The shear viscosity vs. shear rate is presented in Figure . The viscosity decreases when increasing the Si content from 8% to 15%. Similarly to the solid content dependency on the viscosity, at lower shear rates, the differences in viscosity are more significant, increasing doubly from 9, 17, to 34 Pa s for 15%, 11%, and 8% Si, respectively, contrary to at higher shear rates (0.46, 0.54, and 0.63 Pa s for 15%, 11%, and 8% Si, respectively). Compared with pure graphite, the slurry viscosity is at least ten times more than the viscosity in 8% Si/83% Gr formulation for the same solid content. It was necessary to add water (reaching a solid content of 21) to have a viscosity comparable with the 8% and 11% Si formulations at higher shear rates. This difference can be explained by considering that an irregular particle surface or shape such as the one in graphite presents higher viscosity because of more contact points, thus additional inter-particle friction. SEM images evidence the irregular shape of the graphite particles, with sizes ranging from 5 to 20 µm (see Figure ). Si Nps particles are mainly rounded and organized between the graphite grains, with sizes ranging from 5 to 90 nm (see Figure ), indicating the higher viscosity of pure Gr when using SC 26. Figure shows the frequency sweeps for the slurries with different Si content and pure graphite with a solid content of 21. For the pure graphite, G" is higher at a lower frequency, indicating a viscous-like behavior. After 0.88 Hz, this tendency changes to G'> G" with an elastic response. Comparing the slurries with Si content, we observe that the slurry formulation with higher Gr content (8% Si / 83% Gr) presents a tendency closer to pure Gr than the 15% Si/ 76% Gr; only G'> G" happens at a higher frequency. Few studies have reported the rheology changes with Si content variation in the slurry formulation. Andersson et al. reported opposite results, showing an elastic and viscous response when having pure Si and pure Gr, respectively. Nevertheless, the solid content was changed by them when necessary, indicating that particle-particle interactions may change considerably when solid fractions are not controlled, reducing the friction and, thus, affecting the viscous and elastic modulus response.