Crystal Structure of Garnet-Related Li-Ion Conductor Li 7-3 x Ga x La 3 Zr 2 O 12 : Fast Li-Ion Conduction Caused by a Different Cubic Modification?

Li-oxide garnets such as Li ₇ La ₃ Zr ₂ O ₁₂ (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 -3 d , has to be stabilized with supervalent cations. LLZO stabilized with Ga ³⁺ 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 x} Ga _x La ₃ Zr ₂ O ₁₂ with x > 0.07 crystallizes in the acentric cubic space group I -43 d . 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 Ga ³⁺ . ⁷ Li NMR spectroscopy indicates an additional Li-ion diffusion process for LLZO with space group I -43 d compared to space group Ia -3 d . Despite all efforts undertaken to reveal structure-property relationships for this class of materials, this study highlights the potential for new discoveries.