Surface effects on the structure and lithium behavior in lithiated silicon: A first principles study

Silicon anodes with excellent capacity retention and rate capability have been demonstrated utilizing nanoengineered structures, such as nanowires and nanoscale thin films. Here, we present a comparative study using density functional theory calculations to examine the surface effects on the composition, structural evolution, energetics and Li-ion mobility in amorphous Li_xSi alloys (0.42 ≤ x ≤ 3.57). When the Li content is sufficiently low, our calculations predict a slight Li surface enrichment as the presence of Li atoms contributes to the stabilization of the surfaces. As the Li content is further increased, the near-surface structure and alloy composition become similar to that in the bulk, except for the reduction in Si-Si connectivity within the outermost surface layer. The surface effects tend to be very shallow and only extend to the first couple of atomic layers; nonetheless, our ab initio molecular dynamics simulations highlight the improved Li mobility in the near-surface region. Additionally, our calculations show that Li mobility is extremely sensitive to the alloy composition, and Li diffusivity is enhanced by orders of magnitude in the highly lithiated stage.