The electrochemical reaction of lithium with a crystalline Si composite electrode at room temperature was investigated using in situ X-ray diffraction (XRD) techniques. The study confirmed that crystalline Si becomes amorphous during the first lithiation. The range of the coexistence region between crystalline Si and amorphous lithiated Si is 3350 +/- 200 mAh/g. The highly lithiated amorphous LixSi phase was found to crystallize into Li15Si4 rapidly at about 60 mV (vs Li/Li+) and this Li15Si4 phase exists over a relatively narrow range of capacity. During delithiation, the Li15Si4 phase coexists with amorphous LizSi with z = 2.0(-1)(+0). Once all the Li15Si4 phase disappears, the amorphous phase persists as the remaining Li is extracted. The formation of the Li15Si4 phase can be avoided if the potential of the Si electrode is controlled above 70 mV during cycling. Based on the electrochemical and XRD data, a "phase diagram" was constructed to show the phase changes and regions of phase stability that occur during the lithiation and delithiation of the Si electrode. (c) 2007 The Electrochemical Society.