Method for detecting the amount of nmp remaining in a volatile lithium battery cathode

Abstract

The application provides a detection method of NMP residual amount in a volatile lithium battery positive electrode, and the detection method comprises the following steps: the volatile lithium battery positive electrode is subjected to a first heat treatment, and a clear solution is obtained, thereby obtaining a sample after heat treatment; an acidic potassium dichromate solution and the sample after heat treatment are mixed, and a second heat treatment is performed, thereby obtaining a sample to be detected; and the sample to be detected is used for NMP residual amount detection. The detection method provided by the application can be applied to the detection of residual solvent content of a positive electrode material component containing volatile components, and the comprehensive quality level of products is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a detection method for NMP residual amount of volatile lithium battery cathode. BACKGROUND

[0002] In the slurry preparation process of lithium ion battery cathode, the organic solvent N-methyl pyrrolidone (NMP) with high polarity aprotic transfer and excellent compatibility is often used as a dispersant for the mixing and slurry preparation process of the cathode material, and is removed by high-temperature baking in the coating process. However, due to the high boiling point and stability of NMP, a small amount of NMP is usually left in the adhesive layer of the cathode mixture and the gap of the coated foil. When the residual amount is too much, NMP will affect the normal film formation reaction of the electrolyte during the formation stage of the battery, and the by-products of the side reaction will also react with the electrolyte in the later stage, which seriously affects the performance and service life of the battery.

[0003] Therefore, it is very important to control the NMP content left on the electrode sheet for lithium ion batteries.

[0004] Currently, the extraction of residual NMP in the electrode sheet mainly uses direct solvent extraction, which has a long extraction time and low efficiency. The quantitative method mainly uses chromatography, but NMP with high polarity is difficult to be completely desorbed from the chromatographic column, resulting in the failure of the chromatographic column. The existing technology also uses gas chromatography, but the generated tail gas is directly discharged and toxic, which pollutes the environment. Moreover, the market price of chromatographs and mass spectrometers is relatively expensive. The existing technology also uses a gravimetric method to indirectly detect the residual amount of NMP, but this method is only suitable for high-melting-point cathode mixtures and is not suitable for high-temperature volatile cathode mixture components and similar materials. For example, the new pre-lithiated cathode material newly developed and put into production by the applicant contains sulfur, which makes it difficult to detect NMP.

[0005] Therefore, in view of the deficiencies of the prior art, it is necessary to develop a method for efficiently, safely and environmentally determining the residual NMP concentration in the battery electrode sheet. SUMMARY

[0006] To solve the above technical problems, the present application provides a detection method for NMP residual amount of volatile lithium battery cathode, which can realize efficient and accurate detection of NMP residual amount in the cathode sheet under the condition of no mass spectrometry by adding acidic potassium dichromate solution to the sample to be tested, and has a wide application prospect.

[0007] To achieve this purpose, the present application adopts the following technical solutions:

[0008] The present application provides a detection method for NMP residual amount of volatile lithium battery cathode, which comprises the following steps:

[0009] The positive electrode of the volatile lithium battery is subjected to a first heat treatment, and a clear liquid is obtained to obtain a heat-treated sample;

[0010] Mixing the acidic potassium dichromate solution and the heat-treated sample, and then performing a second heat treatment to obtain a sample to be tested;

[0011] The NMP residual amount is detected by using the sample to be tested.

[0012] The present invention utilizes the principle that NMP reacts with acidic potassium dichromate. An acidic potassium dichromate solution is added to the clear liquid after the first heat treatment, and a second heat treatment is performed. During the second heat treatment, potassium dichromate reacts with NMP. This reaction consumes the added acidic potassium dichromate. Therefore, the amount of NMP in the original volatile lithium battery positive electrode can be calculated by detecting the remaining amount of acidic potassium dichromate in the test sample.

[0013] It is worth noting that the first heat treatment process in the present invention can completely leach out the NMP remaining in the volatile lithium battery electrode and transfer it to the aqueous phase. During the second heat treatment process, NMP converts a known amount of hexavalent chromium in acidic potassium dichromate into trivalent chromium. By measuring the remaining amount, the amount consumed in the reaction and the residual amount of organic solvent NMP in the sample are calculated.

[0014] The technical solution of the present invention has been developed with the following key features: 1. Trace amounts of NMP can fully react with the acidic potassium dichromate solution, and the acidic potassium dichromate can completely react with the NMP in the system, thereby achieving accurate detection of NMP; 2. The subsequent detection of trivalent chromium by potassium dichromate is not affected by other metal ions or impurities in the clear liquid system.

[0015] Preferably, the first heat treatment agent in the first heat treatment is water.

[0016] During the drying process of electrode manufacturing, since the residual NMP in the electrode is miscible with water in any proportion, heating can more thoroughly and effectively release the residual NMP in the gaps of the electrode, or even release the crystalline NMP by soaking. Therefore, the present invention preferably uses water as the first heat treatment reagent, which can not only achieve complete dissolution of the NMP component in the lithium-ion battery, but also prevent other impurities in the electrode from entering the clear liquid system.

[0017] Preferably, the volatile lithium battery positive electrode is crushed and then subjected to the first heat treatment.

[0018] Preferably, the mass ratio of the volatile lithium battery positive electrode to the first heat treatment reagent is (0.1-1.0):1, for example, it can be 0.1:1, 0.2:1, 0.3:1, 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1 or 1.0:1.

[0019] It is worth noting that the ratio of added sample to water should be determined based on comprehensive factors such as the detection limit, upper and lower limits of the determination, and sample representativeness of the instrument method subsequently adopted. The UV-visible spectrophotometer used in the present invention has a minimum detection limit of 2 mg / L and an upper detection limit of 700 mg / L. Dilution is required if the concentration exceeds this range. Instrument model: 5B-3BW; the minimum detection limit of the potentiometric titrator is 0.1 mg / L, and the maximum range of the single-tube titration cup is 20 mL. Combined with the known standard concentration of the titration solution and the titration consumption of 30%-70% of the volume in the titration cup, this will produce the best repeatable results. Instrument model: METTLERG10S. In the present invention, 20 g of sample is accurately weighed, and the scale has an accuracy of (0.00001 g) to 35 g of water mass ratio to ensure that the water can cover the sample. In order to make it easier to place the electrode into the digestion tube, the electrode can be cut into pieces.

[0020] Preferably, the temperature of the first heat treatment is 148-152°C, for example, it can be 148°C, 148.5°C, 148.9°C, 149.4°C, 149.8°C, 150.3°C, 150.7°C, 151.2°C, 151.6°C or 152°C, etc., but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0021] Preferably, the time of the first heat treatment is 15 to 40 minutes, for example, it can be 15 minutes, 18 minutes, 21 minutes, 24 minutes, 27 minutes, 29 minutes, 32 minutes, 35 minutes, 38 minutes or 40 minutes, but is not limited to the listed values. Other values ​​not listed within this range are also applicable.

[0022] Preferably, the first heat treatment is followed by cooling and solid-liquid separation to obtain a clear liquid.

[0023] Preferably, the pH of the acidic potassium dichromate solution is 1 to 4, for example, 1, 1.2, 1.3, 1.5, 1.8, 2.0, 2.2, 2.3, 2.5, 2.8, 3, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9 or 4.0.

[0024] Preferably, the acid in the acidic potassium dichromate solution includes sulfuric acid. In the present invention, concentrated sulfuric acid is diluted to 1% and the pH is in the range of 1-4.

[0025] Preferably, the concentration of potassium dichromate in the acidic potassium dichromate solution is 0.2-0.3 mol / L, for example, 0.2 mol / L, 0.21 mol / L, 0.22 mol / L, 0.23 mol / L, 0.24 mol / L, 0.25 mol / L, 0.26 mol / L, 0.27 mol / L, 0.28 mol / L or 0.3 mol / L.

[0026] It is worth noting that a potassium dichromate standard solution is generally used in the present invention. The specific preparation process is exemplified as follows: 12.25800 g of the reference reagent potassium dichromate dried to constant weight at 120° C. is dissolved in ultrapure water, and 10 mL of concentrated sulfuric acid is added to dilute to 1000 mL to obtain a 0.250 mol / L potassium dichromate standard solution.

[0027] Preferably, the volume ratio of the acidic potassium dichromate solution to the clear solution is (0.001-0.1):1, for example, it can be 0.001:1, 0.002:1, 0.003:1, 0.005:1, 0.008:1, 0.009:1, 0.01:1, 0.02:1, 0.03:1, 0.04:1, 0.05:1, 0.06:1, 0.07:1, 0.08:1 or 0.1:1, etc.

[0028] The steps for adding potassium dichromate solution in the present invention are exemplified as follows: remove the electrode from the aqueous solution for soaking the electrode (the electrode is washed with a small amount of ultrapure water, and the washed water is transferred to the soaking solution); use a pipette to accurately take out 5 mL of 0.250 mol / L potassium dichromate standard solution and add it to 1).

[0029] The present invention preferably maintains the above pH, concentration and volume ratio of the acidic potassium dichromate solution system, which can better react with the NMP in the clear solution and make subsequent detection more accurate.

[0030] Preferably, a catalyst is further added to the mixed acidic potassium dichromate solution and the heat-treated sample.

[0031] Preferably, the catalyst comprises a silver sulfate solution.

[0032] Preferably, the concentration of the silver sulfate solution is 0.5-1.5wt%, for example, it can be 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 1.0wt%, 1.1wt%, 1.2wt%, 1.3wt%, 1.4wt% or 1.5wt%.

[0033] Preferably, the added volume of the silver sulfate solution is 0.01 to 1 mL, for example, 0.01 mL, 0.02 mL, 0.1 mL, 0.2 mL, 0.3 mL, 0.4 mL, 0.5 mL, 0.6 mL, 0.7 mL or 1 mL.

[0034] In the present invention, silver sulfate mainly plays a role in catalyzing the reaction, and can ensure that potassium dichromate and NMP fully react.

[0035] Preferably, the temperature of the second heat treatment is 148-152°C, for example, it can be 148°C, 148.5°C, 148.9°C, 149.4°C, 149.8°C, 150.3°C, 150.7°C, 151.2°C, 151.6°C or 152°C, etc., but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0036] Preferably, the time of the second heat treatment is 5 to 30 minutes, for example, it can be 5 minutes, 7 minutes, 9 minutes, 10 minutes, 12 minutes, 14 minutes, 15 minutes, 17 minutes, 19 minutes, 20 minutes, 22 minutes, 25 minutes, 28 minutes or 30 minutes, but is not limited to the listed values. Other values ​​not listed within this range are also applicable.

[0037] Preferably, the method for detecting the residual amount of NMP comprises ultraviolet-visible spectrophotometer detection or potentiometric titration detection.

[0038] The present invention preferably adopts ultraviolet-visible spectrophotometer detection or potentiometric titration detection, which is convenient for detection, has low instrument cost and high detection accuracy.

[0039] Preferably, the spectral range selected in the UV-visible spectrophotometer detection is 420-460 nm, for example, it can be 420 nm, 425 nm, 429 nm, 434 nm, 438 nm, 443 nm, 447 nm, 452 nm, 456 nm or 460 nm, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0040] Preferably, the UV-visible spectrophotometer detection includes: measuring a standard curve of potassium dichromate, and using a UV-visible spectrophotometer to measure the absorbance of the sample to be tested, determining the concentration of potassium dichromate in the sample to be tested, and calculating the residual amount of NMP in the positive electrode of the volatile lithium battery.

[0041] Preferably, the potentiometric titration detection includes: titrating the sample to be tested with an ammonium sulfate solution, and calculating the amount of potassium dichromate in the sample to be tested based on the amount of ammonium sulfate consumed when the titration is completed, to determine the residual amount of NMP in the positive electrode of the volatile lithium battery.

[0042] Preferably, the condition for the completion of the titration is that a sudden peak appears in the potentiometric titration instrument, which is automatically recognized as the titration endpoint.

[0043] Compared with the prior art, the present invention has at least the following beneficial effects:

[0044] (1) The method for detecting the residual amount of volatile NMP in the positive electrode of a lithium battery provided by the present invention can quickly and efficiently monitor the residual amount of NMP in the positive electrode of a lithium battery without a mass spectrometer;

[0045] (2) The method for detecting the residual amount of NMP in the positive electrode of a lithium battery containing volatile components provided by the present invention is particularly suitable for detecting NMP in lithium batteries containing volatile components such as sulfur, and has broad application prospects;

[0046] (3) The detection method for the residual amount of volatile NMP in the positive electrode of a lithium battery provided by the present invention has high accuracy and sensitivity, with an error of ≤5%. The relative standard deviations of the detection using a UV-visible spectrophotometer and potentiometric titration are within 4.18% and 7.60%, respectively. DETAILED DESCRIPTION

[0047] For the convenience of understanding the present invention, the present invention is given below with examples. It should be understood by those skilled in the art that the examples are only for the purpose of helping to understand the present invention and should not be regarded as specific limitations of the present invention.

[0048] The samples used in Examples 1 and 5 are samples with known slurry ratios and theoretical NMP content; the samples used in Examples 2 and 4 are samples baked in the battery electrode manufacturing process.

[0049] Example 1

[0050] This embodiment provides a method for detecting the residual NMP content in a positive electrode of a lithium battery containing volatile substances. To accurately evaluate the accuracy of the method for detecting the residual NMP content in a positive electrode of a lithium battery containing volatile substances, the sample used is specific, specifically a sample with a known slurry ratio and a theoretical NMP content. The detection method includes the following steps:

[0051] Sample pretreatment: Cut the volatile lithium battery positive electrode into small pieces so that it can be placed in the digestion test tube. Here, select 20g of sample, add 35g of first-grade pure water, and the water should cover the electrode. Then tighten the lid and place it in the digestion machine for the first heat treatment at 150℃±2℃ for 30min. After that, cool it to room temperature and filter the clear liquid to obtain the heat-treated sample.

[0052] Filter the cooled sample solution after the heat treatment, take all the filtrate, transfer it to a clean digestion tube, accurately take out 5mL of 0.250mol / L potassium dichromate standard solution with a pipette, and add it to the solution. Then add 5 drops of 1% silver sulfate (about 0.2mL, which acts as a catalyst). Tighten the tube cap and place it in a digestion machine for a second heat treatment at 150℃±2℃ for 10 minutes. After cooling, add the fixed volume to a 25ml volumetric flask to obtain the sample to be tested (simultaneously perform a blank test).

[0053] The NMP residual amount was detected by using the sample to be tested:

[0054] Potassium dichromate is a standard reagent. Before preparing the solution, dry the high-grade pure potassium dichromate in a forced air drying oven at 120±2℃ to constant weight. Weigh 12.25800g and place it in a beaker. Add 600ml of first-grade pure water. Slowly add 100ml of high-grade pure concentrated sulfuric acid (p=1.84g / mL) while stirring. After dissolving and cooling, dissolve it in a 1000ml volumetric flask, shake well, and set aside.

[0055] Construct the standard solution gradient of the curve: Use (1) dried high-purity potassium dichromate to prepare the standard curve concentration (0 mg / L, 25 mg / L, 50 mg / L, 100 mg / L, 150 mg / L, 200 mg / L, 400 mg / L, 600 mg / L)

[0056] The absorbance was measured at 440±20 nm on a UV-visible spectrophotometer, and a potassium dichromate amount versus absorbance curve was drawn based on the absorbance value. The established curve had an effective correlation coefficient R≧0.999 or above.

[0057] Inject the sample to be tested directly into the 3 / 4 height of the cuvette, wipe the outer wall of the cuvette with dust-free paper, and then place it in the instrument to measure its absorbance, and at the same time make a blank sample.

[0058] Calculation result: A=kC+B

[0059] Where A is the absorbance value; k is the slope; C is the concentration; and B is the intercept.

[0060] Example 2

[0061] This embodiment provides a method for detecting the residual amount of volatile NMP in a lithium battery positive electrode. The detection method is the same as that of Example 1, except that the sample is a sample baked during the battery electrode production process. Detailed description thereof will not be given here.

[0062] Example 3

[0063] The embodiment provides a detection method of NMP residual amount of a lithium iron phosphate battery positive electrode, which is the same as that in Embodiment 1 except that the sample is a known sample NMP theoretical proportion content of a traditional industry present lithium iron phosphate battery positive electrode sheet manufacturing sample, which is not repeated here.

[0064] Embodiment 4

[0065] The embodiment provides a detection method of NMP residual amount of a lithium iron phosphate battery positive electrode, which is the same as that in Embodiment 1 except that the sample is a known sample NMP theoretical proportion content of a traditional industry present lithium iron phosphate battery positive electrode sheet manufacturing sample, which is not repeated here.

[0066] Embodiment 5

[0067] The embodiment provides a detection method of NMP residual amount of a lithium iron phosphate battery positive electrode, which is the same as that in Embodiment 1 except that the sample is a known sample NMP theoretical proportion content of a traditional industry present lithium iron phosphate battery positive electrode sheet manufacturing sample, which is not repeated here.

[0068] Sample pretreatment:

[0069] The lithium battery positive electrode containing volatility is cut into small pieces to be placed into a digestion test tube, 20g of the sample is selected, first grade pure water is added, the amount of the positive electrode is covered with water, the cover is tightly screwed, and then the lithium battery positive electrode containing volatility is placed into a digestion machine for first heat treatment at 150℃±2℃ for 30min, and then cooled to room temperature, and the clear solution is filtered to obtain the sample after heat treatment;

[0070] The sample solution after heat treatment is filtered, and the whole filtered solution is transferred into a clean digestion tube, 5mL of 0.250mol / L potassium dichromate standard solution is accurately taken out from the digestion tube by using a syringe and added into the digestion tube, and 5 drops (about 0.2mL) of 1% silver sulfate is added (the function is catalysis). The cover of the digestion tube is tightly screwed, and then the digestion tube is placed into a digestion machine for second heat treatment at 150℃±2℃ for 10min, and then cooled, and then the volume is fixed in a 25ml volumetric flask to obtain a sample to be measured (at the same time, a blank sample is prepared).

[0071] The sample to be measured is used for NMP residual amount detection.

[0072] The concentration of the ammonium ferrous sulfate standard solution is 0.05mol / L; 19.5g of ammonium ferrous sulfate is weighed and dissolved in water, 10ml of the above concentrated sulfuric acid is added, and finally the volume is fixed in a 1000ml volumetric flask, which is ready for use.

[0073] Calibration of the ammonium ferrous sulfate standard solution

[0074] Accurately weigh 12.25800g of the reference reagent potassium dichromate, dried to a constant weight, and dissolve it in water. Then dilute to 1000mL and shake well. Transfer 2mL to the titration cup and add 20mL of water to the titration cup so that the test solution can completely cover the electrodes on the potentiometric titrator. Then fix the titration cup on the instrument and titrate with ammonium ferrous sulfate according to the equivalence method (EQP). When a sudden peak appears, the instrument automatically recognizes the endpoint, reads the volume of ammonium ferrous sulfate consumed, and makes a parallel sample. Calculate the concentration of the ammonium ferrous sulfate standard solution according to the following formula:

[0075]

[0076] Where: C is the concentration of ammonium ferrous sulfate standard solution, mol / L; V is the volume of ammonium ferrous sulfate consumed in titration, mL.

[0077] Transfer all pre-treated samples from a 25ml volumetric flask to the titration cup and titrate with a known concentration of ammonium ferrous sulfate standard solution, also using the equivalence point (EQP) method. When a sudden peak appears, the instrument automatically identifies the endpoint and reads the volume of ammonium ferrous sulfate consumed. The amount of unreacted potassium dichromate can be calculated based on the above. Because the amount of potassium dichromate added to the electrode sample is known, the difference between this known amount and the unreacted amount gives the potassium dichromate consumed. The residual NMP amount is then determined by the stoichiometric relationship of the reaction between potassium dichromate and NMP. A blank sample is also prepared.

[0078] The NMP residue in the sample is expressed as follows:

[0079]

[0080] Where:

[0081] C-------Concentration of ammonium ferrous sulfate standard solution, mol / L;

[0082] V0-------the volume of ammonium ferrous sulfate standard solution consumed by the blank sample, mL;

[0083] V1-------The volume of the standard solution of ammonium ferrous sulfate consumed by the electrode sample, mL;

[0084] 99.13----NMP molar mass;

[0085] 25----is the constant volume, mL;

[0086] m----Weigh the mass of the electrode, g.

[0087] Example 6

[0088] This embodiment provides a method for detecting the residual amount of NMP in a positive electrode of a lithium battery containing volatile substances. The detection method is the same as that of Example 5, except that the sample is a sample baked during the battery electrode production process. Detailed description thereof will not be given here.

[0089] Example 7

[0090] This embodiment provides a method for detecting the residual amount of NMP in the positive electrode of a lithium iron phosphate battery. The detection method is the same as that in Example 5, except that the sample is a known sample with a theoretical NMP content currently used in the traditional industry for making positive electrode sheets for lithium iron phosphate batteries, and is not described again here.

[0091] Example 8

[0092] This embodiment provides a method for detecting the residual amount of NMP in the positive electrode of a lithium iron phosphate battery. The detection method is the same as that of Example 5, except that the sample is a sample baked using the traditional industry's current lithium iron phosphate battery positive electrode sheet production process, and will not be repeated here.

[0093] Comparative Example 1

[0094] This comparative example provides a method for detecting the residual amount of NMP in a positive electrode of a lithium battery containing volatile substances. The detection method is the same as that in Example 1, except that the acidic potassium dichromate solution is replaced by a neutral potassium dichromate solution. Detailed description thereof will not be given here.

[0095] This comparative example is difficult to realize the accurate detection of NMP and cannot be carried out.

[0096] Comparative Example 2

[0097] This comparative example provides a method for detecting the residual amount of NMP in a positive electrode of a lithium battery containing volatile substances. The detection method is the same as that in Example 1 except that the first heat treatment is not performed, and thus will not be further described.

[0098] This comparative example is difficult to realize the accurate detection of NMP and cannot be carried out.

[0099] Comparative Example 3

[0100] This comparative example provides a method for detecting the residual amount of NMP in a positive electrode of a lithium battery containing volatile substances. The detection method is the same as that in Example 2, except that the acidic potassium dichromate solution is replaced by an acidic sodium dichromate solution, and will not be described in detail here.

[0101] This comparative example is difficult to realize the accurate detection of NMP and cannot be carried out.

[0102] The above method was used to detect the positive electrode of lithium-ion batteries and compared with the theoretical specific concentration of known samples.

[0103] The test results of the above embodiments and comparative examples are shown in Table 1.

[0104] Table 1

[0105]

[0106]

[0107] “ / ” in Table 1 indicates that there are no relevant data.

[0108] The above method was used to test the positive electrode of a lithium-ion battery. Taking Example 1 as an example, the test results were repeated 10 times and are shown in Table 2, with the unit being ppm.

[0109] Table 2

[0110]

[0111] The above method was used to test the positive electrode of a lithium-ion battery. Taking Example 5 as an example, the test results were repeated 10 times and are shown in Table 3 (unit: ppm).

[0112] Table 3

[0113]

[0114] From Table 1 we can see that:

[0115] The UV-visible spectrophotometer has the following performance parameters: 200 mg / L to 2000 mg / L, and the indication error is ≤ 5%. The above test results are valid.

[0116] From Table 2 and Table 3 we can see that:

[0117] The calculated relative standard deviations were 4.18 and 7.60, and the quality control RSD was 1.3≦RSD≦8.4%, indicating that the above test results were valid and both detection methods were effective.

[0118] The present invention uses the above-described embodiments to illustrate the detailed features of the present invention. However, the present invention is not limited to the above-described detailed features, which does not mean that the present invention must rely on the above-described detailed features to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent replacements for the selected technical features of the present invention, additions to auxiliary technical features, and selections of specific methods, etc., fall within the scope of protection and disclosure of the present invention.

Claims

1. A method for detecting the residual amount of NMP in a positive electrode of a lithium battery containing volatile components, characterized in that: The detection method comprises the following steps: The positive electrode of the lithium battery containing volatile components is subjected to a first heat treatment, and a clear liquid is obtained to obtain a heat-treated sample; Mixing the acidic potassium dichromate solution and the heat-treated sample, and then performing a second heat treatment to obtain a sample to be tested; Utilizing the sample to be tested to detect the residual amount of NMP; The first heat treatment reagent in the first heat treatment is water; The temperature of the first heat treatment is 148-152° C.; the time of the first heat treatment is 15-40 minutes; The pH of the acidic potassium dichromate solution is 1-4; the acid in the acidic potassium dichromate solution includes sulfuric acid; the concentration of potassium dichromate in the acidic potassium dichromate solution is 0.2-0.3 mol / L; The volume ratio of the acidic potassium dichromate solution to the clear solution is (0.001-0.1):1; The temperature of the second heat treatment is 148-152° C.; the time of the second heat treatment is 5-30 minutes; The method for detecting the residual amount of NMP includes ultraviolet-visible spectrophotometer detection or potentiometric titration detection.

2. The detection method according to claim 1, characterized in that The positive electrode of the lithium battery containing volatile components is crushed and then subjected to a first heat treatment.

3. The detection method according to claim 1 or 2, characterized in that The mass ratio of the lithium battery positive electrode containing volatile components to the first heat treatment reagent is (0.1~1.0):

1.

4. The detection method according to claim 1, wherein After the first heat treatment, cooling and solid-liquid separation are performed to obtain a clear liquid.

5. The detection method according to claim 1, wherein The spectral range selected in the ultraviolet-visible spectrophotometer detection is 420-460 nm.

6. The detection method according to claim 1, characterized in that The UV-visible spectrophotometer detection includes: measuring a standard curve of potassium dichromate, and using a UV-visible spectrophotometer to measure the absorbance of the sample to be tested, determining the concentration of potassium dichromate in the sample to be tested, and calculating the residual amount of NMP in the positive electrode of the lithium battery containing volatile components.

7. The detection method according to claim 1, characterized in that The potentiometric titration detection includes: titrating the sample to be tested with an ammonium sulfate solution, and calculating the amount of potassium dichromate in the sample to be tested based on the amount of ammonium sulfate consumed when the titration is completed, to determine the residual amount of NMP in the positive electrode of the lithium battery containing volatile components.

8. The detection method according to claim 7, characterized in that The condition for the completion of the titration is that a sudden peak appears in the potentiometric titration instrument, which is automatically recognized as the titration end point.

Images