Large-scale atomistic simulations were performed to investigate the fracture behaviors of amorphous lithiated silicon. The simulation results revealed that as the lithium concentration increases, there exists a transition in fracture mechanism from intrinsic nanoscale cavitation to extensive shear banding ahead of the crack tip. It is shown that the observed fracture-mechanism transition can be understood from the changing ratio between critical stresses for cavitation and plastic yield under increasing lithium content. Furthermore, we investigated the mechanistic details of cavitation (i.e. growth of nanovoids) in amorphous lithiated silicon using fully three-dimensional atomistic simulations. It was revealed that in a low lithium concentration environment, an initial void grows heterogeneously by merging with neighboring nucleated voids. However, at high lithium concentrations, the initial void continues to grow in a homogeneous mode. These atomistic mechanisms provide a fundamental understanding of how silicon anodes in lithium-ion batteries fracture during lithiation.