Theoretical study of lithium absorption in silicon nanostructures for application as anodes in lithium-ion batteries

Abstract

The absorption and adsorption of lithium in silicon nanostructures: silicon quantum dots (SiQDs), silicon nanowires (SiNWs) and silicon nanopores (SiNPs) were studied using density functional theory (DFT) with M06-2X hybrid functional and 6-31G+(d) basis set. The tetrahedral sites, both Tdinner and Tdsurface, are the most preferred sites for lithiation due to their favorable binding energy profiles. For single lithium absorption and adsorption, SiQDs exhibit a binding energy of 1 eV, SiNWs demonstrate a binding energy of 1.28 eV, and SiNPs display a binding energy of 0.73 eV. Similarly, multiple lithium adsorptions yield binding energies of 1.12 eV for SiQDs, 1.21 eV for SiNWs, and 0.94 eV for SiNPs. The binding energy is altered more with the adsorption site, than with the cluster size. Molecular volume had been calculated to assess the volume expansion. A volume change of no greater than 2.51% was observed and it does not vary with the number of Li atoms, but depends on the absorption and adsorption sites. The energy gap of silicon nanostructures depends on the size (the larger being more conductive) and lithiation. Thus, large-sized silicon nanostructures are recommended for anode materials of Li-ion batteries, since the materials can yield high energy density and have small volume expansion with reasonable conductivity.