Inorganic lithium solid state electrolytes have been considered as candidates to substitute the organic liquid electrolytes in lithium batteries. However, their low conductivity is the bottleneck of their practical applications. In this paper, composite ceramic electrolytes based on lithium lanthanum titanium oxides were synthesized by adding SiO2, X-ray diffraction, Raman spectroscopy, scanning electron microscopy, energy dispersive spectroscopy and electrochemical impedance spectroscopy were used to characterize their composition, microstructure and ionic conductivity. The results showed that the total ionic conductivity was enhanced to achieve about 10(-4) S/cm at 30 degrees C due to the formation of some amorphous lithium silicate in the grain boundary. (C) 2008 Elsevier B.V. All rights reserved.