An Ab Initio Study of Lithium Diffusion in Titanium Disulfide Nanotubes

Using density functional theory (DFT), we have investigated the mobility and insertion potential of Li on single-walled TiS2 nanotubes as a function of radius. To explore large radii, the nanotube surface is modeled as a curved surface. For small tubes, for which DFT calculations are practical on the complete tube, we find that strain energies, voltages, and activation barriers calculated with the curved surface method are nearly the same as those calculated with a full nanotube. Our results show that, for the range of nanotube radii that are seen experimentally (approximately 50−250 Å), Li diffusion on the nanotube surface is very fast and similar to diffusion on a flat surface. In general, the activation barrier for Li diffusion is 200 meV smaller on the surface than in the bulk, which could result in an improved mobility of Li by a factor of 3000 at room temperature. The effect of tube radius on the Li insertion voltage and migration energy can be explained by the electrostatic repulsion between Li and Ti and by the relaxation of the S atoms.