Metastability in Li–La–Ti–O Perovskite Materials and Its Impact on Ionic Conductivity

A great number of candidates exist for solid electrolytes in all-solid Li batteries. This study represents the first in a series using combinatorial synthesis, X-ray diffraction (XRD), and impedance spectroscopy to screen for better solid electrolytes. Herein, over 576 Li–La–Ti–O samples are synthesized and characterized by XRD. Phase compositions are determined using automated Rietveld refinement, and the resulting phase stabilities provide important insights into this class of materials. This system includes the lithium lanthanum titanate (LLTO) perovskite structures. Of highest importance, we find that the perovskite structure is not stabilized as a pure phase at any composition but rather as composites wherein LLTO is stabilized by the presence of secondary phases at high temperature, and at some compositions, these composites are further favored during slow cooling. This new means of stabilizing metastable phases is of interest in itself, but it also proves important in designing solid electrolytes as the ionic conductivities vary dramatically with changes in the secondary phase content. We find ionic conductivities as high as 5 × 10–5 S cm–1 in total and >10–3 S cm–1 in the bulk in a sample where the secondary phase is TiO2 with a composition of 9 molar %. Both conductivity values are highly competitive with the state-of-the-art, even though more cost-effective sintering protocols are used herein (far shorter heating times and lower temperatures). We find that TiO2 helps lower the grain boundary energy in the composite electrolytes and speculate that it may be acting as a sintering agent. This study therefore helps to decouple the effects of composition and synthesis conditions that have plagued the understanding of this class of material. Thus, this work not only serves as a proof of concept for the use of combinatorial methods in studying solid electrolytes but also gives significant insights into the importance of secondary phases in ionic transport, and this is done for a class of materials that has proven to be particularly challenging. Given the negligible focus on secondary phases in the literature of solid electrolytes, these findings will be of use in further explorations of other classes of solid electrolytes.