Effects of Manufacturing Parameters on Hard Carbon Electrode Microstructures: A Combined FIB-SEM and Discrete Element Method Investigation

The commercialization of the sodium-ion battery is gaining interest due to the now-proven advantages over lithium-ion batteries in low-temperature, high-power, and safety-focused applications. Additionally, Na chemistry offers a more environmentally friendly and ethical alternative due to its predominant materials from sustainable and abundant sources. However, some limitations exist in terms of increasing energy density as hard carbon (HC), the commercially favorable active material (AM) for the negative electrode, faces various obstacles in terms of electrode densification [1]. In the traditional wet processing method of manufacturing electrodes, densification is typically achieved by calendering the electrodes after the coated electrode slurry has been dried. The factors that affect calendering include HC particle morphology, electrode composition, particle size distribution, distribution of the different electrode components, and orientation. These factors are also highly influenced by all the previous manufacturing process parameters, such as the solid content of the slurry, drying rate, and coating thickness, among others. By studying the microstructural properties of the overall electrode after drying and calendering, we can gain a deeper insight into the effects of the different manufacturing steps. Furthermore, a microscale study of the electrode microstructure would provide a more detailed characterization of electrode properties such as the tortuosity of the electrode pore network, the percolation of the carbon-binder domain, and the orientation and distribution of the active material particle. The electrode microstructures are often characterized in detail using imaging techniques such as tomography and Focused Ion Beam – Scanning Electron Microscopy (FIB-SEM) reconstruction, where the last one provides greater detail in terms of resolving the conductive carbon and the AM phases. However, such localized studies require analysis at multiple sites to account for the effects of macroscale heterogeneities. In this work, the processing limitations of the HC electrodes are investigated using FIB-SEM reconstruction of electrode sections obtained from various manufacturing conditions. The impact of manufacturing process parameters on the final electrode properties is studied by characterizing the electrode microstructure. However, since FIB-SEM reconstruction is a destructive and local technique, the microstructure of the calendered electrode is taken at a different region of the electrode, thereby subjecting the study to the effects of macroscale heterogeneity [2]. To mitigate heterogeneity effects, a Discrete Element Method (DEM) model is developed to directly simulate calendering on the microstructure of uncalendered electrodes resulting from different slurry processing parameters. The model preserves the microstructural characteristics of the electrode as obtained from the FIB-SEM reconstruction, allowing a more accurate assessment of how prior manufacturing parameters influence the calendering process than would be possible by comparing FIB-SEM microstructures alone. This provides essential insights into how material properties, such as particle shape, binder type, and carbon additive type, and manufacturing parameters such as electrode composition, solid content and calendering degree, influence the final electrode properties, including particle orientation and distribution, carbon-binder distribution, and electrode thickness. Furthermore, the output microstructures from the DEM model were further voxelized and meshed and used as inputs for a recently developed electrochemical model for HC electrodes. This model explicitly considers two charge storage phenomena: intercalation and pore-filling, enabling the estimation of the resulting electrochemical performance with 3D resolution [3]. The study provides a robust comparison between the FIB-SEM and DEM microstructures of the uncalendered and calendered electrodes, thereby improving confidence in using the microstructure to investigate the effects of calendering. References: [1] H. Raj, S. Saravanan, M. Herrmann Alba, F. Rabuel, D. Huo, and A. A. Franco, “Exploring processability limitations of commercial hard carbon for negative electrodes of Na-ion batteries,” J. Power Sources , vol. 661, p. 238588, Jan. 2026, doi: 10.1016/j.jpowsour.2025.238588. [2] S. Saravanan, I. Cardenas Sierra, P. Sun, H. Raj, D. Duranti, M. Nafees, E. Rossi, M. Sebastiani, P. Vigneaux, M. Petit, M. Moreaud, F. Fernandez, A. A. Franco, “Assessing Mesoscale Heterogeneities in Hard Carbon Electrodes through FIB-SEM Characterization, Manufacturing and Electrochemical Modeling,” Dec. 09, 2025, ChemRxiv . doi: 10.26434/chemrxiv-2025-85sxc. [3] I. Cardenas-Sierra, F. Fernandez, M. Petit, and A. A. Franco, “A microstructure-resolved model of sodium-ion battery hard carbon electrodes,” J. Power Sources , vol. 655, p. 237878, Nov. 2025, doi: 10.1016/j.jpowsour.2025.237878.