8 to 9. In some embodiments, in step (d), evaporation is carried out by means of a falling film evaporator, and the evaporation temperature is controlled between 30 °C and 40 °C, or between 30 °C and 35 °C. In some embodiments, the evaporation of step (d) can be carried out in one or more evaporators. In some embodiments, in step (d), the β1-2 stream includes between 70% and 90% by weight of lithium bis(fluorosulfonyl)imide and between 5% and 25% by weight of water, and the remainder is LiF, LiSO₄ and lithium sulfamate, etc., based on 100% by weight of the β1-2 stream. [Dehydration stage] In the dehydration step (e) of the present invention, the β1-2 stream containing lithium bis(fluorosulfonyl)imide and an ester solvent is added to an evaporator for evaporation, thereby obtaining a β2 stream containing lithium bis(fluorosulfonyl)imide. In some embodiments, in step (e), an aqueous solution of the ester solvent obtained by condensation is supplied for recycling treatment. In some embodiments, in step (e), the β1-2 stream is mixed with the ester solvent in a volumetric ratio of 1:(0.4-0.8), preferably 1:(0.5-0.7). In some embodiments, in step (e), the resulting β2 stream also contains between 0.1% and 2% by volume of water and between 20% and 40% by volume of the ester solvent. In some embodiments, in step (e), the evaporator is a falling film evaporator and the evaporation temperature is controlled between 40 °C and 55 °C, optionally between 45 °C and 50 °C. In some embodiments, the evaporation of step (e) can be carried out in one or more evaporators. Since lithium salts exhibit strong hygroscopicity, it is difficult to reduce the humidity to the required level solely through evaporation. For example, the adsorption of lithium salts to water can be attenuated by adding a large quantity of water-insoluble organic ester solvents (such as carbonates and carboxylates, preferably DEC and EMC) with a boiling point preferably higher than that of water (e.g., between 100 and 130 °C). In this way, evaporation and dehydration, along with the addition of the organic ester solvents, can reduce the moisture content to the required level, for example, below 3000 ppm. A mixture of organic ester solvents and water obtained by condensation is recyclable after purification treatment in the recycling section. In some embodiments, optionally, during steps (d) and (e), the following side reaction takes place: (SO2F-N-SO2F) -Li++4LiOHNH2SO3Li+Li2SO4+2LiF+H2O. Before the desolventization stage (f), a lithium compound byproduct (such as NH2SO3Li, Li2SO4 and 2LiF) is optionally removed by centrifugation and filtration, for example, using a scraping centrifuge or a disc centrifuge. In some embodiments, an ester solvent is mixed with the β1-2 stream, and the pH of the mixture is maintained between 7 and 9 while evaporation and dehydration are carried out. Optionally, centrifugation and filtration are performed to obtain a β2 stream containing between 60% and 80% by weight of lithium bis(fluorosulfonyl)imide, between 20% and 40% by weight of the ester solvent, and between 0.2% and 1.5% by weight of water. In some embodiments, in step (e), the pH of the mixture is maintained between 7 and 9, preferably between 8 and 9, by adding the aqueous lithium hydroxide solution during evaporation. In some embodiments, in step (e), the concentration of the aqueous lithium hydroxide solution is from 1 mol / L to 15 mol / L, optionally from 2 mol / L to 10 mol / L. [Desolventization stage] In the desolventization step (f) of the present invention, the β2 stream obtained in step (e) and an ester solvent are added to an evaporator for evaporation to obtain crude lithium bis(fluorosulfonyl)imide β3. In some embodiments, after the dehydration step (e) and before the desolventization step (f), a lithium compound by-product (such as NHSOLi, LiSO4, and LiF) is optionally removed by centrifugation and filtration, for example, with a scraping centrifuge or a disc centrifuge, to obtain a β-1 stream. In some embodiments, in step (f), the β stream or the β-1 stream is mixed with an ester solvent in a volumetric ratio of 1:(0, 1-0, 4), preferably 1:(0, 2-0, 3). In some embodiments, in stage (f), the evaporator is a scraping evaporator and the evaporation temperature is controlled between 60 and 80 °C, optionally between 65 and 75 °C. In some embodiments, the evaporation in step (f) may be carried out in one or more evaporators. In some embodiments, the lithium bis(fluorosulfonyl)imide β3 obtained in step (f) has a water content of 2000 ppm to 4000 ppm, preferably 3000 ppm. By continuing to mix the β2 stream or the β2-1 stream obtained by filtration with the ester solvent in step (e), and then continuing the evaporation, further water can be removed from the β2 stream or the β2-1 stream. In some embodiments, in step (f), the lithium bis(fluorosulfonyl)imide β3 contains between 80% and 90% by weight of lithium bis(fluorosulfonyl)imide and between 10% and 20% by weight of the ester solvent. In some embodiments, in step (f), an aqueous solution of the ester solvent obtained by condensation is supplied for recycling treatment. In this application, during the alkalization, dehydration and desolventization stages, it is necessary to maintain the pH of the mixture in a range of 7 to 9, preferably 8 to 9. Preferably, by adding lithium hydroxide as a solvent, a slightly alkaline system is maintained and product decomposition is suppressed. In some embodiments, the pH of the evaporated mixture is maintained in a range of 7 to 9, preferably 8 to 9, by the addition of an aqueous solution of lithium hydroxide. In some embodiments, in step (f), the concentration of the aqueous lithium hydroxide solution is from 1 mol / L to 15 mol / L, optionally from 2 mol / L to 10 mol / L. Since lithium salt dissolves in an ester solvent such as diethyl carbonate, it is necessary to evaporate and remove a certain amount of ester solvent; otherwise, crystallization cannot be carried out or the crystallization rate may be very low in the later stage. In some embodiments, optionally, steps (e) and (f) are carried out in the same evaporator, or in different evaporators. [Crystallization step] In the crystallization step (g) of the present invention, the crude lithium bis(fluorosulfonyl)imide β3 obtained in step (f) is pumped into a crystallization reactor and dichloromethane is added to precipitate the lithium bis(fluorosulfonyl)imide crystals. In some embodiments, in the crystallization step (g), the water content of the dichloromethane used is 200 ppm. If the water content of dichloromethane is too high, it will affect the production of lithium bis(fluorosulfonyl)imide crystal nuclei, resulting in crystals that are too small and thin, and making them more difficult to dry later. In some embodiments, optionally, between steps (g) and (h), the method includes pumping a stream containing the ester solvent, dichloromethane, and the lithium bis(fluorosulfonyl)imide crystals obtained in crystallization step (g) to a two-in-one device with filtration and washing functions. In this device, a crystallization liquid containing diethyl carbonate and dichloromethane is supplied for recovery treatment, and the remaining lithium bis(fluorosulfonyl)imide crystals fall by gravity into a drying vessel located in the lower layer of the two-in-one device. [Drying stage] In the drying step (h) of the present invention, the lithium bis(fluorosulfonyl)imide crystals precipitated in step (g) are purged with an inert gas in a dryer to obtain lithium bis(fluorosulfonyl)imide powder crystals with a water content of less than 50 ppm. In some embodiments, in step (h), the liquid phase obtained after drying and condensation includes dichloromethane and water, and this liquid phase is recycled and transferred to the crystallization step (g). [Canning stage] The method of the present invention further includes (i) a canning step after the drying step (h), where, after dissolution with a solvent and, optionally, acid removal, water removal and filtration, the lithium bis(fluorosulfonyl)imide crystals in powder form obtained in the drying step meet the following criteria: HF 50 µg / g and water content 20 µg / g. If the HF content in the dissolved solution is found to exceed the limit (e.g., HF > 50 µg / g), lithium hydroxide is used for acid removal. If the water content in the dissolved solution is found to exceed the limit (e.g., water content > 20 µg / g), a molecular sieve is used for water removal. In some embodiments, the solvent used in the dissolution of step (i) is the same as the ester solvents used in steps (e) and (f). A second unclaimed aspect of this application further provides lithium bis(fluorosulfonyl)imide prepared by the method described in the first aspect of this application. A third, unclaimed aspect of this application further provides an electrolytic solution comprising lithium bis(fluorosulfonyl)imide prepared by the method described in the first aspect of this application. A fourth unclaimed aspect of this application further provides a secondary battery that includes the electrolyte solution described in the third aspect of this application. The electrolyte solution or secondary battery of this application includes lithium bis(fluorosulfonyl)imide prepared according to the first aspect thereof, and therefore has at least the same advantages as the method described in that first aspect. The raw materials and auxiliaries used in the LiFSI synthesis process of the present invention are relatively common chemicals. The production cost is low, high temperatures and pressures are not required in the reaction process, and the exothermic heat of the initial synthesis reaction is cooled by a refrigerator to ensure a low-temperature reaction with a high safety factor. As a result, LiFSI has a low production cost, generates less waste, has high purity, and the raw materials can be fully recycled for reuse. Furthermore, the byproduct can be purified to generate additional economic benefits, making it suitable for industrial production. By recovering and recycling raw materials, the consumption of raw materials and auxiliaries is reduced, the utilization rate of reaction raw materials is improved, the costs of disposal and treatment of compounds are reduced, production costs are effectively reduced, and economic benefits are improved. Examples The following examples describe the application. These examples are illustrative and used solely to explain the application; they should not be interpreted as limiting it. Where specific techniques or conditions are not specified in the examples, the techniques or conditions described in the relevant literature or product specifications apply. The reagents or instruments used, without specifying the manufacturer, are commercially available, conventional products. Example 1 [Synthesis stage - Evaporation stage - Extraction stage] A one-step process (synthesis - evaporation - extraction) is described with reference to Figure 1, in which an extraction tower is used. 40 m3 of triethylamine and 35 m3 of acetonitrile were pumped into a synthesis reactor with a volume of 100 m3. The reactor temperature was reduced to 15 °C. Then, 120 kg of ammonia gas were introduced first, and finally, 2000 kg of ammonia gas and 26000 kg of sulfuryl fluoride were introduced simultaneously, while the pressure in the reactor was maintained at 0.3 MPa and the temperature in the reactor was maintained at 15 °C. After the reaction lasted 4 h, the pressure in the reactor was reduced to 0.1 MPa and stirring was stopped. The reaction mixture stream 1 obtained (containing 40 wt% of (SO2F-NH-SO2F) ·Et3N, 18 wt% of triethylamine hydrogen fluoride salt and 6 wt% of triethylamine) was filtered through a tetrafluorocarbon filter bag with a pore diameter of 5 µm to remove a solid byproduct, sulfonamide.A filtrate was pumped to a falling-film evaporator (with a hot water reservoir at 75 °C), and the solvent present in the filtrate was evaporated under vacuum at -0.02 MPa (the first stage of the vacuum pump was condensed with water at 0 °C, and the second with water at -15 °C) to obtain a stream 2 containing (SOF-NH-SOF)·EtN and the triethylamine hydrogen fluoride salt (with 70 wt% (SOF-NH-SOF)·EtN and 28 wt% the triethylamine hydrogen fluoride salt), and a condensate containing acetonitrile. The condensate was recycled for synthesis in the synthesis reactor in the first stage. Stream 2 was pumped to a rotating disk extraction tower (with a stirring frequency of 15 ± 1 Hz and a controlled flow rate to obtain a weight ratio of deionized water to 2 of 1:. 1, 2). In the tower, it was thoroughly mixed with deionized water to obtain an aqueous phase, containing the triethylamine hydrogen fluoride salt as the top-layer liquid (with 77 wt% water and 22 wt% triethylamine hydrogen fluoride salt), and an oily phase 3, containing (SOF-NH-SOF)·EtN as the bottom-layer liquid (also containing 15 wt% water). The upper aqueous phase was sent to a recycling plant for treatment, and the lower oily phase was sent to an alkalization stage. Upon detection, the oily phase 3 also contained 100 ppm of F-. [Alkalinization stage-dehydration stage-desolventization stage] A β-stage process (alkalinization-dehydration-desolventization) is described with reference to Figure 3. Stream 3 was directly alkalized in a falling-film evaporator B (the steam heating temperature in the hot water tank was 35 °C), and an aqueous lithium hydroxide solution (with a concentration of 5 mol / L, where the volumetric ratio between stream 3 and the aqueous lithium hydroxide solution was 1.1:1) was added dropwise while continuous stirring was maintained. After one hour of reaction, a β1-1 mixture stream (a crude lithium salt) was obtained. Simultaneously, hot water at 35 °C was used for heating, and a vacuum was applied to maintain the pressure in the boiler at -0.08 MPa. The vaporization time was 6 hours. The initial and final stages of the vacuum pump performed a five-stage condensation with water at 25 °C and 0 °C, respectively. The condensate was allowed to settle for liquid separation.The top layer, an aqueous triethylamine solution, was pumped to the recycling workshop, and the bottom layer was recovered for the alkalization process to prepare the aqueous lithium hydroxide solution. After evaporation in the falling film evaporator B, a stream β1-2 containing lithium bis(fluorosulfonyl)imide (85 wt% lithium bis(fluorosulfonyl)imide and 10 wt% water) was obtained. During evaporation in falling film evaporator B, an aqueous solution of lithium hydroxide (with a concentration of 5 mol / L) was added to maintain the pH of the β1-2 stream at 8. The β1-2 stream continued to evaporate in a falling-film evaporator C (with a steam heating temperature of 50 °C in a hot water vessel). Simultaneously, diethyl carbonate (DEC) was dosed and pumped (a flow meter was controlled to ensure a volumetric ratio of DEC to the β1-2 stream of 0.6:1). Heating and evaporation continued under vacuum (-0.08 MPa), and the initial and final stages of the vacuum pump performed five-stage condensation with water at room temperature (25 °C) and 0 °C, respectively. The resulting condensate was an aqueous solution containing DEC. The condensate was sent to the recycling workshop for reuse.After evaporation in the falling film evaporator C, a β2 stream containing lithium bis(fluorosulfonyl)imide was obtained (with a composition of 70 wt% lithium bis(fluorosulfonyl)imide, 29 wt% diethyl carbonate, and 1 wt% water). During evaporation in the falling film evaporator C, an aqueous lithium hydroxide solution (with a concentration of 5 mol / L) was measured and added to maintain a pH of the β2 stream at 8. The β2 stream obtained after evaporation in the falling film evaporator C was centrifuged and filtered using a disc centrifuge (at a rotation speed of 1500 rpm) to remove a lithium compound byproduct, thus obtaining a β2-1 stream with 1 wt% water, 30 wt% diethyl carbonate and 69 wt% lithium bis(fluorosulfonyl)imide. Stream β2-1 was pumped to a scraper evaporator D (heated from a hot water tank at 75 °C). Simultaneously, diethyl carbonate (DEC) was dosed and pumped (using a flow meter that controlled the volumetric ratio of DEC to stream β2-1 at 0.25:1). Vacuum heating (at a vacuum of -0.08 MPa), evaporation, and dehydration then took place. A lithium hydroxide solution (5 mol / L) was added during evaporation using scraper evaporator D to maintain the pH of the resulting stream β3 at 8. The condensate, containing mainly DEC and a small amount of water, was delivered to the recycling workshop for reuse.After 6 hours of evaporation, lithium bis(fluorosulfonyl)imide β3 (containing 85% by weight of lithium bis(fluorosulfonyl)imide and 15% by weight of diethyl carbonate) was obtained with a water content of 3000 ppm. [Crystallization stage, drying stage] A crystallization and drying process is described with reference to Figure 4. β3 was pumped into a crystallization reactor, and dichloromethane with a water content of 100 ppm was pumped at a rate of 20 L / h. After stirring and mixing, the β3 and dichloromethane were pumped into a hybrid device with filtration and washing functions. The dichloromethane (containing DEC) was sent to a recycling facility, while the remaining lithium bis(fluorosulfonyl)imide crystals fell by gravity into a drying reactor located in the lower layer of the device. Nitrogen gas was introduced into a dryer to purge the crystals, and they were then dried at 60 °C. After drying and condensation, a condensate containing 99.5% dichloromethane and 0.5% water was obtained and sent to a crystallization process for recycling. Once the water content of the crystals was reduced to the target limit (50 ppm), the resulting powder was sent to the packaging section. After analysis, the purity of the lithium bis(fluorosulfonyl)imide powder obtained after drying was 99.8%, and the yield reached 92%, with a free acid content of 10 µg / g and a water content of 20 µg / g. [Canning stage] An optional canning process is described with reference to figure 5. Seventy 70 L of diethyl carbonate and 0.1 kg of lithium hydroxide were added to 30 kg of lithium bis(fluorosulfonyl)imide powder obtained after crystallization and drying. The sample was then centrifuged in a disc centrifuge (at a rotation speed of 1500 rpm) to remove the solids. The filtrate was transferred to a dehydration reactor to which 20 kg of molecular sieves were added, where it was stirred at a rotation speed of 800 rpm for 2 hours. Finally, the molecular sieves were filtered out, and the resulting filtrate was transferred to a product preparation reactor. Finally, demagnetization (using an 8000 gauss vertical demagnetizing filter) and filtration (through 1 micron, 0.5 micron and 0.1 micron filters, respectively) were performed to obtain a diethyl carbonate solution containing lithium bis(fluorosulfonyl)imide with a concentration of 28% by weight (HF 50 µg / g and water content 20 µg / g).Finally, the packaging process began. Example 2: The technical solution in Example 2 is the same as in Example 1, differing only in that a static mixer is used in the extraction stage (pipe length / diameter ratio L / D = 10; and the flow is controlled so that the weight ratio of deionized water to stream 2 is 1:1, 2). As shown in Figure 2, stream 2 was pumped to a static mixer to be thoroughly mixed with deionized water. Subsequently, it was introduced into a layer separation tank for 2 hours to obtain an aqueous phase (water) containing a triethylamine hydrogen fluoride salt as the upper layer and an oily phase 3 (SO2F-NH-SO2F) ·Et3N as the lower layer. The upper aqueous phase was sent to a recycling workshop for treatment, and the lower oily phase was sent to an alkalization stage. After analysis, the oily phase 3 contained 1000 ppm of F-. After analysis, the purity of the lithium bis(fluorosulfonyl)imide powder obtained after drying in Example 2 was 99.2%, with a yield of 89%, where the free acid content was 15 µg / g and the water content was 25 µg / g.

Claims

1. A method for the preparation of lithium bis(fluorosulfonyl)imide, comprising the following steps: (a) a synthesis step: reacting sulfuryl fluoride, ammonia gas, and triethylamine in a reactor in the presence of a solvent to obtain a stream 1 containing (SO2F-NH-SO2F)·Et3N, a triethylamine hydrogen fluoride salt, and triethylamine; (b) an evaporation step: evaporating stream 1 to obtain a stream 2 containing (SO2F-NH-SO2F)·Et3N and the triethylamine hydrogen fluoride salt, optionally post-treating the evaporated solvent and the triethylamine, and then returning to step (a); (c) an extraction step: washing the stream 2 obtained in step (b) with water in an extraction tower or static mixer to obtain an oil phase 3 containing (SO2F-NH-SO2F) ·Et3N and an aqueous phase water containing the hydrogen fluoride salt of triethylamine,and separating the oily phase 3; (d) an alkalization step: introducing the oily phase 3 obtained in step (c) into an evaporator to mix it with an aqueous solution of lithium hydroxide and obtain a mixture stream β1-1, and then performing reduced-pressure evaporation on the β1-1 stream to obtain a β1-2 stream containing lithium bis(fluorosulfonyl)imide; (e) a dehydration step: adding the β1-2 stream containing lithium bis(fluorosulfonyl)imide and an ester solvent to an evaporator for evaporation, thereby obtaining a β2 stream containing lithium bis(fluorosulfonyl)imide; (f) a desolventization step: adding the β2 stream obtained in step (e) and an ester solvent to an evaporator for evaporation,obtaining crude lithium bis(fluorosulfonyl)imide β3; and (g) a crystallization step: pumping the crude lithium bis(fluorosulfonyl)imide β3 obtained in step (f) into a crystallization reactor and adding dichloromethane to precipitate the lithium bis(fluorosulfonyl)imide crystals.

2. The method according to claim 1, wherein the method further comprises the following step: (h) a drying step: purging the lithium bis(fluorosulfonyl)imide crystals precipitated in step (g) with inert gas in a dryer, to obtain lithium bis(fluorosulfonyl)imide crystals in powder form with a water content of less than 50 ppm.

3. The method according to any one of claims 1 to 2, wherein in step (a), the molar ratio of sulfuryl fluoride, ammonia gas, and triethylamine is (1.5-3.5) :1: (1-6), optionally (2-3) :1: (1-5).

4. The method according to any one of claims 1 to 3, wherein in step (a),The solvent is selected from acetonitrile, propionitrile, isopropionitrile, diethyl ether, propyl ether, isopropyl ether, tetrahydrofuran, acetone, butanone, methyl isobutyl ketone, methylpyrrolidone, or a mixture of two or more of these, preferably acetonitrile.

5. The method according to any one of claims 1 to 4, wherein in step (a), the reaction temperature is not higher than 25 °C, optionally between 3 °C and 20 °C; and / or, the reaction pressure is not higher than 0.4 MPa, optionally not higher than 0.25 MPa.

6. The method according to any one of claims 1 to 5, wherein between steps (a) and (b), the method further comprises a step of filtering stream 1 to remove a solid sulfonamide byproduct; Optionally, the filtration stage comprises filtration through a tetrafluorocarbon filter bag with a pore diameter of 5 µm to 20 µm; and / or wherein stage (b),Stream 1 is evaporated using a falling film evaporator; and / or wherein in step (c), the aqueous phase (water) containing the triethylamine hydrogen fluoride salt is delivered for recycling treatment, and the triethylamine obtained after subjecting the aqueous phase (water) to alkalization and purification treatment is recyclable; and / or wherein in step (d), the oil phase 3 is mixed and stirred with the aqueous lithium hydroxide solution to react for 0.5–3 hours, optionally 1–2 hours; and / or wherein in step (d), the condensate obtained by evaporation of stream β1-1 is subjected to separation by decantation. The liquid in the upper layer is an aqueous triethylamine solution, which is recycled; and the liquid in the lower layer is condensed water, which is recycled to prepare the aqueous lithium hydroxide solution required for the alkalization step.

7. The method according to any of claims 1 to 6, wherein in step (d),The volumetric ratio between the oil phase 3 and the aqueous lithium hydroxide solution is (0.8-5) :1, or optionally (1-1.2) :1; and / or wherein in steps (e) and (f), the aqueous ester solvent solution obtained by condensation is recycled; and / or wherein in step (e), the resulting β2 stream contains between 0.1% and 2% by volume of water and between 20% and 40% by volume of the ester solvent.

8. The method according to any one of claims 1 to 7, wherein the crude lithium bis(fluorosulfonyl)imide β3 obtained in step (f) has a water content of 2000 ppm to 4000 ppm, preferably 2300 ppm to 2800 ppm; and / or where steps (e) and (f) are carried out in the same evaporator, or in different evaporators; and / or where, during steps (d) and (e), the following side reaction takes place: (SO2F-N-SO2F) -Li++4LiOHNH2SO3Li+Li2SO4+2LiF+H2O,and a lithium composite byproduct is removed by centrifugation and filtration prior to the desolventization step (f), optionally by a scraping centrifuge or a disc centrifuge.

9. The method according to any one of claims 1 to 8, wherein in steps (e) and (f), the ester solvent is selected from an organic solvent having a boiling point above 70 °C, preferably above 80 °C, and preferably between 100 °C and 130 °C, and insoluble in water; optionally, the ester solvent contains a carbonate solvent comprising ethylene carbonate, ethylmethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylenepropyl carbonate, butylene carbonate, fluoroethylene carbonate, or a mixture of two or more thereof; Optionally, the ester solvent contains a carboxylate solvent comprising propyl acetate, methyl propionate, ethyl propionate,propyl propionate, methyl butyrate, ethyl butyrate, or a mixture of two or more of these; and furthermore, optionally, the ester solvent is selected from at least one of the following: ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate.

10. The method according to any one of claims 1 to 9, wherein the evaporation temperature in the alkalization step (d) is controlled between 30°C and 40°C, or between 30°C and 35°C; and / or the evaporation temperature in the dehydration step (e) is controlled between 40°C and 55°C, or between 45°C and 50°C; and / or the evaporation temperature in the desolventization step (f) is controlled between 60°C and 80°C, or between 65°C and 75°C; and / or wherein in steps (d), (e) and (f), the aqueous lithium hydroxide solution is added so that the pH of the mixture in the evaporator is maintained between 7 and 9, preferably between 8 and 9; and / or wherein in steps (d), (e) and (f),the concentration of the aqueous lithium hydroxide solution is from 1 mol / L to 15 mol / L, optionally from 2 mol / L to 10 mol / L; and / or wherein between steps (g) and (h), the method consists of pumping a stream containing the ester solvent, dichloromethane, and the lithium bis(fluorosulfonyl)imide crystals obtained in the crystallization step (g) to a two-in-one device with filtration and washing functions, where a crystallization liquid containing diethyl carbonate and dichloromethane is supplied for recovery treatment, and the remaining lithium bis(fluorosulfonyl)imide crystals fall by gravity into a drying vessel located in the lower layer of the two-in-one device; and / or wherein in step (h), the liquid phase obtained after drying and condensation comprises dichloromethane and water,and said liquid phase is recycled and supplied to the crystallization step (g); and / or wherein the water content of the dichloromethane used in the crystallization step (g) is 200 ppm.

11. The method according to any one of claims 1 to 10, wherein the method further comprises (i) a canning step after the drying step (h), wherein, after dissolution with a solvent and, optionally, removal of acid, water, and filtration, the powdered lithium bis(fluorosulfonyl)imide crystals obtained in step (h) meet the following criteria: HF 50 µg / g, and water content 20 µg / g.

12. The method according to claim 11, wherein the solvent used in the dissolution of step (i) is the same as the ester solvents used in steps (e) and (f).

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