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 LixSi 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.