A method for analyzing lithium-ion battery failure causes

Abstract

The application provides a method for analyzing the failure cause of a lithium ion battery. The method comprises performing an end-of-life stage test and a small current capacity recovery test on the failed lithium ion battery, and preliminarily judging the proportion of reversible loss and irreversible loss of the lithium ion battery according to the small current capacity recovery result; analyzing the capacity increment curve and the differential voltage curve of the failed lithium ion battery in the initial stage and the end-of-life stage, and preliminarily determining the cause of the attenuation of the lithium ion battery; performing appearance detection on the failed lithium ion battery; performing nondestructive testing on the failed lithium ion battery; disassembling the lithium ion battery after the nondestructive testing is completed, and performing electrochemical analysis and physical and chemical analysis on each component of the disassembled lithium ion battery to analyze the final cause of the failure of the lithium ion battery. The method of the application systematically determines the performance degradation mechanism of the lithium ion battery in each attenuation stage, and can accurately trace the failure.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery technology, and specifically to a method for analyzing the causes of lithium-ion battery failure. Background Technology

[0002] Lithium-ion batteries, such as those with prismatic shells, are widely used in electric vehicles and other fields due to their stable structure and high pack efficiency. New-generation products place higher demands on battery energy density, power density, and cycle life, making a systematic analysis of their aging mechanisms and failure causes particularly crucial.

[0003] Lithium-ion battery failures mainly fall into two categories: performance failures and safety failures. Performance failures include capacity decay, internal resistance increase, and cycle failures, while safety failures include bulging, leakage, and short circuits. Capacity decay is the most common failure mode, and its mechanisms can be divided into irreversible thermodynamic losses (such as active lithium loss and degradation of the positive and negative electrode materials) and partially recoverable kinetic losses (such as increased interfacial impedance). While existing non-destructive methods (such as capacity testing and DC internal resistance measurement) can preliminarily determine the main cause of failure, they struggle to accurately quantify lithium loss, identify the location of material degradation, or the source of impedance, thus failing to reveal the in-depth failure mechanism.

[0004] Currently, there is a lack of unified and reliable standard methods for failure analysis of lithium-ion batteries, such as prismatic lithium-ion batteries. Directly using empirical test conditions (such as half-electric state and low-temperature environment) is not only costly and time-consuming, but may also lead to data distortion.

[0005] Therefore, an improved method for analyzing the causes of lithium-ion battery failure is needed. Summary of the Invention

[0006] The following is an overview of the subject matter described in detail herein. This overview is not intended to limit the scope of protection of this application.

[0007] This application provides a method for analyzing the causes of lithium-ion battery failure, including the following steps: S100: Perform end-of-life (EOL) testing and low-current capacity recovery testing on failed lithium-ion batteries, and preliminarily determine the proportion of reversible and irreversible losses of the lithium-ion batteries based on the results of the low-current capacity recovery. S200: Analyze the capacity increment curves and differential voltage curves of failed lithium-ion batteries in the early life stage (BOL) and end life stage (EOL) to preliminarily determine the causes of lithium-ion battery degradation. S300: Visual inspection of failed lithium-ion batteries; S400: Non-destructive testing of failed lithium-ion batteries; and S500: Disassemble lithium-ion batteries that have completed non-destructive testing, and perform electrochemical and physicochemical analyses on each component of the disassembled lithium-ion battery to analyze the ultimate cause of lithium-ion battery failure.

[0008] In an exemplary embodiment, in step S100, the low current capacity recovery test includes testing using a rate of 0.01C-0.1C; alternatively, testing using a rate of 0.05C.

[0009] In an exemplary embodiment, in step S100, determining the ratio of reversible loss to irreversible loss of the lithium-ion battery includes testing the capacity of the failed battery at room temperature and at 0.33C for the initial stage and the end stage of its lifespan, and recording 1-EOL / BOL as the total loss; after the end-of-life stage test is completed, testing the capacity at 0.05C, and recording 1-0.05C capacity / BOL capacity as the irreversible capacity loss; the total loss minus the irreversible capacity loss is recorded as the reversible capacity loss.

[0010] In an exemplary embodiment, in step S200, the capacity increment curve and differential voltage curve of the failed lithium-ion battery in the early stage of its lifespan and the late stage of its lifespan are analyzed, including comparing the peak position shift, peak intensity change and whether a new peak appears between the curves in the early stage and the late stage of its lifespan, in order to determine at least one cause of battery degradation, such as loss of active lithium, loss of active material, material structure phase change or increase in interface impedance.

[0011] In an exemplary embodiment, in step S300, the appearance inspection includes detecting whether the battery is leaking or bulging.

[0012] In an exemplary embodiment, in step S400, the nondestructive testing includes ultrasonic testing, CT scanning, and electrochemical impedance spectroscopy.

[0013] In one exemplary embodiment, the ultrasonic testing includes analyzing whether gas is generated inside the battery, whether there are defects in the electrodes, or whether the electrolyte is poorly wetted.

[0014] In an exemplary embodiment, in step S500, the electrochemical analysis of each component of the disassembled lithium-ion battery includes assembling the positive and negative electrodes of the disassembled lithium-ion battery and the positive and negative electrodes of the fresh battery with lithium sheets to form a single soft-pack battery, and performing charge and discharge tests to compare the charge and discharge curves of the fresh electrode and the failed electrode.

[0015] In an exemplary embodiment, in step S500, the positive electrode of the disassembled lithium-ion battery is assembled with a fresh positive electrode to form a symmetrical battery, or the negative electrode of the disassembled lithium-ion battery is assembled with a fresh negative electrode to form a symmetrical battery, and an electrochemical impedance test is performed to compare the changes in impedance spectrum.

[0016] In an exemplary embodiment, in step S500, the physicochemical analysis includes one or more of the following: scanning electron microscopy, transmission electron microscopy, X-ray diffraction, inductively coupled plasma atomic emission spectroscopy, membrane permeability testing, X-ray energy dispersive spectroscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, mercury porosimetry, gas chromatography-mass spectrometry, nuclear magnetic resonance spectroscopy, thermogravimetric-differential scanning calorimetry, electrochemical impedance spectroscopy, and charge / discharge capacity testing.

[0017] In an exemplary embodiment, before step S100, the battery capacity retention rate of the failed lithium-ion battery from its initial state to the end of its life is obtained as a function of the number of cycles, and the curve is subjected to piecewise linear fitting to determine the battery capacity decay stage and the decay rate corresponding to each stage.

[0018] In an exemplary embodiment, in step S500, the physicochemical analysis includes obtaining gas composition characterization, electrolyte composition analysis, positive and negative electrode morphology, material capacity utilization, and membrane permeability.

[0019] In this application, the term "failed lithium-ion battery" refers to a battery whose capacity retention rate decays to less than or equal to a preset threshold after charge-discharge cycle testing. The preset threshold is set according to the application scenario; for example, in electric vehicle applications, this threshold may be 80%; in other scenarios, such as to obtain complete failure data in a shorter time, the threshold may be set to 83%.

[0020] The method for analyzing the causes of lithium-ion battery failure provided in this application integrates non-destructive battery analysis techniques with electrochemical and physicochemical characterization methods of disassembled components to systematically identify the performance degradation mechanism of the battery in each degradation stage, thereby accurately determining the root cause of its failure.

[0021] The efficient and low-cost method for analyzing the causes of lithium-ion battery failure provided in this application significantly improves analysis efficiency, reduces diagnostic costs, and provides a reliable basis and clear direction for subsequent battery performance improvement.

[0022] The method for analyzing the causes of lithium-ion battery failure provided in this application is of great significance for improving the accuracy of failure diagnosis, reducing analysis costs, and promoting the iteration of battery technology.

[0023] The method for analyzing the causes of lithium-ion battery failure provided in this application combines non-destructive testing with component-level physicochemical analysis after disassembly. This method allows for the characterization of the morphology, structure, composition, and electrochemical performance of the positive electrode, negative electrode, separator, and electrolyte through standardized disassembly, component separation, and cleaning. Furthermore, it utilizes low-current capacity recovery testing to differentiate between thermodynamic and kinetic capacity loss, and employs electrochemical impedance spectroscopy (EIS) and other measurement techniques to identify changes in interfacial reactions and transport processes.

[0024] The method in this application systematically determines the performance degradation mechanism of lithium-ion batteries at each degradation stage by comprehensively applying non-destructive testing technology and analyzing the electrochemical characteristics and physicochemical properties of the disassembled components, so as to accurately trace the source of failure.

[0025] In this application, the battery failure mode includes at least one of cycle failure and storage failure; the failure phenomenon includes at least one of capacity decay, impedance increase, gas expansion, thermal runaway and internal short circuit.

[0026] In this application, performance metrics include, but are not limited to, one or more of capacity retention, energy retention, voltage plateau, and storage performance. For example, analyzing the slope of the capacity retention curve as a function of cycle count to determine the inflection point and predict lifetime, etc.

[0027] In this application, an electrochemical workstation was used to test electrochemical impedance spectroscopy, and DRT technology was used to separate the impedances of various components, including ohmic impedance, SEI film impedance, charge transfer impedance, and diffusion impedance.

[0028] In this application, the disassembly process may include, in a dew-point controlled drying environment, cutting open the battery casing, separating the internal components, and cleaning and drying the resulting components with DMC; the components are at least one of positive electrode, negative electrode, electrolyte, and separator. Further, after being transferred to a glove box for drying, the failed components and fresh components are reassembled into a battery with lithium plates, and electrochemical performance tests are performed to determine the capacity loss of the positive and negative electrodes, etc.

[0029] In this application, electrochemical analysis may employ one or more of the following methods: charge-discharge curve analysis, incremental capacity analysis, or three-electrode electrochemical impedance spectroscopy.

[0030] In this application, physicochemical analysis may include scanning electron microscopy, transmission electron microscopy, X-ray diffraction, inductively coupled plasma atomic emission spectroscopy, membrane permeability testing, X-ray energy dispersive spectroscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, mercury porosimetry, gas chromatography-mass spectrometry, nuclear magnetic resonance spectroscopy, thermogravimetric-differential scanning calorimetry, electrochemical impedance spectroscopy, and charge / discharge capacity testing, etc.

[0031] In this application, SEM was used to observe the side reaction products on the electrode surface and CP cross-section was used to observe the changes in material structure; ICP was used to test the dissolution of transition metals on the surface of the negative electrode; and an air permeability tester was used to test the air permeability value of the diaphragm after cycling to determine whether it was blocked.

[0032] In this application, scanning electron microscopy and transmission electron microscopy combined with EDS are used to observe and semi-quantitatively analyze the structural changes of the positive and negative electrodes and the surface side reaction products, and to determine the degree of pore blockage of the solid electrolyte interphase (SEI) film on the negative electrode surface.

[0033] Other features and advantages of this application will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing the application. Other advantages of this application can be realized and obtained by means of the solutions described in the description and the accompanying drawings. Attached Figure Description

[0034] The accompanying drawings are used to provide an understanding of the technical solutions of this application and constitute a part of the specification. They are used together with the embodiments of this application to explain the technical solutions of this application and do not constitute a limitation on the technical solutions of this application.

[0035] Figure 1 This is a schematic flowchart illustrating the method for analyzing the causes of lithium-ion battery failure according to this application; Figure 2 This is a curve showing the change in battery capacity retention rate with the number of cycles provided in Embodiment 1 of this application; Figure 3 A comparison chart of the capacity increment curve and differential voltage curve of the battery provided in Embodiment 1 of this application at the beginning and end of its lifespan. Figure 4 This is a photograph of the appearance of a failed lithium-ion battery provided in Embodiment 1 of this application; Figure 5 This is an ultrasonic testing scan of a failed lithium-ion battery provided in Embodiment 1 of this application; Figure 6 The left curve shows the charge-discharge capacity comparison of fresh positive electrode and reconstituted negative electrode with failed positive electrode provided in Example 1 of this application, and the right curve shows the charge-discharge capacity comparison of fresh negative electrode and reconstituted negative electrode with failed negative electrode. Figure 7 Scanning electron microscope (SEM) images of a fresh positive electrode cross-section (left) and a failed positive electrode cross-section (right) provided in Embodiment 1 of this application; and Figure 8 A comparison of electrochemical impedance spectroscopy (EIS) of a fresh positive electrode symmetrical cell (left) and a failed positive electrode symmetrical cell (right) provided in Example 2 of this application. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of this application clearer, the embodiments of this application are described in detail below. It should be noted that, unless otherwise specified, the embodiments and features described in these embodiments can be arbitrarily combined with each other.

[0037] Figure 1 This is a schematic flowchart illustrating the method for analyzing the causes of lithium-ion battery failure according to this application. (Reference) Figure 1 The method for analyzing the causes of lithium-ion battery failure in this application may include the following steps: S100: Performing end-of-life stage tests and low-current capacity recovery tests on the failed lithium-ion batteries, and preliminarily determining the proportion of reversible and irreversible losses based on the low-current capacity recovery results; S200: Analyzing the capacity increment curves and differential voltage curves of the failed lithium-ion batteries in the early and late stages of their lifespan to preliminarily determine the causes of lithium-ion battery degradation; S300: Performing visual inspection on the failed lithium-ion batteries; S400: Performing non-destructive testing on the failed lithium-ion batteries; and S500: Disassembling the lithium-ion batteries that have completed non-destructive testing, and performing electrochemical and physicochemical analyses on each component of the disassembled lithium-ion batteries to analyze the ultimate cause of lithium-ion battery failure.

[0038] The following uses a failed prismatic lithium-ion battery as an example to specifically describe the method of analyzing the causes of lithium-ion battery failure in this application.

[0039] Example 1 The method for analyzing the causes of failure in prismatic lithium-ion batteries provided in this embodiment may include the following steps: S100: Perform end-of-life stage testing and low-current capacity recovery testing on failed lithium-ion batteries, and preliminarily determine the proportion of reversible and irreversible losses of lithium-ion batteries based on the results of low-current capacity recovery.

[0040] Before performing the end-of-life stage test (0.33C) and low-current capacity recovery test in step S100, the battery capacity retention rate of the failed lithium-ion battery from its initial state to the end of its life is obtained as a function of the number of cycles. Piecewise linear fitting is then performed on the curve to determine the battery capacity decay stage and the corresponding decay rate for each stage.

[0041] Figure 2 This is a curve showing the change in battery capacity retention as a function of cycle number, provided in Embodiment 1 of this application. From... Figure 2 It can be seen that the capacity of the failed lithium-ion battery mainly decays in three stages, and the equations for the three simulated curves are as follows: y1=-0.0379 x+100.187(100%>y>97%, R 2 =0.996) y2=-0.0245 x+98.983(97%>y>93%, R 2 =0.999) y3=-0.0162 x+96.642(93%>y>83%, R 2 =0.997) The fitting results show that the battery's cycle curve exhibits uniform aging, which is not caused by severe irreversible side reactions.

[0042] In this step, when performing low-current capacity recovery on the failed battery, the rate can be 0.01C-0.1C, preferably 0.05C.

[0043] In this step, based on the low-current capacity recovery results, the method for initially determining the capacity loss percentage of the failed battery is as follows: at room temperature (within the range of 15℃-35℃, for example 20℃), the battery's BOL and EOL capacities are tested at 0.33C respectively, and 1-EOL / BOL is recorded as the total loss; after the EOL test, the capacity is tested at 0.05C, and 1-0.05C capacity / BOL capacity is recorded as the irreversible capacity loss; the total loss minus the irreversible capacity loss is recorded as the reversible capacity loss.

[0044] S200: Analyze the capacity increment curves and differential voltage curves of the initial and final stages of the lifespan of a failed lithium-ion battery to preliminarily determine the causes of lithium-ion battery degradation.

[0045] In this step, the capacity increment curves and differential voltage curves of the early stage (BOL) and late stage (EOL) of the battery life are analyzed, such as... Figure 3 As shown. The main causes of battery degradation can be determined by factors such as changes in peak intensity, peak position shifts, and the characteristics of loss of positive and negative electrodes and active lithium.

[0046] from Figure 3 As can be seen from the curve, the decrease in peak intensity in the capacity increment curve indicates that the overall performance of the battery has deteriorated and is experiencing degradation. At the same time, the shift in peak position indicates an increase in internal polarization of the battery. Furthermore, the differential voltage curve shows that the loss of positive and negative electrode active materials and active lithium has increased, indicating that the active materials have undergone varying degrees of degradation.

[0047] S300: Perform visual inspection on failed lithium-ion batteries.

[0048] In this step, visual inspection can determine whether the lithium-ion battery is leaking, bloating, or otherwise malfunctioning. Figure 4 A photograph of the appearance of the failed battery provided in Embodiment 1 of this application.

[0049] S400: Perform non-destructive testing on failed lithium-ion batteries.

[0050] In this step, non-destructive testing can be performed on the batteries after they come off the production line, such as analyzing internal gas generation, electrode defects, or poor electrolyte wetting.

[0051] In this step, non-destructive testing may include ultrasonic testing, CT scanning, and electrochemical impedance spectroscopy.

[0052] Ultrasonic testing can be used to detect whether the electrolyte has dried up or whether there are air bubbles inside. Figure 5 This is an ultrasonic scan of a failed battery provided in Embodiment 1 of this application.

[0053] S500: Disassemble lithium-ion batteries that have completed non-destructive testing, and perform electrochemical and physicochemical analyses on each component of the disassembled lithium-ion battery to analyze the ultimate cause of lithium-ion battery failure.

[0054] In this step, fresh and failed batteries (capacity retention rate of 83%) are disassembled: Before disassembly, the lithium-ion batteries are discharged, and gas is collected from the explosion-proof valve using a gas bag to help eliminate interference factors such as electrolyte oxidation and excessive moisture; the aluminum shell of the battery is cut open to collect the free electrolyte, and then the positive electrode, negative electrode and separator are separated, cleaned with dimethyl carbonate and vacuum dried at room temperature, and stored in a glove box for subsequent electrochemical and physicochemical analysis.

[0055] In a dry room with dew point control, the material at the tabs of the disassembled positive and negative electrodes were wiped off and dried. Fresh electrodes and failed electrodes were then assembled into single-cell pouch batteries with lithium sheets, and their capacity was analyzed by charge and discharge.

[0056] The dew point control requirement is ≤-50℃; the size of a single soft-pack positive electrode is 45. 65mm, lithium chip size is 48 68mm; after assembly, it was tested after being immersed in high temperature for 24 hours.

[0057] Next, the positive electrode is used to assemble the lithium battery, and the positive electrode is charged with lithium (e.g., charged at a constant current of 0.05C-0.1C to the cutoff voltage), and the positive electrode is de-intercalated with lithium; the negative electrode is used to assemble the lithium battery, and the negative electrode is discharged with lithium (discharged at a constant current of 0.05C-0.1C to the cutoff voltage), and the negative electrode is intercalated with lithium; then, three standard charge-discharge cycles are performed at the same rate, and the discharge capacity of the third cycle is used as the comparable capacity of the reconstituted electrode.

[0058] from Figure 6The charge-discharge curves of the reconstituted positive electrode show that the high-voltage plateau has essentially disappeared, and the capacity cannot be utilized, representing a loss of 13.5% compared to the fresh positive electrode (relative to the initial capacity of the positive electrode itself). The charge-discharge curves of the reconstituted negative electrode show little change in discharge specific capacity, with only about a 2.3% loss in lithium intercalation capacity. The total capacity loss of the entire battery is 17% (100% → 83%). This indicates that the battery failure is mainly caused by the loss of positive electrode active material (i.e., it plays a dominant role); the positive electrode is the primary cause of capacity decay, and the specific reasons require further physicochemical analysis to determine.

[0059] Next, the positive and negative electrodes, electrolyte, and separator of the disassembled battery were characterized for their physicochemical properties. This embodiment mainly studies the positive electrode. First, the cross-section of the electrode was characterized using a scanning electron microscope, such as... Figure 7 As shown. By Figure 7 It can be seen that no particle breakage was observed in the fresh electrode, while obvious cracks appeared in the large particles of the electrode after cycling. This is caused by the volume expansion of the material during cycling, which is related to the irreversible phase transition of the positive electrode material particles under high voltage. At the same time, inductively coupled plasma (ICP) also detected the dissolution of transition metal on the surface of the negative electrode, which further proves that the structure of the positive electrode material has been damaged.

[0060] In summary, under the high-temperature cycling conditions of this embodiment, the main reasons for the battery failure are the irreversible phase transition of the positive electrode, structural collapse leading to kinetic degradation, and the slow loss of active lithium during cycling.

[0061] Example 2 Unlike Example 1, in this example, in addition to assembling a single lithium-ion pouch cell, symmetrical cells with fresh and depleted positive electrodes are also assembled.

[0062] Electrochemical impedance spectroscopy was performed on failed and fresh batteries at room temperature to determine the impedance failure type of the failed batteries. The state of charge of the electrodes used for electrochemical impedance spectroscopy was maintained at 100% SOC. The ambient temperature range is 15℃-35℃.

[0063] Electrochemical impedance spectroscopy (EIS) tests were performed on both failed and fresh batteries at room temperature, including the following steps: 2.1) Connect the electrodes of the electrochemical workstation to the positive and negative terminals of the failed battery; 2.2) Set the test temperature to room temperature and the test frequency range to 100kHz-0.01Hz; 2.3) Using the same method as in steps 2.1) to 2.2), the electrochemical AC impedance spectrum of the fresh battery was obtained. After comparison and analysis with the failed battery, the impedance failure type of the failed battery was determined.

[0064] In the above steps, lithium-ion and negative electrode monocells can be assembled, and the electrochemical workstation can adopt a two-electrode system, a three-electrode system, or a four-electrode system.

[0065] In this embodiment, an electrochemical workstation can be used to perform electrochemical impedance spectroscopy on the battery.

[0066] from Figure 8 The electrochemical impedance spectroscopy (EIS) results of the fresh and failed symmetrical cells show that the interfacial film impedance (R0) in the high-frequency region of the failed cell is... CEI The significant increase in the diameter of the semicircle in the mid-frequency region, representing charge transfer impedance (Rct), indicates a significant increase in positive electrode impedance; this is a key factor leading to increased polarization and performance degradation in the full cell.

[0067] The above analysis shows that the structural degradation of the cathode material, the dissolution of transition metals, and the accumulation of electrolyte by-reaction products all contribute to the deterioration of interfacial kinetics.

[0068] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.

Claims

1. A method for analyzing the causes of lithium-ion battery failure, characterized in that, Includes the following steps: S100: Perform end-of-life stage testing and low-current capacity recovery testing on failed lithium-ion batteries, and preliminarily determine the proportion of reversible and irreversible losses of the lithium-ion batteries based on the results of low-current capacity recovery. S200: Analyze the capacity increment curve and differential voltage curve of the initial stage and the final stage of the life of the failed lithium-ion battery to preliminarily determine the cause of lithium-ion battery degradation. S300: Visual inspection of failed lithium-ion batteries; S400: Non-destructive testing of failed lithium-ion batteries; and S500: Disassemble lithium-ion batteries that have completed non-destructive testing, and perform electrochemical and physicochemical analyses on each component of the disassembled lithium-ion battery to analyze the ultimate cause of lithium-ion battery failure.

2. The method according to claim 1, characterized in that, In step S100, the low-current capacity recovery test includes testing at a rate of 0.01C-0.1C; optionally, testing at a rate of 0.05C; and / or, In step S100, determining the ratio of reversible to irreversible losses of the lithium-ion battery includes testing the capacity of the failed battery at the beginning and end of its lifespan at 0.33C at room temperature, and recording 1-EOL / BOL as the total loss; after the end-of-life test is completed, the capacity is tested at 0.05C, and the 1-0.05C capacity / BOL capacity is recorded as the irreversible capacity loss. Total loss minus irreversible capacity loss is denoted as reversible capacity loss.

3. The method according to claim 1, characterized in that, In step S200, the capacity increment curve and differential voltage curve of the failed lithium-ion battery in the early stage and late stage of its lifespan are analyzed, including comparing the peak position shift, peak intensity change and whether a new peak appears between the curves in the early stage and late stage of the lifespan, in order to determine at least one cause of battery degradation, such as loss of active lithium, loss of active material, material structure phase change or increase of interface impedance.

4. The method according to claim 1, characterized in that, In step S300, the appearance inspection includes detecting whether the battery is leaking or bulging.

5. The method according to claim 1, characterized in that, In step S400, the non-destructive testing includes ultrasonic testing, CT scanning, and electrochemical impedance spectroscopy.

6. The method according to claim 5, characterized in that, The ultrasonic testing includes analyzing whether gas is generated inside the battery, whether there are defects in the electrode sheets, or whether the electrolyte is poorly wetted.

7. The method according to claim 1, characterized in that, In step S500, electrochemical analysis is performed on each component of the disassembled lithium-ion battery, including assembling the positive and negative electrodes of the disassembled lithium-ion battery and the positive and negative electrodes of the fresh battery with lithium sheets to form a single soft-pack battery, and performing charge-discharge tests to compare the charge-discharge curves of the fresh electrodes and the failed electrodes; and / or, The positive electrode of the disassembled lithium-ion battery was assembled with a fresh positive electrode to form a symmetrical battery, or the negative electrode of the disassembled lithium-ion battery was assembled with a fresh negative electrode to form a symmetrical battery, and electrochemical impedance spectroscopy was performed to compare the changes in impedance spectrum.

8. The method according to claim 1, characterized in that, In step S500, the physicochemical analysis includes one or more of the following: scanning electron microscopy, transmission electron microscopy, X-ray diffraction, inductively coupled plasma atomic emission spectroscopy, membrane permeability testing, X-ray energy dispersive spectroscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, mercury porosimetry, gas chromatography-mass spectrometry, nuclear magnetic resonance spectroscopy, thermogravimetric-differential scanning calorimetry, electrochemical impedance spectroscopy, and charge / discharge capacity testing.

9. The method according to claim 1, characterized in that, In step S500, the physicochemical analysis includes obtaining gas composition characterization, electrolyte composition analysis, positive and negative electrode morphology, material capacity utilization, and membrane permeability.

10. The method according to any one of claims 1-9, characterized in that, Before step S100, the battery capacity retention rate of the failed lithium-ion battery from its initial state to the end of its life is obtained as a function of the number of cycles. Piecewise linear fitting is performed on the curve to determine the battery capacity decay stage and the decay rate corresponding to each stage.

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