A method to produce nanocomposite polymer electrolytes consisting of poly(ethylene oxide) (PEO) as the polymer matrix, lithium tetrafluoroborate (LiBF4) as the lithium salt, and TiO2 as the inert ceramic filler is described. The ceramic filler, TiO2, was synthesized in situ by a sol-gel process. The morphology and crystallinity of the nanocomposite polymer electrolytes were examined by scanning electron microscopy and differential scanning calorimetry, respectively. The electrochemical properties of interest to battery applications, such as ionic conductivity, Li+ transference number, and stability window were investigated. The room-temperature ionic conductivity of these polymer electrolytes was an order of magnitude higher than that of the TiO2 free sample. A high Li+ transference number of 0.51 was recorded, and the nanocomposite electrolyte was found to be electrochemically stable up to 4.5 V versus Li+/Li. (C) 2003 Wiley Periodicals, Inc.