Method for evaluating stability of battery sei film

Abstract

The application relates to the field of batteries, and discloses a method for evaluating the stability of a battery SEI film. The method comprises the following steps: obtaining a shoulder peak index SPI of a battery to be tested and comparing the SPI with a first preset value; obtaining a charge transfer resistance Rct of the battery to be tested and comparing the Rct with a second preset value; if the SPI of the battery to be tested is less than the first preset value and the Rct of the battery to be tested is less than the second preset value, it is determined that the battery to be tested meets a preset standard; wherein the shoulder peak index SPI = |d 2 (-Zim) / d(Zre) 2 | max / Rct x 100%. The method can simplify the test process of the battery, and the evaluation result is more accurate.

Description

Technical Field

[0001] This application relates to the field of batteries, and in particular to a method for evaluating the stability of a battery SEI film. Background Technology

[0002] Current methods for evaluating the stability of the SEI film in batteries primarily rely on voltage drop. This requires disassembling the battery and using surface characterization techniques such as XPS, SEM / TEM, and AFM to observe the surface and determine the stability of the SEI film after formation. This testing method is costly, requires destructive testing of the battery, and involves complex processes. Furthermore, under the same voltage drop ΔV, failures may originate from different interface failure processes, which traditional methods cannot distinguish, leading to biased evaluation conclusions and difficulty in guiding precise optimization. Summary of the Invention

[0003] This application discloses a method for evaluating the stability of the SEI film in a battery, which simplifies the battery testing process and provides more accurate evaluation results.

[0004] To achieve the above objectives, this application provides the following technical solution: This application provides a method for evaluating the stability of a battery SEI film, the method comprising: Obtain the shoulder peak index (SPI) of the battery under test and compare it with a first preset value; The charge transfer resistance Rct of the battery under test is obtained and compared with a second preset value; If the SPI of the battery under test is less than the first preset value and the Rct of the battery under test is less than the second preset value, then the battery under test is determined to meet the preset standard. Wherein, the acromion index SPI = |d 2 (-Zim) / d(Zre) 2 | max / Rct×100%; |d 2 (-Zim) / d(Zre) 2 | max This represents the numerical value of the shoulder peak position obtained by taking the second derivative of the function -Zim=f(Zre); the function -Zim=f(Zre) is a function of the negative imaginary part of the battery impedance with respect to the real part of the battery impedance, obtained from electrochemical impedance test data. The first preset value is the shoulder peak index SPI of the reference battery; the second preset value is the charge transfer resistance Rct of the reference battery.

[0005] The beneficial effects of adopting the technical solution of this application are as follows: The evaluation method provided in this application abandons the traditional voltage drop ΔV test method and instead uses Rct and shoulder peak index (SPI) for judgment. Both Rct and SPI can be obtained using battery testing equipment. Research has found that a larger Rct and a higher SPI value indicate SEI film structural defects and difficulties in interface reaction; a larger Rct and a lower SPI value indicate difficulties in interface reaction due to external factors affecting the SEI film structure; a smaller Rct and a lower SPI value indicate a uniform and dense SEI film structure and smooth interface reaction. Therefore, when the SPI of the battery under test is less than a first preset value and the Rct of the battery under test is less than a second preset value, the battery under test is determined to meet the SEI film formation standard. The first and second preset values ​​are the SPI and Rct of the reference battery, respectively. The composition of the battery under test and the reference battery can be different. By comparing the battery under test with the reference battery, the qualification of the SEI film of the battery under test can be quickly determined without further disassembly of the battery. Detailed Implementation

[0006] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, and 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.

[0007] It should be noted that: Unless otherwise specified, all embodiments and preferred methods mentioned herein can be combined to form new technical solutions. Unless otherwise specified, all technical features and preferred features mentioned herein can be combined to form new technical solutions. Unless otherwise specified, percentages (%) or parts refer to weight percentages or parts by weight relative to the composition. Unless otherwise specified, the components involved or their preferred components can be combined to form new technical solutions. Unless otherwise specified, the numerical range "a~b" in this application represents an abbreviation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "6~22" means that all real numbers between "6~22" have been listed herein; "6~22" is merely an abbreviation of these numerical combinations. The "range" disclosed in this application can be in the form of a lower limit and an upper limit, and may be one or more lower limits and one or more upper limits, respectively. Unless otherwise specified, the various reaction or operation steps in this application can be performed sequentially or in order. Preferably, the reaction methods described herein are performed sequentially.

[0008] Unless otherwise stated, the technical and scientific terms used herein have the same meanings as those familiar to a person skilled in the art. Furthermore, any methods or materials similar to or equivalent to those described herein may also be used in this application.

[0009] To improve the accuracy of SEI membrane testing and reduce testing difficulty, this application provides a novel method for evaluating SEI membrane stability.

[0010] The method for evaluating the stability of the battery SEI film in this application includes the following steps: Obtain the shoulder peak index (SPI) of the battery under test and compare it with a first preset value; obtain the charge transfer resistance (Rct) of the battery under test and compare it with a second preset value; if the SPI of the battery under test is less than the first preset value and the Rct of the battery under test is less than the second preset value, then the battery under test is determined to meet the preset standard.

[0011] Wherein, the acromion index SPI = |d 2 (-Zim) / d(Zre) 2 | max / Rct×100%.

[0012] In the above expression for the acromion index (SPI), |d 2 (-Zim) / d(Zre) 2 | max This represents the numerical value of the shoulder peak position obtained by taking the second derivative of the function -Zim=f(Zre); the function -Zim=f(Zre) is a function of the negative imaginary part of the battery impedance with respect to the real part of the battery impedance, obtained from electrochemical impedance test data.

[0013] Among them, d is obtained by taking the second derivative of -Zim=f(Zre). 2 (-Zim) / d(Zre) 2 For a curve with a peak, |d 2 (-Zim) / d(Zre) 2 | max This indicates the value at the peak.

[0014] The first preset value is the shoulder peak index (SPI) of the reference battery; the second preset value is the charge transfer resistance (Rct) of the reference battery. The SPI of the reference battery is determined by testing the electrochemical impedance of the reference battery to obtain the negative imaginary part of the battery impedance as a function of the real part of the battery impedance, -Zim=f(Zre). Then, |d| is calculated based on the function -Zim=f(Zre) of the reference battery. 2 (-Zim) / d(Zre) 2 | max .

[0015] The negative imaginary part of the battery impedance as a function of the real part of the battery impedance, -Zim=f(Zre), and the charge transfer resistance, Rct, can both be obtained by testing with an electrochemical workstation. The electrochemical workstation can be used to obtain the electrochemical impedance spectroscopy (EIS) of the test battery and the reference battery. From the EIS, -Zim=f(Zre) and Rct can be obtained.

[0016] The reference battery can be a battery whose SEI film meets the usage requirements after testing, or a battery whose performance has been verified to be excellent. The battery under test is tested against this type of reference battery. The inventors of this application have discovered that the stability of the SEI film is closely related to the battery's Rct and shoulder peak index SPI. A larger Rct and a higher SPI value indicate SEI film structural defects and difficulties in interface reaction; a larger Rct and a lower SPI value indicate difficulties in interface reaction due to external factors affecting the SEI film structure; a smaller Rct and a lower SPI value indicate a uniform and dense SEI film structure and smooth interface reaction. Therefore, when the SPI of the battery under test is less than a first preset value and the Rct of the battery under test is less than a second preset value, the battery under test is determined to meet the SEI film formation standard. The first and second preset values ​​are the SPI and Rct of the reference battery, respectively. The composition of the battery under test and the reference battery can be different. By comparing the battery under test with the reference battery, the qualification of the SEI film of the battery under test can be quickly determined without further disassembly of the battery.

[0017] In this embodiment of the application, the battery under test meeting the preset standard means that the stability of the battery under test reaches the stability of the reference battery.

[0018] In one embodiment of this application, the test charge control range for the shoulder peak index is 5%~20% SOC, and the test frequency is 10. 2 ~10 4 Hz. The test power control range is controlled within 5%~20% SOC, and the test frequency is controlled at 10 Hz. 2 ~10 4 In the mid-frequency range of Hz, test accuracy can be improved.

[0019] In one embodiment, the battery under test undergoes 10 to 30 charge-discharge cycles before testing. Multiple charge-discharge cycles facilitate the formation of a stable SEI film.

[0020] In an optional embodiment, the evaluation method further includes: The initial shoulder index SPI0 at room temperature and the normalized shoulder index SPI1 of the battery under test under heating environment are obtained. The relationship value (SPI1-SPI0) / SPI0 between the battery under test and SPI1 is compared with a third preset value. If (SPI1-SPI0) / SPI0 of the battery under test is less than the third preset value, then the SEI film of the battery under test meets the thermal stability preset standard. The third preset value is (SPI1-SPI0) / SPI0 of the reference battery.

[0021] The ambient temperature is 25℃±5℃. Under heating conditions, the temperature of the battery under test is 45℃~65℃, and the heating time is 12h~48h. Setting the heating temperature within the range of 45℃~65℃ allows the SEI film to react fully.

[0022] The normalized shoulder peak index SPI1 is based on the battery's SPI at heating temperature and the temperature correction coefficient K = e^[E a The ratio SPI / K of [R (1 / T0-1 / T)] is determined by calculation, where Ea is the charge transfer activation energy obtained by fitting the Arrhenius equation, R=8.314J / (mol·K), T0=298K, and T is the absolute temperature of the heating environment.

[0023] Comparing the (SPI1-SPI0) / SPI0 ratio of the test cell and the reference cell can be used to further evaluate the structural uniformity of the SEI film, thus enabling analysis of SEI film structural uniformity without disassembling the cell. A smaller (SPI1-SPI0) / SPI0 ratio indicates a stronger ability of the SEI film to maintain structural uniformity under thermal disturbances; a larger (SPI1-SPI0) / SPI0 ratio indicates a weaker ability of the SEI film to maintain structural uniformity under thermal disturbances.

[0024] Understandably, when testing SPI1, it can be tested at multiple time points within different time periods. When calculating (SPI1-SPI0) / SPI0, the largest SPI1 value can be selected for calculation.

[0025] In one embodiment, the evaluation method further includes: Obtain the static shoulder peak index SPI2 of the battery under test after it has been heated to room temperature. Compare the relationship value of SPI1 and SPI2, |SPI1-SPI2| / SPI1, with a fourth preset value. If |SPI1-SPI2| / SPI1 of the battery under test is greater than the fourth preset value, then the SEI film of the battery under test meets the preset standard for self-healing function. The fourth preset value is the relationship between the static shoulder peak SPI2 of the reference battery after heat treatment and the normalized shoulder peak index SPI1 of the reference battery, which is |SPI1-SPI2| / SPI1.

[0026] The dynamic repair efficiency of the SEI film can be guaranteed by the relationship value of |SPI1-SPI2| / SPI1. The larger the value of |SPI1-SPI2| / SPI1, the higher the dynamic repair efficiency of the SEI film after thermal disturbance; the smaller the value of |SPI1-SPI2| / SPI1, the lower the repair efficiency of the SEI film after thermal disturbance.

[0027] In one embodiment of this application, the battery under test is left to stand for 10-24 hours after the heat treatment returns to room temperature. This 10-24 hour standing time allows the SEI film to fully stabilize.

[0028] In one embodiment of this application, the evaluation method further includes: The maximum value among SPI0, SPI1, and SPI2 of the battery under test is SPI. max With minimum value SPI min Relationship Values ​​(SPI) max -SPI min The value of (SPI0) is compared with the fifth preset value. If the (SPI0) of the battery under test is... max -SPI min If ) / SPI0 is less than the fifth preset value, then the SEI film of the battery under test meets the preset standard for high-temperature structural stability. The fifth preset value is the maximum value among the reference batteries SPI0, SPI1, and SPI2. max With minimum value SPI min Relationship Values ​​(SPI) max -SPI min ) / SPI0.

[0029] (SPI max -SPI min ) / SPI0 can be used to determine the stress adaptability of the SEI membrane interface: calculate the difference between the maximum and minimum values ​​of SPI0, SPI1, and SPI2, and compare it with SPI0 to obtain the relative fluctuation amplitude of SPI. max -SPI min The smaller the SPI0 ratio, the more stable the SEI film structure is during thermal disturbance and recovery, indicating better interfacial stress adaptability; conversely, the smaller the SPI0 ratio, the better the SEI film structure is during thermal disturbance and recovery. max -SPI min The larger the ) / SPI0, the worse the interfacial stress adaptability of the SEI film during thermal disturbance and recovery processes.

[0030] Among them, the power control range and test frequency are the same for the above SPI values, such as SPI0, SPI1 and SPI2.

[0031] The evaluation method provided in this application is a comprehensive and quantitative method for assessing the thermal stability and dynamic evolution of SEI films, revealing their intrinsic mechanisms and providing direct and reliable data support for the development of electrolyte formulations and formation processes. A stable and high-quality SEI film exhibits a uniform interface, good process coupling, indistinct shoulder peaks, and a low and stable SPI value. SEI films that have undergone thermal disturbance and successfully repaired will experience a characteristic process of a temporary increase in inhomogeneity (SPI increase) followed by a return to homogeneous stability (SPI decrease). A continuously deteriorating SEI film may exhibit chaotic and irregular SPI values, or consistently fail to form significant shoulder peak characteristics (i.e., fail to form an effective repair mechanism). Therefore, the evaluation method in this application uses the process index SPI as an evaluation indicator to capture the "self-healing ability" and final structural strength of the SEI film under thermal stress.

[0032] The following detailed explanation of the battery SEI film stability evaluation method of this application embodiment, with reference to specific examples.

[0033] Example 1: Determination of SEI film thermal stability. The battery fabrication steps in this example include the following: S1. Prepare a soft-pack battery (positive electrode NCM, negative electrode graphite), and inject electrolyte 1 (containing self-healing additives such as FEC and LiPO2F2) and electrolyte 2 (conventional electrolyte) into the battery respectively, and adjust the SOC to 7.5% after capacity separation. S2. Test the EIS of the two batteries at room temperature (25℃) and calculate SPI0; S3. Place at 45℃ for high-temperature aging, test EIS every 12 hours, and record the original SPI1 value at high temperature; S4. After the battery is brought back to room temperature and left to stand for 12-24 hours, test the EIS spectrum in the same SOC range and calculate the SPI value SPI2 after high temperature. S5. Data statistics and processing results are listed in Table 1.

[0034] Table 1

[0035] Referring to the test data in Table 1, the test results for the two sets of batteries are as follows.

[0036] The test data for the battery corresponding to electrolyte 1 are as follows: 12h: Original SPI1 = 11.7%, fitted E a=19.8kJ / mol, K=1.653, normalized SPI1=7.11%, increase (normalized SPI1-SPI0) / SPI0: 1.57%; 24h: Original SPI1 = 12.6%, Normalized SPI1 = 7.6%, Increase of 8.57%; 36h: Original SPI1 = 12.9%, Normalized SPI1 = 7.8%, Increase of 11%; After 36 hours of aging, the sample was allowed to stand at room temperature for 12 hours. The SPI2 value was 7.3%, and the decrease was |SPI1-SPI2| / SPI1: 6%. Relative volatility (SPI) max -SPI min ) / SPI0:11%.

[0037] The test data for the battery corresponding to electrolyte 2 are as follows: 12h: Original SPI1=22%, fitted E a =28.5kJ / mol, K=2.06, normalized SPI1=10.7%, increase (normalized SPI1-SPI0) / SPI0: 25.9%; 24h: Original SPI1 = 23.3%, Normalized SPI1 = 11.3%, Increase of 33.3%; 36h: Original SPI1 = 20.4%, Normalized SPI1 = 9.9%, Increase of 16.4%; After 36 hours of aging, the sample was allowed to stand at room temperature for 12 hours. The SPI2 value was 8.8%, and the decrease was |SPI1-SPI2| / SPI1: 22.3%. Relative volatility (SPI) max -SPI min ) / SPI0: 33.3%.

[0038] In this embodiment of the application, one type of battery can be used as the battery to be tested, and the other type of battery is a control battery.

[0039] determination: 1. Resistance to thermal degradation (ability to maintain structural uniformity) Evaluation metric: The increase in normalized SPI1 relative to the initial SPI0. A smaller increase indicates better maintenance of the SEI film's structural uniformity under thermal disturbance and stronger thermal resistance to degradation. Specifically, the battery corresponding to electrolyte 1 showed a maximum increase of only 11%, while the battery corresponding to electrolyte 2 showed a maximum increase of 33.3%. This indicates that the battery corresponding to electrolyte 1 has significantly better thermal resistance to degradation than the battery corresponding to electrolyte 2.

[0040] 2. Thermal self-healing efficiency (dynamic repair capability)

[0041] Evaluation Metric: The decline in SPI2 relative to the highest normalized SPI1 after high temperature. A larger decline indicates a stronger ability of the SEI film to restore structural uniformity after thermal disturbance, and higher self-healing efficiency. For the battery corresponding to electrolyte 1, the decline was 6%. For the battery corresponding to electrolyte 2, the decline was 22.3%. Summary: Numerically, the decline in the battery corresponding to electrolyte 2 is larger, but its self-healing is a "remedial" behavior based on severe prior degradation (high amplification). The battery corresponding to electrolyte 1, due to very slight degradation at high temperatures, has less room for self-healing.

[0042] Overall assessment: The stability achieved by electrolyte 1 batteries through inhibiting degradation is superior and more reliable than the mode of repair after degradation of electrolyte 2 batteries.

[0043] 3. Interfacial stress adaptability (structural stability)

[0044] Evaluation metric: The relative fluctuation of the SPI value throughout the entire testing process (initial, high temperature, and recovery). A smaller fluctuation indicates a more stable overall structure and better interfacial stress adaptability of the SEI film during thermal expansion / contraction and structural changes. The relative fluctuation of the battery corresponding to electrolyte 1 is 11%. The relative fluctuation of the battery corresponding to electrolyte 2 is 33.3%.

[0045] In summary, the interfacial stress adaptability of the battery using electrolyte 1 is far superior to that of the battery using electrolyte 2. The SEI film of electrolyte 1 exhibits very small changes in its SPI value throughout the entire thermal disturbance and recovery process, demonstrating extremely high structural stability and excellent adaptability to changes in interfacial stress. In contrast, the SEI film of the battery using electrolyte 2 experienced severe structural fluctuations, resulting in poor stability.

[0046] Example 2: Aging Time Optimization. The battery fabrication steps in this example include the following: S1. Prepare a soft-pack battery (positive electrode NCM, negative electrode graphite), inject electrolyte 1, and charge to 7.5% SOC after formation; S2. Test the EIS of the battery at room temperature (25℃) and calculate SPI0; the calculated SPI0 is 7.0%. S3. Place the battery at 45℃ for high-temperature aging, test EIS every 12 hours, and record the original SPI value at high temperature; Ea is 19.8 and K value is 1.653. S4. After the battery is restored to room temperature and left to stand for 12~24 hours, test the EIS spectrum in the same SOC range and calculate the SPI value SPI2 after high temperature. The results of data statistics and processing are listed in Table 2.

[0047] Table 2

[0048] Referring to the test data in Table 2, the battery aging evaluation results are as follows.

[0049] The test data for the battery corresponding to electrolyte 1 are as follows: 12h: Original SPI1 = 11.7%, fitted E a =19.8kJ / mol, K=1.653, normalized SPI1=7.11%, increase of 1.57%; 24h: Original SPI1 = 12.6%, Normalized SPI1 = 7.6%, Increase of 8.57%; 36h: Original SPI1 = 12.9%, Normalized SPI1 = 7.8%, Increase of 11%; 48h: Original SPI1 = 13.1%, Normalized SPI1 = 7.9%, Increase of 12.9%.

[0050] For the battery corresponding to electrolyte 1: Under high-temperature conditions of 45℃, the structural uniformity of the SEI film gradually decreases with the extension of aging time, and the degree of thermal degradation continues to deepen. The longer the aging time, the greater the impact of thermal stress on the SEI film. When the aging time reaches or exceeds 36 hours, the structural changes of the SEI film tend to stabilize. Further extending the aging time may not significantly improve its stability, but will instead increase production time and cost.

[0051] Obviously, those skilled in the art can make various modifications and variations to the embodiments of this application without departing from the spirit and scope of this application. Therefore, if these modifications and variations of this application fall within the scope of the claims of this application and their equivalents, this application also intends to include these modifications and variations.

Claims

1. A method for evaluating the stability of a battery SEI film, characterized in that, include: Obtain the shoulder peak index (SPI) of the battery under test and compare it with a first preset value; The charge transfer resistance Rct of the battery under test is obtained and compared with a second preset value; If the SPI of the battery under test is less than the first preset value and the Rct of the battery under test is less than the second preset value, then the battery under test is determined to meet the preset standard. Wherein, the acromion index SPI = |d 2 (-Zim) / d(Zre) 2 | max / Rct×100%; |d 2 (-Zim) / d(Zre) 2 | max This represents the numerical value of the shoulder peak position obtained by taking the second derivative of the function -Zim=f(Zre); the function -Zim=f(Zre) is a function of the negative imaginary part of the battery impedance with respect to the real part of the battery impedance, obtained from electrochemical impedance test data. The first preset value is the shoulder peak index SPI of the reference battery; the second preset value is the charge transfer resistance Rct of the reference battery. The evaluation method also includes: The initial shoulder peak index SPI0 at room temperature and the normalized shoulder peak index SPI1 of the battery under test under heating environment are obtained. The relationship value (SPI1-SPI0) / SPI0 between the battery under test and SPI1 is compared with a third preset value. If (SPI1-SPI0) / SPI0 of the battery under test is less than the third preset value, then the SEI film of the battery under test meets the preset thermal stability standard. The third preset value is (SPI1-SPI0) / SPI0 of the reference battery; The normalized shoulder peak index SPI1 is based on the battery's SPI at heating temperature and the temperature correction coefficient K = e^[E a The ratio SPI / K of [R (1 / T0-1 / T)] is determined by calculation, where Ea is the charge transfer activation energy obtained by fitting the Arrhenius equation, R=8.314J / (mol·K), T0=298K, and T is the absolute temperature of the heating environment.

2. The evaluation method according to claim 1, characterized in that, The heating environment is characterized by a heating temperature of 45℃ to 65℃ and a heating time of 12h to 48h.

3. The evaluation method according to claim 1, characterized in that, The evaluation method also includes: Obtain the static shoulder peak index SPI2 of the battery under test after it has been heated to room temperature. Compare the relationship value of SPI1 and SPI2, |SPI1-SPI2| / SPI1, with a fourth preset value. If |SPI1-SPI2| / SPI1 of the battery under test is greater than the fourth preset value, then the SEI film of the battery under test meets the preset standard for self-healing function. The fourth preset value is the relationship between the static shoulder peak SPI2 of the reference battery after heat treatment and the normalized shoulder peak index SPI1 of the reference battery, |SPI1-SPI2| / SPI1.

4. The evaluation method according to claim 3, characterized in that, The battery under test was left to stand for 10-24 hours after being heated to room temperature.

5. The evaluation method according to claim 3, characterized in that, The evaluation method also includes: The maximum value among SPI0, SPI1, and SPI2 of the battery under test is SPI. max With minimum value SPI min Relationship Values ​​(SPI) max -SPI min The value of (SPI0) is compared with the fifth preset value. If the (SPI0) of the battery under test is... max -SPI min If ) / SPI0 is less than the fifth preset value, then the SEI film of the battery under test meets the preset standard for high-temperature structural stability. The fifth preset value is the maximum value among the reference batteries SPI0, SPI1, and SPI2. max With minimum value SPI min Relationship Values ​​(SPI) max -SPI min ) / SPI0.

6. The evaluation method according to any one of claims 1-5, characterized in that, The test power control range for the shoulder peak index is 5%~20% SOC.

7. The evaluation method according to any one of claims 1-5, characterized in that, The test frequency is 10. 2 ~10 4 Hz.

8. The evaluation method according to any one of claims 1-5, characterized in that, The battery under test undergoes 10 to 30 charge-discharge cycles before testing.

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