Method for determining the performance of lithium-ion battery seimembranes

Abstract

The application discloses a method for determining the performance of SEI film of a lithium ion battery, comprising the following steps: performing formation treatment on a plurality of battery cells, adding a redox medium to the electrolyte of the battery cells after the formation treatment, and then performing aging treatment; performing cyclic charge-discharge test on the battery cells obtained in the foregoing steps to obtain the coulomb efficiency of the plurality of battery cells; and determining the performance of the SEI film formed by the formation of the battery cells according to the coulomb efficiency of the battery cells. The application can directly characterize the SEI film after the formation of the SEI film, and can quickly obtain qualitative data about whether the SEI film after the formation has obvious electronic channels or lithium salt and solvent molecule channels; when the test result shows that the performance of the SEI film is poor, the quality of the SEI film can be improved by properly adjusting the formation temperature, pressure, current, cutoff voltage or electrolyte component, and the application can be used for screening of electrolyte components, anode materials and formation conditions, greatly reducing the research and development cost and shortening the research and development cycle.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery technology, and in particular to a method for determining the performance of the SEI film in a lithium-ion battery. Background Technology

[0002] The SEI film in lithium-ion batteries can block the electrolyte and anode, preventing excessive electrolyte consumption. Simultaneously, the SEI film is also a Li-type electrolyte separator. + Excellent conductor, allowing Li + The free insertion and extraction of lithium into and out of the anode, as well as the prevention of co-intercalation of solvent molecules, determine the kinetics of lithium insertion / extraction and the stability of the negative electrode electrolyte interface. Currently, the main methods for judging the quality of SEI film formation are the characterization of the material electrode layers and conventional cyclic charge-discharge tests. However, the former is expensive and complex and can only provide a local view. It also needs to consider the sensitivity of the SEI film to air and the low accuracy caused by the damage of the SEI film by high-energy electron beams. The latter is time-consuming and cannot clearly identify the influence of the SEI film, making it impossible to accurately characterize it. Summary of the Invention

[0003] In view of this, this application provides a method for determining the performance of the SEI film in a lithium-ion battery, which can obtain qualitative data on whether the SEI film has excellent electronic insulation properties after formation.

[0004] The method for determining the performance of the SEI film in a lithium-ion battery includes the following steps: S100: performing formation treatment on multiple cells; S200: adding a redox medium to the electrolyte of the formed cells, followed by aging treatment; S300: performing cyclic charge-discharge tests on the cells obtained in step S200 to obtain the coulombic efficiency of the multiple cells; S400: determining the performance of the SEI film formed by the cell formation based on the coulombic efficiency of the cells. A high-quality SEI film should have high electronic insulation, ionic conductivity, and chemical / mechanical stability. When the SEI film thickness is uneven, electron tunneling exists in some areas, or cracks exist in the interface phase, the SEI film will not be completely insulating, and the electron-conducting surface will be directly exposed to the electrolyte, causing irreversible side reactions. During charge and discharge, this manifests as an increase in charging capacity and a decrease in discharging capacity, i.e., low coulombic efficiency.

[0005] This application introduces a redox medium into the electrolyte of the battery cell after formation. This enhances the cell's self-discharge during charge and discharge. Test data from conventional equipment can accurately reflect the SEI film quality in a short time. SEI film quality refers to SEI film performance, which includes at least one of SEI film thickness uniformity and SEI film cracking degree. When SEI film performance is poor, such as uneven thickness or cracks at the interface, the electrolyte continuously decomposes and consumes the material during charge and discharge to repair the SEI film, leading to increased SEI film thickness and irreversible capacity loss, thus affecting the battery's cycle performance and safety. Conversely, when SEI film performance is excellent, it effectively blocks the electrolyte and anode, preventing excessive electrolyte consumption and inhibiting SEI film thickness growth, thereby achieving excellent cycle life and significantly improving battery safety. This application provides qualitative data (determined by the coulombic efficiency of the battery cell) on whether the SEI film exhibits significant electron channels or lithium salt and solvent molecule channels after formation. When test results indicate poor SEI film performance, the quality of the SEI film can be improved by appropriately adjusting formation conditions, battery cell manufacturing processes, and electrolyte components. The formation conditions can include formation temperature, formation pressure, formation current, and formation cutoff voltage.

[0006] In some embodiments, in step S100, the battery cell is a single battery cell or a battery cell group composed of multiple single battery cells. Specifically, when the battery cell is a battery cell group composed of multiple single battery cells, the multiple battery cells are multiple battery cell groups. Each single battery cell in the battery cell group is sequentially subjected to formation processing, and then divided by group. For example, there are three battery cell groups A, B, and C. Each battery cell group includes five single battery cells, that is, A includes A1 to A5, B includes B1 to B5, and C includes C1 to C5. Each single battery cell in A1 to A5, B1 to B5, and C1 to C5 is subjected to formation processing, wherein the formation conditions of A1 to A5 are the same, the formation conditions of B1 to B5 are the same, and the formation conditions of C1 to C5 are the same.

[0007] In some embodiments, in step S200, the redox medium includes any one of inorganic redox medium, organic redox medium, and mixture containing inorganic redox medium and organic redox medium.

[0008] Specifically, in some examples, the redox medium includes an inorganic redox medium; in other examples, the redox medium includes an organic redox medium; and in still other examples, the redox medium includes an inorganic redox medium, and may also include an organic redox medium, depending on the specific circumstances. This application does not limit the choice.

[0009] In some embodiments, the inorganic redox medium includes Fe-containing media. 3+ / Fe 2+ Electrophoretic compounds, Ag-containing compounds + / Ag redox couple compounds or containing Mn 4+ / Mn 2+ At least one of the redox couple compounds.

[0010] The selection of the redox medium needs to take into account the anode delithiation potential (the lithium insertion potential of graphite is 0.1–0.2V). If the difference between the redox potential of the redox medium and the anode delithiation potential is too large, it will significantly enhance the kinetics of charge transfer through the SEI. Excessive driving force can lead to artifacts. Conversely, if the redox potential of the redox medium is low, below the decomposition potential of the electrolyte components (generally occurring in the range of 1.2–0.8V), the electrolyte components will preferentially react at sites where the SEI film protection is insufficient, repairing the SEI film and interfering with the actual results. Therefore, a redox medium with a redox potential close to and slightly higher than the electrolyte decomposition potential is preferred. Furthermore, during battery charging and discharging, to ensure that a corresponding redox medium is available at different potentials to identify SEI film quality and to prevent premature repair of the SEI film by lithium salt decomposition, multiple media with different redox potentials can be mixed and used to improve detection accuracy. For example, the inorganic redox medium includes Mn-containing media... 4+ / Mn 2+ Electrophoretic compounds and Fe-containing compounds 3+ / Fe 2+ Electrophoretic compounds; or, the inorganic redox medium includes Mn-containing compounds. 4+ / Mn 2+ Electrophoretic compounds and Ag-containing compounds + / Ag redox couple compounds, in a mixed medium of various inorganic redox media combinations, wherein the molar ratio of different inorganic redox couple compounds is 1:1.

[0011] For example, the Fe-containing 3+ / Fe 2+ The redox couple compound is selected from at least one of ferrocene, potassium ferricyanide, and Prussian blue; the Ag-containing compound... + The / Ag redox couple compound is selected from at least one of silver nitrate and silver fluoride; the Mn-containing compound... 4+ / Mn 2+ The redox couple compound is selected from at least one of manganese phosphate acid and manganese phthalocyanine.

[0012] In some embodiments, the organic redox medium includes TEMPO. + / TEMPO redox couple compounds, PHT + / PHT electropair compounds or AQ +At least one of the / AQ redox couple compounds. Here, "class" refers to the tunable nature of non-characteristic functional groups in the organic redox medium; for example, the length and number of alkyl groups at the same substituent positions can be adjusted.

[0013] For example, the TEMPO + / TEMPO redox couple compounds are selected from at least one of tetramethylpiperidine oxide, tetraethylpiperidine nitroxide, and tetramethylpiperidine chloride oxide; the PHT + / PHT redox couple compounds are selected from at least one of methylphthalimide and phthalimide; the AQ + / AQ redox couples are selected from at least one of anthraquinones and 1-aminoanthraquinones.

[0014] Furthermore, to prevent the inorganic redox medium and the solvent components in the electrolyte from complexing and affecting the solvation and desolvation processes before the interfacial reaction occurs, organic and inorganic redox media (mixed media) can be used in combination to avoid affecting the battery interfacial reaction. Examples include mixed media containing ferrocene and methylphthalimide, mixed media containing silver nitrate and methylphthalimide, and mixed media containing Mn-ATP, silver nitrate, and anthraquinone; these are merely illustrative examples and are not limiting. The molar ratio of the inorganic redox medium to the organic redox medium is 1:1 to 2.

[0015] In some embodiments, the concentration of the redox medium is 0.01–0.4 mol / L (M, where M represents mol / L in the following text). Adding too much redox medium (greater than 0.4 M) will affect the viscosity and activity of the electrolyte, failing to accurately reflect the film formation quality. Conversely, adding too little redox medium (less than 0.01 M) will make the coulombic efficiency insensitive to the protective properties of the SEI film. Therefore, the concentration of the added redox medium must be within the aforementioned range.

[0016] Specifically, the concentration of the redox medium can be initially defined by the anode material. Depending on the material type, for example, for lithium-ion batteries with graphite anodes, the concentration of the redox medium is 0.01–0.2 M. Exemplarily, for lithium-ion batteries with graphite anodes, the concentration of the redox medium is 0.01 M, 0.03 M, 0.05 M, 0.08 M, 0.1 M, 0.15 M, 0.2 M, or any combination of two of the above values. For lithium-ion batteries with silicon-graphite composite anodes, the concentration of the redox medium is 0.03–0.3 M. Exemplarily, for lithium-ion batteries with silicon-graphite composite anodes, the concentration of the redox medium is 0.03 M, 0.05 M, 0.08 M, 0.1 M, 0.15 M, 0.2 M, 0.25 M, 0.3 M, or any combination of two of the above values. For lithium-ion batteries with silicon anodes, the concentration of the redox medium is 0.15–0.4 M. For example, for lithium-ion batteries with silicon anodes, the concentration of the redox medium is 0.15 M, 0.2 M, 0.25 M, 0.3 M, 0.35 M, 0.4 M, or any two of the above values. The graphite anode refers to an anode in which the mass percentage of graphite is greater than or equal to 90% based on the total mass of the anode active material. The silicon-graphite composite anode refers to an anode in which the mass percentage of both silicon and graphite is greater than 10% and less than 90% based on the total mass of the anode active material. The silicon anode refers to an anode in which the mass percentage of silicon is greater than or equal to 90%. After identifying the anode material, since the specific surface area of ​​the anode material affects the area of ​​the SEI film, a larger anode specific surface area results in a larger SEI film. Therefore, the specific surface area of ​​the material is used to further limit the concentration of the redox medium within the aforementioned range. Specifically, in lithium-ion batteries with graphite anodes, when the specific surface area of ​​graphite is 0.6–1.1 μm... 2 At a concentration of 0.01–0.12 M / g, in lithium-ion batteries with silicon anodes, when the specific surface area of ​​silicon particles is 4.08–6.1 m² / g... 2 When the specific surface area is / g, the concentration of the redox medium is 0.2–0.35 M. When the specific surface area is greater than or less than the above range, the concentration of the redox medium is adjusted accordingly based on the increase or decrease in specific surface area.

[0017] In some embodiments, in step S200, the aging treatment conditions include: aging temperature: 20–60°C, aging time: 48–240 h. For example, the aging temperature is a range of 20°C, 35°C, 40°C, 50°C, 55°C, 60°C, or any two of the above values; the aging time is a range of 48 h, 50 h, 55 h, 60 h, 65 h, 80 h, 100 h, 150 h, 200 h, 240 h, or any two of the above values. The purpose of aging is to completely eliminate polarization caused by formation, while ensuring that the medium (i.e., the redox medium, hereinafter the same) is fully mixed and diffused into the electrolyte at different locations in the cell. If the aging temperature is too low (below 20℃) or the aging time is too short (below 48h), complete deactivation cannot be achieved, and the dispersion of the dielectric in the cell cannot be guaranteed, affecting the cell's self-discharge and ultimately impacting the accuracy of the results. Conversely, if the aging temperature is too high (above 60℃) or the aging time is too long (above 240h), it will affect the activity and stability of the electrolyte and dielectric, impacting the accuracy of the test results and increasing energy consumption and time costs. Therefore, a suitable aging range is beneficial for improving the accuracy of test results.

[0018] In some embodiments, in step S300, the conditions for the cyclic charge-discharge test are: a test temperature of 12–60°C, a charge-discharge cycle rate of 0.01–2C, and 1–10 cycles. Test temperatures that are too high (above 60°C) or too low (below 12°C) will affect the viscosity of the electrolyte and the activity of the medium, and may also trigger unnecessary side reactions. Simultaneously, when the charge-discharge rate is too high (greater than 2C), the battery polarization is large, making it easier for the battery potential to reach the potential for electrolyte solvent decomposition. This leads to a large amount of solvent reacting at the SEI film where electron tunneling and cracks exist, resulting in inaccurate test results. When the charge-discharge rate is too low (less than 0.01C), the charge-discharge time is long. Therefore, cyclic charge-discharge test conditions within the above range are more conducive to improving test accuracy. Furthermore, in the first few cycles after formation (1-10 cycles), there are no significant changes in material structure or side reactions caused by lithium plating. The irreversible capacity loss in these cycles mainly comes from SEI film repair. Within the above-mentioned cycle range, the number of cycles is sufficient to differentiate the quality of the SEI film. However, excessive cycles would lead to unnecessary time waste and increased testing costs. Preferably, the test temperature is 20-55°C, and the charge-discharge cycle rate is 0.02-0.8C.

[0019] In some implementations, in step S300, when the cell is a single cell, the coulombic efficiency of the plurality of cells is the set of the coulombic efficiencies of each single cell; when the cell is a cell group composed of a plurality of single cells, the coulombic efficiency of the plurality of cells is the set of the coulombic efficiencies of each cell group, wherein the coulombic efficiency of each cell group is the average coulombic efficiency of the single cells in that group.

[0020] In some implementations, in step S300, the coulombic efficiency includes the first-cycle coulombic efficiency. That is, when the difference in the first-cycle coulombic efficiency of different cells is higher than a threshold k (0.1% to 0.3%), the first-cycle coulombic efficiencies of multiple cells can be directly compared. The higher the first-cycle coulombic efficiency, the better the performance of the SEI film formed by the formation. When the difference in the first-cycle coulombic efficiency of different cells is not higher than the threshold k, the sum m of the coulombic efficiencies of multiple cells from the 1st to the nth cycle is compared. The higher the sum m, the better the performance of the SEI film formed by the formation. Here, n is any integer from 2 to 10.

[0021] The beneficial effects of the technical solutions provided by some embodiments of this application include at least the following: the determination method described in this application is applicable to batteries with different chemical systems composed of various positive electrodes, negative electrodes, separators, and electrolytes. This application can directly characterize the SEI film after its formation, and can quickly obtain qualitative data on whether there are obvious electron channels or lithium salt and solvent molecule channels in the SEI film after formation. When the test results show that the SEI film performance is poor, the quality of the SEI film can be improved by appropriately adjusting the formation temperature, pressure, current, cutoff voltage, or electrolyte composition. It can be used for screening electrolyte composition / anode materials and formation conditions, greatly reducing R&D costs and shortening the R&D cycle. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 A diagram illustrating the mechanism of the redox medium during the charging and discharging process of a battery cell.

[0024] Figure 2 EIS impedance spectrum of the battery cell after formation;

[0025] Figure 3 The first charge-discharge curves of battery cells with and without ferrocene added;

[0026] Figure 4 The graph shows the first charge-discharge coulombic efficiency test results of the three sets of cells in Example 1 without the addition of a redox medium.

[0027] Figure 5 The graph shows the first charge-discharge coulombic efficiency test results of the three sets of cells in Example 1 after adding redox media.

[0028] Figure 6 The graph shows the test results of three sets of battery cells without added redox media in Example 1 under conventional cycle testing.

[0029] Figure 7 The graph shows the first charge-discharge coulombic efficiency test results of the two sets of cells in Example 2 without the addition of a redox medium.

[0030] Figure 8 The graph shows the first charge-discharge coulombic efficiency test results of the two sets of cells in Example 2 after adding an oxidation-reduction medium. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0032] This application provides a method for determining the performance of an SEI film in a lithium-ion battery, comprising: step S100: performing formation treatment on multiple battery cells; step S200: adding a redox medium to the electrolyte of the formed battery cells, followed by aging treatment; step S300: performing cyclic charge-discharge tests on the battery cells obtained in step S200 to obtain the coulombic efficiency of the multiple battery cells; and step S400: determining the performance of the SEI film formed by the battery cell formation based on the coulombic efficiency of the battery cells. A poorly performing SEI film cannot effectively inhibit electrolyte decomposition, leading to irreversible charge loss and causing differences in charge quantity during charge and discharge (manifested as coulombic efficiency). This difference can be used to determine the quality of the SEI film.

[0033] Technical principles: such as Figure 1 As shown, if the SEI film performance is poor, such as uneven SEI film thickness, electron tunneling in certain areas, or cracks in the interface phase, the conductive surface is directly exposed to the electrolyte, providing a channel for charge transfer reactions between the redox medium and the negative electrode. This leads to irreversible charge loss, thereby reducing the coulombic efficiency, making the coulombic efficiency highly sensitive to the protective properties of the SEI film. For example, as... Figure 2 and Figure 3 As shown, Figure 2 The tests showed that the EIS impedance spectra of the multiple cells after formation did not exhibit significant differences. The ohmic impedance (Rs), SEI film impedance (Rsei), and charge transfer impedance (Rct) were basically consistent, indicating that the multiple cells had good consistency. Figure 2 Two cells with good test consistency are randomly selected for testing. Figure 3 The cyclic test was conducted, with ferrocene used as the redox medium. The cyclic test method can refer to existing conventional test methods, see [link to relevant documentation]. Figure 3 During the initial charge, the battery cell with added ferrocene (FC) had a charging capacity approximately 500mAh higher than the cell without it. However, during discharge, the former's capacity was approximately 146mAh lower than the latter, with a coulombic efficiency difference of about 16%. This indicates that the SEI film has significant cracks that are detected by the ferrocene. During charge and discharge, the ferrocene shuttles between the anode and cathode, continuously undergoing oxidation and reduction at the cracks in the SEI film, consuming electrons. It should be noted that... Figure 2 and Figure 3 The use of ferrocene as a redox medium is merely an example. Other redox media described in this application can also identify and qualitatively detect SEI films, and will not be repeated here.

[0034] Lithium-ion battery capacity test: At 25℃, the lithium-ion battery is first charged to 4.2V with a constant current of 0.1C (160mA), and then further charged with a constant voltage of 4.2V until the current is less than 0.05C (80mA) to obtain the initial charge capacity. Then, the lithium-ion battery is discharged to 3.0V with a constant current of 0.5C (800mA) to obtain the initial discharge capacity.

[0035] Lithium-ion battery AC impedance spectroscopy (EIS) test (testing cell consistency): At 25℃, the EIS of the formed lithium-ion battery was tested using an electrochemical workstation at a test frequency of 10000-0.1Hz and a perturbation voltage of ±5mV.

[0036] The following examples illustrate the implementation of this application in more detail. The battery cells used in step 1 of the following examples are all prepared using the methods described below.

[0037] (I) Cell fabrication

[0038] (1) Preparation of negative electrode sheet:

[0039] Graphite, conductive carbon (Super-P), styrene-butadiene rubber, and sodium carboxymethyl cellulose (CMC) were mixed in deionized water at a mass ratio of approximately 95:2:2:1 and stirred until homogeneous to obtain a negative electrode slurry. The slurry was coated onto a copper foil with a thickness of approximately 12 μm, dried, cold-pressed, and then cut and welded with tabs to obtain the negative electrode sheet.

[0040] (2) Preparation of the positive electrode sheet:

[0041] Lithium cobalt oxide (LiCoO2), conductive carbon (Super-P), and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of approximately 95:2:3 in the solvent N-methylpyrrolidone and stirred until homogeneous to obtain a positive electrode slurry. The slurry was coated onto an aluminum foil with a thickness of approximately 12 μm, dried, cold-pressed, and then cut and welded with tabs to obtain the positive electrode sheet.

[0042] (3) Preparation of electrolyte:

[0043] Lithium salt (LiPF6) was dissolved in a mixed solvent of EC / PC / DEC (mass ratio 1:1:1) to prepare a 1 mol / L LiPF6 electrolyte.

[0044] (4) Preparation of the separating membrane

[0045] PE porous polymer film is used as the separator.

[0046] (5) Electrolyte injection and encapsulation of battery cells

[0047] The obtained positive electrode, negative electrode and separator are wound in sequence, placed in the outer packaging foil, leaving the liquid injection port, and electrolyte is poured in through the liquid injection port and then sealed.

[0048] Example 1

[0049] Step 1: Obtain the three packaged cells and perform formation processing in sequence. The formation current is 1C, and the formation temperature and pressure are 70℃ and 0.9MPa, 70℃ and 1.2MPa, and 60℃ and 0.9MPa, respectively, to obtain the A group, B group, and C group cells after formation.

[0050] Step 2: Add ferrocene, a redox medium, sequentially to the A group, B group, and C group cells after formation. The concentration of the medium is 0.01M for each group.

[0051] Step 3: The three sets of cell gas-cutting bags are formed and aged at a temperature of 45℃ for 60 hours.

[0052] Step 4: Use conventional charging and discharging equipment to perform 5 cycles of cyclic charging and discharging tests on the A group cells, B group cells and C group cells in sequence. The test rate is 0.2C and the test temperature is 25℃.

[0053] Step 5: Determine the quality of the film formation based on the test results from Step 4.

[0054] Figure 4 The results of the first charge-discharge coulombic efficiency test for the three groups of cells (Group A, Group B, and Group C) without the addition of a redox medium (all other steps are the same except for step 2, i.e., the test conditions for other steps can refer to the above range) are provided by Figure 4 The results showed that the coulombic efficiency of the three groups of cells without added redox media did not show a significant difference in the first charge-discharge test. Figure 5 The test results are based on the three sets of battery cells prepared according to the steps in Example 1. Figure 5The results show (CE value): Group B > Group A > Group C (the SEI film of Group B is better than that of Group A, and the SEI film of Group A is better than that of Group C). Among them, the average coulombic efficiency values ​​of Group A cells, Group B cells and Group C cells are calculated to be 0.9877, 0.9888 and 0.9855 respectively.

[0055] Figure 6 To assess the test results of three sets of battery cells without added redox media under routine cycle testing, the test method for routine cycle testing is as follows: at a temperature of 45°C, the battery cells are charged at a constant current rate of 0.5C to 4.45V, then charged at a constant voltage rate to a current of 0.05C, and then discharged at a constant current rate of 0.5C to 3.0V. This charging and discharging cycle is repeated. Figure 6 The results showed that at the same 745cls, the capacity retention rates of the three groups of cells were 84.3%, 86.2%, and 81.2%, respectively, i.e., group B > group A > group C. Figure 5 The CE test results using redox media were consistent.

[0056] Example 2

[0057] Step 1: Obtain two packaged battery cells and perform formation treatments sequentially. The formation temperature and pressure are 75℃ and 0.6MPa, respectively, and the formation currents are 0.1C and 0.2C, respectively, to obtain the A group battery cells and B group battery cells after formation.

[0058] Step 2: Add the redox medium PHT sequentially to the A group cells and B group cells after formation. The concentration of the medium is 0.2M for both groups.

[0059] Step 3: Shape and age the two sets of cell gas-cutting bags. The aging temperature is 60℃ and the aging time is 180h.

[0060] Step 4: Use conventional charging and discharging equipment to perform 3 cycles of cyclic charging and discharging tests on the A group cells and the B group cells in sequence. The test rate is 0.5C and the test temperature is 45℃.

[0061] Step 5: Determine the quality of the film formation based on the test results from Step 4.

[0062] Figure 7 The results of the first charge-discharge coulombic efficiency test for the two groups of cells (Group A and Group B) without the addition of a redox medium (i.e., all other steps are the same except for step 2) are given by [the relevant authority / organization]. Figure 7 The results showed that the coulombic efficiency of the two sets of cells without added redox media did not show a significant difference in the first charge-discharge test. Figure 8 The test results of the two sets of battery cells prepared according to the steps of Example 2 (with the addition of a redox medium) are as follows: Figure 8 The results showed (CE value): Group B > Group A, meaning that the SEI membrane performance of Group B was better than that of Group A.

[0063] Example 3

[0064] Step 1: Obtain two packaged battery cells and perform formation processing sequentially. The formation temperature and pressure are 50℃ and 1.5MPa, respectively, and the formation currents are 1C and 1.5C, respectively, to obtain the A group battery cells and B group battery cells after formation.

[0065] Step 2: Add the redox medium methyl phthalimide sequentially to the formed A group cells and B group cells, with the concentration of the medium being 0.4M for both.

[0066] Step 3: Shape and age the two sets of cell gas-cutting bags. The aging temperature is 20℃ and the aging time is 220h.

[0067] Step 4: Use conventional charging and discharging equipment to perform 5 cycles of cyclic charging and discharging tests on the A group cells and the B group cells in sequence. The test rate is 0.5C and the test temperature is 45℃.

[0068] Step 5: Determine the quality of the film formation based on the test results from Step 4.

[0069] Example 4

[0070] Step 1: Obtain two packaged battery cells and perform formation treatments sequentially. The formation pressure is 1.1MPa, the formation current is 1.2C, and the formation temperatures are 55℃ and 70℃, respectively, to obtain the A group battery cells and B group battery cells after formation.

[0071] Step 2: Add a mixture of redox mediators silver nitrate and Mn-ATP (molar ratio of 1:1) sequentially to the A group and B group cells after formation. The concentration of the mediators is 0.3M.

[0072] Step 3: Shape and age the two sets of cell gas-cutting bags. The aging temperature is 45℃ and the aging time is 60h.

[0073] Step 4: Use conventional charging and discharging equipment to perform 5 cycles of cyclic charging and discharging tests on the A group cells and B group cells in sequence. The test rate is 0.02C and the test temperature is 35℃.

[0074] Step 5: Determine the quality of the film formation based on the test results from Step 4.

[0075] Example 5

[0076] Step 1: Obtain two packaged battery cells and perform formation processing sequentially. The formation temperature is 65℃, the formation current is 0.8C, and the formation voltage is 0.6MPa and 1.1MPa, respectively, to obtain the A group battery cells and B group battery cells after formation.

[0077] Step 2: Add a mixture of ferrocene and TEMPO (molar ratio of 1:2) to the A group and B group cells after formation, with the concentration of the medium being 0.1M.

[0078] Step 3: Shape and age the two sets of cell gas-cutting bags. The aging temperature is 45℃ and the aging time is 60h.

[0079] Step 4: Use conventional charging and discharging equipment to perform 10 cycles of cyclic charging and discharging tests on the A group cells and the B group cells in sequence. The test rate is 0.8C and the test temperature is 20℃.

[0080] Step 5: Determine the quality of the film formation based on the test results from Step 4.

[0081] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method of determining the performance of a lithium-ion battery SEI film, characterized by, The method comprises the following steps: S100: performing formation treatment on a plurality of battery cells; S200: adding a redox medium to the electrolyte of the battery cells after formation treatment, and then performing aging treatment; S300: performing a cycle charge-discharge test on the battery cells obtained in step S200 to obtain the coulomb efficiency of the plurality of battery cells; S400: determining the performance of the SEI film formed by the formation of the battery cells according to the coulomb efficiency of the battery cells; In step S200, the redox medium comprises any one of an inorganic redox medium, an organic redox medium, or a mixture containing an inorganic redox medium and an organic redox medium; The organic redox medium comprises at least one of TEMPO+ / TEMPO, PHT+ / PHT, or AQ+ / AQ.

2. The determination method according to claim 1, characterized in that, At least one of the following conditions is met: (1) In step S100, the battery cell is a single battery cell or a battery cell group composed of a plurality of single battery cells; (2) The concentration of the redox medium is 0.01-0.4 mol / L.

3. The determination method according to claim 2, characterized in that, The inorganic redox mediator includes at least one of Fe 3+ / Fe 2+ electroactive compound, Ag + / Ag electroactive compound, or Mn 4+ / Mn 2+ electroactive compound.

4. The determination method according to claim 3, characterized in that, The inorganic redox mediator includes Mn 4+ / Mn 2+ Electroactive compound and Fe 3+ / Fe 2+ Electroactive compound; or, The inorganic redox mediator includes Mn 4+ / Mn 2+ Electroactive compounds and Ag-containing + / Ag electroactive compounds.

5. The determination method according to claim 2, characterized in that, For a lithium ion battery with a graphite negative electrode, the concentration of the redox medium is 0.01-0.2 mol / L; For a lithium ion battery with a silicon-graphite composite negative electrode, the concentration of the redox medium is 0.03-0.3 mol / L; For a lithium ion battery with a silicon negative electrode, the concentration of the redox medium is 0.15-0.4 mol / L.

6. The determination method according to claim 5, characterized in that, In a lithium ion battery of a graphite negative electrode, when the specific surface area of the graphite is 0.6-1.1 m 2 / g, the concentration of the redox medium is 0.01-0.12 mol / L; In a lithium ion battery of a silicon negative electrode, when the specific surface area of the silicon particles is 4.08 to 6.1 m 2 / g, the concentration of the redox medium is 0.2 to 0.35 mol / L.

7. The determination method of claim 1, wherein, In step S200, the aging treatment conditions include: Aging temperature: 20-60°C; Aging time: 48-240 h.

8. The determination method of claim 1, wherein, In step S300, the cycle charge-discharge test conditions are: Test temperature: 12-60°C; Charge-discharge cycle rate: 0.01-2C; Cycle number: 1-10 cycles.

9. The determination method of claim 1, wherein, In step S300, When the battery cell is a single battery cell, the coulomb efficiency of the plurality of battery cells is the set of coulomb efficiencies of each single battery cell; When the battery cell is a battery cell group composed of a plurality of single battery cells, the coulomb efficiency of the plurality of battery cells is the set of coulomb efficiencies of each battery cell group, wherein the coulomb efficiency of each battery cell group is the average of the coulomb efficiencies of the single battery cells in the group; The coulomb efficiency includes the first cycle coulomb efficiency; and / or The coulomb efficiency includes the sum m of the coulomb efficiencies of the first cycle to the nth cycle, wherein n is any integer in the range of 2 to 10.

10. The determination method of claim 1, wherein, Step S400 includes comparing the first cycle coulomb efficiencies of the plurality of battery cells; When the difference between the first cycle coulomb efficiencies of different battery cells is higher than a threshold value k, the higher the first cycle coulomb efficiency, the better the performance of the SEI film formed by the formation; When the difference between the first cycle coulomb efficiencies of different battery cells is not higher than the threshold value k, then the sum m of the coulomb efficiencies of the first cycle to the nth cycle of the plurality of battery cells is compared, and the higher the sum m value, the better the performance of the SEI film formed by the formation; Wherein, the value range of k is 0.1%-0.3%.

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