Mechanosynthesis of Solid Electrolytes: Preparation, Characterization, and Li Ion Transport Properties of Garnet-Type Al-Doped Li₇La₃Zr₂O₁₂ Crystallizing with Cubic Symmetry

Abstract

Various polycrystalline samples of Al-doped garnet-like Li₇La₃Zr₂O₁₂ crystallizing with cubic symmetry were synthesized from the binary oxides Li₂O, ZrO₂, Al₂O₃, and La₂O₃. The synthesis of phase pure samples was carried out following a two-step preparation route. It consists of an activation step by high-energy ball milling and a subsequent annealing step at elevated temperatures. The synthesis route chosen allows the precise adjustment of the cationic ratios, leading to a garnet which is best described by the formula Li_{7–3<i>x</i>+<i>z</i>}Al_{<i>x</i>+<i>y</i>+<i>z</i>}La_3–<i>y</i>Zr_2–<i>z</i>O₁₂. As confirmed by X-ray powder diffraction and ²⁷Al magic angle spinning nuclear magnetic resonance (NMR), at low Al concentrations the incorporated Al³⁺ ions act as an aliovalent dopant by replacing three Li⁺ ions. However, with increasing Al content, La³⁺ and Zr⁴⁺ ions are progressively replaced by Al ions. It turned out that, in particular, the substitution of La³⁺ and Zr⁴⁺ with Al³⁺ ions stabilizes the cubic modification of the garnet and greatly affects the corresponding Li ion dynamics. The latter has been probed by both impedance and ⁷Li NMR spectroscopy. The high ion conductivity (10^–4 S cm^–1 at 293 K) found does not only depend on the stoichiometry and the annealing conditions chosen but also on the exact kind of Al distribution on the different sites in Li₇La₃Zr₂O₁₂