A method for measuring the three-dimensional spheroidization rate and porosity of spheroidized graphite.

Abstract

This invention relates to the field of spheroidization rate measurement technology, specifically to a method for measuring the three-dimensional spheroidization rate and porosity of spheroidized graphite. This invention achieves stable dispersion of single spheroidized graphite particles with micron to submicron diameters through dispersion pretreatment and directional sample preparation. Based on Nano-CT rapid positioning and confined scanning using optical-X-ray dual-modal coordinate pre-calibration, it can accurately locate and complete high-resolution three-dimensional imaging Nano-CT scanning in a short time. Based on high-iteration reconstruction and adaptive segmentation of three-dimensional image processing, it can completely preserve submicron and nanometer-scale pore information. Finally, spheroidization rate and porosity are calculated based on three-dimensional volume parameters. The method of this invention enables rapid, accurate, and repeatable measurement of the three-dimensional spheroidization rate and porosity of spheroidized graphite, solving the problems of large errors in three-dimensional spheroidization rate measurement results, long scanning times, and low porosity measurement results in existing spheroidization rate measurement methods.

Description

Technical Field

[0001] This invention relates to the field of spheroidization rate measurement technology, specifically to a method for measuring the three-dimensional spheroidization rate and porosity of spheroidized graphite. Background Technology

[0002] Spheroidized graphite, also known as spherical graphite, is a spherical particulate material formed by processing natural flake graphite through mechanical processing and purification. It features high purity, high sphericity, good electrical conductivity, and chemical stability. The unique structure of spheroidized graphite makes it widely used in lithium-ion battery anode materials. Its spherical structure provides a larger specific surface area and a shorter lithium-ion diffusion path, thereby improving the battery's charge-discharge efficiency and cycle life.

[0003] In practical applications, the spheroidization rate and porosity of spheroidized graphite are considered core parameters for evaluating its performance, directly affecting the material's conductivity, mechanical strength, oxidation resistance, and service life. Spheroidization rate characterizes the degree to which graphite particles approximate an ideal spherical shape, while porosity describes the proportion of volume occupied by pores within and between particles. High-spheroidization spheroidized graphite particles can be more uniformly packed, forming a continuous and stable conductive network, reducing internal resistance and improving energy transfer efficiency. Simultaneously, its spherical structure helps disperse mechanical loads and thermal stresses, reducing crack initiation and propagation, and improving the material's stability under high-temperature or rapid temperature change environments. Furthermore, the relatively smooth and dense surface of spherical particles reduces the penetration rate of oxidizing media, thereby improving oxidation resistance and service life.

[0004] As lithium-ion batteries develop towards higher energy density and longer cycle life, spheroidized graphite particles are gradually becoming smaller in size. Currently, the particle size of spheroidized graphite in commercial anode materials is typically concentrated in the range of 14μm to 25μm. Studies have shown that smaller-sized spheroidized graphite is more beneficial for improving the rate performance and cycle stability of batteries. Therefore, accurate measurement of the spheroidization rate and porosity of small-sized spheroidized graphite is of great significance for process optimization and performance evaluation.

[0005] However, existing methods for measuring spheroidization rate are mainly based on two-dimensional images obtained by scanning electron microscopy (SEM), and the spheroidization rate parameter is obtained by geometric calculation of the projected area and the maximum projected size. Since spheroidized graphite particles are essentially three-dimensional structures, and most of them exhibit ellipsoidal or irregular spherical morphologies, two-dimensional projection methods cannot reflect their true three-dimensional morphological characteristics, and the errors are particularly significant under small particle size conditions. In addition, two-dimensional methods have difficulty distinguishing between internal pores and surface depressions of particles, and their ability to evaluate porosity is extremely limited.

[0006] Micro-computed tomography (Micro-CT), a three-dimensional non-destructive imaging technique, has been used for porosity analysis of porous materials and has some reference value in the three-dimensional characterization of spheroidized graphite. However, Micro-CT still has structural limitations when applied to the measurement of small-particle spheroidized graphite. Specifically, high-resolution Micro-CT scans typically require several hours or even longer, and the localization of target particles relies on blind scanning, resulting in low imaging efficiency. Furthermore, limited by spatial resolution, Micro-CT struggles to identify the widespread submicron and even nanoscale pores and defects in spheroidized graphite, easily leading to a systematic underestimation of porosity and spheroidization rate in small-particle spheroidized graphite. These problems are particularly pronounced when fine-scale defects exist on the surface or inside the spheroidized graphite. Summary of the Invention

[0007] To address the problems of existing methods for measuring spheroidization rate failing to reflect the true three-dimensional morphology, resulting in large measurement errors and long scanning times, and making it difficult to accurately identify submicron and nanoscale pores, leading to low porosity measurement results, this invention provides a method for measuring the three-dimensional spheroidization rate and porosity of spheroidized graphite.

[0008] The measurement method of this invention enables rapid localization and high-resolution three-dimensional imaging of small-particle-size spheroidized graphite, and can accurately identify submicron and nanoscale pore structures. This invention employs synchrotron radiation soft X-ray microscopy (Nano-CT) technology to achieve three-dimensional reconstruction of micron- to submicron-sized spheroidized graphite, accurately measuring its spheroidization rate and porosity.

[0009] To achieve the above objectives, the technical solution of the present invention is as follows.

[0010] This invention provides a method for measuring the three-dimensional spheroidization rate and porosity of spheroidized graphite, comprising the following steps: Spheroidized graphite powder was dispersed in an alcohol solvent and then uniformly dispersed with a nonionic surfactant to obtain a spheroidized graphite dispersion. This dispersion was then directionally deposited onto a grid to obtain the sample to be tested. This step helps to obtain individual spheroidized graphite particles for characterizing their spheroidization and porosity.

[0011] A panoramic scan of the sample under test was performed using an optical microscope to establish a grid coordinate system. The correspondence between the two-dimensional optical coordinates of each spheroidized graphite particle within the field of view and the grid reference point was obtained, thus acquiring the coordinate information of the spheroidized graphite particle to be observed. This step is mainly to quickly locate the spheroidized graphite particle to be tested, and to adjust the unobstructed, easily obstructed spheroidized graphite particle to the center of the field of view and record its coordinate information.

[0012] After switching to X-ray imaging mode, the target particles are located based on their coordinate information. Once located, confined field-of-view scanning and axial offset correction are performed on spheroidized graphite particles with diameters ranging from 10 μm to 25 μm to obtain projected images of the target particles.

[0013] The projected image of the target particle is iteratively reconstructed to achieve a submicron pore recognition rate of >90%, resulting in a reconstructed image. Local adaptive thresholding based on gray-level gradients is then applied to the reconstructed image to retain pores <200 nm in size, yielding 3D reconstructed data. Based on this 3D reconstructed data, the 3D sphericity and porosity of the target particle are obtained.

[0014] This invention obtains individual small-diameter spheroidized graphite particles by dispersion with nonionic surfactants and alcohol solutions; and achieves three-dimensional reconstruction imaging of individual particles and accurate characterization of submicron to nanoscale pores in spheroidized graphite through synchrotron radiation soft X-ray imaging.

[0015] Preferably, the nonionic surfactant is polyvinylpyrrolidone, and the nonionic surfactant accounts for 0.05–0.1 wt% of the spheroidized graphite dispersion. This invention achieves better dispersion by selecting polyvinylpyrrolidone as the nonionic surfactant and adjusting its mass percentage, thus solving the problem of finding suitable graphite particles.

[0016] Preferably, the specific method for dispersing spheroidized graphite powder in an alcohol solvent is as follows: Spheroidized graphite powder is placed in an alcohol solvent, centrifuged to remove agglomerated particles, and the supernatant is retained to obtain a monodisperse spheroidized graphite particle with a particle size of 10μm to 25μm, thus obtaining an alcohol solution of spheroidized graphite; the alcohol solvent is ethanol or methanol. This invention mainly utilizes an alcohol solvent to disperse spheroidized graphite for convenient subsequent observation; however, this invention does not limit the specific type of alcohol solvent.

[0017] Preferably, the specific method for directionally depositing spheroidized graphite dispersion onto a carrier grid is as follows: Place the spheroidized graphite dispersion in a grid and let it stand for 20 to 40 seconds to allow the spheroidized graphite particles to disperse in the central area of ​​the grid openings, thus obtaining the sample to be tested.

[0018] Preferably, the scanning range of the limited field-of-view scanning is a region of 20μm × 20μm.

[0019] Preferably, the condition for shaft offset correction is that the rotation shaft deviation is less than 50 nm.

[0020] Preferably, the iterative reconstruction is performed using the SIRT algorithm, and the number of iterations is ≥300. For example, the number of iterations is typically 300.

[0021] The beneficial effects of this invention are: 1. This invention improves the exposure rate of small-diameter spheroidal graphite single particles to 80% through a sample preparation method combining particle size classification and interface wetting control, significantly reducing aggregation and occlusion, and improving the repeatability and accuracy of three-dimensional measurements. Through optical-X-ray dual-modal coordinate pre-calibration and confined scanning strategy, the single-particle three-dimensional imaging time is shortened from >5h in Micro-CT to <30min, and a spatial resolution of 25nm is achieved. Furthermore, through high-iteration SIRT reconstruction and adaptive threshold segmentation algorithm, submicron and nanopores are completely preserved, and the porosity detection accuracy is improved by about 40% compared with Micro-CT.

[0022] 2. The method of the present invention can achieve rapid positioning and high-resolution three-dimensional imaging for small-particle-size spheroidized graphite, and can accurately identify submicron and nanoscale pore structures. It enables rapid, accurate and repeatable measurement of the three-dimensional spheroidization rate and porosity of spheroidized graphite, providing a reliable basis for the process optimization of lithium-ion battery anode materials. Attached Figure Description

[0023] Figure 1 To determine the porosity of graphite particles for synchrotron radiation Nano-CT. Among them, (a) represents the porosity in the Nano-CT graphite particle slice determined by the threshold segmentation method; (b) represents the total area of ​​the Nano-CT graphite particle slice determined by the threshold segmentation method. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0025] Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] Existing two-dimensional SEM projection methods cannot reflect the true three-dimensional morphology, especially showing significant errors for ellipsoidal or surface defect particles. Micro-CT suffers from long scanning times, low positioning efficiency, and insufficient resolution in small-diameter spheroidized graphite, failing to accurately identify submicron and nanoscale pores. Furthermore, existing reconstruction and segmentation algorithms tend to treat small-scale pores as noise and discard them, leading to a systematic underestimation of porosity. Therefore, there is an urgent need for a method that can achieve rapid positioning, high-resolution three-dimensional imaging, and accurate identification of submicron and nanoscale pore structures in small-diameter spheroidized graphite, as well as a method for measuring spheroidization and porosity.

[0027] The technical solution of the present invention will be further described below through specific embodiments. Unless otherwise specified, the methods described in the following embodiments are conventional methods; the reagents and materials described are commercially available unless otherwise specified.

[0028] Example 1 A method for measuring the three-dimensional spheroidization rate and porosity of spheroidized graphite includes the following steps: Step 1, directional dispersion pretreatment method.

[0029] Step 1 mainly addresses the problem of small-particle-size spheroidized graphite easily agglomerating and obscuring in 3D imaging, leading to distortion in the measurement of spheroidization rate and porosity, through a directional dispersion preprocessing method.

[0030] Step 1.1, Particle size classification and agglomerate removal pretreatment: The original spheroidized graphite powder was placed in an ethanol solution and centrifuged at 500 rpm for 2 min to remove obviously agglomerated large particles; the monodisperse spheroidized graphite particles with a particle size of 10 μm to 25 μm were retained in the supernatant to obtain an ethanol solution of spheroidized graphite.

[0031] Based on the percentage of overlapping particles per unit field of view, step 1.1 can reduce the aggregation probability by approximately 35%.

[0032] Step 1.2, Interfacial wetting control and dispersion method: PVP with a mass concentration of 0.05 wt% was introduced into an ethanol solution of spheroidized graphite as a nonionic surfactant. After uniform dispersion, a spheroidized graphite dispersion was obtained.

[0033] Step 1.2 By reducing the interfacial energy between particles, the spatial separation of spheroidized graphite on the copper substrate is improved, increasing the single-particle exposure ratio from 62% to 85%.

[0034] Step 1.3, method of directional deposition on the carrier mesh: The spheroidized graphite dispersion was added dropwise at a rate of 5 μL to the center of a 100-mesh copper mesh. After standing for 30 seconds, excess solution was removed to ensure that the spheroidized graphite particles were preferentially distributed in the central area of ​​the copper mesh openings, reducing edge occlusion and obtaining the sample to be tested for Nano-CT imaging.

[0035] By using the preprocessing methods in steps 1.1 to 1.3, it is possible to prepare test samples for single-particle, unobstructed, and highly repeatable Nano-CT imaging without altering the true pore structure of spheroidized graphite, thus providing a foundation for subsequent high-precision measurements of three-dimensional spheroidization and porosity.

[0036] Step 2: Nano-CT rapid scanning method based on optical-X-ray dual-modal coordinate pre-calibration.

[0037] Step 2 mainly focuses on the Nano-CT rapid scanning method based on optical-X-ray dual-modal coordinate pre-calibration, which solves the problems of long blind scan time, low positioning efficiency, and insufficient resolution in Micro-CT for three-dimensional imaging of small-particle spheroidal graphite.

[0038] Step 2.1, Optical panoramic scanning and coordinate pre-calibration: A panoramic scan of the entire copper grid of the sample under test was performed using optical microscope mode to establish a grid coordinate system. The correspondence between the two-dimensional optical coordinates of each spheroidized graphite particle in the field of view and the grid reference point was obtained. The unobstructed spheroidized graphite particles that were easy to observe were located and adjusted to the center of the field of view, and their coordinate information was recorded.

[0039] Step 2.1 involves switching to optical microscope mode in the Nano-CT system. First, a panoramic scan of the entire copper grid is performed using optical microscope mode to establish the macroscopic coordinate system of the grid. Then, for each representative spheroidized graphite particle in the field of view, the correspondence between its two-dimensional optical coordinates and the grid reference point is recorded.

[0040] Step 2.2, Target Particle Coordinate Mapping and Rapid Localization: After switching to X-ray imaging mode, the sample stage is moved to within ±2μm of the target particle based on the recorded coordinate information of the spheroidized graphite particles in order to locate the target particle.

[0041] Step 2.2 involves switching to X-ray imaging mode in the Nano-CT system. Based on the recorded coordinates of the spheroidized graphite particles, the sample stage can be directly moved within ±2 μm of the target particle, avoiding the layer-by-layer blind scanning positioning method in traditional Micro-CT. This method reduces the single-particle positioning time from 20–40 minutes in Micro-CT to <2 minutes.

[0042] Step 2.3, Limited Field of View Scanning Strategy: After locating the target particles, high-resolution scanning was performed on a local area of ​​20μm×20μm for spheroidized graphite with a particle size of 10μm to 25μm.

[0043] Step 2.3 involves performing high-resolution scanning only on a local area of ​​20μm×20μm for spheroidized graphite with a particle size of 10μm to 25μm, avoiding redundant acquisition of irrelevant areas, and keeping the complete scanning time of a single particle within 8min to 12min.

[0044] Step 2.4, rapid axis alignment and real-time imaging correction: Real-time X-ray imaging is used to quickly align the target particles to reduce the rotation axis deviation to less than 50 nm. Then, the real-time image of the target particles is projected and acquired to obtain a projected image after axis offset correction.

[0045] Before formal projection acquisition, real-time X-ray imaging was used to perform rapid axis alignment on the target particles, so that the rotation axis deviation was less than 50nm, which significantly reduced reconstruction artifacts.

[0046] In summary, the embodiments of the present invention, while ensuring a spatial resolution of 25nm, control the total time for single-particle three-dimensional imaging to within 30 minutes, which is significantly better than the >5h scanning process of Micro-CT.

[0047] Step 3: Introduce a multi-level reconstruction and fidelity segmentation strategy in Nano-CT data processing.

[0048] Step 3 mainly introduces a multi-level reconstruction and fidelity segmentation strategy in Nano-CT data processing to solve the problem of submicron porosity loss caused by resolution and threshold segmentation limitations during the reconstruction process in Micro-CT.

[0049] Step 3.1, SIRT reconstruction optimization with high iteration count: The projected image after axis offset correction is reconstructed using the SIRT algorithm for ≥300 iterations to achieve a submicron pore recognition rate of >90%, thus obtaining the reconstructed image.

[0050] Step 3.1 involves correcting the axial offset of the acquired projection image and then performing at least 300 iterations of reconstruction using the SIRT (Simultaneous Iterative Reconstruction Technique) algorithm to reduce noise and enhance the recognizability of low-contrast pore structures.

[0051] Comparative experiments show that when the number of iterations is less than 100, the submicron pore recognition rate is less than 60%; more than 300 iterations can improve the submicron pore recognition rate to >90%.

[0052] Step 3.2, Multi-threshold adaptive segmentation strategy: The reconstructed image is segmented using a local adaptive threshold based on gray-level gradient, retaining pores with a size <200nm, to obtain three-dimensional reconstructed data.

[0053] In step 3.2, local adaptive thresholding based on gray-level gradient is used for the reconstructed image. The thresholding method used is the software's built-in method, which mainly segments the graphite particles and their pores according to the gray-level differences of different components in the 3D reconstructed structure. Figure 1 Unlike the single global threshold segmentation method commonly used in Micro-CT, this invention employs local adaptive threshold segmentation based on gray-level gradients, effectively avoiding the misclassification and rejection of pores with a size <200nm as noise.

[0054] Step 3.3: Calculate the sphericity and porosity of the target particles based on the 3D reconstruction data.

[0055] Verification and comparison of pore structure fidelity: Using the same batch of spheroidized graphite samples, the porosity obtained by the traditional FIB-SEM profiling method was 2.7%; Micro-CT, limited by its resolution, could not obtain accurate porosity. The Nano-CT method of this embodiment of the invention obtained a porosity of 13.4%.

[0056] The volume definition calculation of the three-dimensional sphericity: Based on the three-dimensional reconstruction data, the true surface area and the true volume V of the target particle are obtained. The surface area of ​​an ideal sphere of equal volume is obtained based on the true volume V. The three-dimensional sphericity is obtained according to the ratio of the true surface area of ​​the target particle to the surface area of ​​the ideal sphere of equal volume, thus avoiding the ellipsoid error caused by two-dimensional projection.

[0057] The formula for calculating the three-dimensional sphericity is: SI = Where SI represents the three-dimensional sphericity; A 真实 A represents the true surface area of ​​the target particle; 理想 A represents the surface area of ​​an ideal sphere of equal volume. 真实 =4πR 2 .

[0058] The formula for calculating three-dimensional porosity is: ;in, Indicates three-dimensional porosity; V represents the pore volume of the target particle, obtained by thresholding to get the pore area and then integrating; 真实 The total area is obtained by threshold segmentation and integration, representing the true volume of the target particle.

[0059] Comparative Example 1 Methods for measuring the porosity and spheroidization rate of target particles using traditional two-dimensional methods (see GB / T 9441-2021): Sphericity: μ= Where μ is the sphericity; The actual area under the 2D SEM image. It is the area of ​​a circle with the diameter of the largest particle.

[0060] Porosity: Where ∂ represents porosity; The pore area is the area of ​​the two-dimensional cross-section. It represents the cross-sectional area of ​​the two-dimensional section.

[0061] In summary, Example 1 of this invention first improves the exposure rate of small-diameter spheroidized graphite single particles to 80% through a sample preparation method combining particle size classification and interface wetting control, significantly reducing aggregation and occlusion, and improving the repeatability and accuracy of three-dimensional measurements. Then, through optical-X-ray dual-modal coordinate pre-calibration and a confined scanning strategy, the three-dimensional imaging time for a single particle is shortened from >5 hours in Micro-CT to <30 minutes, achieving a spatial resolution of 25 nm. Finally, through high-iteration SIRT reconstruction and an adaptive threshold segmentation algorithm, submicron and nanopores are completely preserved. Compared with the traditional two-dimensional method used in Comparative Example 1, the measurement method of Example 1 of this invention can improve the accuracy of porosity detection by approximately 40%.

[0062] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for measuring the three-dimensional spheroidization rate and porosity of spheroidized graphite, characterized in that, Includes the following steps: Spheroidized graphite powder was dispersed in an alcohol solvent and then uniformly dispersed with a nonionic surfactant to obtain a spheroidized graphite dispersion. The spheroidal graphite dispersion was directionally deposited onto a grid to obtain the sample to be tested; A panoramic scan of the sample under test was performed using an optical microscope to establish a grid coordinate system and obtain the correspondence between the two-dimensional optical coordinates of each spheroidized graphite particle in the field of view and the grid reference point, thereby obtaining the coordinate information of the spheroidized graphite particles to be observed. After switching to X-ray imaging mode, the target particle is located based on the coordinate information of the spheroidized graphite particle to be observed. After locating the target particles, a confined field-of-view scan and axis offset correction are performed on spheroidized graphite particles with a particle size of 10μm to 25μm to obtain a projected image of the target particles. The projected image of the target particle is iteratively reconstructed to achieve a submicron pore recognition rate of >90%, resulting in a reconstructed image. The reconstructed image is then segmented using a local adaptive threshold based on gray-level gradient to retain pores with a size <200nm, thus obtaining three-dimensional reconstructed data. Based on the 3D reconstruction data, the 3D sphericity and porosity of the target particles are obtained.

2. The method for measuring the three-dimensional spheroidization rate and porosity of spheroidized graphite according to claim 1, characterized in that, The nonionic surfactant is polyvinylpyrrolidone, and the nonionic surfactant accounts for 0.05 to 0.1 wt% of the spheroidized graphite dispersion.

3. The method for measuring the three-dimensional spheroidization rate and porosity of spheroidized graphite according to claim 1, characterized in that, The specific method for dispersing spheroidized graphite powder in an alcohol solvent is as follows: The spheroidized graphite powder was placed in an alcohol solvent, centrifuged to remove agglomerated particles, and the monodisperse spheroidized graphite particles with a particle size of 10 μm to 25 μm were retained in the supernatant to obtain an alcohol solution of spheroidized graphite; the alcohol solvent was ethanol or methanol.

4. The method for measuring the three-dimensional spheroidization rate and porosity of spheroidized graphite according to claim 1, characterized in that, The specific method for the directional deposition of spheroidized graphite dispersion onto a carrier grid is as follows: Place the spheroidized graphite dispersion in a grid and let it stand for 20 to 40 seconds to allow the spheroidized graphite particles to disperse in the central area of ​​the grid openings, thus obtaining the sample to be tested.

5. The method for measuring the three-dimensional spheroidization rate and porosity of spheroidized graphite according to claim 1, characterized in that, The scanning range of the limited field-of-view scan is a region of 20μm × 20μm.

6. The method for measuring the three-dimensional spheroidization rate and porosity of spheroidized graphite according to claim 1, characterized in that, The condition for shaft offset correction is that the deviation of the rotating shaft is less than 50 nm.

7. The method for measuring the three-dimensional spheroidization rate and porosity of spheroidized graphite according to claim 1, characterized in that, Iterative reconstruction is performed using the SIRT algorithm, with ≥300 iterations.

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