Method and device for testing binary diffusion coefficient of ions in electrolyte

Abstract

The invention provides a method and device for testing the binary diffusion coefficient of ions in electrolyte, and the method comprises the steps: carrying out the short circuit of a symmetric battery, guaranteeing the open-circuit voltage to be zero, carrying out the constant-current excitation-relaxation test of the short-circuited battery at a temperature, enabling the excitation current to be equal to the excitation time to be equal to the excitation time, and enabling the relaxation time to be equal to the excitation time to be equal to the excitation time. The relaxation time is that the relaxation voltage of the battery is sampled and recorded according to a specified sampling interval; and fitting the relaxation voltage according to the following formula I to obtain the ion binary diffusion coefficient in the electrolyte. According to the method, the diffusion coefficient can be obtained according to fitting only by correcting the tortuosity of the diaphragm and measuring the relaxation voltage, and other parameters do not need to be additionally tested and calibrated; in addition, a more accurate diffusion coefficient can be obtained, the excitation amplitude and time required for establishing a steady state do not need to be repeatedly adjusted, and the operation is simple and convenient.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to a method and apparatus for testing the binary diffusion coefficient of ions in an electrolyte. Background Technology

[0002] Electrochemical testing technology primarily studies the advantages of electrodes or batteries, the variation of current under transient and steady-state excitation conditions, and measures the corresponding electrode and electrolyte kinetic parameters. Generally, testing methods can be broadly categorized into two types: time-domain and frequency-domain methods. Time-domain methods utilize measurement data acquired in the time domain, such as charge / discharge curves or pulse test data; frequency-domain methods typically refer to electrochemical impedance spectroscopy, where battery impedance is measured as a function of the excitation signal frequency. Due to the different measurement principles, the identification and accuracy of parameters also differ. The acquisition of key kinetic parameters in lithium-ion battery simulation models is usually conducted on two levels: 1) measurement of the kinetic parameters of the battery electrode active particles, including the lithium diffusion coefficient and liquid-solid interface reaction constant; 2) measurement of the electrolyte characteristic kinetic parameters, including liquid phase conductivity, liquid phase diffusion coefficient, ion transfer number, and thermodynamic factor.

[0003] Currently, several methods exist in this field for determining the diffusion coefficient. For example, Chinese patent application CN118671169A discloses a method that applies a potential to a battery using a constant voltage excitation (constant voltage method) until a stable concentration gradient is established in the system (i.e., steady state is reached). The excitation is then interrupted, and the change in current over time during relaxation is monitored. The diffusion coefficient is calculated by fitting the relationship between the relaxation current and time. However, such methods have significant drawbacks in practical applications: First, it is difficult to confirm whether a steady state has truly been established, which directly determines the effectiveness of the fit; second, the excitation amplitude and time required to establish a steady state need repeated adjustments, resulting in low experimental efficiency and poor repeatability; third, the determination of the initial current and transient response is unclear during the excitation interruption or measurement start stage, leading to unstable fitting results and easy introduction of systematic errors; fourth, interface phenomena (such as the formation of SEI) or small parasitic currents and noise can severely affect the reliability of the current signal, thereby reducing the credibility of the results.

[0004] In summary, there is an urgent need in this field for a new method to obtain the binary diffusion coefficient in electrolytes quickly and accurately, without relying on difficult-to-control steady-state conditions and effectively avoiding and suppressing the interference of artifacts such as parasitic currents and experimental noise. Summary of the Invention

[0005] To address the aforementioned problems, this application provides a method for determining the diffusion coefficient based on the potential relaxation behavior of a two-electrode battery, comprising applying a constant current excitation to the battery for a set time and then stopping the excitation, and recording the open-circuit potential decaying over time. Directly related to relaxation potential and time Fitting the data yields the binary diffusion coefficient. By analyzing the potential decay during the relaxation phase rather than the transient current during the excitation phase, the sensitivity to excitation initiation conditions and excitation interruption transients is significantly reduced.

[0006] To achieve the above objectives, this application adopts the following technical solution:

[0007] In a first aspect, this application provides a method for testing the binary diffusion coefficient of ions in an electrolyte, comprising short-circuiting a symmetrical cell to ensure that the open-circuit voltage is zero, and then subjecting the short-circuited symmetrical cell to a temperature... A constant current excitation-relaxation test was performed, with the excitation current being... The incentive period is The relaxation time is The relaxation voltage of the battery is sampled and recorded at specified sampling intervals. ;

[0008] The relaxation voltage The binary diffusion coefficient of ions in the electrolyte is obtained by fitting the following formula. :

[0009] ,

[0010] in, It represents the sum of the stoichiometric coefficients of anions and cations. Indicates the number of cations charged. Represents the stoichiometric coefficient of cations. The gas constant is It is Faraday's constant. Thermodynamic temperature Indicates the initial electrolyte salt concentration. This indicates that the electrolyte concentration is The cation transference number at that time This indicates that the electrolyte salt concentration is Thermodynamic factors at time The concentration of the electrolyte is Logarithmic concentration gradient of thermodynamic factor This represents the electrolyte salt concentration difference between the positive and negative electrodes of the battery when the current excitation is interrupted. It is pi. Indicates the tortuosity of the diaphragm. The total thickness of the separator used in the symmetrical battery during constant current excitation-relaxation testing. Let n be the concentration of the electrolyte metal salt, and n be the degree of the infinite series.

[0011] Among them, the total thickness of the separator used in the symmetrical battery during the constant current excitation-relaxation test. The unit is All concentrations are expressed in mol / L, temperatures in K, and times (t) in seconds.

[0012] In some implementations, the excitation time ≥30 s, the excitation current is >0.2 The relaxation time The specified sampling interval is <10s. To estimate the diffusion coefficient.

[0013] in, The value is generally taken as 9E-9. .

[0014] In some implementations, a symmetrical cell with a multilayer separator is used in the constant current excitation-relaxation test, wherein the multilayer separator has a thickness range of 50-150 μm.

[0015] In some embodiments, the tortuosity Through Ohmic impedance and diaphragm thickness The following formula (II) was used for fitting: ,in, For the membrane porosity, For the resistance value of mechanical components, To test a specified temperature The electrolyte conductivity with the diffusion coefficient to be measured The area of ​​the metal sheet in a symmetrical battery is... The thickness of the separator used in symmetrical batteries during tortuosity testing.

[0016] The unit for diaphragm porosity is %, and the unit for mechanical component resistance is %. The unit of electrolyte conductivity is . The unit for the area of ​​the battery metal sheet is .

[0017] In some embodiments, the tortuosity test employs at least three sets of symmetrical cells with different numbers of separator layers, and the ohmic impedance... By using a specified temperature The following results were obtained by performing an AC impedance spectroscopy (EIS) test on the battery.

[0018] In some embodiments, the area of ​​the battery metal sheet is >110 mm². 2 It is understandable that the area of ​​the battery metal sheet at this time refers to, for example, the area of ​​the lithium sheet in a lithium battery or the area of ​​the sodium sheet in a sodium battery.

[0019] In some embodiments, the electrolyte includes a lithium-ion electrolyte or a sodium-ion electrolyte.

[0020] In some embodiments, the symmetrical battery is a lithium symmetrical button cell, and the structure of the lithium symmetrical button cell includes a lower casing, a lower stainless steel gasket, a lithium metal sheet, a separator with different layers, a lithium metal sheet, an upper stainless steel gasket, and an upper casing.

[0021] In some embodiments, the diameters of the components in the lithium symmetric coin cell satisfy the following relationship: separator > stainless steel gasket > lithium metal sheet.

[0022] This application also provides a testing device for the binary diffusion coefficient of ions in the aforementioned electrolyte, the testing device including a conductivity testing component, an EIS testing component, a temperature control component, an electrochemical workstation, and a plotting and calculation component.

[0023] This application has the following beneficial effects:

[0024] The first aspect of this application provides a method for testing the binary diffusion coefficient of ions in an electrolyte. The core steps of this testing method include applying a constant current excitation to the battery for a set time and then stopping the excitation, and recording the open circuit potential that decays over time. Directly related to relaxation potential and time Fitting the data yields the binary diffusion coefficient. The principle behind this method is to use particle swarm optimization (PSO) or least squares method to fit the relationship between relaxation voltage and relaxation time to determine the diffusion coefficient. Essentially, it combines a physical model of the relaxation process (diffusion control dynamics) with optimization algorithms. Specifically, based on the physical mechanism of voltage change during relaxation (diffusion control), the theoretical relationship between voltage and time (Formula I) is derived, where the diffusion coefficient is the parameter to be determined. The PSO algorithm and other algorithms are then used to optimize the diffusion coefficient, minimizing the fitting error between the theoretical curve and experimental data, thus obtaining the optimal solution for the diffusion coefficient. The significant advantages of this method are: 1) Only the diaphragm tortuosity needs to be corrected and the relaxation voltage measured to obtain the diffusion coefficient from the fitted data, without the need for additional testing and calibration of other parameters; 2) A more accurate analytical expression for the relationship between relaxation voltage and relaxation time is established, without approximation, resulting in a more accurate diffusion coefficient; 3) Through current excitation, long-term relaxation establishes a steady state, eliminating the need for repeated adjustments to the excitation amplitude and time required to establish a steady state, making the operation simple and convenient.

[0025] The second aspect of this application also provides a testing device for testing the binary diffusion coefficient of ions in the aforementioned electrolyte. This device integrates conductivity and EIS multi-parameter testing, precise temperature control, and data calculation functions, and can efficiently and stably determine the binary diffusion coefficient of ions in the electrolyte. Attached Figure Description

[0026] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the description of the embodiments of this application will be briefly introduced below.

[0027] Figure 1 These are the electrochemical impedance spectroscopy curves obtained from tests at different total membrane thicknesses in Example 1 of this application.

[0028] Figure 2 The results of the ohmic resistance and diaphragm thickness obtained by least squares fitting in Example 1 of this application are the results of the least squares fitting.

[0029] Figure 3 This is a graph showing the voltage, current, and time relationship of the constant current excitation test in Embodiment 1 of this application.

[0030] Figure 4 This is a graph showing the fitting results of relaxation voltage and relaxation time t2 in Embodiment 1 of this application.

[0031] Figure 5 This is a graph showing the voltage, current, and time relationship of the constant current excitation test in Embodiment 2 of this application.

[0032] Figure 6 This is a graph showing the fitting results of relaxation voltage and relaxation time t2 in Embodiment 2 of this application.

[0033] Figure 7 This is a graph showing the voltage, current, and time relationship of the constant current excitation test in Embodiment 3 of this application.

[0034] Figure 8 This is a graph showing the fitting results of relaxation voltage and relaxation time t2 in Embodiment 3 of this application.

[0035] Figure 9 This is a schematic diagram of the symmetrical lithium metal battery assembly of this application. Detailed Implementation

[0036] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application. It should be understood that the specific embodiments described are merely used to explain this application and are not intended to limit this application.

[0037] In the description of the embodiments of this application, it should be noted that all ranges disclosed in this application are to be understood to encompass any and all subranges included therein. For example, the stated range "10-18 kV" should be considered to include any and all subranges that begin with a minimum value of 10 kV or greater and end with a maximum value of 18 kV or less, such as 10 to 12 kV, or 13 to 15 kV, or 12 to 18 kV. Furthermore, all ranges disclosed in this application are also considered to include the endpoints of the ranges, unless otherwise explicitly stated. For example, ranges "between 13 and 15," "13 to 15," or "13-15" should generally be considered to include the endpoints 13 and 15.

[0038] In the description of this embodiment, it should be understood that the orientation descriptions, such as up, down, front, back, left, right, etc., are based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this embodiment and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this embodiment.

[0039] In the description of this embodiment, "several" means one or more, "multiple" means two or more, "greater than," "less than," and "exceeding" are understood to exclude the stated number, while "above," "below," and "within" are understood to include the stated number. The use of "first" and "second" in the description is merely for distinguishing technical features and should not be construed as indicating or implying relative importance, or implicitly indicating the number of indicated technical features, or implicitly indicating the order of the indicated technical features.

[0040] In the description of this embodiment, unless otherwise explicitly limited, terms such as setting, installing, and connecting should be interpreted broadly, and those skilled in the art can reasonably determine the specific meaning of the above terms in this embodiment in conjunction with the specific content of the technical solution.

[0041] This application addresses the significant shortcomings of existing methods for determining diffusion coefficients in practical applications. Primarily, they struggle to confirm the establishment of a true steady state, necessitating repeated adjustments to the excitation amplitude and time required for steady-state establishment, resulting in low efficiency and poor repeatability. Furthermore, the unclear determination of initial current and transient response during excitation interruption or measurement initiation leads to unstable fitting results and introduces systematic errors. Changes in interface phenomena during current flow can severely impact the reliability of the current signal, reducing the feasibility of the results. Therefore, there is an urgent need for a novel method to rapidly and accurately obtain key transport parameters of electrolytes and measure the binary diffusion coefficient of electrolytes, independent of uncontrollable steady-state conditions and capable of effectively avoiding and suppressing interference from parasitic currents, experimental noise, and other artifacts.

[0042] This application proposes a diffusion coefficient testing method based on the potential relaxation behavior of a two-electrode battery. The core steps of this method are: applying a constant current excitation to the battery for a set time and then stopping the excitation, recording the open-circuit potential that decays over time. Regarding the relaxation potential and time The binary diffusion coefficient was obtained by fitting. The principle behind this method is to use particle swarm optimization (PSO) or least squares method to fit the relationship between relaxation voltage and relaxation time to determine the diffusion coefficient. Essentially, it combines a physical model of the relaxation process—diffusion control dynamics—with optimization algorithms. Specifically, based on the physical mechanism of voltage change during relaxation (diffusion control), the theoretical relationship between voltage and time (Formula I) is derived, where the diffusion coefficient is the parameter to be determined. The PSO algorithm and other algorithms are then used to optimize the diffusion coefficient, minimizing the fitting error between the theoretical curve and experimental data, thus obtaining the optimal solution for the diffusion coefficient. The significant advantages of this method include: first, only the diaphragm tortuosity needs to be corrected and the relaxation voltage measured to obtain the diffusion coefficient from the fitted data, without the need for additional testing and calibration of other parameters; second, it establishes a more accurate analytical expression for the relationship between relaxation voltage and relaxation time, without approximation, resulting in a more precise diffusion coefficient; and third, it establishes a steady state through current excitation and long-term relaxation, eliminating the need for repeated adjustments to the excitation amplitude and time required to establish a steady state, making the operation simple and convenient.

[0043] To achieve the above objectives, this application adopts the following technical solution:

[0044] This application provides a method for testing the binary diffusion coefficient of ions in an electrolyte, including short-circuiting a symmetrical cell to ensure that the open-circuit voltage is zero, and then subjecting the short-circuited symmetrical cell to a temperature... A constant current excitation-relaxation test was performed, with the excitation current being... The incentive period is The relaxation time is The relaxation voltage of the battery is sampled and recorded at specified sampling intervals. .

[0045] It should be noted that shorting the symmetrical cells to ensure that the open-circuit voltage is zero is intended to eliminate parasitic reactions in subsequent testing and improve testing accuracy.

[0046] The relaxation voltage The binary diffusion coefficient of ions in the electrolyte is obtained by fitting the following formula. :

[0047] ,in, It represents the sum of the stoichiometric coefficients of anions and cations. Indicates the number of cations charged. Represents the stoichiometric coefficient of cations. The gas constant is It is Faraday's constant. Thermodynamic temperature Indicates the initial electrolyte salt concentration. This indicates that the electrolyte concentration is The cation transference number at that time This indicates that the electrolyte salt concentration is Thermodynamic factors at time The concentration of the electrolyte is Logarithmic concentration gradient of thermodynamic factor This represents the electrolyte salt concentration difference between the positive and negative electrodes of the battery when the current excitation is interrupted. It is pi. Indicates the tortuosity of the diaphragm. The total thickness of the separator used in the symmetrical battery during constant current excitation-relaxation testing. Let n be the concentration of the electrolyte metal salt, and n be the degree of the infinite series.

[0048] Among them, the total thickness of the separator used in the symmetrical battery during the constant current excitation-relaxation test. The unit is All units for concentration are... Temperature is measured in Kelvin (K), and time (t) is measured in seconds (s).

[0049] It should be noted that, , , No precise numerical values ​​are required; only initial values ​​on the order of magnitude of the fitted parameters are needed. Typically, these initial values ​​are: , , The specific numerical value can be obtained by fitting the algorithm through optimization.

[0050] Understandable In electrochemistry, the term is usually expressed as "interdiffusion coefficient" or "binary diffusion coefficient," which is different from the self-diffusion coefficient of a single ion. The self-diffusion coefficient is the diffusion of a single ion within its own solution, while the interdiffusion coefficient (binary diffusion coefficient) is the coefficient when two ions diffuse together, taking into account their interaction and reflecting the diffusion capacity of the entire system. This indicates the initial electrolyte salt concentration. This refers to a binary ionic electrolyte system, such as Li⁺ / PF6⁻, where the electrolyte concentration is... The interdiffusion coefficient, which is the coefficient of co-diffusion of two ions due to the concentration gradient, reflects the overall ion transport capacity of the binary electrolyte caused by the concentration difference.

[0051] In some implementations, the excitation time ≥30 s, the excitation current is >0.2 The relaxation time The specified sampling interval is <10s. To estimate the diffusion coefficient.

[0052] The excitation time can be any value or a range of any two values ​​between 30s, 40s, 50s, 60s, 70s, and 80s.

[0053] It should be noted that, among them, Generally 1E-10 To establish reliable steady-state conditions, further... The specific value can be 9E-10 .

[0054] In some possible implementations, the relaxation time can be any value of 400s, 600s, and 800s, or a range of any two values.

[0055] In some possible implementations, the excitation current can be 0.22. 0.44 0.50 and 0.60 The range of any one or any two values.

[0056] In some possible implementations, the specified sampling interval can be any value or a range of any two values ​​from 1s, 2s, 3s, 4s, 5s, 6s, 7s, 8s, and 9s.

[0057] In some embodiments, the symmetrical battery is a coin cell symmetrical battery including multiple layers of separators, and further, the thickness of the multiple layers of separators is in the range of 50-150 μm.

[0058] It should be noted that a multi-layered symmetrical cell with a separator is used in the constant current excitation test to facilitate the establishment of a stable lithium salt concentration gradient in the electrolyte, thereby improving test accuracy. In theory, increasing the number of separator layers during the excitation-relaxation test should not affect the test results. However, too many layers can affect the assembly accuracy of the coin cell, while too few layers will result in a very small voltage difference in steady state, approaching the lower limit of the measuring instrument. In this case, instrument noise will lead to a decrease in accuracy. Therefore, it is necessary to control the number of separator layers.

[0059] In some embodiments, the tortuosity Through Ohmic impedance and diaphragm thickness The following formula (II) was used for fitting: ,in, For the membrane porosity, For the resistance value of mechanical components, To test a specified temperature The electrolyte conductivity with the diffusion coefficient to be measured The area of ​​the metal sheet in a symmetrical battery is... The thickness of the separator used in symmetrical batteries during tortuosity testing.

[0060] The unit for diaphragm porosity is %, and the unit for mechanical component resistance is %. The unit of electrolyte conductivity is . The unit for the area of ​​the battery metal sheet is .

[0061] In some embodiments, the tortuosity test employs at least three sets of symmetrical cells with different numbers of separators.

[0062] It should be noted that using at least three sets of symmetrical cells with different numbers of separators is intended to eliminate the error caused by the mechanical resistance when directly calculating the tortuosity of the separator.

[0063] Furthermore, at least three types of membrane layers should be selected, such as membranes with 1, 2, 3, 4, and 5 layers.

[0064] It should be noted that in this application, the thickness of the single-layer membrane can be 6-20 μm, for example, it can be any value or any two values ​​of 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm and 20 μm.

[0065] In some embodiments, the ohmic impedance By using a specified temperature The results were obtained by performing an AC impedance spectroscopy (EIS) test on the battery, with the test frequency range being 100 kHz to 0.1 mHz.

[0066] It should be noted that the results were obtained through battery EIS testing. Then, based on the known diaphragm thickness... , The result was obtained by fitting the formula II. and .

[0067] In some embodiments, the area of ​​the battery metal sheet is >110 mm². 2 For example, it can be 150mm 2 200mm 2 227mm2 250mm 2 300mm 2 400mm 2 and 500mm 2 The range of any one or any two values ​​in the range.

[0068] In some possible implementations, the symmetrical battery is a lithium symmetrical coin cell, and the structure of the lithium symmetrical coin cell includes a lower casing, a lower stainless steel gasket, a lithium metal sheet, a separator with different layers, a lithium metal sheet, an upper stainless steel gasket, and an upper casing.

[0069] It should be noted that during battery packaging, it is generally necessary to control the liquid injection volume and packaging pressure to ensure battery consistency. Essentially, this involves controlling the uniformity of key process parameters to reduce initial differences within the same batch of batteries, making them as similar as possible in structure, composition, and performance. Here, "consistency" does not mean absolute equality, but rather a high degree of similarity within reasonable tolerances allowed by the process.

[0070] It is understandable that the assembly of a lithium symmetric coin cell involves: assembling lithium metal sheets into coin half-cells, using lithium hexafluorophosphate (LiPF6) as the electrolyte, a mixed solution of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) as the solvent, and using a common separator to separate the lithium metal sheets and encapsulate them in a coin cell case to obtain a coin half-cell.

[0071] In some possible implementations, the membrane is typically made of a material suitable for the battery industry, such as any one of polypropylene (PP), polyethylene (PE), glass fiber, and aramid.

[0072] In some possible implementations, the diameters of the components in the lithium symmetric coin cell satisfy the following relationship: separator > stainless steel gasket > lithium metal sheet.

[0073] In some possible implementations, the diameter of the diaphragm can be in the range of 14-22 mm, for example, any value or a range of any two of 14 mm, 15 mm, 16 mm, 17 mm, 18 mm, 19 mm, 20 mm, 21 mm and 22 mm.

[0074] The diameter of stainless steel gaskets can range from 12 to 20 mm, for example, any value or any two of 12 mm, 13 mm, 14 mm, 15 mm, 16 mm, 17 mm, 18 mm, 19 mm and 20 mm.

[0075] The diameter of the lithium metal sheet can be in the range of 10-18 mm, for example, any value or any two values ​​of 10 mm, 11 mm, 12 mm, 13 mm, 14 mm, 15 mm, 16 mm, 17 mm and 18 mm.

[0076] In some embodiments, the electrolyte includes a lithium-ion electrolyte or a sodium-ion electrolyte.

[0077] In some possible embodiments, the lithium-ion electrolyte includes a lithium salt and a solvent. Further, the lithium salt may be selected from one or more of LiPF6, lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The solvent is generally selected from one or more of EC, EMC, and diethyl carbonate (DEC), for example, it may be a mixture of EC and EMC in a volume ratio of 3:7.

[0078] In some possible embodiments, the sodium ion electrolyte comprises a sodium salt and a solvent. Further, the sodium salt may be selected from one or more of sodium hexafluorophosphate (NaPF6), sodium perchlorate (NaClO4), and sodium difluorosulfonamide (NaFSI), and the solvent is generally selected from one or more of EC, propylene carbonate (PC), and DEC.

[0079] In some possible implementations, the concentration of the initial electrolyte salt can be 0.1-3 mol / L, for example, any value or a range of any two of 0.1 mol / L, 0.5 mol / L, 1.0 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L and 3 mol / L.

[0080] In some implementations, the fitting method may employ the least squares algorithm or the particle swarm optimization algorithm.

[0081] This application also provides a testing apparatus for the aforementioned method of testing the binary diffusion coefficient of ions in an electrolyte, the testing apparatus comprising a conductivity testing component, an EIS testing component, a temperature control component, an electrochemical workstation, and a plotting and calculation component.

[0082] Understandably, conductivity testing kits are used to accurately measure the conductivity of electrolytes.

[0083] The EIS test kit is used to provide high-precision ohmic resistance values.

[0084] Temperature control components are used to maintain a constant temperature environment.

[0085] The electrochemical workstation is used to apply a short constant current pulse. After the current pulse ends, the electrochemical workstation cuts off the current and monitors the relaxation curve of the open-circuit voltage decaying over time with high precision.

[0086] The plotting and calculation component is used to process voltage-time data, and the calculations are based on different diffusion model formulas.

[0087] The following examples will further illustrate this application.

[0088] All raw materials used in the embodiments of this application are commercially available.

[0089] Example 1

[0090] This embodiment 1 provides a method for testing the binary diffusion coefficient of ions in an electrolyte, specifically including the following steps:

[0091] The electrolyte solution to be tested had a volume ratio of EC:EMC = 3:7, the solute was LiPF6, and the concentration was 1 mol / L. The test temperature was... The temperature is 25℃.

[0092] The conductivity of the electrolyte was measured to be 0.84 S / m using a conductivity meter.

[0093] The lithium metal button cell has a lithium sheet diameter of 17mm and an area A of 2.27E-4m². 2 The diaphragm diameter is 20mm, the stainless steel gasket diameter is 18mm, the diaphragm thickness is 12um, and the diaphragm porosity is... ,according to Figure 9 The symmetrical lithium metal battery assembly sequence is used to assemble a lithium metal coin cell symmetrical battery.

[0094] The number of separator layers in the symmetrical lithium metal batteries used for the tortuosity test were 1, 2, 3, 4, and 5, respectively, and the total thickness of the separator was... The values ​​are 12, 24, 36, 48, and 60 μm, respectively.

[0095] The EIS test frequency was 100kHz-0.1mHz, and the EIS test results for different total diaphragm thicknesses were obtained as follows: Figure 1 The curves shown are used to extract ohmic resistance values, which are listed in Table 1. It can be observed that the greater the total thickness of the diaphragm, the greater the ohmic resistance; and the smaller the thickness, the smaller the resistance.

[0096] The ohmic resistance and diaphragm thickness obtained by fitting using the least squares method according to Formula II are shown below. Figure 2 ,get .

[0097] The parameters of the symmetric lithium metal battery used for constant current excitation testing are: 10 separator layers, total thickness of 120 μm, and excitation current of [value missing]. The incentive period is The relaxation time is The sampling interval for the relaxation process is 5 seconds.

[0098] The voltage, current, and time relationship graph obtained from the test is shown below. Figure 3 .

[0099] The fitting results of the relaxation voltage and relaxation time t2 using the least squares method according to Formula I are shown below. Figure 4 The diffusion coefficient was found to be 2.62E-10m. 2 / s.

[0100] Example 2

[0101] This embodiment 2 provides a method for testing the binary diffusion coefficient of ions in an electrolyte, which differs from embodiment 1 only in that:

[0102] The parameters of the symmetric lithium metal battery used for constant current excitation testing are: 8 separator layers, total thickness of 96 μm, and excitation current of [value missing]. The incentive period is The relaxation time is The sampling interval for the relaxation process is 1 second.

[0103] Other settings are the same as in Example 1.

[0104] The voltage, current, and time relationship graph obtained from the test is shown below. Figure 5 .

[0105] Using the particle swarm optimization algorithm, with Equation I as the objective function, the relaxation voltage and relaxation time are fitted. The fitting results are shown in Figure 6 The diffusion coefficient was found to be 2.65E-10m. 2 / s.

[0106] Example 3

[0107] This embodiment 3 provides a method for testing the binary diffusion coefficient of ions in an electrolyte, which differs from embodiment 1 only in that:

[0108] The parameters of the symmetric lithium metal battery used for constant current excitation testing are as follows: 10 separator layers, total thickness 120 μm, LiPF6 electrolyte concentration 2 mol / L, and excitation current. The incentive period is The relaxation time is The sampling interval for the relaxation process is 2 seconds.

[0109] Other settings are the same as in Example 1.

[0110] The voltage, current, and time relationship graph obtained from the test is shown below. Figure 7 .

[0111] The relaxation voltage and relaxation time are fitted using the least squares method according to Formula I. The fitting results are shown in Figure 8The diffusion coefficient was found to be 1.39E-10m. 2 / s.

[0112] Ohmic resistance value extracted by EIS test in Example 1 See Table 1.

[0113] Table 1

[0114] 12 24 36 48 60 2.461767 2.69775 2.93385 3.169909 3.40591

[0115] from Figure 2 As can be seen from the graph, the horizontal axis represents the diaphragm thickness, and the vertical axis represents the ohmic resistance. The ohmic resistance values ​​extracted through experimental testing can be found in [reference needed]. Figure 2 The hollow circles in the diagram represent experimental data. Assuming diaphragm thickness is the independent variable and ohmic resistance is the dependent variable, a linear fit is obtained using the least squares method to obtain the fitted straight line. From... Figure 2 It can be observed that all measured values ​​fall precisely on the fitted line, and the coefficient of determination R0 2 =1.00, indicating that the error during the experiment was extremely small and the experimental results were reliable. This verifies the rationality of the theoretical model and provides a quantitative basis for the analysis of the tortuosity of the diaphragm microstructure.

[0116] Among them, the coefficient of determination R 2 The calculation method is as follows:

[0117]

[0118] in, This represents the sum of squared residuals. This represents the total sum of squares;

[0119]

[0120]

[0121] The average of the measured values. These are the fitted values. is the measured value, and n is the number of measurements.

[0122] Figure 3 The current curve shows a progression through constant current excitation and relaxation decay stages. The voltage curve rises rapidly during the excitation stage, dominated by battery polarization, and then declines rapidly during the relaxation stage, stabilizing after the polarization effect is eliminated, reflecting the battery's dynamic electrochemical response characteristics. In terms of time, the 30s excitation and 600s relaxation time settings cover the process from transient polarization to quasi-steady state, and the 5s sampling interval effectively captures the details of voltage decay during the relaxation stage.

[0123] Figure 4The horizontal axis represents the relaxation time t2 (s), and the vertical axis represents the relaxation voltage U (V). Hollow circles represent experimental data, and the red solid line represents fitted data. The overall trend of both experimental and fitted data shows that the voltage decreases with increasing time, and most data points closely surround the fitted curve without significant deviation, indicating a good fit. The model accurately describes the change in relaxation voltage of the experimental system over time. Within the observation time range (t≤25s), the relaxation behavior of the system is dominated by the diffusion process, without interference from other unexpected kinetic processes. The obtained diffusion coefficient is reasonable and highly reliable.

[0124] pass Figure 3 , Figure 5 and Figure 7 It can be observed that during the constant current and relaxation phases of the test, differences in separator thickness and number of layers, excitation current, and excitation and relaxation times lead to significant differences in the voltage rise rate during the constant current phase and the voltage drop slope during the relaxation phase. Specifically, a thicker separator with more layers results in higher internal resistance, manifested as a more significant voltage rise during the excitation phase and a slower voltage drop during the relaxation phase; conversely, a thinner separator with fewer layers results in lower internal resistance, weaker polarization, and faster recovery. This pattern provides a direct basis for controlling the dynamic performance of the battery through separator structure design.

[0125] pass Figure 4 , Figure 6 and Figure 8 It can be observed that the experimental points are very closely distributed around the fitted curve, and the fitting effect is the best, indicating that the model can reliably explain the experimental phenomena, and the diffusion coefficient obtained by fitting is effective and reliable.

[0126] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for testing the binary diffusion coefficient of ions in an electrolyte, characterized in that, Short-circuit the symmetrical cells to ensure the open-circuit voltage is zero, and then place the short-circuited symmetrical cells at a temperature... A constant current excitation-relaxation test was performed, with the excitation current being... The incentive period is The relaxation time is The relaxation voltage of the battery is sampled and recorded at specified sampling intervals. ; The relaxation voltage The binary diffusion coefficient of ions in the electrolyte is obtained by fitting the following formula I. : ,in, It represents the sum of the stoichiometric coefficients of anions and cations. Indicates the number of cations charged. Represents the stoichiometric coefficient of cations. The gas constant is It is Faraday's constant. Thermodynamic temperature Indicates the initial electrolyte salt concentration. This indicates that the electrolyte concentration is The cation transference number at that time This indicates that the electrolyte salt concentration is Thermodynamic factors at time The concentration of the electrolyte is Logarithmic concentration gradient of thermodynamic factor This represents the electrolyte salt concentration difference between the positive and negative electrodes of the battery when the current excitation is interrupted. It is pi. Indicates the tortuosity of the diaphragm. The total thickness of the separator used in the symmetrical battery during constant current excitation-relaxation testing. Let n be the concentration of the electrolyte metal salt, and n be the degree of the infinite series.

2. The test method according to claim 1, characterized in that, The tortuosity Through Ohmic impedance and diaphragm thickness The following formula (II) was used for fitting: ,in, For the membrane porosity, For the resistance value of mechanical components, To test a specified temperature Electrolyte conductivity with measured diffusion coefficient The area of ​​the metal sheet in a symmetrical battery is... The thickness of the separator used in symmetrical batteries during tortuosity testing.

3. The test method according to claim 1, characterized in that, The excitation time ≥30 s, the excitation current is >0.2 The relaxation time The specified sampling interval is <10s. To estimate the diffusion coefficient.

4. The test method according to claim 1, characterized in that, Symmetrical cells with multilayer separators are used in constant current excitation-relaxation tests, wherein the thickness of the multilayer separators is in the range of 50-150 μm.

5. The test method according to claim 2, characterized in that, The tortuosity test uses at least three sets of symmetrical cells with different numbers of separators, and the ohmic impedance... By using a specified temperature The following results were obtained by performing an AC impedance spectroscopy (EIS) test on the battery.

6. The test method according to claim 2, characterized in that, The area of ​​the battery metal sheet is >110mm² 2 .

7. The test method according to claim 1, characterized in that, The electrolyte includes lithium-ion electrolyte or sodium-ion electrolyte.

8. The test method according to claim 1, characterized in that, The symmetrical battery is a lithium symmetrical button cell, and the structure of the lithium symmetrical button cell includes a lower shell, a lower stainless steel gasket, a lithium metal sheet, a separator with different layers, a lithium metal sheet, an upper stainless steel gasket, and an upper shell.

9. The test method according to claim 8, characterized in that, The diameters of the components in the lithium symmetric coin cell satisfy the following relationship: separator > stainless steel gasket > lithium metal sheet.

10. A testing apparatus for testing the binary diffusion coefficient of ions in the electrolyte according to any one of claims 1-9, characterized in that, The testing device includes a conductivity testing component, an EIS testing component, a temperature control component, an electrochemical workstation, and a plotting and calculation component.

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