Atomistic Observation of the Lithiation and Delithiation Behaviors of Silicon Nanowires Using Reactive Molecular Dynamics Simulations

Hyun Jung, Minho Lee, Byung Chul Yeo, Kwang-Ryeol Lee, Sang Soo Han

For the practical use of silicon nanowires (Si NWs) as anodes for Li-ion batteries, understanding their lithiation and delithiation mechanisms at the atomic level is of critical importance. Here, we report the mechanisms for the lithiation and delithiation of Si NWs determined using a large-scale molecular dynamics (MD) simulation with a reactive force field (ReaxFF). The ReaxFF is developed in this work using first-principles calculations. Our ReaxFF-MD simulation shows that an anisotropic volume expansion behavior of Si NWs during lithiation is dependent on the surface structures of the Si NWs; however, the volumes of the fully lithiated Si NWs are almost identical irrespective of the surface structures. During the lithiation process, Li atoms penetrate into the lattices of the crystalline Si (c-Si) NWs preferentially along the 110 or 112 direction, and then thec-Si changes into amorphous Li_xSi (a-Li_xSi) phases due to the simultaneous breaking of Si–Si bonds as a result of the tensile stresses between Si atoms. Before the complete amorphization of the Si NWs, we observe the formation of silicene-like structures in the NWs that are eventually broken into low-coordinated components, such as dumbbells and isolated atoms. However, during delithiation of the Li_xSi NWs, we observe the formation of a small amount of c-Si nuclei in the a-Li_xSi matrix below a composition of Li_1.4Si ≈ Li_1.5Si, in which the volume fraction of formed c-Si phases relies on the delithiation rate. We also demonstrate that the two-phase structure can be thermodynamically more favorable than the single-phase a-Li_xSi. We expect that our comprehensive understanding of the lithiation and delithiation mechanisms along with the developed ReaxFF for Li–Si systems will provide helpful guidelines in designing Si anodes to obtain better performing Li-ion batteries.