Low temperature stabilization of cubic (Li7−xAlx/3)La3Zr2O12: role of aluminum during formation

Abstract

The lithium lanthanum zirconium oxide garnet, Li7La3Zr2O12 (LLZ), has received significant attention in recent years due to its high room temperature lithium ion conductivity and its stability against lithium metal. Together these features make it a promising electrolyte candidate for a high energy all solid-state battery. Previous studies have shown that incorporation of aluminum cations during the synthesis stabilizes the higher conductivity cubic phase of LLZ; however the incorporation process and its effect on the phase transition are still unclear. In the present study, we have combined powder X-ray diffraction (XRD), 27Al and 7Li MAS NMR and high-resolution X-ray diffraction (HRXRD) to determine the disposition of Al cations during the formation of low temperature cubic LLZ. At temperatures as low as 700 °C, the aluminum is incorporated into amorphous or nanocrystalline grain boundary phases. Above 700 °C, the Al cations are associated with a poorly crystalline anti-fluorite phase Li5AlO4, composed of molecular [AlO4]5− anions. This phase then reacts with tetragonal LLZ to form cubic LLZ over a 25 hour period at 850 °C. Although the reaction appears complete by powder X-ray diffraction, 27Al NMR spectra showed overlapping resonances suggesting multiple Al environments due to uneven substitution of the 24d Li(1) site. This was confirmed by high-resolution XRD and was consistent with a series of closely related cubic LLZ phases with slightly different Al concentrations, indicating the slower Al(III) diffusion within the lattice has not reached equilibrium in the time allotted. The disorder over the two crystallographic tetrahedral sites by lithium and aluminum cations at this temperature contributes to the observed lattice enlargement associated with the low temperature cubic phase.