Detection of binder gradients using impedance spectroscopy and their influence on the tortuosity of Li-ion battery graphite electrodes

Drying battery electrodes at high rates leads to binder migration and has shown to affect the mechanical as well as the electrochemical properties of Li-ion battery electrodes. Up to now, little evidence has been shown as to why the performance suffers. Here, we investigate the influence of an inhomogeneous binder distribution on the total ionic resistance within an electrode. First, we model the impact that vertical inhomogeneities have on the impedance spectrum and the total resistance of an RC (resistor, capacitor) transmission line model. We show how different resistance profiles lead to characteristic changes in the impedance spectrum, in phase angle and magnitude, with high resistances close to the separator leading to a significantly increased overall resistance. We then show energy dispersive X-ray spectroscopy cross section data for electrodes dried at different temperatures and give experimental evidence for the formation and extent of the binder gradients. These electrodes are then measured using blocking electrolyte electrochemical impedance spectroscopy in a symmetric cell setup. It is demonstrated that, depending on the extent of the binder gradient, the phase angle changes and that just from impedance data one can detect binder gradients qualitatively without the need for other time-consuming analysis methods.