Spheroidization of Graphite As Anode Material for Li-Ion Batteries: Spheroidization Process and Material Texture and Morphology

Natural graphite (NG) is the dominant anode material for lithium ion batteries (LIBs) today, with a market share of approx. ~55%. With the continuous growth of portable, automotive, and other applications the total LIB market is expected to double within in the next decade. In view of this growth scenario, some prognoses predict supply bottlenecks of NG. Furthermore, the NG sources are geographically strongly concentrated today, which poses an immanent risk to supply security. In principle, the increasing demand for graphite as LIB anode material can be met by synthetic graphite, for which an upscaling of the production capacity is possible with economically justifiable effort within short time. Nonetheless, there are strong activities to develop new sources of NG. To benchmark the quality of these new materials, convenient and cost-effective characterization methods and facilities are required. Pristine flat flakes of NG are usually mechanically turned into spherical shapes (“spheroidization”) and coated with a thin film of amorphous carbon, in order to alleviate problems with exfoliation due to electrolyte solvent co-intercalation during lithium intercalation, to decrease the surface area and thus the irreversible capacity in the first charge/discharge cycle, as well as to increase density and thus the volumetric capacity. Recently, spheroidization is also more and more applied to synthetic graphite materials. Characteristic of the spheroidization procedure is that the yields are usually around 50% only. Any process improvement will therefore have a strong economic impact. The present work focuses on the lab-scale reconstruction of the industrial spheroidization and fractionation processes, using a rotor impact mill, an air jet sieve, and an air classifier. The aim is: to obtain a better understanding of the spheroidization process, by identifying critical process parameters and their influence on the resulting products (spherical graphite particles as well as by-products), to identify potentials for improving the process (especially in terms of yield), and to offer the opportunity to benchmark graphitic materials in lab-scale in comparison to industrially manufactured references. The obtained materials are analysed with regard to their electrochemical performance, as well as to their particle morphology (particle size distribution, particle shape, tap density and texture). The microstructure and texture is studied by SEM and FIB/SEM tomography, which provide information on the outer appearance in the nm scale and the open and closed porosity of the materials (Fig. 1). Acknowledgement: The authors are indebted to the German Federal Ministry of Education and Research (BMBF) for financial support (Project: Li-EcoSafe, contract no. 03X4636). Figure 1