Molecular dynamics simulations of plasticity and cracking in lithiated silicon electrodes

Abstract

First-principle calculations have provided critical insights into the deformation behavior of lithiated silicon electrodes in high-capacity lithium ion batteries, but quantitative interpretations have been limited by the size scales of these calculations. Here, we show that large-scale molecular dynamics (MD) simulations, based on the modified embedded atom method (MEAM) potential, are capable of describing the elastic softening and plastic behavior of LixSi alloys. In particular, our MD simulations at 0 K correctly reproduce the stress–strain response of LixSi alloys from Density Functional Theory (DFT) calculations across all Li concentrations, while matching the corresponding yield strength data from existing experiments at 300 K. Results from these MD simulations reveal a sharp transition in the atomic-scale plasticity mechanisms for LixSi with increasing Li content: from the breaking of Li–Si bonds at x<1, to the breaking of Li–Li bonds at x>1. The associated transition in the fracture behavior from brittle to ductile is due to the high stretchability of Li–Li bonds compared to Li–Si and Si–Si bonds.