Synergistic impacts of Ca2+ and Ta5+ dopants on electrical performance of garnet-type electrolytes

Highlights: • LLZO electrolytes co-doped with Ca²⁺ and Ta⁵⁺ were produced. • Synergistic impacts of Ca²⁺ and Ta⁵⁺ dopants were explored. • Prominent promotion of ionic conductivity was achieved by doping Ca²⁺ into LLZTO. • Ionic conductivity of Li_6.42La_2.98Ca_0.02Zr_1.4Ta_0.6O₁₂ reaches 5.69 × 10^–4 S·cm^–1. • Manipulating Li⁺ concentration is crucial to properties of LLZO electrolytes. -- Abstract: The cubic Li₇La₃Zr₂O₁₂ (LLZO) electrolytes co-doped with Ca²⁺ and Ta⁵⁺ dopants were produced successfully through solid method at 1230 °C for 1 h in ambient air atmosphere. The crystal structure, morphology, density and electrical performance of the Li_6.4+xLa_3–xCa_xZr_1.4Ta_0.6O₁₂ (0 ≤ x ≤ 0.08) and Li_6.6+xLa_3–xCa_xZr_1.6Ta_0.4O₁₂ (0 ≤ x ≤ 0.08) ceramics were characterized in detail. The synergistic effects of Ca²⁺ and Ta⁵⁺ dopants were explored. Attributed to the gaps of valence and ion radius between Ca²⁺ and La³⁺, the increasing concentration of transferable Li⁺ and the broader Li⁺ conduction pathway could be obtained in LLCZTO ceramics. A significantly improved electrical performance was achieved in the Li_6.4+xLa_3–xCa_xZr_1.4Ta_0.6O₁₂ (0 ≤ x ≤ 0.08) ceramics. Particularly, the specimen with the constituent of Li_6.42La_2.98Ca_0.02Zr_1.4Ta_0.6O₁₂ exhibited the highest ionic conductivity of 5.69 × 10⁴ S·cm¹ as well as the lowest activation energy of 0.27 eV. Nevertheless, the excessively high Li⁺ concentration would result to inferior ionic conductivities instead ascribed to the serious abnormal grain growth inside the ceramics.