Spinel cobalt ferrite (CoFe2O4) thin films have been fabricated by 355 nm reactive pulsed laser deposition on stainless steel substrates. XRD and SEM analyses showed that the CoFe2O4 films exhibited a polycrystalline structure and were composed of nanoparticles with an average size of 80 nm. At 1C rate, the initial irreversible capacity of polycrystalline CoFe2O4 film electrode cycled between 0.01 and 3.0 V reached 1280 mAh/g. After 20 cycles, the reversible discharge capacities decreased and sustained about 610 mAh/g. The diffusion coefficient of Li ion for CoFe2O4 films was determined by ac impedance method, and the average value was estimated to be 1.1 X 10(-13) cm(2)/S. Based on ex situ XRD, SEM and XPS data, the electrochemical mechanism of CoFe2O4 film with lithium upon cycling was proposed. During the first discharge, the amorphization process of CoFe2O4 film electrode is accompanied with the reduction of Co2+ and Fe3+ into metal Co and Fe, respectively, and then the reversible oxidation/reduction processes of Co, Fe and Li2O take place in the subsequent charge/discharge cycles. (C) 2004 Elsevier Ltd. All rights reserved.