A method for testing the uniaxial compression mechanical properties of battery electrode material particles

Abstract

This invention relates to a method for testing the uniaxial compressive mechanical properties of battery electrode material particles, belonging to the technical field of battery electrode material performance testing. First, the electrode sheet and conductive substrate to be tested are attached one-to-one onto two sample stages of an SEM. Then, the two sample stages are placed on the mechanical platform of a dual-beam electron microscope. Next, flat sheet-like samples are cut from the electrode sheet and welded to the conductive substrate. Subsequently, individual particles are cut on the surface of the sheet-like samples to form micropillars. Finally, a micro-nano mechanical testing device installed inside the SEM cavity is used to perform uniaxial compression tests on the micropillar samples, obtaining stress-strain curves and allowing observation of the microstructure after compression deformation. This method also avoids air contamination of the samples. Compared to previously reported nanoindentation testing methods, the method described in this invention provides more comprehensive data for evaluating the impact of electrochemical cycling on changes in battery mechanical properties.

Description

Technical Field

[0001] This invention relates to a method for testing the uniaxial compressive mechanical properties of battery electrode material particles, belonging to the technical field of battery electrode material performance testing. Background Technology

[0002] Lithium-ion batteries are currently widely used in electronic products and new energy vehicles. During the charge-discharge cycle of lithium-ion batteries, the repeated insertion and extraction of lithium ions between the positive and negative electrode materials often causes structural changes in the electrode materials, such as expansion and contraction, and even cracking. This hinders the insertion and extraction of lithium ions, reduces battery capacity, and shortens its lifespan. Therefore, the stability of the structure and mechanical properties of battery electrode materials plays a crucial role in the electrochemical performance of batteries.

[0003] Currently, the mechanical measurement method for battery electrode materials is based on nanoindentation technology. Existing articles (Xu, R., Sun, H., de Vasconcelos, LS, & Zhao, K. (2017). Mechanical and structural degradation of LiNixMnyCozO2 cathode in Li-ion batteries: an experimental study. Journal of The Electrochemical Society, 164(13), A3333.) and patents (application publication number: CN 112857995 A) have reported on the use of nanoindentation technology to study the changes in hardness and Young's modulus of polycrystalline lithium-ion battery cathode materials with the number of charge-discharge cycles. Battery electrode materials with different electrochemical cycle numbers were cleaned with dimethyl carbonate in a vacuum chamber filled with argon gas, vacuum dried, and then fixed with resin cold mounting. After grinding and polishing, a smooth surface was finally obtained. Nanoindentation was used to test the changes in hardness and modulus of the battery materials with the number of cycles. Although this method can measure the change of hardness and modulus of battery electrode materials with the number of cycles, it has the following problems: (1) It is difficult to obtain the stress-strain curve of the sample by nanoindentation test, and it is difficult to analyze the deformation characteristics of the sample; (2) During nanoindentation test, the sample is under multiaxial stress, and the mechanical analysis is more complicated; (3) The grinding and polishing process of the sample cannot be carried out in the glove box. At this time, the electrode material is in contact with the air for a long time, which makes the method have great limitations for air-sensitive electrode materials. Summary of the Invention

[0004] To address the challenges of complex stress states and the inability to obtain stress-strain curves in nanoindentation testing, and to avoid sample exposure to air during preparation, this invention provides a method for testing the uniaxial compressive mechanical properties of battery electrode material particles. Micropillar samples of battery electrode material particles before and after electrochemical cycling are prepared using a high-energy focused ion beam under a dual-beam scanning electron microscope (DEM). The entire micropillar sample preparation process is conducted under vacuum, thus avoiding air contamination. Uniaxial stress compression testing is performed on the micropillar samples using in-situ nanomechanical testing equipment under the scanning electron microscope. The stress state is simple, allowing direct measurement of the engineering stress-strain curves of the battery electrode material particles. Simultaneously, the scanning electron microscope enables direct observation of the microstructure of the micropillar samples after compression deformation, revealing the influence of charge-discharge on the mechanical stability of the battery electrode material.

[0005] The objective of this invention is achieved through the following technical solutions.

[0006] A method for testing the uniaxial compressive mechanical properties of battery electrode material particles, specifically including the following steps:

[0007] (1) Disassemble the battery that has completed a preset number of cycles under preset operating conditions and remove the electrode plates;

[0008] (2) First, attach the electrode sheet and the conductive substrate one by one to the two sample stages of the scanning electron microscope (SEM), and then place the two sample stages on the mechanical platform of the dual-beam electron microscope.

[0009] (3) First, a flat sheet sample is obtained by cutting the electrode sheet with a high-energy focused ion beam of a dual-beam electron microscope, which is called a separator sheet; then, the separator sheet is moved to a conductive substrate by a mechanical micro-probe of the dual-beam electron microscope, and one surface of the separator sheet is fixed to the surface of the conductive substrate; finally, a single particle is cut on the surface of the separator sheet to form a micropillar.

[0010] (4) Install the micro-nano mechanical testing equipment in the SEM cavity, remove the conductive substrate containing the micropillar sample from the sample stage and transfer it to the micro-nano mechanical testing equipment, and then perform uniaxial compression test to directly obtain the compression load (F)-displacement (x) curve of the battery electrode material. Then, convert the compression load (F)-displacement (x) curve into stress (F / A)-strain (x / h) curve by using the cross-sectional area (A) and height (h) of the micropillar. At the same time, use SEM to observe the micro morphology of the micropillar after compression deformation.

[0011] Furthermore, the specific operations for preparing the particulate micropillars in step (3) are as follows:

[0012] (3.1) Two grooves are symmetrically cut on the surface of the electrode sheet using a high-energy focused ion beam, and a sheet-like structure with two flat surfaces is formed between the two grooves.

[0013] (3.2) A high-energy focused ion beam is used to cut the two sides and the bottom of the separator, so that one side and the bottom are completely separated from the groove, while the other side is partially connected to the groove.

[0014] (3.3) First, use an auxiliary deposition gas to weld the mechanical micro probe in the dual-beam electron microscope to the separator. Then, use a high-energy focused ion beam to cut the other side of the separator to completely separate it from the groove. Finally, use the retracted mechanical micro probe to remove the separator from the groove.

[0015] (3.4) Move the conductive substrate to the center of the dual-beam electron microscope field of view, bring one surface of the separator into contact with the surface of the conductive substrate by moving the mechanical micro probe, and weld the separator and the conductive substrate together by using an auxiliary deposition gas;

[0016] (3.5) A high-energy focused ion beam is used to cut individual particles on the surface of the separator to form micropillars.

[0017] Preferably, the conductive substrate is a silicon wafer, sapphire, quartz, or glass with a gold nanofilm on its surface.

[0018] Preferably, in step (2), the conductive substrate is attached to the inclined surface of the sample stage with a tilt angle of 90°-α, while the electrode sheet is attached to the flat surface of the sample stage without tilt angle. Correspondingly, in step (3.3), the sample stage is tilted by α so that the surface of the separator is parallel to the surface of the conductive substrate on the sample stage, which facilitates the transfer of the separator to the conductive substrate; wherein, α is the angle between the ion beam and the electron beam in a dual-beam electron microscope.

[0019] Preferably, in step (3.1), the thickness of the separator is 7 to 10 μm.

[0020] Preferably, in step (3.1), the process conditions for forming the separator by cutting the groove with an ion beam are as follows: the ion source is Ga, the ion beam acceleration voltage is 30keV, the beam current is 21nA to 80pA, and the beam current gradually decreases as the cutting time progresses. This is because using a large beam current in the early stage can improve the cutting efficiency, while using a small beam current in the later stage is beneficial to correct the cross section and ensure that a separator with a flat surface is obtained.

[0021] Preferably, in step (3.5), the process conditions for cutting to form micropillars are as follows: the ion source is Ga, the ion beam accelerating voltage is 30keV, the beam current is 2.5nA to 7pA, and the beam current gradually decreases as the cutting time progresses. This is because using a large beam current in the early stage can improve the cutting efficiency, while using a small beam current in the later stage is beneficial to reduce damage and improve the dimensional accuracy of the micropillars.

[0022] Preferably, the auxiliary deposition gas is Pt (platinum) or C (carbon).

[0023] Preferably, the cross-sectional shape of the micropillar in step (3.5) is circular or square.

[0024] Beneficial effects:

[0025] (1) The thickness of battery electrode sheets is often on the scale of tens of micrometers, and their size does not change before and after electrochemical cycling. Traditional grinding and polishing operations belong to the category of macroscopic scale processing and are difficult to operate on the micrometer scale. However, the high-energy focused ion beam used in this invention has good micro-nano scale component processing capabilities, which is suitable for cutting electrode sheets and extracting sheet-like structures with flat surfaces.

[0026] (2) If the thickness of the sheet sample is too small, the sheet structure will become unstable and the subsequent sheet structure transfer and micropillar preparation steps cannot be completed; if the thickness of the sheet sample is too large, the processing time of the focused ion beam will be increased. Based on experimental experience, the thickness of the cut sheet sample is preferably 7 to 10 μm.

[0027] (3) The flatness of the sheet-like sample directly affects the flatness of the top of the micropillar. The better the flatness, the smaller the experimental error. Therefore, in the preparation of sheet-like samples, it is necessary to ensure the flatness of the cut surface of the sheet sample.

[0028] (4) During micropillar compression testing, the top plane of the indenter applying the load needs to be parallel to the bottom surface of the micropillar. The higher the parallelism, the smaller the test error. When the conductive substrate is transferred to the micro / nano mechanical testing equipment, the top plane of the indenter automatically maintains a parallel relationship with the upper and lower bottom surfaces of the conductive electrode. The bottom surface of the micropillar prepared on the electrode particles in the sheet sample is theoretically parallel to the upper and lower bottom surfaces of the sheet sample. Therefore, the parallelism between the sheet sample and the conductive substrate directly affects the parallelism between the top plane of the indenter and the bottom surface of the micropillar. Therefore, placing the conductive substrate on the inclined sample stage, so that the sheet sample and the conductive substrate are parallel, can reduce the test error of micropillar compression.

[0029] (5) The method of this invention consists of three main steps: first, disassembling the battery, which is completed in a glove box filled with protective gases such as argon after electrochemical cycling; second, preparing micropillars, where the disassembled electrode sheets are transferred to a dual-beam electron microscope and micropillars are prepared using a high-energy focused ion beam; and third, compression testing, where micro-nano mechanical testing equipment is installed in the SEM chamber to perform micropillar compression testing. Therefore, the method of this invention is performed entirely within an air-isolated glove box or electron microscope chamber, thus avoiding the influence of air on the sample.

[0030] (6) Hardness is the ability of a material to resist indentation by a hard object on its surface. It is not a simple physical concept, but a comprehensive indicator of the material's mechanical properties, such as elasticity, plasticity, strength, and toughness. Strength measures the material's ability to resist failure and is a relatively singular mechanical property indicator. Existing nanoindentation testing methods involve materials subjected to complex triaxial stress states, yielding the hardness of the electrode material. The method described in this invention involves micropillar compression, where the material is subjected to simple uniaxial stress, yielding the strength of the electrode material. Furthermore, the stress zone in nanoindentation testing is much larger than the indentation itself, making the measurement result represent the overall hardness of the electrode material particles and their surrounding area, and difficult to reflect the hardness of the electrode material particles themselves. This effect becomes more pronounced as the size of the electrode particles decreases. In the uniaxial compression test of this invention, the stress is concentrated on the micropillar itself, and the measurement result only represents the strength of the electrode particles themselves, making the mechanical test more accurate. Simultaneously, the method described in this invention can directly reflect the deformation characteristics by observing the changes in the micropillar morphology before and after the uniaxial compression test. Compared to previously reported nanoindentation testing methods, the method described in this invention can provide more comprehensive data for evaluating the impact of electrochemical cycling on changes in the mechanical properties of batteries. Attached Figure Description

[0031] Figure 1 This is a scanning electron microscope (SEM) image of the surface of the positive electrode sheet after battery cycling in step (1) of Example 1.

[0032] Figure 2 This is a scanning electron microscope image of the separator after being cut by a high-energy focused ion beam in step (4) of Example 1.

[0033] Figure 3 This is a scanning electron microscope image of the mechanical micro-probe and the separator after they are welded together and the separator is completely separated from the groove in step (5) of Example 1.

[0034] Figure 4 This is a scanning electron microscope image of the separator and silicon wafer before welding in step (6) of Example 1.

[0035] Figure 5 This is a scanning electron microscope image of the micropillars formed by cutting in step (7) of Example 1.

[0036] Figure 6 The image shows a scanning electron microscope image of the micropillar after compression testing in step (8) of Example 1.

[0037] Figure 7 The stress-strain curve obtained in step (9) of Example 1 is shown.

[0038] Figure 8 The image shows a scanning electron microscope image of a micropillar of the electrode material that was not circulated in step (10) of Example 1 after a compression test.

[0039] Figure 9 The stress-strain curve of the uncycled electrode material obtained in step (10) of Example 1 is shown. Detailed Implementation

[0040] The present invention will be further described below with reference to specific embodiments. Unless otherwise specified, the methods described are conventional methods, and the raw materials described are obtainable from publicly available commercial sources.

[0041] Example 1

[0042] (1) To evaluate the performance of a full cell with Ni90 as the positive electrode, a CR2032 button cell was assembled. This cell used Ni90 as the positive electrode, lithium metal as the negative electrode, and a Celgard 2400 microporous polypropylene membrane (pp) as the separator. The electrolyte was 1.0 M LiPF6 dissolved in ethylene carbonate (EC) / dimethyl carbonate (DMC) / ethyl methyl carbonate (EMC) (volume ratio 1:1:1) and contained 5% FEC. The electrochemical performance of the assembled cell was tested using a LANHE CT2001A series battery testing system. The test involved activation at 0.1C for 3 cycles, followed by constant current charge-discharge testing at 0.5C. The test voltage window was 2.8–4.8 V, and the test temperature was 25°C. After 10 cycles, the cell was disassembled, and the positive electrode was removed. The surface morphology of the positive electrode after cycling was as follows: Figure 1 As shown; the assembly and disassembly of the battery are both carried out in a glove box filled with argon gas;

[0043] (2) Use conductive tape to attach the side of the positive electrode sheet without electrode material taken out in step (1) to the sample stage plane of the SEM without tilt angle. Use conductive adhesive to attach the silicon wafer with a gold film with a thickness of 20nm on the surface to the inclined surface of the sample stage of the SEM with a tilt angle of 38°. Then place the two sample stages on the mechanical platform of the dual-beam electron microscope.

[0044] (3) Tilting the mechanical platform of the dual-beam electron microscope by 52° so that the high-energy focused ion beam is perpendicular to the surface of the electrode sheet on the sample stage, and then using the high-energy focused ion beam to symmetrically cut two grooves on the surface of the electrode sheet, and forming a sheet-like structure with a thickness of about 10 μm and two flat surfaces between the two grooves.

[0045] The process conditions for high-energy focused ion beam cutting are as follows: the ion source is Ga, the ion beam acceleration voltage is 30keV, and the beam current is 21nA to 80pA. In the early stage, a large beam current of 21nA is selected to improve the cutting efficiency. Then, the beam current is gradually reduced to obtain a relatively flat cross section. Finally, a small beam current of 80pA is selected to further correct the cross section to ensure that a flat surface of the separator is obtained.

[0046] (4) Zero the tilt angle of the mechanical platform of the dual-beam electron microscope. Use a high-energy focused ion beam to cut the two sides and bottom of the separator, so that one side and bottom are completely separated from the groove, while the other side is partially connected to the groove. At this time, the separator is in a cantilever beam manner in the groove, such as... Figure 2 As shown;

[0047] (5) Tilt the mechanical platform of the dual-beam electron microscope by 52° so that the mechanical microprobe in the dual-beam electron microscope contacts the suspended side of the separator. Then, use the auxiliary deposition gas Pt to weld the mechanical microprobe to the separator. Finally, use a high-energy focused ion beam to cut the other side of the separator to completely separate it from the groove. Figure 3 As shown; finally, the mechanical micron probe is slowly retracted to remove the separator from the groove, at which point the surface of the separator is parallel to the surface of the silicon wafer;

[0048] (6) Zero the tilt angle of the mechanical platform of the dual-beam electron microscope, move the conductive substrate to the center of the field of view of the dual-beam electron microscope, then bring one surface of the separator into contact with the surface of the silicon wafer by translating the mechanical micron probe, and weld the separator and the silicon wafer together by using the auxiliary deposition gas Pt, such as Figure 4 As shown;

[0049] (7) Tilt the mechanical platform of the dual-beam electron microscope by 14° so that the silicon wafer on the sample stage is perpendicular to the high-energy focused ion beam. Use the high-energy focused ion beam to cut individual particles on the surface of the separator to form micropillars with a cylindrical cross-section, such as... Figure 5 As shown;

[0050] The process conditions for high-energy focused ion beam cutting are as follows: the ion source is Ga, the ion beam acceleration voltage is 30keV, a large beam current of 2.5nA is selected in the early stage to improve the cutting efficiency, the beam current is gradually reduced in the later stage, and a small beam current of 7pA is finally selected to reduce damage and improve accuracy.

[0051] (8) Install the micro-nano mechanical testing equipment (model FT-NMT04 from Switzerland) into the SEM chamber, remove the silicon wafer containing the micropillar sample from the sample stage and transfer it to the micro-nano mechanical testing equipment, and then perform a uniaxial compression test with a quasi-static compression rate of 10. -3 s -1 The compressive load (F)-displacement (x) curve of the battery electrode material was directly obtained, and the microstructure of the micropillars after compression deformation was observed using SEM (e.g., ...). Figure 6 (as shown);

[0052] (9) The compressive load (F)-displacement (x) curve is converted into a stress (F / A)-strain (x / h) curve using the cross-sectional area (A) and height (h) of the micropillar, as shown below. Figure 7 As shown, the stress corresponding to 0.2% strain is taken as the yield strength, that is, the yield strength is 4.27 GPa;

[0053] (10) For the uncirculated positive electrode sheet, perform the corresponding operations according to steps (2) to (9). Accordingly, the microstructure of the micropillars of the uncirculated electrode material particles after compression deformation is as follows: Figure 8 As shown, the stress-strain curves of the uncirculated electrode material particles are as follows: Figure 9 As shown, the stress corresponding to 0.2% strain is taken as the yield strength, that is, the yield strength is 2.98 GPa.

[0054] By comparing the compressive yield strength, stress-strain curve shape, and micropillar morphology of the electrode material particles before and after cycling, it can be seen that the compressive yield strength of the cycled electrode material is significantly increased compared to the uncycled electrode material, from 2.98 GPa to 4.27 GPa, an increase of 1.29 GPa. The compressive stress-strain curve of the uncycled electrode material is a continuous sawtooth shape, while the stress of the cycled electrode material rapidly increases to its maximum value and then drops sharply. Compared with the micropillar morphology of the uncycled electrode material particles, the surface of the micropillars of the cycled electrode material particles forms obvious steps, and the localized strain is more concentrated. Based on the above test results, it can be concluded that after electrochemical cycling, the yield strength of the electrode material increases, the deformation localization is severe, and the mechanical stability decreases.

[0055] In summary, the above are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. 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 of testing the uniaxial compressive mechanical properties of particles of a battery electrode material, characterized by: Specifically, the following steps are included: (1) Disassemble the battery that has completed a preset number of cycles under preset operating conditions and remove the electrode plates; (2) First, attach the electrode sheet and the conductive substrate one by one to the two sample stages of the SEM, and then place the two sample stages on the mechanical platform of the dual-beam electron microscope. (3) First, a flat sheet sample is obtained by cutting the electrode sheet with a high-energy focused ion beam of a dual-beam electron microscope, which is called a separator sheet; then, the separator sheet is moved to a conductive substrate by a mechanical micro-probe of the dual-beam electron microscope, and one surface of the separator sheet is fixed to the surface of the conductive substrate; finally, a single particle is cut on the surface of the separator sheet to form a micropillar. (4) Install the micro-nano mechanical testing equipment in the SEM cavity, remove the conductive substrate containing the micro-pillar sample from the sample stage and transfer it to the micro-nano mechanical testing equipment, then perform uniaxial compression test to directly obtain the compression load-displacement curve of the battery electrode material, and then convert the compression load-displacement curve into a stress-strain curve by the cross-sectional area and height of the micro-pillar, while using SEM to observe the micro morphology after the micro-pillar compression deformation. In step (2), the conductive substrate is attached to the inclined surface of the sample stage with a tilt angle of 90°-α, while the electrode sheet is attached to the flat surface of the sample stage without tilt angle. Correspondingly, in step (3.3), the sample stage is tilted by α so that the surface of the separator sheet is parallel to the surface of the conductive substrate on the sample stage; where α is the angle between the ion beam and the electron beam in the dual-beam electron microscope. The specific steps for preparing the particulate micropillars in step (3) are as follows: (3.1) Two grooves are symmetrically cut on the surface of the electrode sheet using a high-energy focused ion beam, and a sheet-like structure with two flat surfaces is formed between the two grooves. (3.2) A high-energy focused ion beam is used to cut the two sides and the bottom of the separator, so that one side and the bottom are completely separated from the groove, while the other side is partially connected to the groove. (3.3) First, use an auxiliary deposition gas to weld the mechanical micro probe in the dual-beam electron microscope to the separator. Then, use a high-energy focused ion beam to cut the other side of the separator to completely separate it from the groove. Finally, use the retracted mechanical micro probe to remove the separator from the groove. (3.4) Move the conductive substrate to the center of the dual-beam electron microscope field of view, and bring one surface of the separator into contact with the surface of the conductive substrate by moving the mechanical micro probe, and weld the separator and the conductive substrate together by using an auxiliary deposition gas; (3.5) A high-energy focused ion beam is used to cut individual particles on the surface of the separator to form micropillars; In step (3.1), the thickness of the separator is 7–10 μm; In step (3.5), the process conditions for cutting to form micropillars are as follows: the ion source is Ga, the ion beam accelerating voltage is 30keV, the beam current is 2.5nA to 7pA, and the beam current decreases step by step as the cutting time progresses.

2. The method for testing the uniaxial compressive mechanical properties of battery electrode material particles according to claim 1, characterized in that: The conductive substrate is a silicon wafer, sapphire, quartz or glass with a gold nanofilm on its surface.

3. The method for testing the uniaxial compressive mechanical properties of battery electrode material particles according to claim 1, characterized in that: In step (3.1), the process conditions for forming the separator by cutting the groove with an ion beam are as follows: the ion source is Ga, the ion beam acceleration voltage is 30keV, the beam current is 21nA to 80pA, and the beam current decreases step by step as the cutting time progresses.

4. The method for testing the uniaxial compressive mechanical properties of battery electrode material particles according to claim 1, characterized in that: The auxiliary deposition gas is Pt or C.

5. The method for testing the uniaxial compressive mechanical properties of battery electrode material particles according to claim 1, characterized in that: In step (3.5), the cross-sectional shape of the micropillar is circular or square.

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