Colloidal silica gels as composite electrolytes: Rheology and ion transport

This thesis applies aspects of colloidal science, rheology, and electrochemistry to address fundamental issues pertaining to the development of composite electrolytes with desirable conductivity, processability, and mechanical stability. Investigated are colloidal gels comprised of nano-sized fumed silica (SiO₂) with alkyl chains attached to its surface, end capped oligomers of polyethylene oxide, and lithium salts. The interactions between salt, silica, and solvating medium on ion transport characteristics, microstructure, and gel rheology are examined using nuclear magnetic resonance spectroscopy (NMR), electrophoretic NMR, ac impedance spectroscopy, and various rheological techniques.; Electrophoretic NMR is used to determine the lithium transference number (T_Li, charge transport due to lithium cations), an important ion-transport characteristic that is difficult to measure. We independently measure the migration and, hence, the transference numbers of both the cation and anion, and show that they sum to unity, thus validating the technique.; The effect of fumed silica concentration on various ion transport properties (T_Li, ionic conductivity, and diffusivities of ions) reveal minimal interaction between the charged species and the fumed silica network. The large, open structure of the silica network provide unimpeded ionic mobility, which in turn is found to be determined by the solvating oligomer and lithium salt.; We find the gel elastic modulus (G′) and yield stress (τ_y) to exhibit power-law scaling relationships with filler volume fraction (&phis;): G′ ∼ &phis;n and τ_y ∼ &phis;p. The addition of salt at a constant filler volume fraction shows moderate increases in G′ , τ_y, and critical strain (γ_c). We attribute the enhanced gel properties in the presence of salts to the solvating medium becoming more polar and making the alkyl chains attached to the fumed silica “stickier”, thereby increasing particle-particle interactions.; We examine the presence of wall slip and yield stress in the colloidal gels using serrated and smooth test geometries. Tests conducted with smooth geometries exhibit wall-slip at high shear strains. Wall-slip is independent of filler and salt content. However, for salt-free solvents, increasing solvent viscosity reduces wall slip. The use of hydrophobic plates reduces slip, possibly due to the affinity between the plate and hydrophobic silica.