Method for detecting content of NMP in liquid

Abstract

The invention provides a method for detecting the content of NMP (N-Methyl Pyrrolidone) in liquid, which comprises the following steps: 1) preparing a standard curve working solution containing an internal standard substance, carrying out on-machine detection on the standard curve working solution by adopting a liquid chromatograph-mass spectrometer, and fitting according to the detection result of the standard curve working solution to obtain a standard curve equation; and 2) carrying out on-machine detection on the to-be-detected liquid containing the internal standard substance by using a liquid chromatograph-mass spectrometer, and calculating the content of NMP in the to-be-detected liquid according to the standard curve equation and the NMP quantitative ion peak area obtained by detection. Compared with a traditional gas chromatography-mass spectrometry technology, the detection method has the advantages that analysis errors caused by uneven sample preparation, carrier gas flow fluctuation, detector response difference and the like can be effectively eliminated, high-precision measurement is realized, secondly, the influence of external factors such as temperature, humidity and air pressure change on a measurement result is avoided, and the detection accuracy is improved. And the data reliability is enhanced.

Description

Technical Field

[0001] The present invention relates to the field of detection and analysis technology, and in particular to a method for detecting NMP content in a liquid. Background Art

[0002] In the lithium battery industry, N-methylpyrrolidone (NMP) is widely used as a solvent for the preparation of positive electrode slurry and an additive for the preparation of negative electrode slurry. However, after the positive and negative electrode slurries are coated, the NMP solvent needs to be removed as much as possible after being baked in an oven. Once the NMP residue is too high, it will seriously affect the safety performance of the battery. Therefore, it is of great significance to accurately and efficiently detect the NMP content in the electrode sheet.

[0003] Traditional NMP detection methods mostly use the external standard method of gas chromatography-mass spectrometry (GC-MS) technology, that is, comparing the NMP in the sample with a standard substance of known concentration to infer the NMP content in the sample. However, this method has the disadvantages of high sample pretreatment requirements, great influence of the external environment, carrier gas flow fluctuations and detector response differences causing analysis errors, etc. These defects lead to the complexity and unreliability of gas chromatography-mass spectrometry analysis and affect the accuracy of the analysis results.

[0004] Therefore, it is necessary to propose a new method that can accurately and efficiently detect the NMP content in liquid. Summary of the invention

[0005] The present invention provides a method for detecting the content of NMP in a liquid. Compared with gas chromatography-mass spectrometry technology, the detection method can effectively eliminate analysis errors caused by uneven sample preparation, carrier gas flow fluctuations, detector response differences, etc., and achieve high-precision measurement. Secondly, the method avoids the influence of external factors such as temperature, humidity, and air pressure changes on the measurement results, thereby enhancing the reliability of the data.

[0006] In detail, the present invention provides a method for detecting the content of NMP in a liquid, comprising the following steps:

[0007] 1) preparing a standard curve working solution containing an internal standard, using a liquid chromatography-mass spectrometer to perform an on-machine test on the standard curve working solution, and fitting the standard curve equation according to the test result of the standard curve working solution;

[0008] 2) using a liquid chromatography-mass spectrometer to perform on-machine detection on a liquid to be tested containing an internal standard, and calculating the content of NMP in the liquid to be tested by an internal standard method according to the standard curve equation and the detected NMP quantitative ion peak area.

[0009] Furthermore, the standard curve equation is obtained specifically through the following process:

[0010] Prepare a series of standard curve working solutions containing NMP at different concentrations, and use a liquid chromatography-mass spectrometry (LC-MS) instrument to detect the standard curve working solutions. Record the detection results. Take the ratio of the quantitative ion peak area of NMP to the quantitative ion peak area of the internal standard as the ordinate, and the NMP concentration of the corresponding standard curve working solution as the abscissa, and obtain the standard curve equation by fitting.

[0011] Further, the internal standard is selected from diethanolamine or chlorobenzene.

[0012] Further, for the liquid chromatograph of the LC-MS instrument, gradient elution of the chromatographic column is performed with mobile phase A and mobile phase B. Mobile phase A is a formic acid solution with a volume fraction of 0.5%, and mobile phase B is methanol, and / or the flow rate of the gradient elution is 0.2 - 0.4 mL / min.

[0013] Further, the chromatographic time is denoted as t, and the gradient elution program includes:

[0014] 0 min < t < 3.5 min, the chromatographic column is eluted with mobile phase A and mobile phase B, where the volume ratio of mobile phase A to mobile phase B is 9:1;

[0015] 3.5 min ≤ t < 4.5 min, the chromatographic column is eluted with mobile phase A;

[0016] 4.5 min ≤ t < 7 min, the chromatographic column is eluted with mobile phase B;

[0017] 7 min ≤ t < 7.5 min, the chromatographic column is eluted with mobile phase A and mobile phase B, where the volume ratio of mobile phase A to mobile phase B is 9:1;

[0018] 7.5 min ≤ t ≤ 10 min, the chromatographic column is eluted with mobile phase A and mobile phase B, where the volume ratio of mobile phase A to mobile phase B is 9:1.

[0019] Further, the liquid chromatograph of the LC-MS instrument uses a C18 chromatographic column, and the column temperature is 40 - 45 °C.

[0020] Further, the mass spectrometer of the LC-MS instrument is a triple quadrupole mass spectrometer with an electrospray ionization (ESI) source.

[0021] Further, the detection conditions of the mass spectrometer include:

[0022] The ionization voltage is 5500 V;

[0023] and / or the ion source temperature is 250 °C;

[0024] and / or, the spray gas is 60 psi;

[0025] and / or, the auxiliary heating gas is 60 psi;

[0026] And / or, the flow rate is 0.3 mL / min.

[0027] Furthermore, the liquid to be tested is obtained by the following process:

[0028] The electrode material in the electrode sheet is obtained, the electrode material is crushed and placed in a methanol aqueous solution for ultrasonic immersion and pressure extraction treatment, and the liquid to be tested is obtained after filtering to remove solid impurities.

[0029] Furthermore, the liquid to be tested is obtained by the following process:

[0030] The aqueous negative electrode slurry is centrifuged at high speed and then filtered to remove solid impurities to obtain the liquid to be tested. The solvent of the aqueous negative electrode slurry includes water and NMP.

[0031] Compared with the existing gas chromatography-mass spectrometry technology, the method for detecting NMP content provided by the present invention can effectively eliminate the analysis errors caused by uneven sample preparation, carrier gas flow fluctuations, detector response differences, etc., and realize high-precision measurement; and the method avoids the influence of external factors such as temperature, humidity, and air pressure changes on the measurement results, thereby enhancing the reliability of the data; secondly, the detection method also has high selectivity and high sensitivity when facing trace amounts of NMP in complex matrices, and therefore can provide reliable technical support for the production and quality control of battery pole pieces. DETAILED DESCRIPTION

[0032] In order to enable those skilled in the art to better understand the scheme of the present invention, the present invention is further described in detail below. The specific implementation methods listed below are only for describing the principles and features of the present invention. The examples are only used to explain the present invention and are not intended to limit the scope of the present invention. Based on the embodiments of the present invention, all other implementation methods obtained by ordinary technicians in the field without creative work are within the scope of protection of the present invention.

[0033] The present invention provides a method for detecting the content of NMP in a liquid, comprising the following steps:

[0034] 1) preparing a standard curve working solution containing an internal standard, using a liquid chromatography-mass spectrometer to perform an on-machine test on the standard curve working solution, and fitting the standard curve equation according to the test result of the standard curve working solution;

[0035] 2) using a liquid chromatography-mass spectrometer to perform on-machine detection on a liquid to be tested containing an internal standard, and calculating the content of NMP in the liquid to be tested by an internal standard method according to the standard curve equation and the detected NMP quantitative ion peak area.

[0036] The invention detects the NMP content in the test liquid by using a liquid chromatography-mass spectrometer (LC-MS) and an internal standard method, which can not only effectively eliminate the analysis error caused by uneven sample preparation, detector response difference and the like, so as to achieve high-precision measurement; and the detection method can avoid the influence of external factors such as temperature, humidity and air pressure changes on the measurement results, thereby enhancing the reliability of the data; secondly, the detection method also has high selectivity and high sensitivity when facing trace amounts of NMP in a complex matrix, and therefore can provide reliable technical support for the production and quality control of lithium ion battery electrodes.

[0037] In a specific embodiment, the standard curve equation is obtained by the following process:

[0038] A series of standard curve working solutions containing NMP of different concentrations were prepared, and the standard curve working solutions were tested on the machine using a liquid chromatography-mass spectrometer. The test results were recorded, and the ratio of the quantitative ion peak area of ​​NMP to the quantitative ion peak area of ​​the internal standard was used as the ordinate, and the NMP concentration of the corresponding standard curve working solution was used as the abscissa, and the standard curve equation was obtained by fitting.

[0039] It can be understood that the standard curve working solution is a series of internal standard solutions containing different concentrations of NMP. As for the NMP concentration value of each internal standard solution, the technician can adjust it according to the situation. For example, the NMP concentration is any value in 1-500 μg / mL. For example, the internal standard, NMP and solvent methanol are used to prepare standard curve working solutions with NMP concentrations of 1 μg / mL, 10 μg / mL, 50 μg / mL, 100 μg / mL and 200 μg / mL, respectively, to ensure that the internal standard content of the standard curve working solution is consistent. The standard curve working solution is tested on the machine using a liquid chromatography-mass spectrometer. According to the test results, the ratio of the quantitative ion peak area of ​​NMP to the quantitative ion peak area of ​​the internal standard is used as the ordinate, and the NMP concentration of the corresponding standard curve working solution is used as the abscissa. The standard curve equation can be obtained by fitting.

[0040] In some embodiments, the detection method of the present invention is applicable to a test solution having a NMP concentration of 500-1000 ppm.

[0041] In a specific embodiment, the internal standard is selected from diethanolamine or chlorobenzene.

[0042] Among them, the internal standard is preferably diethanolamine. The internal standard provided by the present invention can improve the accuracy and precision of NMP quantitative analysis, and has the advantages of correcting matrix effects and optimizing peak shape and resolution.

[0043] The composition of the eluent (also known as the mobile phase) of the liquid chromatograph directly affects many aspects such as the separation effect, resolution, retention time, peak shape, detection sensitivity, column pressure, reproducibility, and stability of liquid chromatography. Therefore, in order to further improve the separation effect, analysis accuracy, and reliability of liquid chromatography, in a preferred embodiment, the liquid chromatograph of the liquid chromatography-mass spectrometry instrument performs gradient elution on the chromatographic column with mobile phase A and mobile phase B. Mobile phase A is a formic acid solution with a volume fraction of 0.5%, and mobile phase B is methanol. The flow rate of the gradient elution is 0.2 - 0.4 mL / min.

[0044] Furthermore, the chromatographic time is denoted as t, and the gradient elution program includes:

[0045] 0 min < t < 3.5 min, mobile phase A and mobile phase B elute the chromatographic column, wherein the volume ratio of mobile phase A to mobile phase B is 9:1;

[0046] 3.5 min ≤ t < 4.5 min, mobile phase A elutes the chromatographic column;

[0047] 4.5 min ≤ t < 7 min, mobile phase B elutes the chromatographic column;

[0048] 7 min ≤ t < 7.5 min, mobile phase A and mobile phase B elute the chromatographic column, wherein the volume ratio of mobile phase A to mobile phase B is 9:1;

[0049] 7.5 min ≤ t ≤ 10 min, mobile phase A and mobile phase B elute the chromatographic column, wherein the volume ratio of mobile phase A to mobile phase B is 9:1.

[0050] In a specific embodiment, the liquid chromatograph of the liquid chromatography-mass spectrometry instrument uses a C18 chromatographic column, and the column temperature is 45°C.

[0051] In order to further improve the detection accuracy of trace NMP, in a specific embodiment, the mass spectrometer of the liquid chromatography-mass spectrometry instrument is a triple quadrupole mass spectrometer with an electrospray ionization (ESI) source.

[0052] In order to improve the ionization efficiency and signal intensity of the mass spectrometer, in a specific embodiment, the detection conditions of the mass spectrometer include:

[0053] The ionization voltage is 5500 V;

[0054] and / or, the ion source temperature is 250°C;

[0055] and / or, the spray gas is 60 psi;

[0056] and / or, the auxiliary heating gas is 60 psi;

[0057] And / or, the flow rate is 0.3 mL / min.

[0058] Since NMP is widely used as a solvent or cleaning agent in the fields of chemical, pharmaceutical, and electronic industries, the detection method of the present invention is applicable to detecting the residual NMP content of various chemical products, pharmaceutical products, electronic devices, and coating products, for example, including but not limited to: detecting the residual NMP content of polymers such as polyimide and polyurethane, the residual NMP content of electrode sheets, the residual NMP content of pharmaceutical products, etc.

[0059] In a specific embodiment, the detection method of the present invention is used to detect the residual NMP content of the electrode sheet. At this time, the liquid to be tested is obtained by the following process:

[0060] The electrode material in the battery pole piece is obtained, the electrode material is crushed and placed in a methanol aqueous solution for ultrasonic immersion and pressure extraction treatment, and the liquid to be tested is obtained after filtering to remove solid impurities.

[0061] The electrode sheet can be a positive electrode sheet or a negative electrode sheet. The present invention can obtain a liquid to be tested containing NMP through the above pretreatment, and further ensure that the NMP in the obtained liquid to be tested is not interfered by other components, thereby improving the accuracy of detection.

[0062] Since the electrode sheet contains a metal foil and an electrode material layer coated on the metal foil, in order to separate the electrode material from the metal foil, a certain mass of electrode material can be obtained from the electrode sheet by cutting, grinding, scraping, etc. As for the conversion method of the obtained NMP content in the liquid to be tested and the NMP content of the electrode material, the present invention does not make any special limitation, and the technicians can choose any conventional calculation method.

[0063] Exemplarily, the extraction solvent used in the above-mentioned pressurized extraction treatment can be one or more of acetone, ethanol, methanol, ethyl acetate, butyl acetate, n-butanol, etc. As for the above-mentioned ultrasonic immersion time, ultrasonic power, pressurized extraction time and other conditions, the present invention does not specifically limit them, and technicians can make corresponding adjustments according to whether there is a dressing on the surface of the foil. For example, the ultrasonic power is 30-50kHz, for example: 30kHz, 35kHz, 40kHz, 45kHz or 50kHz; the ultrasonic time is 10-60min, for example: 13min, 15min, 20min, 25min, 30min, 35min, 40min, 45min, 50min, 55min, etc.; the pressurized extraction time is 20-60min, for example: 25min, 30min, 35min, 40min, 45min, 50min, 55min, etc.

[0064] In a specific embodiment, the detection method of the present invention is used to detect the NMP content in the aqueous negative electrode slurry. At this time, the liquid to be tested is obtained by the following process:

[0065] The aqueous negative electrode slurry is centrifuged at high speed and then filtered to remove solid impurities to obtain the liquid to be tested. The solvent of the aqueous negative electrode slurry includes water and NMP.

[0066] It can be understood that the main solvent of the aqueous negative electrode slurry is water, and adding a small amount of NMP (generally between 0.5-5% of the total slurry mass) can improve the solubility of the binder, improve the dispersibility and rheology of the slurry, enhance the mechanical and electrochemical properties of the electrode, and promote drying and film formation. In addition, since the NMP concentration of the aqueous negative electrode slurry may be high, in order to further ensure the accuracy of the instrument measurement, the liquid to be tested can be diluted to less than 1ppm, and the dilution multiple can be recorded, and then the actual NMP concentration of the original aqueous negative electrode slurry can be calculated.

[0067] In order to further understand the present invention, the technical solution of the present invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0068] Unless otherwise specified, the reagents involved in the embodiments of the present invention are all commercially available products and can be purchased through commercial channels.

[0069] The following experiments involve diethanolamine (DEA), purity: 99.99% (electronic grade purity); NMP, purity: 99.99% (electronic grade purity).

[0070] The positive electrode sheet tested below includes: a metal current collector and a coating on the surface of the metal current collector, the coating including lithium nickel cobalt manganese oxide (NCM), carbon nanotubes (CNT), conductive carbon black, polyvinylidene fluoride (PVDF) and residual N-methylpyrrolidone (NMP) in a weight ratio of 100:0.37:0.73:0.8.

[0071] The aqueous negative electrode slurry for the following experiments was prepared by a method including the following process: a binder SBR, a dispersant CMC, and a conductive agent conductive carbon black were dissolved in deionized water at a mass ratio of 100:1:1:0.5, and then 0.5 wt % of NMP was added.

[0072] Example 1

[0073] This embodiment uses a UPLC-MS instrument (Shimadzu liquid phase LC-30AD and triple quadrupole Sciex TripleQuad 4500 liquid mass spectrometer) to measure the NPM content of the liquid to be tested, which specifically includes the following steps:

[0074] 1) Get the liquid to be tested: After the positive electrode is crushed, take 5 g, placed in 40mL of chromatographic grade methanol for ultrasonic immersion for 1 hour, with an ultrasonic power of 20kHz; then using chromatographic grade methanol as an extracting liquid, pressurized extraction was carried out for 30 minutes at a pressure of 20MPa, and solid components and insoluble impurities were removed by organic membrane filtration, and 1μL of DEA with a purity of 99.99% was added to the obtained liquid to obtain a liquid to be tested.

[0075] 2) DEA, NMP and solvent methanol were used to prepare standard curve working solutions with NMP concentrations of 1 μg / mL, 10 μg / mL, 50 μg / mL, 100 μg / mL and 200 μg / mL, respectively. The amount of DEA added to each standard curve working solution was 1 μL. After preparation, the standard curve working solutions were stored at 4 degrees Celsius to prevent volatilization and decomposition. The above series of standard curve working solutions were tested on a UPLC-MS instrument, and the quantitative ion peak area of ​​NMP was recorded. The ratio of the quantitative ion peak area of ​​NMP to the quantitative ion peak area of ​​DEA was used as the ordinate, and the NMP concentration of the corresponding standard curve working solution was used as the abscissa. The standard curve equation was obtained by fitting.

[0076] 3) The standard curve working solution of step 2) and the test liquid of step 1) are detected by UPLC-MS coupling instrument. Under the multiple reaction monitoring (MRM) mode, the specific ion fragments of NMP and DEA are selected, and the respective response values ​​are accurately recorded. According to the previously established standard curve equation, the exact concentration value of NMP in the test liquid is calculated.

[0077] In the detection process of step 2) and step 3), the UPLC-MS instrument uses Acquity BEHC18 chromatographic column (external Acquity BEH C18 guard column), column temperature was 45°C. The mass spectrometer adopted positive and negative ionization switching electrospray multiple reaction monitoring mode, the liquid phase and mass spectrometer settings are shown in Table 1, the elution program settings are shown in Table 2, and the mass spectrometer and chromatographic parameters are shown in Tables 3 and 4; wherein, the eluent includes: mobile phase A and mobile phase B: mobile phase A is 0.5% formic acid solution, and mobile phase B is pure methanol solution; the data processing software SCIEX software and Analyst 1.7 software provided by the instrument are used for data extraction and processing.

[0078] Table 1: Measurement condition parameters

[0079]

[0080] Table 2: Mobile phase gradient elution program

[0081]

[0082] In the table, “ / ” means: no organic phase is added.

[0083] Table 3: Mass spectrometry parameters

[0084]

[0085] Table 4: Chromatographic parameters

[0086] name Peak time NMP 3:15min DEA (Diethanolamine) 2:25min

[0087] Example 2

[0088] The method is basically the same as Example 1, except that: Step 1) the aqueous negative electrode slurry is subjected to high-speed centrifugation at a speed of 10,000 rpm for 10 minutes, and filtered with an aqueous filter membrane to remove solid components and insoluble impurities to obtain a test solution.

[0089] Reproducibility and precision testing:

[0090] (1) Positive electrodes of the same battery produced in 10 different batches were collected and analyzed using the sample pretreatment and detection steps in Example 1 and Comparative Example 1; the results showed that the detection method of Example 1 showed excellent reproducibility in the determination of NMP concentration in the samples, with an RSD (relative standard deviation) of less than 2% and a spike recovery rate of between 98% and 102%. It can be seen that the detection method of the present application is significantly superior to the traditional external standard method and has high accuracy and reliability.

[0091] (2) Ten different batches of aqueous negative electrode slurries of the same series were collected and analyzed using the sample pretreatment and detection steps in Example 2 and Comparative Example 2, respectively. The results showed that the detection method of Example 2 showed excellent reproducibility in the determination of NMP concentration in the samples, with an RSD (relative standard deviation) of less than 2% and a spike recovery rate of between 98% and 102%. It can be seen that the detection method of the present application is significantly superior to the traditional external standard method and has high accuracy and reliability.

[0092] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for detecting NMP content in a liquid, characterized in that: It includes the following steps: 1) Prepare a standard curve working solution containing an internal standard, perform on-machine detection of the standard curve working solution using a liquid chromatography-mass spectrometry (LC-MS) instrument, and obtain a standard curve equation by fitting according to the detection results of the standard curve working solution; 2) Perform on-machine detection of the liquid to be measured containing an internal standard using an LC-MS instrument, and calculate the content of NMP in the liquid to be measured by internal standard method according to the standard curve equation and the detected area of the NMP quantitative ion peak.

2. The detection method according to claim 1, characterized in that: The standard curve equation is specifically obtained through the following process: Prepare a series of the standard curve working solutions containing different concentrations of NMP, perform on-machine detection of the standard curve working solutions using an LC-MS instrument, record the detection results, use the ratio of the area of the NMP quantitative ion peak to the area of the quantitative ion peak of the internal standard as the ordinate, and the NMP concentration of the corresponding standard curve working solution as the abscissa, and obtain the standard curve equation by fitting.

3. The detection method according to claim 1 or 2, characterized in that: The internal standard is selected from diethanolamine or chlorobenzene, and / or the solvent of the standard curve working solution is methanol.

4. The detection method according to any one of claims 1 to 3, characterized in that: For the liquid chromatograph of the LC-MS instrument, gradient elution of the chromatographic column is performed with mobile phase A and mobile phase B. Mobile phase A is a formic acid solution with a volume fraction of 0.5%, and mobile phase B is methanol, and / or the flow rate of the gradient elution is 0.2 - 0.4 mL / min.

5. The detection method according to claim 4, characterized in that: The chromatographic time is denoted as t, and the gradient elution program includes: 0 min < t < 3.5 min, the chromatographic column is eluted with mobile phase A and mobile phase B, where the volume ratio of mobile phase A to mobile phase B is 9:1; 3.5 min ≤ t < 4.5 min, the chromatographic column is eluted with mobile phase A; 4.5 min ≤ t < 7 min, the chromatographic column is eluted with mobile phase B; 7 min ≤ t < 7.5 min, the chromatographic column is eluted with mobile phase A and mobile phase B, where the volume ratio of mobile phase A to mobile phase B is 9:1; 7.5 min ≤ t ≤ 10 min, the chromatographic column is eluted with mobile phase A and mobile phase B, where the volume ratio of mobile phase A to mobile phase B is 9:

1.

6. The detection method according to any one of claims 4-5, characterized in that: For the liquid chromatograph of the LC-MS instrument, a C18 chromatographic column is used, and the column temperature is 45°C.

7. The detection method according to any one of claims 1 to 6, characterized in that: The mass spectrometer of the LC-MS instrument is a triple quadrupole mass spectrometer with an electrospray ionization (ESI) source.

8. The detection method according to claim 7, characterized in that: The detection conditions of the mass spectrometer include: The ionization voltage is 5500 V; and / or the ion source temperature is 250°C; and / or the spray gas is 60 psi; and / or the auxiliary heating gas is 60 psi; and / or the flow rate is 0.3 mL / min.

9. The detection method according to any one of claims 1 to 8, characterized in that: The liquid to be measured is obtained through the following process: Obtain the electrode material in the electrode sheet, crush the electrode material and place it in an aqueous methanol solution for ultrasonic soaking and pressure extraction treatment, and filter to remove solid impurities to obtain the liquid to be measured.

10. The detection method according to any one of claims 1 to 8, characterized in that: The liquid to be measured is obtained through the following process: Centrifuge the aqueous negative electrode slurry at high speed, filter to remove solid impurities to obtain the liquid to be measured, and the solvent of the aqueous negative electrode slurry includes water and NMP.

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