Method for evaluating SEI stability of lithium ion battery

Abstract

This invention discloses a method for evaluating the SEI stability of lithium-ion batteries, relating to the field of lithium-ion battery technology. By presetting an over-discharge cutoff voltage and an over-discharge capacity threshold, this invention establishes a dual-threshold control mechanism: either a voltage cutoff or a capacity cutoff, stopping once either is reached. This limits over-discharge stress to a low to moderate level, primarily affecting the negative electrode SEI interface and reducing the risk of excessive copper current collector dissolution and internal micro-short circuits. The capacity loss rate ΔC and the growth rate ΔR of the SEI-related interface impedance parameters are also considered. SEI and self-recovery voltage V rec A weighted scoring and collaborative judgment are performed, with the three factors reflecting the degree of SEI degradation from three independent dimensions: active lithium loss, interface impedance change, and polarization recovery capability. During the capacity recovery charging stage, the constant voltage stage duration tcv or the constant voltage stage capacity ratio Rcv is introduced as an auxiliary judgment parameter. The parameters are mutually corrected to avoid misjudgment caused by fitting error or external interference of a single parameter.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and more specifically, to a method for evaluating the SEI stability of lithium-ion batteries. Background Technology

[0002] The solid electrolyte interphase (SEI) film at the negative electrode of a lithium-ion battery significantly impacts the battery's cycle life, low-temperature performance, rate performance, self-discharge, and safety. The stability of the SEI film directly determines the battery's capacity retention and interfacial impedance growth under long-cycle, high-temperature storage, and abnormal operating conditions. Film-forming additives, lithium salt types, and solvent composition in the electrolyte formulation, as well as the particle size, specific surface area, and surface state of the negative electrode material, all significantly affect the SEI film's chemical composition, thickness, density, and self-healing ability. Therefore, a rapid and quantitative evaluation method to differentiate the stability of different SEI films is needed during electrolyte formulation development and negative electrode material screening.

[0003] Currently, the main methods for evaluating SEI film stability include high-temperature storage testing, long-cycle testing, and electrochemical impedance spectroscopy (EIS). High-temperature storage testing typically requires storing the film at 55°C–60°C for several weeks to months before measuring capacity retention and impedance growth; this process is lengthy and requires a large sample space. Long-cycle testing usually requires hundreds to thousands of charge-discharge cycles, which is even more time-consuming and costly. While EIS can fit interfacial impedance parameters, using the rate of increase of these parameters alone makes it difficult to distinguish between various degradation mechanisms such as SEI re-formation, loss of positive electrode active material, and copper deposition on the negative electrode, leading to potential misjudgments.

[0004] Conventional over-discharge tests are mostly used for battery safety or abuse boundary assessment. Typically, the battery is over-discharged to 0V or close to 0V, and only the presence of fire, swelling, leakage, or failure is observed. These test conditions are severe and can easily cause extensive dissolution of copper current collectors, internal short circuits, and irreversible failures. The tested cells cannot be used for SEI film state assessment, nor can the stability of SEI films formed by different electrolyte formulations or different negative electrode materials be quantitatively graded.

[0005] Therefore, the existing technology has the following main defects: (1) the evaluation cycle of SEI stability is long and the cost is high; (2) the conventional over-discharge stress is uncontrollable and cannot be used as an accelerated stress screen for SEI; (3) the single parameter judgment is not distinguishable enough and cannot accurately reflect the difference between SEI degradation and other failure modes; (4) there is a lack of a rapid SEI screening method that is not limited to specific battery types and capacity specifications, is quantitative, repeatable, and can be extended to different positive and negative electrode systems.

[0006] In view of this, the present invention is proposed. Summary of the Invention

[0007] The purpose of this invention is to provide a method for evaluating the SEI stability of lithium-ion batteries, aiming to provide a reliable, low-damage, and repeatable evaluation method.

[0008] This invention is implemented as follows: In a first aspect, the present invention provides a method for evaluating the SEI stability of a lithium-ion battery, comprising: S1. Initial parameters of the same specification lithium-ion battery under test are measured to obtain the initial capacity C0 and the initial SEI-related interface impedance parameter R. int0 and initial open-circuit voltage; S2. Apply a controlled over-discharge impact to the battery, and over-discharge it at a preset temperature and a preset current. Stop when the preset over-discharge cutoff voltage or preset over-discharge capacity threshold is reached. The preset temperature is 15℃~45℃, the preset current is 0.05C~1C, the preset over-discharge cutoff voltage is 0.5V~1.8V, and the preset over-discharge capacity threshold is 1%~20% of the rated capacity. S3. After stopping the over-discharge, allow the circuit to stand open and collect the self-recovery voltage V. rec Among them, the self-recovery voltage V rec The open-circuit voltage after the over-discharge has stopped and the circuit has been left to stand for 5 to 120 minutes. S4. Charge the battery using a constant current and constant voltage method to restore its capacity, and record the duration t of the constant voltage phase. cv Or the constant voltage section capacity ratio R cv Then, a standard discharge is performed to obtain the recovered capacity C1 after the over-discharge impulse, and the capacity loss rate ΔC is calculated. The capacity recovery charge is constant current charging at 0.05C~1C to the charging cutoff voltage, and then constant voltage charging is performed to the cutoff current at 0.02C~0.1C. The capacity loss rate ΔC = (C0-C1) / C0×100%; S5. Perform EIS testing under the same state of charge and temperature as the initial EIS test, and obtain the SEI-related interface impedance parameter R after the over-discharge impact. int1 Calculate the growth rate ΔR of the SEI-related interface impedance parameter. SEI SEI-related interface impedance parameter growth rate ΔR SEI =(R int1 -R int0 ) / R int0 ×100%; S6. Based on ΔC and ΔR SEI V rec Perform weighted scoring and collaborative judgment, and use t cv Or R cv As an auxiliary judgment parameter, the SEI stability evaluation result is output.

[0009] In an optional implementation, in step S6, the weighted scoring collaborative determination is performed as follows: ΔC and ΔR are... SEI t cv Or R cv According to linear normalization where smaller is better, V rec Linear normalization is applied, with larger values ​​preferred; core parameter weights α=0.40, β=0.35, γ=0.25, and auxiliary parameter weight δ=0.10; the core score S is calculated. core and final score S final ; Core overall score S core Calculate using the following formula: ; Final overall score: S final Calculate using the following formula: ;or, ; The grading criteria are as follows: If S final ≥85, SEI stability is rated as excellent; If 65≤S final <85, SEI stability is rated as good; If 50 ≤ S final <65, SEI stability is judged as medium; If S final If the value is less than 50, the SEI stability is considered poor.

[0010] In optional implementations, parameters that are preferred when smaller include the capacity loss rate ΔC and the growth rate ΔR of the SEI-related interface impedance parameter. SEI Duration t of the constant pressure stage cv and the proportion of constant voltage section capacity R cv The following linear normalization formula is used: ; Where: x is the actual measured value of the parameter to be normalized; x best This is the baseline value of 100 points for the corresponding parameter; x worst This is the baseline value of 0 for the corresponding parameter; Constraints: When x≤x best When f(x) = 100; When x≥x worst When f(x) = 0; When x best <x<x worst When f(x) is calculated according to the formula, its value ranges from 0 to 100.

[0011] In an optional implementation, for the parameter that is larger is better - self-recovery voltage V rec The following linear normalization formula is used: ; V best This is the baseline value for 100 points of the self-recovery voltage; V worst This is the zero-point reference value for the self-recovery voltage; Constraints: When V rec ≥V best When, f(V) rec )=100; When V rec ≤V worst When, f(V) rec )=0; When V worst <V rec <V best When, f(V) rec Calculated according to the formula, the value ranges from 0 to 100.

[0012] In an optional implementation, the method for determining the normalized benchmark value is as follows: Select cells from the same batch or system, pre-store them at 60°C for at least 30 days, pre-classify SEI stability based on capacity retention rate, classify samples with high-temperature storage retention rate ≥90% as excellent, 80%~90% as good, 70%~80% as medium, and <70% as poor; test samples with known high-temperature storage levels under the same controlled over-discharge impact conditions, statistically analyze the distribution range of each parameter in samples of different levels, use the typical lower limit or upper limit of excellent level samples as the 100-point benchmark value, use the typical upper limit or lower limit of poor level samples as the 0-point benchmark value, and ensure that the parameter values ​​of more than 99% of the tested samples fall within the linear scoring interval between the two benchmark values.

[0013] In an optional implementation, the normalized reference values ​​are set as follows: capacity loss rate 1.0% for 100 points, 8.0% for 0 points; SEI-related interface impedance parameter growth rate 10.0% for 100 points, 60.0% for 0 points; self-recovery voltage 3.2V for 100 points, 2.0V for 0 points; constant voltage stage duration 10min for 100 points, 30min for 0 points; constant voltage stage capacity percentage 3.0% for 100 points, 12.0% for 0 points.

[0014] In an optional implementation, in step S2, the preset temperature is 23℃~27℃, the preset current is 0.05C~0.2C, the preset over-discharge cutoff voltage is 1.0V~1.5V, and the preset over-discharge capacity threshold is 5%~10% of the rated capacity; And / or, after a controlled over-discharge shock, the battery terminal voltage rebounds to above 2.0V, and subsequent capacity recovery charging and EIS testing can be completed.

[0015] In an optional implementation, the self-recovery voltage V rec The open-circuit voltage is the voltage after the over-discharge has stopped and the circuit has been left to stand for 30 to 60 minutes.

[0016] In an optional implementation, in step S4, the capacity recovery charge is performed by constant current charging at 0.2C to 0.5C until the charging cutoff voltage.

[0017] In an optional implementation, in step S5, the frequency range of the EIS test is 10kHz to 0.01Hz, and the disturbance voltage is 5mV to 10mV or the disturbance current is 50mA to 200mA.

[0018] This invention has the following beneficial effects: By presetting the over-discharge cutoff voltage and over-discharge capacity threshold, this invention forms a dual-threshold control system of "voltage cutoff or capacity cutoff, stopping when either is reached," thus limiting the over-discharge stress to a low to medium level. It primarily acts on the negative electrode SEI interface, reducing the risk of excessive copper current collector dissolution and internal micro-short circuits. This also reduces the capacity loss rate ΔC and the growth rate ΔR of the SEI-related interface impedance parameters. SEI and self-recovery voltage V rec A weighted scoring and collaborative judgment are performed, with the three factors reflecting the degree of SEI degradation from three independent dimensions: active lithium loss, interface impedance change, and polarization recovery capability. During the capacity recovery charging stage, the constant voltage stage duration tcv or the constant voltage stage capacity ratio Rcv is introduced as an auxiliary judgment parameter. The parameters are mutually corrected to avoid misjudgment caused by fitting error or external interference of a single parameter. Attached Figure Description

[0019] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 The overall flowchart of the lithium-ion battery SEI stability evaluation method based on controlled over-discharge impact provided by the present invention; Figure 2The images show the EIS (Nyquist plot) of the battery in Example 1 in its initial state and after a controlled over-discharge shock. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0022] This invention provides a method for evaluating the SEI stability of lithium-ion batteries, such as... Figure 1 As shown, the steps are as follows: S1, Initial Parameter Measurement Initial parameters were measured on lithium-ion batteries of the same specification to obtain the initial capacity C0 and the initial SEI-related interfacial impedance parameter R. int0 And initial open-circuit voltage. Only lithium-ion batteries under test of the same specification are comparable in stability. Same specification means that the positive electrode material system, negative electrode material system, electrolyte system and separator type are the same, the rated capacity, charging cut-off voltage and discharging cut-off voltage are the same, the battery structure (wound or stacked), the external dimensions and current collector design are the same, and the batteries are manufactured using the same manufacturing process. Only batteries of the same specification have comparable test results for SEI-related interface impedance parameters, capacity, etc., and only then can normalized benchmark values ​​and scoring results be used interchangeably.

[0023] Specifically, the initial capacity C0 and the initial SEI-related interface impedance parameter R int0 The test method for initial open-circuit voltage is a general method. For example, the test method for initial capacity C0 is as follows: At 25℃±2℃, charge at a constant current of 0.5C until the charging cutoff voltage is 3.65V, then switch to constant voltage charging until the current drops to 0.05C cutoff. After standing for 30 minutes, discharge at a constant current of 0.5C until the discharge cutoff voltage is 2.5V. Record the discharge capacity. Repeat 2-3 times, and take the stable value as the initial capacity C0. The initial SEI-related interface impedance parameter R... int0 The test method is as follows: The battery is adjusted to the specified state of charge (27% SOC in this example), and after being left to stand at 25°C until voltage equilibrium is reached, an EIS test is performed (frequency range 10kHz~0.01Hz, disturbance voltage 5mV). R is obtained using equivalent circuit fitting. int0 The initial open-circuit voltage test method is as follows: The open-circuit voltage is measured after the battery has been left to stand at 25℃±2℃ until the voltage stabilizes (the voltage change within 1 hour does not exceed 1mV). This is used to confirm the consistency of the initial state of the batteries under test of the same specification and to eliminate abnormal samples.

[0024] S2, Over-discharge impact A controlled over-discharge impact is applied to the battery, and over-discharge is performed at a preset temperature and a preset current. The over-discharge is stopped when the preset over-discharge cutoff voltage or preset over-discharge capacity threshold is reached, so that the SEI film is damaged to a controlled degree of low to moderate extent, and the battery can still be recovered.

[0025] The preset temperature is 15℃~45℃, such as 15℃, 20℃, 23℃, 25℃, 27℃, 30℃, 35℃, 40℃, 45℃, etc., preferably 23℃~27℃. The preset current is 0.05C~1C, such as 0.05C, 0.10C, 0.20C, 0.30C, 0.40C, 0.50C, 0.60C, 0.70C, 0.80C, 0.90C, 1.00C, etc., preferably 0.05C~0.2C. The preset over-discharge cutoff voltage is 0.5V~1.8V, such as 0.5V, 0.8V, 1.0V, 1.3V, 1.5V, 1.8V, etc., preferably 1.0V~1.5V. The preset over-discharge capacity threshold is 1% to 20% of the rated capacity, such as 1%, 5%, 8%, 10%, 13%, 15%, 18%, 20%, etc., preferably 5% to 10%.

[0026] The preferred stopping conditions are voltage cutoff or over-discharge capacity cutoff; stopping occurs when either condition is met. By controlling the depth of over-discharge, the over-discharge stress is primarily applied to the negative electrode interface, reducing the risk of significant dissolution of the copper current collector and internal short circuits, but the occurrence of localized side reactions cannot be ruled out. After a controlled over-discharge impact, the battery terminal voltage can rebound to above 2.0V, and subsequent capacity recovery charging and EIS testing can be completed.

[0027] S3, Open circuit static setting After the over-discharge stops, the circuit is left open and the self-recovery voltage V is collected. rec Among them, the self-recovery voltage V rec The open-circuit voltage after stopping over-discharge and allowing the circuit to stand for 5 to 120 minutes is preferred, with a standing time of 30 to 60 minutes being ideal, such as 30 minutes or 60 minutes.

[0028] It should be noted that V rec It mainly reflects the electrode polarization relaxation capability and the degree of interface damage after over-discharge impact. V rec A higher value indicates less polarization and interface damage caused by over-discharge shock, and better electrode potential recovery capability; V rec A lower value indicates slower polarization relaxation and more severe interface damage. If copper deposition or a micro-short circuit is present, there will be continuous internal leakage during the battery's open-circuit resting period, manifested as V. rec The voltage is significantly low, and the open-circuit voltage continues to drop or fails to rebound effectively during the resting period.

[0029] S4, Capacity Recovery Charging The battery was charged using a constant current and constant voltage method to restore its capacity, and the duration t of the constant voltage phase was recorded. cv Or the constant voltage section capacity ratio R cv Then, a standard discharge is performed to obtain the recovery capacity C1 after the over-discharge impact. The capacity loss rate ΔC is calculated as follows: Capacity loss rate ΔC = (C0 - C1) / C0 × 100%.

[0030] Specifically, the capacity recovery charging is performed with a constant current of 0.05C to 1C (preferably 0.2C to 0.5C) until the charging cutoff voltage, followed by constant voltage charging until the cutoff current of 0.02C to 0.1C. The duration t of the constant voltage stage is recorded. cv Or, constant voltage phase capacity percentage (the percentage of capacity charged during the constant voltage phase in the capacity recovery charging process relative to the total recovered charging capacity) R cv The standard discharge test method is as follows: discharge at a constant current of 0.5C to the discharge cutoff voltage of 2.5V at 25℃±2℃.

[0031] S5, EIS test EIS testing was conducted under the same state of charge and temperature as the initial EIS test, and the SEI-related interface impedance parameter R after the over-discharge impact was obtained by fitting. int1 Calculate the growth rate ΔR of the SEI-related interface impedance parameter. SEI SEI-related interface impedance parameter growth rate ΔR SEI =(R int1 -R int0 ) / R int0 ×100%.

[0032] In some embodiments, in step S5, the frequency range of the EIS test is 10kHz to 0.01Hz, and the disturbance voltage is 5mV to 10mV or the disturbance current is 50mA to 200mA. That is, either the AC disturbance voltage amplitude or the AC disturbance current amplitude can be used, with the aim of making the battery system operate in the linear region.

[0033] Specifically, the equivalent circuit is R0 + R int / CPE1 + R ct / CPE2 + W, this circuit is R0 and (R int Parallel structure with CPE1), (R) ct The parallel structure with CPE2 and the Warburg impedance W are connected in series. Where: R0 is the ohmic impedance, corresponding to the intersection of the high-frequency end and the real axis of the Nyquist plot, including the ionic resistance of the electrolyte, the ohmic resistance of the positive and negative current collectors and the tabs, and the contact resistance of each component; R int Connected in parallel with the constant-phase element CPE1, corresponding to the first semicircle in the high-frequency region, R intThese are SEI-related interface impedance parameters, primarily characterizing the impedance of lithium ions crossing the negative electrode SEI film; CPE1 characterizes the capacitance characteristics of the SEI film; R ct Connected in parallel with the constant-phase element CPE2, corresponding to the second semicircle in the mid-frequency region, R ct denoted as , where is the charge transfer impedance at the electrode / electrolyte interface, and CPE2 represents the interfacial double-layer capacitance. W is the Warburg impedance, corresponding to the low-frequency slant line, representing the diffusion impedance of lithium ions in the bulk phase of the electrode active material. Impedance analysis software (such as ZView) was used for fitting, and the measured EIS spectrum was fitted using the equivalent circuit described above with least-squares fitting. R0 was taken as . int The fitted value. Since the two semicircles of a real full cell overlap to some extent, R... int It may also include contributions from partial charge transfer processes; therefore, this invention requires that R be tested under the same SOC, temperature, and test conditions. int A relative comparison shows that the changes are mainly related to the state of the SEI film on the negative electrode, but not the only source. The growth rate of the SEI-related interfacial impedance parameters is ΔR. SEI = (R int1 - R int0 ) / R int0 × 100%. The method provided by this invention uses full-cell EIS testing, does not disassemble the battery, and does not prepare symmetric cells.

[0034] S6. Conduct SEI stability evaluation Based on ΔC, ΔR SEI V rec Perform weighted scoring and collaborative judgment, and use t cv Or R cv As an auxiliary judgment parameter, the SEI stability evaluation result is output.

[0035] The weighted scoring collaborative determination method is as follows: ΔC and ΔR are used to determine the weighted scores. SEI t cv Or R cv According to linear normalization where smaller is better, V rec Linear normalization is applied, with larger values ​​indicating better performance. Core parameter weights are α=0.40, β=0.35, γ=0.25, and auxiliary parameter weights are δ=0.10; the core score S is calculated. core and final score S final .

[0036] Among them, the core comprehensive score S core Calculate using the following formula: ; Final overall score: S final Calculate using the following formula: ;or, ; The grading standard is: if S final ≥85, SEI stability is rated as "excellent"; if 65≤S final <85, SEI stability is judged as "good"; if 50≤S final If S < 65, the SEI stability is judged as "medium"; if S final If the value is less than 50, the SEI stability is rated as "poor".

[0037] It should be noted that the above weighting allocation is determined as follows: the capacity loss rate ΔC directly reflects the total loss of active lithium after over-discharge shock, and has the strongest correlation with SEI repair consumption and long-term storage performance, therefore it has the largest weight; the growth rate ΔR of SEI-related interface impedance parameters... SEI It reflects the degree of SEI film damage, but there is some error in the full-cell EIS fitting, so its weight is slightly lower; self-recovery voltage V rec It reflects polarization relaxation and potential recovery capability, but is greatly affected by static conditions and micro-short circuits, exhibiting strong fluctuations; therefore, its weight is considered again. Auxiliary parameter t cv Or R cv It is somewhat correlated with the core parameters. To avoid interfering with the core determination, its weight δ is limited to between 0 and 0.2, and is 0.10 for example.

[0038] Furthermore, for parameters that are better when smaller, such as the capacity loss rate ΔC and the growth rate ΔR of the SEI-related interface impedance parameter, the optimal values ​​are as follows: SEI Duration t of the constant pressure stage cv and the proportion of constant voltage section capacity R cv The following linear normalization formula is used: ; Where: x is the actual measured value of the parameter to be normalized; x best This is the baseline value of 100 points for the corresponding parameter; x worst This is the baseline value of 0 for the corresponding parameter; Constraints: When x≤x best When f(x) = 100; When x≥x worst When f(x) = 0; When x best <x<x worst When f(x) is calculated according to the formula, its value ranges from 0 to 100.

[0039] Furthermore, for the parameter where a larger value is better – the self-recovery voltage V rec The following linear normalization formula is used: ; V best This is the baseline value for 100 points of the self-recovery voltage; V worst This is the zero-point reference value for the self-recovery voltage; Constraints: When V rec ≥V best When, f(V) rec )=100; When V rec ≤V worst When, f(V) rec )=0; When V worst <V rec <V best When, f(V) rec Calculated according to the formula, the value ranges from 0 to 100.

[0040] Furthermore, the method for determining the normalized benchmark value is as follows: Select cells from the same batch or system, and pre-store them at 60℃ for at least 30 days (30 days or longer). Based on the capacity retention rate, perform a pre-classification of SEI stability. Samples with a high-temperature storage retention rate ≥90% are classified as excellent, 80%–90% as good, 70%–80% as medium, and <70% as poor. Test samples with known high-temperature storage grades under the same controlled over-discharge impact conditions, and statistically analyze the distribution range of each parameter in samples of different grades. Use the typical lower or upper limit of the "excellent" grade samples as the 100-point benchmark value, and the typical upper or lower limit of the "poor" grade samples as the 0-point benchmark value. Ensure that the parameter values ​​of most (e.g., above 99%) of the tested samples fall within the linear scoring interval between the two benchmark values ​​to guarantee sufficient discrimination in the scoring. Specifically, for parameters where smaller values ​​are considered better, including capacity loss rate ΔC and the growth rate of SEI-related interface impedance parameters ΔR... SEI Duration t of the constant pressure stage cv and the proportion of constant voltage section capacity R cv The baseline value of 100 points is the typical lower limit, i.e., the minimum value, for a sample of "excellent" grade; for the parameter where a larger value is better - the self-recovery voltage V rec Therefore, a score of 100 is the typical upper limit, or maximum value, for a sample of the "excellent" grade. The principle for determining a score of 0 is similar and will not be explained in detail here.

[0041] The benchmark values ​​will differ depending on the battery specifications. For a 5Ah lithium iron phosphate / artificial graphite soft-pack lithium-ion battery (charging cutoff voltage 3.65V, discharging cutoff voltage 2.5V), the normalized benchmark values ​​are set as follows: capacity loss rate 1.0% for 100 points, 8.0% for 0 points; SEI-related interface impedance parameter growth rate 100% for 100 points, 60.0% for 0 points; self-recovery voltage 3.2V for 100 points, 2.0V for 0 points; constant voltage stage duration 10min for 100 points, 30min for 0 points; constant voltage stage capacity percentage 3.0% for 100 points, 12.0% for 0 points.

[0042] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0043] Battery systems in Examples 1-2 and Comparative Example 1: A 5Ah lithium iron phosphate / artificial graphite pouch cell was used as the subject. The positive electrode active material was lithium iron phosphate. The positive electrode active coating consisted of lithium iron phosphate, conductive carbon black (Super P) as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder in a mass ratio of 94:3:3. The coating was applied to both sides, with a double-sided areal density of 31.5 mg / cm². 2 The compacted density after roller pressing is approximately 2.35 g / cm³, using 13μm thick aluminum foil as the current collector; the negative electrode active material is artificial graphite, and the negative electrode active coating is composed of artificial graphite, conductive carbon black (Super P), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) in a mass ratio of 94.5:1.5:2.0:2.0, double-sided coating, with a double-sided areal density of 16.3 mg / cm³. 2 The compacted density after rolling is approximately 1.45 g / cm³, with an 8 μm thick copper foil as the current collector. The electrolyte formulation is ethylene carbonate (EC): ethyl methyl carbonate (EMC): dimethyl carbonate (DMC) = 3:4:3, with a LiPF6 concentration of 1.0 mol / L. The separator is a ceramic-coated PE membrane (PE base membrane thickness 9 μm, ceramic coating thickness 3 μm). The battery was standardized and used as the test battery.

[0044] The electrolyte formulations of Examples 1-2 and Comparative Example 1 are different, as detailed below: Example 1: Basic electrolyte + 2.5 wt% vinylene carbonate (VC) + 0.5 wt% fluoroethylene carbonate (FEC); Example 2: Basic electrolyte + 2.5 wt% VC; The EIS spectra of the battery in Example 1 before and after the controlled over-discharge impact are as follows: Figure 2As shown, after the controlled over-discharge impact, the first semicircle in the high-frequency region increased significantly, and the fitted SEI-related interface impedance parameters increased from 1.87mΩ to 2.32mΩ, with a growth rate of 24.1%. This indicates that the controlled over-discharge impact caused the negative electrode SEI film to undergo controllable degradation, and the battery did not swell, leak, or experience internal short circuits.

[0045] Comparative Example 1: Basic electrolyte, without VC and FEC.

[0046] Test conditions: Controlled over-discharge impact: Under an environment of 25℃±2℃, constant current discharge at 0.1C to 1.5V or over-discharge capacity reaching 5% of rated capacity, stopping immediately upon either condition is met; Self-recovery voltage: After stopping over-discharge, let stand for 30 minutes and record the open-circuit voltage; Capacity recovery charging: Charge at a constant current of 0.2C to 3.65V, then maintain constant voltage until 0.05C cutoff. Record the constant voltage period t. cv and the proportion of constant voltage section capacity R cv ; EIS test: 27% SOC, 25℃, frequency 10kHz~0.01Hz, disturbance voltage 5mV, equivalent circuit fitting yields R. int .

[0047] Calculate S according to the calculation method described in the instruction manual. final The duration of the constant pressure phase (tcv) was used as an auxiliary judgment parameter. The test results for each parameter are shown in Table 1. Table 1 Test results of Examples 1-2 and Comparative Example 1

[0048] As can be seen from the table, S in Example 1 final The value was 86.0, with the highest retention rate at 60°C; Example 2's S final The retention rate is 66.8%, which is moderate; compared to the S in Comparative Example 1. final With a score of only 6.6, it had the worst storage retention rate. This shows that the weighted score is positively correlated with high-temperature storage performance.

[0049] Application examples 1-3 are for different anode materials, as follows: Using the electrolyte formulation of Example 2, battery cells were prepared with different negative electrode materials: Application Example 1: Artificial graphite A, D50 = 8 μm, specific surface area 4.5 m² 2 / g; Application Example 2: Artificial graphite B, D50 = 18 μm, specific surface area 2.0 m² 2 / g; Application Example 3: Natural graphite C, D50 = 15 μm, specific surface area 5.0 m²2 / g.

[0050] The test conditions were the same as those in the example group. The test results are shown in Table 2: Table 2 Test results of application examples 1-3

[0051] Application Example 4-6 for different formation systems Under the same electrolyte and negative electrode material conditions (using the electrolyte formulation of Example 2, artificial graphite B), different formation regimes were employed: Application Example 4: Normalization, 0.05C to 3.0V, then 0.1C to 3.65V, cutoff at 0.02C; Application Example 5: Slow formation, 0.02C to 3.65V, cutoff at 0.01C; Application Example 6: Fast formation, 0.5C to 3.65V, cutoff at 0.1C.

[0052] The test results of Application Example 4-6 are shown in Table 3: Table 3 Test results of application examples 4-6

[0053] Slower conversion to SEI results in more complete film formation, S final It has the highest performance and the best retention rate during high-temperature storage.

[0054] Comparative Example 2: Conventional over-discharge to 0V To illustrate the difference between this method and conventional over-discharge testing, conventional over-discharge testing was performed on cells of the same specification. The cells were discharged to 0V at a constant current of 0.1C, the cell status was observed, and a capacity recovery charge was attempted.

[0055] The results show that the cell swells significantly after being over-discharged to 0V, and the terminal voltage cannot recover normally, making it impossible to complete normal constant current and constant voltage capacity recovery charging. This cell cannot be used for SEI stability evaluation.

[0056] In contrast, the controlled over-discharge impacts in Examples 1-2 and Comparative Example 1 did not exhibit bulging, leakage, or internal short circuits, and all were able to complete subsequent capacity recovery charging and EIS testing. This indicates that the present method, by controlling the over-discharge cutoff voltage and over-discharge capacity threshold, can avoid irreversible damage caused by conventional over-discharge testing.

[0057] Comparative Example 3: Over-discharge to 2.0V cannot effectively distinguish SEI stability. To further illustrate the impact of over-discharge depth selection on method discrimination, the over-discharge cutoff voltage was adjusted to 2.0V, and the cells of Example 1, Example 2, and Comparative Example 1 were tested, with all other conditions remaining unchanged.

[0058] The test results are shown in Table 4: Table 4. Test results for over-discharge to 2.0V

[0059] As can be seen from the table, when the over-discharge cutoff voltage is 2.0V, the S of the three electrolyte cells... final All values ​​exceeded 85, and all were rated "excellent," making it impossible to distinguish the actual differences in SEI stability. This indicates that when the over-exposure depth is too shallow, the SEI membrane is not fully activated, and the stability differences between different SEIs cannot be effectively amplified, thus losing the screening significance.

[0060] Based on the test results of over-discharge to 1.5V in the example group, when over-discharged to 1.5V, the three S final The values ​​are 86.0, 66.8, and 6.6 respectively, which can effectively distinguish between excellent, good, and poor. Therefore, the over-discharge cutoff voltage range of 0.5V to 1.8V, preferably 1.0V to 1.5V, is selected in this method to achieve a balance between low damage and high distinguishability, thus possessing specificity.

[0061] This invention provides a method for evaluating the SEI stability of lithium-ion batteries, which has the following advantages: (1) Transforming over-discharge testing from destructive safety evaluation to low-damage SEI accelerated stress screening: This invention establishes a dual-threshold control system by presetting an over-discharge cutoff voltage and an over-discharge capacity threshold, allowing the over-discharge stress to be limited to a low to medium level. This primarily affects the negative electrode SEI interface, reducing the risk of massive dissolution of copper current collectors and internal micro-short circuits. After a controlled over-discharge impact, the battery terminal voltage can rebound to above 2.0V, and capacity recovery charging and EIS testing can continue. Comparative Example 2 shows that conventional over-discharge to 0V results in cell bulging and irreversibility; however, the cell does not bulge, leak, or fail under the method of this invention. This demonstrates that this method avoids the irreversible damage of conventional over-discharge testing and achieves low-damage, repeatable SEI accelerated stress screening.

[0062] (2) The three-parameter weighted scoring collaborative judgment improves the accuracy and discrimination of SEI stability evaluation: This invention combines the capacity loss rate ΔC, the growth rate of SEI-related interface impedance parameters ΔR, and the three-parameter weighted scoring collaborative judgment. SEI and self-recovery voltage V rec A weighted scoring system is used for collaborative judgment. The three factors—active lithium loss, interfacial impedance change, and polarization recovery capability—reflect the degree of SEI degradation from three independent dimensions, mutually correcting each other to avoid misjudgments caused by fitting errors or external interference from a single parameter. Examples and comparative cases show that using only V... rec During the judgment, Comparative Example 1 was mistakenly judged as "medium" using only ΔR. SEIIn the judgment, Example 3 was misclassified as "Good," while the three-parameter weighted scoring collaborative judgment could correctly distinguish between Excellent, Good, Average, and Poor. This shows that the proposed method has higher reliability and resolution than single-parameter judgment.

[0063] (3) The auxiliary parameters of the constant voltage stage provide preliminary ability to distinguish failure modes: The present invention introduces the duration t of the constant voltage stage in the capacity recovery charging stage. cv Or the constant voltage section capacity ratio R cv As an auxiliary judgment parameter, ΔR is used when SEI re-formation / repair is significant. SEI Higher and t cv Significantly increased, while V rec Relatively normal; when the possibility of positive electrode damage is high, ΔC is larger but ΔR is smaller. SEI and t cv The change is not significant; when there is a risk of copper deposition or micro-short circuit, V rec The voltage was significantly lower than expected and continued to decrease during the resting period, with a large ΔC. Therefore, this method, while providing the SEI stability level, can also preliminarily determine the main failure direction under the condition of no disassembly of the entire cell, thus improving the information content of the evaluation results.

[0064] (4) The weighted score is positively correlated with high-temperature storage performance and has long-term performance prediction capability: The controlled over-discharge impact weighted score S established in this invention final There is a clear positive correlation between the capacity retention rate and the 30-day high-temperature storage at 60°C of the same batch of cells. Example 1, S final The value is 86.0, corresponding to a memory retention rate of 94%; S in Example 2 final It is 66.8, corresponding to 85%; the S of Comparative Example 1 final The score is 6.6, corresponding to 65%. The benchmark calibration process uses high-temperature storage performance as an independent external basis, ensuring that the score is not arbitrarily set afterward. Therefore, this method can predict the long-term storage performance of battery cells, providing a rapid screening method for long-cycle tests such as high-temperature storage.

[0065] (5) The method is universal and repeatable, applicable to various battery systems and screening scenarios: This invention does not rely on specific battery capacity, size, or specific positive and negative electrode materials. All tests are performed in situ on the battery under test, without disassembling the battery or preparing symmetrical batteries. For different negative electrode materials, adaptive calibration can be achieved by adjusting the over-discharge cutoff voltage. Repeatability tests show that after three consecutive controlled over-discharge shocks to the same cell, S final The variation range does not exceed 2%, and the cell can still continue to complete subsequent tests. Therefore, this method has good low-damage, repeatability, and engineering versatility, and can be used for electrolyte formulation screening, anode material screening, and formation regime screening.

[0066] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for evaluating the SEI stability of a lithium-ion battery, characterized in that, include: S1. Initial parameters of the same specification lithium-ion battery under test are measured to obtain the initial capacity C0 and the initial SEI-related interface impedance parameter R. int0 and initial open-circuit voltage; S2. Apply a controlled over-discharge impact to the battery and over-discharge it at a preset current at a preset temperature. Stop when the preset over-discharge cutoff voltage or preset over-discharge capacity threshold is reached. The preset temperature is 15℃~45℃, the preset current is 0.05C~1C, the preset over-discharge cutoff voltage is 0.5V~1.8V, and the preset over-discharge capacity threshold is 1%~20% of the rated capacity. S3. After stopping the over-discharge, allow the circuit to stand open and collect the self-recovery voltage V. rec Among them, the self-recovery voltage V rec The open-circuit voltage after the over-discharge has stopped and the circuit has been left to stand for 5 to 120 minutes. S4. Charge the battery using a constant current and constant voltage method to restore its capacity, and record the duration t of the constant voltage phase. cv Or the constant voltage section capacity ratio R cv Then, a standard discharge is performed to obtain the recovered capacity C1 after the over-discharge impulse, and the capacity loss rate ΔC is calculated. The capacity recovery charge is constant current charging at 0.05C~1C to the charging cutoff voltage, and then constant voltage charging is performed to the cutoff current at 0.02C~0.1C. The capacity loss rate ΔC = (C0-C1) / C0×100%; S5. Perform EIS testing under the same state of charge and temperature as the initial EIS test, and obtain the SEI-related interface impedance parameter R after the over-discharge impact. int1 Calculate the growth rate ΔR of the SEI-related interface impedance parameter. SEI SEI-related interface impedance parameter growth rate ΔR SEI =(R int1 -R int0 ) / R int0 ×100%; S6. Based on ΔC and ΔR SEI V rec Perform weighted scoring and collaborative judgment, and use t cv or R cv As an auxiliary judgment parameter, the SEI stability evaluation result is output.

2. The evaluation method according to claim 1, characterized in that, In step S6, the weighted scoring collaborative determination is performed as follows: ΔC and ΔR are... SEI t cv or R cv According to linear normalization where smaller is better, V rec Linear normalization is performed with the largest values ​​being considered optimal. The core parameter weights are α=0.40, β=0.35, and γ=0.25, and the auxiliary parameter weight is δ=0.10; the core score S is calculated. core and final score S final ; Core overall score S core Calculate using the following formula: ； Final overall score: S final Calculate using the following formula: ;or, ； The grading criteria are as follows: If S final ≥85, SEI stability is rated as excellent; If 65≤S final <85, SEI stability is rated as good; If 50 ≤ S final <65, SEI stability is judged as medium; If S final If the value is less than 50, the SEI stability is considered poor.

3. The evaluation method according to claim 2, characterized in that, For parameters that are better the smaller they are, including capacity loss rate ΔC and the growth rate of SEI-related interface impedance parameters ΔR, the following parameters are preferred. SEI Duration t of the constant pressure stage cv and the proportion of constant voltage section capacity R cv The following linear normalization formula is used: ； Where: x is the actual measured value of the parameter to be normalized; x best This is the baseline value of 100 points for the corresponding parameter; x worst This is the baseline value of 0 for the corresponding parameter; Constraints: When x≤x best When f(x) = 100; When x≥x worst When f(x) = 0; When x best <x<x worst When f(x) is calculated according to the formula, its value ranges from 0 to 100.

4. The evaluation method according to claim 3, characterized in that, For the parameter where a larger value is better - self-recovery voltage V rec The following linear normalization formula is used: ； V best This is the baseline value for 100 points of the self-recovery voltage; V worst This is the zero-point reference value for the self-recovery voltage; Constraints: When V rec ≥V best When, f(V) rec )=100; When V rec ≤V worst When, f(V) rec )=0; When V worst <V rec <V best When, f(V) rec Calculated according to the formula, the value ranges from 0 to 100.

5. The evaluation method according to claim 4, characterized in that, The method for determining the normalized benchmark value is as follows: Select cells from the same batch or system, pre-store them at 60℃ for at least 30 days, and pre-classify the SEI stability based on the capacity retention rate. Samples with a high-temperature storage retention rate ≥90% are classified as excellent, 80%~90% as good, 70%~80% as medium, and <70% as poor. Test samples with known high-temperature storage grades under the same controlled over-discharge impact conditions, and statistically analyze the distribution range of each parameter in samples of different grades. Use the typical lower or upper limit of the excellent grade sample as the 100-point benchmark value, and the typical upper or lower limit of the poor grade sample as the 0-point benchmark value, and ensure that the parameter values ​​of more than 99% of the tested samples fall within the linear scoring interval between the two benchmark values.

6. The evaluation method according to claim 5, characterized in that, The normalized baseline values ​​are set as follows: Capacity loss rate 1.0% for 100 points, 8.0% for 0 points; SEI-related interface impedance parameter growth rate 10.0% for 100 points, 60.0% for 0 points; Self-recovery voltage 3.2V for 100 points, 2.0V for 0 points; Constant voltage stage duration 10min for 100 points, 30min for 0 points; Constant voltage stage capacity percentage 3.0% for 100 points, 12.0% for 0 points.

7. The evaluation method according to claim 1, characterized in that, In step S2, the preset temperature is 23℃~27℃, the preset current is 0.05C~0.2C, the preset over-discharge cutoff voltage is 1.0V~1.5V, and the preset over-discharge capacity threshold is 5%~10% of the rated capacity; And / or, after a controlled over-discharge shock, the battery terminal voltage rebounds to above 2.0V, and subsequent capacity recovery charging and EIS testing can be completed.

8. The evaluation method according to claim 1, characterized in that, Self-recovery voltage V rec The open-circuit voltage is the voltage after the over-discharge has stopped and the circuit has been left to stand for 30 to 60 minutes.

9. The evaluation method according to claim 1, characterized in that, In step S4, the capacity recovery charge is performed by constant current charging at a current of 0.2C to 0.5C until the charging cutoff voltage.

10. The evaluation method according to claim 1, characterized in that, In step S5, the frequency range of the EIS test is 10kHz to 0.01Hz, and the disturbance voltage is 5mV to 10mV or the disturbance current is 50mA to 200mA.

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