Method for measuring residual NMP (N-Methyl Pyrrolidone) in lithium ion battery pole piece by internal standard method

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a method for determining residual NMP (N-Methyl Pyrrolidone) in a lithium ion battery pole piece by an internal standard method.The method comprises the steps that S1, N-ethyl-2-pyrrolidone serves as an internal standard substance, and multiple groups of standard solutions of NMP and N-ethyl-2-pyrrolidone with different concentration ratios are prepared; s2, analyzing a plurality of groups of standard solutions of NMP and N-ethyl-2-pyrrolidone with different concentration ratios by adopting gas chromatography-mass spectrometry, and drawing a standard curve; s3, carrying out ultrasonic extraction on the lithium ion battery pole piece sample, and filtering to obtain a filtrate; and S4, carrying out gas chromatography-mass spectrometry analysis on the filtrate by using N-ethyl-2-pyrrolidone as an internal standard substance, and determining the content of residual NMP in the lithium ion battery pole piece sample. The method can accurately quantify the residual NMP content in the lithium ion battery pole piece.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a method for determining residual NMP in lithium-ion battery electrodes using the internal standard method. Background Technology

[0002] In the homogenization process of lithium-ion batteries, N-methylpyrrolidone (NMP) acts as a solvent / dispersant, effectively dissolving binders such as PVDF and dispersing lithium salts and conductive materials, ensuring thorough and uniform contact among the components. Due to the porous structure of the electrode, a small amount of NMP inevitably remains. Excessive NMP residue can negatively impact lithium-ion batteries in several ways, including capacity decay, increased internal resistance, and shortened cycle life. Therefore, detecting residual NMP in lithium-ion battery electrodes is crucial. Only by accurately controlling its content can the risk of performance degradation in lithium-ion batteries be effectively mitigated, thereby enhancing their competitiveness in the market.

[0003] Methods for detecting residual NMP in lithium-ion battery electrodes mainly include ultraviolet-visible spectrophotometry (UV-Vis), infrared spectroscopy (IR), high-performance liquid chromatography (HPLC), gas chromatography (GC), and gas chromatography-mass spectrometry (GC-MS). In practical applications, the selection of a detection method must consider factors such as the composition of the lithium-ion battery electrode, the concentration range of residual NMP, and detection efficiency. Among these, GC and GC-MS are currently relatively suitable methods for detecting residual NMP in lithium-ion battery electrodes due to their high sensitivity and accuracy. In the detection of residual NMP in lithium-ion battery electrodes, minimizing the impact of factors such as electrode size inconsistency, solvent evaporation, and instrument stability during the pretreatment stage, thereby improving the accuracy and stability of the analytical results, has become a core focus in this field.

[0004] Patent application CN104535689A discloses a gas chromatography method for the detection of N-methylpyrrolidone (NMP) in lithium-ion battery electrodes. This method employs solvent extraction combined with external standard gas chromatography, and achieves rapid analysis of NMP content through steps such as sample preparation, standard solution preparation, and spike recovery. This method is simple to operate, has good repeatability, and can effectively detect NMP residues in electrodes, providing technical support for optimizing electrode baking processes and controlling battery quality.

[0005] Patent application CN107884484A discloses a method for determining the NMP content in lithium-ion battery electrodes. The method involves ultrasonic extraction of NMP from the lithium-ion battery electrodes, followed by filtration and purification. The solution is then analyzed using gas chromatography-mass spectrometry (GC-MS), and NMP is quantified using an external standard method. This method has a low detection limit, good repeatability, and can accurately determine the NMP content in lithium-ion battery electrodes, playing an important monitoring role in the lithium-ion battery manufacturing process.

[0006] Patent application CN116953103A discloses a method for determining the residual NMP content in lithium-ion battery electrodes using an internal standard method. This method uses chlorobenzene as an internal standard, determines the relative correction factor by preparing multiple sets of standard solutions, and combines this with gas chromatography analysis of the electrode extract. This method uses internal standard quantification, is simple and convenient to operate, and improves the stability and accuracy of the detection.

[0007] However, looking at the existing technologies mentioned above, the field still faces the following challenges in the detection process: 1. External standard method quantification is affected by instrument injection stability and detector sensitivity drift, which easily leads to fluctuations in detection results; 2. The accuracy of gas chromatography (GC) in detecting low levels of residual NMP needs to be improved.

[0008] The specific description is as follows:

[0009] (1) Patent applications CN104535689A and CN107884484A use the external standard method for quantitative analysis. The external standard method relies on the accuracy of the standard curve and is prone to errors due to changes in operating conditions (such as injection volume and chromatographic conditions), resulting in distorted test results. After long-term use, changes in instrument performance require frequent calibration of the standard curve, which increases the complexity of operation.

[0010] (2) Patent application CN116953103A uses gas chromatography with an internal standard method to determine the residual NMP content in lithium-ion battery electrodes. The internal standard used, chlorobenzene (boiling point 132℃), is highly toxic and volatile. This prior art has weak qualitative ability using gas chromatography. When faced with lithium-ion battery electrode samples with complex compositions, gas chromatography has difficulty in effectively distinguishing impurities with retention times similar to NMP. Furthermore, for lithium-ion battery electrodes with low residual NMP content, the detection accuracy of gas chromatography needs to be improved. Summary of the Invention

[0011] In view of this, the technical problem to be solved by the present invention is to provide a method for determining residual NMP in lithium-ion battery electrodes using the internal standard method, which can accurately quantify the residual NMP content in lithium-ion battery electrodes.

[0012] This invention provides a method for determining residual NMP in lithium-ion battery electrodes using the internal standard method, comprising the following steps:

[0013] S1. Using N-ethyl-2-pyrrolidone as an internal standard, prepare multiple sets of standard solutions of NMP and N-ethyl-2-pyrrolidone with different concentration ratios.

[0014] S2. Gas chromatography-mass spectrometry was used to analyze multiple groups of standard solutions of NMP and N-ethyl-2-pyrrolidone with different concentration ratios, and standard curves were plotted.

[0015] S3. The lithium-ion battery electrode sample is subjected to ultrasonic extraction, filtered, and the filtrate is obtained.

[0016] S4. The filtrate was analyzed by gas chromatography-mass spectrometry using N-ethyl-2-pyrrolidone as an internal standard to determine the residual NMP content in the lithium-ion battery electrode sample.

[0017] Preferably, step S1 includes: preparing NMP standard stock solution and N-ethyl-2-pyrrolidone internal standard stock solution, diluting and bringing to volume with organic solvents according to the proportions, shaking and then using them for later use.

[0018] Preferably, step S2 includes:

[0019] Gas chromatography-mass spectrometry analysis was used, and the peak area of ​​NMP was denoted as A. NMP The peak area of ​​the internal standard N-ethyl-2-pyrrolidone is denoted as A. s Calculate the ratio of their areas, denoted as A. NMP / A s ;

[0020] The concentration of NMP is denoted as C. NMP The concentration of the internal standard N-ethyl-2-pyrrolidone is denoted as C. s Calculate the concentration ratio of the two, denoted as C. NMP / C s ;

[0021] The ratio A of the peak areas of each group of standard solutions was obtained. NMP / A s and concentration ratio C NMP / C s Afterwards, with A NMP / A s C is the x-axis. NMP / C s Plot the standard curve of NMP with N-ethyl-2-pyrrolidone as the internal standard, using y as the ordinate.

[0022] Preferably, the conditions for the gas chromatography analysis include:

[0023] The chromatographic column was a DB-WAX, the stationary phase was polyethylene glycol, the column length was 30m, the inner diameter was 0.25mm, and the film thickness was 0.25μm;

[0024] Carrier gas: Helium greater than 99.999%; Inlet temperature: 240℃; Injection volume: 1μL; Split ratio: 5:1; Constant flow rate: 1mL / min; Temperature program: Initial temperature 80℃, hold for 3.5min, increase to 240℃ at a rate of 20℃ / min, and hold for another 2~5min.

[0025] Preferably, the conditions for the mass spectrometry analysis include:

[0026] Ion source temperature 230℃; quadrupole 150℃; transfer line temperature 250℃; solvent delay 4.5 min; dual-channel scanning: Scan range 10~200 m / z, SIM scan, characteristic ions of NMP 99, 98, 44, characteristic ions of N-ethyl-2-pyrrolidone 98, 113, 70.

[0027] Preferably, in step S3, before ultrasonic extraction of the lithium-ion battery electrode sample, the method further includes: cutting the lithium-ion battery electrode sample into round pieces with a diameter of 8-11 mm.

[0028] After cutting the lithium-ion battery electrode sample, the process also includes:

[0029] Weigh a lithium-ion battery electrode sample with mass m, mix it with an organic solvent of volume V1 in a sample bottle, and seal it.

[0030] Preferably, step S4 is as follows:

[0031] Transfer N-ethyl-2-pyrrolidone secondary internal standard stock solution and mix it with the filtrate obtained in step S3 in an equal volume in a gas chromatography sample vial. Shake well to obtain the test solution.

[0032] Gas chromatography-mass spectrometry (GC-MS) was used to analyze the NMP content (X) in the lithium-ion battery electrode sample and, based on the plotted standard curve, the content was calculated. NMP ;

[0033] X NMP =2×C' NMP ×V1 / m;

[0034] Among them, X NMP The NMP content in the electrode sample is expressed in μg / g; C' NMP V1 is the NMP concentration of the test solution in μg / mL; V1 is the volume of the ultrasonic extraction solvent in mL; m is the mass of the lithium-ion battery electrode sample weighed in g; and 2 is the volumetric dilution factor.

[0035] Preferably, the conditions for the gas chromatography analysis include:

[0036] The chromatographic column was a DB-WAX, the stationary phase was polyethylene glycol, the column length was 30m, the inner diameter was 0.25mm, and the film thickness was 0.25μm;

[0037] Carrier gas: Helium greater than 99.999%; Inlet temperature: 240℃; Injection volume: 1μL; Split ratio: 5:1; Constant flow rate: 1mL / min; Temperature program: Initial temperature 80℃, hold for 3.5min, increase to 240℃ at a rate of 20℃ / min, and hold for another 2~5min.

[0038] Preferably, the ultrasonic extraction power is 160~400 W, the temperature is 22~26℃, and the time is 5~45 min;

[0039] After ultrasonic extraction, the process further includes: standing; the standing time is 3-5 minutes.

[0040] Preferably, the filtration uses a 0.45μm organic needle filter.

[0041] This invention optimizes the sample pretreatment process, avoiding the influence of sample inhomogeneity and solvent evaporation during ultrasonic extraction. It employs an internal standard method, selecting N-ethyl-2-pyrrolidone (NEP) as the internal standard, adhering to the principle that the peak position of the internal standard should be as close as possible to the peak position of the analyte, yet completely separate. By using the internal standard method and calculating the peak area ratio of the analyte to the internal standard, the influence of injection volume fluctuations and some chromatographic condition changes on the test results can be effectively offset, improving the accuracy and stability of the test. Combined with gas chromatography-mass spectrometry (GC-MS), it significantly enhances the detection capability of low concentrations of NMP in lithium-ion battery electrodes. Detailed Implementation

[0042] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0043] This invention provides a method for determining residual NMP in lithium-ion battery electrodes using the internal standard method, comprising the following steps:

[0044] S1. Using N-ethyl-2-pyrrolidone (NEP) as an internal standard, prepare multiple sets of standard solutions of N-methylpyrrolidone (NMP) and N-ethyl-2-pyrrolidone (NEP) with different concentration ratios.

[0045] S2. Gas chromatography-mass spectrometry was used to analyze multiple groups of standard solutions of NMP and N-ethyl-2-pyrrolidone with different concentration ratios, and standard curves were plotted.

[0046] S3. The lithium-ion battery electrode sample is subjected to ultrasonic extraction, filtered, and the filtrate is obtained.

[0047] S4. The filtrate was analyzed by gas chromatography-mass spectrometry using N-ethyl-2-pyrrolidone as an internal standard to determine the residual NMP content in the lithium-ion battery electrode sample.

[0048] Regarding step S1:

[0049] Using N-ethyl-2-pyrrolidone (NEP) as an internal standard, several standard solutions of N-methylpyrrolidone (NMP) and N-ethyl-2-pyrrolidone (NEP) with different concentration ratios were prepared.

[0050] Specifically, including:

[0051] Prepare NMP standard stock solution and N-ethyl-2-pyrrolidone internal standard stock solution, respectively, dilute with organic solvent according to the ratio, and shake before use.

[0052] More specifically, including:

[0053] Accurately weigh the NMP standard sample and the NEP standard sample, prepare NMP standard stock solution and NEP internal standard stock solution respectively, and dilute them with organic solvents according to the ratio to six 100mL volumetric flasks, and shake repeatedly.

[0054] This invention uses NEP as an internal standard to ensure the reliability of the peak area ratio during quantitative calculation, and has the characteristics of high boiling point (203.6℃), low volatility, and lower toxicity compared with chlorobenzene.

[0055] In some embodiments of the present invention, the concentration of the NEP internal standard stock solution is 2000 μg / mL. The NEP internal standard stock solution is prepared by mixing NEP with an organic solvent. The standard curve is plotted using a primary NEP internal standard stock solution with a concentration of 100 μg / mL. This primary NEP internal standard stock solution is prepared by mixing the NEP internal standard stock solution with an organic solvent. The residual NMP content in the sample is determined using a secondary NEP internal standard stock solution with a concentration of 20 μg / mL. This secondary NEP internal standard stock solution is prepared by mixing the primary NEP internal standard stock solution with an organic solvent.

[0056] In some embodiments of the present invention, a secondary NMP standard stock solution with a concentration of 100 μg / mL is used to plot the standard curve. The secondary NMP standard stock solution is prepared by mixing a primary NMP standard stock solution with an organic solvent. The primary NMP standard stock solution has a concentration of 1000 μg / mL. The primary NMP standard stock solution is prepared by mixing NMP with an organic solvent.

[0057] In some embodiments of the present invention, the organic solvent is selected from acetone, ethanol, methanol, cyclohexane, toluene, acetonitrile, ethyl acetate, tetrahydrofuran, dichloromethane, chloroform, or petroleum ether, all of which are chromatographic grade.

[0058] Regarding step S2:

[0059] Gas chromatography-mass spectrometry was used to analyze multiple sets of standard solutions of NMP and N-ethyl-2-pyrrolidone at different concentration ratios, and standard curves were plotted, including:

[0060] Gas chromatography-mass spectrometry analysis was used, and the peak area of ​​NMP was denoted as A. NMP The peak area of ​​the internal standard NEP is denoted as A. s Calculate the ratio of their areas, denoted as A. NMP / A s ;

[0061] The concentration of NMP is denoted as C. NMP The concentration of the internal standard NEP is denoted as C. s Calculate the concentration ratio of the two, denoted as C. NMP / C s ;

[0062] Obtain the peak area ratio (A) of each set of standard solutions (which are also the multiple sets of standard solutions with different concentration ratios in step S1). NMP / A s ) and concentration ratio (C NMP / C s After that, with A NMP / A s C is the x-axis. NMP / C s Plot the standard curve of NMP with NEP as the internal standard, using NEP as the ordinate.

[0063] In some embodiments of the present invention, the conditions for the gas chromatography analysis include:

[0064] The chromatographic column was a DB-WAX, the stationary phase was polyethylene glycol (PEG), the column length was 30m, the inner diameter was 0.25mm, and the film thickness was 0.25μm;

[0065] Carrier gas: Helium greater than 99.999%; Inlet temperature: 240℃; Injection volume: 1μL; Split ratio: 5:1; Constant flow rate: 1mL / min; Temperature program: Initial temperature 80℃, hold for 3.5min, increase to 240℃ at a rate of 20℃ / min, and hold for another 2~5min (e.g., 2min).

[0066] In some embodiments of the present invention, the conditions for the mass spectrometry analysis include:

[0067] Ion source temperature 230℃; quadrupole 150℃; transfer line temperature 250℃; solvent delay 4.5 min; dual-channel scanning: Scan range 10~200 m / z, SIM scan, characteristic ions of NMP 99, 98, 44, characteristic ions of N-ethyl-2-pyrrolidone 98, 113, 70. Regarding step S3:

[0068] The lithium-ion battery electrode sample was subjected to ultrasonic extraction, and after filtration, the filtrate was obtained.

[0069] In some embodiments of the present invention, before ultrasonic extraction of the lithium-ion battery electrode sample, the method further includes cutting the lithium-ion battery electrode sample. Specifically, this includes cutting the lithium-ion battery electrode sample into circular pieces with a diameter of 8-11 mm (e.g., 11 mm, 8 mm). This ensures that the surface area of ​​the lithium-ion battery electrode sample is standardized and consistent during ultrasonic extraction, avoiding excessive deviations in extraction effect due to size differences.

[0070] In some embodiments of the present invention, after cutting the lithium-ion battery electrode sample, the method further includes:

[0071] Weigh a lithium-ion battery electrode sample with mass m, mix it with an organic solvent of volume V1 in a sample bottle, and seal it.

[0072] The organic solvent is the same as above.

[0073] In some embodiments of the present invention, m is 0.8~1.3 g, such as 1.0 g or 1.3 g. V1 is 20~80 mL, such as 40 mL.

[0074] In some embodiments of the present invention, the ultrasonic extraction power is 160~400 W, such as 200 W or 300 W; the temperature is 22~26℃, such as 25℃ or 26℃; and the time is 5~45 min, such as 30 min or 45 min. During the ultrasonic extraction process, the temperature of the water in the ultrasonic tank is controlled by water circulation. After the ultrasonic extraction, the process further includes: settling. The settling time is 3~5 min, such as 3 min or 4 min.

[0075] This invention utilizes ultrasonic extraction to extract NMP from lithium-ion battery electrode samples. The extract, containing NMP, is then filtered to obtain an NMP filtrate. In some embodiments of this invention, the ultrasonic extraction is followed by: extracting the extract. Specifically, a disposable syringe is used to extract the ultrasonically extracted solution to obtain the extract.

[0076] The obtained extract was filtered.

[0077] In some embodiments of the present invention, the filtration employs a 0.45 μm organic needle filter.

[0078] Regarding step S4:

[0079] The filtrate was analyzed by gas chromatography-mass spectrometry using NEP as an internal standard to determine the residual NMP content in the lithium-ion battery electrode sample.

[0080] Specifically, including:

[0081] Transfer NEP secondary internal standard stock solution and mix it with the filtrate obtained in step S3 (lithium-ion battery electrode filtrate) in an equal volume in a gas chromatograph vial, shake well, and obtain the test solution;

[0082] Gas chromatography-mass spectrometry (GC-MS) was used to analyze the NMP content (X) in the lithium-ion battery electrode sample and, based on the plotted standard curve, the content was calculated. NMP ;

[0083] X NMP =2×C' NMP ×V1 / m;

[0084] Among them, X NMP The NMP content in the electrode sample is expressed in μg / g, C'. NMP V1 is the NMP concentration of the test solution, in μg / mL; V1 is the volume of ultrasonic extraction solvent, in mL; m is the mass of the lithium-ion battery electrode sample weighed, in g; and 2 is the volumetric dilution factor.

[0085] In some embodiments of the present invention, the volume of NEP secondary internal standard stock solution transferred is 0.5 mL, and the volume of the filtrate (lithium-ion battery electrode filtrate) obtained in step S3 is 0.5 mL.

[0086] In some embodiments of the present invention, the conditions for the gas chromatography analysis include:

[0087] The chromatographic column was a DB-WAX, the stationary phase was polyethylene glycol (PEG), the column length was 30m, the inner diameter was 0.25mm, and the film thickness was 0.25μm;

[0088] Carrier gas: Helium greater than 99.999%; Inlet temperature: 240℃; Injection volume: 1μL; Split ratio: 5:1; Constant flow rate: 1mL / min; Temperature program: Initial temperature 80℃, hold for 3.5min, increase to 240℃ at a rate of 20℃ / min, and hold for another 2~5min (e.g., 2min).

[0089] In some embodiments of the present invention, the conditions for the mass spectrometry analysis include:

[0090] Ion source temperature 230℃; quadrupole 150℃; transfer line temperature 250℃; solvent delay 4.5 min; dual-channel scanning: Scan range 10~200 m / z, SIM scan, characteristic ions of NMP 99, 98, 44, characteristic ions of N-ethyl-2-pyrrolidone 98, 113, 70.

[0091] Beneficial effects:

[0092] This invention discloses a gas chromatography-mass spectrometry (GC-MS) method based on the internal standard method for determining residual NMP in lithium-ion battery electrodes. NMP in the lithium-ion battery electrode is extracted by ultrasonication, filtered, and then an internal standard stock solution is added and shaken well. The sample solution is then analyzed by GC-MS to select characteristic ions of NMP, and quantification is performed using the internal standard method, thereby accurately determining the NMP content in the lithium-ion battery electrode sample. This invention features simple sample processing, strong qualitative capability using GC-MS, minimal susceptibility to instrument fluctuations due to the internal standard method, and stable and reliable test data.

[0093] The present invention does not impose any special restrictions on the source of the raw materials used above, and they can be commercially available.

[0094] To further illustrate the present invention, the following describes in detail, with reference to embodiments, a method for determining residual NMP in lithium-ion battery electrodes using the internal standard method provided by the present invention, but this should not be construed as limiting the scope of protection of the present invention.

[0095] Example 1

[0096] 1. Preparation of standard solutions:

[0097] NEP internal standard stock solution: Weigh 0.2 g (accurate to 0.1 mg) of internal standard NEP and dilute to 100 mL with chromatographic grade ethanol to obtain a concentration of 2000 μg / mL; NEP primary internal standard stock solution: Accurately transfer 5 mL of internal standard stock solution and dilute to 100 mL with chromatographic grade ethanol to obtain a concentration of 100 μg / mL; NEP secondary internal standard stock solution: Accurately transfer 20 mL of NEP primary internal standard stock solution and dilute to 100 mL with chromatographic grade ethanol to obtain a concentration of 20 μg / mL;

[0098] NMP Primary Standard Stock Solution: Accurately weigh 0.1 g (accurate to 0.1 mg) of NMP and dilute to 100 mL with chromatographic grade ethanol to obtain a concentration of 1000 μg / mL; NMP Secondary Standard Stock Solution: Accurately transfer 10 mL of the Primary Standard Stock Solution and dilute to 100 mL with chromatographic grade ethanol to obtain a concentration of 100 μg / mL.

[0099] Accurately transfer 0.25 mL, 0.5 mL, 1 mL, 1.5 mL, 2.5 mL, and 4 mL of 100 μg / mL NMP secondary standard stock solution, and 1 mL of 100 μg / mL NEP primary internal standard stock solution, respectively, to 10 mL volumetric flasks with chromatographic grade ethanol. The concentrations of these standard solutions are 2.5 μg / mL, 5 μg / mL, 10 μg / mL, 15 μg / mL, 25 μg / mL, and 40 μg / mL, respectively, and the internal standard concentration is 10 μg / mL.

[0100] 2. The conditions for gas chromatography analysis are:

[0101] The chromatographic column was a DB-WAX, the stationary phase was polyethylene glycol (PEG), the column length was 30m, the inner diameter was 0.25mm, and the film thickness was 0.25μm;

[0102] Carrier gas: Helium greater than 99.999%; Inlet temperature: 240℃; Injection volume: 1μL; Split ratio: 5:1; Constant flow rate: 1mL / min; Temperature program: Initial temperature 80℃, hold for 3.5min, increase to 240℃ at a rate of 20℃ / min, and hold for another 2min.

[0103] The conditions for mass spectrometry analysis are:

[0104] Ion source temperature 230℃; quadrupole 150℃; transfer line temperature 250℃; solvent delay 4.5 min; dual-channel scanning: Scan range 10~200 m / z, SIM scan, characteristic ions of NMP 99, 98, 44, characteristic ions of N-ethyl-2-pyrrolidone 98, 113, 70.

[0105] Gas chromatography-mass spectrometry was used to analyze multiple sets of standard solutions of NMP and N-ethyl-2-pyrrolidone at different concentration ratios, and standard curves were plotted.

[0106] Let A be the peak area of ​​NMP. NMP The peak area of ​​the internal standard NEP is denoted as A. s Calculate the ratio of their areas, denoted as A. NMP / A s ;

[0107] The concentration of NMP is denoted as C. NMP The concentration of the internal standard NEP is denoted as C. s Calculate the concentration ratio of the two, denoted as C. NMP / C s ;

[0108] Obtain the peak area ratio (A) of each set of standard solutions (which are also the multiple sets of standard solutions with different concentration ratios in step 1). NMP / A s) and concentration ratio (C NMP / C s After that, with A NMP / A s C is the x-axis. NMP / C s Plot the standard curve of NMP with NEP as the internal standard, using NEP as the ordinate.

[0109] 3. Using a circular sampler for lithium-ion battery electrode areal density, cut the coated, rolled, and baked lithium-ion battery electrodes into 11mm diameter discs. Weigh 1.0g of the cut lithium-ion battery electrode disc sample (mass denoted as m) and place it into sample vials with masses of 1.0067g (sample 1), 1.0073g (sample 2), and 1.0075g (sample 3). Add 40mL of chromatographic grade ethanol (volume denoted as V1) to the sample vials, seal them, and place the vials in an ultrasonic cleaner for ultrasonic extraction for 45min at an ultrasonic power of 200W. Use circulating water during ultrasonic extraction and maintain the temperature at 25℃. After ultrasonic extraction, let it stand for 3min. Then, use a disposable syringe to extract the ultrasonically extracted solution and filter it through a 0.45μm organic needle filter into a sample vial to obtain the lithium-ion battery electrode filtrate.

[0110] 4. Transfer 0.5 mL of NEP secondary internal standard stock solution and mix it with 0.5 mL of the filtrate obtained in step 3 (lithium-ion battery electrode filtrate) in a gas chromatograph vial, shake well, and obtain the test solution;

[0111] Gas chromatography-mass spectrometry analysis was used to determine the NMP content X in the lithium-ion battery electrode sample based on the plotted standard curve. NMP ;

[0112] X NMP =2×C' NMP ×V1 / m;

[0113] Among them, X NMP The NMP content in the electrode sample is expressed in μg / g, C'. NMP V1 is the NMP concentration of the test solution, in μg / mL; V1 is the volume of ultrasonic extraction solvent, in mL; m is the mass of the lithium-ion battery electrode sample weighed, in g; and 2 is the volumetric dilution factor.

[0114] The test results of the three samples mentioned in step 3 are shown in Table 1.

[0115] Table 1. Test results of the three samples.

[0116]

[0117] Example 2

[0118] 1. Preparation of standard solutions:

[0119] The difference from Example 1 is that chromatographic grade ethanol is replaced with chromatographic grade acetone; specifically:

[0120] NEP internal standard stock solution: Weigh 0.2 g (accurate to 0.1 mg) of internal standard NEP and dilute to 100 mL with chromatographic grade acetone to obtain a concentration of 2000 μg / mL; NEP primary internal standard stock solution: Accurately transfer 5 mL of internal standard stock solution and dilute to 100 mL with chromatographic grade acetone to obtain a concentration of 100 μg / mL; NEP secondary internal standard stock solution: Accurately transfer 20 mL of NEP primary internal standard stock solution and dilute to 100 mL with chromatographic grade acetone to obtain a concentration of 20 μg / mL;

[0121] NMP Primary Standard Stock Solution: Accurately weigh 0.1 g (accurate to 0.1 mg) of NMP and dilute to 100 mL with chromatographic grade acetone, resulting in a concentration of 1000 μg / mL; NMP Secondary Standard Stock Solution: Accurately transfer 10 mL of the Primary Standard Stock Solution and dilute to 100 mL with chromatographic grade acetone, resulting in a concentration of 100 μg / mL.

[0122] Accurately transfer 0.25 mL, 0.5 mL, 1 mL, 1.5 mL, 2.5 mL, and 4 mL of 100 μg / mL NMP secondary standard stock solution, and 1 mL of 100 μg / mL NEP primary internal standard stock solution, respectively, to 10 mL volumetric flasks with chromatographic-grade acetone. The concentrations of these standard solutions are 2.5 μg / mL, 5 μg / mL, 10 μg / mL, 15 μg / mL, 25 μg / mL, and 40 μg / mL, respectively, and the internal standard concentration is 10 μg / mL.

[0123] 2. The conditions for gas chromatography analysis are:

[0124] Same as Example 1.

[0125] The conditions for mass spectrometry analysis are:

[0126] Same as Example 1.

[0127] Gas chromatography-mass spectrometry was used to analyze multiple sets of standard solutions of NMP and N-ethyl-2-pyrrolidone at different concentration ratios, and standard curves were plotted.

[0128] Let A be the peak area of ​​NMP. NMP The peak area of ​​the internal standard NEP is denoted as A. s Calculate the ratio of their areas, denoted as A. NMP / A s ;

[0129] The concentration of NMP is denoted as C.NMP The concentration of the internal standard NEP is denoted as C. s Calculate the concentration ratio of the two, denoted as C. NMP / C s ;

[0130] Obtain the peak area ratio (A) of each set of standard solutions (which are also the multiple sets of standard solutions with different concentration ratios in step 1). NMP / A s ) and concentration ratio (C NMP / C s After that, with A NMP / A s C is the x-axis. NMP / C s Plot the standard curve of NMP with NEP as the internal standard, using NEP as the ordinate.

[0131] 3. Using a circular sampler for lithium-ion battery electrode areal density, cut the coated, rolled, and baked lithium-ion battery electrodes into 8mm diameter discs. Weigh 1.3g of the cut lithium-ion battery electrode disc sample (mass denoted as m) and place it into sample vials with masses of 1.2920g (sample 1), 1.2842g (sample 2), and 1.2901g (sample 3). Add 40mL of chromatographic grade acetone (volume denoted as V1) to the sample vials, seal them, and place the vials in an ultrasonic cleaner for ultrasonic extraction for 30min at an ultrasonic power of 300W. Use circulating water during ultrasonic extraction and maintain the temperature at 26℃. After ultrasonic extraction, let it stand for 4min. Then, use a disposable syringe to extract the ultrasonically extracted solution and filter it through a 0.45μm organic needle filter into a sample vial to obtain the lithium-ion battery electrode filtrate.

[0132] 4. Transfer 0.5 mL of NEP secondary internal standard stock solution and mix it with 0.5 mL of the filtrate obtained in step 3 (lithium-ion battery electrode filtrate) in a gas chromatograph vial, shake well, and obtain the test solution;

[0133] Gas chromatography-mass spectrometry analysis was used to determine the NMP content X in the lithium-ion battery electrode sample based on the plotted standard curve. NMP ;

[0134] X NMP =2×C' NMP ×V1 / m;

[0135] Among them, X NMP The NMP content in the electrode sample is expressed in μg / g, C'. NMPV1 is the NMP concentration of the test solution, in μg / mL; V1 is the volume of ultrasonic extraction solvent, in mL; m is the mass of the lithium-ion battery electrode sample weighed, in g; and 2 is the volumetric dilution factor.

[0136] The test results of the three samples mentioned in step 3 are shown in Table 2.

[0137] Table 2 Test results of the three samples

[0138]

[0139] Comparative Example 1

[0140] The difference from Example 2 is as follows:

[0141] Replace NEP with chlorobenzene.

[0142] The remaining steps and parameters are the same as in Example 2.

[0143] The test results of the three samples are shown in Table 3.

[0144] Table 3 Test results of the three samples

[0145]

[0146] Comparative Example 2

[0147] The difference from Example 2 is as follows:

[0148] Replace NEP with N,N-dimethylacetamide.

[0149] The remaining steps and parameters are the same as in Example 2.

[0150] The test results of the three samples are shown in Table 4.

[0151] Table 4. Test results of the three samples.

[0152]

[0153] Comparative Example 3

[0154] The difference from Example 2 is as follows:

[0155] Replace the gas chromatography-mass spectrometry analysis method in steps 2 and 4 with gas chromatography. The conditions for gas chromatography analysis are the same as in Example 2.

[0156] The remaining steps and parameters are the same as in Example 2.

[0157] The test results for the three samples are shown in Table 5.

[0158] Table 5. Test results of the three samples.

[0159]

[0160] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for determining residual NMP in lithium-ion battery electrodes using the internal standard method, comprising the following steps: S1. Using N-ethyl-2-pyrrolidone as an internal standard, prepare multiple sets of standard solutions of NMP and N-ethyl-2-pyrrolidone with different concentration ratios. S2. Gas chromatography-mass spectrometry was used to analyze multiple groups of standard solutions of NMP and N-ethyl-2-pyrrolidone with different concentration ratios, and standard curves were plotted. S3. The lithium-ion battery electrode sample is subjected to ultrasonic extraction, filtered, and the filtrate is obtained. S4. The filtrate was analyzed by gas chromatography-mass spectrometry using N-ethyl-2-pyrrolidone as an internal standard to determine the residual NMP content in the lithium-ion battery electrode sample.

2. The method according to claim 1, characterized in that, Step S1 includes: preparing NMP standard stock solution and N-ethyl-2-pyrrolidone internal standard stock solution, diluting and bringing to volume with organic solvents according to the proportions, shaking and then using them for later use.

3. The method according to claim 1, characterized in that, Step S2 includes: Gas chromatography-mass spectrometry analysis was used, and the peak area of ​​NMP was denoted as A. NMP The peak area of ​​the internal standard N-ethyl-2-pyrrolidone is denoted as A. s Calculate the ratio of their areas, denoted as A. NMP / A s ; Let the concentration of NMP be denoted as C. NMP The concentration of the internal standard N-ethyl-2-pyrrolidone is denoted as C. s Calculate the concentration ratio of the two, denoted as C. NMP / C s ; The ratio A of the peak areas of each group of standard solutions was obtained. NMP / A s and concentration ratio C NMP / C s Afterwards, with A NMP / A s C is the x-axis. NMP / C s Plot the standard curve of NMP with N-ethyl-2-pyrrolidone as the internal standard, using y as the ordinate.

4. The method according to claim 3, characterized in that, The conditions for the gas chromatography analysis include: The chromatographic column was a DB-WAX, the stationary phase was polyethylene glycol, the column length was 30m, the inner diameter was 0.25mm, and the film thickness was 0.25μm; Carrier gas: Helium greater than 99.999%; Inlet temperature: 240℃; Injection volume: 1μL; Split ratio: 5:1; Constant flow rate: 1mL / min; Temperature program: Initial temperature 80℃, hold for 3.5min, increase to 240℃ at a rate of 20℃ / min, and hold for another 2~5min.

5. The method according to claim 3, characterized in that, The conditions for the mass spectrometry analysis include: Ion source temperature 230℃; quadrupole 150℃; transfer line temperature 250℃; solvent delay 4.5 min; dual-channel scanning: Scan range 10~200 m / z, SIM scan, characteristic ions of NMP 99, 98, 44, characteristic ions of N-ethyl-2-pyrrolidone 98, 113, 70.

6. The method according to claim 3, characterized in that, In step S3, before ultrasonic extraction of the lithium-ion battery electrode sample, the following steps are also included: cutting the lithium-ion battery electrode sample into round pieces with a diameter of 8-11 mm. After cutting the lithium-ion battery electrode sample, the process also includes: Weigh a lithium-ion battery electrode sample with mass m, mix it with an organic solvent of volume V1 in a sample bottle, and seal it.

7. The method according to claim 6, characterized in that, Step S4 is: Transfer N-ethyl-2-pyrrolidone secondary internal standard stock solution and mix it with the filtrate obtained in step S3 in an equal volume in a gas chromatography sample vial. Shake well to obtain the test solution. Gas chromatography-mass spectrometry (GC-MS) was used to analyze the NMP content (X) in the lithium-ion battery electrode sample and, based on the plotted standard curve, the content was calculated. NMP ; X NMP =2×C' NMP ×V1 / m; Among them, X NMP The NMP content in the electrode sample is expressed in μg / g; C' NMP V1 is the NMP concentration of the test solution in μg / mL; V1 is the volume of the ultrasonic extraction solvent in mL; m is the mass of the lithium-ion battery electrode sample weighed in g; and 2 is the volumetric dilution factor.

8. The method according to claim 7, characterized in that, The conditions for the gas chromatography analysis include: The chromatographic column was a DB-WAX, the stationary phase was polyethylene glycol, the column length was 30m, the inner diameter was 0.25mm, and the film thickness was 0.25μm; Carrier gas: Helium greater than 99.999%; Inlet temperature: 240℃; Injection volume: 1μL; Split ratio: 5:1; Constant flow rate: 1mL / min; Temperature program: Initial temperature 80℃, hold for 3.5min, increase to 240℃ at a rate of 20℃ / min, and hold for another 2~5min.

9. The method according to claim 1, characterized in that, The ultrasonic extraction power is 160~400 W, the temperature is 22~26℃, and the time is 5~45 min; After ultrasonic extraction, the process further includes: standing; the standing time is 3-5 minutes.

10. The method according to claim 1, characterized in that, The filtration uses a 0.45μm organic needle filter.

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