Mechanical properties of amorphous Li_xSi alloys: A reactive force field study

Silicon is a high-capacity anode material for lithium-ion batteries. Electrochemical cycling of Si electrodes usually produces amorphous Li _xSi (a-Li_xSi) alloys at room temperature. Despite intensive investigation of the electrochemical behaviors of a-Li_xSi alloys, their mechanical properties and underlying atomistic mechanisms remain largely unexplored. Here we perform molecular dynamics simulations to characterize the mechanical properties of a-Li_xSi with a newly developed reactive force field (ReaxFF). We compute the yield and fracture strengths of a-Li_xSi alloys under a variety of chemomechanical loading conditions, including the constrained thin-film lithiation, biaxial compression, uniaxial tension and compression. Effects of loading sequence and stress state are investigated to correlate the mechanical responses with the dominant atomic bonding, featuring a transition from the covalent to the metallic glass characteristics with increasing Li concentration. The results provide mechanistic insights for interpreting experiments, understanding properties and designing new experiments on a-Li_xSi alloys, which are essential to the development of durable Si electrodes for high-performance lithium-ion batteries.