Direct-current impedance testing method for secondary battery

Abstract

The embodiment of the invention provides a direct-current impedance testing method of a secondary battery, and belongs to the technical field of battery manufacturing, and the method provided by the embodiment of the invention comprises the following steps: constructing an electrochemical model of the secondary battery; according to the electrochemical model, simulating and calculating solid-phase potential, liquid-phase potential, active ion concentration and interface current density distribution of the secondary battery; according to the electrochemical model, the solid phase potential, the liquid phase potential, the active ion concentration and the interface current density distribution, the direct current impedance of a target assembly in the secondary battery is determined, and the target assembly comprises at least one of a positive plate, a negative plate and a diaphragm. Through the method provided by the embodiment of the invention, the problem that the direct current impedance of the secondary battery cannot be accurately decomposed in the existing direct current impedance decomposition mode can be solved.

Description

Technical Field

[0001] The present invention relates to the technical field of battery manufacturing, and in particular to a method for testing the direct current impedance of a secondary battery. Background Art

[0002] Direct current resistance (DCR) decomposition is a kinetic analysis method widely used in the design optimization of lithium-ion and other secondary battery cells. It identifies the component materials or structures to be optimized by splitting the polarization contributions of various kinetic processes (such as solid-phase ion diffusion, liquid-phase ion transport, interfacial charge transfer, solid-phase electron conduction, etc.) on each component during battery cell charging and discharging.

[0003] DCR decomposition is usually based on DC pulse testing and AC impedance spectroscopy testing, or combined with equivalent circuit model fitting for decomposition. However, due to the coupling effects of multiple dynamic processes such as solid-phase ion diffusion, liquid-phase ion transport, interface charge transfer, and solid-phase electronic conduction on the porous electrodes of lithium-ion batteries (overlap and mutual influence in the time / frequency domain), the above theoretical method based on the linear superposition assumption will have large analysis errors, and sometimes even draw completely wrong conclusions, resulting in half the result with twice the effort for battery cell design optimization. In addition, the existing methods can only roughly decompose DCR into three parts: ohmic impedance, charge transfer impedance, and diffusion impedance, and cannot fully meet the needs of detailed analysis of commercial batteries.

[0004] Providing an accurate and reliable DCR decomposition method can provide clear guidance for the optimization direction of secondary battery kinetic performance. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for testing the DC impedance of a secondary battery, so as to solve the problem that the existing DC impedance decomposition method cannot accurately decompose the DC impedance of the secondary battery.

[0006] In order to solve the above problems, the present invention is achieved through the following technical solutions:

[0007] The present invention provides a method for testing the DC impedance of a secondary battery, which includes:

[0008] Construct electrochemical models of secondary batteries;

[0009] According to the electrochemical model, simulation calculation is performed on the solid phase potential, liquid phase potential, active ion concentration and interface current density distribution of the secondary battery;

[0010] The DC impedance of a target component in the secondary battery is determined according to the electrochemical model, the solid phase potential, the liquid phase potential, the active ion concentration, and the interface current density distribution. The target component includes at least one of a positive electrode sheet, a negative electrode sheet, and a separator.

[0011] Furthermore, in the testing method, the solid phase potential, liquid phase potential, active ion concentration and interface current density distribution of the secondary battery are simulated and calculated, including: simulating and calculating the solid phase potential, liquid phase potential, active ion concentration and interface current density distribution under preset temperature, preset SOC, preset pulse time and preset current conditions.

[0012] Furthermore, in the testing method, the electrochemical model is the Newman electrochemical model.

[0013] Furthermore, in the test method, determining the DC impedance of the target component in the secondary battery according to the electrochemical model, the solid phase potential, the liquid phase potential, the active ion concentration and the interface current density distribution includes:

[0014] The solid-phase ohmic resistance of each electrode is determined based on the total area of ​​the positive electrode, the thickness of the electrode, the solid-phase effective conductivity, the solid-phase current density and the total current of the battery cell.

[0015] Furthermore, in the test method, the solid-phase ohmic resistance of each electrode sheet is determined based on the total area of ​​the positive electrode sheet, the thickness of the electrode sheet, the solid-phase effective conductivity, the solid-phase current density and the total current of the battery cell, including:

[0016] According to the total area of ​​the positive electrode sheet, the thickness of the electrode sheet, the solid phase effective conductivity, the solid phase current density and the total current of the battery cell, the solid phase ohmic resistance of each electrode sheet is determined according to the following formula (1):

[0017]

[0018] Among them, S is the total area of ​​the positive electrode sheet, σ pos / neg is the solid phase effective conductivity, i s is the solid phase current density, L pos / neg is the thickness of the electrode, and I is the total current of the battery cell.

[0019] Furthermore, in the test method, determining the DC impedance of the target component in the secondary battery according to the electrochemical model, the solid phase potential, the liquid phase potential, the active ion concentration and the interface current density distribution includes:

[0020] The liquid phase ohmic resistance of each target component is determined based on the total area of ​​the positive electrode sheet, the thickness of the target component, the effective liquid phase conductivity, the liquid phase current density and the total current of the battery cell.

[0021] Furthermore, in the test method, the liquid phase ohmic resistance of each target component is determined based on the total area of ​​the positive electrode sheet, the thickness of the target component, the liquid phase effective conductivity, the liquid phase current density and the total current of the battery cell, including:

[0022] According to the total area of ​​the positive electrode sheet, the thickness of the target component, the effective conductivity of the liquid phase, the liquid phase current density and the total current of the battery cell, the liquid phase ohmic resistance of each target component is determined according to the following formula (2):

[0023]

[0024] Among them, S is the total area of ​​the positive electrode, κ eff,pos / neg / sep is the effective conductivity of the liquid phase, i l is the liquid phase current density, L pos / neg / sep is the target component thickness, and I is the total current of the battery cell.

[0025] Furthermore, in the test method, determining the DC impedance of the target component in the secondary battery according to the electrochemical model, the solid phase potential, the liquid phase potential, the active ion concentration and the interface current density distribution includes:

[0026] The liquid phase diffusion resistance of each target component is determined based on the total area of ​​the positive electrode sheet, the thickness of the target component, the active ion migration number, the activity coefficient, the active ion concentration, the liquid phase current density and the total current of the battery cell.

[0027] Furthermore, in the test method, the liquid phase diffusion resistance of each target component is determined based on the total area of ​​the positive electrode sheet, the thickness of the target component, the active ion migration number, the activity coefficient, the active ion concentration, the liquid phase current density and the total current of the battery cell, including:

[0028] According to the total area of ​​the positive electrode sheet, the thickness of the target component, the active ion migration number, the activity coefficient, the active ion concentration, the liquid phase current density and the total current of the battery cell, the liquid phase diffusion resistance of each target component is determined according to the following formula (3):

[0029]

[0030] Where S is the total area of ​​the positive electrode sheet, t + is the active ion migration number, f ± is the activity coefficient, c l is the active ion concentration, i l is the liquid phase current density, L pos / neg / sep is the target component thickness, I is the total cell current, F is the Faraday constant, R is the universal gas constant, and T is the temperature.

[0031] Furthermore, in the test method, determining the DC impedance of the target component in the secondary battery according to the electrochemical model, the solid phase potential, the liquid phase potential, the active ion concentration and the interface current density distribution includes:

[0032] The solid phase diffusion resistance of each electrode is determined based on the total area of ​​the positive electrode, volume specific surface area, electrode thickness, solid-liquid interface reaction current density, equilibrium electrode potential and total current of the battery cell.

[0033] Furthermore, in the test method, the solid phase diffusion resistance of each electrode is determined based on the total area of ​​the positive electrode, the volume specific surface area, the electrode thickness, the solid-liquid interface reaction current density, the equilibrium electrode potential and the total current of the battery cell, including:

[0034] According to the total area of ​​the positive electrode, volume specific surface area, electrode thickness, solid-liquid interface reaction current density, equilibrium electrode potential and total current of the battery cell, the solid phase diffusion resistance of each electrode is determined according to the following formula (4):

[0035]

[0036] Among them, S is the total area of ​​the positive electrode sheet, a v is the volume specific surface area, i ct is the solid-liquid interface reaction current density, E eq,ave is the equilibrium electrode potential corresponding to the average lithium ion concentration in the active material of the electrode, E eq,surf is the equilibrium electrode potential corresponding to the lithium ion concentration on the surface of the active material of the electrode, L pos / neg is the thickness of the electrode, and I is the total current of the battery cell.

[0037] Furthermore, in the test method, determining the DC impedance of the target component in the secondary battery according to the electrochemical model, the solid phase potential, the liquid phase potential, the active ion concentration and the interface current density distribution includes:

[0038] The solid-phase charge transfer resistance of each electrode is determined based on the total area of ​​the positive electrode, volume specific surface area, electrode thickness, solid-liquid interface reaction current density, equilibrium surface potential, solid phase potential, liquid phase potential and total current of the battery cell.

[0039] Furthermore, in the test method, the solid-phase charge transfer resistance of each electrode is determined based on the total area of ​​the positive electrode, the volume specific surface area, the electrode thickness, the solid-liquid interface reaction current density, the equilibrium surface potential, the solid phase potential, the liquid phase potential and the total current of the battery cell, including:

[0040] According to the total area of ​​the positive electrode, volume specific surface area, electrode thickness, solid-liquid interface reaction current density, equilibrium surface potential, solid phase potential, liquid phase potential and total current of the battery cell, the solid phase charge transfer resistance of each electrode is determined according to the following formula (5):

[0041]

[0042] Among them, S is the total area of ​​the positive electrode sheet, a v is the volume specific surface area, ict is the solid-liquid interface reaction current density, E eq,surf is the equilibrium electrode potential corresponding to the lithium ion concentration on the surface of the active material of the electrode, φ s is the solid phase potential, φ l is the liquid phase potential, L pos / neg is the thickness of the electrode, and I is the total current of the battery cell.

[0043] Compared with the prior art, the embodiments of the present invention have the following advantages:

[0044] In an embodiment of the present invention, a method for testing the DC impedance of a secondary battery is provided. The method first constructs an electrochemical model of the secondary battery. Based on the electrochemical model, the solid-phase potential, liquid-phase potential, active ion concentration, and interfacial current density distribution of the secondary battery are simulated and calculated. The DC impedance of a target component in the secondary battery is then determined based on the electrochemical model, the solid-phase potential, liquid-phase potential, active ion concentration, and interfacial current density distribution. The target component includes at least one of the positive electrode sheet, the negative electrode sheet, and the separator. This method, which combines experimental measurement with electrochemical model simulation analysis, compares favorably to existing DCR decomposition methods. Firstly, the present embodiment overcomes the difficulty in decoupling the DC impedance of various kinetic processes in a secondary battery through numerical calculations of the electrochemical model, while also enabling a more detailed decomposition of the DCR into multiple components. Secondly, the present embodiment ensures the accuracy of the parameters required for simulation analysis by preparing structures equivalent to secondary battery cells, such as half-cells and symmetrically structured batteries. This solves the problem that existing DC impedance decomposition methods cannot accurately decompose the DC impedance of secondary batteries.

[0045] It should be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 This is a flowchart of the steps of a method for testing the DC impedance of a secondary battery provided by an embodiment of the present invention;

[0047] Figure 2 This is a flow chart of sub-steps of step 101 in the DC impedance testing method for a secondary battery provided by an embodiment of the present invention;

[0048] Figure 3 This is a schematic structural diagram of a battery cell provided by an embodiment of the present invention;

[0049] Figure 4 Schematic diagram of simulation results and measured results of a half-cell rate charge and discharge process provided by an embodiment of the present invention;

[0050] Figure 5 is a schematic diagram of a current collector electric field simulation model provided by an embodiment of the present invention;

[0051] Figure 6 It is a schematic diagram of the decomposition result of the DC impedance of the secondary battery provided by the embodiment of the present invention. DETAILED DESCRIPTION

[0052] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0053] DC impedance is an important indicator of secondary battery performance. The applicants of the present invention have discovered that existing methods for analyzing DC impedance, such as experimental decomposition methods based on DC pulse testing and AC impedance spectrum testing, or decomposition methods combined with equivalent circuit models, can result in significant analytical errors when analyzing the DC impedance of secondary batteries due to the coupling effects of the battery's internal dynamic processes. This can lead to deviations or even errors in the analysis results, which in turn affects the design and optimization of secondary battery cells. Furthermore, existing methods can only roughly decompose DCR into three components: ohmic impedance, charge transfer impedance, and diffusion impedance, and cannot fully meet the needs of detailed analysis of commercial batteries.

[0054] In order to solve the above problems, the embodiment of the present invention provides a method for testing the DC impedance of a secondary battery. Figure 1 As shown, the method includes steps 101 to 103:

[0055] Step 101: Construct an electrochemical model of a secondary battery.

[0056] In response to the problems in the related art, the present invention combines the simulation decomposition method and the experimental decomposition method; in the above step 101, the embodiment of the present invention first constructs an electrochemical model of the secondary battery. The electrochemical model characterizes the kinetic process inside the secondary battery through mathematical equations, etc., including electrochemical reactions, material transfer, charge conservation and other processes inside the secondary battery; common electrochemical models of secondary batteries include single particle model (SPM), quasi-two-dimensional model (P2D), porous electrode theory model (PET), etc.; the above-mentioned secondary batteries mainly include lithium-ion batteries.

[0057] In the above step 101, if the electrochemical model is the Newman model based on a uniform porous electrode, the parameters required for the electrochemical model of the secondary battery are established, such as Figure 2 As shown, it can be obtained specifically through the following sub-steps 1011 to 1017:

[0058] Sub-step 1011: prepare positive and negative electrode half-cells and perform rate testing.

[0059] In order to establish an electrochemical model of a target secondary battery, the present invention first prepares a half-cell of the target secondary battery to obtain the parameters required for establishing the electrochemical model of the target secondary battery during the preparation of the half-cell; in the battery cell, a single independent electrode (including a metal part and its adjacent electrolyte) is called a half-cell.

[0060] Specifically, sub-step 1011 may include: making thin electrodes according to the positive / negative electrode formula of the target cell of the target secondary battery, and assembling them into a half-cell with a separator and lithium metal; the mass proportion of the thin electrode conductive agent (relative to the total solid mass) is not less than 5%, the coating thickness does not exceed 30μm, and the compaction density does not exceed the target cell electrode; the half-cell uses the same electrolyte as the target cell and in sufficient quantity.

[0061] After the half-cell is prepared, the prepared positive / negative electrode half-cell is subjected to a resting-formation-cycling-rate test to obtain the potential-voltage curve of the rate test; the resting step time is not less than 8 hours, and the cycle test is performed for 3-10 cycles of charge and discharge. The highest charge and discharge rate in the rate test is not less than 20C; the battery rate is used to characterize the charge and discharge rate of the battery. For example, for a battery with a capacity of 1Ah, 1C means that the discharge current discharges the entire battery in 1 hour.

[0062] Sub-step 1012: prepare a symmetrical structure battery and perform an AC impedance spectrum test.

[0063] Next, the present invention prepares a symmetrical structure battery using fresh electrodes of the target battery cell and a separator of the target battery cell, and obtains relevant parameters.

[0064] Specifically, sub-step 1012 may include: manufacturing the fresh electrode corresponding to the target battery cell into a symmetrical electrode-diaphragm-electrode structure battery, manufacturing the corresponding diaphragm of the target battery cell into a symmetrical steel sheet-diaphragm-steel sheet structure battery, and injecting sufficient electrolyte into both; the fresh electrode refers to the electrode that has undergone the rolling process but has not been activated (i.e., no ion insertion / extraction has occurred). In a symmetrical battery, both electrodes are positive electrodes or negative electrodes. The manufactured symmetrical battery structure is allowed to stand, and then an AC impedance spectroscopy test is performed to obtain a Nyquist curve; the frequency of the above AC impedance spectroscopy test is not less than 100kHz and not more than 0.1Hz.

[0065] In the related art, since it is difficult to directly obtain parameters such as the potential, concentration, electrochemical reaction rate gradient information inside the target cell of the target secondary battery through experimental measurement, simulation analysis is difficult to perform accurately; the present invention prepares structures (half-cells and symmetrical structure batteries) equivalent to the inside of the target cell through step 101, especially sub-steps 1011 and 1012 therein. These structures are easy to perform experimental measurements, and the measurement results obtained through experimental measurements can also characterize the parameters of the corresponding target cell, thereby being used for electrochemical modeling and simulation analysis of the target secondary battery.

[0066] Sub-step 1013: test and obtain the positive and negative electrode membrane resistances.

[0067] For a separate fresh electrode, the sheet resistance of the electrode also needs to be obtained. To this end, the sheet resistance test needs to be performed on the fresh positive electrode and the fresh negative electrode respectively.

[0068] Sub-step 1014: Obtain the resistance value between the electrode and the tab of the target battery cell.

[0069] The resistance values ​​between the positive electrode column and the positive electrode ear, and between the negative electrode column and the negative electrode ear can be measured by a resistance meter, or the voltage differences between the positive electrode column and the positive electrode ear, and between the negative electrode column and the negative electrode ear can be calculated by electric field simulation, and the resistance values ​​can be indirectly calculated using Ohm's law.

[0070] Sub-step 1015: Obtain the positive and negative electrode solid phase ion diffusion coefficients and interface exchange current densities.

[0071] Use the electrochemical model to simulate and analyze the half-cell prepared in step 1011 to obtain simulation data of the potential-voltage of the half-cell. Fit the potential-voltage curve obtained in step 1011 with the simulation data to determine the positive / negative electrode solid phase ion diffusion coefficient and the interface exchange current density.

[0072] Among them, simulation analysis and fitting can be achieved through commercial software COMSOL, GT-autolion or open source program Pybamm.

[0073] Sub-step 1016: Calculate and obtain the tortuosity of the positive electrode, the negative electrode, and the separator respectively.

[0074] Based on the Nyquist curve obtained in step 1012, for a symmetrical structure battery, the electrolyte ion resistance R in the electrode pores can be obtained by the following formula (11): Ω :

[0075] R Ω =3(Z LF→∞ -R HF ) (11)

[0076] Among them, ZLF→∞ is the intercept of the low-frequency band in the Nyquist curve extended to the real axis (the intercept obtained by virtual extrapolation of the curve to the real axis), R HF is the intercept of the Nyquist curve with the real axis (the actual intercept of the curve on the real axis);

[0077] Furthermore, the tortuosity of the diaphragm can be obtained by the following formula (12):

[0078]

[0079] Where ε is the porosity, κ is the intrinsic ionic conductivity of the electrolyte, A and L are the effective area and thickness of the electrode / diaphragm, respectively.

[0080] The tortuosity of the positive and negative electrodes can be obtained by the following formula (13):

[0081]

[0082] The various parameters have been explained in the above formulas (11) and (12) and will not be repeated here.

[0083] Sub-step 1017: obtain the positive and negative current collector resistances through simulation calculation.

[0084] Two-dimensional modeling and electric field simulation were performed on the single-layer positive and negative current collectors respectively. According to the voltage difference ΔU on the current collector and the total current I of the battery cell, the resistance of the positive and negative current collectors was calculated by the following formula (14):

[0085]

[0086] At this point, the parameters required for simulation analysis based on the electrochemical model of the target secondary battery can be obtained.

[0087] Step 102: simulating and calculating the solid phase potential, liquid phase potential, active ion concentration, and interface current density distribution of the secondary battery according to the electrochemical model.

[0088] Substituting the parameters obtained in substeps 1013 to 1017 above into the electrochemical model, simulation results for the target secondary battery can be obtained. If the electrochemical model of the target secondary battery is the Newman model, the simulation results may include the solid phase potential, liquid phase potential, active ion concentration, and interfacial current density distribution of the target secondary battery cell.

[0089] Step 103: Determine the DC impedance of a target component in the secondary battery according to the electrochemical model, the solid phase potential, the liquid phase potential, the active ion concentration, and the interface current density distribution. The target component includes at least one of a positive electrode sheet, a negative electrode sheet, and a separator.

[0090] Based on the parameters characterized by the simulation results, the DC voltage, DC current and other parameters of the target component can be further determined, so that the DC impedance of each component in the target battery cell of the target secondary battery can be calculated based on Joule's law or Ohm's law, thereby realizing the DC impedance decomposition of the target secondary battery.

[0091] In an embodiment of the present invention, a method combining experimental measurement with electrochemical model simulation analysis is used. Compared with existing DCR decomposition methods, on the one hand, the embodiment of the present invention overcomes the problem of difficulty in decoupling the DC impedance of various kinetic processes in secondary batteries through numerical calculations of electrochemical models, and can more finely decompose the DCR into multiple parts. On the other hand, the embodiment of the present invention ensures the accuracy of the parameters required in the simulation analysis by preparing structures equivalent to secondary battery cells, such as half-cells and symmetrical structure batteries. Therefore, the problem that existing DC impedance decomposition methods cannot accurately and finely decompose the DC impedance of secondary batteries is solved.

[0092] Optionally, in one embodiment, the step 102 specifically includes:

[0093] The solid phase potential, liquid phase potential, active ion concentration and interface current density distribution of the secondary battery are simulated and calculated, including: simulating and calculating the solid phase potential, liquid phase potential, active ion concentration and interface current density distribution under preset temperature, preset SOC, preset pulse time and preset current conditions.

[0094] In this embodiment, when simulating and calculating the solid phase potential, liquid phase potential, active ion concentration, and interfacial current density distribution, the battery operating conditions can be preset to simulate the battery's operation under specific operating conditions. The battery operating conditions can be simulated by presetting operating parameters such as temperature, SOC (state of charge), pulse time, and current.

[0095] For example, the operating conditions of the electrochemical model are set to 25° C., 50% SOC, and 1C discharge for 30 seconds, and then the various parameters obtained in step 101 are substituted into the operating conditions to obtain the simulation analysis results of the secondary battery.

[0096] Optionally, in one embodiment, the electrochemical model in step 101 is a Newman electrochemical model.

[0097] In this embodiment, the electrochemical model constructed in step 101 is preferably the Newman electrochemical model; this model is based on the porous electrode theory and the concentrated solution theory, and quantitatively describes the processes such as the fixed phase charge conservation, liquid phase charge conservation, solid phase mass conservation, liquid phase mass conservation, and electrochemical reaction inside the battery cell through multiple equations.

[0098] The Newman model can characterize processes such as electron conduction (solid phase charge) in battery cell electrodes, ion conduction (liquid phase charge) in electrolyte solutions, lithium ion diffusion in electrode active particles, changes in lithium ion concentration in electrolyte solutions, and lithium ion insertion / extraction at the electrode / electrolyte solution interface. Substituting the parameters obtained in the above sub-steps 1011-1017 into the equation corresponding to the Newman chemical model, the value of the target variable in the equation can be obtained, that is, the simulation calculation result of the target secondary battery can be obtained.

[0099] Optionally, in one embodiment, the above step 103 may specifically include:

[0100] Sub-step 1031 : determining the solid-phase ohmic resistance of each electrode sheet according to the total area of ​​the positive electrode sheet, the electrode sheet thickness, the solid-phase effective conductivity, the solid-phase current density, and the total current of the battery cell.

[0101] Optionally, the above step 1031 specifically includes:

[0102] According to the total area of ​​the positive electrode sheet, the thickness of the electrode sheet, the solid phase effective conductivity, the solid phase current density and the total current of the battery cell, the solid phase ohmic resistance of each electrode sheet is determined according to the following formula (1):

[0103]

[0104] Among them, S is the total area of ​​the positive electrode sheet, σ pos / neg is the solid phase effective conductivity, i s is the solid phase current density, L pos / neg is the thickness of the electrode, and I is the total current of the battery cell.

[0105] In this embodiment, the solid-phase ohmic resistance of the positive electrode sheet or the negative electrode sheet can be obtained based on Ohm's law by the total area of ​​the positive electrode sheet, the thickness of the electrode sheet, the solid-phase effective conductivity, the solid-phase current density and the total current of the battery cell; by substituting L in the above formula (1) pos / neg By adaptively adjusting the thickness of the electrode sheet corresponding to the positive electrode sheet or the negative electrode sheet, the solid-phase ohmic resistance of the positive electrode sheet or the negative electrode sheet can be determined.

[0106] Optionally, in one embodiment, the above step 103 may specifically include:

[0107] Sub-step 1032: Determine the liquid phase ohmic resistance of each target component based on the total area of ​​the positive electrode sheets, the thickness of the target component, the liquid phase effective conductivity, the liquid phase current density, and the total current of the battery cell.

[0108] Optionally, the above step 1032 specifically includes:

[0109] According to the total area of ​​the positive electrode sheet, the thickness of the target component, the effective conductivity of the liquid phase, the liquid phase current density and the total current of the battery cell, the liquid phase ohmic resistance of each target component is determined according to the following formula (2):

[0110]

[0111] Among them, S is the total area of ​​the positive electrode, κ eff,pos / neg / sep is the effective conductivity of the liquid phase, i l is the liquid phase current density, L pos / neg / sep is the target component thickness, and I is the total current of the battery cell.

[0112] In this embodiment, the liquid phase ohmic resistance of the positive and negative electrodes can be obtained by combining Ohm's law with the total area of ​​the positive electrode sheet, the target component thickness, the effective conductivity of the liquid phase, the liquid phase current density, and the total current of the battery cell; by adaptively adjusting L in the above formula (2) pos / neg / sep , making it the thickness of any target component, the liquid phase ohmic resistance of the target component can be obtained. For example, L pos / neg / sep Set as the thickness of the positive electrode sheet, and the liquid phase ohmic resistance of the positive electrode sheet can be obtained through the above formula (2).

[0113] Optionally, in one embodiment, the above step 103 may specifically include:

[0114] Step 1033: Determine the liquid phase diffusion resistance of each target component based on the total area of ​​the positive electrode sheet, the thickness of the target component, the active ion migration number, the activity coefficient, the active ion concentration, the liquid phase current density and the total current of the battery cell.

[0115] Optionally, the above step 1033 specifically includes:

[0116] According to the total area of ​​the positive electrode sheet, the thickness of the target component, the active ion migration number, the activity coefficient, the active ion concentration, the liquid phase current density and the total current of the battery cell, the liquid phase diffusion resistance of each target component is determined according to the following formula (3):

[0117]

[0118] Where S is the total area of ​​the positive electrode sheet, t + is the active ion migration number, f ± is the activity coefficient, c l is the active ion concentration, i l is the liquid phase current density, L pos / neg / sep is the target component thickness, I is the total cell current, F is the Faraday constant, R is the universal gas constant, and T is the temperature.

[0119] In this embodiment, the liquid phase diffusion resistance of the target component can be obtained by combining Ohm's law with the total area of ​​the positive electrode sheet, the thickness of the target component, the active ion migration number, the activity coefficient, the active ion concentration, the liquid phase current density and the total current of the battery cell; by adaptively adjusting L in the above formula (3) pos / neg / sep , making it the thickness of any target component, the liquid phase diffusion resistance of the target component can be obtained. For example, L pos / neg / sep Set to the thickness of the positive electrode sheet, the liquid phase diffusion resistance of the positive electrode sheet can be obtained through the above formula (3).

[0120] Optionally, in one embodiment, the above step 103 may specifically include:

[0121] Step 1034: Determine the solid phase diffusion resistance of each electrode piece based on the total area of ​​the positive electrode piece, the volume specific surface area, the electrode piece thickness, the solid-liquid interface reaction current density, the equilibrium electrode potential and the total current of the battery cell.

[0122] Optionally, the above step 1034 may specifically include:

[0123] According to the total area of ​​the positive electrode, volume specific surface area, electrode thickness, solid-liquid interface reaction current density, equilibrium electrode potential and total current of the battery cell, the solid phase diffusion resistance of each electrode is determined according to the following formula (4):

[0124]

[0125] Among them, S is the total area of ​​the positive electrode sheet, a v is the volume specific surface area, i ct is the solid-liquid interface reaction current density, E eq,ave is the equilibrium electrode potential corresponding to the average lithium ion concentration in the active material of the electrode, E eq,surf is the equilibrium electrode potential corresponding to the lithium ion concentration on the surface of the active material of the electrode, L pos / neg is the thickness of the electrode, and I is the total current of the battery cell.

[0126] In this embodiment, the solid phase diffusion resistance of the positive / negative electrode sheets can be obtained by combining Ohm's law with the total area of ​​the positive electrode sheet, the volume specific surface area, the thickness of the electrode sheet, the solid-liquid interface reaction current density, the equilibrium electrode potential and the total current of the battery cell; by adaptively adjusting L in the above formula (4) pos / neg , let it be the thickness of the positive electrode sheet or the negative electrode sheet, the solid phase diffusion resistance corresponding to the positive electrode sheet or the negative electrode sheet can be obtained. For example, L pos / neg Set to the thickness of the positive electrode sheet, the solid phase diffusion resistance of the positive electrode sheet can be obtained through the above formula (4).

[0127] Optionally, in one embodiment, the above step 103 may specifically include:

[0128] Step 1035: Determine the solid-phase charge transfer resistance of each electrode based on the total area of ​​the positive electrode, volume specific surface area, electrode thickness, solid-liquid interface reaction current density, equilibrium surface potential, solid phase potential, liquid phase potential and total current of the battery cell.

[0129] Optionally, the above step 1035 may specifically include:

[0130] According to the total area of ​​the positive electrode, volume specific surface area, electrode thickness, solid-liquid interface reaction current density, equilibrium surface potential, solid phase potential, liquid phase potential and total current of the battery cell, the solid phase charge transfer resistance of each electrode is determined according to the following formula (5):

[0131]

[0132] Among them, S is the total area of ​​the positive electrode sheet, a v is the volume specific surface area, i ct is the solid-liquid interface reaction current density, E eq,surf is the equilibrium electrode potential corresponding to the lithium concentration on the surface of the active material of the electrode, φ s is the solid phase potential, φ l is the liquid phase potential, L pos / neg is the thickness of the electrode, and I is the total current of the battery cell.

[0133] In this embodiment, the solid-phase charge transfer resistance of the positive / negative electrode can be obtained by combining Ohm's law with the total area of ​​the positive electrode, the volume specific surface area, the electrode thickness, the solid-liquid interface reaction current density, the equilibrium surface potential, the solid phase potential, the liquid phase potential and the total current of the battery cell; by adaptively adjusting L in the above formula (5) pos / neg , let it be the thickness of the positive electrode sheet or the negative electrode sheet, the solid phase charge transfer resistance corresponding to the positive electrode sheet or the negative electrode sheet can be obtained. For example, L pos / neg Set to the thickness of the positive electrode sheet, the solid-phase charge transfer resistance of the positive electrode sheet can be obtained through the above formula (5).

[0134] The following describes the DC impedance decomposition method of the secondary battery provided by the present invention, taking a ternary-graphite square aluminum shell battery cell with a capacity of 150Ah as an example of the target secondary battery cell.

[0135] Step S1: According to the positive and negative electrode formulas of the above square aluminum battery cell, a 10 μm thick ternary thin electrode and a 20 μm thick graphite thin electrode are respectively prepared. The preparation method is exactly the same as the slurry-coating method for preparing conventional electrodes.

[0136] The ternary electrode conductive agent has a mass fraction of 10% and a compacted density of 3.2g / m3; the graphite electrode conductive agent has a mass fraction of 5% and a compacted density of 1.3g / m3. The ternary thin electrode and graphite thin electrode are assembled with a 12μm-thick composite ceramic separator (the same separator as the 150Ah ternary-graphite square aluminum shell battery cell) and a 0.5mm-thick lithium sheet into a 2032 button-type half-cell. The electrolyte used is the same as the target battery cell, with a content of 80μL.

[0137] After preparation, the prepared positive and negative electrode half-cells were subjected to a resting-formation-cycling-rate test. The resting step lasted 24 hours, and the cycle test was performed for five cycles of 0.33C charge and discharge. The rate tests were performed at 0.5C, 1C, 5C, and 10C, with the charge rate tested first and the discharge rate tested later. The positive electrode half-cell voltage window was 2.8-4.4V, and the negative electrode half-cell voltage window was 0.005-0.8V.

[0138] It can be understood that step S1 is equivalent to the above-mentioned sub-step 1011.

[0139] Step S2: A symmetrical electrode-diaphragm-electrode structure battery (i.e., electrodes that have undergone a roll-pressing process but not been activated) identical to those used in the aforementioned square aluminum cell was fabricated. The aforementioned diaphragm was fabricated into a symmetrical steel sheet-diaphragm-steel sheet structure battery. 80 μL of electrolyte was injected into each battery. The fabricated symmetrical batteries were allowed to rest for 24 hours before undergoing an AC impedance spectroscopy test at a frequency of 0.1 Hz to 100 kHz.

[0140] It can be understood that step S2 is equivalent to the above-mentioned sub-step 1012.

[0141] Step S3: Cut the same fresh positive and negative electrode sheets used in the square aluminum battery cells into pieces with an area of ​​1540 mm 2 The disc is placed on a film resistance meter for testing, and the positive and negative film resistances are respectively obtained as: R f,pos =200mΩ, R f,pos =1.5mΩ.

[0142] It can be understood that step S3 is equivalent to the above-mentioned sub-step 1013.

[0143] Step S4: Use a resistance meter to measure the resistance between the positive electrode column and the positive ear, and between the negative electrode column and the negative ear, and obtain the resistance of the positive and negative mechanical parts: R m,pos =0.03mΩ, R m,neg =0.02mΩ, that is, the total resistance R m =0.03+0.02=0.05mΩ. The positional relationship between the electrode and the tab in the battery cell is as follows: Figure 3As shown, 1 represents the electrode pole, 2 represents the upper plastic, 3 represents the connecting piece, 4 represents the tab, and 5 represents the battery cell shell.

[0144] It can be understood that step S4 is equivalent to the above-mentioned sub-step 1014.

[0145] Step S5: The widely used Newman porous electrode model is used to simulate the half-cell rate charge and discharge process in step S1, and the positive / negative electrode solid phase ion diffusion coefficient and interface exchange current density are determined by fitting the model simulation results to the potential-voltage curve of the rate test in step S1; the fitting results are as follows: Figure 4 As shown (the model can be simulated and fitted using the commercial software COMSOL), where the rate charge and discharge simulation results of the half-battery are as follows Figure 4 As shown by the solid line in the figure, the actual test results of the half-cell rate charge and discharge are as follows: Figure 4 The solid phase diffusion coefficients and interface exchange current densities of the positive and negative electrodes are shown in Table 1 below.

[0146] parameter Positive electrode (ternary) Negative electrode (graphite) <![CDATA[Solid-phase diffusion coefficient / m 2 s -1 > 2.3e-14 4.6e-15 <![CDATA[Exchange current density / A m -2 > 5.5 1.2

[0147] Table 1

[0148] It can be understood that step S5 is equivalent to the above-mentioned sub-step 1015.

[0149] Step S6: Based on the AC impedance spectrum test results (Nyquist curve) in step S2, the tortuosity of the diaphragm and the positive and negative electrodes is calculated according to formula (11), formula (12), and formula (13): τ sep =4.3, τ pos =3.1, τ neg =5.7.

[0150] It can be understood that step S6 is equivalent to the above-mentioned sub-step 1016.

[0151] Step S7: Perform two-dimensional modeling and electric field simulation on the single-layer positive and negative electrode current collectors respectively, set the potential condition φ=V0 and the current condition I=I0 at the tab, and then calculate the positive and negative electrode current collector resistances according to the pressure difference ΔU on the current collector and the total current I0 of the battery cell using the above formula (14): cc,pos =0.012mΩ, R cc,neg =0.008mΩ. In this step, the schematic diagram of the electric field simulation modeling of the electrode collector is as follows Figure 5 As shown, the potential condition φ=V0 and the current condition I=I0 are set at position 5, i.e., the upper boundary of the tab. Figure 5 In FIG, 6 represents a current collector.

[0152] It can be understood that step S7 is equivalent to the above-mentioned sub-step 1017.

[0153] Step S8: Establish a Newman electrochemical model of the entire target secondary battery, substitute the parameters obtained in the above steps S3 to S7, and simulate the solid phase potential, liquid phase potential, lithium ion concentration and interface current density distribution of the target secondary battery at 25°C, 50% SOC, and 1C discharge for 30s.

[0154] It can be understood that step S8 is equivalent to the above-mentioned step 102.

[0155] Step S9: Substitute the simulation calculation results of step S8 and the parameters of each component in the target secondary battery into the above formula (1), formula (2), formula (3), formula (4), and formula (5) in order to obtain the DC impedance of each component and its proportion, thereby completing the DC impedance decomposition of the target secondary battery, that is, completing the DC impedance test of the secondary battery. The schematic diagram of the proportional distribution of the DC impedance proportion of each component obtained in step S9 is as follows: Figure 6 As shown, the various resistors have been described in the above step 103 and will not be repeated here.

[0156] It can be understood that step S9 is equivalent to the above step 103.

[0157] In summary, in this embodiment, an electrochemical model of a secondary battery is first constructed; then, based on the electrochemical model, the solid-phase potential, liquid-phase potential, active ion concentration, and interfacial current density distribution of the secondary battery are simulated and calculated; then, based on the electrochemical model, the solid-phase potential, liquid-phase potential, active ion concentration, and interfacial current density distribution, the DC impedance of the target component in the secondary battery is determined, the target component including at least one of the positive electrode sheet, the negative electrode sheet, and the separator. By combining experimental measurement with electrochemical model simulation analysis, on the one hand, the embodiment of the present invention overcomes the problem of difficulty in decoupling the DC impedance of various kinetic processes in the secondary battery through numerical calculation of the electrochemical model, and can more finely decompose the DC impedance DCR into multiple parts; on the other hand, the embodiment of the present invention ensures the accuracy of the parameters required in the simulation analysis by preparing structures equivalent to the battery cells of the secondary battery, such as half-cells and symmetrical structure batteries; thus, solving the problem that the existing DC impedance decomposition method cannot accurately and finely decompose the DC impedance of the secondary battery.

[0158] Although the preferred embodiments of the present invention have been described, those skilled in the art may make additional changes and modifications to these embodiments once they are aware of the basic creative concepts. Therefore, the claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the embodiments of the present invention.

[0159] The above is a detailed introduction to the DC impedance testing method of a secondary battery provided by the present invention. Specific examples are used herein to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the method of the present invention and its core idea. At the same time, for those skilled in the art, according to the ideas of the present invention, there will be changes in the specific implementation methods and application scopes. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A method for testing the DC impedance of a secondary battery, characterized in that: include: Construct electrochemical models of secondary batteries; According to the electrochemical model, simulation calculation is performed on the solid phase potential, liquid phase potential, active ion concentration and interface current density distribution of the secondary battery; The DC impedance of a target component in the secondary battery is determined according to the electrochemical model, the solid phase potential, the liquid phase potential, the active ion concentration, and the interface current density distribution. The target component includes at least one of a positive electrode sheet, a negative electrode sheet, and a separator.

2. The method according to claim 1, characterized in that The simulation calculation of the solid phase potential, liquid phase potential, active ion concentration and interface current density distribution of the secondary battery includes: The simulation calculates the solid phase potential, liquid phase potential, active ion concentration and interface current density distribution under the preset temperature, preset SOC, preset pulse time and preset current conditions.

3. The method according to claim 1, characterized in that The electrochemical model is the Newman electrochemical model.

4. The method according to claim 1, wherein Determining the DC impedance of a target component in the secondary battery according to the electrochemical model, the solid phase potential, the liquid phase potential, the active ion concentration, and the interface current density distribution includes: The solid-phase ohmic resistance of each electrode is determined based on the total area of ​​the positive electrode, the thickness of the electrode, the solid-phase effective conductivity, the solid-phase current density and the total current of the battery cell.

5. The method according to claim 4, characterized in that Determine the solid-phase ohmic resistance of each electrode based on the total area of ​​the positive electrode, electrode thickness, solid-phase effective conductivity, solid-phase current density, and total current of the cell, including: According to the total area of ​​the positive electrode sheet, the thickness of the electrode sheet, the solid phase effective conductivity, the solid phase current density and the total current of the battery cell, the solid phase ohmic resistance of each electrode sheet is determined according to the following formula (1): Among them, S is the total area of ​​the positive electrode sheet, σ pos / neg is the solid phase effective conductivity, i s is the solid phase current density, L pos / neg is the thickness of the electrode, and I is the total current of the battery cell.

6. The method according to claim 1, characterized in that Determining the DC impedance of a target component in the secondary battery according to the electrochemical model, the solid phase potential, the liquid phase potential, the active ion concentration, and the interface current density distribution includes: The liquid phase ohmic resistance of each target component is determined based on the total area of ​​the positive electrode sheet, the thickness of the target component, the effective liquid phase conductivity, the liquid phase current density and the total current of the battery cell.

7. The method according to claim 6, characterized in that Determine the liquid phase ohmic resistance of each target component based on the total area of ​​the positive electrode sheet, the thickness of the target component, the effective liquid phase conductivity, the liquid phase current density, and the total current of the battery cell, including: According to the total area of ​​the positive electrode sheet, the thickness of the target component, the effective conductivity of the liquid phase, the liquid phase current density and the total current of the battery cell, the liquid phase ohmic resistance of each target component is determined according to the following formula (2): Among them, S is the total area of ​​the positive electrode, κ eff,pos / neg / sep is the effective conductivity of the liquid phase, i l is the liquid phase current density, L pos / neg / sep is the target component thickness, and I is the total current of the battery cell.

8. The method according to claim 1, characterized in that Determining the DC impedance of a target component in the secondary battery according to the electrochemical model, the solid phase potential, the liquid phase potential, the active ion concentration, and the interface current density distribution includes: The liquid phase diffusion resistance of each target component is determined based on the total area of ​​the positive electrode sheet, the thickness of the target component, the active ion migration number, the activity coefficient, the active ion concentration, the liquid phase current density and the total current of the battery cell.

9. The method according to claim 8, characterized in that Determine the liquid phase diffusion resistance of each target component based on the total area of ​​the positive electrode sheet, the thickness of the target component, the active ion migration number, the activity coefficient, the active ion concentration, the liquid phase current density, and the total current of the battery cell, including: According to the total area of ​​the positive electrode sheet, the thickness of the target component, the active ion migration number, the activity coefficient, the active ion concentration, the liquid phase current density and the total current of the battery cell, the liquid phase diffusion resistance of each target component is determined according to the following formula (3): Where S is the total area of ​​the positive electrode sheet, t + is the active ion migration number, f ± is the activity coefficient, c l is the active ion concentration, i l is the liquid phase current density, L pos / neg / sep is the target component thickness, I is the total cell current, F is the Faraday constant, R is the universal gas constant, and T is the temperature.

10. The method according to claim 1, characterized in that Determining the DC impedance of a target component in the secondary battery according to the electrochemical model, the solid phase potential, the liquid phase potential, the active ion concentration, and the interface current density distribution includes: The solid phase diffusion resistance of each electrode is determined based on the total area of ​​the positive electrode, volume specific surface area, electrode thickness, solid-liquid interface reaction current density, equilibrium electrode potential and total current of the battery cell.

11. The method according to claim 10, characterized in that According to the total area of ​​the positive electrode, volume specific surface area, electrode thickness, solid-liquid interface reaction current density, equilibrium electrode potential and total current of the battery cell, the solid phase diffusion resistance of each electrode is determined, including: According to the total area of ​​the positive electrode, volume specific surface area, electrode thickness, solid-liquid interface reaction current density, equilibrium electrode potential and total current of the battery cell, the solid phase diffusion resistance of each electrode is determined according to the following formula (4): Among them, S is the total area of ​​the positive electrode sheet, a v is the volume specific surface area, i ct is the solid-liquid interface reaction current density, E eq,ave is the equilibrium electrode potential corresponding to the average lithium ion concentration in the active material of the electrode, E eq,surf is the equilibrium electrode potential corresponding to the lithium ion concentration on the surface of the active material of the electrode, L pos / neg is the thickness of the electrode, and I is the total current of the battery cell.

12. The method according to claim 1, characterized in that Determining the DC impedance of a target component in the secondary battery according to the electrochemical model, the solid phase potential, the liquid phase potential, the active ion concentration, and the interface current density distribution includes: The solid-phase charge transfer resistance of each electrode is determined based on the total area of ​​the positive electrode, volume specific surface area, electrode thickness, solid-liquid interface reaction current density, equilibrium surface potential, solid phase potential, liquid phase potential and total current of the battery cell.

13. The method according to claim 12, characterized in that According to the total area of ​​the positive electrode, volume specific surface area, electrode thickness, solid-liquid interface reaction current density, equilibrium surface potential, solid phase potential, liquid phase potential and total current of the battery cell, the solid phase charge transfer resistance of each electrode is determined, including: According to the total area of ​​the positive electrode, volume specific surface area, electrode thickness, solid-liquid interface reaction current density, equilibrium surface potential, solid phase potential, liquid phase potential and total current of the battery cell, the solid phase charge transfer resistance of each electrode is determined according to the following formula (5): Among them, S is the total area of ​​the positive electrode sheet, a v is the volume specific surface area, i ct is the solid-liquid interface reaction current density, E eq,surf is the equilibrium electrode potential corresponding to the lithium ion concentration on the surface of the active material of the electrode, φ s is the solid phase potential, φ l is the liquid phase potential, L pos / neg is the thickness of the electrode, and I is the total current of the battery cell.

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