Yi Cui, Seok Woo Lee, Hyun Wook Lee
Department of Materials Science and Engineering, Stanford University
Silicon is considered one of the most promising anode materials for Li-ion batteries because of its exceptional specific capacity of 4200 mAh/g, which is about ten times that of commercial graphite anodes. However, conventional Si anodes typically suffer from rapid capacity decay due to mechanical fracture caused by large volume changes (300%) during repeated electrochemical lithium insertion and extraction.
We have reported in various studies how Si anodes expand and fracture during lithiation. First, anisotropic volume expansion and anomalous fracture behavior of crystalline Si nanostructures of various axial orientations is revealed by study of crystalline silicon nanopillars. Single crystal silicon has a fast reaction with lithium along the <110> directions, with favored fracture sites between them, as shown below.
In-situ TEM videos of lithiation of silicon nanoparticles clearly verified this anomalous behavior. above
The video shows Si fracture and expansion during lithiation process. The first part of the video represents the lithiation of Si nanowires, which is the first breakthrough to overcome large volume changes. The second part shows core shell structure of crystalline silicon and Li-Si alloy, bumped morphology at the beginning of lithiation due to anisotropic expansion of Li-Si shell and fracture at the surface due to large tensile hoop stress induced by expansion of newly lithiated silicon at the interface. In-situ video of lithiation of amorphous silicon particle breaks common sense. It is believed that diffusion of lithium plays dominant role of lithiation of amorphous silicon. But the video shows a sharp interface of amorphous silicon and a Li-Si shell during lithiation of an amorphous silicon particle, just as crystalline silicon does. We believe that these fundamental studies of how silicon anode is lithiated will provide better insight to design high performance silicon anodes for lithium-ion batteries.