Revisiting the Original 2003 Garnet-like Li-ion Conducting Solid Electrolytes Li5La3M2O12 (M = Nb, Ta, Nb/Ta): A Look into Phase Formation and the Identification of Carbonate and Alumina Contamination

Garnet-type electrolytes are a promising class of solid-state oxide materials for next-generation lithium batteries. In this study, parent-phase garnets, Li5La3M2O12 (M = Nb, Ta, Nb/Ta), were synthesised via solid-state reaction at 900 °C and 1100 °C, enabling comprehensive structural and chemical stability characterisation. Powder X-ray diffraction (PXRD) and Raman spectroscopy confirmed single-phase garnets with Ia¯3 d symmetry, while scanning electron microscopy (SEM) imaging revealed improved densification at 1100 °C. Electrochemical impedance spectroscopy showed the high ionic conductivities of 4.8 × 10⁻⁵ S cm⁻¹ at 55 °C for Nb and Ta co-substituted Li5La3NbTaO12 prepared at 1100 °C. X-ray photoelectron spectroscopy and thermogravimetric analysis identified a surface lithium carbonate layer formed under ambient conditions, which was not discussed in the original garnet reported in 2003. Solid-state nuclear magnetic resonance spectroscopy provided an insight into the lithium environment in the surface and bulk of the samples and confirmed aluminium contamination in samples sintered at the base of alumina crucibles at 1100 °C, with LaAlO3 identified as the dominant secondary phase, corroborated by PXRD, SEM, and energy dispersive X-ray spectroscopy analysis. Nb-doped garnet showed the most severe reaction with the alumina crucible. The use of a sacrificial mother powder of targeted garnet oxides, an approach commonly used for the preparation of Li-garnets, or sintering at 900 °C effectively reduced Al contamination. This work delivers a detailed evaluation of the parent-phase garnet, offering renewed insights into phase and structure two decades on from its initial development in 2003.