Synthesis, Crystal Chemistry, and Electrochemical Properties of Li7–2xLa3Zr2–xMoxO12 (x = 0.1–0.4): Stabilization of the Cubic Garnet Polymorph via Substitution of Zr4+ by Mo6+

Cubic Li7La3Zr2O12 (LLZO) garnets are exceptionally well suited to be used as solid electrolytes or protecting layers in “Beyond Li-ion Battery” concepts. Unfortunately, cubic LLZO is not stable at room temperature (RT) and has to be stabilized by supervalent dopants. In this study we demonstrate a new possibility to stabilize the cubic phase at RT via substitution of Zr4+ by Mo6+. A Mo6+ content of 0.25 per formula unit (pfu) stabilizes the cubic LLZO phase, and the solubility limit is about 0.3 Mo6+ pfu. Based on the results of neutron powder diffraction and Raman spectroscopy, Mo6+ is located at the octahedrally coordinated 16a site of the cubic garnet structure (space group Ia-3d). Since Mo6+ has a smaller ionic radius compared to Zr4+ the lattice parameter a0 decreases almost linearly as a function of the Mo6+ content. The highest bulk Li-ion conductivity is found for the 0.25 pfu composition, with a typical RT value of 3.4 × 10–4 S cm–1. An additional significant resistive contribution originating from the sample interior (most probably from grain boundaries) could be identified in impedance spectra. The latter strongly depends on the prehistory and increases significantly after annealing at 700 °C in ambient air. Cyclic voltammetry experiments on cells containing Mo6+ substituted LLZO indicate that the material is stable up to 6 V. [ABSTRACT FROM AUTHOR]