Effect of composition on lithium-ion conductivity for perovskite-type lithium–strontium–tantalum–zirconium-oxide solid electrolytes

We synthesized Li2x−ySr1−xTayZr1−yO3 (LSTZ, x=0.75y) with various Ta contents y=0.60, 0.70, 0.75, 0.77 and 0.8 via a conventional solid state reaction method and investigated their crystal phase, microstructure and lithium-ion conductivity. Almost single phase perovskite-type structured LSTZ was obtained at y=0.60–0.75 and their lattice sizes were increased with increasing y, indicating that Sr2+ and Zr4+ are successfully substituted by Li+ and Ta5+ with smaller ionic radii in these three samples. On the other hand, LSTZ with higher Ta contents y=0.77 and 0.8 included some impurity phases such as LiTaO3, SrTa2O6 and Sr2Ta2O7. A solid solution range for Ta into Zr site for LSTZ is expected to be y≤0.75. Both the bulk and total (bulk+grain-boundary) ionic conductivity of LSTZ monotonically increased with y from 0.60 to 0.75. Maximum bulk and total (bulk+grain-boundary) conductivities of 2.8×10−4 S cm−1 and 2.0×10−4 S cm−1 at 27 °C were obtained in LSTZ with y=0.75