Rheo-Impedance Measurements for the Dispersibility Evaluation of Electrode Slurries

The dispersibility of carbon dispersions (slurries) used in lithium-ion and fuel cells considerably affects the electrode properties. The uniform dispersion of conductive materials in the slurries used for electrodes has a strong influence on battery performance; hence, evaluation of the dispersibility of electrode slurries is important. We developed a rheometer-based method to evaluate the dispersibility of a carbon slurry by measuring its electrochemical impedance while applying shear stress to the dispersed particles. We measured the changes in the rheo-impedance spectra of acetylene black dispersions to which methylcellulose (MC) was added as a dispersant. We evaluated the dispersibility of a concentrated carbon slurry based on the measured rheo-impedance by observing the relationship between the MC concentration and impedance. For a carbon slurry with good dispersibility, the spectra did not change substantially under the influence of shear stress. In addition, we observed three capacitive semicircles in the Nyquist plots of impedance. Based on an equivalent circuit model, we attributed the semicircles in the high-frequency range to particle-particle contact resistance and capacitance, those in the mid-frequency range to particle bulk resistance and capacitance, and those in the low-frequency range to device-derived resistance and capacitance. The bulk resistance of acetylene black showed no shear rate dependence, although the resistance of acetylene black decreased as the MC concentration increased. The resistance measured at each MC concentration increased with the shear rate. This is attributed to the partial breakdown of the carbon-carbon network and decreasing conductivity with increasing shear rate. Our results show that a combination of viscosity and electrochemical impedance measurements can be used to evaluate the dispersibility of an electrode slurry in greater detail than was previously possible. This method may be applicable to not only carbon dispersions but also to slurries composed of a wide variety of non-carbon particles.