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 Li 1 5 Si 4 . This phase is the fully lithiated phase for silicon at room temperature, not Li 2 1 Si 5 as is widely believed. Delithiation of the Li 1 5 Si 4 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.