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 Li 0.344 Sr 0.433 Ca 0.02 Ta 3/4 Hf 1/4 O 3 , which we compare with Li 3/8 Sr 7/16 Ta 3/4 Hf 1/4 O 3. 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 (E a = 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 Li 0.344 Sr 0.433 Ca 0.02 Ta 3/4 Zr 1/4 O 3 and led to the same conclusion compared to Li 3/8 Sr 7/16 Ta 3/4 Zr 1/4 O 3. Elemental analysis by energy-dispersive X-ray (EDX) of Li 0.344 Sr 0.433 Ca 0.02 Ta 3/4 Hf 1/4 O 3 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 Li 0.344 Sr 0.433 Ca 0.02 Ta 3/4 Hf 1/4 O 3. 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. For table of Contents Only: Enhanced conductivities with calcium-substituted Li 3/8 Sr 7/16 Ta 3/4 Hf 1/4 O 3 perovskite. Thorough structural characterization reveals intermediary phase at grain boundaries. [Display omitted] • A new calcium-substituted perovskite-type electrolyte has been synthetized. • Li-ion conductivity is two times higher than the analogous calcium-free phase. • The Li-ion conductivity of both grain and grain boundary is increased. • Neutron powder diffraction was used to determine the Li positions in the structure. • An intermediary phase at grain boundaries containing calcium is revealed. [ABSTRACT FROM AUTHOR]