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

Abstract

This invention provides a method for obtaining the electrochemical active area of ​​an electrode, the method comprising the following steps: (1) testing the double-layer capacitance value Q1 of a single active particle; (2) testing the double-layer capacitance value Q2 of the electrode to obtain the electrochemical active area S of the electrode. E =[(Q2 / 2) / Q1]*S p , among which, S p The surface area of ​​a single active particle in step (1) is the electrode sheet, which includes the active particle in step (1). This invention obtains a highly accurate electrochemical active area by testing the double-layer capacitance of a single particle and the double-layer capacitance of the electrode sheet, based on the definition of capacitance, the double-layer capacitance of a single particle, the surface area, and the double-layer capacitance of the electrode sheet, thus eliminating errors caused by factors such as binders and particle extrusion.

Description

Technical Field

[0001] This invention belongs to the field of battery technology and relates to a method for obtaining the electrochemical active area of ​​an electrode. Background Technology

[0002] With the large-scale application of lithium-ion batteries, the demand for them has increased significantly. Simultaneously, the reaction mechanism of lithium-ion batteries is receiving increasing attention. Since the performance and lifespan of lithium-ion batteries are ultimately related to electrode reactions, obtaining the parameters of lithium-ion battery electrode reactions not only reveals the performance of electrode materials but also serves as input parameters for simulation modeling, thereby improving simulation accuracy. Among the many parameters, the electrochemical reactive area of ​​the electrode, as a fundamental parameter of the electrode material, is a key parameter controlling the analysis of electrode reaction rates. Factors such as electrode reaction current density, lithium-ion diffusion coefficient, and the rate of parasitic reactions on the electrode material surface are all related to this parameter.

[0003] However, accurately testing or calculating the values ​​of the above parameters is very difficult. Therefore, the industry currently uses the specific surface area of ​​the electrode material to calculate the electrochemical active area of ​​the material. However, this method has a significant drawback: inside the electrode sheet, the electrode material is mixed with conductive agents and binders and then pressed onto the surface of the current collector. This results in the electrode material particles not being in complete contact with the electrolyte, and some particle surfaces cannot undergo electrochemical reactions. Therefore, the electrochemical active area calculated using the BET method is much larger than the true value.

[0004] Existing technologies use electrode area or the BET method to obtain the chemically active area of ​​the electrode. However, due to the porosity of the electrode, the electrolyte can penetrate into the interior of the electrode. After the material is compacted and the binder affects the electrode material, the actual area where the electrochemical reaction occurs is much smaller than the sum of the surface areas of the electrode material particles. This leads to an overestimation of the area calculated using the BET method. When using the electrode area for calculation, each active particle may come into contact with the electrolyte when it penetrates the electrode. Directly using the electrode area as the sum of the surface areas of the active particles has a large error and is significantly understated. This results in an electrochemically active area that is smaller than the true value and cannot accurately reflect the electrochemically active area.

[0005] Based on the above research, there is a need to provide a method for obtaining the electrochemical active area of ​​an electrode. Compared with the traditional electrode area or BET method, the method can accurately reflect the electrochemical active area of ​​the electrode and effectively avoid erroneous predictions of the electrochemical active area due to factors such as binders and particle extrusion. Summary of the Invention

[0006] The purpose of this invention is to provide a method for obtaining the electrochemical active area of ​​an electrode. This method obtains the electrochemical active area with high accuracy by testing the double-layer capacitance and surface area of ​​a single particle and combining this with the double-layer capacitance of the electrode, thus eliminating errors caused by factors such as binders and particle extrusion.

[0007] To achieve this objective, the present invention employs the following technical solution:

[0008] This invention provides a method for obtaining the electrochemical active area of ​​an electrode, the method comprising the following steps:

[0009] (1) Test the double-layer capacitance value Q1 of a single active particle;

[0010] (2) The double-layer capacitance Q2 of the electrode is measured to obtain the electrochemical active area S of the electrode. E =[(Q2 / 2) / Q1]*S p , of which S p The surface area of ​​the single active particle in step (1) is given by the electrode, which includes the active particle in step (1).

[0011] This invention first tests the double-layer capacitance of a single active particle to calculate the double-layer capacitance Q1 of a single particle under conditions without conductive agent or particle compression. Then, it tests the double-layer capacitance Q2 of the electrode. Using the definition of capacitance C = εS / 4πkd (where ε is the dielectric constant, S is the electrode area, k is the Boltzmann constant, and d is the double-layer thickness), it is found that the double-layer capacitance Q1 at the particle interface is proportional to the particle surface area. Therefore, by using the double-layer capacitance Q1 of a single particle and its surface area S... p The electrochemical reaction active area of ​​the electrode layer was obtained by measuring the double layer capacitance value Q2 of the electrode. Since the test in step (1) is for a single active particle that has not yet been coated, it effectively avoids the incorrect prediction of the electrochemical active area due to binder and particle extrusion, thus improving the accuracy of obtaining the electrochemical active area of ​​the electrode.

[0012] Preferably, step (1) of testing the double-layer capacitance value Q1 of a single active particle includes: using a single-particle microelectrode system to assist in testing the AC impedance of a single active particle, and fitting the obtained AC impedance test results to an equivalent circuit to obtain the double-layer capacitance value Q1 of the single active particle.

[0013] This invention employs a single-particle microelectrode system, which can perform electrochemical characterization on a single particle of an electrode material, as an auxiliary method. The AC impedance of a single active particle is measured, and then the double-layer capacitance value, which is positively correlated with the electrochemical active area of ​​the electrode, is obtained through equivalent circuit fitting. The combination of the single-particle microelectrode system and AC impedance testing provides a more accurate understanding of the electrochemical reaction active area of ​​the electrode material. It is also suitable for calculating the variation of the electrochemical active area with the degree of lithium intercalation in the material.

[0014] The single-particle microelectrode system of the present invention uses a single particle as an electrode to perform AC impedance testing. Specifically, a single active particle is placed in an electrolytic cell containing an electrolyte, and two electrodes are connected to the active particle respectively.

[0015] Preferably, the test results are fitted using an equivalent circuit R1 (CPE1R2)(CPE2(R3W)), wherein the value of CPE2 obtained by fitting is Q1.

[0016] The single active particles described in this invention can be selected using a single-particle microelectrode system or other feasible devices. Preferably, the single active particles in step (1) are selected using a single-particle microelectrode system.

[0017] Preferably, the sphericity of the selected individual active particles in step (1) is ≥0.8, for example, it can be 0.87, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0018] The present invention utilizes individual active particles with high sphericity, which improves the accuracy of test results and avoids errors caused by low sphericity or irregular particle shapes. p The problem stems from inaccurate calculations or inaccurate Q1 testing.

[0019] Preferably, the diameter of the selected single active particle in step (1) is d≈D50=1nm-100μm, for example, it can be 100nm, 1μm, 5μm, 25μm or 50μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0020] Where d≈D50, for example, it can be d=D50±0.5μm, where D50 represents the particle size corresponding to when the cumulative particle size distribution percentage of the sample reaches 50%.

[0021] The diameter d of a single active particle described in this invention also affects the results. Testing representative particles within a specific range, or taking a weighted average of particles from multiple particle size grades, can further improve the accuracy of the test results.

[0022] Preferably, the charge state of the individual active particle in step (1) is a non-fully intercalated state and a non-completely delithiated state, and is the same as the charge state of the electrode in step (2). For example, it can be 10% SOC, 20% SOC, 30% SOC, 40% SOC, 50% SOC, 60% SOC, 70% SOC, 80% SOC, 90% SOC or 95% SOC, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0023] Fully inserted state refers to a state of charge of 100% SOC, while fully delithiated state refers to a state of charge of 0% SOC.

[0024] This invention relates to the AC impedance testing of individual active particles in a non-fully intercalated and non-fully delithiated state. To ensure testing accuracy, symmetrical cell testing is selected for electrode capacitance testing. The double-layer capacitance response of the active material only appears when the electrode is in a non-fully intercalated (or non-fully delithiated) state. Furthermore, to eliminate the difference in double-layer capacitance between the individual active particle and the electrode due to varying lithium intercalation amounts, the AC impedance testing of the individual active particle must be performed under non-fully intercalated conditions, and the lithium intercalation state must be identical to that of the electrode under test.

[0025] Preferably, step (2) of testing the double-layer capacitance value Q2 of the electrode includes: preparing the electrode into a symmetrical cell, testing the AC impedance of the symmetrical cell, and then performing equivalent circuit fitting on the obtained AC impedance test results to obtain the double-layer capacitance value Q2 of the electrode.

[0026] This invention first prepares the electrode into a symmetrical cell, and then tests the AC impedance. In this case, preparing it into a symmetrical cell instead of a half cell can further improve the accuracy of the test. Since a half cell generally requires a lithium metal sheet as the counter electrode, and the lithium metal sheet itself has capacitance, it will lead to inaccurate Q2 measurement.

[0027] The symmetrical battery described in this invention refers to a battery with two identical electrodes. The method for preparing a symmetrical battery is not specifically limited in this invention, and those skilled in the art can make reasonable choices according to their needs. For example, the published patent CN 109585932A can be referred to.

[0028] Preferably, the S in step (2) p =πd 2 .

[0029] Preferably, the active material of the electrode in step (2) is the same type as the single active particle in step (1).

[0030] The electrode sheet in step (2) of this invention is prepared using the same material as the active particles in step (1). This invention does not impose specific limitations on the parameters and other conditions during preparation, and those skilled in the art can make reasonable selections according to their needs.

[0031] As a preferred embodiment of the present invention, the method includes the following steps:

[0032] (1) A single active particle with a sphericity ≥ 0.8 and a diameter d ≈ D50 is obtained by using a single particle microelectrode system for selection. Then, the AC impedance of the single active particle in the non-fully inserted and non-completely delithiated state is tested by using a single particle microelectrode system. The AC impedance test results are fitted with an equivalent circuit to obtain the double layer capacitance value Q1 of the single active particle.

[0033] (2) Prepare a symmetrical battery by using an electrode with the same active material as the active particles described in step (1). Then test the AC impedance of the electrode in the non-fully inserted and non-completely delithiated state. Then perform equivalent circuit fitting on the AC impedance test results to obtain the double layer capacitance value Q2 of the electrode. In steps (1) and (2), the charge state of the electrode is the same as the charge state of the single active particle described in step (1).

[0034] (3) Based on Q1 and d described in step (1) and Q2 described in step (2), the electrochemical active area S of the electrode is obtained. E =[(Q2 / 2) / Q1]*S p , of which S p =πd 2 , is the surface area of ​​a single active particle described in step (1).

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] This invention tests the double-layer capacitance of a single particle and the double-layer capacitance of an electrode. Utilizing the definition of capacitance, and based on the double-layer capacitance and surface area of ​​the single particle, combined with the double-layer capacitance of the electrode, it obtains a result that approximates the true electrochemical active area of ​​the electrode as closely as possible. The overall method is not only simple and fast, but also eliminates errors caused by factors such as binders and particle compression. Compared to using electrode area or the BET method to test the electrochemical active area, the method described in this invention significantly improves the accuracy of testing the electrochemical active area of ​​the electrode. Attached Figure Description

[0037] Figure 1 This is a schematic diagram of the single-particle microelectrode system described in Embodiment 1 of the present invention;

[0038] Figure 2 The AC impedance diagram of a single active particle as described in Embodiment 1 of the present invention;

[0039] Figure 3 The AC impedance diagram of the electrode sheet described in Embodiment 1 of the present invention;

[0040] Figure 4 The AC impedance diagram of the electrode sheet described in Embodiment 4 of the present invention;

[0041] Among them, 1-active particles, 2-electrolyte, 3-electrolytic cell, and 4-electrode. Detailed Implementation

[0042] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0043] Example 1

[0044] This embodiment provides a method for obtaining the electrochemical active area of ​​an electrode, the method comprising the following steps:

[0045] (1) A single active particle 1 with a sphericity of 0.95 and a diameter d of 3.71 μm was obtained by using a single-particle microelectrode system for selection. Then, the AC impedance of the single active particle 1 at 50% SOC was tested using the single-particle microelectrode system. The AC impedance test results were fitted with the equivalent circuit R1 (CPE1R2)(CPE2(R3W)) to obtain the double-layer capacitance value Q1 of the single active particle 1, which is 0.002197 μS·sec. n ;

[0046] Among them, active particle 1 is NCM613 material (LiNi 0.6 Co 0.1 Mn 0.3 O2); The disturbance voltage for AC impedance testing is 5mV, and the test range is 100kHz-20mHz;

[0047] (2) Prepare a symmetrical battery by using an electrode with the same active material as the active particles 1 described in step (1). Then, test the AC impedance of the electrode at 50% SOC. Fit the AC impedance test results to the equivalent circuit R1 (CPE1R2)(CPE2 (R3W)) to obtain the double-layer capacitance value Q2 of the electrode, which is 0.1758 S·sec. n ;

[0048] The active material of the electrode is NCM613; the disturbance voltage for AC impedance testing is 5mV, and the test range is 100kHz-20mHz.

[0049] (3) Based on Q1 and d described in step (1) and Q2 described in step (2), the electrochemical active area S of the electrode is obtained. E =[(Q2 / 2) / Q1]*Sp =17.2cm 2 , of which S p =πd 2 =4.3×10 -7 cm 2 , is the surface area of ​​a single active particle 1 described in step (1);

[0050] A schematic diagram of the single-particle microelectrode system is shown below. Figure 1 As shown, the device includes a single active particle 1 in the electrolyte 2, a bottom electrolytic cell 3, and an electrode 4; the AC impedance diagram of the single active particle 1 is shown below. Figure 2 As shown, the AC impedance diagram of the electrode is as follows: Figure 3 As shown.

[0051] Example 2

[0052] This embodiment provides a method for obtaining the electrochemical active area of ​​an electrode. The method, except that in step (1), the individual active particle is a rectangular particle with a sphericity of 0.04 and a side length of 1 μm, results in an electrochemical active area S. E Apart from the adaptive changes, everything else is the same as in Example 1.

[0053] The sphericity of the active particles selected in this embodiment is too low, so S is used. p =πd 2 The calculated surface area is πμm 2 Its actual surface area is approximately 6 μm. 2 At this point, the error is relatively large, which reduces the accuracy of the obtained electrochemical active area.

[0054] Example 3

[0055] This embodiment provides a method for obtaining the electrochemical active area of ​​an electrode. The method, except that in step (2), the electrode is prepared as a half-cell (using a lithium sheet as the counter electrode) and not a symmetric cell, resulting in an electrochemical active area S... E Apart from the adaptive changes, everything else is the same as in Example 1.

[0056] The electrode fabrication process described in step (2) of this invention, which prepares the electrode into a symmetrical battery, can more accurately measure its double-layer capacitance value. However, in this embodiment 3, since there is a lithium metal sheet in the half-cell, the AC impedance test is for the test results including the lithium sheet. At this time, the half-cell capacitance is the result of the electrode capacitance and the negative electrode lithium sheet capacitance connected in series. Therefore, the capacitance value obtained by fitting includes the capacitance of the lithium metal sheet, thereby reducing the accuracy of the obtained electrochemical active area.

[0057] Example 4

[0058] This embodiment provides a method for obtaining the electrochemical active area of ​​an electrode. The method includes step (1) testing the AC impedance of a single active particle at 100% SOC, and step (2) testing the AC impedance of the electrode of the symmetrical battery at 100% SOC (test diagram shown). Figure 4 As shown), the resulting electrochemically active area S E Apart from the adaptive changes, everything else is the same as in Example 1.

[0059] In this embodiment, the AC impedance of the single particle tested in step (1) and the AC impedance of the electrode tested in step (2) are both AC impedances under full insertion state. Since the AC impedance spectrum of the electrode under full insertion state cannot show the electrode de-intercalation behavior, the value of CPE2 cannot be obtained by fitting the equivalent circuit diagram R1(CPE1R2)(CPE2(R3W)).

[0060] Comparative Example 1

[0061] This comparative example provides a method for obtaining the electrochemical active area of ​​an electrode, which is obtained by the BET method, yielding S... E =37.5cm 2 Specifically, it includes the following steps:

[0062] The specific surface area of ​​the powder, as determined by BET testing, is 0.75 m². 2 / g, the symmetrical battery's single electrode is a circular piece with a diameter of 10mm, and the active material mass is 5mg. Its BET calculated active area is: active material mass × specific surface area = 37.5cm². 2 .

[0063] Comparative Example 2

[0064] This comparative example provides a method for obtaining the electrochemical active area of ​​an electrode, the method being obtained through the electrode area, and the resulting S E =0.785cm 2 Specifically, it includes the following steps:

[0065] The diameter of the electrode was measured to be 10 mm, S E =πR 2 =0.785cm 2 .

[0066] As can be seen from Example 1 and Comparative Example 1, the method of obtaining the electrochemical active area of ​​the electrode using the BET method is larger than the true value. Since the electrode is made by mixing electrode material with conductive agent and binder and pressing it onto the surface of the current collector, the electrode material particles are not completely in contact with the electrolyte. Some particles cannot undergo electrochemical reaction on their surface, so the obtained result is too large. However, the present invention obtains the electrochemical active area by testing the capacitance, surface area and electrode capacitance of a single active particle that has not been coated into the electrode, thus overcoming the influence of particles inside the electrode and improving the accuracy of the test.

[0067] As can be seen from Example 1 and Comparative Example 2, since the electrode will be wetted by the electrolyte, theoretically each particle may come into contact with the electrolyte after immersion in the electrolyte. Therefore, using the area of ​​the electrode to replace the sum of the surface areas of the active particles is obviously too small, thus resulting in a smaller result. Compared with using the electrode area or the BET method to test the electrochemical active area, the present invention greatly improves the accuracy of testing the electrochemical active area of ​​the electrode by combining the double layer capacitance value and surface area of ​​a single particle with the double layer capacitance of the electrode.

[0068] In summary, this invention provides a method for obtaining the electrochemical active area of ​​an electrode. This method combines the double-layer capacitance of the particles and the surface area with the double-layer capacitance of the electrode to obtain a highly accurate electrochemical active area, eliminating errors caused by factors such as binders and particle extrusion.

[0069] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for obtaining the electrochemically active area of ​​an electrode, characterized in that, The method includes the following steps: (1) Test the double-layer capacitance value Q1 of a single active particle; the sphericity of the single active particle is ≥0.8; the diameter of the single active particle is 1nm-100μm; (2) test the double-layer capacitance Q2 of the electrode sheet, and obtain the electrochemical active area S of the electrode sheet E = [(Q2 / 2) / Q1] * S p , wherein S p is the surface area of the single active particle in step (1), and the electrode sheet comprises the active particle in step (1); The charge state of the single active particle in step (1) is not fully inserted and not completely delithiated, and is the same as the charge state of the electrode in step (2).

2. The method according to claim 1, characterized in that, Step (1) of testing the double-layer capacitance value Q1 of a single active particle includes: using a single-particle microelectrode system to assist in testing the AC impedance of a single active particle, fitting the obtained AC impedance test results to an equivalent circuit, and obtaining the double-layer capacitance value Q1 of the single active particle.

3. The method according to claim 1 or 2, characterized in that, In step (1), the individual active particles are selected with the assistance of a single-particle microelectrode system.

4. The method according to claim 1, characterized in that, Step (2) of testing the double-layer capacitance value Q2 of the electrode includes: preparing the electrode into a symmetrical cell, testing the AC impedance of the symmetrical cell, and then fitting the AC impedance test results to obtain the double-layer capacitance value Q2 of the electrode.

5. The method according to claim 1, characterized in that, Step (2) S p =πd 2 .

6. The method according to claim 1, characterized in that, The active material of the electrode in step (2) is the same as that of the single active particle in step (1).

7. The method according to claim 1, characterized in that, The method includes the following steps: (1) A single active particle with a sphericity ≥ 0.8 and a diameter d ≈ D50 is obtained by using a single particle microelectrode system for selection. Then, the AC impedance of the single active particle in the non-fully inserted and non-completely delithiated state is tested by using a single particle microelectrode system. The AC impedance test results are fitted with an equivalent circuit to obtain the double layer capacitance value Q1 of the single active particle. (2) Prepare a symmetrical battery by using an electrode with the same active material as the active particles described in step (1). Then test the AC impedance of the electrode in the non-fully inserted and non-completely delithiated state. Then fit the AC impedance test results to the equivalent circuit to obtain the double layer capacitance value Q2 of the electrode. In steps (1) and (2), the charge state of the electrode is the same as that of the single active particle described in step (1) during the AC impedance test. (3) Based on Q1 and d described in step (1) and Q2 described in step (2), the electrochemical active area S of the electrode is obtained. E =[(Q2 / 2) / Q1]*S p , among which, S p =πd 2 , is the surface area of ​​a single active particle in step (1).

Images