We report here the performance of FeSb2 used as negative electrode for Li-ions batteries. A capacity of 400 mAh g(-1) and 3500 mAh cm(-3) can be retained after 10 cycles at a C/10 cycling rate. We studied in particular the electrochemical mechanism during the first discharge of the FeSb2/Li battery. Both Mossbauer spectroscopy (Fe-57 and Sb-121) and in situ XRD studies show the formation of an unknown fcc-phase during the first step. A second step shows the conversion process leading to Li3Sb at the end of the discharge. The structure of the intermediate new phase Li4FeSb2 is isotype to the face-centered-cubic Li3Sb phase in which one Fe atom is substituted by two Li atoms. (C) 2008 Elsevier B.V. All rights reserved.