Mechanism of lithium ion diffusion in the hexad substituted Li7La3Zr2O12 solid electrolytes

• Partial substitutions by W and Mo ions can stabilize cubic phase of Li–La–Zr–O.
• W substituted sample with x = 0.2 has the highest total conductivity (8.7 × 10− 5 S/cm).
• Two prominent relaxation-type IF peaks were observed in each Li–La–Zr–M–O sample.
• Two possible lithium ion diffusion routes via vacancies were proposed.

The Li6.4La3Zr1.7M0.3O12 (M = Mo and Cr) and Li7 − 2xLa3Zr2 − xWxO12 (x = 0.1–0.5) solid lithium-ion conductors were prepared by conventional solid state reaction method. The results indicate that partial substitutions by W and Mo ions can stabilize the garnet-like cubic phase of Li–La–Zr–O at room temperature, while the Cr ions substitution cannot even at a content of 15 mol%. The total conductivity of the Li7 − 2xLa3Zr2 − xWxO12 lithium conductor is the highest at x = 0.2 (8.7 × 10− 5 S/cm), and decreases with the increasing W substituting concentration. The Li6.4La3Zr1.7Mo0.3O12 has a higher total conductivity than that of Li6.4La3Zr1.7W0.3O12. Different from one apparent peak in the Li7La3Zr2O12 samples, two prominent relaxation-type internal friction peaks related to the short range diffusion of lithium ions were observed in each W or Mo substituted Li7La3Zr2O12 compounds. The high-temperature IF peaks locate at about 365.9 K and 358.9 K at 1 Hz, corresponding to an activation energy of 0.49 and 0.51 eV for the W and Mo substituted Li7La3Zr2O12 compounds, respectively. Considering the crystalline structure, it was suggested that the lithium ionic diffusion between 48g(96h)-24d corresponds to the low-temperature peak and 48g(96h)-48g(96h) to the high-temperature peak.