Brittle-to-ductile transition of lithiated silicon electrodes: Crazing to stable nanopore growth

Using first principle calculations, we uncover the underlying mechanisms explaining the brittle-to-ductile transition of Li_xSi electrodes in lithium ion batteries with increasing Li content. We show that plasticity initiates at x = ∼ 0.5 with the formation of a craze-like network of nanopores separated by Si-Si bonds, while subsequent failure is still brittle-like with the breaking of Si-Si bonds. Transition to ductile behavior occurs at x ≥ 1 due to the increased density of highly stretchable Li-Li bonds, which delays nanopore formation and stabilizes nanopore growth. Collapse of the nanopores during unloading of the Li_xSi alloys leads to significant strain recovery.