A new class of ionic conducting organic/inorganic hybrid composite electrolyte with high conductivity, better electrochemical stability and mechanical behavior was prepared through the sol-gel processing between ethylene-bridged polysilsesquioxane and poly(ethylene glycol) (PEG). The composite electrolyte with 0.05 LiCIO4 per PEG repeat unit has the best conductivity up to 10(-4) S/cm at room temperature with the transference number up to 0.48 and an electrochemical stability window as high as 5.5 V versus Li/Li+. Moreover, the effect of the PEG chain length on the properties of the composite electrolyte has also been studied. The interactions between ions and polymer have also been investigated for the composite electrolyte in the presence of LiClO4 by means of FTIR, DSC, and TGA. The results indicated the interaction of Li+ ions with the ether oxygen of the PEG, and the formation of transient crosslinking with LiClO4, resulting in an increase of the T-g of the composite electrolyte. The VTF-type behavior of the ionic conductivity implied that the diffusion of the charge carriers was assisted by the segmental motions of the polymer chains. (c) 2006 Wiley Periodicals, Inc.