Method for screening high-rate-performance electrode and application thereof

Abstract

The invention relates to the field of electrochemical energy storage, in particular to a method for screening a high-rate-performance electrode and application thereof. The method comprises the following steps: (1) testing an electrode to obtain the double-electric-layer capacitance Cdl of the electrode; (2) testing the electrode to obtain an SCV curve graph of the electrode at different scanning speeds v; (3) calculating the area enclosed by the SCV curve graph, calculating the proportion Cdl% of the double-layer capacitance through the double-layer capacitance Cdl, and calculating the proportion Cp% of the pseudocapacitance through the SCV curve graphs with different scanning speeds v; and (4) screening to obtain an electrode with high pseudocapacitance proportion, namely the electrode with high rate performance. The method disclosed by the invention is short in time consumption, can optimize the current pseudocapacitance measurement method, separates the ratio of pseudocapacitance to double-layer capacitance, and can effectively screen electrode materials suitable for batteries or capacitors.

Description

Technical Field

[0001] The invention relates to the field of electrochemical energy storage, and in particular to a method for screening high-rate performance electrodes and an application thereof. Background Art

[0002] The development of electrode materials with both high capacity and high rate performance is crucial for the current electrochemical energy storage field. Among them, the rate performance of the electrode is closely related to the electrochemical polarization and concentration polarization of the material, and pseudocapacitance behavior is a Faraday behavior occurring on the surface of the material, which is closely related to the electrochemical polarization and concentration polarization of the material. By establishing a clear relationship between electrode materials and pseudocapacitance, it is helpful to screen high rate performance electrode materials, so that the material selection can be completed at the front end.

[0003] Current electrode materials require cycle testing at different current densities to determine their rate performance, which is time-consuming. In addition, the existing methods for testing the capacity of electrode materials are unclear and cannot distinguish the proportion of double-layer capacitance, pseudocapacitance, and diffusion current, resulting in large-scale trial and error in different applications of electrode materials in the field of energy storage. Summary of the invention

[0004] The purpose of the present invention is to overcome the problems of the existing methods, such as long testing time, unclear composition of each component of the electrode material capacity, inability to distinguish the proportion of double-layer capacitance, pseudocapacitance and diffusion current, and provide a method for screening high-rate performance electrodes and its application. The method is short in time and has good consistency, can optimize the current pseudocapacitance measurement method, and separates the proportion of pseudocapacitance and double-layer capacitance, and can effectively screen electrode materials suitable for batteries or capacitors.

[0005] In order to achieve the above-mentioned object, the first aspect of the present invention provides a method for screening high-rate performance electrodes, the method comprising: (1) testing the electrode to obtain the double-layer capacitance C of the electrode; dl ; (2) Testing the electrode to obtain the SCV curves of the electrode at different scanning speeds v; (3) Calculating the area enclosed by the SCV curves, and using the double layer capacitance C dl Calculate the double layer capacitance ratio C dl %The pseudocapacitance ratio C is calculated by fitting the SCV curves at different scan rates v. p %; (4) The electrode with a high pseudocapacitance ratio is screened as a high-rate performance electrode.

[0006] Preferably, the method for testing the electrode in step (1) is EIS, with a frequency f of 0.1 Hz to 200 kHz and an amplitude of 3 to 15 mV, to obtain the ohmic internal resistance R of the electrode. Ω , through the ohmic internal resistance R Ω Calculate the double layer capacitance C dl .

[0007] Preferably, the method for testing the electrode in step (2) is to perform SCV scanning by an electrochemical workstation, and the ohmic internal resistance R obtained by the EIS test in step (1) is Ω Deduct IR drop.

[0008] Preferably, in step (2), SCV scanning is performed at different scan rates using an electrochemical workstation, the scan rate is 0.1 to 100 mV / s, and the step potential is 0.1 to 10 mV.

[0009] Preferably, in step (3), an SCV curve diagram of a suitable scan rate that satisfies a linear relationship between the peak potential displacement and the scan logarithm lgv and between the current logarithm lgI and the scan logarithm lgv is selected for calculation.

[0010] Preferably, in step (3), the pseudocapacitance ratio C is calculated by fitting p % will delete the residual current.

[0011] Preferably, in step (3), the double-layer capacitor C dl Or ohmic internal resistance R Ω Calculate the double layer capacitance ratio C dl %, and the calculation formula is as follows:

[0012]

[0013] Where E is voltage, E1 is the lowest potential, E2 is the highest potential, I is current, v is the scan rate, C dl is the double layer capacitance, f is the frequency, R Ω is the ohmic internal resistance.

[0014] Preferably, in step (3), the pseudocapacitance ratio C p %The fitting calculation process is as follows:

[0015] (a) After deleting the residual current of the SCV curve, the linear relationship between the current I and the scan rate v was analyzed by linear regression using the least squares method to obtain the best fitting straight line and the slope k1. The linear relationship formula is as follows:

[0016]

[0017] Where, I is the current, v is the scan rate, k1 is the slope, and k2 is the intercept;

[0018] (b) Fitting the k1v curve to the SCV curve graph through the slope k1;

[0019] (c) Calculate the pseudocapacitance ratio C p %, and the calculation formula is as follows:

[0020]

[0021] Where E is voltage, I is current, k1 is slope, v is sweep speed, f is frequency, R Ω Preferably, step (3) also includes calculating the diffusion current proportion C b %, and the calculation formula is as follows:

[0022] C b % = 1-C dl %-C p %

[0023] Among them, C dl % is the percentage of double layer capacitance, C p % is the proportion of pseudocapacitance.

[0024] The method of the present invention adopts EIS combined with SCV, and the ohmic internal resistance RΩ obtained by EIS test not only eliminates the influence of ohmic polarization, but also simplifies the processing process. The existing test method eliminates ohmic polarization by calibrating the peak shift, and the peak shift is jointly affected by ohmic polarization, electrochemical polarization and diffusion polarization. The existing method simply attributes the peak shift at a large scan rate to ohmic polarization, which is not reasonable enough.

[0025] The method of the present invention optimizes the current pseudocapacitance measurement method, proposes a more reasonable test and calculation method for eliminating polarization, eliminating residual current interference, and separating the double-layer capacitance ratio, and can effectively screen electrode materials suitable for batteries or capacitors.

[0026] The second aspect of the present invention provides application of the method for screening high-rate performance electrodes according to the present invention in the field of electrochemical energy storage.

[0027] The present invention constructs the relationship between pseudocapacitance and the rate performance of electrode materials, distinguishes the proportions of pseudocapacitance and double-layer capacitance, and provides a new solution for screening high-rate electrode materials.

[0028] The present invention optimizes the current pseudocapacitance measurement method and can distinguish the proportion of pseudocapacitance, double-layer capacitance, and diffusion current, providing convenience for the application of new materials in the field of electrochemical energy storage, and is expected to reduce the number of trial and error times for new materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a schematic diagram of k1v curve fitting;

[0030] Figure 2 Schematic diagram of range selection for pseudocapacitance calculation;

[0031] Figure 3 The EIS graphs of Examples 1-3 (Groups A, B, and C);

[0032] Figure 4 The SCV curves of Examples 1-3 (Group A, 3-8 mV / s);

[0033] Figure 5 The SCV curves of Examples 1-3 (Group A, 9-100 mV / s);

[0034] Figure 6 The reduction peak potential shift-lgv relationship diagram of Examples 1-3 (Group A, 0.1-2 mV / s);

[0035] Figure 7 The linear relationship diagram of the reduction peak potential shift ~lgv of Examples 1-3 (Group A, 3-100 mV / s);

[0036] Figure 8 The lgI-lgv linear relationship diagram of Examples 1-3 (Group A, 0.1-2 mV / s);

[0037] Fig. 9 The lgI-lgv linear relationship diagram of Examples 1-3 (Group A, 3-100 mV / s);

[0038] Fig.10 This is a graph showing the pseudocapacitance contribution ratios of Examples 1-3. DETAILED DESCRIPTION

[0039] The specific implementation of the present invention is described in detail below in conjunction with the accompanying drawings. It should be understood that the specific implementation described here is only used to illustrate and explain the present invention, and is not used to limit the present invention.

[0040] The first aspect of the present invention provides a method for screening high-rate performance electrodes, the method comprising: (1) testing the electrode to obtain the double-layer capacitance C of the electrode; dl ; (2) Testing the electrode to obtain the SCV curves of the electrode at different scanning speeds v; (3) Calculating the area enclosed by the SCV curves, and using the double layer capacitance C dl Calculate the double layer capacitance ratio C dl %The pseudocapacitance ratio C is calculated by fitting the SCV curves at different scan rates v. p %; (4) The electrode with a high pseudocapacitance ratio is screened as a high-rate performance electrode.

[0041] In the present invention, the double layer capacitance accounts for C dl % is the ratio of the area occupied by the double layer capacitor to the area enclosed by the SCV curve. The pseudocapacitance accounts for C p% is the ratio of the area occupied by the pseudocapacitor to the area enclosed by the SCV curve, and the ratio of the remaining area of ​​the SCV curve to the area enclosed by the SCV curve is the diffusion current ratio C b %.

[0042] The working principle of the present invention is as follows:

[0043] In terms of kinetics, there are two types of electrode reaction rates: surface control and diffusion control. Regarding surface control, it can be described by the following equation:

[0044]

[0045] Among them, I is the current, A is the contact area between the electrode and the electrolyte, n is the number of electron transfers, F is the Faraday constant, c is the electrolyte concentration, R is the ideal gas state constant, T is the temperature, and v is the scan rate. This represents the behavior of pseudocapacitance and double-layer capacitance.

[0046] Regarding diffusion control, it can be described by the following equation:

[0047]

[0048] Where D is the ion diffusion coefficient, which represents the diffusion behavior.

[0049] In general, the charge storage mechanism is controlled by these three behaviors. Equations (1) and (2) can be combined to simplify the expression:

[0050]

[0051] Among them, k1v includes the current contributed by the double-layer capacitance k1”v and the pseudocapacitance k1'v, k2v 0.5 represents the diffusion current.

[0052] When the pseudocapacitive behavior is dominant, the reaction is surface controlled, the material can undergo a rapid redox reaction, and can exhibit a higher rate performance. That is, by screening electrodes with a large current contribution from the pseudocapacitor k1'v, high rate performance electrodes can be screened out.

[0053] In the SCV curve, the area enclosed by the SCV curve represents the capacitance of the three behaviors: pseudocapacitance, double layer capacitance, and diffusion control. The ratio of the current area contributed by the pseudocapacitance k1'v to the area enclosed by the SCV curve can be calculated by fitting, that is, by calculating the pseudocapacitance ratio C dl %, and then screen out the pseudocapacitor proportion C dl % high electrode.

[0054] In the SCV curve, the area Area enclosed by the curve and the current I satisfy the following equation:

[0055] Area=∫IdE (4)

[0056] Double layer capacitance ratio C dl % can be calculated by the following equation:

[0057]

[0058] Among them, Area1 is the area current enclosed by the curve of double-layer capacitance behavior, Area3 is the area enclosed by the SCV curve, C dl is the double layer capacitance, E1 is the lowest potential and E2 is the highest potential.

[0059] Through equations (3) and (4), the pseudocapacitance ratio C p % can be calculated by the following equation:

[0060]

[0061] Wherein, Area2 is the area current enclosed by the curves of the double-layer capacitance k1”v and the pseudocapacitance k1’v behavior.

[0062] By transforming equation (3), we can get:

[0063]

[0064] The slope k1 corresponding to the same voltage E can be obtained through the SCV curves of different scanning speeds v, and the k1v curve can be further fitted on the SCV curves, such as Figure 1 shown.

[0065] The pseudocapacitance ratio C can be calculated by equation (6): p %.

[0066] In addition, the diffusion current accounts for C b %, can be calculated by the following equation:

[0067] C b % = 1-C dl %-C p %(8)

[0068] Then, the pseudocapacitor proportion C is screened out. dl Electrodes with high % can be used to screen out electrodes with high rate performance.

[0069] In some embodiments, the method for testing the electrode in step (1) is EIS, the frequency f is 0.1 Hz to 200 kHz, the amplitude is 3-15 mV, and the ohmic internal resistance R of the electrode is obtained by fitting in Nova 2.1.7 software. Ω , through the ohmic internal resistance R ΩCalculate the double layer capacitance C dl . Double layer capacitance C dl It can be calculated by the following equation:

[0070]

[0071] Among them, R Ω is the ohmic internal resistance and f is the frequency.

[0072] Through the ohmic internal resistance R Ω The double layer capacitance C can be calculated dl .

[0073] In the process of processing the contribution of the double-layer capacitance, the method of the present invention converts the scan rate by using EIS measurement data to reflect the actual proportion of the double-layer capacitance.

[0074] The existing calculation method for the double-layer contribution in battery systems is based on the CV test results, which are roughly treated as ideal rectangles and considered to be the contribution of double-layer capacitance. However, for battery systems, most of the area enclosed by this rectangle is contributed by pseudocapacitance, and only a small part is contributed by double-layer capacitance. Therefore, the proportion of double layer calculated by the existing method is often too large.

[0075] According to a particularly preferred embodiment of the present invention, the method for testing the electrode in step (2) is to perform SCV scanning through an electrochemical workstation, and based on the ohmic internal resistance R obtained by the EIS test in step (1), Ω By adopting the above preferred solution, the interference of ohmic polarization on the results presented in the electrochemical reaction process can be eliminated.

[0076] The method of the present invention adopts the SCV measurement method, which is more suitable for studying the system where the Faraday reaction occurs than the cyclic voltammetry method, and can measure the Faraday current more accurately.

[0077] According to a particularly preferred embodiment of the present invention, in step (2), SCV scanning at different scan rates is performed using INTELLO software using a VIONIC electrochemical workstation of Metrohm, Switzerland, with a scan rate of 0.1 to 100 mV / s and a step potential of 0.1 to 10 mV.

[0078] According to a particularly preferred embodiment of the present invention, in step (3), an SCV curve diagram of a suitable scan rate that satisfies a linear relationship between the peak potential displacement and the scan logarithm lgv and between the current logarithm lgI and the scan logarithm lgv is selected for calculation.

[0079] It should be understood that the peak potential shift in the present invention refers to the offset potential of the peak current at each scan rate relative to 0.1 mV / s.

[0080] When ohmic polarization is eliminated, that is, after deducting the IR drop, the peak potential displacement and the sweep logarithm lgv satisfy a linear relationship, and at the same time, the current logarithm lgI and the sweep logarithm lgv satisfy a linear relationship, the electrochemical reaction can be considered to have less thermodynamic influence and only kinetic control is considered.

[0081] According to a particularly preferred embodiment of the present invention, in step (3), the pseudocapacitance ratio C is calculated by fitting p By adopting the above preferred solution, the calculation results can be made to conform to the surface control and diffusion control equations.

[0082] like Figure 2 As shown, since the voltammetric curve turns immediately when it reaches the specified potential, there will be a relatively large residual current caused by the reverse reaction at a large scan rate, and the current must pass through the zero point when it turns from positive to negative or from negative to positive.

[0083] This part of the residual current can be described by the exponential current decay equation:

[0084]

[0085] Where I1 is the current generated by forward cyclic voltammetry, I2 is the current generated by reverse cyclic voltammetry, a and b are the endpoints where the current starts to turn from the forward scan to the reverse scan, and c and d are the endpoints where the current starts to turn from the reverse scan to the forward scan.

[0086] Since this part of the residual current conforms to equations (10) and (11) but not to equation (3), it is necessary to select and discard this part of the data during the calculation process. That is, during the fitting process, the data between a and c and the data between b and d need to be deleted.

[0087] According to a particularly preferred embodiment of the present invention, in step (3), the double-layer capacitor C dl Or ohmic internal resistance R Ω Calculate the double layer capacitance ratio C dl %, after solving equations (5) and (9), the double layer capacitance accounts for C dl %The calculation formula is as follows:

[0088]

[0089] According to a particularly preferred embodiment of the present invention, in step (3), the pseudocapacitance accounts for C p %The fitting calculation process is as follows:

[0090] (a) After deleting the residual current of the SCV curve, the linear relationship between the current I and the scan rate v is analyzed by linear regression using the least squares method. The linear relationship formula is equation (7), and the best fitting straight line is obtained, and then the slope k1 is calculated;

[0091] (b) Fitting the k1v curve to the SCV curve using the slope k1;

[0092] (c) Calculate the pseudocapacitance ratio C p %, after solving equations (6) and (9), the pseudocapacitance ratio C p %The calculation formula is as follows:

[0093]

[0094] According to a particularly preferred embodiment of the present invention, in step (2), SCV scanning is performed by an electrochemical workstation with a scanning rate of 3 to 100 mV / s and a step potential of 2 to 3 mV. By adopting the above preferred solution, more accurate k1 and k2 values ​​can be further obtained.

[0095] Through a large number of experimental tests, the inventors found that when the scan rate is 3 to 100 mV / s and the step potential is 2 to 3 mV, the peak potential displacement and the scan logarithm lgv, as well as the current logarithm lgI and the scan logarithm lgv, are more in line with the linear relationship. When testing other electrode materials, selecting this range can reduce the number of tests.

[0096] The method of the present invention adopts EIS combined with SCV, and the ohmic internal resistance RΩ obtained by EIS test not only eliminates the influence of ohmic polarization, but also simplifies the processing process. The existing test method eliminates ohmic polarization by calibrating the peak shift, and the peak shift is jointly affected by ohmic polarization, electrochemical polarization and diffusion polarization. The existing method simply attributes the peak shift at a large scan rate to ohmic polarization, which is not reasonable enough.

[0097] The method of the present invention optimizes the current pseudocapacitance measurement method, proposes a more reasonable test and calculation method for selecting a suitable scan rate, eliminating polarization, eliminating residual current interference, and separating the double-layer capacitance ratio, which can effectively screen electrode materials suitable for batteries or capacitors.

[0098] The second aspect of the present invention provides application of the method for screening high-rate performance electrodes according to the present invention in the field of electrochemical energy storage.

[0099] The present invention constructs the relationship between pseudocapacitance and the rate performance of electrode materials, distinguishes the proportions of pseudocapacitance and double-layer capacitance, and provides a new solution for screening high-rate electrode materials.

[0100] The present invention optimizes the current pseudocapacitance measurement method and can distinguish the proportion of pseudocapacitance, double-layer capacitance, and diffusion current, providing convenience for the application of new materials in the field of electrochemical energy storage, and is expected to reduce the number of trial and error times for new materials.

[0101] The present invention will be described in detail below through examples. In the following examples, the device involved in the test electrode is as follows:

[0102] Electrochemical workstation: VIONIC from Metrohm, Switzerland;

[0103] Test software: INTELLO from Swiss Metrohm;

[0104] Analysis software: Nova 2.1.7.

[0105] Examples 1-3

[0106] Conventional graphite, 2C fast-charging graphite and 4C fast-charging graphite were selected as research objects Group A, Group B and Group C, respectively, and button batteries were assembled using metallic lithium as the counter electrode and reference electrode.

[0107] The EIS test was performed at open circuit potential (OCP) with a frequency range of 0.1 Hz to 200 kHz and an amplitude of 5 mV to obtain the ohmic internal resistance R Ω The EIS test results are as follows: Figure 3 shown.

[0108] The SCV scan was performed using INTELLO on the VIONIC electrochemical workstation of Metrohm, Switzerland, and the R values ​​were obtained by analyzing the EIS test using the Nova 2.1.7 software. Ω After deducting the IR drop, the scanning rate interval was 0.1-100 mV / s, and the step potential was 2.44 mV. The SCV scanning results of group A are as follows Figure 4 , 5 shown.

[0109] According to the SCV curve, a graph is made between the peak potential displacement after ohm calibration (deducting IR drop) and the sweep logarithm lgv to observe whether it satisfies the linear relationship; a graph is made between the sweep current lgI and the sweep logarithm lgv to observe whether it satisfies the linear relationship. The results are as follows Figure 6 , 7 As shown in , 8 and 9, it can be seen that there is a linear relationship between the peak potential displacement with a sweep value of 3 to 100 mV / s and the sweep logarithm lgv, as well as between the current logarithm lgI and the sweep logarithm lgv.

[0110] The SCV curve diagram (3-100 mV / s) with a suitable scanning speed that satisfies the linear relationship between the peak potential displacement and the scan logarithm lgv and between the current logarithm lgI and the scan logarithm lgv is selected for calculation.

[0111] The area Area3 enclosed by the SCV curve at different scan rates is calculated by equation (4), and the ohmic internal resistance R is measured by the EIS results. Ω According to equation (12), the double layer capacitance ratio C of group A is calculated dl %, and the results are shown in Table 1.

[0112] After deleting the residual current of the SCV curve, the linear relationship between the current I and the scan rate v was analyzed by linear regression using the least squares method. The linear relationship formula is equation (7). The best fitting straight line is obtained, and the slope k1 is calculated. The k1v curve is fitted on the SCV curve using the slope k1, and then the pseudocapacitance ratio C of group A is calculated according to equation (13): p %, the results are as follows Fig.10 , as shown in Table 1.

[0113] The diffusion current proportion C of group A is calculated by equation (8): b %. The results are shown in Table 1.

[0114] The double-layer capacitance ratio C of group B and group C was calculated using the same method. dl %, Pseudocapacitance ratio C p %, diffusion current ratio C b %. The results are as follows Fig.10 , as shown in Table 1.

[0115] Table 1 Capacitance contribution of batteries

[0116]

[0117] From Table 1, Fig.10 It can be seen that the proportion of pseudocapacitance and double-layer capacitance increases with the increase of scan rate, indicating that the material is mainly controlled by the surface; and it can also be seen that group C (4C graphite) has higher pseudocapacitance.

Claims

1. A method for screening high rate performance electrodes, characterized in that: The method comprises: (1) Test the electrode and obtain the double-layer capacitance C of the electrode dl ; (2) testing the electrode to obtain SCV curves of the electrode at different scanning speeds v; (3) Calculate the area enclosed by the SCV curve and calculate the double layer capacitance C dl Calculate the double layer capacitance ratio C dl %The pseudocapacitance ratio C is calculated by fitting the SCV curves at different scan rates v. p %; (4) The electrodes with a high proportion of pseudocapacitance are screened as high-rate performance electrodes.

2. The method according to claim 1, characterized in that: The method for testing the electrode in step (1) is EIS, with a frequency f of 0.1 Hz to 200 kHz and an amplitude of 3 to 15 mV, and the ohmic internal resistance R of the electrode is measured. Ω , through the ohmic internal resistance R Ω Calculate the double layer capacitance C dl .

3. The method according to claim 2, characterized in that The method for testing the electrode in step (2) is to perform SCV scanning through an electrochemical workstation, and the ohmic internal resistance R obtained by the EIS test in step (1) is Ω Deduct IR drop.

4. The method according to claim 3, characterized in that In step (2), SCV scanning is performed at different scanning rates using an electrochemical workstation, with the scanning rate being 0.1 to 100 mV / s and the step potential being 0.1 to 10 mV.

5. The method according to claim 4, characterized in that In step (3), an SCV curve diagram of an appropriate scan rate that satisfies a linear relationship between the peak potential displacement and the scan logarithm lgv and between the current logarithm lgI and the scan logarithm lgv is selected for calculation.

6. The method according to claim 5, characterized in that In step (3), the pseudocapacitance ratio C is calculated by fitting p % will delete the residual current.

7. The method according to claim 2, characterized in that In step (3), the double layer capacitor C dl Or ohmic internal resistance R Ω Calculate the double layer capacitance ratio C dl %, and the calculation formula is as follows: Where E is voltage, E1 is the lowest potential, E2 is the highest potential, I is current, v is the scan rate, C dl is the double layer capacitance, f is the frequency, R Ω is the ohmic internal resistance.

8. The method according to claim 6, characterized in that In step (3), the pseudocapacitance ratio C p %The fitting calculation process is as follows: (a) After deleting the residual current of the SCV curve, the linear relationship between the current I and the scan rate v was analyzed by linear regression using the least squares method to obtain the best fitting straight line and the slope k1. The linear relationship formula is as follows: Where, I is the current, v is the scan rate, k1 is the slope, and k2 is the intercept; (b) Fitting the k1v curve to the SCV curve graph through the slope k1; (c) Calculate the pseudocapacitance ratio C p %, and the calculation formula is as follows: Where E is voltage, I is current, k1 is slope, v is sweep speed, f is frequency, R Ω is the ohmic internal resistance.

9. The method according to claim 1, characterized in that: Step (3) also includes calculating the diffusion current ratio C b %, and the calculation formula is as follows: C b %=1-C dl %-C p % Among them, C dl % is the percentage of double layer capacitance, C p % is the proportion of pseudocapacitance.

10. Application of the method according to any one of claims 1 to 9 in the field of electrochemical energy storage.

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