Crystal Structure of Garnet-Related Li-Ion Conductor Li_7–3xGa_xLa₃Zr₂O₁₂: 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 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 Li7–3xGaxLa3Zr2O12 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.

Ion dynamics in solid electrolytes for lithium batteries

“…Another almost universal feature of R 1 ρ diff in the case of cation-disordered LLZO is the broad relaxation rate peak that spans an extraordinary large temperature range [39,117]. It is definitely the result of a superposition of distinct local and long-range jump processes.…”

“…[39,117,118] obtained. Importantly, the low sintering temperatures particularly prevent Li loss during the final annealing step.…”

Interface Instability of Fe-Stabilized Li7La3Zr2O12 versus Li Metal

“…3−5 The Li-ion conductivities of the cubic polymorphs are 2 orders of magnitude higher (σ total ≈ 10 –3 to 10 –4 S·cm –1 at room temperature (RT)) compared to that of the tetragonal polymorph (σ total ≈ 10 –6 S·cm –1 at RT), and the electrochemical properties of the acentric cubic modification seem to be better than the centric one. 6,7 …”

“…8−10 By using a more repulsive cation, i.e., using Ga 3+ instead of Al 3+ , the phase transformation from the centric SG Ia -3 d to the acentric SG I -43 d can be observed. 3 Very recently, some of us have shown that the acentric cubic phase can also be obtained by substituting some of the Li + with Fe 3+ . 5 …”

A novel low-temperature solid-state route for nanostructured cubic garnet Li7La3Zr2O12 and its application to Li-ion battery

“…9 Recently, single crystal studies by Robben et al and briefly after by Wagner et al Ga-doping is accompanied by a reduction of the LLZO cubic garnet symmetry. 11,12 Buschmann et al reported that Ga substituted LLZO/Li interface showed higher area specific interfacial resistance (ASR) (6000 Ω•cm 2 ) than 0.9 wt% Al substituted LLZO/interface (2800 Ω•cm 2 ). 13 Geiger et al also suggested that a minor concentration of Al in LLZO (0.10 to 0.15 Al per formula unit) as stabilizing agent could compensate the additional charge emergence due to Li + vacancies and then reduce the free energy associated with ordering of the Li sub-lattice in LLZO core symmetry.…”