Calcium substitution to improve the total ionic conductivity of the Li3/8Sr7/16Ta3/4Hf1/4O3 perovskite-type electrolyte

We report novel calcium-substituted perovskite-type solid state electrolyte with nominal composition Li0.344Sr0.433Ca0.02Ta3/4Hf1/4O3, which we compare with Li3/8Sr7/16Ta3/4Hf1/4O3. The compounds were synthesized via solid-state reaction and studied by X-ray and neutron powder diffraction and electrochemical impedance spectroscopy. Neutron powder diffraction allowed the Li position in the structure to be accurately determined. Calcium-substituted phase showed higher Li-ion conductivity than the analogous calcium-free phase obtained with our synthesis method. High total Li-ion conductivities of 3.6 ± 1.0 × 10−4 S cm−1 (Ea = 431 meV) at 30 °C were reached for calcium-substituted phase, and both bulk and grain-boundary conductivities increased compared to that of the calcium-free phase. The same experiment was conducted on Li0.344Sr0.433Ca0.02Ta3/4Zr1/4O3 and led to the same conclusion compared to Li3/8Sr7/16Ta3/4Zr1/4O3. Elemental analysis by energy-dispersive X-ray (EDX) of Li0.344Sr0.433Ca0.02Ta3/4Hf1/4O3 showed the formation of an intermediary phase at grain boundaries, which contained essentially strontium, calcium, and oxygen. To better understand the increased bulk conductivity, neutron diffraction was performed on Li0.344Sr0.433Ca0.02Ta3/4Hf1/4O3. The results demonstrate the importance of understanding and controlling the grain boundary composition, as much as the bulk composition, to improve the total ionic conductivity of solid electrolytes.