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 Li 7 La 3 Zr 2 O 12 crystallizing with cubic symmetry were synthesized from the binary oxides Li 2 O, ZrO 2, Al 2 O 3, and La 2 O 3 . 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 x + z Al x + y + z La 3– y Zr 2– z O 12 . As confirmed by X-ray powder diffraction and 27 Al magic angle spinning nuclear magnetic resonance (NMR), at low Al concentrations the incorporated Al 3+ ions act as an aliovalent dopant by replacing three Li + ions. However, with increasing Al content, La 3+ and Zr 4+ ions are progressively replaced by Al ions. It turned out that, in particular, the substitution of La 3+ and Zr 4+ with Al 3+ 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 7 Li 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 Li 7 La 3 Zr 2 O 12 .