Synthesis and Lithium-ion Conductivity of Sr(La_1−xLi_3x)ScO₄ with a K₂NiF₄ Structure

抄録

K₂NiF₄-type oxides are expected to be potential lithium-ion conductors because they have a structure similar to that of perovskites, which provide a reasonably flexible framework for accommodating defects such as charge carriers within the lattice. However, the K₂NiF₄-type structure as a framework for lithium conductors is scarcely reported. This article presents the preparation of Sr(La_1−xLi_3x)ScO₄ with a K₂NiF₄ structure by a solid-state reaction at a high pressure of 2 GPa, and elucidates its lithium-ion-conducting properties. Sr(La_1−xLi_3x)ScO₄ forms solid solutions in the x range of 0.05–0.20. Its orthorhombic lattice expands with lithium doping, indicating the incorporation of lithium ions as interstitial species in the structure. The highly doped samples exhibit high ionic conductivities (e.g., 4.66 × 10⁻⁶ S cm⁻¹ at 250 °C for x = 0.15 and 4.29 × 10⁻² S cm⁻¹ at 375 °C for x = 0.2) with an activation energy of ∼100 kJ mol⁻¹. The samples show an abnormal increase in the ionic conductivity at ∼300 °C, possibly due to the increase in the orthorhombicity of Sr(La_1−xLi_3x)ScO₄. As the electronic conductivities of the developed oxide materials are a few orders of magnitude lower than their total conductivities, they can be used as solid electrolytes in all-solid-state lithium batteries. The study reveals that K₂NiF₄-type oxides are attractive candidates for developing novel lithium-ion conductors.