| Summary: | Perovskite-type lithium ionic conductors were explored in the (Li<i><sub>x</sub></i>La<sub>1−<i>x</i>/3</sub>)ScO<sub>3</sub> system following their syntheses via a high-pressure solid-state reaction. Phase identification indicated that a solid solution with a perovskite-type structure was formed in the range 0 ≤ <i>x </i>< 0.6. When <i>x</i> = 0.45, (Li<sub>0.45</sub>La<sub>0.85</sub>)ScO<sub>3</sub> exhibited the highest ionic conductivity and a low activation energy. Increasing the loading of lithium as an ionic diffusion carrier expanded the unit cell volume and contributed to the higher ionic conductivity and lower activation energy. Cations with higher oxidation numbers were introduced into the <i>A</i>/<i>B</i> sites to improve the ionic conductivity. Ce<sup>4+</sup> and Zr<sup>4+</sup> or Nb<sup>5+</sup> dopants partially substituted the <i>A</i>-site (La/Li) and <i>B</i>-site Sc, respectively. Although <i>B</i>-site doping produced a lower ionic conductivity, <i>A</i>-site Ce<sup>4+</sup> doping improved the conductive properties. A perovskite-type single phase was obtained for (Li<sub>0.45</sub>La<sub>0.78</sub>Ce<sub>0.05</sub>)ScO<sub>3</sub> upon Ce<sup>4+</sup> doping, providing a higher ionic conductivity than (Li<sub>0.45</sub>La<sub>0.85</sub>)ScO<sub>3</sub>. Compositional analysis and crystal-structure refinement of (Li<sub>0.45</sub>La<sub>0.85</sub>)ScO<sub>3</sub> and (Li<sub>0.45</sub>La<sub>0.78</sub>Ce<sub>0.05</sub>)ScO<sub>3</sub> revealed increased lithium contents and expansion of the unit cell upon Ce<sup>4+</sup> co-doping. The highest ionic conductivity of 1.1 × 10<sup>−3</sup> S cm<sup>−1</sup> at 623 K was confirmed for (Li<sub>0.4</sub>Ce<sub>0.15</sub>La<sub>0.67</sub>)ScO<sub>3</sub>, which is more than one order of magnitude higher than that of the (Li<i><sub>x</sub></i>La<sub>1−<i>x</i>/3</sub>)ScO<sub>3</sub> system.
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