Novel nano-structured composite ionic conductors were synthesized in the system of LiI-SiO2 by impregnating molten LiI into highly ordered mesoporous SiO2 with several porous structures. Composite solid electrolytes were characterized by measurements of XRD, geometric density, and a.c. impedance. Almost all the LiI was impregnated into pores and there existed about 20-vol.% of void among composite particles. Maximum ionic conductivity of composite solid electrolytes was observed when LiI was a little less than the volume fraction at which LiI is just enough to fill pores. Ionic conductivity was the highest in the composite synthesized from mesoporous SiO2 with 3-D pores, the largest surface area and the largest pore volume. The maximal conductivity of 5.8 x 10(-6) S cm(-1) was obtained for the composite synthesized by impregnating LiI into cubic mesoporous SiO2 (Ia (3) over bard) with volume fraction of SiO2 of 0.173. Activation energy was 0.32-0.36 eV and was independent Of SiO2 composition. These results suggested that the ionic conductivity was enhanced not by deformation-induced defects but by space-charge layer at interfaces. (c) 2004 Elsevier B.V. All rights reserved.