Lithium ion conductivity of oriented Li_0.33La_0.56TiO₃ solid electrolyte films prepared by a sol-gel process

Lithium lanthanum titanates (Li_3xLa_{2/3 - x}TiO₃, LLTOs) having a-axis orientations were synthesized via a simple sol-gel route on SrTiO₃ and Nb-doped SrTiO₃ substrates. The lithium ion conduction behaviors of the LLTO films were determined. Annealing the films above 1200 °C yielded strong a-axis growth orientation, while heating samples above 1300 °C led to the reduction of the Li transference number, T_Li, which was attributed to Li evaporation. The optimized annealing temperature was 1200 °C, where both the bulk conductivity, σ_dc, and the T_Li exhibited their maxima. The bulk conductivity was 4.42 × 10^{- 5} S/cm for a 411-nm-thick film, which was almost 10-times-higher than that previously reported for sol-gel-derived films. A key reason for the enhancement in conductivity is the expanded bottleneck of the LLTO lattice because of a tensile stress induced by the substrate. Optimization of the annealing temperature and the film thickness further improved the conductivity.