High lithium ion conducting solid electrolytes based on NASICON Li1+xAlxM2−x(PO4)3 materials (M = Ti, Ge and 0 ≤ x ≤ 0.5)

Fast ion conductors Li1+xAlxTi2−x(PO4)3 (LATP) and Li1+xAlxGe2−x(PO4)3 (LAGP) with 0 ≤ x ≤ 0.5 have been successfully prepared by the solid state reaction method and characterized using X-ray diffraction (XRD), nuclear magnetic resonance (NMR) and impedance spectroscopy (IS) techniques. The structural analysis showed that the main crystalline phase detected in the XRD patterns of prepared samples displays the rhombohedral NASICON-type structure (space group R-3c). From the analysis of the 27Al and 31P MAS-NMR spectra, octahedra occupation and cation distribution have been investigated; from 7Li MAS-NMR spectra, structural sites and local mobility of Li have been analyzed. Information about long-range lithium mobility has been deduced from the analysis of IS data recorded in the frequency 20 Hz to 3 GHz and temperature 100–500 K intervals. The use of the derivative log σ vs. log ω function has enabled the detection of two high frequency responses that have been associated, according to the core-shell model, to the heterogeneous distribution of Al at surface and inside LATP and LAGP particles. IS measurements showed a higher bulk conductivity (3.4 × 10−3 and ∼10−4 S cm−1 at RT) and lower activation energy (0.28 and 0.38 eV) in LATP and LAGP samples, respectively.