Abstract
Li7La3Zr2O12 (LLZO) garnets are among the most promising solid electrolytes for next-generation all-solid-state Li-ion battery applications due to their high stabilities and ionic conductivities. To help determine the influence of different supervalent dopants on the crystal structure and site preferences, we combine solid-state 17O, 27Al, and 71Ga magic angle spinning (MAS) NMR spectroscopy and density-functional theory (DFT) calculations. DFT-based defect configuration analysis for the undoped and Al and/or Ga-doped LLZO variants uncovers an interplay between the local network of atoms and the observed NMR signals. Specifically, the two characteristic features observed in both 27Al and 71Ga NMR spectra result from both the deviations in the polyhedral coordination/site-symmetry within the 4-fold coordinated Li1/24d sites (rather than the doping of the other Li2/96h or La sites) and with the number of occupied adjacent Li2 sites that share oxygen atoms with these dopant sites. The sharp 27Al and 71Ga resonances arise from dopants located at a highly symmetric tetrahedral 24d site with four corner-sharing LiO4 neighbors, whereas the broader features originate from highly distorted dopant sites with fewer or no immediate LiO4 neighbors. A correlation between the size of the 27Al/71Ga quadrupolar coupling and the distortion of the doping sites (viz. XO4/XO5/XO6 with X = {Al/Ga}) is established. 17O MAS NMR spectra for these systems provide insights into the oxygen connectivity network: 17O signals originating from the dopant-coordinating oxygens are resolved and used for further characterization of the microenvironments at the dopant and other sites.
Highlights
Li-stuffed garnets have received widespread attention as solid electrolytes for next-generation Li-ion batteries and have been studied extensively since the discovery of Li7La3Zr2O12 (LLZO).[1]
In the case of Ga0.3Al0.1LLZO no component A was observed in the 11.8 T field, but considering the low intensity of this component for the same compound in the higher field measurement, it was most likely too low in intensity in this case to be detected above the noise
In order to provide an atomic-level understanding of the source of the 17O, 27Al, and 71Ga NMR peaks, we have performed extensive density-functional theory (DFT)-gauge-including projector augmented wave (GIPAW)
Summary
Li-stuffed garnets have received widespread attention as solid electrolytes for next-generation Li-ion batteries and have been studied extensively since the discovery of Li7La3Zr2O12 (LLZO).[1]. The low-temperature tetragonal phase is fully ordered comprising full occupancy (site-occupancy factor, g = 1) of the three lithium sites, namely, the tetrahedral Li1/T(8a), and the octahedral Li2/O(16f) and Li3/O(32g) sites. Cubic LLZO features two Li site groups: the tetrahedral Li1/24d and the octahedral Li2/[48g/96h] (Figure 1). The Li2 site is a split position with the two 96h sites separated by 0.77 Å, and the 48g site sitting in the center. This site group cannot be fully occupied, resulting in a partial occupancy, contributing to the Li mobility.[3,7,9−11,15]
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