Vacancies in Si Can Improve the Concentration-Dependent Lithiation Rate: Molecular Dynamics Studies of Lithiation Dynamics of Si Electrodes

The study of lithiation dynamics is important because it affects both stress generation and rate performance of electrodes for Li-ion batteries. This topic becomes more crucial for Si anodes because its high capacity is accompanied by dramatic volume and structural changes, which lead to mechanical fracture, capacity loss, and limited cycle life. To provide fundamental insights into the lithiation dynamics, determine the rate-limiting process of lithiation, and investigate the effect of concentration on Li diffusivity, molecular dynamics along with reactive force field was used to simulate the lithiation process of both amorphous and crystalline Si electrodes. The local Li concentration evolution shows that lithiation dynamics can be characterized as occurring in two stages: an initial mixing stage followed by a subsequent random walk diffusion stage. The Li diffusion is demonstrated to be concentration-dependent as Li diffuses faster with higher Li concentration, opposite to many intercalation compounds. The degree of Li diffusivity increment with respect to Li concentration increases dramatically up to Li_0.8Si. This relationship provides an underlying reason for the experimentally observed two-phase lithiation in both c-Si and a-Si. Furthermore, it is found that the lithiation rate during the initial mixing stage increases exponentially with vacancy concentration in Si. This relationship reveals that the Si-Si bond breaking is the rate-limiting factor for Si lithiation.