Structural and Electronic Properties of Lithiated SnO₂. A Periodic DFT Study

The structural and electronic properties of the intercalation compound Li_xSnO₂ (x = 1/16, 1/8, 1/4, 1/2, 1) as well as the inherent diffusion mechanism of Li ion into the rutile SnO₂ were investigated by means of periodic density functional calculations. Optimized structural parameters, cohesive energies, electronic band structure, and density-of-states and Mulliken charges for the Li_xSnO₂ system at different Li ordering for each Li content are reported. The energetic profiles for the Li diffusion process into rutile SnO₂ are also presented. Our calculation indicates substantial host distortion around intercalation sites, predominantly along the ab-planes. These deformations are found to be related to the soft B_1g, E_u, A_2g, and A_1g vibrational modes of very low frequency and therefore easy to be achieved. The corresponding variation in volume monotonically increases with the Li concentration. Cohesive energies are consistent with continuous and reversible intercalation process. In lithiated SnO₂, lithium is significantly ionized; however, the distribution pattern of the charge transferred from the lithium to the host is very dependent upon the ion concentration. By increasing the Li content, the relative amount of charge transferred to the Sn atoms decreases whereas the charge transferred to oxygen atoms increases. Lithium intercalation causes a chemical reduction of SnO₂ and yields metallic properties. Effects induced by Li intercalation on the electronic band structures of SnO₂ were assessed according to their origins, i.e., if they originate from lattice expansion or from chemical reduction. The energy difference between the valence-band maximum and conduction-band minimum of lithiated SnO₂ decreases with increasing Li content. Lithium diffusion along the c-direction demands significantly lower activation energy than the energy required for diffusion along ab-planes. Energetic barriers related to the lithium diffusion into SnO₂ were found to be dependent upon the Li content.