Mechanism of mixed conductivity in crystalline and amorphous lithium lanthanum titanate

Lithium Lanthanum Titanate (LLTO) is a Li-ion conducting perovskite-type oxide ceramic which has attracted significant attention as a prospective solid electrolyte for use in all-solid-state Li-ion batteries. However, recent investigations have shown that the material exhibits electronic conductivity under certain conditions. Notably, the amorphous phase of the material does not seem to suffer from this deficiency. In this work density functional theory calculations are used to elucidate the exact nature of electronic conductivity in LLTO, and comparisons are drawn between the crystalline and amorphous cases. Results show that Li interstitial defects form sponta-neously within LLTO, and that even slight variations in the Li content of the material will introduce electronically conductive states. This behavior is the same in the crystalline and amorphous phases. However, in the amorphous phase the band edge states are highly localized, so electronic conduction does not occur in the material. For this reason, amorphous LLTO may serve as a viable solid electrolyte for all-solid-state batteries despite the known problems with the crystalline phase.