Crystal Structure of Garnet-Related Li-Ion Conductor Li_{7–3<i>x</i>}Ga_<i>x</i>La₃Zr₂O₁₂: Fast Li-Ion Conduction Caused by a Different Cubic Modification?

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 <i>Ia</i>-3<i>d</i>, 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_7-3<i>x</i> Ga _<i>x</i> La₃Zr₂O₁₂ with <i>x</i> > 0.07 crystallizes in the acentric cubic space group <i>I</i>-43<i>d</i>. 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>I</i>-43<i>d</i> compared to space group <i>Ia</i>-3<i>d</i>. Despite all efforts undertaken to reveal structure-property relationships for this class of materials, this study highlights the potential for new discoveries.