Composite polymer electrolyte (CPE) membranes, comprising poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP), aluminum oxyhydroxide (AlO[OH](n)) of two different sizes 7 mu m/14 nm and LiN(C2F5SO2)(2) as the lithium salt were prepared using a solution casting technique. The prepared membranes were subjected to XRD, impedance spectroscopy, compatibility and transport number studies. Also LiCr0.01Mn1.99O4/CPE/Li cells were assembled and their charge-discharge profiles made at 70 degrees C. The incorporation of nanotiller greatly enhanced the ionic conductivity and the compatibility of the composite polymer electrolyte. The film which possesses a nanosized filler offered better electrochemical properties than a film with micron sized fillers. The results are discussed based on Lewis acid-base theory. (c) 2005 Elsevier B.V. All rights reserved.