A method for evaluating the interfacial film quality of solid electrolytes in lithium batteries

Abstract

This invention discloses a method for evaluating the quality of solid electrolyte interphase (SEI) film formation in lithium-ion batteries. The method includes: applying an AC perturbation signal with an amplitude of 1-20 mV during the lithium-ion battery formation stage or cycling process, and real-time acquisition of electrochemical impedance spectroscopy (EIS) data across the entire frequency band; decoupling the mid-frequency EIS using the relaxation time distribution method; constructing an equivalent circuit model incorporating SEI film characteristics, which is then fitted to the decoupled EIS curve; calculating the SEI film compactness index based on the SEI film resistance and capacitance, and using the compactness index to determine whether the SEI film compactness meets the standard; and calculating the lithium-ion diffusion coefficient by combining charge transfer resistance and Warburg diffusion impedance to evaluate the SEI film ion transport efficiency. This invention supports non-destructive monitoring and enables quantitative assessment of SEI film compactness, ion conductivity, and short-circuit faults within the module, thereby improving the cycle life and safety of lithium-ion batteries.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery technology, specifically a method for evaluating the interfacial film quality of solid electrolytes in lithium batteries based on electrochemical impedance spectroscopy (EIS) and relaxation time distribution (DRT) analysis. Background Technology

[0002] The solid electrolyte interphase (SEI) film is a passivation layer formed on the negative electrode surface during the first charge and discharge of a lithium-ion battery. Its density and the distribution of inorganic / organic components directly affect lithium-ion transport efficiency and battery cycle life. Traditional SEI film evaluation methods (such as scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy) require damaging the battery structure and cannot monitor the film formation dynamics in real time. Among currently used non-invasive detection methods, traditional electrochemical impedance spectroscopy (EIS) is simple and fast, suitable for continuous monitoring throughout the battery's entire life cycle (from formation to aging), but it still has the following drawbacks:

[0003] 1) Model limitations: Traditional equivalent circuits (such as the Randle model) do not consider the porous layer structure and dynamic evolution characteristics of the SEI film, resulting in parameter extraction errors exceeding 10%;

[0004] 2) Lack of dynamic correlation: Existing methods struggle to establish SEI film parameters (such as film resistance R). SEI Film capacitor C SEI The dynamic mapping relationship between battery performance (such as capacity decay rate and internal short circuit risk);

[0005] 3) Difficulty in module-level diagnosis: Multiple cells connected in series and parallel within a lithium battery module cause impedance coupling, and existing methods cannot accurately locate the faulty cell. Summary of the Invention

[0006] To address the shortcomings of the existing technologies, this invention provides a method for detecting the quality of solid electrolyte interfacial film (SEI) formation based on electrochemical impedance spectroscopy and relaxation time distribution analysis. By monitoring the changes in impedance parameters during the SEI film formation process in real time and combining them with an equivalent circuit model, the method achieves quantitative assessment of the SEI film's compactness and ion conductivity, providing data support for optimizing electrolyte formulations and formation processes, and improving the cycle life and safety of lithium batteries.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a method for evaluating the quality of solid electrolyte interfacial film formation in lithium batteries, comprising the following steps:

[0008] 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.

[0009] 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;

[0010] 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.

[0011] 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.

[0012] 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.

[0013] Furthermore, 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, the second constant phase angle element CPE2 is connected in parallel with a second resistor R2, and the second resistor R2 is connected in series with a Warburg diffusion impedance Zw.

[0014] The inductance L is used to characterize the contact inductance in the high-frequency region, which is the curve in the electrochemical impedance spectrum located below the real axis. This part is a straight line that is almost perpendicular to the real axis.

[0015] 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.

[0016] 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 resistance of the solid electrolyte interface film, 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.

[0017] 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-low frequency region semicircle in the electrochemical impedance spectroscopy curve.

[0018] Furthermore, 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 a lithium-ion battery, corresponding to a straight line in the low-frequency region of the electrochemical impedance spectroscopy curve.

[0019] Furthermore, the step of decoupling the electrochemical impedance spectroscopy in the mid-frequency region using the relaxation time distribution method includes:

[0020] Preprocess the electrochemical impedance spectroscopy data to remove high-frequency inductance and low-frequency diffusion noise;

[0021] Based on regularization parameters Calculate the relaxation time distribution function ;

[0022] 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:

[0023] , .

[0024] Furthermore, the density and ionic conductivity of the SEI membrane were optimized by dynamically adjusting the concentration of fluoroethylene carbonate in the electrolyte.

[0025] Furthermore, the compactness index of the SEI film is <20 Ω·μF. -1 When the density index is lower, the SEI membrane is judged to meet the standard. The lower the density index, the lower the porosity of the SEI membrane.

[0026] Furthermore, the electrochemical impedance spectroscopy curves were fitted using the Levenberg-Marquardt algorithm, and the parameter extraction error was <5%.

[0027] Furthermore, the lithium-ion diffusion coefficient was calculated by combining the charge transfer resistance and Warburg diffusion impedance parameters. The formula is:

[0028]

[0029] 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.

[0030] Furthermore, when the method is used for short-circuit fault diagnosis within a lithium battery module, the fault coefficient k is defined as k = (R ct / R SEI ) / (R ct0 / R SEI0 When k exceeds the threshold, a faulty unit is determined to exist, where 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.

[0031] Furthermore, the failure coefficient of the series module 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.

[0032] Furthermore, the detection temperature for lithium batteries is 25°C.

[0033] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0034] This invention monitors the changes in impedance parameters during the SEI film formation process in real time, and combines an equivalent circuit model to achieve quantitative assessment of the SEI film's compactness and ion conductivity, providing data support for optimizing electrolyte formulation and formation process, and improving the cycle life and safety of lithium batteries.

[0035] This invention can track the dynamic formation process of the SEI film in real time, avoiding battery disassembly and enabling non-destructive monitoring. It can reduce the DRT decoupling error from 10% to 3%, achieving high-precision parameter extraction through the compactness index (i.e., DI=R). SEI / C SEI The dynamic correlation between the electrolyte composition and the module identification process shortens the formulation development cycle and achieves a faulty cell identification accuracy of >98% in module identification. Attached Figure Description

[0036] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0037] Figure 1 This is a flowchart of the lithium battery solid electrolyte interface film quality detection method of the present invention;

[0038] Figure 2 This is a flowchart of the decoupling of the lithium battery relaxation time distribution according to the present invention;

[0039] Figure 3 This is a schematic diagram of the equivalent circuit model of the lithium battery of the present invention. Detailed Implementation

[0040] Specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings. While specific embodiments of the invention are shown in the drawings, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the invention and to fully convey the scope of the invention to those skilled in the art.

[0041] To facilitate understanding of the embodiments of the present invention, further explanations and descriptions will be provided below with reference to the accompanying drawings, and the drawings do not constitute a limitation on the embodiments of the present invention.

[0042] like Figure 1 As shown in the figure, this embodiment provides a method for detecting the quality of the solid electrolyte interface film in lithium batteries based on electrochemical impedance spectroscopy and relaxation time distribution analysis. The steps are as follows:

[0043] S1 applies an AC perturbation signal with an amplitude of 1~20mV during the lithium battery formation stage or cycling process, and collects electrochemical impedance spectroscopy data across the entire frequency band in real time.

[0044] During the lithium battery formation stage or cycling process, an AC signal with an amplitude of 1~20mV is applied to collect EIS data across the entire frequency band (0.01Hz~1MHz).

[0045] S2, the relaxation time distribution method was used to decouple the electrochemical impedance spectroscopy in the mid-frequency region (1~1000Hz) and separate the SEI film resistance (R). SEI ) and charge transfer resistance (R ct The characteristic peak corresponding to ).

[0046] 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 SEI film resistance, film capacitance, charge transfer resistance and Warburg diffusion impedance.

[0047] The equivalent circuit model is formed as follows: high-frequency inductor, R SEI -C SEI Parallel structure (SEI membrane characteristics), R ct -C dlParallel structure (charge transfer process) and Z w (Warburg diffusion impedance).

[0048] 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.

[0049] The inductance L is used to characterize the contact inductance in the high-frequency region, which is the curve in the electrochemical impedance spectrum located below the real axis. This part is a straight line that is almost perpendicular to the real axis.

[0050] 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.

[0051] 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 resistance of the solid electrolyte interface film, 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.

[0052] 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-low frequency region semicircle in the electrochemical impedance spectroscopy curve.

[0053] 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.

[0054] like Figure 2 As shown, the steps for decoupling the electrochemical impedance spectroscopy in the mid-frequency region using the relaxation time distribution method include:

[0055] Preprocess the electrochemical impedance spectroscopy data to remove high-frequency inductance and low-frequency diffusion noise;

[0056] Based on regularization parameters =10 -5 Calculate the relaxation time distribution function , This refers to the relaxation time;

[0057] 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:

[0058] , .

[0059] The density and ionic conductivity of the SEI membrane were optimized by dynamically adjusting the concentration of fluoroethylene carbonate in the electrolyte.

[0060] The compactness index of the SEI film is <20 Ω·μF -1 When the density index is lower, the SEI membrane is judged to meet the standard. The lower the density index, the lower the porosity of the SEI membrane.

[0061] Electrochemical impedance spectroscopy curves were fitted using the Levenberg-Marquardt algorithm, with parameter extraction error <5%. The objective function was to minimize the weighted sum of squared residuals.

[0062]

[0063] 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.

[0064] Calculate the lithium-ion diffusion coefficient by combining charge transfer resistance and Warburg diffusion impedance parameters. The formula is:

[0065]

[0066] 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.

[0067] 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.

[0068] 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.

[0069] When the method is used for short-circuit fault diagnosis within a lithium battery module, the fault coefficient k is defined as (R... ct / R SEI ) / (R ct0 / R SEI0 When k exceeds the threshold, a faulty unit is determined to exist, where 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.

[0070] 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.

[0071] The above embodiments constitute the complete technical solution of the present invention. This embodiment can obtain the state of the SEI film of a lithium battery in a timely manner without damaging the battery. The battery can still be used after the test. It can fully obtain the dynamics and electrode interface structure information of the battery, and can detect the internal SEI film information of the battery more specifically and comprehensively.

[0072] In another embodiment, such as Figure 2 As shown, the DRT decoupling process includes: data preprocessing, regularization calculation, and characteristic peak integration.

[0073] In this embodiment, data preprocessing specifically involves removing high-frequency inductance (>10kHz) and low-frequency diffuse noise (<0.1Hz), and regularization calculation specifically involves using the Python library "DRTtools" to set regularization parameters. =10 -5 Calculate the relaxation time distribution function In the characteristic peak integral, R SEI The integration interval is =10 -3 s to =10 -2 s, R ct Integration interval =10 -2 s to =10 -1 s, the calculation formula is:

[0074] ,

[0075] 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.

[0076] In this embodiment, the components in the equivalent circuit model are explained as follows:

[0077] (1) Inductor L

[0078] 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, it has a large degree of uncertainty. This invention uses an inductor L as an approximate equivalent to this part, but does not study this part in detail.

[0079] (2) Resistor R0 element

[0080] 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.

[0081] (3) The first resistive element R1 and the first constant phase angle element CPE1

[0082] 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.

[0083] (4) The second resistive element R2 and the second constant phase angle element CPE2

[0084] The second resistive element R2 and the second constant phase angle element CPE2 are used to characterize the reaction impedance between the electrolyte solution and the positive and negative electrodes of the lithium-ion battery. R2 represents the charge transfer resistance of the interfacial reaction, and CPE2 represents the double-layer capacitance of the solid electrode due to the "diffusion effect". This structure is similar to R1 and CPE1, and it is also a semicircle in the electrochemical impedance spectroscopy curve, corresponding to the mid-to-low frequency region. However, due to the influence of experimental temperature and the internal structure of the battery, the semicircle in the mid-to-high frequency region of the electrochemical impedance spectrum measured in this experiment coincides with the semicircle in the mid-to-low frequency region. In order to better reflect the internal structure of the battery, this invention uses two sets of R and CPE to represent one semicircle being measured.

[0085] (5) Warburg impedance Zw

[0086] The Warburg impedance (Zw) is used to characterize the impedance formed by the diffusion of lithium ions within the solid active material of a lithium-ion battery due to concentration polarization. It corresponds to a straight line in the low-frequency region of the electrochemical impedance spectroscopy curve, with an angle of approximately 45° to the real axis.

[0087] Furthermore, this embodiment uses Nova fitting software to fit the electrochemical impedance spectroscopy curves using an equivalent circuit model to determine the parameters of each component. Table 1 shows the parameter fitting results of the lithium-ion battery after 60 over-discharge aging cycles at a lower cutoff voltage of 2.0V. As can be seen from Table 1, the errors of the parameter values ​​in the equivalent circuit model are small and within an acceptable range.

[0088] Table 1. Fitting results of equivalent circuit model parameters after 60 over-discharge aging cycles with a lower cutoff voltage of 2.0V.

[0089]

[0090] The method described in this invention will be verified below with reference to specific embodiments.

[0091] In terms of battery configuration, LiNi is selected as the cathode material. 0.8 Co 0.1 Mn 0.1 O2 (NCM811), active material coating amount 12 mg / cm² 2 The negative electrode material is artificial graphite (coating amount 8 mg / cm). 2 The electrolyte was 1M LiPF6 in EC / DMC (volume ratio 1:1) containing 2% FEC additive, and the separator was Celgard 2400 with a thickness of 20μm. The cells were grouped according to the formation process: Group A: constant current charging (0.1C to 4.2V) followed by constant voltage charging until the current <0.05C; Group B: stepped current charging (0.1C→0.2C→0.5C, each stage charged to 4.2V followed by constant voltage charging until the current <0.05C).

[0092] 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.

[0093] Table 2 Parameter statistics for different formation processes

[0094]

[0095] 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 film formation quality of a lithium battery solid electrolyte interface, characterized by, The method comprises the steps of: S1, during the formation or cycling of the lithium battery, an AC perturbation signal with an amplitude of 1-20 mV is applied, and the electrochemical impedance spectrum data in the full frequency band is collected in real time; S2, the relaxation time distribution method is used to decouple the electrochemical impedance spectrum in the medium frequency band, and the characteristic peaks corresponding to the SEI film resistance and the charge transfer resistance are separated; S3, an equivalent circuit model containing the SEI film characteristics is constructed, the decoupled electrochemical impedance spectrum curve is fitted, and the element parameters corresponding to the SEI film resistance, the film capacitance, the charge transfer resistance and the Warburg diffusion impedance are extracted; S4, 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 meets the standard is judged by using the compactness index, wherein the compactness index = SEI film resistance / film capacitance; S5, the lithium ion diffusion coefficient is calculated by combining the charge transfer resistance and the Warburg diffusion impedance, and the ion transport efficiency of the SEI film is evaluated.

2. The method for evaluating the quality of the solid electrolyte interface film of a lithium battery according to claim 1, characterized by, The equivalent circuit model comprises an inductor L, a resistor R0, a first constant phase element CPE1 and a second constant phase element CPE2 connected in series; the first constant phase element CPE1 is connected in parallel with a first resistor R1, the second constant phase element CPE2 is connected in parallel with a second resistor R2, and the second resistor R2 is connected in series with a Warburg diffusion impedance Zw; The inductor 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 spectrum; The resistor 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 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 material with the surrounding electrolyte solution; the first resistor R1 is used to represent the film resistance of the solid electrolyte interface, and the first constant phase element CPE1 is used to represent the film capacitance of the solid electrolyte interface film, and this parallel structure corresponds to the high-frequency half circle in the electrochemical impedance spectrum curve; The second resistor R2 is connected in parallel with the second constant phase 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 element CPE2 is used to represent the double-layer capacitance of the solid electrode, and this parallel structure corresponds to the low-frequency half circle in the electrochemical impedance spectrum curve.

3. The method for evaluating the quality of the solid electrolyte interface film of a lithium battery according to claim 2, characterized by, 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, and corresponds to the straight line in the low frequency region of the electrochemical impedance spectrum curve.

4. The method for evaluating the quality of the solid electrolyte interface film of a lithium battery according to claim 1, characterized by, The step of decoupling the electrochemical impedance spectrum in the medium frequency band by using the relaxation time distribution method comprises: Pretreating the electrochemical impedance spectrum data to remove high-frequency inductance and low-frequency diffusion noise; Based on a regularization parameter = 10 -5 , calculate the relaxation time distribution function ; SEI film resistance R is calculated by integral formula SEI charge transfer resistance R ct SEI film resistance R is calculated by integral formula SEI charge transfer resistance R ct Corresponding characteristic peak separation: ， 。 5. The method for evaluating the quality of the solid electrolyte interface film of a lithium battery according to claim 1, characterized by, Optimizing the compactness and ion conductivity of the SEI film by dynamically adjusting the concentration of fluoroethylene carbonate in the electrolyte.

6. The method for evaluating the quality of the solid electrolyte interface film of a lithium battery according to claim 1, characterized by, The SEI film has a compactness index < 20 Ω.μF -1 When the compactness of the SEI film is determined to be satisfactory.

7. The method for evaluating the quality of the solid electrolyte interface film of a lithium battery according to claim 1, characterized by, Fitting the electrochemical impedance spectrum curve based on the Levenberg-Marquardt algorithm.

8. The method for evaluating the quality of the solid electrolyte interface film of a lithium battery according to claim 1, characterized by, Combining the charge transfer resistance and Warburg diffusion impedance parameters to calculate the lithium ion diffusion coefficient The formula is: wherein, R is the gas constant, T is the temperature, A is the electrode area, n is the number of electrons in the reaction, F is the Faraday constant, C is the lithium ion concentration, is the Warburg coefficient.

9. The method for evaluating the quality of the solid electrolyte interface film of a lithium battery according to claim 1, characterized by, The method is used for diagnosing short circuit fault in lithium battery module, defines a fault coefficient k=(R ct / R SEI ) / (R ct0 / R SEI0 ), and determines that there is a fault cell when k exceeds a threshold value. where R SEI is the SEI film resistance, R ct is the charge transfer resistance, R SEI0 is the initial SEI film resistance, serving as a reference baseline for evaluating subsequent changes in the quality of the SEI film; and R ct0 is the initial charge transfer resistance, serving as a reference baseline for evaluating subsequent changes in the electrochemical reaction kinetics.

10. The method for evaluating the quality of the solid electrolyte interface film of a lithium battery according to claim 9, characterized by, Series module failure coefficient When the failure coefficient k>k s , a faulty monomer is determined to exist; parallel module failure coefficient When the failure coefficient k>k p , a faulty monomer is determined to exist; m, n are the number of batteries in series module and parallel module respectively and ≤8.

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