An In Situ X-Ray Diffraction Study of the Reaction of Li with Crystalline Si

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 . The highly lithiated amorphous phase was found to crystallize into rapidly at about (vs ) and this phase exists over a relatively narrow range of capacity. During delithiation, the phase coexists with amorphous with . Once all the phase disappears, the amorphous phase persists as the remaining Li is extracted. The formation of the phase can be avoided if the potential of the Si electrode is controlled above 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.