Crystal Structure of Garnet-Related Li-Ion Conductor Li7-3 x Ga x La3Zr2O12: Fast Li-Ion Conduction Caused by a Different Cubic Modification?

Reinhard Wagner; Günther J Redhammer; Daniel Rettenwander; Anatoliy Senyshyn; Walter Schmidt; Martin Wilkening; Georg Amthauer

doi:10.1021/acs.chemmater.6b00038

Crystal Structure of Garnet-Related Li-Ion Conductor Li_{7-3 x} Ga _x La₃Zr₂O₁₂: Fast Li-Ion Conduction Caused by a Different Cubic Modification?

Chem Mater. 2016 Mar 22;28(6):1861-1871. doi: 10.1021/acs.chemmater.6b00038. Epub 2016 Feb 10.

Authors

Reinhard Wagner¹, Günther J Redhammer¹, Daniel Rettenwander¹, Anatoliy Senyshyn², Walter Schmidt³, Martin Wilkening³, Georg Amthauer¹

Affiliations

¹ Department of Chemistry and Physics of Materials, University of Salzburg , Hellbrunnerstrasse 34, 5020 Salzburg, Austria.
² Heinz Maier-Leibnitz Zentrum, Technische Universität München , Lichtenbergstrasse 1, 85748 Garching b. München, Germany.
³ Institute for Chemistry and Technology of Materials, Graz University of Technology , Stremayrgasse 9, 8010 Graz, Austria.

Abstract

Li-oxide garnets such as Li₇La₃Zr₂O₁₂ (LLZO) are among the most promising candidates for solid-state electrolytes to be used in next-generation Li-ion batteries. The garnet-structured cubic modification of LLZO, showing space group Ia-3d, has to be stabilized with supervalent cations. LLZO stabilized with Ga³⁺ shows superior properties compared to LLZO stabilized with similar cations; however, the reason for this behavior is still unknown. In this study, a comprehensive structural characterization of Ga-stabilized LLZO is performed by means of single-crystal X-ray diffraction. Coarse-grained samples with crystal sizes of several hundred micrometers are obtained by solid-state reaction. Single-crystal X-ray diffraction results show that Li_7-3x Ga _x La₃Zr₂O₁₂ with x > 0.07 crystallizes in the acentric cubic space group I-43d. This is the first definite record of this cubic modification for LLZO materials and might explain the superior electrochemical performance of Ga-stabilized LLZO compared to its Al-stabilized counterpart. The phase transition seems to be caused by the site preference of Ga³⁺. ⁷Li NMR spectroscopy indicates an additional Li-ion diffusion process for LLZO with space group I-43d compared to space group Ia-3d. Despite all efforts undertaken to reveal structure-property relationships for this class of materials, this study highlights the potential for new discoveries.