A method for evaluating the effect of aluminum oxide coating on the surface of lithium cobalt oxide

Abstract

The present application provides a method for evaluating the effect of aluminum oxide coating on the surface of lithium cobalt oxide. The method removes the surface-coated aluminum oxide in the sample to be tested by solid alkali melting digestion, and tests the Al content of the filter residue after filtration. The better the aluminum oxide coating effect on the surface of lithium cobalt oxide, the more aluminum-containing alkaline oxides are generated at the coating interface, and the more Al content in the filter residue. Therefore, the aluminum oxide coating effect on the surface of lithium cobalt oxide can be reflected by measuring the Al content of the filter residue. By comparing with the standard Al content measured from the filter residue of the standard sample, the aluminum oxide coating effect on the surface of the sample to be tested can be clearly and intuitively understood without the need for complex electrical performance testing. It has the advantages of short detection time, simple process and low cost.

Description

Technical Field

[0001] The present application belongs to the technical field of coating effect detection of electrode active materials, and specifically relates to a method for evaluating the coating effect of aluminum oxide on the surface of lithium cobalt oxide. Background Art

[0002] Lithium-containing transition metal oxides have been widely used in commercial lithium-ion batteries. For example, lithium cobalt oxide (LCO) has the advantages of simple synthesis, high operating voltage, and good rate performance, but its structure and interface are unstable during the charge-discharge cycle, which greatly affects the performance of lithium cobalt oxide. Lithium cobalt oxide is usually coated and modified to improve the electrochemical properties of lithium cobalt oxide. Among them, coating lithium cobalt oxide with aluminum oxide (Al2O3) can establish a physical barrier between lithium cobalt oxide and the electrolyte, thereby effectively improving the stability of the interface between the electrode and the electrolyte, and improving the cycle life and high-voltage stability of lithium-ion batteries.

[0003] When coated, aluminum oxide forms an unevenly distributed, island-like, dot-like coating structure on the surface of lithium cobalt oxide. Different coating effects can significantly affect the performance of lithium cobalt oxide. Currently, the surface coating effect of lithium cobalt oxide is generally evaluated directly by testing the electrochemical performance of the battery. This evaluation method is indirect, has a long cycle, and cannot eliminate the impact of the battery preparation process and other factors on battery performance. In addition, existing technologies also use transmission electron microscopy for microscopic observations. However, this method is not only costly, but also can only analyze local microscopic areas, making it difficult to accurately reflect the overall coating effect. Summary of the Invention

[0004] The purpose of this application is to provide a method for evaluating the effect of aluminum oxide coating on the surface of lithium cobalt oxide. The surface coating aluminum oxide in the sample to be tested is removed by solid alkali melting digestion, and then the Al content in the digestion residue is tested. The aluminum oxide coating effect is determined after comparison with the standard Al content. The method has the advantages of short detection time, low cost, simplicity and effectiveness.

[0005] The method for evaluating the effect of aluminum oxide coating on the surface of lithium cobalt oxide provided in this application adopts the following scheme:

[0006] A method for evaluating the surface aluminum oxide coating effect of lithium cobalt oxide comprises the following steps: step 1, mixing a standard sample with a solid alkali, and performing a solid alkali melting digestion treatment on the surface aluminum oxide in the standard sample to obtain a digestion residue; step 2, adding deionized water to the digestion residue, filtering and collecting the filter residue, and testing the Al content of the filter residue, which is recorded as the standard Al content; step 3, repeating steps 1 and 2 on a sample to be tested to obtain an Al content to be tested, wherein the standard sample refers to a sample of lithium cobalt oxide with a desired content of aluminum oxide coated on the surface; step 4, comparing the Al content to be tested with the standard Al content, and when the Al content to be tested is greater than or equal to the standard Al content, determining that the surface aluminum oxide coating effect of the sample to be tested is qualified; when the Al content to be tested is less than the standard Al content, determining that the surface aluminum oxide coating effect of the sample to be tested is unqualified.

[0007] The method for evaluating the effect of aluminum oxide coating on the surface of lithium cobalt oxide in the present application is to perform an alkali melting digestion treatment on the sample to be tested or the standard sample by melting solid alkali. In the present application, the alkali melting digestion treatment refers to the process of melting and digesting the surface-coated aluminum oxide in the sample to be tested or the standard sample after mixing the solid alkali with the sample to be tested. When aluminum oxide coats lithium cobalt oxide, aluminum-containing alkaline oxides will be generated at the coating interface, wherein aluminum oxide is an amphoteric oxide, and lithium cobalt oxide and aluminum-containing alkaline oxides are alkaline oxides. By performing a melting digestion treatment on the sample to be tested by solid alkali, the solid alkali can react with aluminum oxide to form a metaaluminate that is easily soluble in water or acid, while maintaining the integrity of the lithium cobalt oxide. At this time, the aluminum oxide molten digestate contains a high concentration of alkali and salt. Direct testing may affect the operating state of the test instrument, resulting in poor accuracy of the test results, so dilution treatment is usually required; however, in the diluted aluminum oxide molten digestate, the relative content of Al is less than the salt concentration and alkali concentration, so there is a problem of large relative error in testing the Al content in the diluted aluminum oxide molten digestate. Therefore, this application adopts an indirect determination of the aluminum content in the filter residue. The aluminum content comes from the two parts of LCO doped with Al and the aluminum-containing alkaline oxide. The coating effect is evaluated by indirectly measuring the Al content in the filter residue, which has the advantage of small test error. The digestion residue obtained after the sample to be tested is melted and digested includes alumina molten digestate, lithium cobalt oxide and aluminum-containing alkaline oxide. Deionized water is added to dissolve the alumina digestate and then filtered to obtain a filter residue. At this time, the aluminum element in the filter residue only comes from the aluminum-containing alkaline oxide at the coating interface and the aluminum doped with lithium cobalt oxide. The better the coating effect of aluminum oxide, the larger the contact area with lithium cobalt oxide, and the more aluminum-containing alkaline oxide is generated at the coating interface. Therefore, the Al content of the filter residue can reflect the coating effect of aluminum oxide. This application uses a standard sample as an evaluation indicator for whether the aluminum oxide coating effect on the surface of lithium cobalt oxide is qualified. The standard sample refers to a sample with a desired content of aluminum oxide coated on the surface of lithium cobalt oxide. The desired content can be determined according to actual needs, and can specifically refer to cost, process efficiency, battery performance, etc. The standard sample is a sample with a known alumina coating effect on the surface of lithium cobalt oxide, for example, a lithium cobalt oxide with a qualified coating effect determined through electrochemical performance testing. By comparing the Al content to be tested with the standard Al content, the alumina coating effect of the sample under test can be clearly and intuitively understood, eliminating the need for complex electrochemical performance testing. This has the advantages of short testing time, simple process, and low cost.

[0008] When aluminum oxide is coated on lithium cobalt oxide (LiCoO2, abbreviated as LCO), aluminum-containing alkaline oxides are formed. Both aluminum-containing alkaline oxides and lithium cobalt oxide are alkaline oxides and do not react with solid alkali. After being treated with solid alkali melting and digestion, the filter residue is filtered and collected. The aluminum content in the filter residue includes the aluminum-containing alkaline oxide LiAlCoO2 generated at the coating interface and the aluminum element doped with lithium cobalt oxide. A higher Al content indicates that the aluminum oxide coating effect is good, which can be used to evaluate the aluminum oxide coating effect on the surface of lithium cobalt oxide.

[0009] Preferably, the conditions for the solid alkali melting digestion treatment include: a digestion temperature T2 of 400 to 600°C, and a digestion time of 100 to 200 minutes. The present application adopts a temperature above 400°C to allow the solid alkali to fully react with the aluminum oxide. When the temperature T2 is too high, it will cause the lithium cobalt oxide to melt and crack, and a small amount of lithium cobalt oxide will be melted, thereby affecting the accuracy of this evaluation method. By controlling the solid alkali melting digestion temperature T2 to 400 to 600°C, the present application can ensure that the alkali and aluminum oxide fully react while avoiding the melting and cracking of lithium cobalt oxide, thereby improving the measurement precision and accuracy.

[0010] Preferably, in order to avoid dissolution and cracking of lithium cobalt oxide and further improve detection accuracy, the digestion temperature T2 is 400 to 550°C.

[0011] Preferably, the solid base comprises an inorganic base; the inorganic base is selected from at least one of KOH, NaOH, Na2CO3, K2CO3, and Na2O2.

[0012] Preferably, the solid alkali melt digestion treatment further comprises: first mixing the test sample or standard sample with the solid alkali to obtain a mixture, and then preheating the mixture to dry it; the drying temperature T1 is between 150 and 300°C; and the heating rate from T1 to T2 is between 5 and 50°C / min. Considering that solid inorganic alkalis such as KOH and NaOH are easily deliquescent, and that LCO may also absorb a small amount of water during storage, rapid evaporation of water during heating can easily lead to splashing. The present application controls the drying temperature T1 to between 150 and 300°C; and the heating rate from T1 to T2 is between 5 and 50°C / min, effectively avoiding splashing and sample loss caused by excessive heating. Preferably, the mass ratio of the test sample or standard sample to the solid alkali is between 1:20 and 1:10. Controlling the mass ratio of the test sample or standard sample to the solid alkali within this range ensures sufficient sample dissolution while avoiding reagent waste and environmental pollution caused by excessive alkali, and also helps reduce the amount of acid reagent required for acid treatment of the filter residue.

[0013] Preferably, in step 2, the temperature T3 of the deionized water is 95 to 100° C. Deionized water is used to dissolve the alumina molten digestate after solid alkali molten digestion treatment. By controlling the temperature T3 of the deionized water to 95 to 100° C., sufficient dissolution of the alumina molten digestate is ensured, thereby avoiding errors caused by sample loss.

[0014] Preferably, in step 2, the filtration membrane used has a pore diameter of 0.22 to 0.45 μm. Selecting a pore diameter of 0.22 to 0.45 μm effectively filters out lithium cobalt oxide containing aluminum-containing alkaline oxides at the interface, improving the accuracy of aluminum measurement in the filter residue, shortening filtration time, and improving detection efficiency.

[0015] Preferably, in step 2, testing the Al content of the filter residue comprises: adding the filter residue to an acid for acid treatment, and then performing an inductively coupled plasma (ICP) test to obtain the Al content of the filter residue. The Al content of the filter residue is obtained by chemically reacting the aluminum-containing alkaline oxide in the filter residue with the acid and then digesting it, and then measuring it by ICP. This simple and efficient measurement method facilitates rapid and accurate determination of the Al content of the filter residue.

[0016] Preferably, the volume of acid used for each 0.2 g of filter residue is 10 to 15 mL; the acid is selected from at least one of hydrochloric acid, nitric acid, and sulfuric acid. The present application uses an acid to digest the filter residue, for example, concentrated hydrochloric acid (volume concentration of 36-38%) or concentrated nitric acid (volume concentration of 65-68%).

[0017] Preferably, the acid is a mixed acid of concentrated hydrochloric acid and concentrated nitric acid in a volume ratio of 1:5 to 5:1, for example, aqua regia (a volume ratio of concentrated hydrochloric acid to concentrated nitric acid of 3:1). Exemplarily, the mixed acid can be diluted with water in a volume ratio of 1:1 to 1:4 before use.

[0018] Preferably, the temperature T4 of the acid treatment is 180 to 220°C.

[0019] Based on the above-mentioned evaluation method of the aluminum oxide coating effect on the surface of lithium cobalt oxide of the present application, aluminum oxide forms aluminum-containing alkaline oxide at the coating interface when coating lithium cobalt oxide, and the surface-coated aluminum oxide in the sample to be tested is removed by solid alkali melting digestion, and a filter residue is obtained by filtration. The filter residue contains lithium cobalt oxide with aluminum-containing alkaline oxide at the interface, and the filter residue is acid-treated and the Al content in the lithium cobalt oxide and the aluminum-containing alkaline oxide is measured. The higher the Al content, the more aluminum-containing alkaline oxide is produced at the coating interface, that is, the better the aluminum oxide coating effect. The present application uses the Al content after filtration and separation to evaluate the aluminum oxide coating effect on the surface of lithium cobalt oxide, which is conducive to reducing the interference of other factors and the characterization results are highly accurate. Lithium cobalt oxide with the expected aluminum oxide coating content is used as a standard sample. By comparing it with the standard sample, the qualified status of the aluminum oxide coating effect of the sample to be tested can be quickly judged. This evaluation method is accurate and intuitive, and can comprehensively reflect the overall surface aluminum oxide coating of lithium cobalt oxide. It has the advantages of fast detection speed and simple process, and is conducive to promotion and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic diagram of the aluminum oxide coating effect on the surface of lithium cobalt oxide in a specific embodiment of the present application;

[0021] Figure 2 This is a photo of the filter residue obtained by filtering after digestion treatment of alumina with 30 wt% KOH solution in a specific embodiment;

[0022] Figure 3 This is a photograph of the filter residue obtained by filtering after solid alkaline fusion digestion of aluminum oxide at 400° C. using solid KOH in a specific embodiment;

[0023] Figure 4 This is a photograph of the filter residue obtained by filtering alumina after solid alkali fusion digestion treatment using solid KOH at 700°C in a specific embodiment;

[0024] Figure 5 This is the microscopic morphology of lithium cobalt oxide;

[0025] Figure 6 This is a microscopic morphology of lithium cobalt oxide after solid alkali dissolution treatment using solid KOH at 400°C in a specific embodiment;

[0026] Figure 7 This is a microscopic morphology of lithium cobalt oxide after solid alkali dissolution treatment using solid KOH at 700°C in a specific embodiment;

[0027] Reference numerals: 101 , lithium cobalt oxide; 102 , LiAlCoO 2 ; 103 , aluminum oxide. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical solutions and advantages of this application more clearly understood, the present application is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0029] When coated, alumina will form an unevenly distributed island-like point coating structure on the surface of lithium cobalt oxide. Different coating effects will have a significant impact on the performance of lithium cobalt oxide. Figure 1 A schematic diagram showing the effect of aluminum oxide coating on the surface of lithium cobalt oxide is shown in FIG. Figure 1 As shown, when the coating structure aluminum oxide 103 is coated on the lithium cobalt oxide 101, an island-like point coating structure is formed, and aluminum-containing alkaline oxide LiAlCoO2102 is formed at the interface contact between the aluminum oxide 103 and the lithium cobalt oxide 101.

[0030] Existing methods for testing the surface coating effect of lithium cobalt oxide (LCO) are time-consuming and fail to eliminate the impact of battery manufacturing processes and other factors on battery performance. Furthermore, existing techniques use transmission electron microscopy for microscopic observations, but this method is not only costly but also only analyzes localized microscopic areas, making it difficult to comprehensively evaluate the LCO coating effect.

[0031] In view of this, the present application proposes a method for evaluating the effect of aluminum oxide coating on the surface of lithium cobalt oxide, comprising the following steps: step 1, mixing a standard sample with a solid base, and performing a solid base melting digestion treatment on the surface-coated aluminum oxide in the standard sample to obtain a digestion residue; step 2, adding deionized water to the digestion residue, filtering and collecting the filter residue, and testing the Al content of the filter residue, which is recorded as the standard Al content; step 3, repeating steps 1 and 2 on the sample to be tested to obtain the Al content to be tested, wherein the standard sample refers to a sample of lithium cobalt oxide with a desired content of aluminum oxide coated on the surface; step 4, comparing the Al content to be tested with the standard Al content, and when the Al content to be tested is greater than or equal to the standard Al content, determining that the aluminum oxide coating effect on the surface of the sample to be tested is qualified;

[0032] In some embodiments, the standard sample can be a sample that has passed electrochemical performance testing; illustratively, the electrochemical performance can be selected from at least one of internal resistance, cycle performance, and rate performance. It is understood that the listed performances are for reference only, and those skilled in the art can select any sample that has passed electrochemical performance testing related to the alumina coating effect as the standard sample based on testing needs.

[0033] In some embodiments, the solid base includes an inorganic base; the inorganic base is selected from at least one of KOH, NaOH, Na2CO3, K2CO3, and Na2O2. The conditions for the solid base melting digestion treatment include: the digestion temperature T2 is 400 to 600°C. This application must use solid base for melting digestion treatment, and any concentration of aqueous solution of the base cannot achieve the purpose of fully dissolving the coated alumina. The alumina is sintered at 800°C for 10 hours according to the sintering process of alumina in the preparation method of alumina-coated lithium cobaltate to prepare di-burned alumina. The di-burned alumina is digested under different conditions, Figures 2 to 4 The following figure shows the photos of the residues obtained after 0.2g of bis-burned alumina was digested under different conditions and filtered through a filter membrane with a pore diameter of 0.22um. Figure 2 To use a 30% KOH solution to heat 0.2g of di-burned alumina at 220℃ for 2h, Figure 3 Solid KOH was used for alkali dissolution at 400℃ for 2h. Figure 4 Solid KOH was used for solid alkali melting digestion at 700℃ for 2h. Figure 2 As shown, after the alkaline solution was kept warm for 2 hours and filtered using a filter membrane, a large amount of filter residue remained, and some of the residue remained undissolved. This shows that the aluminum oxide could not be completely dissolved by the alkaline solution. Figure 3 and Figure 4 It is shown that when solid alkali melt digestion treatment is carried out at above 400°C, the alumina powder can be completely dissolved.

[0034] In some embodiments, the mass ratio of the sample to be tested or the standard sample to the solid base is 1:20 to 1:10. Exemplarily, the mass of the sample to be tested or the standard sample can be selected from 0.1 to 2 g, and the corresponding mass of the solid base used is 1 to 40 g. In contrast, when 0.2 g of di-burned alumina and 1 g of solid base KOH are subjected to alkali melting treatment using the method of the present application, a small amount of particles will precipitate, indicating that the sample is not completely dissolved. According to the chemical reaction stoichiometric ratio of alumina and KOH, the di-burned alumina is still not completely dissolved when an excess of KOH is used as the base. The reason may be that there are multiple crystal phases in the di-burned alumina after high-temperature sintering. According to Figure 2 It can be seen that some residue remains after the alkaline solution digests the di-calcined alumina, suggesting that different alumina crystal phases have different solubility difficulties in the alkaline solution. The mass ratio of the test sample or standard sample to the solid base is 1:20 to 1:10, preferably 1:16 to 1:14. Adding solid base that meets this ratio ensures complete digestion of the alumina while avoiding excessive use of solid base, facilitating subsequent testing.

[0035] In some exemplary embodiments, the solid alkali melt digestion treatment is performed in a muffle furnace. After the solid alkali melt digestion treatment, the digestion residue is cooled to room temperature.

[0036] In some embodiments, the conditions for the solid alkali melting digestion treatment also include: first drying the mixture of the sample to be tested or the standard sample and the solid alkali by preheating; the drying temperature T1 is 150 to 300°C; the drying time is 5 to 60 minutes; and the heating rate from T1 to T2 is 5 to 50°C / min.

[0037] Optionally, in step 2, after adding deionized water to the digestion residue, the residue may be allowed to stand and / or heated to completely dissolve the alkali-fused alumina digestate; the standing time may be 0.5 to 2 hours; and the heating time may be 2 to 30 minutes. For example, the residue may be allowed to stand for 1 hour to dissolve most of the alumina digestate, and then heated on a hot plate for about 5 minutes to completely dissolve the alumina digestate.

[0038] In some embodiments, in step 2, collecting the filter residue after filtering can be performed by suction filtration.

[0039] In some embodiments, in step 2, testing the Al content of the filter residue includes: adding the filter residue to acid for acid treatment, and then performing an ICP test to obtain the Al content of the filter residue.

[0040] Optionally, the temperature T4 of the acid treatment is 180 to 220° C.; the time of the acid treatment is 10 to 60 min, preferably 30 min.

[0041] In some embodiments, the ICP test may be performed using ICP-OES, and the contents of elements such as Li and Co in the filter residue may be simultaneously tested when testing the Al element.

[0042] In some embodiments, the step 4 of comparing the Al content to be measured with the standard Al content includes normalizing the standard Al content and calculating the normalization coefficient of the sample to be measured. When the normalization coefficient a of the sample to be tested is ≥1, it means that the Al content to be tested is greater than or equal to the standard Al content, and the surface alumina coating effect of the sample to be tested is determined to be qualified; when the normalization coefficient a of the sample to be tested is <1, it means that the Al content to be tested is less than the standard Al content, and the surface alumina coating effect of the sample to be tested is determined to be unqualified.

[0043] Example

[0044] The technical solutions of the present application are described below with reference to specific embodiments. The raw materials used in the following embodiments are all from common commercially available products, and the devices or equipment used are all purchased from conventional market sales channels.

[0045] Example 1

[0046] The evaluation method of the aluminum oxide coating effect on the surface of lithium cobalt oxide in this embodiment is based on the synthesis of lithium cobalt oxide by high-temperature solid-phase sintering, and specifically includes the following steps: 820.1g of Co3O4 and 377.5g of Li2CO3 raw materials are weighed and sintered at 1000°C for 5h to obtain LiCoO2; then LiCoO2 and aluminum oxide are mixed at a molar ratio of 98:2 for 7h, and then sintered at 800°C for 10h to obtain lithium cobalt oxide coated with aluminum oxide as the sample to be tested.

[0047] The assessment methodology includes the following steps:

[0048] Step 1. Weigh 5g of potassium hydroxide and spread it on the bottom of a nickel crucible. Then weigh 1g of the sample to be tested and place it in the nickel crucible. Then weigh 10g of potassium hydroxide and cover the sample. Place the nickel crucible in a muffle furnace preheated to 200°C for 15 minutes, then heat it to 400°C and maintain it for 120 minutes for solid alkali melting digestion treatment.

[0049] Step 2: Remove the crucible and, after cooling, add deionized water at 95°C (T3) to the digestion residue in the crucible. Allow to stand for 1 hour to completely dissolve the digestate. After complete dissolution, rinse the crucible several times with deionized water at 95°C (T3). Transfer the rinse solution to a 200mL polytetrafluoroethylene beaker. Heat the beaker on a hotplate at 100°C for approximately 5 minutes to ensure complete dissolution of the digestate. Filter through a filter membrane with a pore diameter of 0.45 μm and collect the residue.

[0050] 0.2 g of filter residue was weighed and added with 10 ml of aqua regia (aqua regia was diluted with deionized water in a volume ratio of 1:1) and acid-treated at 180°C. The content of the coating element Al and the main elements Li and Co in the filtrate was then tested using ICP-OES. The test result showed that the Al content to be tested was 6569 ppm.

[0051] Step 3: Repeat steps 1 and 2 with the standard sample to obtain a standard Al content of 6441 ppm.

[0052] Step 4: Normalize according to the standard Al content and calculate the normalization coefficient of the sample to be tested The normalization coefficient a of the sample to be tested is greater than 1, indicating that the Al content to be tested is greater than the standard Al content, and it is determined that the surface aluminum oxide coating effect of the sample to be tested is qualified.

[0053] Example 2

[0054] The method for evaluating the effect of aluminum oxide coating on the surface of lithium cobalt oxide in this embodiment differs from that in Example 1 only in that: when preparing the sample to be tested, LiCoO2 and aluminum oxide are mixed at a molar ratio of 98:2 for 2 hours, and then a subsequent sintering treatment is performed. The sample to be tested is tested, and the result of the content test in step 2 is the measured Al content: 6119 ppm. Compared with the standard Al content in Example 1, the normalization coefficient a=0.95. The normalization coefficient a of the sample to be tested is less than 1, indicating that the measured Al content is less than the standard Al content, and the surface aluminum oxide coating effect of the sample to be tested is determined to be unqualified.

[0055] Comparing the results of Example 1 and Example 2, it can be seen that although the raw material amounts of LiCoO2 and alumina are the same, the alumina coating content and coating effect may differ due to interference factors that may exist during the preparation process. The method for evaluating the alumina coating effect on the surface of lithium cobalt oxide provided in this application tests the Al content of the filter residue after solid alkali melting digestion. This can eliminate the interference of the alumina dosage during coating and intuitively and accurately obtain the alumina coating effect.

[0056] Example 3

[0057] The method for evaluating the effect of aluminum oxide coating on the surface of lithium cobalt oxide in this embodiment is performed on the same batch of test samples as in Example 1. The only difference from Example 1 is that the temperature of the alkali dissolution treatment in step 1 is 600°C.

[0058] Example 4

[0059] The method for evaluating the effect of aluminum oxide coating on the surface of lithium cobalt oxide in this embodiment is performed on the same batch of test samples as in Example 1. The only difference from Example 1 is that the temperature of the alkali dissolution treatment in step 1 is 700°C.

[0060] Example 5

[0061] The method for evaluating the effect of aluminum oxide coating on the surface of lithium cobalt oxide in this embodiment is to test the samples of the same batch as that in Example 1. The only difference from Example 1 is that in step 2, a filter membrane with a pore diameter of 0.60 μm is used to filter the filter residue.

[0062] Test Example 1

[0063] The micromorphology of the lithium cobalt oxide positive electrode materials without alkali dissolution treatment and after digestion treatment of Example 1 and Example 3 was tested using a Zeiss scanning electron microscope. The test results are as follows: Figures 5-7 As shown. Figure 5 The original LCO morphology, Figure 6The morphology of the lithium cobalt oxide cathode material shows that the particle morphology is basically the same after alkaline dissolution at 400 degrees, with sharp edges and corners, smooth surface, and no melting marks; Figure 7 The alkaline dissolution treatment at 700°C is too high, causing some LCO particles to dissolve on their surfaces. Therefore, the present invention controls the dissolution temperature T2 to 400-600°C to reduce the occurrence of dissolution and cracking of lithium cobalt oxide, thereby improving measurement precision and accuracy.

[0064] Test Example 2

[0065] Using the method for evaluating the effect of aluminum oxide coating on the surface of lithium cobalt oxide (LCO) described in Example 1, samples LCO-1, LCO-2, and LCO-3 were tested. LCO-1 is a standard lithium cobalt oxide with an internal resistance of 50.2 mΩ. LCO-2 is prepared using the same method as LCO-1, differing only in that the aluminum oxide coating on the surface of the LCO is reduced by 20% by weight. LCO-3 is prepared using the same method as LCO-1, differing only in that the aluminum oxide coating on the surface of the LCO is increased by 20% by weight. The test results are shown in Table 1 below.

[0066] Table 1

[0067]

[0068] The aluminum content data in Table 1 above show that for samples with alumina coating on the surface of lithium cobalt oxide (LCO) that differ by ±20% in mass, the varying coating effects lead to different Al contents in the filter residue measured after alkaline fusion digestion. Since the aluminum content of LCO-2 (-20% Al) in Table 1 is lower than that of the corresponding standard sample, LCO-1, the coating effect of LCO (-20% Al) in Table 1 is inferior to that of the standard sample, indicating that the coating effect is unqualified. Similarly, LCO-3 (+20% Al) exhibits a better coating effect than the standard sample, indicating that the coating effect is qualified. This test result is consistent with the alumina coating situation and shares the same trend as the internal resistance performance of the LCO positive active material, demonstrating the reliability and practicality of this method. This also proves that the evaluation method of this application can accurately measure the coating effect of aluminum oxide by removing the surface-coated aluminum oxide through solid alkali melting digestion treatment and then testing the Al content in the remaining solid. The electrochemical properties such as the internal resistance of the lithium cobalt oxide positive electrode active material can be evaluated subsequently through the coating effect results. At the same time, it is also possible to establish a relationship between the aluminum oxide coating effect and electrochemical properties such as internal resistance, and improve the coating process accordingly. By comparing with the standard Al content measured from the filter residue of the standard sample, the present application can clearly and intuitively understand the aluminum oxide coating effect of the sample to be tested. The process is simple and the results are accurate.

[0069] Table 2

[0070] serial number Al content / ppm Coefficient a Eligibility Example 1 6569 1.02 qualified Example 2 6119 0.95 Unqualified Standard samples 6441 1 -

[0071] As shown in Table 2, the Al contents obtained in Example 1, as evaluated by the method for evaluating the aluminum oxide coating effect on the surface of lithium cobalt oxide, were 6569 ppm and 6119 ppm, respectively, which were higher than the standard Al content measured for the standard sample, indicating that the coating effect was acceptable. The Al content obtained in Example 2 was 6119 ppm, which was lower than the standard Al content measured for the standard sample, indicating that the coating effect was unacceptable.

[0072] The evaluation method in Example 4 used an alkaline fusion digestion treatment temperature of 700°C, which resulted in the surface melting of a small number of LCO particles, resulting in an underestimation of the measured Al content. This suggests that by controlling the digestion temperature T2 to 400-600°C, the present invention reduces the melting and cracking of lithium cobalt oxide, thereby improving measurement precision and accuracy.

[0073] In Example 5, the filter residue was filtered using a filter membrane with a pore diameter of 0.60 μm, which also resulted in a lower measured value. This may be because, although the particle size of lithium cobalt oxide is generally in the micron range, the use of a filter membrane with a pore diameter of 0.60 μm results in the loss of lithium cobalt oxide below the micron level and the aluminum-containing alkaline oxide at the coating interface, thus causing the measured Al content to be lowered.

[0074] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present application should be included in the scope of protection of the present application.

Claims

1. A method for evaluating the effect of aluminum oxide coating on the surface of lithium cobalt oxide, characterized in that: The following steps are involved: Step 1: mixing a standard sample with a solid base, and performing a solid base melting digestion treatment on the surface-coated aluminum oxide in the standard sample to obtain a digestion residue; Step 2: adding deionized water to the digestion residue, filtering and collecting the filter residue, and testing the Al content of the filter residue, which is recorded as the standard Al content; Step 3: Repeat steps 1 and 2 on the sample to be tested to obtain the Al content to be tested, wherein the standard sample refers to a sample in which the surface of lithium cobalt oxide is coated with a desired content of aluminum oxide; Step 4: comparing the Al content to be measured with the standard Al content; when the Al content to be measured is greater than or equal to the standard Al content, determining that the surface alumina coating effect of the sample to be measured is qualified; When the Al content to be measured is less than the standard Al content, it is determined that the surface aluminum oxide coating effect in the sample to be measured is unqualified.

2. The evaluation method according to claim 1, wherein: The standard sample refers to a sample in which the surface of lithium cobalt oxide is coated with a desired content of aluminum oxide; The conditions for the solid alkali melting digestion treatment include: a digestion temperature T2 of 400 to 600° C. and a digestion time of 100 to 200 min.

3. The evaluation method according to claim 1, wherein: The digestion temperature T2 is 400 to 550°C.

4. The evaluation method according to claim 2 or 3, characterized in that: The solid base includes an inorganic base; the inorganic base is selected from at least one of KOH, NaOH, Na2CO3, K2CO3, and Na2O2.

5. The evaluation method according to claim 2 or 3, characterized in that: The conditions for the solid alkali melting digestion treatment also include: first mixing the sample to be tested or the standard sample with the solid alkali to obtain a mixture, and then drying the mixture by preheating; the drying treatment temperature T1 is 150 to 300°C; and the heating rate from T1 to T2 is 5 to 50°C / min.

6. The evaluation method according to claim 1, wherein: The mass ratio of the sample to be tested or the standard sample to the solid base is 1:20 to 1:

10.

7. The evaluation method according to claim 1, wherein: In step 2, the temperature T3 of the deionized water is 95 to 100°C.

8. The evaluation method according to claim 1, wherein: In step 2, the pore diameter of the filter membrane used for filtration is 0.22 to 0.45 um.

9. The evaluation method according to claim 1, wherein: In step 2, the testing of the Al content of the filter residue includes: adding the filter residue to an acid for acid treatment, and then performing an inductively coupled plasma test to obtain the Al content of the filter residue.

10. The evaluation method according to claim 9, wherein: The volume of acid used for every 0.2 g of filter residue is 10 to 15 mL; the acid is selected from at least one of hydrochloric acid, nitric acid, and sulfuric acid.

11. The evaluation method according to claim 9, wherein: The temperature T4 of the acid treatment is 180 to 220°C.

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