Order vs. disorder—a huge increase in ionic conductivity of nanocrystalline LiAlO2 embedded in an amorphous-like matrix of lithium aluminate

Abstract

Coarse grained, well crystalline γ-LiAlO₂ (P4₃2₁2) is known as an electronic insulator and a very poor ion conductor with the lithium ions occupying tetrahedral voids in the oxide structure. The introduction of structural disorder such as point defects or higher-dimensional defects, however, may greatly affect ionic conduction on both short-range as well as long-range length scales. In the present study, we used high-energy ball milling to prepare defect-rich, nanocrystalline LiAlO₂ that was characterized from a structural point of view by powder X-ray diffraction, TEM as well as small angle X-ray scattering (SAXS). Temperature-dependent conductivity spectroscopy revealed an increase of the room-temperature ionic conduction by several orders of magnitude when going from microcrystalline γ-LiAlO₂ to its nanocrystalline form. The enhanced ion transport found is ascribed to the increase of Li ions near defective sites both in the bulk as well as in the large volume fraction of interfacial regions in nano-LiAlO₂. The nanocrystalline ceramic prepared at long milling times is a mixture of γ-LiAlO₂ and the high-pressure phase δ-LiAlO₂; it adapts an amorphous like structure after it has been treated in a planetary mill under extremely harsh conditions.