The effect of lanthanoid defects on anionic solvation of Li in Li6.5La2+xZr1.5Ta0.5O12 from x = 0 to x = 1.2 garnet

Abstract

The cubic phase of lithium lanthanum zirconium oxide (LLZO) is a solid electrolyte based on the garnet crystal structure which has shown promise in enabling batteries with significantly higher energy density compared to lithium-ion. Owing to its stability against lithium, LLZO is compatible with metallic lithium electrodes that, when paired with state-of-the-art cathodes, can achieve >1000 Wh/l. The preferred cubic LLZO phase is stabilized at room temperature by doping LLZO, which results in the formation of lithium vacancies (VLi′) that can achieve ionic conductivities in the 1 mS/cm range. While much attention has been paid to optimizing VLi′, discrepancies in lanthanum to zirconium (La:Zr) ratios provide an indication of the existence of vacancies at one or both of these sites. As La is the largest cation present, La vacancies should show the most dramatic effects on lattice parameter, which should affect lithium-ion transport within the LLZO sublattice. In turn, changes in lithium-ion site occupancy and solvation should affect activation energy and conductivity. However, the effects of the sub stoichiometric use of La on the crystal structure of LLZO have yet to be characterized.In this work, La content is varied in the LLZO structure Li6.5La2+xZr1.5Ta0.5O12 from x = 0 to x = 1.2. X-ray diffraction, SEM imaging, Raman Spectroscopy, electrochemical impedance measurements, and DC cycling were performed in order to correlate the effect of La deficiency on crystal structure and electrochemical properties of LLZO. Vacancy and decomposition mechanism are proposed. A linear increase in lattice parameter is observed over the range of La content measured. An increase in conductivity and CCD are also observed ranging from 0.649 mS/cm and 0.5 mA/cm2 at x = 0.2 to 0.789mS/cm and 0.80 mA/cm2 at x = 1.0.