Al 2 O 3 was doped (0.5, 1.0. and 2.0 wt%) into the garnet-type lithium ion-conducting solid electrolyte with a nominal composition of Li 7 La 3 Zr 2 O 12 (LLZ) by solid state reaction at 1,180 °C. The effect of the Al 2 O 3 content in LLZ on the interface behavior with lithium metal was investigated. The lowest interface resistance was observed for 0.5 wt% Al 2 O 3-doped LLZ, which had the highest relative density of 93.7%. The Li/Al 2 O 3-doped LLZ/Li cell showed a short circuit after a limited polarization period. Short circuiting is considered to be due to lithium dendrite formation. The longest period until short circuit was 1,000 s. polarization at 0.5 mA cm-2 and room temperature for the 0.5 wt% Al 2 O 3 doped LLZ.

The garnet-type Nb-doped cubic lithium ion-conducting solid Li6.75-xLa3Zr1.75Nb0.25O12–0.5x (LLZ-Nb) was prepared with 0.46 wt% Al2O3 using a sol-gel precursor. LLZ-Nb sintered at 1,150°C for 36 h exhibited a bulk conductivity of 6.13 × 10−4 S cm−1 and a grain boundary conductivity of 7.76 × 10−3 S cm−1 at 25°C. The cell resistance of Li/LLZ-Nb/Li was gradually increased with storage time over 5 months. The increase in cell resistance was attributed to both the grain boundary and interfacial resistance between LLZ-Nb and the lithium electrode. The Li/LLZ-Nb/Li cell showed a short-circuit for 280 s polarization at 0.5 mA cm−2 and no short-circuit for 41 h polarization at 0.1 mA cm−2. LLZ-Nb was stable in saturated LiOH and LiCl aqueous solution.

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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