As a core morphological indicator of graphite anode materials, particle circularity directly determines powder tap density, specific surface area and electrolyte compatibility, and further affects the volumetric specific capacity, rate performance and cycle life of lithium-ion battery anodes. In the industrial production of graphite anodes, circularity — the two-dimensional projection index corresponding to three-dimensional sphericity — is the key parameter to evaluate the effect of grinding, shaping and spheroidization processes. JACAN Powder Equipment, a leading provider of graphite processing technology, sets a strict process target of ≥ 0.85 sphericity for its spheroidization modification procedure, and accurate circularity measurement is an essential foundation for process debugging, product quality control and performance verification.
Core definition and calculation principles of circularity
In the field of powder characterization, circularity describes how close the projected shape of a particle is to a perfect circle on a two-dimensional plane, and is widely used in industry to indirectly reflect the three-dimensional sphericity of graphite particles. There are two mainstream calculation systems adopted by different standards and application scenarios.
The most widely used definition in the lithium battery material industry is the perimeter-based circularity, calculated by the formula Circularity = 4πA / P², where A stands for the projected area of a single graphite particle and P stands for its actual projected perimeter. 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 method is sensitive to edge roughness and contour deformation, and matches well with the performance influence mechanism of graphite anode particles, so it is usually adopted to mark the sphericity index of finished graphite products such as JACAN’s ≥ 0.85 sphericity standard.
Another definition adopted by ISO 945 and related metallographic standards is the Feret diameter-based roundness, calculated as Roundness = A / (π × (Fmax/2)²), where Fmax refers to the maximum Feret diameter (the longest distance between any two points on the particle contour). This method calculates the ratio of the actual particle area to the area of its circumscribed circle, and is less sensitive to micro-scale edge roughness. It is more commonly used in the metallographic evaluation of spheroidal graphite cast iron, and can also be used for auxiliary characterization of graphite powder morphology.
Laboratory high-precision measurement methods
For R&D scenarios that require in-depth analysis of particle morphology details, static image analysis is the most classic and accurate measurement approach.
Static optical/electron microscopy image analysis is the benchmark method for circularity calibration. The standard testing procedure includes four steps: first, prepare the sample by uniformly dispersing graphite powder on the substrate to avoid particle overlap and agglomeration; second, collect clear particle projection images through an optical microscope or scanning electron microscope (SEM), with the magnification matched to the 10–50μm particle size range of anode graphite to ensure accurate contour recognition; third, perform image binarization processing to extract the complete contour of each individual particle; finally, use professional image analysis software to calculate parameters such as area, perimeter and Feret diameter of each particle, and output statistical results including median circularity (C50) and circularity distribution. This method has high resolution and can observe the morphological details of individual particles, making it suitable for process mechanism research and new product development, but its statistical representativeness is limited due to the small number of particles measured in a single test.
For scenarios that require true three-dimensional sphericity data, X-ray micro-computed tomography (Micro-CT) can be used for non-destructive testing. This technology reconstructs the complete three-dimensional structure of graphite particles through layered scanning, and directly calculates the three-dimensional sphericity without relying on two-dimensional projection approximation. Although it has the highest accuracy, its high equipment cost and long testing cycle make it mainly used for cutting-edge material research rather than daily production testing.
Industrial high-throughput measurement methods
For mass production quality control that requires high efficiency and strong statistical representativeness, dynamic image analysis has become the mainstream industrial testing method, and is also specified in the Chinese standard GB/T 38887-2020 for lithium battery graphite materials.
Dynamic image analysis works by making graphite particles pass through the detection area uniformly and dispersively driven by airflow or liquid flow, and uses a high-speed camera to continuously capture the projection images of thousands of particles in real time. The built-in algorithm automatically identifies each particle and calculates its circularity, particle size and other morphological parameters synchronously. A single test can count tens of thousands of particles, which has excellent statistical representativeness and can complete the full analysis within a few minutes. For JACAN’s classification and post-treatment process, this method can quickly detect the circularity level of each batch of products, verify whether the spheroidization modification process stably meets the ≥ 0.85 index, and ensure batch-to-batch consistency of product morphology.
In addition, there are indirect characterization methods suitable for rapid on-site inspection in production workshops. Since higher particle circularity corresponds to closer packing and higher tap density, the tap density or compaction density test can be used as a rapid auxiliary evaluation method to roughly judge the spherical level of graphite powder through density indicators. Some laser diffraction particle size analyzers can also estimate particle sphericity through scattering signal characteristics, and realize synchronous detection of particle size distribution and shape information.
Key control points for accurate measurement
To ensure reliable and comparable circularity test results, the following key links must be strictly controlled during measurement.
Sample preparation is the primary premise. Graphite particles must be fully dispersed before testing; particle overlap or agglomeration will be recognized as a single large irregular particle by the system, resulting in significantly lower measured circularity. For powder samples, dry dispersion or wet dispersion with appropriate dispersant can be selected according to the test instrument.
Imaging and parameter settings need to be standardized. The image resolution must be high enough to clearly identify the fine edges of particles, and the binarization threshold for image processing should be kept consistent to avoid contour deviation caused by different brightness and contrast. For graphite particles in the 10–50μm range, the selected magnification should ensure that each particle occupies enough pixels to ensure calculation accuracy.
Statistical rules must be unified. Circularity is a statistical indicator, and a single particle value has no practical significance. The test must count a sufficient number of particles (usually no less than 1000 effective particles) and report the median circularity (C50) and particle size distribution interval. When comparing data between different batches, the same test method, instrument and statistical standard must be adopted.
Application value in graphite anode manufacturing
Accurate circularity measurement runs through the whole process of graphite anode production and quality control. In the process development stage, by measuring the circularity 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, circularity 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.
From the perspective of battery performance, circularity data can establish a quantitative corresponding relationship with electrochemical indicators such as tap density, first-cycle coulombic efficiency and cycle life, providing data support for material performance matching and electrode process design. For the whole lithium battery anode industry, standardized circularity measurement is not only a basic means of morphological quality control, but also an important technical support for promoting the upgrading of graphite spheroidization technology and improving the energy density of anode materials.
