Lithium battery solid electrolyte interface film forming quality evaluation method

Abstract

The invention discloses a lithium battery solid electrolyte interface film forming quality evaluation method. The method comprises the following steps: in a lithium battery formation stage or a circulation process, applying an alternating current disturbance signal with an amplitude of 1-20mV, and collecting full-band electrochemical impedance spectroscopy data in real time; a relaxation time distribution method is adopted to decouple the electrochemical impedance spectrum of the intermediate frequency region; constructing an equivalent circuit model containing SEI film characteristics, and fitting the decoupled electrochemical impedance spectrum curve by the equivalent circuit model; the compactness index of the SEI film is calculated according to the SEI film resistance and the film capacitance, and whether the compactness of the SEI film reaches the standard or not is judged according to the compactness index; and calculating a lithium ion diffusion coefficient by combining the charge transfer resistance and the Warburg diffusion impedance, and evaluating the SEI membrane ion transmission efficiency. According to the invention, non-destructive monitoring is supported, quantitative evaluation of SEI membrane compactness, ionic conductivity and short circuit faults in the module can be realized, and the cycle life and safety of the lithium battery are improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium batteries, and particularly relates to a lithium battery solid electrolyte interface film formation quality evaluation method based on electrochemical impedance spectroscopy (EIS) and relaxation time distribution (DRT) analysis. BACKGROUND

[0002] The solid electrolyte interface (SEI) film is a passivation layer formed on the negative electrode surface in the first charging and discharging process of a lithium battery, and the compactness and inorganic / organic component distribution of the SEI film directly affect the lithium ion transmission efficiency and the cycle life of the battery. Traditional SEI film evaluation methods (such as scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy) need to damage the battery structure and cannot monitor the film formation dynamic process in real time. Among the commonly used non-invasive detection methods, the traditional electrochemical impedance spectroscopy (EIS) detection is simple and fast, and is suitable for continuous monitoring of the whole life cycle (from formation to aging) of the battery, but still has the following defects: 1) Model limitation: the traditional equivalent circuit (such as the Randle model) does not consider the porous layer structure and dynamic evolution characteristics of the SEI film, resulting in an error of more than 10% in parameter extraction; 2) Lack of dynamic correlation: existing methods cannot establish a dynamic mapping relationship between SEI film parameters (such as film resistance R SEI , film capacitance C SEI ) and battery performance (such as capacity decay rate and internal short circuit risk); 3) Difficulty in module-level diagnosis: the impedance coupling of multiple batteries in series and parallel in a lithium battery module makes it difficult for existing methods to accurately locate the fault single body. SUMMARY

[0003] In view of the above-mentioned defects of the prior art, the application provides a solid electrolyte interface film formation quality detection method based on electrochemical impedance spectroscopy and relaxation time distribution analysis, which realizes quantitative evaluation of the compactness and ion conductivity of the SEI film by real-time monitoring of the impedance parameter changes in the SEI film formation process, provides data support for optimizing the electrolyte formula and formation process, and improves the cycle life and safety of the lithium battery.

[0004] To achieve the above-mentioned purpose, the application provides the following technical solution: a lithium battery solid electrolyte interface film formation quality evaluation method, which comprises the following steps: S1, during the formation stage or the cycle process of the lithium battery, an alternating current disturbance signal with an amplitude of 1-20 mV is applied, and electrochemical impedance spectroscopy data in the full frequency band are collected in real time; S2, the relaxation time distribution method is used to decouple the electrochemical impedance spectroscopy in the medium frequency region, and separate the characteristic peaks corresponding to the SEI film resistance and the charge transfer resistance; S3, construct an equivalent circuit model containing SEI film characteristics, which fits the decoupled electrochemical impedance spectrum curve, extracts the element parameters corresponding to SEI film resistance, film capacitance, charge transfer resistance and Warburg diffusion impedance; S4, calculate the compactness index of SEI film according to SEI film resistance and film capacitance, and determine whether the compactness of SEI film meets the standard by using the compactness index; S5, calculate the lithium ion diffusion coefficient by combining the charge transfer resistance and the Warburg diffusion impedance, and evaluate the ion transport efficiency of the SEI film.

[0005] Further, the equivalent circuit model comprises inductance L, resistance R0, first constant phase angle element CPE1 and second constant phase angle element CPE2 connected in series; the first constant phase angle element CPE1 is connected in parallel with the first resistance R1, the second constant phase angle element CPE2 is connected in parallel with the second resistance R2, and the second resistance R2 is connected in series with the Warburg diffusion impedance Zw; The inductance L is used to represent the contact inductance in the high frequency region, that is, the curve below the real axis in the electrochemical impedance spectrum, which is a straight line nearly perpendicular to the real axis; The resistance R0 is used to represent the ohmic impedance of the lithium battery, that is, the intersection of the electrochemical impedance spectrum curve and the real axis; The first constant phase angle element CPE1 is connected in parallel with the first resistance R1, which is used to represent the impedance of the solid electrolyte interface film formed at the solid-liquid interface between the negative electrode material of the lithium ion battery and the surrounding electrolyte solution; the first resistance R1 is used to represent the resistance of the solid electrolyte interface film, and the first constant phase angle element CPE1 is used to represent the film capacitance of the solid electrolyte interface film, and this parallel structure corresponds to the semicircle in the medium-high frequency region of the electrochemical impedance spectrum curve; The second resistance R2 is connected in parallel with the second constant phase angle element CPE2, which is used to represent the reaction impedance between the electrolyte solution and the positive and negative electrodes of the lithium ion battery; the second resistance R2 is used to represent the charge transfer resistance of the interface reaction, and the second constant phase angle element CPE2 is used to represent the double-layer capacitance of the solid electrode, and this parallel structure corresponds to the semicircle in the medium-low frequency region of the electrochemical impedance spectrum curve; Further, the Warburg diffusion impedance Zw is used to represent the impedance formed by the diffusion process of lithium ions in the solid active material due to concentration polarization in the lithium ion battery, which corresponds to the straight line in the low frequency region of the electrochemical impedance spectrum curve.

[0006] Further, the step of decoupling the medium frequency region electrochemical impedance spectrum by using the relaxation time distribution method comprises: Pretreatment of electrochemical impedance spectrum data, eliminating high frequency inductance and low frequency diffusion noise; Based on the regularization parameter , the relaxation time distribution function is calculated ; SEI film resistance R is calculated by integral formula SEI and charge transfer resistance R ct , SEI film resistance R is realized SEI and charge transfer resistance R ct Corresponding characteristic peak separation: , .

[0007] Further, by dynamically adjusting the concentration of fluoroethylene carbonate in the electrolyte, the compactness and ionic conductivity of the SEI film are optimized.

[0008] Further, the compactness index of the SEI film is <20Ω·μF -1 , it is judged that the compactness of the SEI film meets the standard, and the smaller the compactness index, the lower the porosity of the SEI film.

[0009] Further, based on the Levenberg-Marquardt algorithm to fit the electrochemical impedance spectrum curve, the parameter error is <5%.

[0010] Further, the lithium ion diffusion coefficient D is calculated by combining the charge transfer resistance and Warburg diffusion impedance parameters

[0011] Among them, R is the gas constant, T is the temperature, A is the electrode area, n is the number of reaction electrons, F is the Faraday constant, C is the lithium ion concentration, is the Warburg coefficient.

[0012] Further, when the method is used for short circuit fault diagnosis in a lithium battery module, a fault coefficient k=(R ct / R SEI ) / (R ct0 / R SEI0 ) is defined, and when k exceeds the threshold value, it is judged that there is a fault cell, wherein R SEI is the SEI film resistance, R ct is the charge transfer resistance, R SEI0 is the initial SEI film resistance, which is used as a reference baseline for evaluating the subsequent quality change of the SEI film; R ct0 is the initial charge transfer resistance, which is used as a reference baseline for evaluating the subsequent change of electrochemical reaction kinetics.

[0013] Further, the series module fault coefficient When the fault coefficient k>k s , a fault cell is determined to exist; the fault coefficient of the parallel module When the fault coefficient k>k p , a fault cell is determined to exist; m and n are respectively the number of batteries in the series module and the parallel module and are ≤8.

[0014] Further, the detection temperature of the lithium battery is 25 DEG C.

[0015] Compared with the prior art, the present application has the beneficial effects of: The present application can realize real-time monitoring of the impedance parameter change in the SEI film formation process, and realize quantitative evaluation of the compactness and ion conductivity of the SEI film in combination with the equivalent circuit model, so as to provide data support for optimization of the electrolyte formula and formation process, and improve the cycle life and safety of the lithium battery.

[0016] The present application can realize real-time tracking of the dynamic formation process of the SEI film, avoid battery disassembly, realize non-destructive monitoring, reduce the DRT decoupling error from 10% to 3%, realize high-precision parameter extraction, and shorten the formula development cycle through the dynamic correlation of the compactness index (i.e., DI=R SEI / C SEI ) and the electrolyte composition, and realize fault cell identification accuracy >98% in module identification. BRIEF DESCRIPTION OF DRAWINGS

[0017] To make the technical solutions in the specific embodiments or the prior art clearer, the accompanying drawings needed in the following specific embodiments or prior art description will be briefly introduced. Obviously, the accompanying drawings in the following description are some embodiments of the present application, and other accompanying drawings can be obtained by those skilled in the art without any creative effort.

[0018] Figure 1 is a flowchart of the lithium battery solid-state electrolyte interface film quality detection method of the present application; Figure 2 is a flowchart of the lithium battery relaxation time distribution decoupling of the present application; Figure 3 is a structural schematic diagram of the equivalent circuit model of the lithium battery of the present application. DETAILED DESCRIPTION

[0019] The specific embodiments of the present application will be described in detail below with reference to the accompanying drawings. Although the specific embodiments of the present application are shown in the accompanying drawings, it should be understood that the present application can be implemented in various forms and should not be limited by the embodiments described herein. On the contrary, these embodiments are provided to enable a more thorough understanding of the present application and to fully convey the scope of the present application to those skilled in the art.

[0020] For the convenience of understanding the embodiments of the present application, the following will be further explained by examples in conjunction with the drawings of the specification, and each drawing does not constitute a limitation on the embodiments of the present application.

[0021] As shown in the figure, the present embodiment provides a lithium battery solid electrolyte interface film quality detection method based on electrochemical impedance spectroscopy and relaxation time distribution analysis, and the steps are as follows: Figure 1 S1, during the formation stage or the cycling process of the lithium battery, an alternating current disturbance signal with an amplitude of 1-20 mV is applied, and the electrochemical impedance spectroscopy data of the full frequency band is collected in real time.

[0022] During the formation stage or the cycling process of the lithium battery, an alternating current signal with an amplitude of 1-20 mV is applied, and the EIS data of the full frequency band (0.01 Hz-1 MHz) is collected.

[0023] S2, the relaxation time distribution method is used to decouple the electrochemical impedance spectroscopy in the intermediate frequency band (1-1000 Hz), and separate the characteristic peaks corresponding to the SEI film resistance (R SEI ) and the charge transfer resistance (R ct ).

[0024] S3, an equivalent circuit model containing SEI film characteristics is constructed, which is used to fit the decoupled electrochemical impedance spectroscopy curve, and extract the SEI film resistance, film capacitance, charge transfer resistance and Warburg diffusion impedance.

[0025] The equivalent circuit model forms: high-frequency inductance, R SEI -C SEI parallel structure (SEI film characteristics), R ct -C dl parallel structure (charge transfer process) and Z w (Warburg diffusion impedance).

[0026] The equivalent circuit model includes inductance L, resistance R0, first constant phase angle element CPE1 and second constant phase angle element CPE2 connected in series; the first constant phase angle element CPE1 is connected in parallel with the first resistance R1, the second constant phase angle element CPE2 is connected in parallel with the second resistance R2, and the second resistance R2 is connected in series with the Warburg diffusion impedance Zw; The inductance L is used to represent the contact inductance in the high-frequency band, that is, the curve below the real axis in the electrochemical impedance spectroscopy, and this part is a straight line nearly perpendicular to the real axis. The resistance R0 is used to represent the ohmic impedance of the lithium battery, that is, the intersection of the electrochemical impedance spectroscopy curve and the real axis. ​The first constant phase angle element CPE1 is connected in parallel with the first resistor R1, and is used to represent the impedance of the solid electrolyte interface film formed at the solid-liquid interface by the reaction of the lithium ion battery negative electrode material with the surrounding electrolyte solution; the first resistor R1 is used to represent the resistance of the solid electrolyte interface film, and the first constant phase angle element CPE1 is used to represent the film capacitance of the solid electrolyte interface film, and this parallel structure corresponds to the high-frequency region semicircle in the electrochemical impedance spectrum curve. The second resistor R2 is connected in parallel with the second constant phase angle element CPE2, and is used to represent the reaction impedance between the electrolyte solution and the positive and negative electrodes of the lithium ion battery; the second resistor R2 is used to represent the charge transfer resistance of the interface reaction, and the second constant phase angle element CPE2 is used to represent the double-layer capacitance of the solid electrode, and this parallel structure corresponds to the low-frequency region semicircle in the electrochemical impedance spectrum curve. The Warburg diffusion impedance Zw is used to represent the impedance formed by the diffusion process of lithium ions in the solid active material due to concentration polarization in the lithium ion battery, and corresponds to the straight line in the low-frequency region of the electrochemical impedance spectrum curve.

[0027] As shown in Figure 2 , the step of decoupling the intermediate frequency region electrochemical impedance spectrum by using the relaxation time distribution method includes: Pretreatment of electrochemical impedance spectrum data, eliminating high-frequency inductance and low-frequency diffusion noise; Based on the regularization parameter =10 -5 , the relaxation time distribution function , is calculated as the relaxation time; The SEI film resistance R SEI and the charge transfer resistance R ct are calculated by the integral formula, the SEI film resistance R SEI and the charge transfer resistance R ct corresponding to the characteristic peak separation: , .

[0028] By dynamically adjusting the concentration of fluoroethylene carbonate in the electrolyte, the compactness and ion conductivity of the SEI film are optimized.

[0029] When the compactness index of the SEI film is less than 20Ω·μF -1 , it is determined that the compactness of the SEI film meets the standard, and the smaller the compactness index, the lower the porosity of the SEI film.

[0030] Based on the Levenberg-Marquardt algorithm, the electrochemical impedance spectrum curve is fitted, and the parameter error is less than 5%, and the objective function is to minimize the weighted residual sum of squares:

[0031] in, and These are the experimental values ​​of the real and imaginary parts of the impedance at the i-th frequency point, respectively. and Calculate the values ​​for the corresponding circuit model; and The standard deviation estimate of the experimental data is used for weighting. The convergence condition of the algorithm is set as: the objective function of two consecutive iterations... The relative change is less than 1×10 -8 Or reach the maximum number of iterations of 1000.

[0032] Calculate the lithium-ion diffusion coefficient by combining charge transfer resistance and Warburg diffusion impedance parameters. The formula is:

[0033] in, R The gas constant is... T For temperature, A Where n is the electrode area and n is the number of electrons in the reaction. F It is Faraday's constant. C Lithium ion concentration, This is the Warburg coefficient.

[0034] S4. Calculate the density index of the SEI film based on the SEI film resistance and film capacitance, and use the density index to determine whether the density of the SEI film meets the standard.

[0035] S5. The lithium-ion diffusion coefficient is calculated by combining the charge transfer resistance and Warburg diffusion impedance to evaluate the ion transport efficiency of the SEI membrane. The specific evaluation method is as follows: the calculated lithium-ion diffusion coefficient... A comparative analysis was performed with a benchmark value. The benchmark value is the typical lithium-ion diffusion coefficient of the same batch of healthy batteries under full charge (SOC=100%). When measured >0.8× When the SEI membrane exhibits excellent ion transport efficiency, it is determined that the ion transport efficiency is excellent; when 0.5× < ≤0.8× At that time, it was determined that the SEI membrane exhibited mild aging, resulting in a decrease in ion transport efficiency; when ≤0.5× When the SEI film is severely aged or structurally damaged, its ion transport efficiency is significantly deteriorated. This evaluation logic is based on the fact that the lithium-ion diffusion coefficient directly reflects the migration rate of lithium ions in the SEI film and electrode materials, and is a core kinetic parameter characterizing the interfacial ion transport capability.

[0036] The method is used for diagnosing short circuit faults in a lithium battery module, and a fault coefficient k=(R ct / R SEI ) / (R ct0 / R SEI0 ) is defined, where R SEI is the SEI film resistance, R ct is the charge transfer resistance, R SEI0 is the initial SEI film resistance, which is used as a reference baseline for evaluating subsequent changes in the quality of the SEI film; and R ct0 is the initial charge transfer resistance, which is used as a reference baseline for evaluating subsequent changes in the kinetics of electrochemical reactions.

[0037] The fault coefficient of the series module is k s , and when the fault coefficient k>k s , it is determined that there is a fault cell; the fault coefficient of the parallel module is k p , and when the fault coefficient k>k p , it is determined that there is a fault cell; m and n are respectively the number of cells in the series module and the parallel module and are ≤8. The above embodiments constitute the complete technical solution of the present application. The present embodiment can timely obtain the state of the SEI film of the lithium battery, does not destructively test the battery, and the battery can still be used after detection, can fully obtain the kinetics and electrode interface structure information of the battery, and can more specifically and comprehensively detect the SEI film information inside the battery.

[0038] In another embodiment, as shown in FIG. 2, the DRT decoupling process includes data preprocessing, regularization calculation, and feature peak integration.

[0039] In the present embodiment, the data preprocessing is specifically to eliminate high-frequency inductance (>10 kHz) and low-frequency diffusion noise (<0.1 Hz), the regularization calculation is specifically to use the Python library “DRTtools” to set the regularization parameter Figure 2 =10 -5 , calculate the relaxation time distribution function , and in the feature peak integration, the integral interval of R SEI is from

[0040] =10 -3 s to =10 -2 s, the integral interval of R ct is from =10 -2 s to =10 -1 s, and the calculation formula is: , , ,

[0041] In another embodiment, such as Figure 3 As shown, the equivalent circuit model of the lithium battery includes: an inductor L, inductor L connected in series with a resistor R0, a first constant phase angle element CPE1 and a second constant phase angle element CPE2, the first constant phase angle element CPE1 connected in parallel with a first resistor R1, the second constant phase angle element CPE2 connected in parallel with a second resistor R2, and the second resistor R2 connected in series with an impedance Zw.

[0042] In this embodiment, the components in the equivalent circuit model are explained as follows: (1) Inductor L The inductor element L is used to characterize the contact inductance in the high-frequency region, which is the curve below the real axis in the electrochemical impedance spectroscopy; this part is a straight line almost perpendicular to the real axis. This part is caused by the contact inductance between the battery electrode and the measuring electrode. Since the position between the battery and the measuring electrode cannot be controlled during each measurement, there is a great deal of uncertainty. This invention uses an inductor L as an approximate equivalent to this part, but does not study this part in detail.

[0043] (2) Resistor R0 element The resistor R0 element is used to characterize the ohmic impedance of a lithium-ion battery, which is the intersection of the electrochemical impedance spectroscopy with the real axis. Through fitting simulation verification, the fitting result is close to the value of the intersection with the real axis. Therefore, the ohmic impedance value can be directly obtained from the electrochemical impedance spectroscopy curve without fitting simulation, simplifying the analysis of experimental results.

[0044] (3) The first resistive element R1 and the first constant phase angle element CPE1 The first resistive element R1 is connected in parallel with a constant phase angle element CPE1 to characterize the impedance of the solid electrolyte interphase (SEI) film formed at the solid-liquid interface due to the reaction between the lithium-ion battery negative electrode material and the surrounding electrolyte solution. R1 represents the resistance of the SEI film, and CPE1 represents the capacitance of the SEI film due to the "diffusion effect". This parallel structure corresponds to the mid-to-high frequency region semicircle in the electrochemical impedance spectroscopy curve, and the parameters of each element need to be obtained through fitting simulation.

[0045] (4) The second resistive element R2 and the second constant phase angle element CPE2 The second resistance element R2 and the second constant phase angle element CPE2 are used to represent the reaction impedance between the lithium ion battery electrolyte solution and the positive and negative electrodes. R2 represents the charge transfer resistance of the interface reaction, and CPE2 represents the double-layer capacitance of the solid electrode affected by the "dispersion effect". The structure is similar to R1 and CPE1, and is also a semicircle in the electrochemical impedance spectrum curve, corresponding to the medium-low frequency region. However, due to the influence of the experimental temperature and the internal structure of the battery, the semicircle in the medium-high frequency region of the electrochemical impedance spectrum measured in the experiment coincides with the semicircle in the medium-low frequency region. In order to better reflect the internal structure of the battery, the present application selects two groups of R and CPE to represent one semicircle.

[0046] (5) Warburg impedance Zw The Warburg impedance Zw is used to represent the impedance formed by the diffusion process of lithium ions in the solid-state active material due to the concentration polarization of the lithium ion battery. In the electrochemical impedance spectrum curve, it corresponds to a straight line in the low frequency region, and the angle between the straight line and the real axis is about 45°.

[0047] In addition, in this embodiment, Nova fitting software is used to fit the electrochemical impedance spectrum curve with an equivalent circuit model to determine the parameters of each element. Table 1 shows the parameter fitting results of the equivalent circuit model when the lithium ion battery is subjected to over-discharge aging cycles for 60 times at a lower limit cutoff voltage of 2.0V. As can be seen from Table 1, the error of the parameter values in the equivalent circuit model is small and within an acceptable range.

[0048] Table 1: Parameter fitting results of the equivalent circuit model when the lithium ion battery is subjected to over-discharge aging cycles for 60 times at a lower limit cutoff voltage of 2.0V

[0049] The method described in the present application is verified by combining specific embodiments.

[0050] In terms of battery configuration, the positive electrode material is selected to be LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811), the active material coating amount is 12 mg / cm 2 , the negative electrode material is selected to be artificial graphite (coating amount 8 mg / cm 2 ), the electrolyte is 1M LiPF6 in EC / DMC (volume ratio 1:1) containing 2% FEC additive, the separator is Celgard2400 with a thickness of 20μm. According to the formation process, group A: constant current charging (0.1C to 4.2V) followed by constant voltage charging until the current is <0.05C, group B: step current charging (0.1C→0.2C→0.5C, charging to 4.2V in each stage followed by constant voltage charging until the current is <0.05C).

[0051] After impedance spectroscopy testing, DRT analysis, equivalent circuit modeling, and parameter extraction, a quality evaluation system for SEI films based primarily on compactness index (DI) and ion transport efficiency was established. The results showed that the stepped-charged group B R... SEI Reduced by 27.6%, film capacitance C SEI The DI value decreased by 40.6% and increased by 22.1%, indicating that the SEI membrane was thinner but the porosity increased slightly; Group B It is 5.3 × 10 -10 cm 2 The efficiency was 152% higher than that of group A, demonstrating that the stepped charging method optimized the ion transport channels of the SEI membrane. The statistical results of the obtained parameters are shown in Table 2.

[0052] Table 2 Parameter statistics for different formation processes

[0053] The foregoing general description of the invention and its specific embodiments should not be construed as a limitation on the technical solution of the invention. Those skilled in the art, based on the disclosure of this application, can add, reduce, or combine the disclosed technical features in the foregoing general description and / or specific embodiments (including examples) without departing from the constituent elements of the invention, to form other technical solutions within the scope of protection of this application.

Claims

1. A method for evaluating the quality of interfacial film formation in lithium-ion batteries solid electrolytes, characterized in that, Including the following steps: S1, during the lithium battery formation stage or cycling process, applies an AC perturbation signal with an amplitude of 1~20mV and collects electrochemical impedance spectroscopy data across the entire frequency band in real time. S2, the relaxation time distribution method is used to decouple the electrochemical impedance spectrum in the mid-frequency region and separate the characteristic peaks corresponding to the SEI film resistance and charge transfer resistance; S3. Construct an equivalent circuit model that includes the characteristics of the SEI film. This equivalent circuit model is used to fit the decoupled electrochemical impedance spectroscopy curve to extract the component parameters corresponding to the SEI film resistance, film capacitance, charge transfer resistance and Warburg diffusion impedance. S4. Calculate the density index of the SEI film based on the SEI film resistance and film capacitance, and use the density index to determine whether the density of the SEI film meets the standard. S5. The lithium-ion diffusion coefficient is calculated by combining the charge transfer resistance and Warburg diffusion impedance to evaluate the ion transport efficiency of the SEI membrane.

2. The method for evaluating the film quality at the interface of a lithium-ion battery solid electrolyte according to claim 1, characterized in that, The equivalent circuit model includes an inductor L, a resistor R0, a first constant phase angle element CPE1, and a second constant phase angle element CPE2 connected in series. The first constant phase angle element CPE1 is connected in parallel with a first resistor R1, and the second constant phase angle element CPE2 is connected in parallel with a second resistor R2. The second resistor R2 is connected in series with a Warburg diffusion impedance Zw. The inductance L is used to characterize the contact inductance in the high-frequency region, i.e., the curve located below the real axis in the electrochemical impedance spectrum; The resistor R0 is used to characterize the ohmic impedance of the lithium battery, that is, the intersection of the electrochemical impedance spectroscopy curve and the real axis. The first constant phase angle element CPE1 is connected in parallel with the first resistor R1 to characterize the impedance of the solid electrolyte interface film formed at the solid-liquid interface by the reaction of the lithium-ion battery negative electrode material with the surrounding electrolyte solution; the first resistor R1 is used to characterize the film resistance of the solid electrolyte interface, and the first constant phase angle element CPE1 is used to characterize the film capacitance of the solid electrolyte interface film. This parallel structure corresponds to the mid-to-high frequency region semicircle in the electrochemical impedance spectroscopy curve. The second resistor R2 is connected in parallel with the second constant phase angle element CPE2 to characterize the reaction impedance between the lithium-ion battery electrolyte solution and the positive and negative electrodes; the second resistor R2 is used to characterize the charge transfer resistance of the interface reaction, and the second constant phase angle element CPE2 is used to characterize the double layer capacitance of the solid electrode. This parallel structure corresponds to the mid-to-low frequency region semicircle in the electrochemical impedance spectroscopy curve.

3. The method for evaluating the film quality at the interface of a solid electrolyte in a lithium battery according to claim 2, characterized in that, The Warburg diffusion impedance Zw is used to characterize the impedance formed by the diffusion process of lithium ions inside the solid active material due to concentration polarization in lithium-ion batteries, corresponding to the straight line in the low-frequency region of the electrochemical impedance spectroscopy curve.

4. The method for evaluating the film quality at the interface of a solid electrolyte in a lithium battery according to claim 1, characterized in that, The steps for decoupling the electrochemical impedance spectroscopy in the mid-frequency region using the relaxation time distribution method include: Preprocess the electrochemical impedance spectroscopy data to remove high-frequency inductance and low-frequency diffusion noise; Based on regularization parameters =10 -5 Calculate the relaxation time distribution function ; The SEI film resistance R is calculated using an integral formula. SEI With charge transfer resistance R ct To achieve SEI film resistance R SEI With charge transfer resistance R ct Corresponding characteristic peak separation: ， 。 5. The method for evaluating the film quality at the interface of a solid electrolyte in a lithium battery according to claim 1, characterized in that, The density and ionic conductivity of the SEI membrane were optimized by dynamically adjusting the concentration of fluoroethylene carbonate in the electrolyte.

6. The method for evaluating the film quality at the interface of a solid electrolyte in a lithium battery according to claim 1, characterized in that, The compactness index of the SEI film is <20 Ω·μF -1 When the SEI membrane is deemed to have met the density standard, it is determined that the density of the SEI membrane is within acceptable limits.

7. The method for evaluating the film quality at the interface of a solid electrolyte in a lithium battery according to claim 1, characterized in that, Electrochemical impedance spectroscopy curves were fitted using the Levenberg-Marquardt algorithm.

8. The method for evaluating the film quality at the interface of a lithium-ion battery solid electrolyte according to claim 1, characterized in that, Calculate the lithium-ion diffusion coefficient by combining charge transfer resistance and Warburg diffusion impedance parameters. The formula is: in, R The gas constant is T For temperature, A Where n is the electrode area and n is the number of electrons in the reaction. F It is Faraday's constant. C Lithium ion concentration, This is the Warburg coefficient.

9. The method for evaluating the quality of solid electrolyte interfacial film formation in lithium batteries according to claim 1, characterized in that, The method is used for diagnosing short-circuit faults within lithium battery modules. Define the failure coefficient k = (R ct / R SEI ) / (R ct0 / R SEI0 When k exceeds the threshold, a faulty unit is determined to exist. Among them, R SEI R is the SEI film resistance. ct R is the charge transfer resistor. SEI0 R represents the initial SEI film resistance, serving as a reference baseline for evaluating subsequent quality changes in the SEI film. ct0 The initial charge transfer resistance serves as a reference baseline for evaluating subsequent changes in electrochemical reaction kinetics.

10. The method for evaluating the film quality of the solid electrolyte interface in lithium batteries according to claim 9, characterized in that, Series module failure coefficient When the failure coefficient k > k s The system determines the presence of a faulty individual unit; the failure coefficient of the parallel module. When the failure coefficient k > k p The system determines if a faulty cell exists; m and n are the number of batteries in the series module and parallel module, respectively, and are ≤8.

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