A method for detecting the content of impurity tricobalt tetraoxide in lithium cobaltate

Abstract

The application belongs to the technical field of impurity detection, and discloses a method for detecting the content of impurity tricobalt tetraoxide in lithium cobalt oxide, which comprises the following steps: dissolving lithium cobalt oxide and a reducing agent by using mixed acid liquid with a volume concentration of 20% after mixing, filtering to obtain powder, and determining the content of tricobalt tetraoxide by using ICP-OES method; the reducing agent is ferrous ammonium sulfate hexahydrate; the mixed acid liquid is a mixture of phosphoric acid solution with a mass concentration of 85% and sulfuric acid solution with a mass concentration of 95%; and the volume proportion of the phosphoric acid solution with a concentration of 85% in the mixed acid liquid is 65-90%. The lithium cobalt oxide is dissolved by using a medium with moderate acidity mainly composed of phosphoric acid and supplemented by sulfuric acid, so that the reactivity of the acid to tricobalt tetraoxide in the dissolving process is reduced, and the specific reducing agent is combined to effectively inhibit the reaction loss of tricobalt tetraoxide in the lithium cobalt oxide dissolving process, thereby improving the test accuracy of the content of tricobalt tetraoxide.

Description

Technical Field

[0001] This invention relates to the field of impurity detection technology, and specifically to a method for detecting the content of cobalt tetroxide, an impurity, in lithium cobalt oxide. Background Technology

[0002] In most high-temperature solid-state synthesis processes, cobalt tetroxide is the main raw material for lithium cobalt oxide. However, in actual production, in order to ensure complete reaction, a small amount of cobalt tetroxide is often added in excess to prevent the formation of a cobalt-deficient phase. This unreacted material is the byproduct.

[0003] Since cobalt tetroxide itself does not participate in the lithium-ion insertion / extraction process, its presence will reduce the proportion of lithium active material in the material. Although it may seem like there is "more cobalt" in theory, it actually dilutes the available lithium ions, resulting in a higher nominal capacity, a significant drop in voltage plateau during discharge, and an actual energy density that is far lower than expected. Excessive cobalt tetroxide or its decomposition products (such as cobalt suboxide CoO) will introduce monoclinic phase impurities. These impurities will destroy the orderliness of the LiCoO2 layered structure, making it more prone to irreversible phase transitions during charge-discharge cycles, leading to rapid capacity decay and a significant decrease in cycle life.

[0004] Therefore, to avoid excessive cobalt tetroxide content in the produced lithium cobalt oxide, testing is necessary for quality control. Existing technology 1: Chinese Patent 202211134504.3 discloses a method for determining the residual cobalt tetroxide content in lithium cobalt oxide materials, including the following steps: mixing the sample to be tested with a hydrochloric acid solution of a first concentration and performing a first heating treatment to prepare a mixture; separating the liquid and residue in the mixture; mixing the residue with a perchloric acid solution and performing a second heating treatment until perchloric acid fumes are generated to prepare a test solution; determining the cobalt content in the test solution using inductively coupled plasma atomic emission spectrometry; determining the mass percentage of cobalt tetroxide in the sample based on the cobalt content in the test solution; utilizing the different reactivity of lithium cobalt oxide and residual cobalt tetroxide to hydrochloric acid at low temperatures, dissolving lithium cobalt oxide in hydrochloric acid, thereby separating lithium cobalt oxide and cobalt tetroxide, facilitating subsequent quantitative testing of residual cobalt tetroxide. This method uses few reagents, is simple to operate, has a short testing cycle, and possesses high accuracy.

[0005] Existing technology 1 uses hydrochloric acid of different concentrations to dissolve lithium cobalt oxide, thereby separating lithium cobalt oxide from cobalt tetroxide. However, hydrochloric acid is a strong acid and still has a certain degree of solubility for cobalt tetroxide. 3+ It exhibits strong oxidizing properties in acidic media and can oxidize Cl. - The ions are converted to Cl2, therefore using hydrochloric acid or chloride salts will cause Co in cobalt tetroxide to be converted to Cl2. 3+ Some of the material is lost through reduction and dissolution, which in turn affects the accuracy of the test. Summary of the Invention

[0006] One of the objectives of this invention is to provide a method for detecting the content of cobalt tetroxide, an impurity, in lithium cobalt oxide. This method uses a mixed acid solution with phosphoric acid as the main component to dissolve lithium cobalt oxide during sample processing, and employs a specific reducing agent to stabilize the entire dissolution system. This effectively suppresses the reaction loss of cobalt tetroxide during the dissolution process of lithium cobalt oxide and improves the accuracy of cobalt tetroxide content testing.

[0007] To achieve the above objectives, the present invention provides a method for detecting the cobalt tetroxide content in lithium cobalt oxide. The method involves mixing lithium cobalt oxide with a reducing agent, dissolving the mixture in a diluted acid solution, filtering to obtain powder, and then determining the cobalt tetroxide content in the powder using the ICP-OES method. The reducing agent is ferrous ammonium sulfate hexahydrate, and the mass ratio of the reducing agent to lithium cobalt oxide is 9-11:1. The mixed acid solution is a mixture of an 85% phosphoric acid solution and a 95% sulfuric acid solution, with the 85% phosphoric acid solution accounting for 65-90% of the volume of the mixed acid solution.

[0008] Furthermore, the volume concentration of the diluted mixed acid solution is diluted to 20%.

[0009] Preferably, the method for preparing the mixed acid solution diluted to 20% by volume is as follows: mix an 85% phosphoric acid solution and a 95% sulfuric acid solution to obtain a mixed acid solution, and then add deionized water in a volume equal to four times that of the mixed acid solution.

[0010] Furthermore, the method specifically includes the following steps: Step 1: Mix lithium cobalt oxide and reducing agent, then add a mixed acid solution diluted to 20% by volume and stir to dissolve to obtain a mixed solution; Step 2: The mixed solution is vacuum filtered and the solid is collected. Then, it is washed with deionized water to obtain powder. Step 3: Digest the powder to prepare the sample to be tested; Step 4: Test the sample using ICP-OES and calculate the cobalt tetroxide content.

[0011] Preferably, step 2 is specifically performed as follows: using a 0.22μm mixed cellulose ester (MEC) aqueous filter membrane, the solid is collected by filtration in a vacuum filtration device, and then the solid is repeatedly washed with deionized water to obtain powder.

[0012] Preferably, in step 2, the number of times the deionized water is rinsed is ≥3 times, and the amount of deionized water used is ≥1L.

[0013] Preferably, step 3 is specifically performed as follows: adding perchloric acid to the powder, then digesting it using a graphite digester, and finally adding deionized water to make up the volume to obtain the sample to be tested.

[0014] Beneficial effects Compared with existing technologies, this application uses a moderately acidic medium, mainly phosphoric acid and supplemented with sulfuric acid, to dissolve lithium cobalt oxide. This reduces the reactivity of the acid to cobalt tetroxide during the dissolution process. Combined with a specific reducing agent, the reducing agent can only react with the chemically active lithium cobalt oxide, thus completely dissolving the lithium cobalt oxide without dissolving the cobalt tetroxide. This effectively suppresses the reaction loss of cobalt tetroxide during the dissolution process of lithium cobalt oxide and improves the accuracy of cobalt tetroxide content testing. Detailed Implementation

[0015] The present invention will be further described below with reference to embodiments, but this does not constitute any limitation on the present invention. Any limited modifications made within the scope of the claims of the present invention are still within the scope of the claims of the present invention.

[0016] To illustrate the technical content of the present invention in detail, the following description is provided in conjunction with the embodiments.

[0017] The following examples and comparative examples all used the same batch of lithium cobalt oxide, with the particle size ground to D50=15μm; the mixed cellulose ester (MEC) aqueous filter membrane was purchased from the "Green Mall" brand under Jiangsu Green Alliance Scientific Instruments Co., Ltd.

[0018] Example 1 A method for detecting the cobalt tetroxide content, an impurity, in lithium cobalt oxide includes the following steps: Step 1: Mix 0.2g of lithium cobalt oxide and 2g of ferrous ammonium sulfate hexahydrate powder in a screw-top glass flask; Step 2: Mix an 85% phosphoric acid solution and a 95% sulfuric acid solution to obtain a mixed acid solution, wherein the volume of the 85% phosphoric acid solution is 65%. Then add 4 times the volume of the mixed acid solution of deionized water to obtain a mixed acid solution with a volume concentration of 20%. Step 3: Add 20 ml of 20% (v / v) mixed acid solution to the screw-top glass flask from Step 1, and stir for 2 hours to dissolve the lithium cobalt oxide to obtain a mixed solution; Step 4: Vacuum filter the mixed solution using a 0.22μm aqueous cellulose filter membrane and collect the solid. Then, rinse the solution three times with deionized water to obtain powder. The amount of deionized water used is ≥1L. Step 5: Transfer the collected powder to a 50ml glass graduated digestion tube, add 3ml of perchloric acid, turn on the graphite digester, set the temperature to 200℃, place the 50ml glass graduated digestion tube in the heating hole, heat for 30 minutes, remove and cool to room temperature, then add deionized water to make up to 50ml to obtain the sample to be tested. Step 6: The Co content of the sample was obtained by testing the sample using the ICP-OES method. The instrument parameters were the recommended parameters, and the test wavelength was 237.862 nm. The recommended standard solutions for the working curve test are Co concentrations of 0 mg / L, 20 mg / L, 40 mg / L, 60 mg / L, and 80 mg / L. Each concentration of standard solution is prepared by diluting the Co national standard solution with the code GSB G 62021-90. The testable cobalt tetroxide content ranges from 0% to 2.0%. 4) Substitute the measured Co value into the following formula to calculate the cobalt tetroxide content: ;

[0019] In the formula, w （Co3O4） This refers to the relative mass content of cobalt tetroxide, expressed in ppm; C Co The concentration of Co measured by ICP-OES is in mg / L; V refers to the volume of the test solution after dilution in L; 1.3621 is the mass conversion factor between Co and Co3O4; m 样品 This refers to the mass of the weighed lithium cobalt oxide sample.

[0020] Example 2 It is largely the same as Example 1, except that the volume percentage of the 85% phosphoric acid solution in the mixed acid solution is 75%.

[0021] Example 3 It is largely the same as Example 1, except that the volume percentage of the 85% phosphoric acid solution in the mixed acid solution is 80%.

[0022] Example 4 It is largely the same as Example 1, except that the volume percentage of the 85% phosphoric acid solution in the mixed acid solution is 90%.

[0023] Comparative Example 1 It is largely the same as Example 1, except that: Step 2 is changed to: mixing hydrochloric acid with a mass concentration of 38% with deionized water to obtain a hydrochloric acid solution with a mass concentration of 15%. Step 3 is modified as follows: Add 20 ml of 15% hydrochloric acid solution to the screw-top glass flask from step 1, and stir for 2 hours to dissolve the lithium cobalt oxide to obtain a mixed solution.

[0024] Comparative Example 2 Step 1 is changed to: Mix 0.2g of lithium cobalt oxide and 2g of ferrous carbonate (FeCO3) in a screw-top glass flask.

[0025] Comparative Example 3 Step 1 is changed to: Mix 0.2g of lithium cobalt oxide and 2g of ferrous acetate tetrahydrate (C4H6FeO4·4H2O) in a screw-top glass flask.

[0026] Comparative Example 4 It is largely the same as Example 1, except that: Step 1 is changed to: Mix 0.2g of lithium cobalt oxide and 20ml of FeSO4 solution with a concentration of 200g / L in a screw-top glass flask.

[0027] Comparative Example 5 It is largely the same as Example 1, except that: Step 2 is modified as follows: a mixed acid solution is obtained by mixing an 85% phosphoric acid solution and a 95% sulfuric acid solution, wherein the volume percentage of the 85% phosphoric acid solution is 95%. Then, deionized water with a volume of 4 times that of the mixed acid solution is added to the mixed acid solution to obtain a mixed acid solution with a volume concentration of 20%.

[0028] Comparative Example 6 It is largely the same as Example 1, except that: Step 2 is modified as follows: a mixed acid solution is obtained by mixing an 85% phosphoric acid solution and a 95% sulfuric acid solution, wherein the volume percentage of the 85% phosphoric acid solution is 55%. Then, deionized water with a volume of 4 times that of the mixed acid solution is added to the mixed acid solution to obtain a mixed acid solution with a volume concentration of 20%.

[0029] I. Validation of repeatability and accuracy of 5000ppm spike. The same batch of lithium cobalt oxide products was used as sample A, and the tests were performed according to the methods of Example 1 and Comparative Examples 1-6. Meanwhile, high-purity cobalt tetroxide powder with a purity of ≥99.9% was thoroughly mixed with sample A to prepare sample A-1 with a theoretical cobalt tetroxide content of 5000ppm, and the sample was tested according to the methods of Example 1 and Comparative Examples 1-6. The difference between the test results of sample A-1 and sample A was calculated to determine the test recovery rate and verify the test accuracy. The results are shown in Table 1.

[0030] Table 1. Results of Repeatability and Accuracy Verification of the Detection Method

[0031] According to the results in Table 1: The spiked recoveries of Examples 1-4 were consistently >90%, indicating high accuracy; among them, the spiked recovery of Example 1 was 96.78%, demonstrating good accuracy.

[0032] Compared to Example 1, the recovery rate of Comparative Example 1, which used hydrochloric acid, was <80%. It is speculated that this is because the use of hydrochloric acid caused cobalt tetroxide to dissolve, resulting in component loss and thus affecting the accuracy of detection.

[0033] The recovery rate of Comparative Example 2, which used ferrous carbonate, was 85.60%, which is significantly low. Comparative Examples 3 and 4 both had recoveries >90%, indicating high accuracy; however, the ferrous acetate tetrahydrate used in Comparative Example 3 has an irritating odor, which is unfriendly to laboratory personnel. Preferably, Examples 1 and Comparative Example 4 represent more accurate testing methods.

[0034] However, the accuracy of Comparative Example 4 was also significantly lower than that of Example 1, indicating that the selection of the reducing agent is quite important in the technical solution of this application.

[0035] Based on the comparison of the results of Example 1 and Comparative Examples 5 and 6, it can be seen that in the technical solution of this application, a high or low proportion of sulfuric acid in the mixed acid solution will significantly affect the test results.

[0036] II. Stability determination of cobalt tetroxide content The same batch of lithium cobalt oxide products was used as sample B, and the cobalt tetroxide content was tested according to the detection methods of Example 1 and Comparative Example 4. The sample was prepared and tested again every day, and data were collected for a total of 15 days. The test stability of the two methods was compared, and the results are shown in Table 2.

[0037] Table 2 Stability Test of Detection Method Example 1 Comparative Example 4 day 1 7557 6988 Day 2 7255 7022 Day 3 7324 7211 day4 6973 6993 day5 7224 7386 day6 7084 7343 day 7 7387 7421 day8 7459 7338 day9 7099 7985 day10 7209 7437 day11 7129 6747 day 12 7166 6862 Day 13 7116 6902 day14 7173 6762 day15 7134 7450 average value 7219 7190 Standard deviation 155 336 RSD% 2.15 4.67

[0038] According to the results in Table 2: Compared with Comparative Example 4, Example 1 showed a lower RSD% and better test stability.

[0039] III. Accuracy Verification of Multi-gradient Cobalt Tetraoxide with Standard Lithium cobalt oxide from the same batch was used as sample C, and the detection method of Example 1 was employed. Simultaneously, high-purity cobalt tetroxide powder with a purity ≥99.9% was thoroughly mixed with sample C to prepare three samples with different concentration gradients of cobalt tetroxide: 1000 ppm, 3000 ppm, and 5000 ppm, respectively, and named C-1, C-2, and C-3. The differences between the results of C-1 to C-3 and those of sample C were compared with the theoretical addition values ​​to verify the accuracy of the test method at different concentration gradients. Nine parallel samples were prepared for each sample; the results are shown in Table 3.

[0040] Table 3 Test Results

[0041] sample <![CDATA[Average Co3O4 content measurement (n = 9, ppm)]]> Test RSD (%) <![CDATA[Theoretical addition value of Co3O4 (ppm)]]> Measured difference (ppm) Spike recovery rate (%) Sample C 3679 1.44 - - - Sample C-1 4732 2.86 1000 1053 105.30 Sample C-2 6527 1.25 3000 2848 94.93 Sample C-3 8396 1.96 5000 4717 94.34

[0042] According to the results in Table 3: The spiked recoveries of the three gradient spiked tests were all within the range of 90-110%, indicating that the test method of the present invention has good test accuracy for lithium cobalt oxide samples containing cobalt tetroxide in each gradient.

[0043] The embodiments presented herein are merely selected implementations based on combinations of all possible embodiments. The appended claims should not be limited to the embodiments described herein. Some numerical ranges used in the claims include sub-ranges within them, and variations within these ranges should also be covered by the appended claims.

Claims

1. A method for detecting the content of cobalt tetroxide, an impurity, in lithium cobalt oxide, characterized in that, Lithium cobalt oxide was mixed with a reducing agent and dissolved in a diluted mixed acid solution. The resulting powder was then filtered, and the cobalt tetroxide content was determined by ICP-OES. The reducing agent is ferrous ammonium sulfate hexahydrate, and the mass ratio of the reducing agent to lithium cobalt oxide is 9-11:

1. The mixed acid solution is a mixture of an 85% phosphoric acid solution and a 95% sulfuric acid solution, with the 85% phosphoric acid solution accounting for 65-90% of the volume of the mixed acid solution.

2. The method for detecting the content of cobalt tetroxide, an impurity, in lithium cobalt oxide according to claim 1, is characterized in that, Specifically, the steps include the following: Step 1: Mix lithium cobalt oxide and reducing agent, then add mixed acid solution diluted to 20% by volume and stir to dissolve to obtain a mixed solution; Step 2: Vacuum filter the mixed solution and collect the solid, then wash with deionized water to obtain powder; Step 3: Digest the powder to prepare the sample to be tested; Step 4: Test the sample using ICP-OES and calculate the cobalt tetroxide content.

3. The method for detecting the content of cobalt tetroxide, an impurity, in lithium cobalt oxide according to claim 2, is characterized in that, The specific operation of step 2 is as follows: using a 0.22μm cellulose aqueous filter membrane, the solid is filtered and collected on a vacuum filtration device, and then the solid is repeatedly washed with deionized water to obtain powder.

4. A method for detecting the content of cobalt tetroxide, an impurity, in lithium cobalt oxide according to claim 2 or 3, characterized in that, In step 2, the number of times the deionized water is rinsed is ≥3, and the amount of deionized water used is ≥1L.

5. The method for detecting the content of cobalt tetroxide, an impurity, in lithium cobalt oxide according to claim 2, characterized in that, The specific operation of step 3 is as follows: add perchloric acid to the powder, then digest it using a graphite digester, and after digestion, add deionized water to make up the volume to obtain the sample to be tested.

6. The method for detecting the content of cobalt tetroxide, an impurity, in lithium cobalt oxide according to claim 1, characterized in that, The volume concentration of the diluted mixed acid solution was diluted to 20%.

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