Structural Complexity and Electrical Properties of the Garnet-Type Structure LaLi0.5Fe0.2O2.09 Studied by Li-7 and La-139 Solid State NMR Spectroscopy and Impedance Spectroscopy

Garnet-like structures containing lithium are of interest for applications in lithium ion batteries because of their inherent lithium ion conductivity and stability against chemical reaction with Li. Here, a series of materials, with parent composition LaLi0.5Fe0.2O2.09, are synthesized using solid-state chemistry, and characterized, in terms of their structure, using a combination of powder X-ray diffraction (PXRD), Li-7, and La-139 solid-state NMR, which reveal disorder on the Li and Fe sites in the lattice. The Li-7 spectra comprise a set of peaks that are distinguished based on their T-1 relaxation properties, as a diamagnetic set and a paramagnetic set of peaks. The La-139 spectra include two La environments, one well-defined, with a C-Q of 56 MHz +/- 1 MHz and asymmetry parameter, eta of 0.05 +/- 0.05, and a second, which experiences a range of local environments, because of the Li/Fe substitution, and has a C-Q of 29 MHz +/- 2 MHz, and eta of 0.6 +/- 0.1. The dynamics within the materials were characterized using impedance spectroscopy, and trends were correlated with the lithium content and structural features. The best conductivity was determined for the parent material, LaLi0.5Fe0.2O2.09, after sintering at 850 degrees C. The complex Li-7 and La-139 NMR spectra, interpreted together with (PXRD) data, indicate that the increasing concentration of lithium in the material populates an iron site with excess lithium, in a range of possible local environments, which appears to decrease the total ionic and electronic conductivity.