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 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 7−3x+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 .

Li Ion Dynamics in Al-Doped Garnet-Type Li7La3Zr2O12 Crystallizing with Cubic Symmetry

“…LLZ [12] while the line with a larger chemical shift value might be related to the replacement of La and Zr by Al [19]. Most likely, Al resides on a tetrahedral site near a La or Zr vacancy.…”

Insights into the Lithium-Ion Conduction Mechanism of Garnet-Type Cubic Li5La3Ta2O12 by ab-Initio Calculations

“…39 Duvel et al prepared Al-doped LLZrO samples by a mechanochemical route and discovered that with increasing Al content, Al ions can also substitute at non-Li cation sites, leading to the formation of La-and Zr-deficient LLZrO. 40 Huang et al indicated a similar effect for Ge-doped LLZrO, according to a comparison of their experimental and simulated XANES spectra. 41 Recently, Ga-doped cubic LLZrO was successfully prepared by standard solid state synthesis 42−44 and a low-temperature sol gel approach.…”

Li+ transport properties of W substituted Li7La3Zr2O12 cubic lithium garnets

“…Reports indicated that the unintentional incorporation of Al 3+ from alumina crucibles during the high-temperature solid state synthesis helps to stabilize the cubic phase against the tetragonal one. [12][13][14][15] The inclusion of Al 2 O 3 from alumina crucible during the preparation of lithium garnets at high sintering temperature around 1225 • C is inevitable. However it is not known completely whether the included Al 3+ occupies the Li + sites or an interstitial site.…”