Ionic conduction in LiI–α,γ-alumina: molecular dynamics study

Abstract

Molecular dynamics simulations were performed on LiI–α,γ-alumina. The LiI–α,γ-alumina systems were constructed by sandwiching a LiI crystal between respectively two α- and two γ-alumina crystals. Two interfaces were formed by the LiI (001), (002) and the α-alumina (0001) and γ-alumina (001) planes. Structural, vibrational and transport properties of the Li + ions in the bulk as well as at the interfaces were investigated. A fraction of Li + ions migrated towards the alumina surfaces occupying tetrahedral sites near the interfaces. An increase in the ionic mobility was observed which was mainly caused by the diffusion along the interfaces. The density of Li + ions near the α-alumina surfaces was larger than at the γ-alumina surfaces. The charged layer composed by the Li + ions on the alumina surfaces results in a net enhanced electric field perpendicular to the interfaces. The migration of Li + ions to the alumina surfaces leads to a higher concentration of vacancies in the LiI crystal and gives therefore an increased ionic mobility. The mechanism of diffusion was the same for the LiI–α-alumina interface as well as the LiI–γ-alumina interface. This explains the fact that the activation energy was the same, 0.38 eV. The relatively larger fraction of Li + ions which has moved to the α-alumina surfaces leads to a higher pre-exponential factor for the LiI–α-alumina interfaces than for the LiI–γ-alumina interfaces.