Structural changes in silicon anodes during lithium insertion/extraction

The structural changes in silicon electrochemically lithiated and delithiated at room temperature were studied by X-ray powder diffraction. Crystalline silicon becomes amorphous during lithium insertion, confirming previous studies. Highly lithiated amorphous silicon suddenly crystallizes at 50 mV to form a new lithium-silicon phase, identified as Li15Si4. This phase is the fully lithiated phase for silicon at room temperature, not Li21Si5 as is widely believed. Delithiation of the Li15Si4 phase results in the formation of amorphous silicon. Cycling silicon anodes above 50 mV avoids the formation of crystallized phases completely and results in better cycling performance. (C) 2004 The Electrochemical Society.