We report here the performance of FeSb2 used as negative electrode for Li-ions batteries. A capacity of 400mAhg−1 and 3500mAhcm−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 Mössbauer spectroscopy (57Fe and 121Sb) 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.