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

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 Al-27 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 Li-7 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.