Metal foreign matter detection method and application

Abstract

The invention belongs to the technical field of battery sample detection, and relates to a metal foreign matter detection method and application. The technical problems that in the prior art, inductively coupled plasma is adopted for testing metal foreign matter, cost is high, consumed time is long, testing steps are complex, a large number of metal particles are filtered out through pretreatment when the metal foreign matter is measured through a developing method, and accuracy is low are solved. The invention provides a method for detecting metal foreign matters, which comprises the following steps: placing a magnetic rod with a coating layer in slurry to be detected, carrying out ball milling on the slurry, removing the magnetic rod to obtain the coating layer adsorbing the metal foreign matters, and cleaning with a solvent to obtain a cleaning solution; filtering with a filter membrane to obtain a filtered filter membrane; the clean filter membrane is taken, the acid solution and the oxidizing solution are dropwise added to the surface in sequence, the filtered filter membrane is flatly laid to the surface of the clean filter membrane, standing is carried out, the color developing agent is dropwise added, the content range of the metal foreign matter is judged according to the number of color developing points, and the detection accuracy of the metal foreign matter is improved through the scheme.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery sample detection, and in particular, relates to a method for detecting metal foreign matter and an application thereof. Background Art

[0002] Lithium-ion batteries are widely used in many fields due to their high energy density, long cycle life, high coulomb efficiency and wide operating temperature range. However, during the manufacturing process, metal impurities may be mixed into the electrode materials, resulting in micro short circuits and battery self-discharge. At present, most production lines use inductively coupled plasma (ICP) testing to detect the content of magnetic impurities in the slurry. Although ICP testing is accurate, it is costly, time-consuming and has complex steps. In addition to ICP testing, there are some other detection methods. For example, the method based on the incremental capacity curve can diagnose the micro internal short circuit fault of lithium-ion batteries. When a micro internal short circuit occurs, part of the charging current generates heat due to the short circuit resistance instead of participating in the electrochemical reaction, resulting in the incremental capacity value of the short-circuited battery being higher than that of the normal battery. By calculating the mean square error of the incremental capacity values ​​of the short-circuited battery and the normal battery, the deviation can be evaluated to diagnose the micro internal short circuit fault. In addition, there are methods such as deviation identification based on measured data, abnormal voltage signal identification and battery self-discharge identification. These methods have their own advantages and disadvantages. For example, the identification based on measured data deviation has limited effect in early internal short circuit identification. The voltage signal anomaly identification method can only identify the internal short circuit of a specific type of battery. The battery self-discharge identification method cannot be used for real-time identification during battery operation.

[0003] For example, the Chinese invention patent application publication number is CN110779781A, the application date is November 12, 2019, and the invention name is "Method for determining the total amount of magnetic foreign matter in battery-grade lithium hydroxide monohydrate by ICP method". The disclosed determination method includes the following steps: first pre-treat the sample with a cylindrical strong magnet of 4000-6000 gauss, 1.2 cm high and 2.5 cm in diameter, and then use Avio 500 series inductively coupled plasma emission spectrometer with S10 automatic sampler to detect the solution, and use PEEK nebulizer with glass cyclonic nebulizer chamber and PerkinElmer original quartz rectangular tube with alumina center tube, and set the instrument parameters. Although this method has high accuracy, the test steps are complicated and increase the equipment cost. Summary of the invention

[0004] 1. Problems to be solved

[0005] In view of the technical problems that the inductively coupled plasma test method used in the prior art for determining whether the content of metal foreign matter exceeds the standard is high in cost, time-consuming, and has complicated test steps, the present application provides a method for detecting metal foreign matter, which improves the accuracy of metal foreign matter detection. At the same time, the present application also provides an application of the detection method.

[0006] 2. Technical solution

[0007] In order to achieve the above purpose, the technical solution provided is:

[0008] A method for detecting metallic foreign matter comprises the following steps:

[0009] A magnetic rod provided with a coating layer is placed in the slurry to be tested, and the slurry is ball-milled and then the magnetic rod is removed to obtain a coating layer for absorbing metal foreign matter;

[0010] The coating layer for absorbing the metal foreign matter is cleaned with a solvent to obtain a cleaning solution;

[0011] filtering the cleaning liquid through a filter membrane to obtain a filtered filter membrane;

[0012] Take a clean filter membrane, drip an acidic solution and an oxidizing solution on the surface in sequence, spread the filtered filter membrane on the surface of the clean filter membrane, let it stand, drip a color developer, and determine the content range of metal foreign matter according to the number of color points.

[0013] Preferably, the coating layer is cleaned by ultrasonic cleaning, and the ultrasonic cleaning time is 3 minutes to 5 minutes.

[0014] Furthermore, the slurry to be tested comprises positive electrode slurry and / or negative electrode slurry, and a solvent; the solvent is deionized water or N,N-dimethylformamide, and the material-liquid ratio is 1:1-1.5 calculated by mass ratio.

[0015] The purpose of adding solvent to the slurry is to dilute the slurry, effectively reduce the viscosity of the slurry, and enhance the fluidity of the slurry. The magnetic rod with the coating layer can contact the slurry more fully and more effectively adsorb the metal foreign matter therein, avoiding the metal foreign matter being wrapped and difficult to be attracted by the magnet due to the excessively high viscosity of the slurry. When the material-liquid ratio is 1:1-1.5, the metal foreign matter can be better dispersed in the slurry, thereby improving the efficiency of the magnetic rod in adsorbing the metal foreign matter and ensuring the accuracy of the detection results.

[0016] Furthermore, the coating layer is a heat shrink tube.

[0017] Preferably, the heat shrink tube is made of polyethylene.

[0018] Furthermore, the acidic solution is dilute hydrochloric acid and / or dilute nitric acid.

[0019] Furthermore, the oxidizing solution is a hydrogen peroxide solution.

[0020] Furthermore, the developer is one or more of potassium thiocyanate, dicarbohydrazide and / or dimethylglyoxime.

[0021] The color reaction can detect whether there is metal foreign matter in the slurry. Taking Fe as an example, if there is Fe in the slurry, it will adhere to the filter membrane through the above operation, and add dilute hydrochloric acid or dilute nitric acid to convert the metal Fe into Fe 2+ , that is, Fe+2H + =Fe 2+ +H2; then add hydrogen peroxide to make Fe 2+ Oxidized to Fe 3+ , i.e. 2Fe 2+ +H2O2+2H + =2Fe 3+ +2H2O; finally, KSCN color developer is added to make Fe 3+ Converted into red ferric thiocyanate, Fe 3+ +3SCN - =Fe(SCN)3.

[0022] Furthermore, the pore size of the filter membrane is ≤10 μm.

[0023] If the pore size of the filter membrane is too large, metal foreign matter smaller than the pore size of the filter membrane will be filtered out, affecting the test results. When the pore size of the filter membrane is ≤10μm, 90% of the metal foreign matter particles can be filtered onto the filter membrane.

[0024] Preferably, the filter membrane is selected from a polypropylene filter membrane or a nylon filter membrane.

[0025] Furthermore, the ball milling time is 10 min to 30 min.

[0026] Ball milling can make the magnetic bar better and more evenly adsorb metal foreign bodies. If the ball milling time is too short (e.g., less than 10 minutes), the particles in the slurry cannot be fully dispersed, causing some metal foreign bodies to be wrapped or hidden by other particles and unable to be effectively adsorbed by the magnetic bar. If the ball milling time is too long (e.g., more than 30 minutes), the particles in the slurry will be over-refined, and even cause the particles to re-aggregate or change their chemical properties, which is not conducive to the detection of metal foreign bodies. The ball milling time of 10 to 30 minutes can ensure that the particles in the slurry are fully dispersed, so that the metal foreign bodies are evenly distributed in the slurry, which is convenient for the magnetic bar to adsorb, and at the same time, avoid the negative effects of excessive ball milling.

[0027] Preferably, the ball milling uses a jar mill.

[0028] Furthermore, the filtration is performed using a suction filtration method; the filter membrane is rinsed with dilute hydrochloric acid and then a solvent.

[0029] The invention discloses an application of a method for detecting metallic foreign matter, wherein the method is applied in monitoring metallic foreign matter in the manufacturing process of lithium-ion batteries.

[0030] 3. Beneficial effects

[0031] Compared with the existing known technology, the technical solution provided by the present invention has the following beneficial effects:

[0032] (1) A method for detecting metal foreign matter of the present invention comprises placing a magnetic rod provided with a coating layer in a slurry to be tested, ball milling the slurry and removing the magnetic rod, adsorbing the metal foreign matter in the slurry to the surface of the coating layer by the magnetic rod with the coating layer, that is, extracting the metal foreign matter, and ball milling can make the magnetic rod adsorb the metal foreign matter more evenly, removing the magnetic rod, and obtaining a coating layer for adsorbing the metal foreign matter; washing the coating layer with a solvent to obtain a cleaning liquid, which contains the extracted metal foreign matter; filtering the cleaning liquid with a filter membrane to obtain a filtered filter membrane; taking a clean filter membrane, dripping an acidic solution and an oxidizing solution on the surface of the filter membrane in turn, spreading the filtered filter membrane on the surface of the clean filter membrane, letting it stand, dripping a color developer, and determining the content range of the metal foreign matter according to the number of color development points. The appearance of colored dots on the surface of the filter membrane indicates the presence of metal foreign matter, and the number of dots is positively correlated with the content of the metal foreign matter. Whether the slurry is qualified can be preliminarily determined according to the number of dots. Taking 200ppb of the production line as the standard, for the positive electrode slurry, when the number of points is greater than or equal to 8, the ICP test result is greater than 200ppb, the metal foreign matter exceeds the standard, and the slurry is abnormal; for the negative electrode slurry, when the number of points is greater than 10, the ICP test result is greater than 200ppb, the metal foreign matter exceeds the standard, and the slurry is abnormal. The detection method of this application is simple to operate, highly accurate, and takes less time.

[0033] (2) The metal foreign matter detection method of the present invention is applied to the monitoring of metal foreign matter in the lithium-ion battery manufacturing process. The slurry of the production line can be inspected every shift, and abnormalities can be quickly discovered and handled in time, thereby improving production efficiency and production stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a correlation distribution diagram of the negative electrode slurry test results and ICP test results of this application;

[0035] Figure 2 This is a correlation distribution diagram of the positive electrode slurry test results and ICP test results of this application;

[0036] Figure 3 A diagram of the filtration device used in this application;

[0037] Figure 4 Comparison diagram of heat shrink tubing wrapped around magnetic rods;

[0038] Figure 5 This is the operation diagram of the color development experiment for this application.

[0039] Among them: 1. Vacuum filter pump; 2. Wash bottle; 3. Buchner funnel; 4. Magnetic bar; 5. Heat shrink tube; 6. Surface dish; 7. Clean filter membrane; 8. Filter membrane after filtration. DETAILED DESCRIPTION

[0040] In order to further understand the content of the present invention, the present invention is described in detail in conjunction with embodiments.

[0041] Example 1

[0042] A method for detecting metal foreign matter in this embodiment is a preliminary experiment to determine the size distribution of metal foreign matter in the slurry. The operation steps are as follows:

[0043] S1. Take 200g of negative electrode slurry from the main screw into a wide-mouth bottle, add 200mL of deionized water to dilute it, and obtain the slurry to be tested;

[0044] S2. Take a clean magnetic rod 4, put on a heat shrink tube 5, seal both ends with a sealing machine, put it into a wide-mouth bottle containing the slurry to be tested, and place the wide-mouth bottle in a ball mill for 20 minutes;

[0045] Ball milling can make the magnetic rod 4 better and more evenly adsorb metal foreign matter. The ball milling time of 20 minutes can ensure that the particles in the slurry are fully dispersed, so that the metal foreign matter is evenly distributed in the slurry, which is convenient for the magnetic rod to adsorb, and at the same time, avoid the negative effects caused by excessive ball milling.

[0046] S3. After the ball milling is completed, the slurry in the wide-mouth bottle is poured out, leaving the magnetic rod 4 with the heat shrink tube 5, and the surface of the heat shrink tube 5 is repeatedly cleaned with deionized water until no obvious black main material remains, and one end of the heat shrink tube 5 is cut with a ceramic blade, and the magnetic rod 4 is taken out to obtain the heat shrink tube 5 that absorbs the metal foreign matter;

[0047] S4. Place the heat shrink tube 5 into a clean beaker, add deionized water to immerse the heat shrink tube 5, and ultrasonically clean it for 3 minutes to obtain a cleaning solution containing metal foreign matter;

[0048] S5: Take a 10 μm filter membrane, preferably a mesh nylon filter membrane, rinse and rinse with dilute hydrochloric acid and deionized water, put it into a Buchner funnel 3, turn on the vacuum filter pump 1, slowly pour the cleaning solution containing metal foreign matter, and the filtrate flows into the washing bottle 2. The filtered filter membrane 8 is to be tested;

[0049] The filter membrane 8 was placed in an oven and dried at 70° C. for 15 min. After drying, an optical microscope was used to observe and count the membranes.

[0050] The percentage of metal foreign bodies of different sizes is shown in Table 1:

[0051] Table 1 Size and quantity of metal foreign matter in slurry

[0052] Length(μm) Number of metals per filter membrane >1000 0 600～1000 0 400～600 0 200～400 0 150～200 0 100～150 0 50～100 1 25～50 28 15～25 97 5～15 50

[0053] Note: The width of the metal foreign matter particles in this table only applies to the orthogonal width.

[0054] As can be seen from Table 1, the size and quantity of metal foreign matter particles in the slurry provide a basis for the selection of filter membrane pore size during filtration.

[0055] Example 2

[0056] A method for detecting metal foreign matter in this embodiment includes the following steps:

[0057] S1. Take 200g of negative electrode slurry from the main screw into a wide-mouth bottle, add 200mL of deionized water to dilute it, and obtain the slurry to be tested;

[0058] S2. Take a clean magnetic rod 4, put on a heat shrink tube 5, seal both ends with a sealing machine, put it into a wide-mouth bottle containing the slurry to be tested, and place the wide-mouth bottle in a ball mill for 20 minutes;

[0059] S3. After the ball milling is completed, the slurry in the wide-mouth bottle is poured out, leaving the magnetic rod 4 with the heat shrink tube 5, and the surface of the heat shrink tube 5 is repeatedly cleaned with deionized water until no obvious black main material remains, and one end of the heat shrink tube 5 is cut with a ceramic blade, and the magnetic rod 4 is taken out to obtain the heat shrink tube 5 that absorbs the metal foreign matter;

[0060] S4. Place the heat shrink tube 5 that absorbs metal foreign matter into a clean beaker, add deionized water to immerse the heat shrink tube 5, and ultrasonically clean it for 3 minutes to obtain a cleaning solution containing metal foreign matter;

[0061] S5: Take a 10 μm filter membrane, preferably a mesh nylon filter membrane, rinse and rinse with dilute hydrochloric acid and deionized water, put it into a Buchner funnel 3, turn on the vacuum filter pump 1, slowly pour the cleaning solution containing metal foreign matter, and the filtrate flows into the washing bottle 2. The filtered filter membrane 8 is to be tested;

[0062] Selecting a 10μm filter membrane can accurately determine the content of metal foreign matter, and a large filter membrane pore size can increase the filtration efficiency.

[0063] S6: Take a clean filter membrane 7 of 0.22 μm, preferably a polypropylene filter membrane, place a glass surface dish 6, wet it with dilute hydrochloric acid and deionized water, then drip dilute hydrochloric acid to cover the filter membrane, then drip hydrogen peroxide to cover the filter membrane, spread the filtered filter membrane 8 on the surface of the clean filter membrane 7, and let it stand for 1 minute;

[0064] Selecting a smaller filter membrane of 0.22μm as the reaction background plate can prevent the solution from spreading and making it difficult to count the points.

[0065] S7: Use KSCN as the color developer and add it dropwise onto the surface of two overlapping filter membranes. Wait for 3 to 5 minutes until 18 red dots appear on the filter membranes.

[0066] The principle of the color reaction in this application is: adding dilute hydrochloric acid converts the metal Fe into Fe 2+, that is, Fe+2H + =Fe 2+ +H2; then add hydrogen peroxide to make Fe 2+ Oxidized to Fe 3+ , i.e. 2Fe 2+ +H2O2+2H + =2Fe 3+ +2H2O; finally, KSCN color developer is added to make Fe 3+ Converted into red ferric thiocyanate, Fe 3+ +3SCN - =Fe(SCN)3.

[0067] Example 3

[0068] S1. Take 200g of negative electrode slurry from the main screw into a wide-mouth bottle, add 200mL of deionized water to dilute it, and obtain the slurry to be tested;

[0069] S2. Take a clean magnetic rod 4, put on a heat shrink tube 5, seal both ends with a sealing machine, put it into a wide-mouth bottle containing the slurry to be tested, and place the wide-mouth bottle in a ball mill for 20 minutes;

[0070] S3. After the ball milling is completed, the slurry in the wide-mouth bottle is poured out, leaving the magnetic rod 4 with the heat shrink tube 5, and the surface of the heat shrink tube 5 is repeatedly cleaned with deionized water until no obvious black main material remains, and one end of the heat shrink tube 5 is cut with a ceramic blade, and the magnetic rod 4 is taken out to obtain the heat shrink tube 5 that absorbs the metal foreign matter;

[0071] S4. Place the heat shrink tube 5 that absorbs metal foreign matter into a clean beaker, add deionized water to immerse the heat shrink tube 5, and ultrasonically clean it for 3 minutes to obtain a cleaning solution containing metal foreign matter;

[0072] S5. Take a 10 μm filter membrane, preferably a mesh nylon filter membrane, rinse and rinse with dilute hydrochloric acid and deionized water, put it into a Buchner funnel 3, turn on the vacuum filter pump 1, slowly pour the cleaning solution containing metal foreign matter, the filtrate flows into the wash bottle 2, and the filtered filter membrane 8 is tested;

[0073] S6. Take a clean filter membrane 7 of 0.22 μm, preferably a polypropylene filter membrane, place a glass surface dish 6, wet it with dilute hydrochloric acid and deionized water, then add dilute hydrochloric acid to cover the filter membrane, then add hydrogen peroxide to cover the filter membrane, spread the filtered filter membrane 8 on the surface of the clean filter membrane 7, and let it stand for 1 minute;

[0074] S7. Use KSCN as the color developer and drop it onto the surface of two overlapping filter membranes. Wait for 3 to 5 minutes until 8 red dots appear on the filter membranes.

[0075] Example 4

[0076] A method for detecting metal foreign matter in this embodiment includes the following steps:

[0077] S1. Pour 200g of nickel-cobalt-manganese cathode slurry into a wide-mouth bottle, and then add 200mL of N,N-dimethylformamide (NMP) to dilute to obtain the slurry to be tested;

[0078] S2. Take a clean magnetic rod 4, put on a heat shrink tube 5, seal both ends with a sealing machine, put it into a wide-mouth bottle containing the slurry to be tested, and place the wide-mouth bottle in a ball mill for 20 minutes;

[0079] S3. After the ball milling is completed, the slurry in the wide-mouth bottle is poured out, leaving the magnetic rod 4 with the heat shrink tube 5, and the surface of the heat shrink tube 5 is repeatedly cleaned with N, N-dimethylformamide until there is no obvious black main material remaining, and one end of the heat shrink tube 5 is cut with a ceramic blade, and the magnetic rod 4 is taken out to obtain the heat shrink tube 5 that adsorbs the metal foreign matter;

[0080] S4. Place the heat shrink tube 5 adsorbing metal foreign matter into a clean beaker, add N,N-dimethylformamide to immerse the heat shrink tube, and ultrasonically clean for 3 minutes to obtain a cleaning solution containing metal foreign matter;

[0081] S5. Take a 5μm filter membrane, preferably a polypropylene filter membrane, rinse and rinse with dilute hydrochloric acid and N,N-dimethylformamide, put into the Buchner funnel 3, turn on the vacuum filter pump 1, slowly pour the cleaning solution containing metal foreign matter, the filtrate flows into the wash bottle 2, and the filtered filter membrane 8 is tested;

[0082] Since the minimum size of metal foreign matter particles is 5μm, and because the content of metal foreign matter in the positive electrode slurry is lower than that in the negative electrode slurry, and the size of the positive electrode main material is less than 1μm, in order to ensure the accuracy of the detection, a 5μm filter membrane is selected, which can not only filter out the residual positive electrode main material, but also allow the metal foreign matter to be filtered and retained on the filter membrane to the maximum extent.

[0083] S6. Take a clean filter membrane 7 of 0.22 μm, preferably a polypropylene filter membrane, place a glass surface dish 6, wet it with dilute hydrochloric acid and N,N-dimethylformamide, then drip dilute hydrochloric acid to cover the filter membrane, then drip hydrogen peroxide to cover the filter membrane, spread the filtered filter membrane 8 on the surface of the clean filter membrane 7, and let it stand for 1 minute;

[0084] S7. Select KSCN as the color developer and drop it onto the surface of two overlapping filter membranes. Wait for 3 to 5 minutes until 9 red dots appear on the filter membranes.

[0085] Example 5

[0086] A method for detecting metal foreign matter in this embodiment includes the following steps:

[0087] S1. Pour 200g of nickel-cobalt-manganese cathode slurry into a wide-mouth bottle, and then add 300mL of N,N-dimethylformamide (NMP) to dilute to obtain the slurry to be tested;

[0088] S2. Take a clean magnetic rod 4, put on a heat shrink tube 5, seal both ends with a sealing machine, put it into a wide-mouth bottle containing the slurry to be tested, and place the wide-mouth bottle in a ball mill for 20 minutes;

[0089] S3. After the ball milling is completed, the slurry in the wide-mouth bottle is poured out, leaving the magnetic rod 4 with the heat shrink tube 5, and the surface of the heat shrink tube 5 is repeatedly cleaned with N, N-dimethylformamide until there is no obvious black main material remaining, and one end of the heat shrink tube 5 is cut with a ceramic blade, and the magnetic rod 4 is taken out to obtain the heat shrink tube 5 that adsorbs the metal foreign matter;

[0090] S4. Place the heat shrink tube 5 adsorbing metal foreign matter into a clean beaker, add N,N-dimethylformamide to immerse the heat shrink tube, and ultrasonically clean for 3 minutes to obtain a cleaning solution containing metal foreign matter;

[0091] S5. Take a 5μm filter membrane, preferably a polypropylene filter membrane, rinse and rinse with dilute hydrochloric acid and N,N-dimethylformamide, put into the Buchner funnel 3, turn on the vacuum filter pump 1, slowly pour the cleaning solution containing metal foreign matter, the filtrate flows into the wash bottle 2, and the filtered filter membrane 8 is tested;

[0092] S6. Take a clean filter membrane 7 of 0.22 μm, preferably a polypropylene filter membrane, place a glass surface dish 6, wet it with dilute hydrochloric acid and N,N-dimethylformamide, then drip dilute hydrochloric acid to cover the filter membrane, then drip hydrogen peroxide to cover the filter membrane, spread the filtered filter membrane 8 on the surface of the clean filter membrane 7, and let it stand for 1 minute;

[0093] S7. Use KSCN as the color developer and drop it onto the surface of two overlapping filter membranes. Wait for 3 to 5 minutes until 4 red dots appear on the filter membranes.

[0094] Example 6

[0095] A method for detecting metal foreign matter in this embodiment, including determining accuracy, comprises the following steps:

[0096] Multiple batches of negative electrode slurry were continuously sampled and divided into two equal parts. One part was used to detect the number of red spots of metal foreign matter using the method of Example 2, and the other part was sent for ICP testing. The results are shown in Table 2:

[0097] Table 2 Comparison of the color reaction test results of metallic foreign matter in the negative electrode slurry and the ICP test results

[0098] Sample No. Color reaction (number of red dots) ICP(ppb) 1 22 330 2 30 372.1 3 7 117 4 9 161.5 5 11 215.6

[0099] From Table 2 and Figure 1It can be seen that the detection results of the color development reaction of metallic foreign matter in the negative electrode slurry are linearly related to the ICP test results. Taking the production line 200ppb as the standard, when the ICP test result is greater than 200ppb, that is, when the number of red dots of the color development reaction is greater than 10, the metallic foreign matter in the negative electrode slurry exceeds the standard and it is an abnormal slurry.

[0100] Example 6

[0101] A method for detecting metal foreign matter in this embodiment, including determining accuracy, comprises the following steps:

[0102] Multiple batches of positive electrode slurry were continuously sampled and divided into two equal parts. One part was used to detect the number of red spots of metal foreign matter using the method of Example 4, and the other part was sent for ICP testing. The results are shown in Table 3:

[0103] Table 3 Comparison of the color reaction test results of metal foreign matter in the positive electrode slurry and the ICP test results

[0104] Sample No. Color reaction (number of red dots) ICP(ppb) 1 7 173.5 2 6 156.9 3 5 81.3 4 8 245.7 5 7 171

[0105] From Table 3 and Figure 2 It can be seen that the detection results of the color development reaction of metallic foreign matter in the positive electrode slurry are linearly related to the ICP test results. Taking the production line 200ppb as the standard, when the ICP test result is greater than 200ppb, that is, when the number of red dots of the color development reaction is greater than or equal to 8, the metallic foreign matter in the positive electrode slurry exceeds the standard and is an abnormal slurry.

[0106] Comparative Example 1

[0107] A method for detecting metal foreign matter in this comparative example comprises the following steps:

[0108] Take 200g of negative electrode slurry from the main screw outlet into a wide-mouth bottle and add 200mL of deionized water to dilute it;

[0109] Take a 30μm filter membrane, wet it with dilute hydrochloric acid and deionized water, put it into a Buchner funnel and filter it. After filtering, rinse the surface with deionized water several times.

[0110] Place the above filter membrane in a beaker filled with deionized water and clean it ultrasonically for 5 minutes;

[0111] Take another 30μm filter membrane, wet it with dilute hydrochloric acid and deionized water, put it into the Buchner funnel for the second filtration, and rinse the filter membrane surface with deionized water several times after the filtration is completed;

[0112] Take a 0.22 μm filter membrane, wet it with dilute hydrochloric acid and deionized water, then add dilute hydrochloric acid to cover the filter membrane, then add hydrogen peroxide to cover the filter membrane, spread the filtered filter membrane on the surface of the filter membrane, and let it stand for 1 minute;

[0113] Select KSCN as the color developer and add it dropwise to the surface of the filter membrane. Wait for 3 to 5 minutes and three red dots will appear on the surface of the filter membrane.

[0114] This comparative example and Example 2 were sampled at the same time. When detecting metal foreign matter in the slurry, the slurry was directly filtered twice with a 30μm filter membrane during the pretreatment process. Although the main material in the slurry (the size of the negative electrode main material is 25μm to 50μm) was filtered out, metal foreign matter smaller than the main material was also filtered out, resulting in inaccurate final color development reaction results. Directly filtering the slurry and detecting metal foreign matter with the filter membrane after filtration by color development method is simple to operate, but the pretreatment will filter out a large number of metal particles, and it is impossible to accurately determine whether the slurry with a metal foreign matter particle size smaller than the main material exceeds the standard.

[0115] It can be seen from Examples 6 and 7 that when the number of red dots in the color development reaction of the filter membrane is greater than 10, the ICP test result is greater than 200ppb, the metal foreign matter in the negative electrode slurry exceeds the standard, and it is an abnormal slurry; when the number of red dots in the color development reaction of the filter membrane is greater than or equal to 8, the ICP test result is greater than 200ppb, and the metal foreign matter in the positive electrode slurry exceeds the standard, which is an abnormal slurry. It can be judged that: in Example 2, 18 red dots appeared on the filter membrane, the metal foreign matter in the negative electrode slurry exceeded the standard, which is an abnormal slurry; in Example 3, 8 red dots appeared on the filter membrane, and the metal foreign matter in the negative electrode slurry was normal; in Example 4, 9 red dots appeared on the filter membrane, and the metal foreign matter in the positive electrode slurry exceeded the standard, which is an abnormal slurry; in Example 5, 4 red dots appeared on the filter membrane, and the metal foreign matter in the positive electrode slurry was normal.

[0116] The above-mentioned embodiments only express the preferred implementation modes of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the present invention. It should be pointed out that, for a person skilled in the art, several modifications, improvements and substitutions can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the attached claims.

Claims

1. A method for detecting metal foreign matter, characterized in that: The following steps are involved: placing a magnetic rod (4) provided with a coating layer in a slurry to be tested, ball-milling the slurry and removing the magnetic rod to obtain a coating layer for absorbing metallic foreign matter; The coating layer for absorbing the metal foreign matter is cleaned with a solvent to obtain a cleaning solution; Filtering the cleaning liquid through a filter membrane to obtain a filtered filter membrane (8); Take a clean filter membrane (7), drip an acidic solution and an oxidizing solution onto the surface in sequence, spread the filtered filter membrane (8) onto the surface of the clean filter membrane (7), let it stand, drip a color developer, and determine the content range of the metal foreign matter based on the color development points.

2. A method for detecting metal foreign matter according to claim 1, characterized in that: The slurry to be tested comprises positive electrode slurry and / or negative electrode slurry, and a solvent; the solvent is deionized water or N,N-dimethylformamide, and the material-liquid ratio is 1:1-1.5 calculated by mass ratio.

3. A method for detecting metal foreign matter according to claim 1, characterized in that: The coating layer is a heat shrink tube (5).

4. A method for detecting metal foreign matter according to claim 1, characterized in that: The acidic solution is dilute hydrochloric acid and / or dilute nitric acid.

5. A method for detecting metal foreign matter according to claim 1, characterized in that: The oxidizing solution is a hydrogen peroxide solution.

6. A method for detecting metal foreign matter according to claim 1, characterized in that: The color developer is one or more of potassium thiocyanate, dicarbohydrazide and / or dimethylglyoxime.

7. A method for detecting metal foreign matter according to any one of claims 1 to 6, characterized in that: The pore size of the filter membrane is ≤10 μm.

8. A method for detecting metallic foreign matter according to claim 7, characterized in that: The ball milling time is 10 min to 30 min.

9. A method for detecting metallic foreign matter according to claim 8, characterized in that: The filtration is carried out by suction filtration; the filter membrane is rinsed with dilute hydrochloric acid and then with a solvent.

10. Application of a method for detecting metallic foreign matter, characterized in that: Application of the method according to any one of claims 1 to 9 in monitoring metallic foreign matter in the manufacturing process of lithium-ion batteries.

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