Filler effects of layer-structured lithium montmorillonite (Li-MMT) clays on the structure and ion conductivity of poly(ethylene oxide) (PEO)based electrolytes were investigated. Sodium cations in MMT were exchanged by Li cations for a usage as Li ion sources in Li batteries. With the increase of filler content to 20%, melting temperature and crystallinity of polymer composite electrolytes (PCEs) had been gradually decreased. Accordingly, the PCEs containing 20% Li-MMT showed the highest conductivity of 5.3 x 10(-6) (S/cm) and the highest lithium transference number of 0.55. These enhanced conducting behaviors were probably attributed to the decreased crystallinity of PEO, resulting from the structural and morphological changes by addition of Li-MMT. The change of conductivity with temperature was found to be related to the polymer segmental motion by an Arrhenius plot analysis. (C) 2007 Elsevier B.V. All rights reserved.