Migration of binder and conductive agent during drying process of Li-ion battery cathodes

Drying of an electrode film during a wet coating process for Li-ion batteries often leads to a heterogeneous distribution of the binder and conductive agent in the film thickness direction. Because this heterogeneous distribution affects battery performance and durability, understanding and controlling the migration behavior are important. Herein, the distribution of the components (active material/conductive agent/binder) of a cathode is investigated by a novel method and their relationship with the electrode properties is determined. The binder distribution is investigated via two methods: pyrolysis gas chromatography mass spectrometry (Py-GC/MS) analysis of cut electrodes, and low-voltage scanning electron microscopy with energy-dispersive X-ray spectroscopy. In the sample dried at 30 °C, more binder and conductive agent exists in the bottom layer. With increasing drying temperature, the binder and conductive agent migrate from the bottom to the surface because they are transported by capillary-driven convective flow through the pores between the active material particles. Drying at high temperatures causes migration, resulting in reduced adhesion between the electrode film and the current collector, and increases the electrode resistance. The Py-GC/MS method of characterizing the cut electrodes is shown to be a complementary method that can analyze a wider area than cross-sectional observation.