Electrode structure evolution test method before disassembly and thermal runaway of single power battery

Abstract

The invention relates to a method for testing the evolution of an electrode structure before disassembly and thermal runaway of a single power battery, and belongs to the technical field of safety performance testing of the power battery. The method comprises the following steps: discharging the single power battery to a corresponding cut-off voltage, and screening the single power battery to be qualified through appearance and voltage retest; disassembling in a low-humidity inert atmosphere to prepare a standardized sample; performing a multi-dimensional test on the sample to obtain thermal decomposition, morphology, lattice and interface information of the electrode; and data are synchronously calibrated and fused for modeling, so that the disassembly and test safety is improved, the data accuracy is ensured, the research blank of the microcosmic evolution of the electrode before thermal runaway is filled, and a support is provided for the research of a power battery safety mechanism, the material optimization and the development of an early warning model.

Description

Technical Field

[0001] This invention belongs to the field of power battery safety performance testing technology, specifically relating to a method for testing the evolution of electrode structure before thermal runaway in a single power battery cell. Background Technology

[0002] With the rapid development of new energy vehicles, energy storage power stations, and other fields, the safety of power batteries has become a core bottleneck restricting the industry's development. Among them, thermal runaway of power batteries is the main cause of safety accidents. Therefore, in-depth research into the initiation mechanism and evolution law of thermal runaway is a key prerequisite for improving battery safety.

[0003] Current research on thermal runaway in power batteries primarily focuses on two areas: first, the propagation path and protective measures for thermal runaway across the entire battery pack, such as blocking heat spread by adding heat-insulating materials and designing explosion-proof structures; and second, the exploration of thermal runaway triggering conditions, such as the impact of external stimuli like overcharging, over-discharging, short circuits, and high temperatures on thermal runaway. However, these studies have overlooked a crucial aspect—the microstructural evolution of electrode materials before thermal runaway. In fact, thermal runaway is not triggered instantaneously, but rather is the cumulative result of gradual microscopic changes in electrode materials during heating, including thermal decomposition, lattice structure destruction, and failure of the solid electrolyte interface film. Accurately capturing this microscopic evolution information is essential to understanding the initiation mechanism of thermal runaway at its root, and subsequently optimizing electrode materials and battery structure accordingly. Summary of the Invention

[0004] To address the aforementioned problems in the existing technology, this invention provides a method for disassembling a single power battery cell and testing the evolution of its electrode structure before thermal runaway.

[0005] The objective of this invention can be achieved through the following technical solutions: A method for testing the evolution of electrode structure before thermal runaway in a single power battery cell, comprising: The power battery cells are discharged to the cutoff voltage set by their corresponding system, and the discharged batteries are screened to obtain qualified batteries that meet the safety conditions. The qualified battery was disassembled in a controlled environment with low water content, the electrode core was removed, and the electrode sheet with a flat and undamaged surface was selected. After cutting, solvent soaking and drying, standardized positive and negative electrode test samples were obtained. Differential scanning calorimetry, scanning electron microscopy, transmission electron microscopy, in-situ X-ray diffraction and atomic force microscopy were performed on the positive and negative electrode test samples to obtain test information on thermal decomposition, surface morphology, crystal structure and interface film evolution of the electrodes during the heating process. The test information is integrated and analyzed to establish a multi-dimensional evolution model of the electrode before thermal runaway.

[0006] In a further embodiment of the present invention, the controlled environment is a low-humidity inert gas environment.

[0007] In a further embodiment of the present invention, the electrode is cut into a sheet-like structure of a preset standardized size.

[0008] In a further embodiment of the present invention, the solvent soaking step includes multiple soakings to remove residual electrolyte and byproducts from the electrode surface.

[0009] In a further embodiment of the invention, the drying step is carried out under low pressure or vacuum conditions to avoid the influence of moisture in the air on the electrode.

[0010] In a further embodiment of the invention, the differential scanning calorimetry test is performed at a controlled heating rate to obtain the thermal decomposition characteristics of the electrode material.

[0011] In a further embodiment of the present invention, the in-situ X-ray diffraction test is performed under gradually increasing temperature conditions to obtain information on the lattice structure changes and phase transitions of the electrode material.

[0012] In a further embodiment of the present invention, the atomic force microscopy test includes a conductivity mode to detect changes in the conductivity of the solid electrolyte interface film on the electrode surface.

[0013] In a further embodiment of the present invention, the test information is fused in multiple modes after synchronous calibration of temperature and time to form a multi-dimensional database of thermal, morphological, structural and interface electrical properties.

[0014] In a further embodiment of the present invention, the step of screening the discharged batteries includes: detecting whether the battery appearance has bulging, leakage or shell damage, and rejecting batteries with abnormal appearance; retesting the open circuit voltage of batteries that are qualified in appearance after standing for 12-24 hours, and selecting batteries with an open circuit voltage fluctuation value ≤0.03V after retesting as qualified batteries.

[0015] The present invention has at least the following beneficial effects: disassembly and appearance screening in a low-humidity inert gas environment can avoid the risks of fire and corrosion during the disassembly process; vacuum drying avoids the hydrolysis of the SEI membrane caused by moisture and ensures the integrity of the sample structure. Attached Figure Description

[0016] To facilitate understanding by those skilled in the art, the present invention will be further described below with reference to the accompanying drawings.

[0017] Figure 1 This is a flowchart of a method provided in one embodiment of the present invention; Figure 2This is a flowchart of the core disassembly and standardized sample preparation process provided in one embodiment of the present invention.

[0018] Figure 3 This is a schematic diagram of the heated negative electrode surface provided in one embodiment of the present invention; Figure 4 This is a schematic diagram of the heated positive electrode surface provided in one embodiment of the present invention; Figure 5 This is a schematic diagram of the negative electrode side of the diaphragm provided in one embodiment of the present invention; Figure 6 This is a schematic diagram of the positive electrode side of the diaphragm provided in one embodiment of the present invention; Figure 7 This is a negative electrode SEM image provided in one embodiment of the present invention; Figure 8 This is a positive electrode SEM image provided in one embodiment of the present invention; Figure 9 This is a negative electrode DSC diagram provided in one embodiment of the present invention; Figure 10 This is a positive electrode DSC diagram provided in one embodiment of the present invention; Figure 11 This is a negative electrode XRD pattern provided in one embodiment of the present invention; Figure 12 This is a positive electrode XRD pattern provided in one embodiment of the present invention. Detailed Implementation

[0019] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided.

[0020] Please refer to Figure 1-2 In one embodiment, the present invention provides a method for disassembling a single power battery cell and testing the evolution of its electrode structure before thermal runaway, comprising: The power battery cells are discharged to the cutoff voltage set by their corresponding system, and the discharged batteries are screened to obtain qualified batteries that meet the safety conditions. The qualified battery was disassembled in a controlled environment with low water content, the electrode core was removed, and the electrode sheet with a flat and undamaged surface was selected. After cutting, solvent soaking and drying, standardized positive and negative electrode test samples were obtained. Differential scanning calorimetry, scanning electron microscopy, transmission electron microscopy, in-situ X-ray diffraction and atomic force microscopy were performed on the positive and negative electrode test samples to obtain test information on thermal decomposition, surface morphology, crystal structure and interface film evolution of the electrodes during the heating process. The test information is integrated and analyzed to establish a multi-dimensional evolution model of the electrode before thermal runaway.

[0021] In existing technologies, the testing process often involves arbitrarily disassembling the components and then conducting individual tests with isolated data analysis. This makes it impossible to compare test results from different laboratories and to correlate the synergistic evolution of heat, shape, structure, and interface, which is insufficient to support research on thermal runaway mechanisms.

[0022] This embodiment covers the entire process, including: battery discharge screening, sample preparation through disassembly in a low-water-content environment, multi-dimensional testing, and data integration modeling. It aims to solve the core problems of existing technologies, such as the lack of standardized processes, data fragmentation, and the inability to construct evolution patterns.

[0023] In this embodiment, a lithium iron phosphate power battery with a capacity of 50Ah and a nominal voltage of 3.2V and a ternary lithium power battery NCM622 system with a capacity of 50Ah and a nominal voltage of 3.6V were used as test objects. The specific steps are as follows: Battery Discharge and Screening: A dedicated charge-discharge tester for power batteries was used to perform constant current discharge on both types of batteries: lithium iron phosphate batteries were discharged at 10A and 0.2C to a cutoff voltage of 2.5V, and ternary lithium batteries were discharged at 10A to a cutoff voltage of 3.0V. After discharge, the batteries were allowed to stand for 30 minutes before the first round of voltage testing. The voltage rebound was checked to see if it was within the normal range (the acceptable range for lithium iron phosphate battery voltage rebound is 0.08–0.25V, and for ternary lithium battery voltage rebound is 0.05–0.20V). Batteries that failed the test were discarded.

[0024] Subsequently, the remaining batteries underwent visual inspection, and batteries with bulging, leakage, or damaged casings were removed. Finally, the batteries that passed the visual and voltage tests were left to stand for 24 hours, and their open-circuit voltage was retested using a high-precision multimeter. Batteries with voltage fluctuations ≤0.03V were selected as qualified batteries.

[0025] Disassembly and sample preparation in low-moisture environments: Disassembly was carried out in an inert gas environment. After the separator of the electrode core was peeled off, the positive and negative electrode sheets located in the middle of the electrode core and with no scratches, wrinkles, or electrolyte residue were selected and cut using a laser cutting machine.

[0026] The cut electrode sheets were placed in a polytetrafluoroethylene container, and anhydrous dimethyl carbonate was added. After the initial 20-minute soaking, the electrode sheets were removed and the surface solvent was blotted dry with clean filter paper. Fresh anhydrous dimethyl carbonate was then added, and the electrode sheets were soaked for another 20 minutes to thoroughly remove residual electrolyte and byproducts. After soaking, the electrode sheets were placed in a vacuum drying oven to obtain standardized test samples. After drying, the electrode sheets were transferred back to an inert gas environment to avoid contact with air.

[0027] Multi-dimensional testing: DSC test: Differential scanning calorimeter was used, with a sample amount of 3-5 mg and a test atmosphere of high-purity nitrogen. The heat flow-temperature curve was recorded to obtain the thermal decomposition onset temperature, peak temperature and heat release of the electrode material.

[0028] SEM and TEM tests: Scanning electron microscopy was used to observe the surface morphology of the electrode sheet; transmission electron microscopy was used to prepare ultrathin sections of the electrode sheet to observe the microstructure of the electrode material.

[0029] In-situ XRD testing: XRD patterns were recorded at different temperatures using a X-ray diffractometer to analyze changes in lattice parameters.

[0030] AFM testing: Using an atomic force microscope and a conductive probe, the electrode surface was scanned in conductive mode to record surface morphology and current images, and to analyze the thickness variation and conductivity of the SEI film.

[0031] Data integration modeling: Test data from DSC (temperature-heat flow), in-situ XRD (temperature-lattice parameters), AFM (temperature-conductivity), and SEM / TEM (temperature-morphology features) were calibrated to the same time axis. Subsequently, data fusion was performed to generate correlation maps of temperature, thermal decomposition characteristics, lattice structure, SEI film performance, and surface morphology. Based on machine learning algorithms, a multi-dimensional evolution model of the electrode before thermal runaway was constructed. For example, when the temperature of the ternary lithium cathode rises to 210℃, XRD shows that the peak intensity of the (003) crystal plane decreases by 30%, DSC shows the first exothermic peak, AFM shows that the conductivity of the SEI film decreases by 50%, and SEM observes slight cracking of the cathode particles. These features are correlated as early warning signals for the early stage of thermal runaway.

[0032] This embodiment addresses four major issues of existing technologies through standardized operations throughout the entire process: First, screening by combining appearance and voltage retesting ensures that test samples come from batteries in a stable state, eliminating initially defective batteries and avoiding data deviations; Second, disassembly and vacuum drying in a low-humidity inert atmosphere ensure that the electrode structure is not damaged by moisture or oxygen, and the SEI film thickness observed by SEM / TEM is consistent with the actual thickness under battery operating conditions, with no traces of hydrolysis or oxidation; Third, simultaneous calibration of multi-dimensional testing enables collaborative analysis of thermal, morphological, structural, and interface aspects; Fourth, a complete evolutionary chain is constructed through modeling, providing quantitative evidence for the study of thermal runaway mechanisms. Compared to the single data in existing technologies, the data in this embodiment is more systematic and instructive.

[0033] In a further embodiment of the present invention, the controlled environment is a low-humidity inert gas environment.

[0034] In existing technologies, some studies disassemble the electrodes in low-humidity air. However, the oxygen in the air will still react with the negative electrode graphite and the positive electrode active material to cause oxidation reactions, which will lead to changes in the original structure of the electrodes and distortion of the test data.

[0035] In this embodiment, the use of an inert gas environment can isolate oxygen and prevent electrode oxidation. XPS test shows that the oxygen content of the electrode in the inert atmosphere group is consistent with the original state, with no oxidation products. At the same time, low humidity can prevent SEI film hydrolysis and effectively ensure the integrity of the electrode structure.

[0036] In a further embodiment of the present invention, the electrode is cut into a sheet-like structure of a preset standardized size.

[0037] In this embodiment, the electrodes are cut into sheet-like structures of a preset standardized size. The electrodes used for scanning electron microscopy (SEM) and transmission electron microscopy (TEM) testing are 1cm×1cm, the electrodes used for differential scanning calorimetry (DSC) testing are circular pieces with a diameter of 0.3cm, the electrodes used for in-situ X-ray diffraction (XRD) testing are 2cm×2cm, and the electrodes used for atomic force microscopy (AFM) testing are 0.8cm×0.8cm. This effectively improves the repeatability and comparability of the test data.

[0038] In a further embodiment of the present invention, the solvent soaking step includes multiple soakings to remove residual electrolyte and byproducts from the electrode surface.

[0039] In a further embodiment of the present invention, the drying step is carried out under low pressure or vacuum conditions to avoid the influence of moisture in the air on the electrode.

[0040] In a further embodiment of the present invention, the differential scanning calorimetry test is performed at a controlled heating rate to obtain the thermal decomposition characteristics of the electrode material.

[0041] In a further embodiment of the present invention, the in-situ X-ray diffraction test is performed under gradually increasing temperature conditions to obtain information on the lattice structure changes and phase transitions of the electrode material.

[0042] In a further embodiment of the present invention, the atomic force microscopy test includes a conductivity mode to detect changes in the conductivity of the solid electrolyte interface film on the electrode surface.

[0043] In a further embodiment of the present invention, the test information is fused in multiple modes after synchronous calibration of temperature and time to form a multi-dimensional database of thermal, morphological, structural and interface electrical properties.

[0044] In this embodiment, A data synchronization platform was established to perform time-temperature calibration on data from other tests, using the heating time of in-situ XRD as a benchmark. DSC data calibration: The in-situ XRD heating rate was 4℃ / min, and the DSC heating rate was kept consistent with the in-situ XRD heating rate (4℃ / min) to avoid kinetic deviation. Simultaneously, the melting point of In was used for temperature calibration to reduce the error between the programmed temperature and the actual sample temperature, and the enthalpy of fusion of In was used for heat flux calibration adjustment, thereby aligning the DSC time-temperature-heat flux data with the time-temperature axis of the in-situ XRD.

[0045] AFM data calibration: AFM testing involves intermittent heating, and the temperature point of each test is mapped to the same temperature point in the in-situ XRD to ensure that the AFM conductivity and XRD lattice parameters at the same temperature can be correlated.

[0046] SEM data calibration: Using a SEM with a heating stage, observations were made at three temperature points: 25℃, 150℃, and 250℃. The morphological data at these three temperature points were mapped to the synchronized time-temperature axis to fill the data gap between static SEM and dynamic DSC / XRD.

[0047] Multimodal data fusion to construct temperature-multi-parameter correlation maps: Thermal-structure correlation: The calibrated DSC heat flux curves are superimposed with the peak intensity curves of in-situ XRD; Structure-interface correlation: Correlation between changes in lattice parameters from in-situ XRD and changes in AFM conductivity; Interface-morphology correlation: Correlation between AFM conductivity data and heated SEM morphology data revealed that AFM showed localized no current signal at 150℃, corresponding to localized depressions on the negative electrode surface observed by SEM (pore diameter approximately 10nm), verifying the morphological characteristics of SEI film rupture. Multi-dimensional database construction: All calibrated test data are entered into the database. Each data entry includes fields such as temperature, test method, parameter value, error range, and test time, forming a queryable and analyzable multi-dimensional database.

[0048] By calibrating the time and temperature, the problem of temperature asynchrony in different tests is solved, enabling the correlation of temperature, shape, structure and interface data under the same temperature, improving data utilization, and the analysis of the fused data can reveal hidden correlations, providing a brand-new analysis mode for power battery safety research.

[0049] In a further embodiment of the present invention, the step of screening the discharged batteries includes: detecting whether the battery appearance has bulging, leakage or shell damage, and rejecting batteries with abnormal appearance; retesting the open circuit voltage of batteries with qualified appearance after standing for 12-24 hours, and selecting batteries with open circuit voltage fluctuation value ≤0.03V after retesting as qualified batteries.

[0050] In this embodiment, batteries with potential safety hazards are eliminated through visual inspection to avoid the risk of fire and corrosion during disassembly and improve operational safety. At the same time, batteries with continuous internal reactions are eliminated through static retesting to ensure the stability of the electrode structure of the test samples.

[0051] In some embodiments of the present invention, in engineering practice and experimental research, a large number of battery cells are in an intermediate state of "having experienced a certain amount of thermal stress but not yet thermal runaway," such as: early heating tests; environmental thermal exposure; localized thermal abuse but without triggering thermal runaway. These cells can still be considered "intact" macroscopically, but irreversible electrode structure and interface degradation have occurred internally, significantly affecting subsequent thermal runaway behavior and safety boundaries. This supplementary document addresses these intermediate-state cells (early heating test batteries).

[0052] Please refer to the details. Figure 3-12 .

[0053] Figure 3 The surface of the negative electrode after heating is mainly characterized by surface wrinkles caused by thermal shrinkage of the separator on the macroscopic surface of the electrode sheet after heating. Figure 4 The surface of the positive electrode after heating may exhibit problems such as localized coating peeling, flaking, or exposed foil.

[0054] Figure 5 On the negative electrode side of the diaphragm, there are no obvious deposited particles on its surface, indicating that the negative electrode coating is still relatively intact with only a small amount or no coating transfer; on the positive electrode side of the diaphragm, there are a large number of black areas on the surface, which are caused by the transfer of the positive electrode coating.

[0055] Figure 7 The negative electrode SEM image shows a significant increase in surface deposits due to thermal evolution defects in the negative electrode interface film (SEI). Figure 8 The SEM images of the positive electrode show numerous μm-sized blocky / plate-like clusters in the high-magnification images. Fine particles are "agglomerated" into clumps, and local particle boundaries become indistinct. The clusters are surrounded by fine particles, indicating "secondary aggregation" rather than uniform dispersion. Furthermore, issues such as pore structure degradation are also present.

[0056] Figure 9 The image shows the negative electrode DSC pattern. The thick SEI film formed after heating results in a broad exothermic peak. However, the positive electrode on the right (10) does not exhibit any exothermic decomposition reaction because the LFP crystal structure is relatively stable.

[0057] Figure 11 The image shows the XRD pattern of the negative electrode. The XRD pattern is mainly used to observe the changes in the crystal structure of the battery after heating. It can be seen that the graphite lattice remains stable overall and no obvious structural changes have occurred. Figure 12It appears to be a mixed phase of lithium iron phosphate and iron phosphate, but does not show strong crystal collapse or severe amorphization. The low-angle background rise suggests the possible presence of amorphous carbon / polymer residues or decomposition products.

[0058] After completing the initial heating test, based on the specific test procedure, multiple physical quantities (including but not limited to the heating power-time curve P(t), cumulative input energy E, temperature rise rate dT / dt, and maximum temperature T) were obtained. max The exothermic onset temperature T in the differential scanning calorimetry results of the electrode material onset Peak temperature T peak The pre-heated test cells and normal cells are compared and normalized using the following parameters: heat release per unit mass ΔH; coating peeling area ratio A, crack density ρ, and surface roughness R from scanning electron microscopy and atomic force microscopy results; and DC internal resistance / AC impedance characteristic parameters from electrochemical tests. The resulting reproducible safety status characterization results (such as risk level or safety status index) are then directly used for subsequent test condition control, including determining the upper limit of heating power P for subsequent heating abuse tests. lim Termination threshold (T) stop E stop or dT / dt stop The test path selection (continue / downgrade / terminate) and the corresponding protection level (isolation, inerting, ventilation and explosion-proof configuration) transform the evaluation results from "judgment conclusions" into actionable "control parameters", avoiding subjective inferences about safety based on a single phenomenon or single indicator.

[0059] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for testing the evolution of electrode structure before thermal runaway in a single power battery cell, characterized in that, include: The power battery cells are discharged to the cutoff voltage set by their corresponding system, and the discharged batteries are screened to obtain qualified batteries that meet the safety conditions. The qualified battery was disassembled in a controlled environment with low water content, the electrode core was removed, and the electrode sheet with a flat and undamaged surface was selected. After cutting, solvent soaking and drying, standardized positive and negative electrode test samples were obtained. Differential scanning calorimetry, scanning electron microscopy, transmission electron microscopy, in-situ X-ray diffraction and atomic force microscopy were performed on the positive and negative electrode test samples to obtain test information on thermal decomposition, surface morphology, crystal structure and interface film evolution of the electrodes during the heating process. The test information is integrated and analyzed to establish a multi-dimensional evolution model of the electrode before thermal runaway.

2. The method for disassembling a single power battery cell and testing the evolution of its electrode structure before thermal runaway, as described in claim 1, is characterized in that... The controlled environment is a low-humidity inert gas environment.

3. The method for disassembling a single power battery cell and testing the evolution of its electrode structure before thermal runaway, as described in claim 1, is characterized in that... The electrode is cut into a sheet-like structure of a preset standardized size.

4. The method for disassembling a single power battery cell and testing the evolution of its electrode structure before thermal runaway, as described in claim 1, is characterized in that... The solvent soaking step includes multiple soakings to remove residual electrolyte and byproducts from the electrode surface.

5. The method for disassembling a single power battery cell and testing the evolution of its electrode structure before thermal runaway, as described in claim 1, is characterized in that... The drying step is carried out under low pressure or vacuum conditions to avoid the influence of moisture in the air on the electrode.

6. The method for disassembling a single power battery cell and testing the evolution of its electrode structure before thermal runaway, as described in claim 1, is characterized in that... The differential scanning calorimetry test is performed at a controlled heating rate to obtain the thermal decomposition characteristics of the electrode material.

7. The method for disassembling a single power battery cell and testing the evolution of its electrode structure before thermal runaway, as described in claim 1, is characterized in that... The in-situ X-ray diffraction test was conducted under gradually increasing temperature conditions to obtain information on the lattice structure changes and phase transitions of the electrode material.

8. The method for disassembling a single power battery cell and testing the evolution of its electrode structure before thermal runaway, as described in claim 1, is characterized in that... The atomic force microscopy test includes a conductivity mode to detect changes in the conductivity of the solid electrolyte interface film on the electrode surface.

9. The method for disassembling a single power battery cell and testing the evolution of its electrode structure before thermal runaway, as described in claim 1, is characterized in that... The test information is fused in multiple modes after being calibrated synchronously with temperature and time to form a multi-dimensional database of thermal, morphological, structural and interfacial electrical properties.

10. The method for disassembling a single power battery cell and testing the evolution of its electrode structure before thermal runaway, as described in claim 1, is characterized in that... The steps for screening discharged batteries include: checking whether the battery appearance is bulging, leaking, or damaged, and removing batteries with abnormal appearance; retesting the open circuit voltage of batteries that are qualified in appearance after standing for 12-24 hours, and selecting batteries with an open circuit voltage fluctuation value ≤0.03V after retesting as qualified batteries.

Images