Garnet-type high lithium ion conductivity solid electrolytes with nominal compositions of Li 7-x La 3 Zr 2-x Ta x O 12 (x = 0-0.7) were prepared by using a sol-gel precursor and are proposed as the protective layer for a water-stable lithium electrode for lithium-air rechargeable batteries. The Li 6.75 La 3 Zr 1.75 Ta 0.25 O 12 (LLZ-0.25Ta) composition sintered at 1,180 • C for 36 h showed the highest relative density of 96.7% and resistance to water permeation. The LLZ-0.25Ta was stable in contacting with lithium metal and also in a saturated LiOH/10 M LiCl aqueous solution. The total and bulk conductivities of the sintered LLZ-0.25Ta pellet were 5.20 × 10 −4 and 6.55 × 10 −4 S cm −1 at 25 • C, respectively. The Li/LLZ-0.25Ta/Li cell showed a stable cell resistance of 310 cm 2 at 25 • C for 4 months. However, polarization experiments on the Li/LLZ-0.25Ta/Li cell suggest a lithium dendrite short-circuit after 100 s at 0.5 mA cm −2 and after 2000 s of polarization at 0.1 mA cm −2 at room temperature.

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The Al-free cubic garnet-like Li 5+2x La 3 Nb 2-x Sc x O 12 series with x = 0-0.75 was successfully synthesized by solid state reaction. Final sintering at 1150 °C for 12 h in a magnesia crucible with the mother powder method resulted in stoichiometric single phases that corresponded to each nominal composition. The relationship between the lithium content and ionic conductivity was investigated for each composition. The conductivity of the samples showed a positive dependency on the Li content and the optimum Li content for Li conduction was 6.25-6.5 in the formula unit. The highest bulk conductivity of 3.70×10 -4 S cm -1 was observed for x = 0.625 at 25 °C and that for x = 0.75 at 50 °C was 6.05×10 -4 S cm -1 .