Electrode material surface coating effect evaluation method

Abstract

The invention relates to the technical field of batteries, in particular to a method for evaluating the surface coating effect of an electrode material. The evaluation method for the surface coating effect of the electrode material comprises the following steps: respectively preparing an electrode material, a coating material and a coated electrode material to be tested into electrode plates under the same condition; respectively preparing the batteries from the electrode plates under the same condition; performing electrochemical impedance spectroscopy test on each battery; analyzing the obtained electrochemical impedance spectrum of each battery based on a relaxation time distribution method to obtain a relaxation time distribution curve; taking the relaxation time distribution curve of the battery containing the electrode material and the battery containing the coating material as a standard curve, and comparing and analyzing the relaxation time distribution curve of the battery containing the to-be-detected coating electrode material with the standard curve so as to evaluate the coating effect of the to-be-detected coating electrode material. According to the method, the surface coating effect of the electrode material can be simply, effectively and accurately evaluated.

Description

Technical Field

[0001] The present invention relates to the field of battery technology, and in particular to a method for evaluating the surface coating effect of an electrode material. Background Art

[0002] The structure and properties of electrode materials, such as cathode and anode materials, are key factors in improving battery electrochemical performance. Developing high-performance electrode materials has become a research hotspot. Surface coating of cathode and anode materials is one method for improving their performance. For example, coating the surface of a material plays a crucial role in improving its interfacial properties.

[0003] The completeness and effectiveness of coating directly impacts the electrochemical performance of electrode materials. However, the related art lacks effective methods for characterizing the effectiveness of coating. For example, some existing methods for testing the coating effect on the surface of positive electrode materials rely on direct evaluation of the coating quality by observing the coated material using a scanning electron microscope. However, this method only allows for micro-area analysis, making it difficult to objectively or accurately reflect the overall coating effect.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] In view of this, the present invention aims to solve at least one of the technical problems in the related art to a certain extent. To this end, the present invention provides a method for evaluating the surface coating effect of an electrode material, which can simply, effectively and accurately evaluate the quality of the surface coating effect of the electrode material.

[0006] In order to solve the above technical problems, this application is implemented as follows:

[0007] According to one aspect of the present application, an embodiment of the present application provides a method for evaluating the surface coating effect of an electrode material, the method comprising:

[0008] The electrode material, the coating material and the coated electrode material to be tested are respectively made into electrode sheets under the same conditions;

[0009] Each of the electrode sheets is prepared under the same conditions to obtain a battery;

[0010] Performing electrochemical impedance spectroscopy testing on each of the batteries;

[0011] Analyzing the electrochemical impedance spectra of each of the batteries obtained based on a relaxation time distribution method to obtain a relaxation time distribution curve;

[0012] The relaxation time distribution curves of the battery containing the electrode material and the battery containing the coating material are used as standard curves. The relaxation time distribution curve of the battery containing the coated electrode material to be tested is compared with the standard curve to evaluate the coating effect of the coated electrode material to be tested.

[0013] In some embodiments, the thickness of the electrode sheet is 120 μm to 180 μm.

[0014] In some embodiments, the preparation of the battery includes: assembling the electrode sheet and the counter electrode, and then performing a formation treatment to adjust the battery charge to 10% to 50% SOC; wherein the charge and discharge rate in the formation treatment is 0.05C to 0.5C.

[0015] In some embodiments, the electrochemical impedance spectroscopy test conditions include: voltage perturbation amplitude of 3 to 5 mV, frequency range of 10 ^6 Hz~10 ^-2 Hz.

[0016] In some embodiments, the comparative analysis of the relaxation time distribution curve of the battery containing the coated electrode material to be tested and the standard curve includes: selecting the peak parameters under a preset time constant as characteristic parameters; constructing an evaluation formula using the characteristic parameters; inputting the characteristic parameters of the coated electrode material to be tested into the evaluation formula to obtain a numerical value for evaluating whether the coating effect is good or poor.

[0017] In some embodiments, the characteristic parameters include peak resistance and time constant difference;

[0018] The evaluation formula includes Formula 1; Formula 1 satisfies: a=(R max -R) / (R max -R min );

[0019] Among them, R max is the resistance value of the second peak of the electrode material in the standard curve, R min is the resistance value of the second peak of the coating material in the standard curve, and R is the resistance value of the second peak of the coating electrode material to be measured in the relaxation time distribution curve; when a≤0.1, the coating effect is judged to be poor; when 0.1<a<0.5, the coating effect is judged to be good; when a≥0.5, the coating effect is judged to be excellent.

[0020] In some embodiments, the evaluation formula includes Formula 2;

[0021] The formula 2 satisfies: b=(Δt-Δt min ) / (Δtmax -Δt min );

[0022] Where, Δt max is the time constant difference between the third and fourth peaks of the coating material in the standard curve, Δt min is the time constant difference between the third peak and the fourth peak of the electrode material in the standard curve, and Δt is the time constant difference between the third peak and the fourth peak of the coated electrode material to be tested in the relaxation time distribution curve; when b is less than 0.4, the coating effect is judged to be poor; when b is greater than or equal to 0.4, the coating effect is judged to be excellent.

[0023] In some embodiments, when a≥0.5 and b≥0.4, the coating effect is judged to be excellent.

[0024] In some embodiments, the second peak has a time constant of 10 ^-4 s~10 ^-2 Peaks within the s range.

[0025] In some embodiments, the third peak and the fourth peak are peaks with a time constant ranging from 1 s to 200 s.

[0026] In some embodiments, the coated electrode material to be tested includes multiple materials made of the same coating material to be tested under different coating processes, which are used to compare the coating effects of different coating processes.

[0027] In some embodiments, the different coating processes include at least two of mechanical coating, solvent coating, or solid phase reaction coating.

[0028] In some embodiments, the coated electrode material to be tested includes a plurality of materials prepared using different coating materials to be tested under the same coating process, so as to compare the coating effects of different coating materials to be tested.

[0029] In some embodiments, the electrode material includes at least two different electrode materials, and the coated electrode material to be tested includes multiple materials prepared using the same coating material to be tested and under the same coating process, which are used to compare the coating effects of different electrode materials under the same coating material to be tested and the same coating process.

[0030] The implementation of the technical solution of the present invention has at least the following beneficial effects:

[0031] In the embodiments of the present application, a method for evaluating the surface coating effect of an electrode material is provided. By assembling the materials into a battery, testing their electrochemical impedance, and analyzing their impedance behavior through the distribution of relaxation time (DRT), the coating effect of the materials can be qualitatively compared. Thus, the current lack of effective technical means to characterize the coating effect is alleviated, and the quality of the surface coating effect of the electrode material can be simply, effectively, and accurately evaluated. The evaluation method of the present invention has the advantages of being simple, easy to use, low cost, and high accuracy.

[0032] Additional aspects and advantages of the present application will be given in part in the following description and in part will become obvious from the following description or will be learned through practice of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 Schematic diagram of DRT results of electrode material / coating material calibration provided by some embodiments of the present invention;

[0034] Figure 2 Schematic diagram showing DRT results of coated electrode materials provided in some embodiments of the present invention;

[0035] Figure 3 Schematic diagram of the DRT results of the electrode material / coating material calibration provided in Example 1 of the present invention;

[0036] Figure 4 Shown are DRT diagrams of the coated electrode materials to be tested using different coating processes provided in Example 1 of the present invention. DETAILED DESCRIPTION

[0037] The present application will be further described below with reference to specific examples. It should be understood that these examples are only used to illustrate the present application and are not intended to limit the scope of the present application.

[0038] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range or the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein.

[0039] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution. Unless otherwise specified, all technical features and optional technical features of the present application can be combined with each other to form a new technical solution.

[0040] Unless otherwise specified, the terms used in this application have the common meanings that are commonly understood by those skilled in the art.

[0041] Unless otherwise stated, the numerical values ​​of the various parameters mentioned in this application can be measured using various test methods commonly used in the art, for example, they can be measured according to the test methods given in the examples of this application. Unless otherwise stated, the test temperature of each parameter can be room temperature.

[0042] In the field of batteries, the coating layer on the surface of electrode materials such as positive electrode materials or negative electrode materials plays an extremely important role in improving the interfacial properties of the materials. However, there are still certain deficiencies in the accurate characterization or judgment of the coating effect of electrode materials in practical applications. For example, in the related art, the patent with publication number CN105301194A discloses a method for detecting the surface coating effect of positive electrode active materials, which is to form a solid-liquid mixture by adding an acid solution of a predetermined concentration to a container until the coated positive electrode active material to be tested is completely immersed; the container is sealed, the solid-liquid mixture is heated and stirred, and the pH value of the liquid phase in the solid-liquid mixture at different times is recorded, i.e., the pH value to be tested; and the surface coating effect of the coated positive electrode active material to be tested is judged by comparing the pH value to be tested with the standard pH value. However, this method is cumbersome to operate, easily leads to certain errors, and has low accuracy. For example, in related art, patent publication number CN113358434A discloses a method for evaluating the surface coating of silicon anode materials. This method involves transferring silicon material and a KOH solution into a dry aluminum-plastic film bag at room temperature. The bag is then evacuated and heat-sealed to isolate the silicon material and KOH solution from the outside world. The bag is then heated to 60°C and held for 5 hours, and its expansion rate is measured. The expansion rate of the bag is used to characterize the effectiveness of the carbon material coating on the silicon material surface. The lower the expansion rate of the bag, the better the coating effect. Conversely, the higher the expansion rate of the bag, the worse the coating effect. However, this method also suffers from operational difficulties, large errors, and low accuracy.

[0043] In view of this, the inventors of this application have found through extensive research that by assembling electrode materials into batteries such as buckled half-cells, testing their electrochemical impedance spectroscopy and analyzing their impedance behavior through distribution relaxation time (DRT), it is possible to qualitatively compare their coating effects, which can more simply, effectively, and accurately evaluate the quality of the coating effect. The following is a more detailed description of this application and optional embodiments.

[0044] In some embodiments, the present application provides a method for evaluating the surface coating effect of an electrode material, the evaluation method comprising the following steps:

[0045] S10, preparing electrode sheets from the electrode material, the coating material, and the coating electrode material to be tested under the same conditions;

[0046] S20, preparing batteries by respectively preparing the electrode sheets under the same conditions;

[0047] S30, performing electrochemical impedance spectroscopy testing on each battery;

[0048] S40, analyzing the electrochemical impedance spectra of each battery obtained based on a relaxation time distribution method to obtain a relaxation time distribution curve;

[0049] S50, using the relaxation time distribution curves of the battery containing the electrode material and the battery containing the coating material as standard curves, and comparing and analyzing the relaxation time distribution curve of the battery containing the coated electrode material to be tested with the standard curves to evaluate the coating effect of the coated electrode material to be tested.

[0050] In this paper, the term “distribution relaxation time (DRT)” is a technique for interpreting AC impedance spectra that does not rely on prior knowledge of the object under investigation and can be used to separate and interpret overlapping physical and chemical processes in impedance spectra.

[0051] In this article, the term "electrochemical impedance spectroscopy (EIS)" refers to the impedance spectrum obtained by applying a small-amplitude sinusoidal potential (or current) perturbation signal to an electrochemical system and measuring the resulting current (or potential) response. This spectrum reflects the frequency-dependent impedance of the electrochemical system, providing rich information on interfacial structure and kinetics.

[0052] In this article, the term "coated electrode material" refers to an electrode active material having a coating on its surface, which includes an electrode active material matrix and a coating layer coated on at least part of the surface of the electrode active material, wherein the electrode active material can be a positive electrode active material or a negative electrode active material.

[0053] It should be noted that the electrode material to which the evaluation method provided in the present application is applicable can be a positive electrode material (positive electrode active material) or a negative electrode material (negative electrode active material), and the present application has no particular limitation on this. Furthermore, based on the problems existing in the evaluation of the coating effect of existing positive electrode materials, it is more preferred that the electrode material be selected from the positive electrode material. For example, the method of the present invention can be used to evaluate the effect of the surface coating of the positive electrode material, or the effect of the positive electrode material coated with the coating material to be tested with different properties (ionic conductivity, particle size).

[0054] The present invention tests the electrochemical impedance spectroscopy of the battery and analyzes the data through DRT to qualitatively compare the coating effect. The method of the present invention can objectively and effectively reflect the overall coating effect on the surface of the electrode material. It is not only simple and effective, but also low-cost and highly adaptable.

[0055] Specifically, the present invention first weighs different materials according to the same formula, stirs the slurry, and coats the electrode sheets to prepare electrode sheets. The electrode sheets are then assembled into batteries such as button half-cells. After the battery is formed, the charge is adjusted to a predetermined SOC. The electrochemical impedance spectrum of the battery is tested using an electrochemical workstation, and the data is then analyzed using the DRTtool program to characterize the coating effect by identifying the impedance behavior of the diffusion part.

[0056] Thus, the present invention simply assembles the materials into a button electrode, tests the electrochemical impedance spectrum, analyzes and processes the data, and identifies the coating effect of the material. And through the DRT data of the coating layer, the bulk and surface diffusion behaviors of the material are distinguished in the time constant, and the well-coated material has a smaller charge transfer impedance, thereby effectively characterizing the quality of the coating effect, alleviating the current lack of effective technical means to characterize the coating effect, and can simply, effectively and accurately evaluate the quality of the surface coating effect of the electrode material. The evaluation method of the present invention has the advantages of being simple and easy to implement, low cost and high accuracy.

[0057] In some embodiments, in step S10, the electrode material, the coating material and the coated electrode material to be tested are respectively made into electrode sheets under the same conditions, specifically including: mixing the electrode material, the coating material and the coated electrode material to be tested with the same binder, the same conductive agent and the same solvent in the same proportion to obtain electrode slurries, and coating each electrode slurry on the same current collector surface and drying them to obtain electrode sheets of the same specifications.

[0058] Among them, the coated electrode material to be tested may include multiple different coated electrode materials to be tested, and different coated electrode materials may include coated materials to be tested with different coating processes, and / or different types of electrode materials, and / or different types of coating materials to be tested. One or more can be selected from the coated materials to be tested with different coating processes, different types of electrode materials, and different types of coating materials to be tested, and combined to determine different coated electrode materials to be tested, for example, coated electrode materials to be tested with different coating processes, the same coating material to be tested, and the same type of electrode material, to evaluate the influence of different coating processes on the coating effect; or, coated electrode materials to be tested with the same coating process, different coating materials to be tested, and the same electrode material, to evaluate the coating effect of different coating materials to be tested; or, coated electrode materials to be tested with the same coating process, the same coating material to be tested, and different electrode materials, to evaluate the influence of different electrode materials on the coating effect. They can be combined as needed, and the present invention is not limited to this.

[0059] It should be understood that in the process of preparing electrode sheets, electrode sheets containing electrode materials, electrode sheets containing coating materials (i.e., electrode sheets with coating materials coated on positive electrode materials), and electrode sheets containing coated electrode materials to be tested (i.e., electrode sheets with coating materials to be tested coated on positive electrode materials) are prepared respectively. Except for the different electrode materials, coating materials and coating electrode materials to be tested used, the other preparation conditions such as additive types, dosage ratios, etc. are the same.

[0060] Optionally, depending on the electrode material, the electrode sheet may be a positive electrode sheet or a negative electrode sheet, preferably a positive electrode sheet.

[0061] The present application embodiment has no particular restrictions on the types of electrode materials, conductive agents and binders in the electrode sheet, as long as the purpose of the present application can be achieved. For example, in some embodiments, the electrode material is a positive electrode active material, which may include but is not limited to one or more of lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium cobaltate, lithium manganate, lithium nickel manganate, lithium nickel cobalt manganate, lithium nickel cobalt aluminum oxide and other positive electrode materials. The binder may include but is not limited to polyvinylidene fluoride, polyacrylic acid, polyacrylate, polyimide, polyvinyl alcohol, polyamide, polyamide-imide, polystyrene butadiene copolymer (styrene butadiene rubber), polytetrafluoroethylene, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose or sodium hydroxymethyl cellulose. The conductive agent may include but is not limited to at least one of conductive carbon black (such as acetylene black, Ketjen black), carbon nanotubes (CNTs), carbon fiber, graphene, etc. The above-mentioned carbon nanotubes may be single-walled carbon nanotubes and / or multi-walled carbon nanotubes.

[0062] It should also be noted that the embodiments of the present application do not particularly limit the type of coating material, as long as the purpose of the present application can be achieved. For example, in some embodiments, the coating material can be an inorganic substance (such as an oxide, a phosphide, etc.) or an organic substance. At the same time, there is no particular limitation on the coating process of the coating material, as long as the purpose of the present application can be achieved. For example, in some embodiments, the coating process of the coating material can be a mechanical coating method, a solvent coating method, a solid phase reaction coating method, a chemical vapor deposition method, or an in-situ coating method.

[0063] The ratio of the electrode material, the conductive agent, and the binder in the electrode sheet can also be selected and set according to actual conditions, as long as the purpose of the present application can be achieved. For example, in some embodiments, the mass ratio of the electrode material, the conductive agent, and the binder is (85-99): (0.5-8): (0.5-8).

[0064] In some embodiments, in step S10, the thickness of the electrode sheet is 120 μm to 180 μm. As an example, the thickness of the electrode sheet can be any one of 120 μm, 130 μm, 140 μm, 150 μm, 160 μm, and 180 μm, or a range of values ​​therebetween. Preferably, the thickness of the electrode sheet is 150 μm.

[0065] According to this application, the thickness of the electrode sheet, that is, the thickness of the electrode sheet containing the above-mentioned materials, will affect the ohmic impedance, charge transfer impedance, and diffusion impedance of the battery to a certain extent. Therefore, this application uniformly adopts a thinner coating thickness, such as a electrode sheet thickness of 150μm, to avoid the impact of the electrode sheet thickness on material characterization.

[0066] In some embodiments, in step S20, each electrode sheet is separately prepared under the same conditions to form a battery. The same conditions, such as the same type of counter electrode, the same assembly method, and the same formation conditions, are used to prepare the electrode sheet containing the electrode material, the electrode sheet containing the coating material, and the electrode sheet containing the coating electrode material to be tested. The battery is preferably a button-type half-cell, which is convenient for operation and testing.

[0067] Optionally, in the above battery, metallic lithium may be used as the counter electrode.

[0068] At the same time, on the basis of ensuring the consistency of the thickness of each positive electrode sheet, the thickness of the coating layer of the electrode sheet containing the coating material and the electrode sheet containing the coated electrode material to be tested is kept consistent to improve the accuracy of the evaluation and avoid deviations in various parameters. At the same time, the uniformity of the coating layer is ensured to avoid the problem of inaccurate evaluation conclusions caused by local uneven coating.

[0069] It should be noted that the specific preparation method or conditions of the above-mentioned battery can be carried out in a conventional manner in the art, and this application has no special restrictions on this. As long as the purpose of this application can be achieved, it will not be described in detail here.

[0070] In some embodiments, preparing the battery includes: assembling the electrode sheet and the counter electrode, and then performing a formation treatment to adjust the battery charge to 10% to 50% SOC; wherein the charge and discharge rate during the formation treatment is 0.05C to 0.5C.

[0071] In the above formation treatment, the charge and discharge rate range is within the range of 0.05C to 0.5C, preferably 0.05C. After formation, the battery is charged (adjusted to the state of charge) to 10% to 50% SOC, preferably 50% SOC.

[0072] In this article, the state of charge (SOC), also known as the remaining capacity, represents the ratio of the remaining capacity of a battery after a period of use or long-term storage to its fully charged capacity. It is usually expressed as a percentage. Its value range is 0-100%. When SOC = 0, the battery is fully discharged, and when SOC = 100%, the battery is fully charged.

[0073] The SOC value can be adjusted adaptively based on the type of electrode material. For example, when using a ternary cathode material, the SOC value is preferably adjusted to 50%; or when using lithium iron phosphate, the SOC value can be adjusted to 10%.

[0074] The present application adjusts the charge to a predetermined SOC, such as 50% SOC, through chemical formation treatment, which can better reflect the interface behavior and diffusion behavior of the material and is conducive to more accurate characterization of the coating effect.

[0075] In some embodiments, in step S30, an electrochemical impedance spectroscopy test is performed on each battery, that is, an electrochemical impedance spectroscopy test is performed on a battery comprising an electrode sheet of an electrode material, a battery comprising an electrode sheet of a coating material, and a battery comprising an electrode sheet of a coating electrode material to be tested. For example, the electrochemical impedance spectroscopy of a lithium battery can be obtained by an impedance analyzer (EIS) to obtain an electrochemical impedance spectroscopy of a lithium ion battery. Then, the obtained electrochemical impedance spectroscopy of the lithium battery is analyzed based on the relaxation time distribution method to obtain a relaxation time distribution curve.

[0076] It should be understood that a battery's impedance characteristics are not only closely linked to different frequency ranges but also highly dependent on its state of charge (SOC). When a battery is at different SOC points, the corresponding electrochemical impedance spectra will also vary due to the different lithium insertion states of its positive and negative electrode materials. In this application, it is preferred to charge the battery to 50% SOC before performing the EIS test.

[0077] In some embodiments, the conditions for the electrochemical impedance spectroscopy test include: a voltage perturbation amplitude of 3 to 5 mV, a frequency range of 10 ^6 Hz~10 ^-2 Preferably, the voltage perturbation amplitude is 5 mV.

[0078] As an example, the present application measures the electrochemical impedance spectroscopy of a button half-cell charged to 50% SOC at room temperature (such as about 25°C). The test conditions are 5mV voltage perturbation amplitude and 10 ^6 Hz~10 ^-2 Hz. In this way, by limiting the working conditions of the electrochemical workstation, the accuracy of measuring the charging resistance can be improved.

[0079] In the present application, when testing electrochemical impedance spectroscopy, the test spectrum is performed at room temperature or high temperature. Otherwise, the electrochemical process is greatly affected by temperature, and spontaneous separation occurs in the electrochemical process at low temperatures, which will affect the analysis results.

[0080] In this application, when testing electrochemical impedance spectroscopy, the frequency range needs to be consistent, otherwise it will affect the time constant range. Keeping the frequency range consistent can compare the time responses of different materials.

[0081] In this application, during the electrochemical impedance spectroscopy test, the disturbance conditions need to remain consistent; otherwise, changes in the disturbance conditions will affect the R value.

[0082] In some embodiments, in step S40, after measuring the electrochemical impedance spectroscopy, the impedance data obtained from the test is subjected to DRT processing to obtain a DRT spectrum; that is, based on the electrochemical impedance spectrum of each battery obtained by the relaxation time distribution method, a relaxation time distribution curve can be obtained.

[0083] In some embodiments, in step S50, the relaxation time distribution curve of the battery containing the coated electrode material to be tested is compared with the standard curve, including: selecting the peak parameters under the preset time constant as characteristic parameters; constructing an evaluation formula using the characteristic parameters; inputting the characteristic parameters of the coated electrode material to be tested into the evaluation formula to obtain a numerical value for evaluating whether the coating effect is good or poor.

[0084] In the present application, a standard curve can be constructed first, which includes standard curves of electrode materials and coating materials. That is, DRT treatment is performed on batteries containing electrode sheets of electrode materials and batteries containing electrode sheets of coating materials to obtain calibration DRT result graphs of electrode materials and coating materials.

[0085] Then, the DRT result graph of the battery containing the electrode sheet of the coated electrode material to be tested is compared with the above-mentioned calibrated DRT result graph to obtain the coating effect of the coated electrode material to be tested. In some embodiments of the present application, the DRT data of different coating processes can be used to compare, that is, the DRT data can be used to compare the coating effects of the coated electrode materials to be tested prepared by different coating processes.

[0086] For example, in some embodiments, the coated electrode materials to be tested include multiple materials made of the same coating material to be tested under different coating processes, for comparing the coating effects of different coating processes. The multiple materials can be two or more.

[0087] Optionally, the different coating processes include at least two of mechanical coating, solvent coating or solid phase reaction coating.

[0088] It should be noted that when the electrode material is a positive electrode material, the coating process mainly includes three methods: mechanical coating, solvent coating, and solid-phase reaction coating. Among them, the mechanical method mainly uses mechanical forces such as impact and friction to make the coating material particles grow on the surface of the positive electrode material, which is suitable for small-batch, low-cost coating production; the solvent method mainly dissolves the coating material in a solvent and immerses the positive electrode material in the solvent for coating, which has the advantages of low cost and high coating efficiency; the solid-phase reaction method mainly uses heat treatment or acid-base reaction to make the coating material and the positive electrode material react chemically to form a coating layer, which is suitable for large-scale, high-cost coating production. Generally, the coating process of positive electrode materials mainly adopts the solvent method.

[0089] In some embodiments of the present application, DRT data of different electrode materials can be used to compare, that is, the DRT data can be used to compare the coating effects of different electrode materials prepared by the same coating process on the coated electrode material to be tested.

[0090] For example, in some embodiments, the electrode material includes at least two different electrode materials, the coating material includes materials prepared by different electrode materials, the same coating material, and the same coating process, and the coated electrode material to be tested includes materials prepared by different electrode materials, using the same coating material to be tested, and the same coating process, which is used to compare the coating effects of different electrode materials under the same coating material to be tested and the same coating process.

[0091] It should be understood that when comparing the coating effects of different electrode materials, when providing a standard curve, it is necessary to provide a calibration DRT result graph containing the different electrode materials, that is, the calibration DRT result graph needs to include at least two different electrode materials.

[0092] Therefore, the method of the present application can be used not only to compare the coating effects of different coating materials to be tested, but also to compare the coating effects of different coating processes. Furthermore, it can be used to compare the coating effects of different electrode materials. For example, when the electrode material, such as the positive electrode material, is the same, the coating effects of different coating materials to be tested and different processes can be compared. If the electrode materials, such as the positive electrode materials, are different, the coating effects of the same coating material to be tested and the same process can be compared.

[0093] like Figure 1 and Figure 2 As shown, in some embodiments, in step S50, in the DRT diagram, the horizontal axis is the time constant τ, reflecting the response time of the electrochemical process, and the vertical axis is the resistance value, reflecting the resistance value of the battery electrochemical process.

[0094] In some embodiments, in step S50, the peak parameters under the preset time constant are selected as the characteristic parameters, and the characteristic parameters include the peak resistance value and the time constant difference; that is, the characteristic parameters include the resistance value of the vertical axis and the time constant difference of the horizontal axis in the DRT diagram.

[0095] As an example, first, different characteristic peaks are distinguished according to the time constant segment, for example, the second peak, the third peak and the fourth peak are distinguished according to the time constant segment, wherein the second peak is the time constant 10 ^-4 s~10 ^-2 s range, that is, the second peak falls within 10 ^-4 s~10 ^-2 s time constant, the main reaction material interfacial charge transfer process. The third and fourth peaks are peaks with a time constant range of 1s to 200s, that is, the third and fourth peaks fall within the time constant range of 1s to 200s. The selection of the third and fourth peaks is mainly based on the time period determined by the battery capacity. The capacity of the button half-cell prepared in this application is basically within this range.

[0096] In this application, the criteria for determining the coating effect are primarily the resistance value R of the second peak and / or the time constant difference Δt between the third and fourth peaks. The electrode material curve and coating material curve in the standard curve can be used as a benchmark to evaluate the coating effect of the coated electrode material to be tested. It should be noted that, to improve the accuracy of the evaluation results, the coating material curve can be determined using a coating material and coating process with known good coating effects.

[0097] It should be understood that in the DRT diagram, there is a first peak before the second peak, and this application does not limit the first peak; in addition, there may be other peaks between the second peak and the third peak, and this application does not limit these other peaks.

[0098] In some embodiments, in step S50, an evaluation formula is constructed using characteristic parameters, wherein the evaluation formula includes Formula 1 and / or Formula 2.

[0099] Specifically, Formula 1 satisfies: a=(R max -R) / (R max -R min );

[0100] Among them, R max is the resistance value of the second peak of the electrode material in the standard curve, R min is the resistance value of the second peak of the coating material in the standard curve, and R is the resistance value of the second peak of the coating electrode material to be measured in the relaxation time distribution curve; when a≤0.1, the coating effect is judged to be poor; when 0.1<a<0.5, the coating effect is judged to be good; when a≥0.5, the coating effect is judged to be excellent.

[0101] In this application, according to the material calibration of the actual coating effect, that is, the calibration results of the materials coated with the process and materials with poor coating effect and the process and materials with good coating effect are verified, it is shown that a is 0.1 or a is 0.5 can be used as the endpoint value. When a≤0.1, the coating effect is considered to be poor; when 0.1<a<0.5, the coating effect is considered to be good; when a≥0.5, the coating effect is considered to be excellent.

[0102] Coating electrode materials can improve the interfacial contact between the electrode and the electrolyte, making the migration of lithium ions at the interface smoother, thereby reducing the resistance at the interface. If the coating electrode material is not selected properly or the coating thickness is not appropriate, the resistance value of the second peak will increase, affecting battery performance. Therefore, a is set to represent the variation in the resistance value of the second peak of the current coating electrode material relative to the maximum and minimum values. Through this formula, data of different orders of magnitude and dimensions can be converted to the same interval, facilitating unified comparison and analysis of different indicators, eliminating the influence of dimensional and order of magnitude differences, and more clearly reflecting the relative performance of the resistance value of the current coating electrode material, rather than focusing solely on the absolute value. This allows for dynamic monitoring of the degree of improvement in the coating effect, especially under different temperatures and currents.

[0103] Formula 2 satisfies: b=(Δt-Δt min ) / (Δt max -Δt min );

[0104] Where, Δt max is the time constant difference between the third and fourth peaks of the coating material in the standard curve, Δt min b is the time constant difference between the third and fourth peaks of the electrode material in the standard curve, and Δt is the time constant difference between the third and fourth peaks of the coated electrode material to be tested in the relaxation time distribution curve. When b is less than 0.4, the coating effect is judged to be poor; when b is greater than or equal to 0.4, the coating effect is judged to be excellent. By setting b to normalize the time difference between the third and fourth peaks, the coating material's ability to regulate ion diffusion can be intuitively evaluated, facilitating comparisons of different coating effects. This eliminates the absolute time difference differences between different battery systems or test conditions, eliminating the influence of absolute values. It also clearly reflects the optimization direction of the coated electrode material for ion diffusion, providing data support for subsequent design. A smaller b indicates faster ion diffusion behavior, which is due to the coating electrode material's insufficient regulation of ion transport, which can easily affect the battery's cycling stability and safety. Therefore, b is set to ≥ 0.4 to ensure the coating electrode material's regulatory effect on ion transport, prevent rapid ion impact from damaging the battery structure, and help improve stability.

[0105] In this application, evaluation formula 2 can be used as a basis for judgment, and evaluation formula 1 can be used for further verification, that is, first judge formula 2. If formula 2 judges that the coating effect is excellent, then judge formula 1 to improve the evaluation efficiency and further ensure the accuracy of the evaluation. Specifically, when judging the coating effect, the diffusion behavior of the interface must be different. When the coating effect is not good, the interface behaves as the surface diffusion of the original electrode material; when the coating effect is good, it behaves as a transition behavior between the diffusion behavior of the coating material and the original electrode material; and the reaction-diffusion behavior corresponds to the third peak and the fourth peak. Therefore, this application gives priority to the diffusion behavior as the basis for judgment (i.e., formula 2), and the interface charge transfer process is affected by the diffusion behavior. Therefore, the information of the second peak corresponding to the interface charge transfer can be used as a further verification method.

[0106] In the present application, the resistance value (peak value) of the second peak of the material with good coating effect is small and forms a third peak and a fourth peak that are separated due to a large difference in time constants. The resistance value (peak value) of the second peak of the material with poor coating effect is large and forms a third peak and a fourth peak that are close due to a similar difference in time constants.

[0107] In some embodiments, when a≥0.5 and b≥0.4, the coating effect is determined to be excellent.

[0108] By ensuring that the material simultaneously satisfies a ≥ 0.5 and b ≥ 0.4, that is, simultaneously satisfies the corresponding R and Δt, the coating effect can be more accurately judged. This is because coating should improve the interface. Generally, coating materials facilitate the interfacial charge transfer reaction and accelerate the diffusion behavior between the interface and the bulk phase. For a good coating effect, it should be effective in reducing R and increasing Δt at the same time. In other words, it is necessary to simultaneously satisfy a ≥ 0.5 and b ≥ 0.4. Under this condition, the coating effect can be considered good.

[0109] The following is further described with reference to specific embodiments and drawings.

[0110] Evaluation methods for the surface coating effect of electrode materials include:

[0111] S10. The positive electrode material (lithium iron phosphate), coating material (aluminum oxide), first coating electrode material, second coating electrode material are mixed with conductive agent conductive carbon black (SP), and binder polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1, and an appropriate amount of solvent N-methylpyrrolidone (NMP) is added to prepare a slurry. Each slurry is coated on the same current collector surface and dried and rolled to obtain positive electrode sheets of the same specifications. The thickness of the positive electrode sheet is 150μm. Among them, the thickness of the positive electrode sheet of the positive electrode material is consistent with the thickness of the positive electrode sheet coated with the coating layer, that is, the positive electrode sheet of the positive electrode material not coated with the coating layer is 150μm, and the thickness of the positive electrode sheet coated with the coating material is also 150μm. At the same time, the positive electrode sheet of the first coating electrode material is 150μm, and the positive electrode sheet of the second coating electrode material is 150μm. Among them, the coating material can be coated on the surface of the positive electrode material by an in-situ coating method,

[0112] Wherein, the first coating electrode material can be aluminum fluoride, and the second coating electrode material can be lithium phosphate, wherein aluminum fluoride can effectively inhibit side reactions and enhance structural stability, but its ionic conductivity is not as good as lithium phosphate. Specifically, aluminum fluoride can contact the plating solution containing aluminum fluoride with the positive electrode material through a chemical reaction, and deposit an aluminum fluoride coating layer on the surface of the positive electrode material. For lithium phosphate, the positive electrode material is reacted with a solution containing a lithium phosphate precursor under high temperature and high pressure hydrothermal or solvent thermal reaction conditions, so that lithium phosphate is uniformly deposited on the surface of the positive electrode material. For the coating process, it can be set according to the different coating electrode materials. S20, the above-mentioned positive electrode sheets are respectively prepared under the same conditions to obtain button-type half-cells, wherein metallic lithium is used as the counter electrode.

[0113] The button half-cell was charged and discharged at a rate of 0.05C, and after formation, the battery was charged to 50% SOC.

[0114] S30, at room temperature (25 ° C), the button half-cell charged to 50% SOC was measured for electrochemical impedance spectroscopy. The test conditions were 5mV voltage perturbation amplitude and 10 ^6 Hz~10 ^-2 Hz.

[0115] S40: Perform DRT processing on the impedance data obtained from the test to obtain a DRT spectrum.

[0116] S50. Comparing and analyzing the relaxation time distribution curve of the battery containing the coated electrode material to be tested with the standard curve, including: selecting peak parameters under a preset time constant as characteristic parameters; constructing an evaluation formula using the characteristic parameters; inputting the characteristic parameters of the coated electrode material to be tested into the evaluation formula to obtain a numerical value for evaluating whether the coating effect is good or poor.

[0117] The evaluation formula 1 satisfies: a=(R max -R) / (R max -R min ); where R max is the resistance value of the second peak of the electrode material in the standard curve, R min is the resistance value of the second peak of the coating material in the standard curve, and R is the resistance value of the second peak of the coating electrode material to be measured in the relaxation time distribution curve.

[0118] Evaluation formula 2 satisfies: b=(Δt-Δt min ) / (Δt max -Δt min ); where Δt max is the time constant difference between the third and fourth peaks of the coating material in the standard curve, Δt min is the time constant difference between the third peak and the fourth peak of the electrode material in the standard curve, and Δt is the time constant difference between the third peak and the fourth peak of the coated electrode material to be tested in the relaxation time distribution curve.

[0119] like Figure 3 and Figure 4 As shown, in this embodiment, the time constant difference between the third peak and the fourth peak of the coating material is the maximum value Δt max , the Δt max =49.72-3.25=46.47s, the time constant difference between the third peak and the fourth peak of the positive electrode material is the minimum value Δt min , the Δt min =37.63-4.40=33.23s. Figure 4 As can be seen from the graph, the time constant difference between the third and fourth peaks of the first coated electrode material is Δt = 41.24 - 3.85 = 37.39 s, while the time constant difference between the third and fourth peaks of the second coated electrode material is Δt = 42.71 - 3.42 = 39.29 s. It can be seen that Δt increases with the increase in the material coating amount. The first coated electrode material with poor coating effect has a smaller Δt of 37.39 s, while the second coated electrode material with better coating effect has a larger Δt of 39.29 s.

[0120] Further, if Figure 3 and Figure 4 As shown in the figure, in this embodiment, the peak value R of the second peak is reduced due to the good coating of the ion conductor, and the resistance value of the second peak of the positive electrode material is the maximum value R max , the R max The resistance of the second peak of the coating material is the minimum value R min , the R min is 8.70Ω. Figure 4As can be seen from the graph, the second peak resistance value R of the first coating electrode material is 13.76Ω, while the second peak resistance value R of the second coating electrode material is 11.16Ω. It can be seen that as the coating becomes more complete and uniform, R decreases. The second peak resistance value R of the first coating electrode material with poor coating effect is larger, at 13.76Ω, while the second peak resistance value R of the second coating electrode material with poor coating effect is smaller, at 11.16Ω.

[0121] And according to a=(R max -R) / (R max -R min ) shows that for the first coating electrode material, a is 0.08, according to b = (Δt-Δt min ) / (Δt max -Δt min ) It can be seen that for the first coating electrode material, b is 0.3; a of the first coating electrode material is less than 0.1, and b is less than 0.4, and the coating effect of the first coating electrode material is poor.

[0122] For the first coating electrode material, a is 0.54 and b is 0.46; for the second coating electrode material, a>0.5 and b>0.4, and the coating effect of the second coating electrode material is better.

[0123] In other embodiments, the negative electrode material (graphite), the coating material (amorphous carbon), the first coating electrode material (silver), and the second coating electrode material (copper oxide) can be prepared according to the same standards to obtain negative electrode sheets of the same specifications to determine the coating effect of the first coating electrode material and the second coating electrode material. The evaluation method is as described above and will not be repeated here.

[0124] Parts of the present invention that are not described in detail are well known to those skilled in the art.

[0125] The basic principles of the present invention have been described above in conjunction with specific embodiments. However, it should be noted that the advantages, strengths, and effects mentioned in the present invention are merely illustrative and non-limiting, and should not be construed as necessarily possessed by each embodiment of the present invention. Furthermore, the specific details disclosed above are provided for illustrative purposes and to facilitate understanding, and are not intended to be limiting. These details do not necessarily limit the present invention to being implemented using these specific details.

[0126] It should be noted that the terms "and / or" or " / " used herein are merely a description of an association relationship between associated objects, indicating that three relationships may exist. For example, A and / or B may represent three situations: A exists alone, A and B exist at the same time, and B exists alone. The singular forms "a," "the," and "the" used in the embodiments of the present invention and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0127] In the detailed description and claims, a list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single element or multiple elements. Item B may contain a single element or multiple elements. Item C may contain a single element or multiple elements.

[0128] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A method for evaluating the surface coating effect of an electrode material, characterized in that: The method comprises: The electrode material, the coating material and the coated electrode material to be tested are respectively made into electrode sheets under the same conditions; Each of the electrode sheets is prepared under the same conditions to obtain a battery; Performing electrochemical impedance spectroscopy testing on each of the batteries; Analyzing the electrochemical impedance spectra of each of the batteries obtained based on a relaxation time distribution method to obtain a relaxation time distribution curve; The relaxation time distribution curves of the battery containing the electrode material and the battery containing the coating material are used as standard curves. The relaxation time distribution curve of the battery containing the coated electrode material to be tested is compared with the standard curve to evaluate the coating effect of the coated electrode material to be tested.

2. The method for evaluating the surface coating effect of an electrode material according to claim 1, wherein: The thickness of the electrode sheet is 120 μm to 180 μm; and / or, The prepared battery comprises: After assembling the electrode sheet and the counter electrode, performing a formation treatment to adjust the battery charge to 10% to 50% SOC; The charge and discharge rate in the chemical formation treatment is 0.05C to 0.5C.

3. The method for evaluating the surface coating effect of an electrode material according to claim 1, wherein: The conditions of the electrochemical impedance spectroscopy test include: The voltage disturbance amplitude is 3 to 5 mV and the frequency range is 10 ^6 Hz~10 ^-2 Hz.

4. The method for evaluating the surface coating effect of an electrode material according to any one of claims 1 to 3, characterized in that: The comparative analysis of the relaxation time distribution curve of the battery containing the coated electrode material to be tested with the standard curve includes: Select the peak parameter under the preset time constant as the characteristic parameter; constructing an evaluation formula using the characteristic parameters; The characteristic parameters of the coated electrode material to be tested are input into the evaluation formula to obtain a numerical value for evaluating whether the coating effect is good or bad.

5. The method for evaluating the surface coating effect of an electrode material according to claim 4, wherein: The characteristic parameters include peak resistance value and time constant difference; The evaluation formula includes formula 1; The formula 1 satisfies: a=(R max -R) / (R max -R min ); Among them, R max is the resistance value of the second peak of the electrode material in the standard curve, R min is the resistance value of the second peak of the coating material in the standard curve, and R is the resistance value of the second peak of the coating electrode material to be measured in the relaxation time distribution curve; When a≤0.1, the coating effect is judged to be poor; when 0.1<a<0.5, the coating effect is judged to be good; when a≥0.5, the coating effect is judged to be excellent.

6. The method for evaluating the surface coating effect of an electrode material according to claim 5, wherein: The evaluation formula includes formula 2; The formula 2 satisfies: b=(Δt-Δt min ) / (Δt max -Δt min ); Where, Δt max is the time constant difference between the third and fourth peaks of the coating material in the standard curve, Δt min is the time constant difference between the third peak and the fourth peak of the electrode material in the standard curve, and Δt is the time constant difference between the third peak and the fourth peak of the coated electrode material to be tested in the relaxation time distribution curve; When b is less than 0.4, the coating effect is judged to be poor; when b is greater than or equal to 0.4, the coating effect is judged to be excellent.

7. The method for evaluating the surface coating effect of an electrode material according to claim 6, wherein: When a≥0.5 and b≥0.4, the coating effect is judged to be excellent; and / or, The second peak has a time constant of 10 ^-4 s~10 ^-2 peaks within the range of s; and / or, The third peak and the fourth peak are peaks with a time constant within a range of 1 s to 200 s.

8. The method for evaluating the surface coating effect of an electrode material according to any one of claims 1 to 3, characterized in that: The coated electrode material to be tested includes multiple materials made of the same coating material to be tested under different coating processes, which are used to compare the coating effects of different coating processes; Optionally, the different coating processes include at least two of mechanical coating, solvent coating or solid phase reaction coating.

9. The method for evaluating the surface coating effect of an electrode material according to any one of claims 1 to 3, characterized in that: The coated electrode material to be tested includes a plurality of materials prepared using different coating materials to be tested under the same coating process, which are used to compare the coating effects of different coating materials to be tested.

10. The method for evaluating the surface coating effect of an electrode material according to any one of claims 1 to 3, characterized in that: The electrode material includes at least two different electrode materials, and the coated electrode material to be tested includes multiple materials prepared using the same coating material to be tested and under the same coating process, which are used to compare the coating effects of different electrode materials under the same coating material to be tested and the same coating process.

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