The performance of natural graphite-fibers/PEO-based gel electrolyte/LiFePO4 cells (7 mAh, 4 cm(2)) is reported. The gel polymer electrolytes were produced by electron-beam irradiation and then soaked in a liquid electrolyte. The natural graphite-fiber composite anode in gel electrolyte containing 1.5 M LiFSI-EC/GBL (1:3) exhibited high reversible capacity (361 mAh g(-1)) and high Coulombic efficiency (92%). The LiFePO4 cathode in the same gel polymer exhibited a reversible capacity of 161 mAh g(-1) and 93% Coulombic efficiency. A 1.5 M solution of LiFSI in ethylene carbonate (EC)/gamma-butyrolactone (GBL) (1:3, v/v) mixed solvent is advantageous for use as the electrolyte in the laminated film bag because of its high flame point (135 degrees C), high boiling point (219 degrees C), low vapor pressure and high conductivity (10.2 mS cm(-1) at 20 degrees C). The Li-ion gel polymer battery shows a very low capacity fade of 5% after 500 cycles and also has high-rate capability. The Li-ion gel polymer cell using LiFePO4 cathodes is suitable for HEV applications. (c) 2005 Elsevier B.V. All rights reserved.