Sphericity is a core quality indicator of graphite anode materials for lithium-ion batteries, which directly determines powder tap density, electrode compaction performance, electrolyte compatibility and final electrochemical performance. For the graphite processing industry, standardized sphericity testing is the basis for process debugging, product quality grading and batch consistency control. As a leading provider of graphite processing technology with 19 years of engineering experience, JACAN Powder Equipment sets a strict process target of ≥ 0.85 sphericity for its spheroidization modification procedure, and strictly follows domestic and international standard testing systems in both R&D and mass production quality control links to ensure accurate and comparable morphology evaluation results.
Standard definition and calculation formula of graphite sphericity
In the field of powder characterization, sphericity describes the degree of similarity between a particle and a perfect sphere in three-dimensional space. In the actual industrial testing of graphite powder, since direct three-dimensional measurement is costly and inefficient, the industry universally uses the two-dimensional projected circularity of particles to indirectly reflect three-dimensional sphericity, and this indicator has been incorporated into formal product standards for lithium battery graphite.
According to the general framework specified in ISO 9276-6:2008 Representation of results of particle size analysis — Part 6: Descriptive and quantitative representation of particle shape and morphology, the most widely used circularity calculation formula in the lithium battery graphite industry is the perimeter-based definition:
Circularity = 4πA / P²
Where A is the projected area of a single graphite particle, and P is the actual perimeter of its two-dimensional projection contour. For a perfectly circular projection, the circularity value equals 1; the more irregular the particle edge or the more elongated the shape, the lower the circularity value. This algorithm is highly sensitive to edge roughness and overall contour deformation, matches well with the performance influence mechanism of graphite anode particles, and is the unified calculation basis for the ≥ 0.85 sphericity index adopted by JACAN and mainstream anode manufacturers in the industry.
It should be noted that the spheroidization rate test standards represented by ISO 945-4:2019 and GB/T 9441-2021 are specially formulated for metallographic graphite in ductile cast iron, and their grading rules and application scenarios are not applicable to powdery graphite anode materials for lithium batteries.
Mainstream standard systems for graphite sphericity testing
At present, the sphericity testing of lithium battery graphite follows a multi-level standard system covering national standards, international general standards and industry group standards, which jointly standardize test methods and index requirements.
National standard for dedicated products: GB/T 38887-2020 Spherical Graphite
As the first special national standard for spherical graphite products in China, GB/T 38887-2020 clearly specifies the dynamic imaging method as the standard test method for the sphericity of spherical graphite for lithium batteries. The standard stipulates that the test shall use a dynamic particle image analyzer, collect projection images of particles in motion through a high-speed camera system, calculate the circularity of each particle through image processing algorithms, and output statistical results such as average circularity and circularity distribution. This standard unifies the testing methods and data expression specifications for the spherical graphite industry, and is the basic basis for factory inspection and trade delivery of finished products.
International general standard: ISO 9276-6:2008
This standard is a general international specification for particle shape characterization, which establishes a systematic framework for the classification, calculation and result expression of particle shape descriptors. It standardizes the sources of errors in image analysis, the algorithm principles of shape parameters and the statistical rules of results, and provides a unified methodological basis for the R&D and comparison of spherical graphite testing equipment. All mainstream dynamic and static image analysis instruments in the industry are designed and calibrated in accordance with the principles of this standard.
Industry application standard: T/CNIA 0061-2020 Spherical graphite for lithium ion batteries
Released by the China Nonferrous Metals Industry Association, this group standard further refines the test details and index requirements for spherical graphite used in lithium battery scenarios on the basis of the national standard. It clarifies the sample preparation requirements, effective particle count and result reporting specifications for sphericity testing, and is more in line with the actual quality control needs of the lithium battery anode industry.
Standard test methods and operation specifications
According to different application scenarios and accuracy requirements, the standard testing methods for graphite sphericity are mainly divided into two categories: dynamic image analysis for industrial batch quality inspection and static image analysis for laboratory precision calibration.
Dynamic image analysis: standard method for industrial mass quality control
Recommended by GB/T 38887-2020 as the preferred method for routine inspection, dynamic image analysis is the mainstream testing scheme for graphite production lines.
The standard operation process is as follows:
- Sample dispersion: The graphite powder sample is uniformly dispersed by dry airflow or mechanical vibration feeding to ensure that particles enter the detection area individually without overlapping and agglomeration. For graphite with a particle size range of 10–50μm, the feeding speed and dispersion air pressure need to be adjusted to an appropriate range to avoid particle overlap.
- High-speed imaging: Particles fall freely through the detection area driven by gravity or airflow, and the high-speed camera system continuously captures clear projection images of each particle at a frame rate of no less than 100 frames per second.
- Image recognition and calculation: The built-in software automatically performs binarization processing on the image, extracts the contour of each particle, calculates the circularity value of a single particle according to the 4πA/P² formula, and counts the distribution of all effective particles.
- Result output: Report the average circularity, median circularity (C50) and circularity distribution curve of the sample. The standard requires that the number of effective statistical particles shall not be less than 1000, and industrial-grade instruments usually count tens of thousands of particles in a single test to ensure strong statistical representativeness.
This method has the advantages of fast detection speed, large statistical sample size and good repeatability, and is suitable for rapid quality inspection of products after the classification and post-treatment process. For JACAN’s production lines, this method can quickly verify whether each batch of products stably meets the ≥ 0.85 sphericity standard after spheroidization modification, and ensure batch-to-batch consistency.
Static image analysis: benchmark method for laboratory calibration
Static image analysis based on optical microscope or scanning electron microscope (SEM) is the benchmark method for sphericity calibration and mechanism research, and is also an important supplement to dynamic image method testing.
The standard operation process includes:
- Sample preparation: The graphite powder is uniformly dispersed on the sample stage or conductive adhesive to avoid particle overlap and stacking as much as possible, so as to obtain clear independent particle projection.
- Image acquisition: Select appropriate magnification according to the particle size range to ensure that each particle occupies enough pixels to accurately identify the edge contour. SEM imaging has higher resolution and can observe the details of particle edge morphology at the same time.
- Manual / automatic image analysis: Use professional image analysis software to extract particle contours one by one and calculate circularity parameters. Since the number of particles that can be counted in a single field of view is limited, it is necessary to take images at multiple points to ensure that the total number of effective particles meets the statistical requirements.
This method has high measurement accuracy and can observe the morphological details of individual particles. It is mostly used in the R&D stage to verify the effect of grinding and spheroidization processes, and is also used for the calibration and verification of dynamic image analysis instruments.
Indirect auxiliary characterization method
In the production workshop, tap density and BET specific surface area can be used as rapid auxiliary evaluation means. Higher sphericity corresponds to higher tap density and moderate specific surface area. However, these indirect indicators can only roughly reflect the overall spherical level, and cannot replace the image analysis method as the standard basis for product sphericity judgment.
Key quality control points for standard testing
To ensure the accuracy and comparability of test results, the following key links must be strictly controlled in accordance with standard specifications during testing.
First, sample dispersion quality is the primary premise. Graphite particles have a strong tendency to agglomerate. If particle overlap occurs during testing, the system will recognize multiple small particles as a single large irregular particle, resulting in a significantly lower measured sphericity value. For dry test systems, the dispersion air pressure and feeding speed must be optimized to ensure that particles enter the detection area in a single dispersed state.
Second, sufficient statistical sample size must be guaranteed. Sphericity is a statistical indicator, and the value of a single particle has no practical significance. The test must count enough effective particles to reflect the overall morphological level of the powder. According to conventional requirements, the number of effective particles for routine testing shall not be less than 1000, and for high-precision testing, it shall be increased to more than 5000.
Third, standardize image processing parameters. The resolution of the imaging system must meet the requirements, and the binarization threshold for image processing shall be kept consistent between different tests. ISO 9276-6 specifically points out that unreasonable threshold settings will cause deviation in contour extraction, which in turn affects the accuracy of perimeter and area calculation.
Finally, unify result expression methods. The test results shall be reported by statistical characteristic values such as average circularity or median circularity (C50), and the particle size range corresponding to the test shall be indicated at the same time. When comparing data between different batches or different laboratories, the same test method, instrument model and statistical standard must be adopted.
Application of standard testing in graphite production quality control
Standardized sphericity testing runs through the whole process of graphite anode production. In the process development stage, by testing the sphericity of products under different grinding and spheroidization parameters, manufacturers can optimize the process scheme and stably achieve the high sphericity target like JACAN’s ≥ 0.85 standard. In the mass production stage, sphericity is used as a routine quality inspection index after the classification process to monitor the stability of the spheroidization process and avoid performance fluctuation of finished products caused by morphological differences.
For production lines equipped with online detection systems, the standard test method can be transplanted to the online detection unit to realize real-time monitoring of product sphericity. Combined with the intelligent closed-loop control system, when the sphericity index fluctuates, the process parameters can be adjusted dynamically in time to ensure the stable quality of mass-produced products.
At present, the standard testing system for graphite sphericity has been relatively mature: dynamic image analysis is the national standard recommended method for industrial batch quality inspection, which has the advantages of high efficiency and good statistical representativeness; static image analysis is used as the laboratory benchmark method for R&D verification and instrument calibration. All methods follow the unified circularity calculation principle specified in ISO 9276-6:2008, and are refined and implemented in the special product standard GB/T 38887-2020.
Relying on 19 years of technical accumulation and verification by more than 1,200 global customers, JACAN’s spheroidization modification and classification process strictly follows the standard testing system for full-process quality control, ensuring that each batch of graphite products can stably meet the ≥ 0.85 sphericity index. For the whole lithium battery graphite industry, standardized sphericity testing is not only the basis of quality control, but also an important technical support for promoting the upgrading of spheroidization technology and improving the overall performance level of anode materials.
