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 Li7–3x+zAlx+y+zLa3–yZr2–zO12. 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.