A method for the continuous synthesis of LiFSI using reaction-extraction technology

Abstract

The application provides a method for continuously synthesizing LiFSI by using a reaction-extraction technology. Under normal temperature conditions, in the process of synthesizing LiFSI by reacting HFSI·NEt3 and LiOH, an alkane solvent is added as a first extraction agent, the generated by-product triethylamine is extracted into an organic phase, then a halogenated hydrocarbon is added as a second extraction agent, the residual triethylamine and unreacted HFSI·NEt3 are removed, and thus the aqueous solution of LiFSI is obtained. The conversion rate of the reaction can reach 99.5%, the hydrolysis rate of LiFSI is controlled below 0.1%, and the yield of LiFSI can be greatly improved. The reaction-extraction technology is used to synthesize LiFSI, through simple normal-temperature extraction operation, compared with the original reaction-distillation technology, the reaction energy consumption can be reduced, the yield of LiFSI can be ensured, and industrialization is easy to realize, and a new process route is provided for the efficient synthesis of LiFSI.

Description

Technical Field

[0001] This invention relates to the fields of energy chemical engineering and green chemical engineering, and specifically to a method for the continuous synthesis of LiFSI using reaction-extraction technology. Background Technology

[0002] With the rapid development of global industry, human demand for non-renewable energy sources such as oil and coal is constantly increasing, making energy consumption a major issue for human survival and development. Lithium-ion batteries, due to their high energy density, high conversion efficiency, and long cycle life, are widely used in new energy vehicles, small-scale grid energy storage, aerospace, military, and industrial fields. A lithium-ion battery consists of a positive electrode, a negative electrode, a separator, and an electrolyte, with the electrolyte being one of the most crucial components. Lithium bisfluorosulfonylimide (LiFSI) is a promising new electrolyte lithium salt with broad application prospects. Compared to the widely used electrolyte lithium hexafluorophosphate (LiPF6), LiFSI is considered the most likely replacement for LiPF6 due to its better thermal and hydrolytic stability, higher ionic conductivity, and lower aluminum corrosion potential. Therefore, the efficient synthesis of LiFSI is receiving increasing attention.

[0003] Currently, the common synthesis method for LiFSI is the ammonia-thioyl fluoride method, which involves reacting NH3, SO2F2, and triethylamine (NEt3) to produce HFSI·NEt3, followed by ion exchange of LiOH with HFSI·NEt3 to obtain LiFSI, as illustrated in patents CN101980955 and WO2020099527. Many other processes have also been reported, such as CN111620315A, which discloses the synthesis of bis(fluorosulfonyl)imide salts using an organic base and SO2F2, followed by the addition of lithium oxide to the intermediate in the presence of an organic solvent, and finally, filtration, concentration, and crystallization to obtain LiFSI. However, these methods generally suffer from low product yields, high energy consumption, and long reaction times.

[0004] HFSI·NEt3 undergoes ion exchange with LiOH to generate LiFSI and NEt3, which is the final step in the synthesis of LiFSI via the ammonia-thioacryl fluoride method (as shown in the following formula). This process is reversible; timely removal of the generated NEt3 can promote the reaction towards the formation of LiFSI.

[0005]

[0006] In existing industrial production of LiFSI, reactive distillation is typically used to remove NEt3 generated during the reaction and improve the reaction conversion rate. However, reactive distillation usually takes place at temperatures above 50°C, leading to significant hydrolysis of the generated LiFSI and resulting in low product yield.

[0007] To address the above issues, this invention proposes a reaction-extraction coupling technology for the continuous synthesis of LiFSI. By using a low-temperature extraction process, NEt3 is rapidly removed during the reaction, avoiding the high-temperature-promoted hydrolysis of LiFSI, reducing product loss, and increasing the LiFSI yield. Summary of the Invention

[0008] The main objective of this invention is to provide a method for the continuous synthesis of LiFSI using reaction-extraction technology. This method improves the conversion rate of the reaction and reduces the hydrolysis rate of LiFSI, thereby effectively improving the production efficiency and product yield of LiFSI.

[0009] The method proposed in this invention is as follows: HFSI·NEt3 and LiOH undergo a reversible chemical reaction in an aqueous solution to generate LiFSI and triethylamine (NEt3). An extractant is added to the reaction solution of HFSI·NEt3 and LiOH, and the generated NEt3 is transferred to the organic phase through reaction-extraction. The organic phase is separated to obtain the aqueous phase. LiFSI is then extracted from the aqueous phase.

[0010] In a preferred embodiment, the extractant includes a first extractant and a second extractant. The first extractant is added to the aqueous reaction solution of HFSI·NEt3 and LiOH to perform reaction-extraction and separation. The reaction-extraction and separation steps are performed at least once until the total extraction rate is ≥90% to obtain the first aqueous phase. The second extractant is then added to the first aqueous phase, and after further extraction and separation, the second aqueous phase is obtained. The obtained second aqueous phase is the LiFSI aqueous solution.

[0011] The method for continuous synthesis of LiFSI using the above reaction-extraction technique includes the following steps:

[0012] (1) Add the first extractant to the reaction solution of HFSI·NEt3 and LiOH to carry out reaction-extraction. After standing, separate the aqueous phase and the organic phase. The organic phase contains the by-product NEt3.

[0013] (2) Repeat the above reaction-extraction and separation steps at least once with the aqueous reaction solution obtained in step (1), and separate the aqueous phase and organic phase after standing.

[0014] (3) Add the second extractant to the aqueous reaction solution obtained in step (2), stir at room temperature for 5~60 min, and separate the liquid to obtain an organic phase and an aqueous phase. The organic phase contains unreacted HFSI·NEt3 and residual NEt3, and the aqueous phase is an aqueous solution of LiFSI.

[0015] Further, the reaction solution of HFSI·NEt3 and LiOH in step (1) is a mixed solution of HFSI·NEt3 and LiOH aqueous solution; the mass fraction of the LiOH aqueous solution is 5%~11%, and the molar ratio of HFSI·NEt3 to LiOH is 1: (0.9~1.3).

[0016] Furthermore, in steps (1), (2), and (3), the volume ratio of the extractant to the reaction solution is 1: (0.3~3), and the reaction solution in steps (2) and (3) is the aqueous phase obtained by separation.

[0017] Furthermore, the first extractant is one of cyclohexane, n-heptane, n-octane, n-hexane, isooctane, dodecane, or tridecane.

[0018] Furthermore, the reaction-extraction process using the first extractant is carried out at a temperature of 0–50°C and a reaction time of 5–120 min. Preferably, the reaction temperature is 30°C and the reaction time is 5–30 min.

[0019] Furthermore, in step (3), the second extractant is a haloalkane solvent.

[0020] Furthermore, the second extractant is one of dichloromethane, dichloroethane, dichloropropane, or trichloroethane.

[0021] The reaction-extraction process is carried out in a continuous extraction reactor at room temperature to achieve continuous reaction. The continuous extraction reactor includes a continuous extraction tower or a membrane separation continuous extraction device.

[0022] The first extractant is an alkane solvent, including but not limited to cyclohexane, n-heptane, n-octane, n-hexane, isooctane, dodecane, and tridecane. These solvents have good solubility for NEt3, but poor solubility for HNEt3·FSI and LiFSI. They are insoluble in water and easily separate into phases. Using an alkane solvent as the first extractant allows for the reaction-extraction synthesis of LiFSI.

[0023] The second extractant is a halocarbon solvent, including but not limited to dichloromethane, dichloroethane, dichloropropane, and trichloroethane. This type of solvent can effectively transfer almost all of the free NEt3 to the organic phase, while simultaneously extracting unreacted HFSI·NEt3 into the organic phase. Therefore, using it as the second extractant allows for the complete removal of NEt3 from the system, yielding a high-purity LiFSI aqueous solution.

[0024] Compared with the prior art, the advantages of the present invention are as follows:

[0025] By utilizing reaction-extraction technology, the byproduct NEt3 generated in the reaction can be transferred to the extractant at room temperature using a common extractant. This not only separates the byproduct but also promotes the conversion rate of the reaction and reduces the hydrolysis rate of LiFSI under alkaline conditions, thereby effectively improving the yield of LiFSI. This process has mild reaction conditions, simple operation procedures, and is environmentally friendly, showing good application prospects in the synthesis of LiFSI. Attached Figure Description

[0026] Figure 1 The reaction equation is for HFSI·NEt3 and LiOH.

[0027] Figure 2 FSI at 50℃ - Partial hydrolysis ion chromatogram (HFSI·NEt3: LiOH molar ratio = 1:1, LiOH aqueous solution mass fraction 9.5%).

[0028] Figure 3 The process flow diagram of the preferred embodiment of the present invention is as follows: a first extractant is added to the reaction solution of HFSI·NEt3 and LiOH, and after three reactions-extractions and separations, a second extractant is added to the resulting aqueous phase for extraction. The aqueous phase obtained after separation is the LiFSI aqueous solution.

[0029] Figure 4 This is a schematic diagram of the continuous synthesis of LiFSI via multi-stage dual-solvent tandem extraction. Detailed Implementation

[0030] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments, but this does not limit the scope of protection of the present invention. Unless otherwise specified, the experimental methods used in the present invention are all conventional methods, and the experimental equipment, materials, reagents, etc. used can all be obtained commercially.

[0031] The HFSI·NEt3 used in the following examples was purchased from Times Sikang New Materials Co., Ltd., with a concentration of 88 wt.% HFSI·NEt3 (12 wt.% water); the LiOH aqueous solution was prepared by adding water to LiOH·H2O to form a LiOH aqueous solution with a mass fraction of 9.5 wt.%.

[0032] A systematic study was conducted on the equilibrium and hydrolysis behavior of the ion exchange reaction between HFSI·NEt3 and LiOH. The reaction between HFSI·NEt3 and LiOH is shown in the attached figure. Figure 1As shown, this reaction is reversible. NEt3 reacts with LiFSI to generate HFSI·NEt3 and LiOH, consuming LiFSI and thus reducing the yield. Therefore, removing NEt3 from the reaction system is an effective way to promote the reaction towards the formation of LiFSI. The hydrolysis behavior of this lithiation reaction was also studied. 10 mL of the reaction solution of HFSI·NEt3 and 9.5 wt.% LiOH aqueous solution was placed at 50 °C. o The reaction was carried out in a constant temperature stirred bath at C, where the molar ratio of HFSI·NEt3:LiOH was 1:1, and the mass fraction of LiOH aqueous solution was 9.5%. Ion chromatographic analysis results were obtained. Figure 2 This indicates that under strong alkaline conditions (pH ≥ 12) at 50℃, the hydrolysis products of LiFSI include FSO2 and NSO3. 2- SO3NHSO3 2- and F - To address this, the present invention proposes a reaction-extraction method for synthesizing LiFSI.

[0033] The analysis method in this embodiment is as follows:

[0034] Quantitative analysis was performed using an ion chromatography (IC-D150) from Qingdao Shenghan and a gas chromatography (GC-2014) from Shimadzu. Ion chromatography employed a SH-AC-3 (4×250 mm) anion exchange column with a conductivity detector. KOH aqueous solution was used as the mobile phase, employing a gradient elution program as follows: 0–10 min, KOH concentration maintained at 3 mmol / L; 10–30 min, KOH concentration maintained at 30 mmol / L; 30–45 min, KOH concentration maintained at 55 mmol / L; 45–55 min, KOH concentration maintained at 3 mmol / L. Gas chromatography used an SH-I-17 (30.0 m × 0.25 mm × 0.25 µm) column. Data were corrected using the external standard method. The extraction rate of triethylamine, the conversion rate of HFSI·NEt3, and the hydrolysis rate of LiFSI were calculated using the following formula:

[0035]

[0036]

[0037]

[0038] The following describes embodiments 2-3 of this application, with the process described in the appendix. Figure 3 The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application.

[0039] Example 1: Effect of the first extractant extraction stage

[0040] The reaction solution was a mixture of 500 mL HFSI·NEt3 and LiOH aqueous solution, with a molar ratio of HFSI·NEt3 to LiOH of 1:1. The mass fraction of the LiOH aqueous solution was 9.5%. One to four stages of extraction were performed using n-octane as the first extractant. The volume ratio of n-octane added in each stage to the volume of the aqueous reaction solution was 1:1. Each extraction stage lasted for 30 minutes. o The reaction-extraction at C was carried out for 20 min, and the extraction results are shown in Table 1.

[0041] Table 1 Extraction rates for different extraction stages

[0042]

[0043] The results showed that as the number of extraction stages increased, the total extraction rate also increased, but the extraction rate of subsequent single stages decreased. The three-stage extraction process removed most of the NEt3. When the number of extraction stages reached four, the NEt3 content in the aqueous phase was low, and the mass transfer between the two phases was affected by equilibrium. The extraction rate of the fourth stage was only 0.56%, indicating that the effect of the fourth stage extraction was very limited. To avoid wasting the extractant, the number of extraction stages was set to three, at which point the total extraction rate was 93.64%.

[0044] Example 2 Effect of Extractant

[0045] In the reaction-extraction process, the extractant should possess significant characteristics such as chemical stability, excellent product solubility, and insolubility in water and raw materials. After analyzing the solubility of various solvents, alkane solvents were selected as suitable extractants. Further experiments were conducted using different alkane solvents.

[0046] The molar ratio of HFSI·NEt3 to LiOH was 1:1, and the mass fraction of the LiOH aqueous solution was 9.5%. Under room temperature conditions, 570 mL of the first extractant was added to 500 mL of the reaction solution of HFSI·NEt3 and LiOH aqueous solution, and the mixture was stirred for 20 min. After stirring, the mixture was allowed to stand and separate into layers, yielding an aqueous phase and an organic phase. Another 500 mL of the first extractant was added to the aqueous phase, and the mixture was stirred for 5 min. After stirring, the mixture was allowed to stand and separate into layers, yielding an aqueous phase and an organic phase. Finally, 350 mL of dichloromethane was added to the aqueous phase, and the mixture was stirred for 10 min. After stirring, the mixture was allowed to stand and separate into layers, yielding the aqueous phase, which was the LiFSI aqueous solution. The conversion rate of HFSI·NEt3, the extraction rate of NEt3, and the hydrolysis rate of LiFSI were analyzed by different extractants, as shown in Table 2.

[0047] Table 2. HFSI·NEt3 conversion, NEt3 extraction rate, and LiFSI hydrolysis rate for different first extractants.

[0048]

[0049] The results showed that alkane extractants had good extraction performance for NEt3, and that water and alkane solvents were almost insoluble, which could effectively avoid introducing new impurities into the LiFSI solution.

[0050] Example 3 Effect of extraction temperature

[0051] The reaction solution was a mixture of 500 mL HFSI·NEt3 and LiOH aqueous solution, with a molar ratio of HFSI·NEt3 to LiOH of 1:1 and a mass fraction of 9.5% in the LiOH aqueous solution. Octane was used as the extractant, with an extractant volume to reaction solution volume ratio of 1:1 for each stage. The reaction-extraction time for each stage was 20 min, and a three-stage extraction experiment was conducted at 0–40 °C. The conversion rate of HFSI·NEt3, the extraction rate of NEt3, and the hydrolysis rate of LiFSI at different extraction temperatures are shown in Table 3. The results indicate that with increasing temperature, the mass transfer between the two phases is better, the solubility of NEt3 in the organic phase increases, the extraction rate increases, and NEt3 can be removed from the reaction system more effectively, thus promoting the forward reaction and improving the conversion rate. At 20 °C and 30 °C, the hydrolysis rate remained at a low level, but the conversion rate at 30 °C was significantly higher than that at 20 °C. With increasing temperature, the hydrolysis rate of LiFSI in the reaction system increased, but the reaction equilibrium shifted to the right, resulting in an increased conversion rate. Therefore, setting the reaction temperature to 30℃ is more reasonable.

[0052] Table 3. HFSI·NEt3 conversion rate, NEt3 extraction rate, and LiFSI hydrolysis rate at different temperatures.

[0053]

[0054] Example 4: Continuous Synthesis of LiFSI by Multi-Stage Dual-Solvent Tandem Extraction

[0055] This invention achieves continuous synthesis of LiFSI using a continuous extraction reactor. This embodiment employs a Zaiput membrane separator for multi-stage continuous extraction. The multi-stage continuous extraction experiment was conducted in a modular continuous flow apparatus, consisting of a multi-stage extraction platform (Zaiput Flow Technologies, USA), a four-channel independently controlled syringe pump, and two collection containers. The multi-stage extraction platform has five stages of countercurrent extraction, each stage including a mass transfer tube, membrane separator, interstage pump, and pressure sensor. Interstage pumping is regulated by the pressure sensor. All tubing is PFA, 1 / 8-in and 1 / 16-in. The separator channels are perfluoropolymer with a volume of 400 μL; considering all tubing and pressure sensors, the total volume per stage is 2 mL. Figure 4 As shown, the reaction solution and extractant are simultaneously added to the system from different feed points via syringe pumps for multi-stage countercurrent extraction, with the reaction solution and extractant flowing in opposite directions. After the first extractant performs primary, secondary, and tertiary reaction-extraction on the reaction solution, the first extractant and its extracted NEt3 are separated from the system by a membrane separator. The reaction solution after reaction-extraction continues to flow into the next module, where it is extracted by the second extractant and then separated by a membrane separator to obtain a LiFSI aqueous solution. The second extractant and its extracted HFSI·NEt3 and NEt3 flow out from another outlet. The startup procedure is as follows: 9.5 wt.% LiOH aqueous solution, HFSI·NEt3 aqueous solution, and extractant are injected into 50 mL syringes respectively. The LiOH aqueous solution and HFSI·NEt3 are mixed in the front-end tubing before entering the membrane separator. The flow rate of each phase is controlled by changing the syringe pump flow rate. The interstage pump is controlled by a pressure sensor to balance the pressure of each module. The separated phases flow out of the device and into the corresponding collection containers. Twenty minutes after feeding, water samples were taken from the effluent to determine the concentrations of NEt3 and HFSI·NEt3; organic phase samples were also taken to analyze the NEt3 content. All experiments were conducted at room temperature.

[0056] Under the conditions that the extractant volume is 1.5 times that of the aqueous phase, with n-octane as the first extractant and dichloromethane as the second extractant, an experimental study was conducted using this apparatus on the coupling of three-stage countercurrent extraction of n-octane with one-stage extraction of dichloromethane. The feed rate of HFSI·NEt3 was set at 0.50 mL / min, the feed rate of 9.5 wt.% LiOH aqueous solution was set at 0.39 mL / min, and the feed rates of n-octane and dichloromethane were both 1.34 mL / min. After feeding at room temperature for 20 min, water samples were taken from the outlet to detect the concentrations of NEt3 and HFSI·NEt3. The NEt3 content was analyzed from the organic phase. The conversion rate obtained by this multi-stage dual-solvent continuous reaction-extraction was calculated to reach 99.73%, which is close to complete conversion. This is a significant improvement compared to the less than 90% conversion rate of reactive distillation. At the same time, the hydrolysis rate is only 0.05%, which is also much lower than the 5% hydrolysis rate in current industrial processes, proving that this invention has the potential for industrial application.

[0057] Comparative Example 1: Vacuum Distillation Comparison Experiment

[0058] The reaction solution of 500 ml HFSI·NEt3 and LiOH aqueous solution was added to a flask and subjected to vacuum distillation. The molar ratio of HFSI·NEt3 to LiOH was 1:1, and the mass fraction of LiOH aqueous solution was 9.5%. Distillation was carried out at 50 °C and 20 kPa for 1 h to obtain LiFSI aqueous solution. The conversion rate of the reaction was 92% and the hydrolysis rate of LiFSI was 4%.

[0059] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for the continuous synthesis of LiFSI using a reaction-extraction technique, characterized in that: adding an extractant to the aqueous phase reaction solution of HFSI·NEt3 and LiOH to carry out reaction-extraction, and obtaining an aqueous phase and an organic phase after separation; the reaction-extraction and separation steps are carried out at least once; the obtained aqueous phase is a LiFSI aqueous solution; The extractant includes a first extractant and a second extractant, and the specific operation includes the following steps: (1) adding a first extractant to the reaction solution of HFSI·NEt3 and LiOH to carry out reaction-extraction, and obtaining an aqueous phase and an organic phase after standing and separation, and the organic phase is a by-product triethylamine; (2) repeating the above reaction-extraction and separation steps at least once for the aqueous phase obtained in step (1), and obtaining an aqueous phase and an organic phase after standing and separation; (3) adding a second extractant to the aqueous phase obtained in step (2), stirring at room temperature for 5-60 min, and obtaining an organic phase and an aqueous phase after standing and separation, and the organic phase is unreacted HFSI·NEt3 and residual triethylamine, and the aqueous phase is a LiFSI aqueous solution; In step (1), the reaction solution of HFSI·NEt3 and LiOH is a mixed solution of HFSI·NEt3 and LiOH aqueous solution; the mass fraction of the LiOH aqueous solution is 5%-11%, and the molar ratio of HFSI·NEt3 to LiOH is 1:(0.9-1.3); In step (1), the first extractant is one of cyclohexane, n-heptane, n-octane, n-hexane, isooctane, dodecane or tridecane; In step (3), the second extractant is a halogenated alkane solvent.

2. The process for the continuous synthesis of LiFSI using reaction-extraction technique according to claim 1, characterized in that: The volume ratio of each addition of extractant to reaction solution is 1:(0.3-3).

3. The method for continuous synthesis of LiFSI using reaction-extraction technique according to claim 1, characterized in that: The reaction temperature for the reaction-extraction with the first extractant is 0-50 o C. The reaction time is 5-120 min.

4. The method for continuous synthesis of LiFSI using reaction-extraction technique according to claim 1, characterized in that: The second extractant is one of dichloromethane, dichloroethane, dichloropropane or trichloroethane.

5. The method for continuous synthesis of LiFSI using reaction-extraction technique according to claim 1, characterized in that: The reaction-extraction process is carried out continuously at room temperature using a continuous extraction reactor, and the continuous extraction reactor includes a continuous extraction column or a membrane separation continuous extraction device.

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