Method for evaluating stability of lithium bis (fluorosulfonyl) imide

Abstract

The invention discloses a method for evaluating the stability of lithium bis (fluorosulfonyl) imide, which is characterized in that the content of anion impurities in LiFSI is detected by using an ion chromatography, and when the main content meets the national standard, the content of SO4 < 2-> is greater than 50ppm and the content of FSO3 is greater than 0ppm, the stability of lithium bis (fluorosulfonyl) imide is unqualified and expired; when the content of SO4 < 2-> is less than or equal to 50 ppm and the content of FSO3 is greater than 0 ppm, the stability of the lithium bis (fluorosulfonyl) imide is qualified, and the shelf life is 5-15 days; the content of SO4 < 2-> is smaller than or equal to 50 ppm, FSO3 is equal to 0, the stability of the lithium bis (fluorosulfonyl) imide is qualified, and the shelf life is larger than or equal to 77 days. The method is simple, the result is accurate, research and development, production and storage of the LiFSI can be promoted, and the application effect of the LiFSI in the lithium ion battery is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of electrolyte salt detection, and in particular relates to a method for evaluating the stability of lithium bis(fluorosulfonyl)imide. Background Art

[0002] At present, the most important electrolyte lithium salt used in lithium-ion batteries is lithium hexafluorophosphate (LiPF6), but due to its poor thermal stability (begins to decompose at around 60°C); it quickly decomposes and produces hydrogen fluoride in the presence of trace amounts of water, which greatly limits the application range of lithium-ion batteries. In extreme conditions, it may not only cause battery performance deterioration and environmental pollution, but also cause harm to personnel. Therefore, as the energy density and safety requirements of lithium-ion batteries become higher and higher, LiPF6 has gradually failed to meet the demand.

[0003] Lithium bis(fluorosulfonyl)imide (LiFSI) is an ionic liquid electrolyte with the advantages of high isolation, high stability, high conductivity and low volatility. It can improve the cycle life, power performance and safety performance of lithium batteries. Therefore, it is often used as a part of the electrolyte solution in lithium batteries and has become one of the most promising electrolyte lithium salts to replace LiPF6.

[0004] However, the preparation process of LiFSI is complicated and the purification is difficult. In addition, during the lithiation and dehydration and drying process in the later stage of synthesis, the bis(fluorosulfonyl)imide ion (FSI − ) reacts with alkali or water at high temperature to decompose and produce trace amounts of NH2SO3 − 、FSO3 − 、SO4 2− These trace decomposition products will corrode the positive electrode substrate aluminum foil, limiting the application of LiFSI in lithium-ion batteries. Therefore, it is particularly important to monitor the content of impurity anions and do a good job in quality control of LiFSI.

[0005] According to the requirements of YS / T 1302-2019, it is necessary to test the moisture, dimethyl carbonate (DMC) insoluble matter, free acid, chloride (Cl − ), sulfate (as SO4 2− ) and fluoride (in F − The main content of LiFSI is calculated by subtraction method, that is, 100% minus the above impurity content. This may cause some impurities that have not been paid attention to to be ignored, resulting in errors in the calculation of the main content of LiFSI. It is more likely that the presence of these ignored impurities will deteriorate the stability of LiFSI, or even decompose and deteriorate, resulting in product failure and unusability.

[0006] Chinese patent CN108387674A discloses a method for determining the purity of lithium bis(fluorosulfonyl)imide, which detects the concentration of bis(fluorosulfonyl)imide by ion chromatography and calculates the purity of lithium bis(fluorosulfonyl)imide. The method has the advantages of simple pretreatment, simple operation, and accurate results, and can be used for commercial rapid determination of the purity of lithium bis(fluorosulfonyl)imide.

[0007] However, the purity test can only help determine the content of lithium bis(fluorosulfonyl)imide, but cannot help determine its stability or find out the factors that affect the stability. Therefore, it is necessary to develop an evaluation method that can directly determine the stability of lithium bis(fluorosulfonyl)imide to promote the research and development, mass production, storage and application of LiFSI. Summary of the invention

[0008] In view of the above technical problems, the present invention provides a method for evaluating the stability of lithium bis(fluorosulfonyl)imide. Ion chromatography is used to detect and calculate the changes in the main content and impurity anion content of lithium bis(fluorosulfonyl)imide before and after storage, so as to evaluate the quality of LiFSI in the early stage of synthesis and the key factors affecting its stability, which has guiding significance for the research and development, mass production, storage and application of LiFSI.

[0009] In order to achieve the above object, the present invention provides a method for evaluating the stability of lithium bis(fluorosulfonyl)imide, comprising the following steps: (1) After storing lithium bis(fluorosulfonyl)imide, a liquid sample is obtained after quantification; (2) Ion chromatography detection of the main content of LiFSI and the content of anionic impurities in liquid samples; (3) The stability of lithium bis(fluorosulfonyl)imide was evaluated based on the content of anionic impurities in the main content. The evaluation criteria were: SO4 2- Content>50ppm, FSO3 − When the content is >0ppm, the stability of lithium bis(fluorosulfonyl)imide is unqualified and expired; SO4 2- Content ≤50ppm, FSO3 − When the concentration is >0ppm, the stability of lithium bis(fluorosulfonyl)imide is acceptable and the shelf life is 5-15 days; SO4 2- Content ≤50ppm, FSO3 − =0, lithium bis(fluorosulfonyl)imide has qualified stability and the shelf life is ≥77 days.

[0010] Preferably, the storage method in step (1) is wet storage and dry storage.

[0011] Further preferably, the wet storage method is to dissolve lithium bis(fluorosulfonyl)imide in ultrapure water and store it in a sealed state at room temperature; the dry storage method is to seal lithium bis(fluorosulfonyl)imide and control the water and oxygen content in the environment and store it at room temperature, wherein the water content is ≤0.1 ppm and the oxygen content is ≤0.1 ppm.

[0012] More preferably, the lithium bis(fluorosulfonyl)imide after dry storage is dissolved in ultrapure water and then quantitatively obtained as a liquid sample.

[0013] Preferably, the concentration of the liquid sample in step (1) is 0.2-0.5 g / mL.

[0014] Preferably, the mobile phase of the ion chromatography in step (2) is a sodium carbonate-sodium bicarbonate-acetonitrile solution filtered through a sand core.

[0015] Further preferably, the concentration of sodium carbonate in the sodium carbonate-sodium bicarbonate-acetonitrile solution is 3-4 mM, the concentration of sodium bicarbonate is 4-8 mM, and the volume of acetonitrile is 25-30%.

[0016] Preferably, the detector of the ion chromatography described in step (2) is a point-to-point detector, and the chromatographic column is any one of Thermo Fisher AS-22, Metrosep Supp 5-250 or Shenghan SH-AS-52 chromatographic columns; the electrolytic cell temperature is 35°C, the column temperature is 30°C, the injection volume is 25 μL, and the mobile phase speed is 0.2-1.0 mL / min.

[0017] Preferably, the anionic impurity in step (3) is F − , Cl − 、SO4 2− and FSO3 − .

[0018] Further preferably, when FSO3 − The content is >0, lithium bis(fluorosulfonyl)imide is unstable and the shelf life is 10-15 days.

[0019] The beneficial effects of the present invention are: using ion chromatography to establish F − , Cl − 、SO4 2− Standard curve of impurity anions, combined with FSO3 − The content can determine the stability of lithium bis(fluorosulfonyl)imide and determine the shelf life. When the main content meets the national standard, the stability of lithium bis(fluorosulfonyl)imide can be accurately determined and the shelf life can be estimated, which will help promote the research and development, production and storage of LiFSI and avoid the results being affected by the loss of stability of LiFSI. At the same time, it can also help improve the application effect in lithium-ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 F in Example 1 − , Cl − 、SO4 2− The standard curve of F − The standard curve of B is Cl − Standard curve, C is SO4 2− The standard curve of

[0021] Figure 2 This is the ion chromatogram of the three samples in Example 2. DETAILED DESCRIPTION

[0022] The technical solution of the present invention is further explained below in conjunction with the accompanying drawings and specific embodiments. It is worth noting that the following embodiments are only preferred embodiments of the present invention and should not be understood as limiting the present invention. The protection scope of the present invention shall be based on the contents recorded in the claims. Any modification or replacement made by those skilled in the art to the technical solution of the present invention without creative work shall fall within the protection scope of the present invention.

[0023] Example 1 Drawing of standard curve (1) Configuration containing F − , Cl − 、SO4 2− The mixed standard solution is divided into four groups: A, B, C, and D. − and Cl − Concentration is 0.1ppm, SO4 2− The concentration of F in group B is 1.0ppm. − and Cl − Concentration is 0.5ppm, SO4 2− The concentration of F in group C is 2.0ppm. − and Cl − Concentration is 1.0ppm, SO4 2− The concentration of F in group D is 5.0ppm. − and Cl − Concentration is 2.0ppm, SO4 2− The concentration was 10.0ppm; in addition, ultrapure water was used as a blank group; (2) Prepare the mobile phase: weigh 0.3816 g of anhydrous sodium carbonate standard material (purity ≥99.9%) and 1.0080 g of sodium bicarbonate (chromatographic grade, purity ≥99.8%) into a 1 L volumetric flask, add ultrapure water to make up to volume, dissolve by ultrasonication, and then filter through a sand core filter device equipped with a 47 mm*0.45 μm hydrophilic PTFE filter membrane to obtain the mobile phase; (3) Thermo Fisher AS-22 was used as the chromatographic column. The ion chromatograph was turned on at room temperature (20°C). The electrolytic cell temperature was set to 35°C; column temperature: 30°C; injection volume: 25 μL; mobile phase speed: 0.5 mL / min. After the instrument baseline was stable, ultrapure water, A, B, C, and D group samples were injected in turn to obtain F. − , Cl − 、SO4 2− The impurity anion standard curve, where F − The regression equation is y1=0.1544x-0.0021, R 2 =0.99999;Cl − The regression equation is y2=0.1191x+0.0005, R 2 =0.99983; SO4 2− The regression equation is y3=0.0903x-0.0025, R 2 =0.99981.

[0024] Example 2 Detection of LiFSI (1) Take three LiFSI samples with different stabilities and number them 1, 2, and 3 in descending order of stability; (2) In a glove box, accurately weigh 0.3001 g of sample and place it in a 15 mL sample tube, then quantify it to 10.0001 g with ultrapure water to obtain sample solution No. 1, sample solution No. 2, and sample solution No. 3, respectively; (3) Perform ion chromatography according to the method described in Example 1 to obtain an ion chromatogram, and then − , Cl − 、SO4 2− Substitute the value of into the standard curve obtained in Example 1 to calculate its content (Table 1); (4) Analysis of the ion chromatogram revealed that an impurity peak appeared at 8.139 min in the three samples. The impurity peak was determined to be FSO3 by the spike method. − , and then the impurity anion FSO3 is obtained by area integration − The results of the main components are shown in Table 1.

[0025] Table 1 Anionic impurities and main component contents of samples

[0026] As shown in Table 1, although the fluoride ion content of the three samples is quite different, the change of fluoride ion is easily ignored because the range specified by the industry standard is ≤100ppm; therefore, we pay more attention to the sulfate ion with a smaller control range, so as to more sensitively control the quality of LiFSI within the qualified range. Sample 1 has no anionic impurity FSO3 after synthesis.− The generation of anionic impurities SO4 2− The content is within the range of ≤50 ppm specified by the industry standard and is qualified according to the industry standard; Sample 2 contains anionic impurities FSO3 after synthesis − The generation of anionic impurities SO4 2− The content is within the range specified by the industry standard and is qualified according to the industry standard; sample 3 contains anionic impurities FSO3 after synthesis − The generation of anionic impurities SO4 2− If the content exceeds the range specified in the industry standard, it will be directly judged as unqualified.

[0027] Example 3 Stability of LiFSI under dry storage The powders of Sample 1 and Sample 2 in Example 2 were taken, placed in sample tubes, sealed with sealing film, and stored in an argon-sealed glove box. Samples were taken at regular intervals for ion chromatography to detect the main content and the content of anionic impurities. The results are shown in Table 2-3. Table 2 Anion impurity content and main content data of sample 1 after dry storage

[0028] Table 3 Anion impurity content and main content data of sample 2 after dry storage

[0029] As shown in Table 2, under dry storage, sample 1 can remain stable after being stored in argon sealed conditions for 49 days; when stored in the glove box for 77 days, a weak FSO3 − The impurity peak, area integration, shows that its content is 9 ppm; at this time SO4 2− The impurity content of FSO3 − The impurity content continues to increase to 14 ppm, at which point SO4 2− The impurity content has exceeded the standard and is judged to be unqualified.

[0030] As shown in Table 3, under dry storage, the FSO3 of sample 2 was − The impurity content is as high as 189 ppm. When stored in a glove box for 16 days, SO4 2− The impurity content has exceeded the standard and is judged to be unqualified.

[0031] It can be seen that when FSO3 appears in the LiFSI sample − When the product is stable, the shelf life is only 15 days. − It is the key factor affecting the stability of LiFSI.

[0032] Example 4 Stability of LiFSI under wet storage Take 0.3002 g of the powder of sample 1 and sample 2 in Example 2, quantify to 10.0002 g with ultrapure water, ultrasonically dissolve and seal for storage, take samples at regular intervals to detect the main content and anionic impurity content in the samples under wet storage, the results are shown in Tables 4 and 5; Table 4 Anion impurity content and main content data of sample 1 after wet accelerated storage

[0033] Note: Due to FSO3 - It is very easy to decompose in solution, so FSO3 cannot be accurately and effectively detected after wet accelerated storage. - content, so only SO4 is detected 2- content and make a judgment.

[0034] Table 5 Anion impurity content and main content data of sample 2 after wet accelerated storage

[0035] As shown in Table 4-5, under the wet storage condition, SO4 was detected in the solution of sample 1 in just 14 days. 2− The content has exceeded the standard, but sample 2 only needs 4 days, which greatly shortens the detection time.

[0036] FSO3 was detected in sample 2 at day 0 − Anionic impurities, although the current SO4 2− The anion impurity content is qualified, but SO4 2− The anion impurity content exceeded the standard, and SO4 2− The content of anionic impurities exceeds the standard, so early warning can be given through the wet accelerated method.

Claims

1. A method for evaluating the stability of lithium bis(fluorosulfonyl)imide, characterized in that: The steps include: (1) After storing lithium bis(fluorosulfonyl)imide, a liquid sample is obtained after quantification; (2) Ion chromatography detection of the main content of LiFSI and the content of anionic impurities in liquid samples; (3) The stability of lithium bis(fluorosulfonyl)imide was evaluated based on the content of anionic impurities in the main content. The evaluation criteria were: SO4 2- Content>50ppm, FSO3 − When the content is >0ppm, the stability of lithium bis(fluorosulfonyl)imide is unqualified and expired; SO4 2- Content ≤50ppm, FSO3 − When the concentration is >0ppm, the stability of lithium bis(fluorosulfonyl)imide is acceptable and the shelf life is 5-15 days; SO4 2- Content ≤50ppm, FSO3 − =0, lithium bis(fluorosulfonyl)imide has qualified stability and the shelf life is ≥77 days.

2. The method for evaluating the stability of lithium bis(fluorosulfonyl)imide according to claim 1, characterized in that: The storage method described in step (1) is wet storage and dry storage.

3. The method for evaluating the stability of lithium bis(fluorosulfonyl)imide according to claim 2, characterized in that: The wet storage method is to dissolve lithium bis(fluorosulfonyl)imide in water and store it in a sealed state at room temperature; the dry storage method is to store lithium bis(fluorosulfonyl)imide in a sealed state at room temperature.

4. The method for evaluating the stability of lithium bis(fluorosulfonyl)imide according to claim 3, characterized in that: The lithium bis(fluorosulfonyl)imide stored in the dry method is dissolved and then quantitatively obtained to obtain a liquid sample.

5. The method for evaluating the stability of lithium bis(fluorosulfonyl)imide according to claim 1, characterized in that: The concentration of the liquid sample in step (1) is 0.2-0.5 g / mL.

6. The method for evaluating the stability of lithium bis(fluorosulfonyl)imide according to claim 1, characterized in that: The mobile phase of the ion chromatography described in step (2) is a sodium carbonate-sodium bicarbonate-acetonitrile solution filtered through a sand core.

7. The method for evaluating the stability of lithium bis(fluorosulfonyl)imide according to claim 6, characterized in that: The sodium carbonate-sodium bicarbonate-acetonitrile solution has a sodium carbonate concentration of 3-4 mM, a sodium bicarbonate concentration of 4-8 mM, and an acetonitrile volume ratio of 25-30%.

8. The method for evaluating the stability of lithium bis(fluorosulfonyl)imide according to claim 1, characterized in that: The detector of the ion chromatography described in step (2) is a point-to-point detector, and the chromatographic column is any one of Thermo Fisher AS-22, Metrosep Supp5-250 or Shenghan SH-AS-52 chromatographic columns; the electrolytic cell temperature is 35°C, the column temperature is 30°C, the injection volume is 25 μL, and the mobile phase speed is 0.2-1.0 mL / min.

9. The method for evaluating the stability of lithium bis(fluorosulfonyl)imide according to claim 1, characterized in that: The anionic impurity in step (3) is F − , Cl − 、SO4 2− and FSO3 − .

10. The method for evaluating the stability of lithium bis(fluorosulfonyl)imide according to claim 9, characterized in that: When FSO3 − The content is >0, lithium bis(fluorosulfonyl)imide is unstable and the shelf life is 5-15 days.

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