(1) During lithiation, c-Si phase transforms into amorphous LixSi (a-LixSi) phase, and finally converts into c-Li15Si4 phase at low voltages around 50 mV vs Li/Li + . There are two clear voltage plateaus at around 0.18 and 0.05 V vs Li/Li + . (2) During delithiation, the c-Li15Si4 phase converts back to a-LixSi phase, and then a-Si is formed after complete lithium removal. The a-Si never transforms back to c-Si. (3) The lithiation of a-Si is similar to that of c-Si, except a deviation on the height of the first plateau. (4) An evident voltage hysteresis (about 350 mV after the first cycle) is observed between the charging and discharging curves, indicating different reaction pathways of lithiation and delithiation. What remains unclear is the underlying atomic scale process behind these phenomena, e.g. the mechanism of the finite voltage hysteresis, the difference in the plateau height between a-Si and c-Si lithiation, the stress difference between a-Si and c-Si. These problems depend strongly on the lithiation/delithiation paths of Si anode, especially the structural evolution of crystalline to the intermediate a-LixSi phases. Even though considerable efforts have been made to study these issues, there are still many controversies including the compositions and structure of the intermediate a-LixSi phases: