Method for evaluating the surface coating effect of electrode materials

Abstract

This invention relates to the field of battery technology, specifically to a method for evaluating the surface coating effect of electrode materials. The provided method includes: fabricating electrode materials, coating materials, and the coated electrode material to be tested into electrode sheets under identical conditions; preparing batteries from each electrode sheet under identical conditions; performing electrochemical impedance spectroscopy (EIS) on each battery; analyzing the obtained EIS of each battery using the relaxation time distribution method to obtain relaxation time distribution curves; 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 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. This invention can simply, effectively, and accurately evaluate the quality of the surface coating effect of electrode materials.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically, to a method for evaluating the surface coating effect of electrode materials. Background Technology

[0002] The structure and properties of electrode materials, such as positive or negative electrode materials, are key factors affecting the improvement of battery electrochemical performance. Currently, the development of high-performance electrode materials has become a research hotspot. Surface coating of positive or negative electrode materials is one method to improve their performance; for example, setting a coating layer on the surface of the material plays a crucial role in improving the interfacial properties of the material.

[0003] The completeness and effectiveness of coating directly affect the electrochemical performance of electrode materials. However, effective methods for characterizing coating effectiveness are lacking in related technologies. For example, some existing methods for detecting the coating effect on the surface of cathode materials involve observing the coated material using a scanning electron microscope to directly determine the coating effect. However, this method can only perform micro-area analysis during the detection process and is difficult to objectively or accurately reflect the overall coating effect.

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

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

[0006] To solve the above-mentioned technical problems, this application is implemented as follows:

[0007] According to one aspect of this application, embodiments of this application provide a method for evaluating the surface coating effect of an electrode material, the method comprising:

[0008] Electrode materials, coating materials, and the coating electrode material to be tested are respectively fabricated into electrode sheets under the same conditions;

[0009] Batteries were prepared by preparing each of the aforementioned electrode sheets under the same conditions;

[0010] Electrochemical impedance spectroscopy was performed on each of the aforementioned batteries;

[0011] The relaxation time distribution curves were obtained by analyzing the electrochemical impedance spectra of each battery based on the relaxation time distribution method.

[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. By comparing and analyzing the relaxation time distribution curve of the battery containing the coating electrode material to be tested with the standard curve, the coating effect of the coating electrode material to be tested can be evaluated.

[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 / discharge rate in the formation treatment is 0.05C to 0.5C.

[0015] In some embodiments, the conditions for the electrochemical impedance spectroscopy test include: a voltage perturbation amplitude of 3–5 mV and a 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 with a standard curve includes: selecting the peak parameter under a preset time constant as a characteristic parameter; constructing an evaluation formula using the characteristic parameter; and inputting the characteristic parameter of the coated electrode material to be tested into the evaluation formula to obtain a value for evaluating whether the coating effect is good or bad.

[0017] In some of these implementations, the characteristic parameters include the resistance value of the peak and the 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 R represents the resistance value of the second peak of the electrode material in the standard curve. min R 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 tested 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 of these embodiments, the evaluation formula includes Formula 2;

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

[0022] Where, Δt max Δt represents the time constant difference between the third and fourth peaks of the coated material in the standard curve. min Δt 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 < 0.4, the coating effect is judged to be poor; when b ≥ 0.4, the coating effect is judged to be excellent.

[0023] In some of these implementations, when a ≥ 0.5 and b ≥ 0.4, the coating effect is considered excellent.

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

[0025] In some of these embodiments, the third and fourth peaks are peaks with time constants ranging from 1 s to 200 s.

[0026] In some embodiments, the coating electrode material to be tested includes multiple materials prepared using the same coating material but with different coating processes, for comparing the coating effects of different coating processes.

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

[0028] In some embodiments, the coating electrode material to be tested includes multiple materials prepared using different coating materials under the same coating process, for comparing 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 coating electrode material to be tested includes multiple materials prepared using the same coating material to be tested and under the same coating process, for comparing the coating effect of different electrode materials under the same coating material to be tested and the same coating process.

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

[0031] In the embodiments of this application, the method for evaluating the surface coating effect of electrode materials provides a qualitative comparison of the coating effect by assembling the materials into a battery, testing its electrochemical impedance, and analyzing its impedance behavior through relaxation time distribution (DRT). This alleviates the current deficiency of lacking effective technical means to characterize the coating effect, and enables a simple, effective, and accurate evaluation of the surface coating effect of electrode materials. The evaluation method of this invention has the advantages of being simple, easy to implement, low-cost, and highly accurate.

[0032] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0033] Figure 1 The diagram shown is a schematic diagram of the DRT calibration results of electrode materials / coating materials provided in some embodiments of the present invention;

[0034] Figure 2 The diagram shown is a schematic diagram of the DRT results of the coated electrode material provided in some embodiments of the present invention;

[0035] Figure 3 The diagram shown is a schematic diagram of the DRT calibration results of the electrode material / coating material provided in Embodiment 1 of the present invention;

[0036] Figure 4 The image shows the DRT diagrams of the coated electrode materials under test with different coating processes provided in Embodiment 1 of the present invention. Detailed Implementation

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

[0038] The endpoints and any values ​​of the ranges 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 endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges or individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0039] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions. Unless otherwise specified, all technical features and optional technical features of this application can be combined to form new technical solutions.

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

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

[0042] In the battery field, the coating layer on the surface of electrode materials, such as positive or negative electrode materials, plays a crucial role in improving the interfacial properties of the materials. However, current methods for accurately characterizing or judging the coating effect of electrode materials in practical applications still have certain shortcomings. For example, in related technologies, patent CN105301194A discloses a method for detecting the surface coating effect of positive electrode active materials. This method involves adding an acid solution of a predetermined concentration to a container until the coated positive electrode active material to be tested is completely immersed, forming a solid-liquid mixture; sealing the container, heating and stirring the solid-liquid mixture, and simultaneously recording the pH value of the liquid phase in the solid-liquid mixture at different times, i.e., the pH value to be tested; and judging the surface coating effect of the coated positive electrode active material to be tested by comparing the pH value to be tested with a standard pH value. However, this method is cumbersome to operate, prone to certain errors, and has low accuracy. For example, in related technologies, patent CN113358434A discloses an evaluation method for coating the surface of silicon anode materials. This method involves transferring silicon material and KOH solution into a dry aluminum-plastic film bag at room temperature; evacuating and heat-sealing the aluminum-plastic film bag to isolate the silicon material, KOH solution, and external environment; heating the aluminum-plastic film bag to 60°C and holding it at that temperature for 5 hours, then testing the expansion rate of the aluminum-plastic film bag. The expansion rate of the aluminum-plastic film bag is used to characterize the effect of coating carbon material onto the silicon material surface; a lower expansion rate indicates a better coating effect, and vice versa. However, this method also suffers from drawbacks such as cumbersome operation, large errors, and low accuracy.

[0043] In view of this, the inventors of this application have discovered through extensive research that by assembling electrode materials into batteries such as coin cells, testing their electrochemical impedance spectroscopy, and analyzing their impedance behavior using relaxation time distribution (DRT), the coating effect can be qualitatively compared, allowing for a simpler, more effective, and accurate assessment of the coating effect. The following provides a more detailed description of this application and its optional embodiments.

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

[0045] S10. Electrode sheets are prepared by preparing the electrode material, coating material and the coating electrode material to be tested under the same conditions.

[0046] S20. Batteries are prepared by preparing each electrode sheet under the same conditions;

[0047] S30. Perform electrochemical impedance spectroscopy tests on each battery.

[0048] S40. Based on the relaxation time distribution method, the electrochemical impedance spectra of each battery are obtained, and the relaxation time distribution curves are obtained.

[0049] S50. The relaxation time distribution curves of the battery containing electrode material and the battery containing coating material are used as standard curves. The relaxation time distribution curve of the battery containing the coating electrode material to be tested is compared with the standard curve to evaluate the coating effect of the coating electrode material to be tested.

[0050] In this paper, the term "relaxation time distribution (DRT)" refers to a technique for resolving AC impedance spectra that does not rely on prior knowledge of the research object and can be used to separate and resolve overlapping physicochemical processes in the impedance spectrum.

[0051] In this paper, the term "electrochemical impedance spectroscopy (EIS)" refers to the measurement of the corresponding current (or potential) response of an electrochemical system by applying a small-amplitude sinusoidal potential (or current) perturbation signal, thereby obtaining an impedance spectrum. This spectrum reflects the impedance of the electrochemical system as a function of frequency, providing rich information on interface structure and kinetics.

[0052] In this document, 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 covering at least a portion 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 evaluation method provided in this application is applicable to both positive electrode materials (positive active materials) and negative electrode materials (negative active materials), and this application does not impose any special restrictions on this. Furthermore, based on the existing problems in evaluating the coating effect of positive electrode materials, it is more preferable to select positive electrode materials as the electrode material. For example, the method of this invention can be used to evaluate the effect of coating on the surface of positive electrode materials, or the effect of coating positive electrode materials with different properties (ionic conductivity, particle size).

[0054] This invention qualitatively compares coating effects by testing the electrochemical impedance spectroscopy of the battery and processing the data using DRT (Dielectric-Resistant Thermography). The method of this invention can objectively and effectively reflect the overall coating effect on the electrode material surface; 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 sheet to prepare the electrode sheet. Then, the electrode sheet is assembled into a battery such as a coin cell. After the battery is formed, it is charged to a predetermined SOC. The electrochemical impedance spectroscopy of the battery is tested using an electrochemical workstation. Then, the data is analyzed by the DRTtool program, and the coating effect is characterized by identifying the impedance behavior of the diffusion part.

[0056] Therefore, this invention identifies the coating effect of materials by simply assembling them into button cells, testing electrochemical impedance spectroscopy (EIS), and analyzing the data. Furthermore, by using the DRT data of the coating layer, the bulk and surface diffusion behavior of the material is distinguished in terms of time constant. Well-coated materials exhibit lower charge transfer impedance, thus effectively characterizing the coating effect and alleviating the current lack of effective techniques for characterizing coating effects. This invention provides a simple, effective, and accurate assessment of the surface coating effect of electrode materials. The evaluation method of this invention has the advantages of simplicity, 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, this includes: 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; coating each electrode slurry onto the same current collector surface and drying them to obtain electrode sheets of the same specifications.

[0058] The coating electrode material to be tested may include multiple different coating electrode materials. These different coating electrode materials may include coating materials with different coating processes, and / or different types of electrode materials, and / or different types of coating materials to be tested. One or more of these different coating processes, different types of electrode materials, and different types of coating materials to be tested can be selected and combined to determine different coating electrode materials to be tested. For example, coating electrode materials with different coating processes, the same coating material to be tested, and the same type of electrode material can be used to evaluate the influence of different coating processes on the coating effect; or, coating electrode materials with the same coating process, different coating materials to be tested, and the same electrode material can be used to evaluate the coating effect of different coating materials; or, coating electrode materials with the same coating process, the same coating material to be tested, and different electrode materials can be used to evaluate the influence of different electrode materials on the coating effect. These combinations can be made as needed, and this invention does not impose any limitations on them.

[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 the coating electrode material to be tested (i.e., electrode sheets with the coating material 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, the preparation conditions such as additive types and dosage ratios are the same for these electrode sheets.

[0060] Optionally, depending on the electrode material, the above-mentioned electrode sheet can be a positive electrode sheet or a negative electrode sheet, preferably a positive electrode sheet.

[0061] This application does not impose any particular limitations on the types of electrode materials, conductive agents, and binders used in the electrode sheet, as long as they achieve the purpose of this application. 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 the following positive electrode materials: lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium cobalt oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide. The binder may include, but is not limited to, one or more of the following: 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 carboxymethyl cellulose. The conductive agent may include, but is not limited to, at least one of the following: conductive carbon black (such as acetylene black, Ketjen black), carbon nanotubes (CNTs), carbon fibers, and graphene. The aforementioned carbon nanotubes may be single-walled carbon nanotubes and / or multi-walled carbon nanotubes.

[0062] It should also be noted that the embodiments of this application do not impose any particular restrictions on the type of coating material, as long as it can achieve the purpose of this application. For example, in some embodiments, the coating material can be inorganic (such as oxides, phosphides, etc.) or organic. At the same time, there are no particular restrictions on the coating process of the coating material, as long as it can achieve the purpose of this application. For example, in some embodiments, the coating process of the coating material can be mechanical coating, solvent coating, solid-phase reaction coating, chemical vapor deposition, or in-situ coating.

[0063] The ratio of electrode material, conductive agent, and binder in the above-mentioned electrode sheet can also be selected and set according to the actual situation, as long as the purpose of this application can be achieved. For example, in some embodiments, the mass ratio of electrode material, conductive agent, and 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, or 180 μm, or a range between any two. 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 an electrode sheet thickness of 150 μm, in order to avoid the influence of the electrode sheet thickness on the material characterization.

[0066] In some embodiments, in step S20, each electrode sheet is prepared into a battery under the same conditions, such as using the same type of counter electrode, the same assembly method, and the same formation conditions. Electrode sheets containing electrode material, electrode sheets containing coating material, and electrode sheets containing the coating electrode material to be tested are respectively prepared into batteries. This battery is preferably a coin cell, which is convenient to operate and easy to test.

[0067] Optionally, lithium metal can be used as the counter electrode in the above-mentioned battery.

[0068] Meanwhile, while ensuring that the thickness of each positive electrode sheet is consistent, the thickness of the coating layer is kept consistent for both the electrode sheet containing the coating material and the electrode sheet containing the coating electrode material to be tested, in order to improve the accuracy of the evaluation, avoid deviations in various parameters, and ensure the uniformity of the coating layer to avoid the problem of inaccurate evaluation conclusions caused by uneven local coating.

[0069] It should be noted that the specific preparation methods or conditions of the above-mentioned batteries can be operated in a conventional manner in the field. This application does not have any particular restrictions on this, as long as the purpose of this application can be achieved, and will not be described in detail here.

[0070] 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 / discharge rate in the formation treatment is 0.05C to 0.5C.

[0071] In the above formation process, the charge / discharge rate range is between 0.05C and 0.5C, with 0.05C being the preferred rate. After formation, the battery is conditioned (adjusted to state of charge) to 10% to 50% SOC, preferably to 50% SOC.

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

[0073] Depending on the type of electrode material, the SOC value can be adjusted accordingly. For example, when using ternary cathode material, it is preferable to adjust the load to 50% SOC; or, when using lithium iron phosphate electrode material, the load can be adjusted to 10% SOC.

[0074] This application uses a formation process to adjust the load to a predetermined SOC, such as adjusting it to 50% SOC, which can better reflect the interfacial and diffusion behaviors of the material and facilitate a more accurate characterization of the coating effect.

[0075] In some embodiments, in step S30, electrochemical impedance spectroscopy (EIS) testing is performed on each battery. Specifically, EIS testing is performed on batteries containing electrode sheets with electrode material, batteries containing electrode sheets with coating material, and batteries containing electrode sheets with the coating electrode material to be tested. For example, the EIS of a lithium-ion battery can be obtained using an electrochemical impedance spectroscopy (EIS) instrument. Then, the obtained lithium-ion battery EIS is analyzed using the relaxation time distribution method to obtain a relaxation time distribution curve.

[0076] It should be understood that the impedance characteristics of a battery are not only closely related 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 are also different due to the different lithium intercalation states of its positive and negative electrode materials. In this application, it is preferable to adjust the battery to 50% SOC before performing electrochemical impedance spectroscopy testing.

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

[0078] As an example, this application measures the electrochemical impedance spectroscopy of a coin cell charged to 50% SOC at room temperature (around 25°C). The test conditions are a voltage perturbation amplitude of 5mV and a frequency range of 10. ^6 Hz~10 ^-2 Hz. Thus, by limiting the operating conditions of the electrochemical workstation, the accuracy of measuring charging resistance can be improved.

[0079] In this application, the electrochemical impedance spectroscopy is performed at room temperature or high temperature. Otherwise, the electrochemical process is greatly affected by temperature. At low temperatures, spontaneous separation occurs in the electrochemical process, which will affect the analytical results.

[0080] In this application, the frequency range needs to be kept consistent during electrochemical impedance spectroscopy testing; otherwise, it will affect the time constant range. Keeping the frequency range consistent allows for comparison of the time response of different materials.

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

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

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

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

[0085] Then, the DRT result image of the battery containing the electrode sheet with the coating electrode material to be tested is compared and analyzed with the above-mentioned calibrated DRT result image to obtain the coating effect of the coating electrode material to be tested. In some embodiments of this application, it can be used to compare DRT data of different coating processes, that is, the coating effect of the coating electrode material to be tested prepared by different coating processes can be compared using DRT data.

[0086] For example, in some embodiments, the coating electrode material to be tested includes multiple materials prepared using the same coating material but with different coating processes, for comparing the coating effects of different coating processes. These multiple materials can be two or more.

[0087] Optionally, 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 types: mechanical coating, solvent coating, and solid-state reaction coating. Mechanical coating primarily uses mechanical forces such as impact and friction to cause the coating material particles to attach to the surface of the positive electrode material, suitable for small-batch, low-cost coating production. Solvent coating mainly involves dissolving the coating material in a solvent and immersing the positive electrode material in the solvent for coating, offering advantages such as low cost and high coating efficiency. Solid-state reaction coating mainly uses heat treatment or acid-base reactions to cause a chemical reaction between the coating material and the positive electrode material, forming a coating layer, suitable for high-volume, high-cost coating production. Typically, the coating process for positive electrode materials mainly employs the solvent method.

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

[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 coating electrode material to be tested includes materials prepared by different electrode materials, using the same coating material to be tested, and under the same coating process, for comparing the coating effect 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, a calibration DRT result graph containing the different electrode materials needs to be provided when providing the standard curve. That is, the calibration DRT result graph needs to include at least two different electrode materials.

[0092] Therefore, the method of this application can be used not only to compare the coating effects of different coating materials, but also to compare the coating effects of different coating processes, and also to compare the coating effects of different electrode materials. For example, when the electrode materials are the same as the positive electrode material, the coating effects of different coating materials and different processes can be compared; if the electrode materials are different as the positive electrode material, the coating effects of the same coating material 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 graph, 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 parameter under a preset time constant is selected as the characteristic parameter. The characteristic parameter includes the resistance value of the peak and the time constant difference; that is, the characteristic parameter includes the resistance value on the vertical axis and the time constant difference on the horizontal axis of the DRT graph.

[0095] As an example, firstly, different characteristic peaks are distinguished based on time constant segments. For instance, the second, third, and fourth peaks are distinguished based on time constant segments, where the second peak has a time constant of 10. ^-4 s~10 ^-2 The peak within the range of s, that is, the second peak falls at 10. ^-4 s~10 ^-2 Within the s time constant range, the main reaction occurs during the interfacial charge transfer process of the reaction materials. The third and fourth peaks are within the time constant range of 1s to 200s, meaning they fall within this range. The selection of the third and fourth peaks is primarily based on the time period determined by the battery capacity, and the capacity of the coin cell half-cell obtained in this application is basically within this range.

[0096] In this application, the criteria for judging the coating effect are mainly 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 the coating material curve in the standard curve can be used as evaluation benchmarks to assess the coating effect of the coated electrode material under test. It should be noted that, to improve the accuracy of the evaluation results, the coating material curve can be determined by selecting a known coating material and coating process with good coating effect.

[0097] It should be understood that in the DRT plot, 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 either.

[0098] In some embodiments, in step S50, an evaluation formula is constructed using feature 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 R represents the resistance value of the second peak of the electrode material in the standard curve. min R 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 tested 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, the material calibration based on the actual coating effect, that is, the material calibration results of coating with known poor coating process and material and coating with good coating effect, show that a is 0.1 or a is 0.5 as the endpoint value. When a≤0.1, the coating effect is considered poor; when 0.1<a<0.5, the coating effect is considered good; when a≥0.5, the coating effect is considered excellent.

[0102] Electrode coating materials improve the interfacial contact between the electrode and the electrolyte, facilitating smoother lithium-ion migration at the interface and thus reducing interfacial resistance. However, improper selection of the coating material or inappropriate coating thickness can lead to an increase in the resistance value of the second peak, affecting battery performance. Therefore, we define 'a' to characterize the variation in the resistance value of the second peak of the current coated electrode material relative to its maximum and minimum values. This formula allows data of different orders of magnitude and dimensions to be converted to the same range, facilitating unified comparison and analysis of different indicators. It eliminates the influence of differences in dimensions and orders of magnitude, and more clearly reflects the relative performance of the current coated electrode material's resistance value, rather than focusing solely on the absolute value. This allows for dynamic monitoring of the improvement in coating effectiveness, especially under different temperatures and currents.

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

[0104] Where, Δt max Δt represents the time constant difference between the third and fourth peaks of the coated material in the standard curve. min Δt represents the time constant difference between the third and fourth peaks of the electrode material in the standard curve, and Δt represents the time constant difference between the third and fourth peaks of the coated electrode material under test in the relaxation time distribution curve. When b < 0.4, the coating effect is considered poor; when b ≥ 0.4, the coating effect is considered excellent. By setting b, the time difference between the third and fourth peaks is normalized, which allows for a direct assessment of the coated electrode material's ability to regulate ion diffusion. This facilitates comparison of different coating effects, eliminates the absolute time difference differences under different battery systems or test conditions, and eliminates the influence of absolute values. Simultaneously, it clearly reflects the optimization direction of the coated electrode material for ion diffusion, providing data support for subsequent design. If b is small, it indicates rapid ion diffusion, because the coating electrode material's regulation of ion transport is insufficient, which can easily affect the cycle stability and safety of the battery. Therefore, b ≥ 0.4 is used to ensure the coating electrode material's regulation of ion transport, avoid rapid ion impacts that could damage the battery structure, and help improve stability.

[0105] In this application, evaluation formula two can be used as the basis for judgment, while evaluation formula one can be used for further verification. That is, formula two is judged first. If formula two indicates that the coating effect is excellent, then formula one is judged to improve the evaluation efficiency and further ensure the evaluation accuracy. Specifically, when judging the coating effect, the difference in diffusion behavior at the interface is first required. When the coating effect is poor, the interface exhibits surface diffusion of the original electrode material; when the coating effect is good, it exhibits transitional diffusion behavior between the coating material and the original electrode material. Since the reactive diffusion behavior corresponds to the third and fourth peaks, this application prioritizes diffusion behavior as the basis for judgment (i.e., formula two). The interface charge transfer process is affected by diffusion behavior, so the information of the second peak corresponding to interface charge transfer can be used as a further verification method.

[0106] In this application, the second peak of the material with good coating effect has a smaller resistance value (peak value) and forms a third peak and a fourth peak that are separated due to the large difference in time constant. The second peak of the material with poor coating effect has a larger resistance value (peak value) and forms a third peak and a fourth peak that are close together due to the similar difference in time constant.

[0107] In some embodiments, when a≥0.5 and b≥0.4 at the same time, the coating effect is considered excellent.

[0108] By ensuring that the material simultaneously satisfies a≥0.5 and b≥0.4, i.e., simultaneously satisfies the corresponding R and Δt, the coating effect can be judged more accurately. 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 coating effect to be good, it should be effective in both reducing R and increasing Δt, i.e., it needs to simultaneously satisfy a≥0.5 and b≥0.4. Under this condition, the coating effect can be considered good.

[0109] The following description, in conjunction with specific embodiments and accompanying drawings, provides further details.

[0110] Methods for evaluating the surface coating effect of electrode materials include:

[0111] S10. The positive electrode material (lithium iron phosphate), coating material (alumina), first coated electrode material, and second coated electrode material are mixed with conductive carbon black (SP) as a conductive agent and polyvinylidene fluoride (PVDF) as a binder in a mass ratio of 8:1:1. An appropriate amount of solvent N-methylpyrrolidone (NMP) is added to prepare a slurry. Each slurry is coated onto the same current collector surface and then dried and rolled to obtain positive electrode sheets of the same specifications. The thickness of the positive electrode sheet is 150 μm. Specifically, the thickness of the positive electrode sheet made of the original positive electrode material is the same as the thickness of the positive electrode sheet with the coating layer; that is, the thickness of the positive electrode sheet without the coating layer is 150 μm, and the thickness of the positive electrode sheet with the coating material is also 150 μm. Simultaneously, the thickness of the positive electrode sheet made of the first coated electrode material is 150 μm, and the thickness of the positive electrode sheet made of the second coated electrode material is 150 μm. The coating material can be applied to the surface of the positive electrode material using an in-situ coating method.

[0112] The first coating electrode material can be aluminum fluoride, and the second coating electrode material can be lithium phosphate. Aluminum fluoride can effectively suppress side reactions and enhance structural stability; however, its ionic conductivity is lower than that of lithium phosphate. Specifically, aluminum fluoride can be deposited on the surface of the positive electrode material by contacting the aluminum fluoride-containing plating solution with the positive electrode material through a chemical reaction. 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 solvothermal reaction conditions, so that lithium phosphate is uniformly deposited on the surface of the positive electrode material. The coating process can be set according to the different coating electrode materials. S20: Coin cell half-cells are prepared by preparing the above positive electrode sheets under the same conditions, wherein lithium metal is used as the counter electrode.

[0113] The button cell was formed by charging and discharging at a rate of 0.05C, and then the battery was adjusted to 50% SOC.

[0114] S30. Electrochemical impedance spectroscopy (EIS) was performed on a coin cell charged to 50% SOC at room temperature (25℃). The test conditions were a 5mV voltage perturbation amplitude and a frequency range of 10. ^6 Hz~10 ^-2 Hz.

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

[0116] S50. Compare and analyze the relaxation time distribution curve of the battery containing the coating electrode material to be tested with the standard curve, including: selecting the peak parameter under the preset time constant as the characteristic parameter; constructing an evaluation formula using the characteristic parameter; inputting the characteristic parameter of the coating electrode material to be tested into the evaluation formula to obtain the value used to evaluate whether the coating effect is good or bad.

[0117] The evaluation formula 1 satisfies: a = (R max -R) / (R max -R min ); where R max R represents the resistance value of the second peak of the electrode material in the standard curve. min R 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 tested in the relaxation time distribution curve.

[0118] Evaluation formula two satisfies: b=(Δt-Δt) min ) / (Δt max -Δt min ); where Δt max Δt represents the time constant difference between the third and fourth peaks of the coated material in the standard curve. min Δt represents the time constant difference between the third and fourth peaks of the electrode material in the standard curve, and Δt represents the time constant difference between the third and fourth peaks 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 and fourth peaks of the coating material is taken as the maximum value Δt. max Δt max =49.72-3.25=46.47s, with the time constant difference between the third and fourth peaks of the cathode material as the minimum value Δt. min Δt min =37.63 - 4.40 = 33.23 s. From Figure 4 It can be seen that 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. This indicates that Δt increases with increasing coating amount; the first coated electrode material, with poorer 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] Furthermore, such as Figure 3 and Figure 4 As shown, in this embodiment, the peak value R of the second peak decreases due to the well-coated ionic conductor, with the resistance value of the second peak of the positive electrode material reaching its maximum value R. max The R max The resistance is 14.25Ω, with the minimum value R at the second peak of the coating material. min The R min It is 8.70Ω. From Figure 4As can be seen, the second peak resistance value R of the first coated electrode material is 13.76Ω, while that of the second coated electrode material is 11.16Ω. This indicates that as the coating becomes more complete and uniform, R decreases. The first coated electrode material, with its poorer coating effect, has a larger second peak resistance value R of 13.76Ω, while the second coated electrode material, with its poorer coating effect, has a smaller second peak resistance value R of 11.16Ω.

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

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

[0123] In other embodiments, the negative electrode material (graphite), coating material (amorphous carbon), first coating electrode material (silver), and second coating electrode material (copper oxide) can be prepared into a negative electrode sheet of the same specifications according to the same standard 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] The parts of this invention not described in detail are techniques known to those skilled in the art.

[0125] The basic principles of the present invention have been described above with reference to specific embodiments. However, it should be noted that the advantages, benefits, and effects mentioned in the present invention are merely examples and not limitations, and should not be considered as essential features of each embodiment of the present invention. Furthermore, the specific details disclosed above are for illustrative and facilitative purposes only, and are not limitations. These details do not limit the present invention to the necessity of employing the aforementioned specific details.

[0126] It should be noted that the terms "and / or" or " / " used herein are merely descriptions of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. The singular forms "a," "described," and "the" used in the embodiments of the 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, then 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, then 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, and not to limit them; although the present invention 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 of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for evaluating the surface coating effect of an electrode material, characterized by, The method includes: Electrode materials, coating materials, and the coating electrode material to be tested are respectively fabricated into electrode sheets under the same conditions; Batteries were prepared by preparing each of the aforementioned electrode sheets under the same conditions; Electrochemical impedance spectroscopy was performed on each of the aforementioned batteries; The relaxation time distribution curves were obtained by analyzing the electrochemical impedance spectra of each battery based on the relaxation time distribution method. The relaxation time distribution curves of the battery containing the electrode material and the battery containing the coating material are used as standard curves. By comparing and analyzing the relaxation time distribution curve of the battery containing the coating electrode material to be tested with the standard curve, the coating effect of the coating electrode material to be tested can be evaluated. The comparative analysis of the relaxation time distribution curve of the battery containing the coating electrode material to be tested with the standard curve includes: The peak parameter under a preset time constant is selected as the characteristic parameter; An evaluation formula is constructed using the aforementioned feature parameters; The characteristic parameters of the coating electrode material to be tested are input into the evaluation formula to obtain a value used to evaluate whether the coating effect is good or bad; The characteristic parameters include the peak resistance value and the time constant difference; The evaluation formulas include Formula 1 and Formula 2; Formula 1 satisfies: a=(R) max -R) / (R max -R min ); Among them, R max R represents the resistance value of the second peak of the electrode material in the standard curve. min R 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 considered poor; when 0.1 < a < 0.5, the coating effect is considered good; when a ≥ 0.5, the coating effect is considered excellent. The formula two satisfies: b = (Δt - Δt min ) / (Δt max -Δt min ); Where, Δt max Δt represents the time constant difference between the third and fourth peaks of the coated material in the standard curve. min Δt represents the time constant difference between the third and fourth peaks of the electrode material in the standard curve, and Δt represents 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 < 0.4, the coating effect is considered poor; when b ≥ 0.4, the coating effect is considered excellent.

2. The method of evaluating the surface coating effect of an electrode material according to claim 1, characterized by, The thickness of the electrode sheet is 120μm to 180μm; And / or, The battery prepared includes: After assembling the electrode sheet and the counter electrode, a formation process is performed to adjust the battery charge to 10%–50% SOC. The charge / discharge rate during the formation process is 0.05C to 0.5C.

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

4. The method of evaluating the surface coating effect of an electrode material according to claim 1, characterized by, When a ≥ 0.5 and b ≥ 0.4, the coating effect is considered excellent; and / or, The second peak is a time constant 10 ^-4 s ~ 10 ^-2 a peak in the range of s; and / or, The third and fourth peaks are peaks with time constants ranging from 1s to 200s.

5. The method of evaluating the surface coating effect of an electrode material according to any one of claims 1 to 3, characterized in that, The coating electrode material to be tested includes multiple materials prepared using the same coating material but with different coating processes, used to compare the coating effects of different coating processes; The different coating processes include at least two of the following: mechanical coating, solvent coating, or solid-phase reaction coating.

6. The method of evaluating the surface coating effect of an electrode material according to any one of claims 1 to 3, characterized in that, The coating electrode material to be tested includes multiple materials prepared using different coating materials under the same coating process, used to compare the coating effects of different coating materials to be tested.

7. The method of 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 coating electrode material to be tested includes multiple materials prepared using the same coating material to be tested and the same coating process, for comparing the coating effect of different electrode materials under the same coating material to be tested and the same coating process.

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