Microscopic model for Fracture of Crystalline Si-NWs for Lithium Ion Batteries
Ill Ryu (Stanford University), Seok Woo Lee (Stanford University), Yi Cui (Stanford University), William Nix (Stanford University)
Lithium ion batteries: When Chemistry meets Mechanics
Mon 4:20 - 5:40
Salomon 003
Silicon (Si) nanostructures are attractive candidates for electrodes for Li-ion batteries because they provide both large specific charging capacity and less constraint on the volume changes that occur during Li charging. Recent experiments show that crystalline Si anodes expand highly anisotropically through the motion of a sharp phase boundary between the crystalline Si core and the lithiated amorphous Si shell. Here, we present a microscopic model to describe the size-dependent fracture of crystalline Si nanopillars (NPs) during lithiation. We derive a traction-separation law based on the plastic growth of voids, which, in turn, is used in a cohesive zone-finite element model to describe fracture. The model allows for both the initiation of cracking and crack growth. The initial size and spacing of the nanovoids, assumed to be responsible for the fracture, together with the computed facture toughness, are chosen to conform to recent experiments which showed the critical diameter of Si NPs to be 300~400nm. The anisotropy of the expansion is taken into account and that leads naturally to the observed anisotropy of fracture. It may be possible to use the computed work of fracture found here to describe the failure of lithiated Si for other loadings and geometries, such as the failure of Si nanostructures during Li-ion battery cycling.