Lithium Ion Conductivity in New Perovskite Oxides [AgyLi1-y]3xLa2/3-x1/3-2xTiO3 (x = 0.09 and 0 ≤ y ≤ 1)

Abstract

New perovskite-type lithium ion conductors [Ag y Li 1-y ] 3x La 2/3-x TiO 3 were synthesized and their crystal structure and ionic conductivity were characterized. The La 3- and vacancies contents have been fixed (x = 0.09) and the substitution of Ag + for Li + was varied from 0 to 1. The X-ray powder diffraction analysis revealed that the symmetry of the doubled-perovskite lattice changes from tetragonal for the nonsubstituted oxide (y = 0) to orthorhombic when substitution occurs (0 < y < 1), to come back to tetragonal when substitution is complete (y = 1). The substitution leads to changes in the bottleneck sizes and form of the two different A-cages of the structure and consequently to the lowest activation energy of the conduction process ever found in these titanates (E a = 0.28 eV for y = 0.5). The immobile ions (La 3+ and Ag + ) can be considered as obstacles for the lithium motion through the conduction pathways. Conductivity measurements show that lithium and vacancies no longer percolate when the number of immobile ions per Ti in these oxides is greater than 75%. These immobile ions are distributed differently in the two perovskite subcells of the unit cell, leading to an immobile-ions(La 3+ /Ag + ).-poor layer and an immobile-ions-rich layer. The ionic conductivity is found to follow the occupancy of the poor layer; that is, the higher the occupancy, the smaller the lithium conductivity. The lithium motion occurs then mainly in the (La 3+ /Ag + )-poor layer of these oxides.