(1) For the lithiation of c-Si, Dahn et al. suggested that the intermediate phase was a-Li3.5Si by in-situ X-ray diffraction (XRD). Grey et al. reported that an a-Li7Si3 phase with distributed small Si clusters and Si-Si dumbbells was detected by XRD and ex-situ nuclear magnetic resonance (NMR). Density functional theory (DFT) calculations revealed that the six-membered ring structure in c-Si was destroyed by the insertion of Li atoms, breaking the Si-Si network into smaller rings and chains with star and boomerang structures, and finally splitting into Si-Si dumbbells and isolated Si atoms. (2) For the lithiation of a-Si, the a-Si phase transformed to a new amorphous phase (a-Li2.5Si) via a two-phase mechanism as observed by transmission electron microscope (TEM). The lithiation occurred extremely fast, making it impossible to obtain a series of other intermediate structures. In addition, in-situ NMR showed that a-Si first converted into a-Li2.0Si and then converted into a-Li3.5Si. Theoretical study also suggested that a-Li2.0Si is a key structure, in which large Si clusters were transformed into small ones . Yet other studies suggested that the intermediate phases were Li2.17Si or a-Li2.9Si 20 .