Synthesis, Crystal Structure, and Stability of Cubic Li _{7– x} La ₃Zr _{2– x} Bi _x O ₁₂

Abstract

Li oxide garnets are among the most promising candidates for solid-state electrolytes in novel Li ion and Li metal based battery concepts. Cubic Li ₇La ₃Zr ₂O ₁₂ stabilized by a partial substitution of Zr ⁴⁺ by Bi ⁵⁺ has not been the focus of research yet, despite the fact that Bi ⁵⁺ would be a cost-effective alternative to other stabilizing cations such as Nb ⁵⁺ and Ta ⁵⁺. In this study, Li _{7– x} La ₃Zr _{2– x} Bi _x O ₁₂ ( x = 0.10, 0.20, ..., 1.00) was prepared by a low-temperature solid-state synthesis route. The samples have been characterized by a rich portfolio of techniques, including scanning electron microscopy, X-ray powder diffraction, neutron powder diffraction, Raman spectroscopy, and ⁷Li NMR spectroscopy. Pure-phase cubic garnet samples were obtained for x ≥ 0.20. The introduction of Bi ⁵⁺ leads to an increase in the unit-cell parameters. Samples are sensitive to air, which causes the formation of LiOH and Li ₂CO ₃ and the protonation of the garnet phase, leading to a further increase in the unit-cell parameters. The incorporation of Bi ⁵⁺ on the octahedral 16 a site was confirmed by Raman spectroscopy. ⁷Li NMR spectroscopy shows that fast Li ion dynamics are only observed for samples with high Bi ⁵⁺ contents.

Abstract

The cubic modification of Li ₇La ₃Zr ₂O ₁₂ can be stabilized by a by a partial substitution of Zr ⁴⁺ by Bi ⁵⁺. The incorporation of Bi ⁵⁺ leads to an increase in the unit-cell parameters. Samples prepared by a low-temperature preparation route are sensitive to CO ₂ and H ₂O from air, causing a protonation of the garnet phase. ⁷Li NMR spectroscopy shows that fast translational Li ion dynamics are only observed for samples with high Bi ⁵⁺ contents.