Method for comparing the coating integrity of composite cathode active materials

Abstract

The application relates to the battery technical field, in particular to a method for comparing the coating integrity of composite positive electrode active materials, which comprises the following steps: (1) taking at least two groups of composite positive electrode active materials, wherein the positive electrode active materials in each group of composite positive electrode active materials comprise manganese elements; (2) forming test samples of battery monomers according to the same method for each group of composite positive electrode active materials with the same mass; (3) respectively performing charge-discharge cycle performance tests on the test samples in each group, and the number of charge-discharge cycles is at least three cycles; (4) according to the test data of the charge-discharge cycle performance of the test samples in each group, obtaining the gram capacity of the test samples in each group in the 4.05-4.13V charging voltage platform or the 4.0-4.03V discharging voltage platform, and evaluating the coating integrity of the composite positive electrode active materials. The method is simple and easy to operate, has high accuracy, and is convenient for batch testing of multiple groups of composite positive electrode active materials.

Description

Technical Field

[0001] This application relates to the field of batteries, and specifically to a method for comparing the coating integrity of composite positive electrode active materials. Background Technology

[0002] With the increasing demand for high-energy-density batteries in the power and energy storage markets, improving battery performance has become a top priority, and developing new materials is a key way to achieve this goal. For cathode active materials containing transition metals such as manganese, the dissolution of transition metals can significantly affect battery performance. Therefore, the integrity of the coating layer has a significant impact on the manufacturing process and electrochemical characteristics of electrode materials.

[0003] However, there is currently a lack of simple and efficient methods to directly compare the coating integrity of different composite positive electrode active materials, which is an urgent problem to be solved. Summary of the Invention

[0004] In view of the above problems, this application provides a method for comparing the coating integrity of composite positive electrode active materials. The method described in this application is simple to operate, highly accurate, and convenient for batch testing of multiple groups of composite positive electrode active materials.

[0005] This application provides a method for comparing the coating integrity of composite positive electrode active materials, the method comprising the following steps:

[0006] (1) Take at least two sets of composite positive electrode active materials, wherein the composite positive electrode active materials include positive electrode active materials and a coating layer disposed on at least a portion of the surface of the positive electrode active materials;

[0007] The positive electrode active materials in each group of composite positive electrode active materials have the same general chemical formula, and the positive electrode active materials include manganese.

[0008] (2) Test samples of battery cells are formed independently from at least two groups of composite positive electrode active materials of the same mass using the same method;

[0009] (3) Perform charge-discharge cycle performance tests on each group of test samples, with a minimum of three charge-discharge cycles;

[0010] (4) Based on the test data of charge-discharge cycle performance of each group of test samples, obtain the specific capacity of each group of test samples within the charging voltage platform of 4.05-4.13V or the discharging voltage platform of 4.0-4.03V, and evaluate the coating integrity of the composite positive electrode active material.

[0011] In the technical solution of this application, for composite cathode active materials, during battery cycling, the material structure changes due to factors such as the Jan-Taylor effect, leading to the dissolution of metal ions. Among these, the dissolution of manganese ions causes battery capacity decay. If a coating layer is provided on the composite cathode active material, the dissolution of manganese will be reduced to a certain extent. Therefore, the better the integrity of the coating layer, the less manganese is dissolved, resulting in greater capacity utilization during actual charge and discharge, a larger actual capacity contribution from the Mn platform, and a greater specific capacity of the composite cathode active material.

[0012] Specifically, this application uses a series of processes to form battery cell test samples from various composite positive electrode active materials, and then conducts charge-discharge cycle performance tests. Manganese ions gradually dissolve during the cycle. The integrity of the coating layer is determined by the capacity difference between the test samples within the 4.05-4.13V charging voltage plateau or the 4.0-4.03V discharging voltage plateau. The number of cycles is at least three because a solid electrolyte film forms during the first cycle, which causes initial capacity loss and affects the judgment results. This application adjusts the number of cycles to at least three to essentially eliminate the influence of the solid electrolyte film formation process on the judgment results, improving the accuracy of the test. The 4.05-4.13V charging voltage plateau or the 4.0-4.03V discharging voltage plateau is chosen because during battery charge-discharge cycles, factors such as chemical reactions and internal resistance can lead to a decrease in battery voltage and a weakening of current output capability, i.e., battery polarization. The 4.05-4.13V charging voltage plateau or the 4.0-4.03V discharging voltage plateau is selected, which belongs to a relatively stable voltage plateau region in the battery charge-discharge curve. In this region, the battery polarization is small, and the battery's capacity and voltage retention capabilities are good. Moreover, the voltage plateau in this region is mainly derived from manganese, and the possibility of other transition elements such as iron is low. Therefore, the capacity contribution mainly comes from manganese ions, reducing the influence of irrelevant factors on the evaluation results and improving the accuracy of the test. Therefore, the comparison method of this application is simple to operate, highly accurate, and convenient for batch testing of multiple groups of composite positive electrode active materials.

[0013] In some embodiments, in step (1), the chemical formula of the positive electrode active material is Li. m Fe 1-x-y Mn x M y PO4, 0.8≤m≤1, 0<x≤1, 0≤y≤1, M is selected from any one or at least two combinations of V, Nb, Ti, Co, Ni, Sc, Ge, Mg, Al, Zr, Mn, Hf, Ta, Mo, W, Ru, Ag, Sn and Pb; optionally, the positive electrode active materials in the at least two groups of composite positive electrode active materials are of the same type.

[0014] In the technical solution of this application, the chemical formula of the positive electrode active material is selected as described above. The aforementioned material itself has unique advantages in terms of cost and safety, but it requires a coating layer to improve its conductivity in order to increase energy density. Specifically, the higher the manganese content, the higher the voltage platform. In the specific process, a stable manganese platform is selected based on the actual manganese voltage platform of materials with different manganese contents.

[0015] In addition, selecting the same positive electrode active material for each group can reduce the impact of different types of positive electrode active materials on the comparison results during the test, and improve the accuracy of the test.

[0016] In some embodiments, in step (1), the coating layer includes a carbon coating layer.

[0017] In the technical solution of this application, the carbon coating layer is beneficial for forming a stable interface for the positive electrode active material, which can alleviate electrolyte decomposition, reduce hydrofluoric acid attack on the positive electrode active material, dissolution of transition metal ions, gas generation and other structural problems, and improve the overall electrochemical performance of the material.

[0018] In some implementations, in step (2), the test sample of the battery cell includes a button cell.

[0019] In the technical solution of this application, using button cells as test samples can reduce sample preparation steps, save testing costs, and improve testing efficiency.

[0020] In some embodiments, in step (3), the temperature of the charge-discharge cycle performance test is ≤45°C, and optionally 15-45°C.

[0021] In the technical solution of this application, the temperature for charge-discharge cycle performance testing is adjusted to be within the aforementioned range. This can reduce the possibility of accelerated manganese leaching due to excessively high test temperatures, such as Mn in the positive electrode active material. 4+ Electrochemical reactions occur with Mn 2+ The form of the substance dissolves into the electrolyte, increasing the probability of electrolyte decomposition and thus affecting the accuracy of the test results.

[0022] In some implementations, in step (3), the charging rate of the charge-discharge cycle performance test is 0.02-0.1C.

[0023] In the technical solution of this application, the charging rate of the charge-discharge cycle performance test is within the above-mentioned range. Within this range, the current density is appropriate, the electrode reaction kinetics are relatively mild, and the polarization effect is small, which can reduce capacity errors caused by battery polarization and improve test accuracy. Furthermore, within this range, the battery's charge-discharge behavior is highly stable, the test repeatability is high, and the test accuracy is further improved.

[0024] In some implementations, in step (3), the discharge rate of the charge-discharge cycle performance test is 0.02-0.1C.

[0025] In the technical solution of this application, the discharge rate of the charge-discharge cycle performance test is within the above-mentioned range. Within this range, the current density is appropriate, the electrode reaction kinetics are relatively mild, and the polarization effect is small, which can reduce capacity errors caused by battery polarization and improve test accuracy. Furthermore, within this range, the battery's charge-discharge behavior exhibits high stability, resulting in high test repeatability and further improving test accuracy.

[0026] In some implementations, in step (3), the number of charge-discharge cycle performance tests is ≥3 cycles, optionally 3-200 cycles.

[0027] In the technical solution of this application, the number of charge-discharge cycle performance tests is within the aforementioned range. A solid electrolyte membrane has already been formed, and its structure is relatively stable, reducing the impact of electrolyte infiltration due to membrane rupture on the test results and improving test accuracy. Furthermore, with the number of charge-discharge cycle performance tests within the aforementioned range, the overall structural stability of the battery is good, reducing the impact of changes in the morphology or structure of the positive electrode active material and battery expansion on the test results, thus improving test accuracy.

[0028] In some implementations, step (3) of the charge-discharge cycle performance test includes the following steps:

[0029] Each group of test samples was charged to 100% state of charge (SOC) and then discharged to 0% state of charge (SOC). This process was repeated at least three times, optionally 3-200 times.

[0030] In the technical solution of this application, the charge-discharge cycle performance test is performed according to the above process because the above test is a typical battery charge-discharge cycle test, which can be well used to evaluate the battery's performance, lifespan and consistency.

[0031] In some embodiments, the charge-discharge cycle performance test includes the following steps:

[0032] The test sample of the battery cell is first charged with constant current at 0.02-0.1C until 100% SOC, then charged with constant voltage until the current density is ≤0.02C, and then discharged with constant current at 0.02-0.1C until 0% SOC. The above process is repeated at least three times to complete the charge-discharge cycle performance test.

[0033] In the technical solution of this application, the charge-discharge cycle performance test is performed according to the above process because: a solid electrolyte film is formed during the first cycle of the battery. The film is fully formed during the first low-current cycle, and the effect of the solid electrolyte film formation process on the judgment result can be ignored, thus improving the accuracy of the test; multiple cycles promote the increase of manganese dissolution, and the amplified material specific capacity is further improved by the difference in manganese dissolution, thus improving the accuracy of the test.

[0034] In some implementations, in step (4), the difference in specific capacity of each group of test samples under any voltage range within the charging voltage platform of 4.05-4.13V or the discharging voltage platform of 4.0-4.03V is calculated, and / or the ratio between the difference in specific capacity under any voltage range and the difference in voltage is used to evaluate the coating integrity of the composite positive electrode active material.

[0035] In the technical solution of this application, within a charging voltage platform of 4.05-4.13V or a discharging voltage platform of 4.0-4.03V, the coating integrity of the composite positive electrode active material can be simply and directly evaluated based on the difference in specific capacity under any voltage range, and / or the ratio between the difference in specific capacity under any voltage range and the difference in voltage. That is, the larger the difference in specific capacity under any voltage range, the higher the coating integrity of the composite positive electrode active material; or, the larger the ratio between the difference in specific capacity under any voltage range and the difference in voltage, the higher the coating integrity of the composite positive electrode active material.

[0036] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0037] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of this application. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0038] Figure 1 The graph shows the charge / discharge test results for Example 1;

[0039] Figure 2 The graph shows the charge / discharge test results for Example 6. Detailed Implementation

[0040] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0041] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0042] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0043] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0044] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60–120 and 80–110 are listed for a specific parameter, it is also expected that ranges of 60–110 and 80–120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1–3, 1–4, 1–5, 2–3, 2–4, and 2–5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "2-10" indicates that all real numbers between "2-10" have been listed in this article; "2-10" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0045] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent three cases: A exists, A and B exist simultaneously, and B exists. In addition, the character " / " in this document generally indicates that the related objects before and after it have an "or" relationship.

[0046] In secondary batteries, for positive electrode active materials containing transition metals such as manganese, the dissolution of these transition metals can significantly impact battery performance. Therefore, the integrity of the coating layer has a significant influence on the manufacturing process and electrochemical characteristics of the electrode material. However, a simple and efficient method is currently lacking to directly compare the coating integrity of different composite positive electrode active materials.

[0047] This application assesses the coating integrity of composite cathode active materials by conducting charge-discharge cycle performance tests on battery cells prepared from different groups of composite cathode active materials. The tests compare the specific capacity of each group of samples within a charging voltage plateau of 4.05-4.13V or a discharging voltage plateau of 4.0-4.03V. The comparison method used in this application is simple to operate, highly accurate, and facilitates batch testing of multiple groups of composite cathode active materials.

[0048] [Methods for comparing the coating integrity of composite positive electrode active materials]

[0049] This application provides a method for comparing the coating integrity of composite positive electrode active materials, the method comprising the following steps:

[0050] (1) Take at least two sets of composite positive electrode active materials, wherein the composite positive electrode active materials include positive electrode active materials and a coating layer disposed on at least a portion of the surface of the positive electrode active materials;

[0051] The positive electrode active materials in each group of composite positive electrode active materials have the same general chemical formula, and the positive electrode active materials include manganese.

[0052] (2) Test samples of battery cells are formed independently from at least two groups of composite positive electrode active materials of the same mass using the same method;

[0053] (3) Conduct charge-discharge cycle performance tests on each group of test samples. The number of charge-discharge cycles should be at least three (e.g., four, five, ten, etc.).

[0054] (4) Based on the test data of charge-discharge cycle performance of each group of test samples, obtain the specific capacity of each group of test samples within the charging voltage platform of 4.05-4.13V or the discharging voltage platform of 4.0-4.03V, and evaluate the coating integrity of the composite positive electrode active material.

[0055] In this application, the instrument used for testing the charge-discharge cycle performance of each group of test samples is a Lamborghini tester, model BT-2018A-DK.

[0056] In the technical solution of this application embodiment, for composite cathode active materials, during battery cycling, the material structure changes due to factors such as the Jan-Taylor effect, leading to the dissolution of metal ions. Among these, the dissolution of manganese ions causes battery capacity decay. If a coating layer is provided on the composite cathode active material, the dissolution of manganese will be reduced to a certain extent. Therefore, the better the integrity of the coating layer, the less manganese is dissolved, resulting in greater capacity utilization during actual charge and discharge, a larger actual capacity contribution from the Mn platform, and a greater specific capacity of the composite cathode active material.

[0057] Specifically, this application uses a series of processes to form battery cell test samples from various composite positive electrode active materials, and then conducts charge-discharge cycle performance tests. Manganese ions gradually dissolve during the cycle. The integrity of the coating layer is determined by the capacity difference between the test samples within the 4.05-4.13V charging voltage plateau or the 4.0-4.03V discharging voltage plateau. The number of cycles is at least three because a solid electrolyte film forms during the first cycle, which causes initial capacity loss and affects the judgment results. This application adjusts the number of cycles to at least three to essentially eliminate the influence of the solid electrolyte film formation process on the judgment results, improving the accuracy of the test. The 4.05-4.13V charging voltage plateau or the 4.0-4.03V discharging voltage plateau is chosen because during battery charge-discharge cycles, factors such as chemical reactions and internal resistance can lead to a decrease in battery voltage and a weakening of current output capability, i.e., battery polarization. The 4.05-4.13V charging voltage plateau or the 4.0-4.03V discharging voltage plateau is selected, which belongs to a relatively stable voltage plateau region in the battery charge-discharge curve. In this region, the battery polarization is small, and the battery's capacity and voltage retention capabilities are good. Moreover, the voltage plateau in this region is mainly derived from manganese, and the possibility of other transition elements such as iron is low. Therefore, the capacity contribution mainly comes from manganese ions, reducing the influence of irrelevant factors on the evaluation results and improving the accuracy of the test. Therefore, the comparison method of this application is simple to operate, highly accurate, and convenient for batch testing of multiple groups of composite positive electrode active materials.

[0058] In some embodiments, in step (1), the chemical formula of the positive electrode active material is Li m Fe 1-x-y Mn x M y PO4, 0.8≤m≤1 (e.g., 0.82, 0.84, 0.86, 0.88, 0.9, 0.92, 0.94, 0.96, 0.98, etc.), 0<x≤1 (e.g., 0.2, 0.4, 0.6, 0.8, etc.), 0≤y≤1 (e.g., 0.2, 0.4, 0.6, 0.8, etc.), M is selected from any one or at least two combinations of V, Nb, Ti, Co, Ni, Sc, Ge, Mg, Al, Zr, Mn, Hf, Ta, Mo, W, Ru, Ag, Sn and Pb; optionally, the positive electrode active materials in the at least two groups of composite positive electrode active materials are of the same type.

[0059] In the technical solution of this application embodiment, the chemical formula of the positive electrode active material adopts the above-mentioned selection. The above-mentioned material itself has unique advantages in terms of cost and safety, but it is necessary to improve its conductivity by setting a coating layer to increase energy density; among them, the higher the manganese content, the higher the voltage platform. In the specific process, a stable manganese platform is selected according to the actual manganese voltage platform of materials with different manganese contents.

[0060] In addition, selecting the same positive electrode active material for each group can reduce the impact of different types of positive electrode active materials on the comparison results during the test, and improve the accuracy of the test.

[0061] In some embodiments, in step (1), the coating layer includes a carbon coating layer.

[0062] In the technical solution of this application, the carbon coating layer is beneficial for forming a stable interface for the positive electrode active material, which can alleviate electrolyte decomposition, reduce hydrofluoric acid attack on the positive electrode active material, dissolution of transition metal ions, gas generation and other structural problems, and improve the overall electrochemical performance of the material.

[0063] In some embodiments, based on the mass of the composite positive electrode active material as 100%, the mass content of the coating layer is 0.8%-2.4%, for example 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, 2.2%, etc.

[0064] In this application, the carbon content of the carbon coating layer can be designed as needed. For example, based on the total mass of the carbon coating layer being 100%, the mass content of the carbon element is greater than or equal to 90%, such as 92%, 94%, 96%, 98%, etc.

[0065] In some embodiments, in step (2), the test sample of the battery cell includes a button cell.

[0066] In the technical solution of this application, using button cells as test samples can reduce sample preparation steps, save testing costs, and improve testing efficiency.

[0067] In some embodiments, in step (3), the temperature of the charge-discharge cycle performance test is ≤45℃, and optionally 15-45℃, such as 15℃, 20℃, 25℃, 30℃, 35℃, 40℃, etc.

[0068] In the technical solution of this application embodiment, the temperature for charge-discharge cycle performance testing is adjusted to be within the aforementioned range. This can reduce the possibility of accelerated manganese leaching due to excessively high test temperatures, such as Mn in the positive electrode active material. 4+ Electrochemical reactions occur with Mn 2+The form of the substance dissolves into the electrolyte, increasing the probability of electrolyte decomposition and thus affecting the accuracy of the test results.

[0069] In some embodiments, in step (3), the charging rate of the charge-discharge cycle performance test is 0.02-0.1C, such as 0.04C, 0.06C, 0.08C, etc.

[0070] In the technical solution of this application embodiment, the charging rate of the charge-discharge cycle performance test is within the above-mentioned range. Within this range, the current density is appropriate, the electrode reaction kinetics are relatively mild, and the polarization effect is small, which can reduce capacity errors caused by battery polarization and improve test accuracy. Furthermore, within this range, the battery's charge-discharge behavior is highly stable, the test repeatability is high, and the test accuracy is further improved.

[0071] In some embodiments, in step (3), the discharge rate of the charge-discharge cycle performance test is 0.02-0.1C, such as 0.04C, 0.06C, 0.08C, etc.

[0072] In the technical solution of this application embodiment, the discharge rate of the charge-discharge cycle performance test is within the above-mentioned range. Within this range, the current density is appropriate, the electrode reaction kinetics are relatively mild, and the polarization effect is small, which can reduce capacity errors caused by battery polarization and improve test accuracy. Furthermore, within this range, the battery's charge-discharge behavior exhibits high stability, resulting in high test repeatability and further improving test accuracy.

[0073] In some embodiments, in step (3), the number of charge-discharge cycle performance tests is ≥3 cycles, optionally 3-200 cycles, such as 5 cycles, 10 cycles, 50 cycles, 100 cycles, 150 cycles, etc.

[0074] In the technical solution of this application embodiment, the number of charge-discharge cycle performance tests is within the aforementioned range. A solid electrolyte membrane has already been formed, and its structure is relatively stable, reducing the impact of electrolyte infiltration due to solid electrolyte membrane rupture on the test results and improving test accuracy. Furthermore, with the number of charge-discharge cycle performance tests within the aforementioned range, the overall structural stability of the battery is good, reducing the impact of changes in the morphology or structure of the positive electrode active material and battery expansion on the test results, thus improving test accuracy.

[0075] In some embodiments, step (3) of the charge-discharge cycle performance test includes the following steps:

[0076] Each group of test samples is charged to 100% SOC and then discharged to 0% SOC. This process is repeated at least three times, optionally 3-200 times, such as 5 times, 10 times, 50 times, 100 times, 150 times, etc.

[0077] In this application, 100% state of charge, i.e., 100% SOC, means that the state of charge (SOC) of the battery has reached 100%, that is, the battery is in a fully charged state; 0% state of charge means that the negative electrode is in a completely delithiated state, that is, the battery is in a completely depleted state.

[0078] In the technical solution of this application embodiment, the charge-discharge cycle performance test is performed according to the above process because the above test is a typical battery charge-discharge cycle test, which can be well used to evaluate the battery's performance, lifespan and consistency.

[0079] In some embodiments, the charge-discharge cycle performance test includes the following steps:

[0080] The test sample of the battery cell is first charged with constant current at 0.02-0.1C until 100% SOC, then charged with constant voltage until the current density is ≤0.02C, optionally 0.01-0.02C (e.g., 0.005C, 0.01C, 0.015C, etc.), and then discharged with constant current at 0.02-0.1C until 0% SOC. The above process is repeated at least three times, optionally 3-200 times, to complete the charge-discharge cycle performance test.

[0081] In the technical solution of this application embodiment, the charge-discharge cycle performance test is performed according to the above process because: a solid electrolyte film is formed in the battery during the first cycle. The film is fully formed in the first cycle with a small current. The effect of the solid electrolyte film formation process on the judgment result can be ignored, which improves the accuracy of the test; multiple cycles promote the increase of manganese dissolution. Due to the difference in manganese dissolution, the material specific capacity is amplified, which further improves the accuracy of the test.

[0082] As an example, based on the data from the charge-discharge cycle performance test described above, the specific capacity of the test sample within the 4.05-4.13V charging voltage plateau or the 4.0-4.03V discharging voltage plateau is calculated using the following method:

[0083] Before assembling the composite positive electrode active material into a test sample (e.g., a coin cell), the mass of the composite positive electrode active material on the positive electrode sheet is accurately weighed and input into the testing software. The electrochemical testing software then calculates the specific capacity based on the capacity / mass of the composite positive electrode active material.

[0084] In some embodiments, in step (4), the difference in specific capacity of each group of test samples under any voltage range within the charging voltage platform of 4.05-4.13V or the discharging voltage platform of 4.0-4.03V is calculated, and / or the ratio between the difference in specific capacity under any voltage range and the difference in voltage is calculated, to evaluate the coating integrity of the composite positive electrode active material.

[0085] In the technical solution of this application embodiment, within a charging voltage platform of 4.05-4.13V or a discharging voltage platform of 4.0-4.03V, the coating integrity of the composite positive electrode active material can be simply and directly evaluated based on the difference in specific capacity under any voltage range, and / or the ratio between the difference in specific capacity under any voltage range and the difference in voltage. That is, the larger the difference in specific capacity under any voltage range, the higher the coating integrity of the composite positive electrode active material; or, the larger the ratio between the difference in specific capacity under any voltage range and the difference in voltage, the higher the coating integrity of the composite positive electrode active material.

[0086] In some embodiments, the method for comparing the coating integrity of the composite positive electrode active material includes the following steps:

[0087] (1) Take at least two sets of composite positive electrode active materials, wherein the composite positive electrode active materials include positive electrode active materials and a coating layer disposed on at least a portion of the surface of the positive electrode active materials;

[0088] The positive electrode active materials in each group of composite positive electrode active materials have the same general chemical formula, and the positive electrode active materials include manganese.

[0089] (2) Test samples of battery cells are formed independently from at least two groups of composite positive electrode active materials of the same mass using the same method;

[0090] (3) Under the conditions of ≤45℃ and charge / discharge rate of 0.02-0.1C, the test samples of each group were subjected to cycle performance test, and the number of charge / discharge cycles was ≥3.

[0091] Specifically, the charge-discharge cycle performance test includes the following steps:

[0092] 1) Form a positive electrode sheet from the composite positive electrode active material and assemble it into a test sample of a battery cell (e.g., a coin cell). Keep it at a constant temperature (e.g., 10-40℃, optionally 25℃) for more than 3 hours, optionally 3-5 hours, to allow the electrolyte to fully impregnate it.

[0093] 2) Charge the test sample with a constant current within the current density range mentioned above until it reaches 100% SOC;

[0094] 3) Use constant voltage charging until the current density is ≤0.02C, optionally 0.01-0.02C;

[0095] 4) Perform constant current discharge on the test sample using a current within the above current density range until 0% SOC;

[0096] 5) Repeat steps 2)-4) at least 3 times.

[0097] (4) Based on the test data of charge-discharge cycle performance of each group of test samples, and based on the specific capacity of each group of test samples within the charging voltage platform of 4.05-4.13V or the discharging voltage platform of 4.0-4.03V, calculate the difference in specific capacity of each group of test samples within any voltage range of the charging voltage platform of 4.05-4.13V or the discharging voltage platform of 4.0-4.03V, and / or the ratio between the difference in specific capacity and the difference in voltage within any voltage range, to evaluate the coating integrity of the composite positive electrode active material.

[0098] [Preparation of test samples for individual battery cells]

[0099] In some embodiments, the test sample of the battery cell can be prepared by the following method:

[0100] (1) Electrode preparation: The composite positive electrode active material and conductive agent are dissolved in a solvent (e.g., N-methylpyrrolidone, etc.), and optionally, a binder and other arbitrary components are also included to form a positive electrode slurry. The positive electrode slurry is coated on the positive electrode current collector to form a positive electrode film layer. After drying, cold pressing and slitting, a positive electrode sheet is obtained.

[0101] The negative electrode active material and conductive agent are dissolved in a solvent (such as water), and optionally, a binder and other arbitrary components are also included to form a negative electrode slurry. The negative electrode slurry is coated on a negative electrode current collector to form a negative electrode film layer. After drying, cold pressing and slitting, the negative electrode sheet is obtained.

[0102] (2) Preparation of test samples for battery cells: The positive electrode, separator, and negative electrode are stacked and wound in sequence to obtain the battery cell; the battery cell is placed in the outer packaging, electrolyte is added, and after processes such as encapsulation, standing, formation, and aging, a test sample of the battery cell is obtained. As a test sample of the battery cell, it can be assembled into a button cell.

[0103] As an example, in the positive electrode sheet, the mass ratio of composite positive electrode active material, conductive agent and binder can be (80-99.8):(0.1-10):(0.1-10), where 80-99.8 can be 85, 90, 95, 96, 97, 98, 99, etc., and 0.1-10 can be 0.5, 1, 1.5, 2, 2.5, 3, 4, 5, 6, 7, 8, 9, etc. The amount of conductive agent and binder added can be the same or different.

[0104] As an example, in the negative electrode sheet, the mass ratio of negative electrode active material, conductive agent and binder can be (80-99.8):(0.1-10):(0.1-10), where 80-99.8 can be 85, 90, 95, 96, 97, 98, 99, etc., and 0.1-10 can be 0.5, 1, 1.5, 2, 2.5, 3, 4, 5, 6, 7, 8, 9, etc. The amount of conductive agent and binder added can be the same or different.

[0105] The following example illustrates the composition of the various parts of a test sample of a battery cell.

[0106] Positive electrode sheet

[0107] In some embodiments, the positive electrode includes a positive current collector and a positive film layer disposed on at least one surface of the positive current collector.

[0108] In some embodiments, the positive current collector includes any one or a combination of at least two of the following: a metal foil, a conductive polymer material, a carbon material, or a composite current collector. For example, the metal foil can be a pure metal, an alloy, or a surface-treated metal, including but not limited to stainless steel, copper, aluminum, nickel, titanium, or silver. The composite current collector may include a polymeric material substrate and a metal layer. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymeric material substrate (such as a substrate of polypropylene, polyethylene terephthalate, polybutylene terephthalate, polystyrene, polyethylene, etc.). This application does not impose any particular limitation on the type of positive current collector; any known material can be selected.

[0109] In some embodiments, the positive electrode film layer comprises a composite positive electrode active material (as previously described, not repeated here) and a conductive agent, and optionally, it also comprises a binder and other arbitrary additives (e.g., surfactants).

[0110] In some embodiments, the conductive agent includes any one or a combination of at least two of carbon black (such as acetylene black, Ketjen black, Super P, etc.), graphite, superconducting carbon, carbon dots, carbon nanotubes, graphene, or carbon nanofibers. This application does not impose any particular limitation on the type of conductive agent; any known material can be selected.

[0111] In some embodiments, the adhesive comprises any one or a combination of at least two of polyvinylidene fluoride, polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymer, fluorinated acrylate resin, styrene-butadiene rubber, or polyacrylic acid. This application does not impose any particular limitation on the type of adhesive; any known material may be used.

[0112] In some embodiments, the method for preparing the positive electrode sheet includes the following steps:

[0113] The positive electrode active material and conductive agent are dissolved in a solvent (such as N-methylpyrrolidone, etc.), and optionally, a binder and other arbitrary components are also included to form a positive electrode slurry. The positive electrode slurry is coated on a positive electrode current collector, and after drying, cold pressing and slitting, the positive electrode sheet is obtained.

[0114] Negative electrode sheet

[0115] In some embodiments, the negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector.

[0116] In some embodiments, the negative electrode current collector may be a metal foil, a conductive polymer material, a carbon material, or a composite current collector. For example, as a metal foil, pure metals, alloys, or surface-treated metals may be used, including but not limited to stainless steel, copper, aluminum, nickel, titanium, or silver. The composite current collector may include a polymer material base layer and a metal layer. The composite current collector can be formed by forming a metal material (copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys, etc.) on a polymer material substrate (such as a substrate of polypropylene, polyethylene terephthalate, polybutylene terephthalate, polystyrene, polyethylene, etc.). This application does not have a particular limitation on the type of negative electrode current collector, and any known material can be selected.

[0117] In some embodiments, the negative electrode film layer includes a negative electrode active material and a conductive agent, and optionally, it also includes a binder and other arbitrary additives (such as thickeners).

[0118] In some embodiments, the negative electrode active material includes any one or a combination of at least two of artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, or lithium titanate. This application does not impose any particular limitation on the type of negative electrode active material; any known material can be selected.

[0119] In some embodiments, the adhesive comprises any one or a combination of at least two of polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl alcohol, polyethylene glycol, carboxymethyl cellulose and its salts, or polyethylene oxide. This application does not impose any particular limitation on the type of adhesive; any known material may be used.

[0120] In some embodiments, the conductive agent includes any one or a combination of at least two of carbon black (such as acetylene black, Ketjen black, Super P, etc.), graphite, superconducting carbon, carbon dots, carbon nanotubes, graphene, or carbon nanofibers. This application does not impose any particular limitation on the type of conductive agent; any known material can be selected.

[0121] In some embodiments, the method for preparing the negative electrode sheet includes the following steps:

[0122] The negative electrode active material and conductive agent are dissolved in a solvent (e.g., water), and optionally, a binder and other arbitrary components are also included to form a negative electrode slurry. The negative electrode slurry is coated onto a negative electrode current collector, and after drying, cold pressing, and slitting, the negative electrode sheet is obtained.

[0123] In some embodiments, the negative electrode may be a lithium sheet.

[0124] electrolytes

[0125] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0126] Liquid electrolytes include electrolyte salts and solvents.

[0127] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0128] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone. The solvent may also be an ether solvent. Ether solvents may include at least one of ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,3-dioxolane, tetrahydrofuran, methyl tetrahydrofuran, or diphenyl ether and crown ether.

[0129] In some embodiments, the gel electrolyte comprises a polymer as a backbone network and can be used in conjunction with an ionic liquid-lithium salt.

[0130] In some embodiments, the solid electrolyte includes a polymer solid electrolyte, an inorganic solid electrolyte, and a composite solid electrolyte.

[0131] In some embodiments, the polymer of the polymer solid electrolyte may include at least one of polyether (polyoxyethylene), polysiloxane, polycarbonate, polyacrylonitrile, polyvinylidene fluoride, polymethyl methacrylate, monoionic polymer, polyionic liquid or cellulose.

[0132] In some embodiments, the inorganic solid electrolyte may be at least one of oxide solid electrolytes (crystalline perovskite, sodium superconducting ion conductor, garnet, amorphous LiPON thin film), sulfide solid electrolytes (crystalline lithium superconducting ion conductor (lithium germanium phosphate sulfide, silver sulfide germanium ore), amorphous sulfides), halide solid electrolytes, nitride solid electrolytes, or hydride solid electrolytes.

[0133] In some embodiments, the composite solid electrolyte is formed by adding an inorganic solid electrolyte filler to a polymer solid electrolyte.

[0134] Separating membrane

[0135] This application does not impose any particular restrictions on the type of separator; any known porous separator used in secondary batteries can be selected.

[0136] In some embodiments, the separator includes a glass fiber separator, a nonwoven fabric film, a polyethylene film, a polypropylene film, a polyvinylidene fluoride film, and one or more of the following multilayer composite films.

[0137] [Example]

[0138] Example 1

[0139] (1) Take two sets of composite positive electrode active materials, wherein the composite positive electrode active materials include positive electrode active materials and a carbon coating layer disposed on at least part of the surface of the positive electrode active materials;

[0140] In both groups of composite cathode active materials, the type of cathode active material is the same, with the chemical formula LiFe. 0.5 Mn 0.5 PO4, referred to as materials 1 and 2 respectively, with material 1 having a higher coating integrity than material 2; material 1 is named high-carbon content coated lithium manganese iron phosphate, and based on the mass of the composite positive electrode active material as 100%, the coating layer has a mass content of 2.2%, used to prepare parallel samples of the 596 series, namely, 596-1, 596-2, 596-3, and 596-4; material 2 is named low-carbon content coated lithium manganese iron phosphate, and based on the mass of the composite positive electrode active material as 100%, the coating layer has a mass content of 1.8%, used to prepare parallel samples of the 563 series, namely, 563-1, 563-2, 563-3, and 563-4;

[0141] (2) Test samples of battery cells were formed independently from two groups of composite positive electrode active materials of the same mass using the same method.

[0142] Specifically, the preparation method for test samples of battery cells includes the following steps:

[0143] Positive electrode sheet: The composite positive electrode active material, conductive agent (SP) and binder (PVDF) are dissolved in a solvent (N-methylpyrrolidone) at a mass ratio of 90:5:5 to form a positive electrode slurry. The positive electrode slurry is coated on the positive electrode current collector (aluminum foil) to form a positive electrode film layer. After drying, cold pressing and slitting, the positive electrode sheet is obtained.

[0144] Negative electrode: Lithium sheet is used as the negative electrode.

[0145] Electrolyte: Ethyl carbonate, diethyl carbonate, and dimethyl carbonate are mixed in a volume ratio of 1:1:1. Lithium hexafluorophosphate (LiPF6) is dissolved in the above solution to obtain the electrolyte. The concentration of LiPF6 in this electrolyte is 1 mol / L.

[0146] Preparation of test samples for battery cells: The positive electrode, separator, and negative electrode are stacked and wound in sequence to obtain the cell; the cell is placed in the outer packaging, electrolyte is added, and assembled into a button cell to obtain the test sample of the battery cell.

[0147] (3) Under the conditions of 45℃ and charge / discharge rate of 0.1C, the test samples of each group were subjected to cycle performance test. The number of charge / discharge cycles was 3. The instrument used was a Lamborghini tester, model BT-2018A-DK.

[0148] Specifically, the charge-discharge cycle performance test includes the following steps:

[0149] 1) After the button battery is assembled, let it stand at a constant temperature of 25℃ for 4 hours to allow the electrolyte to fully penetrate.

[0150] 2) Charge the battery with a constant current within the current density range mentioned above until it reaches 100% SOC;

[0151] 3) Use constant voltage charging until the current density reaches 0.02C;

[0152] 4) Discharge the battery with a constant current within the current density range mentioned above until 0% SOC is reached;

[0153] 5) Repeat steps 2)-4) 3 times.

[0154] (4) Based on the test data of charge-discharge cycle performance of each group of test samples, obtain the specific capacity of each group of test samples within the charging voltage platform of 4.05-4.13V, calculate the ratio between the difference in specific capacity and the difference in voltage within the charging voltage platform of 4.05-4.13V for each group of test samples, and evaluate the coating integrity of the composite positive electrode active material.

[0155] Specifically, the specific capacity is obtained by the following method:

[0156] Before assembling the button cell, the mass of the composite positive electrode active material on the positive electrode sheet is accurately weighed and input into the testing software. The electrochemical testing software calculates the specific capacity based on the capacity / mass of the composite positive electrode active material.

[0157] Examples 2-6

[0158] Except for the parameters in Table 1, the comparisons were made according to the method described in Example 1.

[0159] Table 1

[0160] serial number Temperature / °C Multiplier / C Charging voltage platform / V Discharge voltage plateau / V Number of cycles Example 1 45 0.1 4.05-4.13 —— 3 Example 2 20 0.05 4.05-4.13 —— 5 Example 3 15 0.02 4.05-4.13 —— 200 Example 4 50 0.1 4.05-4.13 —— 3 Example 5 45 0.2 4.05-4.13 —— 3 Example 6 45 0.1 —— 4.0-4.03 3

[0161] In the table, "—" indicates that no data is involved.

[0162] Comparative Example 1

[0163] The integrity of the coating layer of the two groups of composite positive electrode active materials in Example 1 was analyzed using the iron leaching method. The specific process is as follows:

[0164] Take the same mass and type of powder sample from Example 1, mix them with the same concentration, volume and type of acidic solution, and react them at the same temperature and time. The acidic solution is a 0.1M sulfuric acid solution.

[0165] Solid-liquid separation was performed on the mixtures after each reaction to obtain filtrate and insoluble matter;

[0166] The concentration of Fe in each group of filtrates was measured, and the coating integrity of the two groups of positive electrode active materials was evaluated based on the concentration of Fe.

[0167] Specifically, 0.5g of disodium ethylenediaminetetraacetate complexing agent was added to an acidic solution, and then 1g of material 1 and material 2 were mixed with 50mL of 0.1M sulfuric acid solution. The acid dissolution reaction was carried out at 25℃ for 60min.

[0168] After 60 min, the iron leaching amounts of Sample 1 (596) and Sample 2 (563) were 78362 ppm and 79540 ppm, respectively. The iron leaching concentration of Sample 1 was lower than that of Sample 2, that is, the coating integrity of Sample 1 was higher than that of Sample 2.

[0169] [Outcome Evaluation]

[0170] The specific capacity of the test samples of each embodiment and comparative example was calculated in accordance with step (4). Specifically, based on the charge-discharge cycle performance test data above, the specific capacity of the test samples in different voltage platforms was calculated by the following method:

[0171] Then calculate the ratio between the difference in specific capacity and the difference in voltage for each group of test samples within the corresponding voltage plateau.

[0172] The test results are summarized in Table 2 and Figure 1-2 middle.

[0173] Table 2

[0174]

[0175] In Table 2, the number of cycles for the specific capacity difference / voltage difference in each embodiment and comparative example corresponds to the number of cycles in Table 1.

[0176] The following text combines Table 2 and Figure 1-2 The various embodiments and comparative examples are described below:

[0177] Analysis of Comparative Example 1 and Example 1 shows that the method described in this application, compared with existing traditional methods, can simply, efficiently and cost-effectively evaluate the coating integrity of different groups of composite positive electrode active materials, facilitating batch testing of multiple groups of composite positive electrode active materials.

[0178] The method described in Examples 2-3 can simply, efficiently, and cost-effectively evaluate the coating integrity of different groups of composite positive electrode active materials. A smaller rate can shorten the charge and discharge time, improve testing efficiency, and facilitate batch testing of multiple groups of composite positive electrode active materials.

[0179] Analysis of Examples 4 and 1 shows that adjusting the test temperature to within 45°C using the method described in this application can better evaluate the coating integrity of different groups of composite positive electrode active materials, facilitating batch testing of multiple groups of composite positive electrode active materials.

[0180] Analysis of Examples 5 and 1 shows that the method described in this application, which adjusts the test current density to the range of 0.02-0.1C, can better evaluate the coating integrity of different groups of composite positive electrode active materials and facilitate batch testing of multiple groups of composite positive electrode active materials.

[0181] Analyze Examples 1 and 6, combined with Figure 1 and Figure 2 It can be seen that, among them, Figure 1 In the meantime, taking 3 cycles as the standard, the curves of the 596 series from top to bottom are 596-4, 596-3, 596-1 and 596-2, and the curves of the 563 series from top to bottom are 563-1, 563-4, 563-2 and 563-3; Figure 2 In the middle, taking 3 cycles as the standard, the curves of the 596 series from top to bottom are 596-4, 596-3, 596-1 and 596-2, and the curves of the 563 series from top to bottom are 563-4, 563-2, 563-1 and 563-3; Figure 1 This represents the oxidation of manganese during the charging process. Figure 2 For the reduction of manganese during discharge, the redox platform can be used to determine the manganese content in the material, and thus determine the integrity of the material coating. The method described in this application can simply, efficiently and cost-effectively evaluate the coating integrity of different groups of composite positive electrode active materials, which is convenient for batch testing of multiple groups of composite positive electrode active materials.

[0182] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and not to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application, and they should all be covered within the scope of the claims and specification of this application. In particular, as long as there is no structural conflict, the various technical features mentioned in the embodiments can be combined in any way. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A method for comparing the coating integrity of composite positive electrode active materials, characterized in that, The method includes the following steps: (1) Take at least two sets of composite positive electrode active materials, wherein the composite positive electrode active materials include positive electrode active materials and a coating layer disposed on at least a portion of the surface of the positive electrode active materials; The positive electrode active materials in each group of composite positive electrode active materials have the same general chemical formula, and the positive electrode active materials include manganese. (2) Test samples of battery cells are formed independently from at least two groups of composite positive electrode active materials of the same mass using the same method; (3) Perform charge-discharge cycle performance tests on each group of test samples, with a minimum of three charge-discharge cycles; (4) Based on the test data of charge-discharge cycle performance of each group of test samples, obtain the specific capacity of each group of test samples within the charging voltage platform of 4.05-4.13V or the discharging voltage platform of 4.0-4.03V, and evaluate the coating integrity of the composite positive electrode active material.

2. The method according to claim 1, characterized in that, In step (1), the chemical formula of the positive electrode active material is Li m Fe 1-x-y Mn x M y PO4, 0.8≤m≤1, 0<x≤1, 0≤y≤1, M is selected from any one or at least two combinations of V, Nb, Ti, Co, Ni, Sc, Ge, Mg, Al, Zr, Mn, Hf, Ta, Mo, W, Ru, Ag, Sn and Pb; optionally, the positive electrode active materials in the at least two groups of composite positive electrode active materials are of the same type.

3. The method according to claim 1 or 2, characterized in that, In step (1), the coating layer includes a carbon coating layer.

4. The method according to any one of claims 1-3, characterized in that, In step (2), the test samples of the battery cells include button cells.

5. The method according to any one of claims 1-4, characterized in that, In step (3), the temperature for the charge-discharge cycle performance test is ≤45℃, and optionally 15–45℃.

6. The method according to any one of claims 1-5, characterized in that, In step (3), the charging rate of the charge-discharge cycle performance test is 0.02-0.1C.

7. The method according to any one of claims 1-6, characterized in that, In step (3), the discharge rate of the charge-discharge cycle performance test is 0.02-0.1C.

8. The method according to any one of claims 1-7, characterized in that, In step (3), the number of charge-discharge cycle performance tests is ≥3 cycles, and optionally 3-200 cycles.

9. The method according to any one of claims 1-8, characterized in that, In step (3), the charge-discharge cycle performance test includes the following steps: Each group of test samples is charged to 100% state of charge and then discharged to 0% state of charge. This process is repeated at least three times, or optionally 3-200 times.

10. The method according to claim 9, characterized in that, The charge-discharge cycle performance test includes the following steps: The test sample of the battery cell is first charged with constant current at 0.02-0.1C until it reaches 100% state of charge, then charged with constant voltage until the current density is ≤0.02C, and then discharged with constant current at 0.02-0.1C until it reaches 0% state of charge. The above process is repeated at least three times to complete the charge-discharge cycle performance test.

11. The method according to any one of claims 1-10, characterized in that, In step (4), the difference in specific capacity of each group of test samples under any voltage range within the charging voltage platform of 4.05-4.13V or the discharging voltage platform of 4.0-4.03V is calculated, and / or the ratio between the difference in specific capacity under any voltage range and the difference in voltage is calculated to evaluate the coating integrity of the composite positive electrode active material.

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