Process-property relationships in dry powder extrusion and dry electrode coating of lithium-iron-phosphate cathodes

This study investigates correlations between the process steps of dry mixing and dry electrode coating for lithium‑iron-phosphate (LFP) cathodes. Active material, conductive additive and PTFE binder were mixed in a continuous twin-screw extrusion process at different intensities via variation of the screw configuration. Suitable characterization methods for the dry mixed powders were applied to conclude on process-property relationships between kneading intensity and granule properties. Intensive kneading with a high specific mechanical energy input resulted in a more homogeneous and fine PTFE fibril network and in more compact and size-reduced granules. These differences in micro- and macrostructure lead to a higher electrical resistivity, higher bulk density and better flowability. The processing window concerning the applicable roll shearing ratio for one-step calendering in the DRYtraec® process broadened for high kneading intensity compared to softer kneading. Thinner electrodes could be obtained due to the higher sensitivity of a fine PTFE fibril network to shearing forces within the calender gap. Resulting electrodes demonstrated a higher adhesion strength and higher electrical resistivity as well as slight breakage of LFP particles and a lower C-rate performance. These results provide detailed insights into the dry processing chain of LFP mixtures including intermediate products and their processability, highlighting the importance of understanding the correlation between different dry coating process steps. With extrusion mixing it is possible to tailor granule properties concerning their processability, electrode structure and electrochemical performance via applying an optimal amount of mechanical energy during mixing for each specific material system.