Battery binder distribution relative uniformity evaluation method and medium

Abstract

The application discloses a battery binder distribution relative uniformity evaluation method and medium. The method can include: using a plurality of groups of different drying process parameters to respectively manufacture pole pieces of symmetrical batteries, performing electrochemical impedance testing on the symmetrical batteries, and obtaining electrochemical impedance spectra of the symmetrical batteries; using a transmission line model to fit the electrochemical impedance spectra, and calculating electrochemical impedance spectra containing only electrode impedance; obtaining phase angle recess characteristic values corresponding to different drying process parameters according to the electrochemical impedance spectra and comparing the phase angle recess characteristic values, and evaluating the relative uniformity of the binder distribution. The application manufactures symmetrical batteries by using pole pieces with different drying process parameters, collects characteristic parameters through conventional electrochemical impedance spectra, and determines the relative uniformity of the binder distribution under different process parameters based on comparison of the characteristic parameters.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery manufacturing processes, and more specifically, to a method and medium for evaluating the relative uniformity of battery binder distribution. Background Technology

[0002] Lithium-ion battery technology originated in the 1950s, boasting a long history of development. It matured and began large-scale commercialization in the 1970s and 80s. In the past decade or so of power battery development, market demands for battery energy density and fast-charging performance have been steadily increasing. Binder flotation is a common problem in battery electrode fabrication; uneven distribution of the binder along the coating thickness direction degrades the electrode's peel strength and electrochemical performance. Binder flotation is usually related to process parameters during the drying stage. Therefore, evaluating the uniformity of binder distribution is crucial for optimizing the electrode fabrication process design.

[0003] Existing methods for evaluating binders involve elemental characterization of electrode cross-sections using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). Fluorine is typically used to characterize the distribution of the positive electrode binder, while tracer elements, such as bromine, are added to the negative electrode binder to characterize its distribution. However, these characterization methods are costly and time-consuming.

[0004] Therefore, it is necessary to develop a method and medium for evaluating the relative uniformity of battery binder distribution.

[0005] The information disclosed in the background section of this invention is intended only to enhance the understanding of the general background of this invention, and should not be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art. Summary of the Invention

[0006] This invention proposes a method and medium for evaluating the relative uniformity of binder distribution in batteries. It can fabricate symmetrical batteries using electrodes with different drying process parameters, collect characteristic parameters through conventional electrochemical impedance spectroscopy, and determine the relative uniformity of binder distribution under different process parameters based on the comparison of characteristic parameters.

[0007] In a first aspect, embodiments of this disclosure provide a method for evaluating the relative uniformity of battery binder distribution, including: Symmetrical cells were fabricated using electrodes with different drying process parameters. Electrochemical impedance spectroscopy was performed on the symmetric cells to obtain the electrochemical impedance spectra of each symmetric cell. The electrochemical impedance spectrum was fitted using a transmission line model to calculate the electrochemical impedance spectrum containing only electrode impedance. The phase angle depression characteristic values ​​corresponding to different drying process parameters are obtained from the electrochemical impedance spectroscopy and compared to evaluate the relative uniformity of binder distribution.

[0008] Preferably, in the transmission line model, the impedance of the symmetrical cell is: , Where Z is the impedance of the symmetrical battery. For ohmic impedance, This represents the electrode impedance.

[0009] Preferably, the analytical solution for the electrode impedance is: , Where x1 represents the ion resistance per unit thickness of the liquid phase. x2 represents the electronic resistance per unit thickness of the solid phase. L is the coating thickness, λ is an intermediate variable, and ξ represents the interfacial impedance. , R ct Represents the interface resistance, j is the imaginary unit, ω is the angular velocity, p is the non-ideal capacitance coefficient, and C is the capacitance coefficient. dl Here, S is the interface capacitance, S is the area of ​​the positive electrode material region, ρ is the solid-phase resistivity, κ is the electrolyte conductivity at a specific temperature, ε is the porosity, and pa is the phase angle. , Z real It is the real part impedance, Z imag It is the imaginary part of the impedance.

[0010] Preferably, the optimization objective of the fitting is: , Among them, Z exp_real Z is the real impedance of the full cell obtained by testing. exp_imag It is the imaginary impedance of the full cell obtained by testing, and k represents the amount of data for each data point in the electrochemical impedance spectroscopy.

[0011] Preferably, after confirming that the fitting accuracy meets the requirements, the ohmic impedance is set to zero to obtain an electrochemical impedance spectrum that only includes the electrode impedance.

[0012] Preferably, the evaluation of the relative uniformity of binder distribution includes obtaining and comparing the phase angle dip characteristic values ​​corresponding to different drying process parameters based on the electrochemical impedance spectroscopy, as well as the following: Bode plots are plotted based on the electrochemical impedance spectroscopy, and phase angle depression feature values ​​within a preset frequency range are extracted from the Bode plots. The relative uniformity of binder distribution is evaluated by comparing the level of the phase angle depression feature values ​​corresponding to different drying process parameters.

[0013] Preferably, the preset frequency range is 10-1000Hz.

[0014] Preferably, the phase angle concavity characteristic value is the phase angle at the lowest point of the concavity.

[0015] Preferably, the higher the phase angle, the more uniform the adhesive distribution.

[0016] Secondly, embodiments of this disclosure also provide a computer-readable storage medium storing a computer program that, when executed by a processor, implements the method for evaluating the relative uniformity of battery binder distribution.

[0017] Its beneficial effects are as follows: This invention, based on the transmission line model, utilizes electrochemical impedance data of electrodes with different drying process parameters to extract characteristic parameters to evaluate the relative uniformity of binder distribution. It eliminates the need for elemental characterization of the electrode coating using scanning electron microscopy and energy dispersive spectroscopy; only conventional electrochemical workstations are required to characterize the electrochemical behavior of the electrodes through electrochemical impedance spectroscopy. By processing the data and extracting characteristic parameters, the relative uniformity of binder distribution in both positive and negative electrodes can be evaluated, which is of great help in rapidly optimizing drying process parameters.

[0018] The methods and apparatus of the present invention have other features and advantages that will be apparent from or will be set forth in detail in the accompanying drawings and following detailed description, which together serve to explain the particular principles of the invention. Attached Figure Description

[0019] The above and other objects, features and advantages of the present invention will become more apparent from the more detailed description of exemplary embodiments of the invention in conjunction with the accompanying drawings, wherein the same reference numerals generally represent the same parts.

[0020] Figure 1 A flowchart illustrating the steps of a method for evaluating the relative uniformity of battery binder distribution according to an embodiment of the present invention is shown.

[0021] Figure 2 A schematic diagram showing the test and fitting results of electrochemical impedance spectroscopy of graphite symmetric cells at different drying temperatures according to Example 1 of the present invention is presented.

[0022] Figure 3 The Bode plots of graphite symmetric cells at different drying temperatures, calculated based on the transmission line model according to Embodiment 1 of the present invention, are shown.

[0023] Figure 4 A schematic diagram showing the test and fitting results of the electrochemical impedance spectroscopy of lithium iron phosphate symmetric batteries with different drying wind speeds according to Example 2 of the present invention is presented.

[0024] Figure 5The Bode plots of lithium iron phosphate symmetric batteries with different drying wind speeds, calculated according to the transmission line model based on Embodiment 2 of the present invention, are shown.

[0025] Figure 6 A binder content distribution diagram along the thickness direction of a lithium iron phosphate cathode coating according to the present invention is shown. Detailed Implementation

[0026] Preferred embodiments of the invention will now be described in more detail. While preferred embodiments of the invention are described below, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein.

[0027] Figure 1 A flowchart illustrating the steps of a method for evaluating the relative uniformity of battery binder distribution according to an embodiment of the present invention is shown.

[0028] like Figure 1 As shown, the method for evaluating the relative uniformity of battery binder distribution includes: Step 101: Use multiple sets of electrodes with different drying process parameters to fabricate symmetrical cells, and perform electrochemical impedance spectroscopy on the symmetrical cells to obtain the electrochemical impedance spectra of each symmetrical cell. Step 102: Fit the electrochemical impedance spectrum using a transmission line model to calculate the electrochemical impedance spectrum containing only electrode impedance. Step 103: Obtain and compare the phase angle depression characteristic values ​​corresponding to different drying process parameters based on electrochemical impedance spectroscopy to evaluate the relative uniformity of binder distribution.

[0029] In one example, the impedance of a symmetrical cell in a transmission line model is: , Where Z is the impedance of the symmetrical battery. For ohmic impedance, This represents the electrode impedance.

[0030] In one example, the analytical solution for the electrode impedance is: , Where x1 represents the ion resistance per unit thickness of the liquid phase. x2 represents the electronic resistance per unit thickness of the solid phase. L is the coating thickness, λ is an intermediate variable, and ξ represents the interfacial impedance. , R ct Represents the interface resistance, j is the imaginary unit, ω is the angular velocity, p is the non-ideal capacitance coefficient, and C is the capacitance coefficient. dlHere, S is the interface capacitance, S is the area of ​​the positive electrode material region, ρ is the solid-phase resistivity, κ is the electrolyte conductivity at a specific temperature, ε is the porosity, and pa is the phase angle. , Z real It is the real part impedance, Z imag It is the imaginary part of the impedance.

[0031] In one example, the optimization objective of the fit is: , Among them, Z exp_real Z is the real impedance of the full cell obtained by testing. exp_imag It is the imaginary impedance of the full cell obtained by testing, and k represents the amount of data for each data point in the electrochemical impedance spectroscopy.

[0032] In one example, after confirming that the fitting accuracy meets the requirements, the ohmic impedance is set to zero to obtain an electrochemical impedance spectrum that only includes the electrode impedance.

[0033] In one example, the phase angle dip characteristic values ​​corresponding to different drying process parameters are obtained from the electrochemical impedance spectroscopy and compared to evaluate the relative uniformity of binder distribution, including: Bode plots are plotted based on the electrochemical impedance spectroscopy, and phase angle depression feature values ​​within a preset frequency range are extracted from the Bode plots. The relative uniformity of binder distribution is evaluated by comparing the level of the phase angle depression feature values ​​corresponding to different drying process parameters.

[0034] In one example, the preset frequency range is 10-1000Hz.

[0035] In one example, the phase angle concavity eigenvalue is the phase angle at the lowest point of the concavity.

[0036] In one example, the higher the phase angle, the more uniform the adhesive distribution.

[0037] Specifically, electrochemical impedance spectroscopy (EIS) testing of symmetrical cells was conducted. Symmetrical cells were fabricated using electrodes with multiple sets of drying process parameters, and their EIS was then tested.

[0038] Electrochemical impedance spectroscopy (EIS) data processing. The EIS is fitted using a transmission line model. The impedance Z of a symmetrical cell is derived from the ohmic impedance Z0. ohm and twice the electrode impedance Z elect Adding them together, we get:

[0039] The analytical solution for the electrode impedance is as follows:

[0040] In the formula, x1 represents the ionic resistance per unit thickness of the liquid phase (Ω / m), x2 represents the electronic resistance per unit thickness of the solid phase (Ω / m), L is the coating thickness (m), λ is an intermediate variable, ξ represents the interfacial impedance, Rct represents the interfacial resistance (Ωm), j is the imaginary unit, ω is the angular velocity, p is the non-ideal capacitance coefficient, Cdl is the interfacial capacitance (F / m), and S is the area of ​​the positive electrode material region of the battery (m²). 2 ρ is the solid resistivity in Ωm, κ is the electrolyte conductivity at a specific temperature in S / m, ε is the porosity (dimensionless), pa is the phase angle in °, Zreal is the real impedance in Ω, and Zimag is the imaginary impedance in Ω.

[0041] The model parameters are fitted using optimization algorithms such as heuristics, least squares method, and conjugate gradient method. The optimization objective is the impedance error Object, defined as follows:

[0042] In the formula Z exp_real Z is the real impedance of the full cell obtained by testing. exp_imag It is the imaginary impedance of the full cell obtained by testing, and k represents the amount of data for each data point in the electrochemical impedance spectroscopy.

[0043] After confirming that the fitting accuracy meets the requirements, the ohmic impedance is set to zero, and the electrochemical impedance spectrum containing only the electrode impedance is calculated.

[0044] Feature data were extracted and the relative uniformity of binder distribution was compared. A Bode plot of the electrochemical impedance spectroscopy was plotted, and the location of the curve dip in the 10-1000 Hz range was identified. The phase angle of the lowest point of this dip was recorded. The higher the phase angle, the more uniform the binder distribution.

[0045] The present invention also provides a computer-readable storage medium storing a computer program that, when executed by a processor, implements the above-described method for evaluating the relative uniformity of battery binder distribution.

[0046] To facilitate understanding of the solutions and effects of the embodiments of the present invention, four specific application examples are given below. Those skilled in the art should understand that these examples are merely for the purpose of understanding the present invention, and any specific details therein are not intended to limit the present invention in any way.

[0047] Example 1

[0048] Figure 2 A schematic diagram showing the test and fitting results of electrochemical impedance spectroscopy of graphite symmetric cells at different drying temperatures according to an embodiment of the present invention is presented.

[0049] Step 1: Electrochemical Impedance Testing of Symmetrical Cells. Three groups of graphite negative electrode sheets dried at different temperatures were used to construct symmetrical cells, and electrochemical impedance spectroscopy was performed at 25°C. The frequency range was 1MHz-100mHz, and the excitation voltage was 10mV. The obtained Nyquist plots are shown below. Figure 2 As shown.

[0050] Step 2: Electrochemical Impedance Spectroscopy Data Processing. The electrochemical impedance spectroscopy is fitted using a transmission line model. The impedance Z of a symmetrical cell is derived from the ohmic impedance Z0. ohm and twice the electrode impedance Z elect Adding them together, we get:

[0051] The analytical solution for the electrode impedance is as follows:

[0052] In the formula, x1 represents the ionic resistance per unit thickness of the liquid phase (Ω / m), x2 represents the electronic resistance per unit thickness of the solid phase (Ω / m), L is the coating thickness (m), λ is an intermediate variable, ξ represents the interfacial impedance, Rct represents the interfacial resistance (Ωm), j is the imaginary unit, ω is the angular velocity, p is the non-ideal capacitance coefficient, and C... dl It is the interface capacitance, measured in F / m; S is the area of ​​the positive electrode region of the battery, measured in m². 2 ρ is the solid resistivity, in Ωm; κ is the electrolyte conductivity at a specific temperature, in S / m; ε is the porosity, dimensionless; pa is the phase angle, in °; Z real This is the real impedance, measured in Ω (Z). imag It is the imaginary part of the impedance, and the unit is Ω.

[0053] The model parameters are fitted using heuristic algorithms, least squares method, conjugate gradient method, etc. The optimization objective is impedance error Object, defined as follows:

[0054] In the formula Zexp_real Z is the real impedance of the full cell obtained by testing. real Z is the analytical solution to the real part impedance of the full cell. exp_imag Z is the imaginary impedance of the full cell obtained from the test. imag This is the analytical solution to the imaginary part impedance of the full cell. k represents the data quantity of each data point in the electrochemical impedance spectroscopy.

[0055] To perform the fitting, some electrode parameters need to be collected, as shown in Table 1. The remaining parameters in the model are determined based on the fitting. The fitted result is as follows: Figure 2 As shown, the fitting errors for 50℃, 75℃ and 100℃ are 1.25%, 1.12% and 0.74%, respectively.

[0056] Table 1. Parameters required for fitting the electrochemical impedance spectroscopy of graphite symmetric cells.

[0057] After confirming that the fitting accuracy meets the requirements, the ohmic impedance in the transmission line model is set to zero, while other parameters remain unchanged. The electrochemical impedance spectrum in the same frequency range is then calculated to obtain the electrochemical impedance spectrum containing only the electrode impedance.

[0058] Step 3: Extract feature data and compare the relative uniformity of binder distribution. Plot a Bode plot of the electrochemical impedance spectroscopy, find the depression in the curve between 10-1000 Hz, and record the phase angle of the lowest point of the depression. The higher the phase angle, the more uniform the binder distribution.

[0059] Figure 3 Bode plots of graphite symmetric cells at different drying temperatures, calculated based on a transmission line model according to an embodiment of the present invention, are shown.

[0060] Bode plots of graphite anode symmetric cells at different drying temperatures are shown below. Figure 3 As shown, the Bode plots for 50℃, 75℃, and 100℃ all exhibit a dip around 100Hz, with the lowest points of the dips being 41°, 40.2°, and 39.3°, respectively. Therefore, as the drying temperature increases, the upward movement of the negative electrode binder becomes more pronounced, and its distribution becomes more uneven.

[0061] Example 2

[0062] Figure 4 A schematic diagram showing the test and fitting results of electrochemical impedance spectroscopy of lithium iron phosphate symmetric batteries at different drying velocities according to an embodiment of the present invention is presented.

[0063] Step 1: Electrochemical Impedance Testing of Symmetrical Cells. Three sets of lithium iron phosphate cathode sheets with different drying velocities were used to construct symmetrical cells, and electrochemical impedance spectroscopy was performed at 25°C. The frequency range was 1MHz-100mHz, and the excitation voltage was 10mV. The obtained Nyquist plots are shown below. Figure 4 As shown.

[0064] Step 2: Electrochemical Impedance Spectroscopy Data Processing. The electrochemical impedance spectroscopy (EIS) spectra are fitted using a transmission line model. For fitting, some electrode parameters need to be collected, as shown in Table 2. The remaining parameters in the model are determined based on the fitting process. The fitted results are shown below. Figure 4 As shown, the fitting errors for 2m / s, 3m / s and 4m / s are 1.33%, 0.85% and 1.07%, respectively.

[0065] Table 2. Parameters required for fitting the electrochemical impedance spectroscopy of lithium iron phosphate symmetric batteries.

[0066] After confirming that the fitting accuracy meets the requirements, the ohmic impedance in the transmission line model is set to zero, while other parameters remain unchanged. The electrochemical impedance spectrum in the same frequency range is then calculated to obtain the electrochemical impedance spectrum containing only the electrode impedance.

[0067] Step 3: Extract feature data and compare the relative uniformity of binder distribution. Plot a Bode plot of the electrochemical impedance spectroscopy, find the depression in the curve between 10-1000 Hz, and record the phase angle of the lowest point of the depression. The higher the phase angle, the more uniform the binder distribution.

[0068] Bode plots of lithium iron phosphate cathode symmetric batteries with different drying velocities are shown below. Figure 5 As shown, the Bode plots for 2m / s, 3m / s, and 4m / s all exhibit dips around 70Hz, with the lowest points of these dips being 41.4°, 40.7°, and 39.8°, respectively. Therefore, as the drying air velocity increases, the upward movement of the positive electrode binder becomes more pronounced, and its distribution becomes more uneven.

[0069] Comparative Example 1

[0070] Figure 6 A binder content distribution diagram along the thickness direction of a lithium iron phosphate cathode coating according to the present invention is shown.

[0071] Using the electrode from Example 2 as the analytical object, energy dispersive spectroscopy was used to characterize the distribution of fluorine in the electrode at different drying velocities. Since the positive electrode binder is composed of PVDF, and fluorine is only distributed in the binder, the uniformity of fluorine distribution represents the uniformity of the positive electrode binder distribution. Figure 6As shown, the left end of the horizontal axis represents the bottom surface of the coating, and the right end represents the surface of the coating. The vertical axis has been normalized. The average fluorine content of the electrode at a drying wind speed of 2 m / s is 1. The higher the drying wind speed, the more obvious the floating of the positive electrode binder and the more uneven its distribution. This conclusion is the same as that obtained from the analysis based on the electrochemical impedance spectroscopy of symmetric cells. Therefore, the evaluation method proposed in this invention is feasible and can be used to optimize the drying process parameters in the electrode preparation process.

[0072] Example 3

[0073] This disclosure provides a computer-readable storage medium storing a computer program that, when executed by a processor, implements the method for evaluating the relative uniformity of battery binder distribution.

[0074] A computer-readable storage medium according to embodiments of the present disclosure stores non-transitory computer-readable instructions. When these non-transitory computer-readable instructions are executed by a processor, all or part of the steps of the methods described in the foregoing embodiments of the present disclosure are performed.

[0075] The aforementioned computer-readable storage media include, but are not limited to: optical storage media (e.g., CD-ROM and DVD), magneto-optical storage media (e.g., MO), magnetic storage media (e.g., magnetic tape or portable hard drive), media with built-in rewritable non-volatile memory (e.g., memory card), and media with built-in ROM (e.g., ROM cartridge).

[0076] Those skilled in the art should understand that the above description of the embodiments of the present invention is only intended to illustrate the beneficial effects of the embodiments of the present invention, and is not intended to limit the embodiments of the present invention to any of the examples given.

[0077] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A method for evaluating the relative uniformity of battery binder distribution, characterized in that, include: Symmetrical cells were fabricated using electrodes with different drying process parameters. Electrochemical impedance spectroscopy was performed on the symmetric cells to obtain the electrochemical impedance spectra of each symmetric cell. The electrochemical impedance spectrum was fitted using a transmission line model to calculate the electrochemical impedance spectrum containing only electrode impedance. The phase angle depression characteristic values ​​corresponding to different drying process parameters are obtained from the electrochemical impedance spectroscopy and compared to evaluate the relative uniformity of binder distribution.

2. The method for evaluating the relative uniformity of battery binder distribution according to claim 1, characterized in that, In the transmission line model, the impedance of a symmetrical cell is: ， Where Z is the impedance of the symmetrical battery. For ohmic impedance, This represents the electrode impedance.

3. The method for evaluating the relative uniformity of battery binder distribution according to claim 2, characterized in that, The analytical solution for the electrode impedance is: ， Where x1 represents the ion resistance per unit thickness of the liquid phase. x2 represents the electronic resistance per unit thickness of the solid phase. L is the coating thickness, λ is an intermediate variable, and ξ represents the interfacial impedance. , R ct Represents the interface resistance, j is the imaginary unit, ω is the angular velocity, p is the non-ideal capacitance coefficient, and C is the capacitance coefficient. dl Here, S is the interface capacitance, S is the area of ​​the positive electrode material region, ρ is the solid-phase resistivity, κ is the electrolyte conductivity at a specific temperature, ε is the porosity, and pa is the phase angle. , Z real It is the real part impedance, Z imag It is the imaginary part of the impedance.

4. The method for evaluating the relative uniformity of battery binder distribution according to claim 1, characterized in that, The optimization objective for the fit is: ， Among them, Z exp_real Z is the real impedance of the full cell obtained by testing. exp_imag It is the imaginary impedance of the full cell obtained by testing, and k represents the amount of data for each data point in the electrochemical impedance spectroscopy.

5. The method for evaluating the relative uniformity of battery binder distribution according to claim 2, characterized in that, After confirming that the fitting accuracy meets the requirements, the ohmic impedance is set to zero to obtain an electrochemical impedance spectrum that only includes the electrode impedance.

6. The method for evaluating the relative uniformity of battery binder distribution according to claim 1, characterized in that, The phase angle dip characteristic values ​​corresponding to different drying process parameters are obtained from the electrochemical impedance spectroscopy and compared to evaluate the relative uniformity of binder distribution, including: Bode plots are plotted based on the electrochemical impedance spectroscopy, and phase angle depression feature values ​​within a preset frequency range are extracted from the Bode plots. The relative uniformity of binder distribution is evaluated by comparing the level of the phase angle depression feature values ​​corresponding to different drying process parameters.

7. The method for evaluating the relative uniformity of battery binder distribution according to claim 6, characterized in that, The preset frequency range is 10-1000Hz.

8. The method for evaluating the relative uniformity of battery binder distribution according to claim 6, characterized in that, The characteristic value of the phase angle concavity is the phase angle at the lowest point of the concavity.

9. The method for evaluating the relative uniformity of battery binder distribution according to claim 8, characterized in that, The higher the phase angle, the more uniform the adhesive distribution.

10. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a computer program that, when executed by a processor, implements the method for evaluating the relative uniformity of battery binder distribution as described in any one of claims 1-9.

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