Method for testing metal impurities in negative electrode material and application thereof

Abstract

The invention discloses a method for testing metal impurities in a negative electrode material, which comprises the following steps of: weighing a negative electrode material sample, and putting the negative electrode material sample into a digestion tank; adding a digestion solution into the digestion tank, and carrying out pre-digestion; carrying out microwave digestion on the pre-digested sample, and fixing the volume; detecting the constant-volume sample to obtain the content of metal impurities; wherein the digestion solution is nitric acid, hydrochloric acid and perchloric acid. The test method can accurately and efficiently test the content of the metal impurities in the negative electrode material, can solve the problem that the current negative electrode material is difficult to digest, greatly reduces the influence of the digestion process on the test result, and improves the accuracy of the method.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a method for testing metal impurities in negative electrode materials and uses thereof. Background Art

[0002] With the development of science and technology, new energy technology is constantly improving, and battery materials can be fully utilized. Porous carbon, as a raw material for batteries, may introduce some metal impurities during the production process. Metal impurities affect the performance of the battery. If the metal impurities exceed the standard, it will cause fatal safety hazards to the battery. Therefore, the content of metal impurities in porous carbon must be strictly controlled. The key to accurately testing the content of metal impurities in the product is the pre-treatment of the sample to ensure that the sample is completely digested and no impurity elements are introduced during the treatment process.

[0003] The digestion method in the existing national standard cannot completely dissolve the sample. The content of metal impurities measured by the porous carbon products processed by this method is inaccurate, and there is a certain difference between the test results and the true value. It is these trace differences that may cause problems such as battery short circuit, poor conductivity, unstable output power, electrochemical damage, etc. Therefore, it is necessary to develop a method that can completely digest porous carbon products to accurately measure the content of metal impurities and provide protection for the production safety and use safety of batteries and other accessories. Summary of the invention

[0004] In order to solve the above problems, the present application provides a method for testing the content of metal impurities in a negative electrode material. The testing method can accurately test the content of metal impurities in the negative electrode material, and the testing method is simple and efficient to operate.

[0005] Specifically, this application involves the following aspects:

[0006] 1. A method for testing metal impurities in a negative electrode material, comprising the following steps:

[0007] Weigh the negative electrode material sample and put it into the digestion tank;

[0008] Adding digestion solution into the digestion tank for pre-digestion;

[0009] The pre-digested samples were subjected to microwave digestion and fixed to volume;

[0010] Testing the fixed volume sample to obtain the content of metal impurities;

[0011] Wherein, the digestion solution is nitric acid, hydrochloric acid and perchloric acid.

[0012] 2. The test method according to item 1, wherein the volume ratio of nitric acid, hydrochloric acid and perchloric acid is (4-6):(4-6):(2-3).

[0013] 3. The test method according to item 1 or 2, wherein the mass volume ratio of the sample to the digestion solution is 0.1:10-15 (g / mL).

[0014] 4. The test method according to any one of items 1 to 3, wherein the maximum temperature during microwave digestion is at least 230°C.

[0015] 5. The test method according to any one of items 1 to 4, wherein the maximum temperature is maintained for 30 min to 40 min during microwave digestion.

[0016] 6. The test method according to any one of items 1 to 5, wherein the heating temperature during pre-digestion is 80°C-100°C.

[0017] 7. The test method according to any one of items 1 to 6, wherein the heating time during pre-digestion is 30 min to 50 min.

[0018] 8. The test method according to any one of items 1 to 7, wherein the metal impurities are selected from one or more of iron, cobalt, chromium, nickel, zinc, copper, aluminum, sodium and manganese.

[0019] 9. The test method according to any one of items 1 to 8, wherein the detection is performed by inductively coupled plasma-mass spectrometry, inductively coupled plasma-atomic emission spectroscopy or inductively coupled plasma-atomic absorption spectroscopy.

[0020] 10. The test method according to any one of items 1 to 9, wherein the negative electrode material is porous carbon.

[0021] 11. Nitric acid, hydrochloric acid and perchloric acid are used together to digest negative electrode materials.

[0022] 12. The use according to item 11, wherein the volume ratio of nitric acid, hydrochloric acid and perchloric acid is (4-6):(4-6):(2-3).

[0023] The testing method described in the present application can accurately and efficiently test the content of metal impurities by completely digesting the negative electrode material samples, greatly reducing the impact of the digestion process on the test results, providing key data support for subsequent research and development and production optimization, and ensuring the production safety and use of batteries and other accessories, playing a positive role in promoting the development of new energy. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The figure below is a comparison of the digestion effects of different digestion solutions. The left side of the figure shows the digestion effects of nitric acid and hydrochloric acid, and the right side shows the digestion effects of nitric acid, hydrochloric acid and perchloric acid. DETAILED DESCRIPTION

[0025] The embodiments described below are described in detail for the present application. Although specific embodiments of the present application are shown, it should be understood that the present application can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided to enable a more thorough understanding of the present application and to fully convey the scope of the present application to those skilled in the art.

[0026] It should be noted that certain words are used in the specification and claims to refer to specific components. Those skilled in the art should understand that technicians may use different nouns to refer to the same component. This specification and claims do not use the difference in nouns as a way to distinguish components, but use the functional differences of components as the criterion for distinction. As mentioned throughout the specification and claims, "including" or "comprising" are open-ended terms and should be interpreted as "including but not limited to". The subsequent description of the specification is a preferred embodiment of the present application, but the description is based on the general principles of the specification and is not intended to limit the scope of the present application. The scope of protection of the present application shall be determined by the attached claims.

[0027] The present application provides a method for testing metal impurities in negative electrode materials, which comprises the following steps: weighing a negative electrode material sample and placing it in a digestion tank; adding a digestion solution to the digestion tank for pre-digestion; performing microwave digestion on the pre-digested sample and fixing the volume; and detecting the fixed volume sample to obtain the content of metal impurities. In a specific embodiment, the digestion solution is nitric acid, hydrochloric acid and perchloric acid.

[0028] In this application, the metal impurities refer to metal elements that remain unintentionally inside or on the surface of the battery during the production, use or recycling of the battery, which may have an adverse effect on the performance, life and safety of the battery. For example, the metal impurities may be iron, cobalt, chromium, nickel, zinc, copper, aluminum, sodium and manganese.

[0029] In the present application, the digestion refers to dissolving the negative electrode material sample using a solvent to obtain a dissolved solution. In the present application, the solvent refers to a digestion solution composed of nitric acid, hydrochloric acid and perchloric acid.

[0030] In the present application, the negative electrode material sample refers to the raw material constituting the negative electrode in the battery, for example, it can be a carbon material (such as porous carbon). The structure of the porous carbon mainly includes a carbon matrix and a pore structure, and its pore size can be macropores (pore size>50nm), mesopores (mesopores) (2nm pore size≤50nm), or micropores (pore size<2nm). The porous carbon can be prepared by conventional methods in the art, such as template method and activation method. The porous carbon can be, for example, spherical porous carbon, porous carbon composite material, porous carbon film or coating, and porous carbon fiber or carbon cloth.

[0031] For the negative electrode material sample, the conventional digestion method in the existing national standard cannot completely digest it, but the testing method described in the present application can effectively digest the negative electrode material sample and accurately test the content of metal impurities in the negative electrode material sample.

[0032] In the present application, the nitric acid refers to trace metal grade nitric acid with a mass fraction of about 68% and a content of each element in the metal impurities of ≤1ppb;

[0033] The hydrochloric acid refers to trace metal grade hydrochloric acid with a mass fraction of about 35% and a content of each element in the metal impurities of ≤1.5ppb;

[0034] The perchloric acid refers to a trace metal grade perchloric acid with a content of each element in the metal impurities of ≤1.5ppb and a mass fraction of about 69%.

[0035] The present application uses nitric acid, hydrochloric acid and perchloric acid to digest the negative electrode material sample, so that the content of metal impurities in the negative electrode material sample can be accurately tested.

[0036] In a specific embodiment, the volume ratio of nitric acid, hydrochloric acid and perchloric acid in the digestion solution is (4-6):(4-6):(2-3).

[0037] For example, the volume ratio of nitric acid, hydrochloric acid, and perchloric acid in the digestion solution can be 4:4:2, 4:5:2, 4:6:2, 4:4:3, 4:5:3, 4:6:3, 5:4:2, 5:5:2, 5:6:2, 5:4:3, 5:5:3, 5:6:3, 6:4:2, 6:5:2, 6:6:2, 6:4:3, 6:5:3, 6:6:3, etc.

[0038] In a specific embodiment, nitric acid, hydrochloric acid and perchloric acid in the digestion solution are added to the digestion tank separately, and preferably, perchloric acid is added to the digestion tank last. In a specific embodiment, nitric acid, hydrochloric acid and perchloric acid in the digestion solution are added to the digestion tank in sequence. In a specific embodiment, hydrochloric acid, nitric acid and perchloric acid in the digestion solution are added to the digestion tank in sequence.

[0039] In a specific embodiment, the mass volume ratio of the negative electrode material sample to the digestion solution is 0.1:10-15 (g / mL).

[0040] For example, the mass volume ratio of the negative electrode material sample to the digestion solution can be 0.1:10 (g / mL), 0.1:11 (g / mL), 0.1:12 (g / mL), 0.1:13 (g / mL), 0.1:14 (g / mL), 0.1:15 (g / mL), etc.

[0041] In a specific embodiment, the maximum temperature during microwave digestion is at least 230°C; preferably, the maximum temperature is maintained for 30 min-40 min during digestion.

[0042] For example, the maximum temperature during microwave digestion can be 230° C., 235° C., 240° C., etc.; the holding time of the maximum temperature during digestion can be 30 min, 35 min, 40 min, etc.

[0043] In a specific embodiment, the heating temperature during pre-digestion is 80° C.-100° C.; preferably, the heating time during pre-digestion is 30 min-50 min.

[0044] For example, the heating temperature during pre-digestion can be 80°C, 90°C, 100°C, etc.; the heating time during pre-digestion can be 30 min, 38 min, 46 min, etc.

[0045] In a specific embodiment, the detection is performed by inductively coupled plasma-mass spectrometry, inductively coupled plasma-atomic emission spectrometry or inductively coupled plasma-atomic absorption spectrometry.

[0046] In the present application, inductively coupled plasma-mass spectrometry, inductively coupled plasma-atomic emission spectrometry or inductively coupled plasma-atomic absorption spectrometry are all conventional methods in the art. The present application does not impose any restrictions on the parameters in inductively coupled plasma-mass spectrometry, inductively coupled plasma-atomic emission spectrometry or inductively coupled plasma-atomic absorption spectrometry, and the conventional parameters in the art are used for testing.

[0047] In a specific embodiment, the detection is performed using inductively coupled plasma-atomic emission spectroscopy.

[0048] In the test using inductively coupled plasma-atomic emission spectrometry, contamination introduced from the outside (such as bottles and jars used to process samples) may lead to inaccurate results. To ensure the accuracy of the test results, an acid countercurrent cleaning machine is introduced in the present application. The used digestion tanks are boiled with 200°C acid steam for 300 minutes in the acid countercurrent cleaning machine and then cleaned with 18.2MΩ-cm ultrapure water. The cleanliness of the digestion tanks after cleaning is 300% higher than that of manual cleaning, ensuring the cleanliness of the bottles and jars used to process the samples.

[0049] In a specific embodiment, the method for testing the content of metal impurities in the negative electrode material comprises the following steps: weighing the negative electrode material sample and placing it in a digestion tank; adding a digestion solution to the digestion tank for pre-digestion; performing microwave digestion on the pre-digested sample and fixing the volume; detecting the fixed volume sample to obtain the content of metal impurities. In a specific embodiment, the digestion solution is nitric acid, hydrochloric acid and perchloric acid. In a specific embodiment, the volume ratio of the nitric acid, hydrochloric acid and perchloric acid is (4-6): (4-6): (2-3). In a specific embodiment, nitric acid, hydrochloric acid and perchloric acid in the digestion solution are added to the digestion tank in sequence. In a specific embodiment, the mass volume ratio of the negative electrode material sample to the digestion solution is 0.1: 10-15 (g / mL). In a specific embodiment, the maximum temperature during microwave digestion is at least 230°C. In a specific embodiment, the holding time of the maximum temperature during microwave digestion is 30min-40min. In a specific embodiment, the heating temperature during pre-digestion is 80°C-100°C. In a specific embodiment, the heating time during pre-digestion is 30min-50min. In a specific embodiment, the metal impurities are selected from one or more of iron, cobalt, chromium, nickel, zinc, copper, aluminum, sodium, and manganese. In a specific embodiment, the detection is tested by inductively coupled plasma-atomic emission spectroscopy. In a specific embodiment, the negative electrode material is porous carbon.

[0050] The present application adopts the above-mentioned test method to digest the negative electrode material sample, which solves the current problem of incomplete digestion of the negative electrode material, and can accurately test the content of metal impurities in the negative electrode material sample. The test method has good stability and high accuracy, and provides support for environmental protection and resource utilization to a certain extent.

[0051] The present application also provides the use of nitric acid, hydrochloric acid and perchloric acid as digestion solutions for digesting negative electrode materials.

[0052] In a specific embodiment, the volume ratio of nitric acid, hydrochloric acid and perchloric acid in the digestion solution is (4-6):(4-6):(2-3).

[0053] For example, the volume ratio of nitric acid, hydrochloric acid, and perchloric acid in the digestion solution can be 4:4:2, 4:5:2, 4:6:2, 4:4:3, 4:5:3, 4:6:3, 5:4:2, 5:5:2, 5:6:2, 5:4:3, 5:5:3, 5:6:3, 6:4:2, 6:5:2, 6:6:2, 6:4:3, 6:5:3, 6:6:3, etc.

[0054] The present application uses nitric acid, hydrochloric acid and perchloric acid as digestion solutions to digest the negative electrode material sample, which can completely digest it, thereby accurately testing the content of metal impurities in the negative electrode material sample.

[0055] Example

[0056] The present application provides a general and / or specific description of the materials and test methods used in the experiments. In the following examples, unless otherwise specified, % represents wt%, i.e., weight percentage. The reagents or instruments used without indicating the manufacturer are all conventional reagent products that can be obtained commercially.

[0057] Example 1

[0058] (1) Sample weighing

[0059] Preheat the ten-thousandth balance for more than 30 minutes (min), take a clean weighing paper and accurately weigh 22 portions of spherical microporous carbon samples of negative electrode materials (0.1000 g / portion) by differential subtraction, record the sample mass and place the samples in polytetrafluoroethylene digestion tanks respectively.

[0060] (2) Preparation of digestion solution

[0061] Prepare trace metal grade nitric acid, hydrochloric acid, and perchloric acid (the volume ratios are 3:3:2, 3:4:2, 4:4:2, 4:5:2, 4:6:2, 4:4:3, 4:5:3, 4:6:3, 5:4:2, 5:5:2, 5:6:2, 5:4:3, 5:5:3, 5:6:3, 6:4:2, 6:5:2, 6:6:2, 6:4:3, 6:5:3, 6:6:3, 6:7:2, and 7:7:2, respectively) (mL). Use a bottle-mouth dispenser to first add nitric acid into a polytetrafluoroethylene digestion tank, then slowly add hydrochloric acid and mix, and then slowly add perchloric acid after complete cooling. During the acid addition process, the sample on the wall of the digestion tank will be flushed to the bottom, and there should be no residual sample on the wall of the digestion tank.

[0062] (3) Pre-digestion

[0063] Heat the sample at 100°C on a heating plate for 30 min, observe the sample until no yellow acid gas is emitted, take out the digestion tank and let it stand to room temperature.

[0064] (4) Dissolution

[0065] Place the sample into the microwave digestion instrument according to the instrument operating procedures and set the heating program: heat to 150°C for 25 minutes → hold for 5 minutes → heat to 200°C for 5 minutes → hold for 10 minutes → heat to 230°C for 10 minutes → keep constant at 230°C for 30 minutes.

[0066] (5) Acid removal, volume determination, and testing

[0067] Remove the lid of the digested sample and place it in the acid-driving plate for 30 minutes until no acid gas is discharged; place the sample at room temperature and dilute it to 25mL or 50mL (digestion solution volume ≥15mL) in a volumetric flask with ultrapure water; turn on the plasma emission spectrometer, balance the instrument until it is stable, prepare standard solution according to the required test elements (nickel Ni, manganese Mn, iron Fe, chromium Cr), register and run the standard solution and sample in turn, and issue a report based on the measured data; at the same time, perform a clarity (NTU) test on the digested sample.

[0068] The test results of each element content are shown in Table 1 below.

[0069] Table 1 Results of the tests on each element in Example 1

[0070]

[0071]

[0072] The results of the NTU test of the samples are shown in Table 2 below.

[0073] Table 2 Results of NTU test of samples in Example 1

[0074]

[0075]

[0076] The above results show that when the volume ratio of nitric acid, hydrochloric acid and perchloric acid is 3:3:2 and 3:4:2, the clarity is poor and the sample digestion is incomplete; when the volume ratio of nitric acid, hydrochloric acid and perchloric acid is (4-7):(4-7):(2-3), the sample can be basically digested and is close to clarity; however, with the increase of the volume ratio, the test results of the content of each element and NTU do not change significantly, and when the volume is greater than 15mL, the acid content is too high and the plasma emission spectrometer cannot tolerate it. The error will also increase with the increase of the dilution factor. After testing, when the volume ratio is 6:7:2 and 7:7:2, the repeatability is poor, and the RSD values ​​of Cr are 8.78% and 8.56%, respectively.

[0077] In summary, considering the stability of subsequent tests and the tolerance of the plasma spectrometer, the preferred volume ratio of nitric acid, hydrochloric acid and perchloric acid is (4-6):(4-6):(2-3).

[0078] Example 2

[0079] (1) Sample weighing

[0080] Preheat the ten-thousandth balance for more than 30 minutes, take a clean weighing paper and accurately weigh 30 portions of spherical microporous carbon samples of negative electrode materials (0.1000g / portion) by differential subtraction, divide them into 10 groups according to the volume ratio of nitric acid, hydrochloric acid and perchloric acid, and conduct repeatability test on 3 samples in each group; record the sample mass and place the sample in a polytetrafluoroethylene digestion tank.

[0081] (2) Preparation of digestion solution

[0082] Prepare trace metal grade nitric acid, hydrochloric acid, and perchloric acid (the volume ratios are 3:3:2, 3:4:2, 4:4:2, 4:5:3, 5:5:2, 5:6:3, 6:5:3, 6:6:2, 6:7:2, and 7:7:2, respectively). Use a bottle-mouth dispenser to first add nitric acid into the polytetrafluoroethylene digestion tank, then slowly add hydrochloric acid and mix well. After it is completely cooled, slowly add perchloric acid. During the acid addition process, flush the sample on the wall of the digestion tank to the bottom. No residual sample should be left on the wall of the digestion tank.

[0083] (3) Pre-digestion

[0084] Heat the sample at 100°C on a heating plate for 30 min, observe the sample until no yellow acid gas is emitted, take out the digestion tank and let it stand to room temperature.

[0085] (4) Dissolution

[0086] Place the sample into the microwave digestion instrument according to the instrument operating procedures and set the heating program: heat to 150°C for 25 minutes → hold for 5 minutes → heat to 200°C for 5 minutes → hold for 10 minutes → heat to 230°C for 10 minutes → keep constant at 230°C for 30 minutes.

[0087] (5) Acid removal, volume determination, and testing

[0088] Remove the lid of the digested sample and place it in the acid-removing plate for 30 minutes until no acid gas is discharged; place the sample at room temperature and dilute it to 25mL or 50mL (digestion solution volume ≥15mL) in a volumetric flask with ultrapure water; take the more common Ni, Mn, Fe, and Cr as examples, and perform plasma spectrometer testing. The RSD results are shown in Table 3 below.

[0089] Table 3 Repeatability results of each element in Example 2

[0090]

[0091] The above results show that when the volume ratio of nitric acid, hydrochloric acid and perchloric acid is (4-6):(4-6):(2-3), the repeatability is good; when the volume ratio is 5:5:2, the RSD values ​​of the tested elements are all less than 2.0%, and the repeatability is the best. Therefore, when digesting the porous carbon sample of the negative electrode material, the preferred volume ratio of nitric acid, hydrochloric acid and perchloric acid is (4-6):(4-6):(2-3).

[0092] Example 3

[0093] (1) Sample weighing

[0094] Preheat the ten-thousandth balance for more than 30 minutes, take a clean weighing paper and accurately weigh 5 portions of spherical microporous carbon samples of negative electrode materials (0.1000 g / portion) by differential subtraction, record the sample mass and place the samples in polytetrafluoroethylene digestion tanks respectively.

[0095] (2) Preparation of digestion solution

[0096] Prepare trace metal grade nitric acid, hydrochloric acid, and perchloric acid (in a volume ratio of 5:5:2). Use a bottle-mouth dispenser to first add nitric acid into the polytetrafluoroethylene digestion tank, then slowly add hydrochloric acid and mix. After it is completely cooled, slowly add perchloric acid. During the acid addition process, flush the sample on the wall of the digestion tank to the bottom. No residual sample should be left on the wall of the digestion tank.

[0097] (3) Pre-digestion

[0098] Heat the sample at 100°C on a heating plate for 30 min, observe the sample until no yellow acid gas is emitted, take out the digestion tank and let it stand to room temperature.

[0099] (4) Dissolution

[0100] Place the samples in the microwave digestion instrument according to the instrument operating procedures and set the heating program: heat to 150°C for 25 minutes → hold for 5 minutes → heat to 200°C for 5 minutes → hold for 10 minutes. Heat the 5 samples to 220°C, 225°C, 230°C, 235°C, and 240°C for 10 minutes respectively and hold for 30 minutes.

[0101] (5) Acid removal, volume determination, and testing

[0102] Remove the lid of the digested sample and place it in the acid-driving plate to remove the acid for 30 minutes until no acid gas is discharged; place the sample at room temperature and dilute it to a 25mL volumetric flask with ultrapure water; take the more common Ni, Mn, Fe, and Cr as examples to perform plasma spectrometer tests, and perform NTU tests on the digested samples at the same time.

[0103] The results of the test of each element content are shown in Table 4 below.

[0104] Table 4 Results of the test of each element in Example 3

[0105]

[0106]

[0107] The results of the NTU test of the samples are shown in Table 5 below.

[0108] Table 5 Results of NTU test of samples in Example 3

[0109] Microwave digestion maximum temperature (℃) NTU 220 ＜5 225 ＜3.5 230 ＜1 235 ＜1 240 ＜1

[0110] The above results show that when the digestion temperature is above 230°C, it is beneficial to sample digestion. Considering the tolerance of the microwave digestion instrument, the preferred maximum microwave digestion temperature is 230°C.

[0111] Example 4

[0112] (1) Sample weighing

[0113] Preheat the ten-thousandth balance for more than 30 minutes, take a clean weighing paper and accurately weigh 5 portions of spherical microporous carbon samples of negative electrode materials (0.1000 g / portion) by differential subtraction, record the sample mass and place the samples in polytetrafluoroethylene digestion tanks respectively.

[0114] (2) Preparation of digestion solution

[0115] Prepare trace metal grade nitric acid, hydrochloric acid, and perchloric acid (in a volume ratio of 5:5:2). Use a bottle-mouth dispenser to first add nitric acid into the polytetrafluoroethylene digestion tank, then slowly add hydrochloric acid and mix. After it is completely cooled, slowly add perchloric acid. During the acid addition process, flush the sample on the wall of the digestion tank to the bottom. No residual sample should be left on the wall of the digestion tank.

[0116] (3) Pre-digestion

[0117] Heat the sample at 100°C on a heating plate for 30 min, observe the sample until no yellow acid gas is emitted, take out the digestion tank and let it stand to room temperature.

[0118] (4) Dissolution

[0119] Place the samples in the microwave digestion instrument according to the instrument operating procedures, and set the heating program: heat to 150°C for 25 minutes → hold for 5 minutes → heat to 200°C for 5 minutes → hold for 10 minutes → heat to 230°C for 10 minutes. Keep the five samples at 230°C for 20 minutes, 25 minutes, 30 minutes, 35 minutes, and 40 minutes, respectively.

[0120] (5) Acid removal, volume determination, and testing

[0121] Remove the lid of the digested sample and place it in the acid-driving plate to remove the acid for 30 minutes until no acid gas is discharged; place the sample at room temperature and dilute it to a 25mL volumetric flask with ultrapure water; take the more common Ni, Mn, Fe, and Cr as examples to perform plasma spectrometer tests, and perform NTU tests on the digested samples at the same time.

[0122] The results of the test of each element content are shown in Table 6 below.

[0123] Table 6 Results of the test of each element in Example 4

[0124]

[0125] The results of the NTU test of the samples are shown in Table 7 below.

[0126] Table 7 Results of NTU test of samples in Example 4

[0127] Microwave digestion maximum temperature holding time (min) NTU 20 ＜5 25 ＜3.5 30 ＜1 35 ＜1 40 ＜1

[0128] The above results show that maintaining the temperature for more than 30 min is beneficial to sample digestion. To save test time, it is preferably maintained at 230°C for 30 min.

[0129] Example 5

[0130] (1) Sample weighing

[0131] Preheat the ten-thousandth balance for more than 30 minutes, take a clean weighing paper and accurately weigh 3 portions of spherical microporous carbon samples of negative electrode materials (0.1000 g / portion) by differential subtraction, record the sample mass and place the samples in polytetrafluoroethylene digestion tanks respectively.

[0132] (2) Preparation of digestion solution

[0133] Prepare trace metal grade nitric acid, hydrochloric acid, and perchloric acid (in a volume ratio of 5:5:2). Use a bottle-mouth dispenser to first add nitric acid into the polytetrafluoroethylene digestion tank, then slowly add hydrochloric acid and mix. After it is completely cooled, slowly add perchloric acid. During the acid addition process, flush the sample on the wall of the digestion tank to the bottom. No residual sample should be left on the wall of the digestion tank.

[0134] (3) Pre-digestion

[0135] Heat 5 samples on a heating plate at 80°C, 90°C and 100°C respectively, record the time, observe the samples until no yellow acid vapor is discharged, take out the digestion tank and let it stand to room temperature; in the pre-digestion, it was found that no yellow acid vapor was produced after the sample was heated at 100°C for 30 minutes, and no yellow acid vapor was produced after heating at 80°C and 90°C for 38 minutes and 46 minutes respectively.

[0136] (4) Dissolution

[0137] Place the sample into the microwave digestion instrument according to the instrument operating procedures and set the heating program: heat to 150°C for 25 minutes → hold for 5 minutes → heat to 200°C for 5 minutes → hold for 10 minutes → heat to 230°C for 10 minutes → keep constant at 230°C for 30 minutes.

[0138] (5) Acid removal, volume determination, and testing

[0139] Remove the lid of the digested sample and place it in the acid-driving plate to remove the acid for 30 minutes until no acid gas is discharged; place the sample at room temperature and dilute it to a 25mL volumetric flask with ultrapure water; take the more common Ni, Mn, Fe, and Cr as examples to perform plasma spectrometer tests, and perform NTU tests on the digested samples at the same time.

[0140] Among them, the results of the test of each element content are shown in Table 8 below.

[0141] Table 8 Results of the test of each element in Example 5

[0142]

[0143] The results of the NTU test of the samples are shown in Table 9 below.

[0144] Table 9 Results of NTU test of samples in Example 5

[0145]

[0146]

[0147] The above results show that the samples can be pre-digested under different temperature conditions. In order to save test time, the preferred pre-digestion heating temperature is 100°C and the heating time is 30 minutes.

[0148] Comparative Example 1

[0149] Preheat the ten-thousandth balance for more than 30 minutes, take a clean weighing paper and accurately weigh 2 portions of spherical microporous carbon samples (0.1000 g / portion) of the negative electrode material by differential subtraction, record the sample mass and place the sample in a polytetrafluoroethylene digestion tank, add nitric acid and hydrochloric acid (the volume ratio is 9:3) to one sample, add nitric acid, hydrochloric acid and perchloric acid (the volume ratio is 4:5:2) to the other sample, and perform pre-digestion → digestion → acid removal, constant volume, and plasma emission spectrometer test in sequence according to the steps in Example 1. The results of the element content test are shown in Table 10 below.

[0150] Table 10 Results of the test of each element in Comparative Example 1

[0151]

[0152] The digestion of the two samples was observed with the naked eye. When nitric acid and hydrochloric acid were used for digestion, a sample solution containing black particles was finally obtained, that is, the sample was not completely digested, and the result of the sample NTU test was >5; while when nitric acid, hydrochloric acid and perchloric acid (the volume ratio was 4:5:2) were used for digestion, the obtained solution was clear.

[0153] The above results show that the samples were digested completely using nitric acid, hydrochloric acid and perchloric acid, and the digestion effect was far better than the existing digestion method (the ratio of aqua regia, nitric acid: hydrochloric acid was 9:3).

[0154] Comparative Example 2

[0155] Comparative Example 2 Compared with Example 1, nitric acid, hydrochloric acid and perchloric acid (the volume ratio is 5:5:2) are pre-mixed and then added into the digestion tank together. After the digestion is completed, a plasma spectrometer test is performed, and an NTU test is performed on the digested sample.

[0156] Among them, the results of the test of each element content are shown in Table 11 below.

[0157] Table 11 Results of the test of each element in Comparative Example 2

[0158] Test Elements ppm Ni 3.98 Mn 1.24 Fe 51.24 Cr 21.12

[0159] The result of the sample NTU test is <1.5. Under the condition of the same volume ratio of nitric acid, hydrochloric acid and perchloric acid, the digestion effect of the pre-mixed digestion solution is poorer than that of separate addition, and the nitric acid and hydrochloric acid need to be mixed and cooled before adding perchloric acid to avoid the danger of heat generation, boiling, etc. when adding them at the same time.

[0160] The above is only the preferred embodiment of the present application, and does not limit the present application in other forms. Any technician familiar with the profession may use the above disclosed technical content to change or modify it into an equivalent embodiment with equivalent changes. However, any simple modification, equivalent change and modification made to the above embodiment based on the technical essence of the present application without departing from the technical solution of the present application still falls within the protection scope of the technical solution of the present application.

Claims

1. A method for testing metal impurities in a negative electrode material, comprising the following steps: Weigh the negative electrode material sample and put it into the digestion tank; Adding digestion solution into the digestion tank for pre-digestion; The pre-digested samples were subjected to microwave digestion and fixed to volume; Testing the fixed volume sample to obtain the content of metal impurities; Wherein, the digestion solution is nitric acid, hydrochloric acid and perchloric acid.

2. The testing method according to claim 1, wherein: The volume ratio of the nitric acid, hydrochloric acid and perchloric acid is (4-6):(4-6):(2-3).

3. The test method according to claim 1 or 2, wherein: The mass volume ratio of the sample to the digestion solution is 0.1:(10-15) (g / mL).

4. The test method according to any one of claims 1 to 3, wherein: The maximum temperature during microwave digestion is at least 230°C.

5. The test method according to any one of claims 1 to 4, wherein: The maximum temperature is maintained for 30-40 minutes during microwave digestion.

6. The test method according to any one of claims 1 to 5, wherein: The heating temperature during pre-digestion is 80°C-100°C.

7. The test method according to any one of claims 1 to 6, wherein: The heating time during pre-digestion is 30min-50min.

8. The test method according to any one of claims 1 to 7, wherein: The metal impurities are selected from one or more of iron, cobalt, chromium, nickel, zinc, copper, aluminum, sodium and manganese.

9. The test method according to any one of claims 1 to 8, wherein: The detection is carried out by inductively coupled plasma-mass spectrometry, inductively coupled plasma-atomic emission spectrometry or inductively coupled plasma-atomic absorption spectrometry.

10. Nitric acid, hydrochloric acid and perchloric acid are used together to digest negative electrode materials.

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