Solid state Li ion electrolytes based on the garnet type crystal structure have been successfully synthesized using the ceramic method. The approach employed in this thesis was doping of the Nb-site in Li5La3Nb2O12 with Sm and Gd and Li stuffing into the garnet-like oxides for charge balance. The resulting family of compounds have a nominal formula Li5+2xLa3Nb2-xSmxO12 (0 ≤ x ≤ 0.7) and Li5+2xLa3Nb2-xGdxO12 (0 ≤ x ≤ 0.45).
Experimental techniques used for the characterization of the solid state materials include powder X-ray diffraction (PXRD) to determine the crystal structure, scanning electron microscopy (SEM) to analyze the microstructure, energy dispersive spectroscopy (EDS) to confirm the elemental composition, AC impedance spectroscopy to determine the lithium ion conductivity Fourier transform infrared spectroscopy (FTIR) to confirm the presence of OH- and CO32- groups in the samples and thermogravimetric analysis (TGA) to test the thermal stability of the compound.
The most promising samples were the x = 0.3 member of the Sm-doped family and the x = 0.45 member of the Gd-doped family. Li5.6La3Nb1.7Sm0.3O12 showed a conductivity of 5.84 x 10-5 S cm-1 at room temperature, with an activation energy of 0.38 eV in the 25-225 °C. temperature range Li5+2xLa3Nb2-xGdxO12 showed the highest conductivity of 1.91 x 10-5 S cm-1 at room temperature with an activation energy of 0.38 eV in the temperature range 25-225 °C. Both show an order of magnitude higher conductivity than the parent compound, Li5La3Nb2O12.