A method for testing the electrochemical active area of ​​an electrode

Abstract

The present invention provides a method for testing the electrochemical active area of ​​an electrode, comprising the following steps: S1: preparing a battery with a three-electrode structure; S2: performing cycle activation on the battery; S3: performing an EIS test on the battery to obtain an EIS test curve; S4: performing equivalent circuit fitting using simulation software based on the curve obtained by the EIS test and calculating the double-layer capacitance C according to a formula dl #imgabs0#Where f is the frequency, Z” is the imaginary component of EIS, and n is the degree of non-ideal behavior. S5: Calculate the electrochemically active area ESCS of the electrode using the formula: #imgabs1#Where ECSA is the electrochemical surface area (cm) contained in each geometric area 2 cm ‑2 ), C*=60μF cm ‑2 (surface capacitance of a smooth surface), A is the geometric area of ​​the electrode (cm 2 The method for testing the electrochemically active area of ​​an electrode provided by the present invention overcomes the defect that the existing method for testing the electrochemically active area of ​​an electrode is easily interfered by external factors, and improves the accuracy, reliability and reproducibility of the method for testing the electrochemically active area of ​​an electrode.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion batteries, and in particular to a method for testing the electrochemical active area of ​​an electrode. Background Art

[0002] Electrochemically active area (ECSA) refers to the effective area involved in electrochemical reactions, which is crucial for the evaluation of reaction activity, the normalization of active sites, and the calculation of electrochemical reaction kinetic parameters. How to accurately and non-destructively measure electrode ECSA is a research hotspot in the current energy field.

[0003] At present, the main methods for measuring the electrochemical active area ECSA include hydrogen zone integration method, carbon monoxide stripping method, cyclic voltammetric redox peak integration method, cyclic voltammetric peak current method, copper underpotential deposition method, cyclic voltammetric double layer method, etc. For example, the hydrogen zone integration method has been widely used in acidic water electrolysis system. This method requires cyclic voltammetric scanning in acidic aqueous solution medium, and the H + The ECSA is calculated by integrating the adsorption and desorption peak areas. However, this method is somewhat subjective in the selection of baselines, and for some materials with rich functional groups on their surfaces (such as MXene), it may interfere with the selection of baselines. In addition, for some electrode materials that are unstable in acidic aqueous solutions, this method cannot be used for accurate measurement.

[0004] The carbon monoxide dissolution method is widely used to determine the ECSA of membrane electrodes in fuel cell systems. The testing principle is to allow the electrode to chemically adsorb CO, then chemically desorb CO by cyclic voltammetry, and finally calculate the ECSA based on the amount of electricity desorbed from the electrode. However, this method is only applicable to special electrode materials (such as Pt) that can strongly chemically adsorb CO.

[0005] The cyclic voltammetry double-layer method is currently a recognized method for testing the electrochemical active area of ​​electrodes in the field of electrocatalysis. It uses the Faraday region where no reaction occurs and has high accuracy. However, in the field of secondary battery energy storage such as lithium ion, the double layer with surface reaction characteristics and pseudocapacitance often belong to the same region during the cyclic voltammetry (CV) measurement process, interfering with the calculation results of ECSA, resulting in inaccurate calculation results of ECSA.

[0006] However, all of the above methods have certain limitations and are affected by many factors such as electrode particle morphology, electrolyte reaction and surface adsorption, and most tests are only applicable to specific materials or specific reactions. Other methods such as cyclic voltammetry redox peak integration method, cyclic voltammetry peak current method and copper underpotential deposition method also have their own limitations and cannot be widely used in various electrochemical systems. Summary of the invention

[0007] The purpose of the present invention is to provide a method for testing the electrochemical active area of ​​an electrode, so as to overcome the defect that the existing method for testing the electrochemical active area of ​​an electrode is easily interfered by external factors, and to improve the accuracy, reliability and reproducibility of the method for testing the electrochemical active area of ​​an electrode.

[0008] The method for testing the electrochemical active area of ​​an electrode of the present invention comprises the following steps:

[0009] S1: preparing a battery with a three-electrode structure;

[0010] S2: cyclic activation of the battery;

[0011] S3: Performing EIS test on the battery to obtain an EIS test curve;

[0012] S4: According to the curve obtained by EIS test, the equivalent circuit is fitted by simulation software and the double layer capacitance C is calculated according to the formula dl :

[0013]

[0014] Where f is the frequency, Z” is the imaginary component of the EIS, and n is the degree to which the non-ideal behavior is described.

[0015] S5: Calculate the electrochemically active area ESCS of the electrode using the formula:

[0016]

[0017] Where ECSA is the electrochemical surface area per geometric area (cm 2 cm -2 ), C*=60μF cm -2 (surface capacitance of a smooth surface), A is the geometric area of ​​the electrode (cm 2 ).

[0018] Furthermore, the battery prepared in step S1 includes a positive electrode, a negative electrode and a reference electrode, and the reference electrode is prepared by an electrodeposition method.

[0019] Furthermore, the reference electrode is constructed by electrodepositing lithium on the outer layer of the copper wire.

[0020] Furthermore, the step S2 uses 0.2-1 mA cm -2 The battery was cycled and activated at a current density of .

[0021] Furthermore, the step S3 is configured to test at an open circuit potential, and the frequency range during the test is 10 kHz to 10 mHz.

[0022] Furthermore, the positive electrode is composed of lithium iron phosphate.

[0023] Furthermore, the negative electrode is composed of graphite

[0024] Beneficial effects of the method for testing the electrochemical active area of ​​an electrode of the present invention:

[0025] The electrochemically active area test method of the present invention can monitor the electrochemical behavior of the positive and negative electrodes separately by introducing a third electrode as a reference electrode, thereby avoiding the phenomenon that the reference potential is unstable due to the presence of an electrochemical reaction in a two-electrode system. In addition, it also avoids the problem that the process of presenting similar time constants on the two electrodes makes it difficult to quantify a single electrode process and brings uncertainty. The use of a three-electrode system can effectively decouple the individual electrochemical reactions of the positive and negative electrodes. The use of an in-situ electrodeposition method ensures that the loss of battery capacity and the steric effect are sufficiently small, and the stability of the deposited layer can be ensured by controlling the current density and deposition thickness of the in-situ deposition, thereby achieving a longer service life. In addition, after the deposited lithium is consumed by the reaction, the reference electrode can be prepared by electrochemical deposition again, and the stability is good.

[0026] The method for testing the electrochemically active area of ​​an electrode of the present invention obtains the value of the electrochemically active area of ​​the electrode by analyzing the electrochemical impedance spectroscopy (EIS) of the battery. Because electrochemical impedance spectroscopy (EIS) is a non-destructive testing technology that uses a small-amplitude sinusoidal AC signal to disturb the electrochemical system and measure the response, it successfully separates the double-layer part, which can not only effectively reveal the reaction kinetics of the electrode; but also is accurate, reliable, and has good reproducibility. At the same time, compared with most current testing methods (such as CV method), it avoids the interference of Faraday reaction on the measurement results, and can be widely used in ECSA determination of various electrochemical systems.

[0027] EIS can be divided into Nyquist diagram and Bode diagram. Nyquist diagram only shows distinguishable processes related to electrochemical reactions. Bode diagram can correspond to the time constant of electrochemical processes that Nyquist diagram does not have. Its low frequency and high frequency have obvious changes in amplitude and position. The two are introduced at the same time for fitting, so that the electrochemical reaction can be associated with time. The present invention uses a fitting method based on time constant to fit EIS: by adding different time constants to gradually fit the equivalent circuit to match it with the real spectrum, the contribution of each element in the equivalent circuit can be distinguished, thereby integrating the contribution of the working electrode to the impedance in different processes. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 A schematic diagram of a battery structure used in the electrode electrochemical active area testing method of the present invention;

[0029] Figure 2 The Bode diagram of the graphite electrode in EIS obtained by the testing method of the present invention;

[0030] Figure 3 The Nyquist diagram of the graphite electrode obtained by the test method of the present invention in the EIS;

[0031] Figure 4 Another Bode diagram of EIS of a graphite electrode obtained by the testing method of the present invention;

[0032] Figure 5 Another Nyquist diagram of EIS of a graphite electrode obtained by the testing method of the present invention;

[0033] Figure 6 The Bode diagram of lithium iron phosphate obtained by the testing method of the present invention in the EIS;

[0034] Figure 7 The Nyquist diagram of lithium iron phosphate obtained by the testing method of the present invention in EIS;

[0035] Figure 8 This is another Bode diagram of lithium iron phosphate in EIS obtained by the testing method of the present invention;

[0036] Fig. 9 This is another Nyquist diagram of EIS of lithium iron phosphate obtained by the testing method of the present invention;

[0037] Fig.10 This is an EIS equivalent circuit diagram for testing the negative pole piece graphite electrode in the testing method of the present invention;

[0038] Fig.11 It is the EIS equivalent circuit of the negative electrode graphite parallel sample in the test method of the present invention;

[0039] Fig.12 It is the EIS equivalent circuit diagram of the positive electrode lithium iron phosphate in the testing method of the present invention;

[0040] Fig.13 This is the EIS equivalent circuit diagram of the positive electrode lithium iron phosphate parallel sample in the testing method of the present invention. DETAILED DESCRIPTION

[0041] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0042] This embodiment provides a method for testing the electrochemical active area of ​​an electrode plate, and the specific steps are as follows:

[0043] S1: Assemble the positive electrode sheet, negative electrode sheet, separator and other components to prepare a battery. In this embodiment, the battery is configured as a battery with three electrodes wrapped by an aluminum-plastic film. The schematic diagram of the battery is as follows: Figure 1 As shown. The battery has a positive electrode 1, a negative electrode 2 and a reference electrode 3. The reference electrode 3 is made by electrochemical in-situ deposition of metallic lithium: first, an insulating enameled copper wire with a diameter of 40 μm is placed between the positive electrode 1 and the negative electrode 2 of the battery, then the insulating layer with a length of 0.5 mm is stripped off at the end of the copper wire and the oxide layer on the surface of the copper wire is removed, and finally, the copper wire is subjected to 10 μA constant current electroplating in two directions of the positive electrode 1 and the negative electrode 2, respectively. To ensure that metallic lithium can be evenly deposited on the surface of the copper electrode, the set time is more than 2 hours. Since the battery of this embodiment introduces the reference electrode 3 of the above structure, it is possible to monitor the electrochemical behavior of the positive electrode 1 and the negative electrode 2 separately, avoiding the phenomenon of unstable reference potential due to the electrochemical reaction in the battery in the two-electrode system of conventional batteries. In addition, it can also avoid the problem that a single electrode process is difficult to quantify and brings uncertainty due to the similar time constants on the two electrodes in the existing battery. In this embodiment, the reference electrode 3 is generated by in-situ electrodeposition, which ensures that the loss of battery capacity and steric effect are sufficiently small, and the stability of the deposited layer can be ensured by controlling the current density and deposition thickness of the in-situ deposition, thereby achieving a longer service life. In addition, after the deposited lithium is consumed by the reaction, the reference electrode can be prepared by electrochemical deposition again, and the stability is better. Therefore, the three-electrode system of this embodiment is used to effectively decouple the separate positive and negative electrode electrochemical reactions.

[0044] S2: cyclically activate the battery.

[0045] The three-electrode cell was placed in a Swiss Metrohm VIONIC electrochemical workstation. The voltage electrode clamp and current electrode clamp of the working electrode were clamped at the pole ear at position 1, the counter electrode was clamped at the pole ear at position 2, and the reference electrode was clamped at position 3. 0.2-1 mA cm -2 The battery was activated for 2 to 10 cycles at a current density of .

[0046] S3: Perform an EIS test on the battery to obtain an EIS test curve.

[0047] First, the negative electrode of the battery was subjected to EIS test. The Kelvin four-terminal clamp method was used. The voltage electrode clamp and the current electrode clamp of the working electrode were clamped at the pole ear at position 2, the counter electrode clamp was clamped at the pole ear at position 1, and the reference electrode clamp was clamped at position 3. Under open circuit potential, the frequency range was 10kHz~10mHz, the amplitude was 5mV, and the following was obtained: Figure 2 The Nyquist diagram shown in Figure 3 The Bode diagram shown in FIG. 1 is shown in FIG. 1 ; the same method is used again to measure the parallel negative electrode pieces of the same group for EIS test, and the following is obtained: Figure 4 The Nyquist diagram shown in Figure 5 The Bode diagram shown in the figure is then subjected to an EIS test on the positive electrode of the battery, and the electrode clips at positions 1 and 3 are exchanged to obtain the following Figure 6 The Nyquist diagram and Figure 7 The Bode diagram shown in Figure 2 is as follows; at the same time, the same method is used to measure the positive electrode plates of the parallel samples in the same group, and the following is obtained: Figure 8 The Nyquist diagram and Fig. 9 The Bode plot shown.

[0048] S4: According to the data obtained from the EIS test, the EIS data obtained from the test are fitted using an equivalent circuit diagram in the Nova software, and the following can be obtained: Fig.10 The equivalent circuit diagram of the negative electrode plate shown in FIG. Fig.11 The equivalent circuit diagram of the parallel negative electrode sheet shown in FIG. Fig.12 The equivalent circuit diagram of the positive electrode piece shown in FIG. Fig.13 The equivalent circuit diagram of the parallel negative electrode sheet is shown.

[0049] Calculate the double layer capacitance C according to the formula dl :

[0050] Where f is the peak frequency, Z' is the imaginary component of the peak frequency, and n is the degree of non-ideal behavior. S5: Electrochemical active area of ​​the electrode obtained using the following formula 2:

[0051]

[0052] Where ECSA is the electrochemical surface area per geometric area (cm 2 cm -2 ), C*=60μF cm -2 (surface capacitance of a smooth surface), A is the geometric area of ​​the electrode (cm 2 ).

[0053] Through the above formula, the electrochemical active area of ​​the electrodes in the corresponding battery can be calculated respectively:

[0054]

[0055] The electrochemically active area of ​​the electrode used in the present invention is tested by the EIS test method and an equivalent circuit is fitted. In the test method of the present invention, EIS can be divided into Nyquist diagram and Bode diagram, and the Nyquist diagram only shows distinguishable processes related to the electrochemical reaction. The Bode diagram can correspond to the time constant of the electrochemical process that the Nyquist diagram does not have, and its low frequency and high frequency have obvious changes in amplitude and position. The two are introduced at the same time for fitting, so that the electrochemical reaction can be associated with time. The present invention uses a fitting method based on time constant to fit EIS: by adding different time constants to gradually fit the equivalent circuit to match it with the real spectrum, so as to distinguish the contribution of each element in the equivalent circuit, thereby integrating the contribution of the working electrode to the impedance in different processes.

[0056] The present invention uses the EIS method and takes the lithium ion battery system as an example to successfully separate the double electric layer part. The calculation results of this method are not only accurate and reliable, but also have good reproducibility, and can also be widely used in the ECSA determination of various electrochemical systems.

[0057] The above embodiments of the present invention are only examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the embodiments here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A method for testing the electrochemical active area of ​​an electrode, characterized in that: The following steps are involved: S1: preparing a battery with a three-electrode structure; S2: cyclic activation of the battery; S3: Performing EIS test on the battery to obtain an EIS test curve; S4: According to the curve obtained by EIS test, the equivalent circuit is fitted by simulation software and the double layer capacitance C is calculated according to the formula dl : Where f is the frequency, Z” is the imaginary component of the EIS, and n is the degree to which the non-ideal behavior is described. S5: Calculate the electrochemically active area ESCS of the electrode using the formula: Where ECSA is the electrochemical surface area per geometric area (cm 2 cm -2 ), C*=60μF cm -2 (surface capacitance of a smooth surface), A is the geometric area of ​​the electrode (cm 2 ).

2. The method for testing the electrochemical active area of ​​an electrode as claimed in claim 1, characterized in that: The battery prepared in step S1 includes a positive electrode, a negative electrode and a reference electrode, and the reference electrode is prepared by an electrodeposition method.

3. The method for testing the electrochemical active area of ​​an electrode as claimed in claim 2, characterized in that: The reference electrode is constructed by electrodepositing lithium on the outer layer of a copper wire.

4. The method for testing the electrochemical active area of ​​an electrode according to claim 1, characterized in that: The step S2 uses 0.2-1 mA cm -2 The battery was cycled and activated at a current density of .

5. The method for testing the electrochemical active area of ​​an electrode according to claim 1, characterized in that: The step S3 is set to test at open circuit potential, and the frequency range during the test is 10kHz to 10mHz.

6. The method for testing the electrochemical active area of ​​an electrode according to claim 1, characterized in that: The positive electrode is composed of lithium iron phosphate.

7. The method for testing the electrochemical active area of ​​an electrode according to claim 1, characterized in that: The negative electrode is composed of graphite.

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