Interaction of Poly(vinylidene fluoride) with Graphite Particles. 2. Effect of Solvent Evaporation Kinetics and Chemical Properties of PVDF on the Surface Morphology of a Composite Film and Its Relation to Electrochemical Performance

We have determined the dependence of the surface distribution of poly(vinylidene fluoride) (PVDF) on molecular weight and functionality of PVDF and solvent evaporation kinetics in a graphite composite film analogous to that used as an anode in a lithium ion battery. The homogeneity of the PVDF surface distribution on the graphite particles is determined from fluorine dot mappings, which are detected using energy-dispersive spectroscopy and electron probe X-ray microanalysis. The results are quantified with a standard deviation method and a spatial autocorrelation function approach, which yield consistent results. We have also examined the electrochemical performance of the anodes with cyclic voltammetry and impedance spectroscopy and correlated the electrochemical properties with the homogeneity of PVDF distribution in the final film. Hydroxyl-modified PVDF shows more homogeneous distribution compared to unmodified PVDF. Furthermore, the homogeneity of PVDF distribution on the graphite increases as the solvent evaporation rate increases. Samples with homogeneous PVDF distribution show higher electrochemical capacity and lower resistance, which can be explained in terms of solid electrolyte interphase film formation, binding capability, binder swelling, and electrochemically active sites of the modified PVDF.