Synthesis, Crystal Chemistry, and Electrochemical Properties of Li_7-2xLa₃Zr_2-xMo_xO₁₂ (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.