XPS, NMR and electrical studies of Li-ion conducting yttrium doped NASICON ceramic compounds

Abstract

Lithium ion conducting NASICON material of the form Li1.3Al0.3-xYxTi1.7(PO4)3 (where x = 0, 0.01 and 0.07) considered as promising electrolyte materials for solid state lithium ion batteries [1, 2] were synthesized via solid state reaction method. The compound was found to have a rhombohedral symmetry with space group R-3c from the x-ray diffraction studies. Further, a partial replacement of Y3+ in the lattice increased the lattice parameters. The room temperature Li+ ion conductivity of the yttrium doped samples deduced from the impedance spectroscopy, in the frequency range 20 MHz to 1 Hz is found to be ∼10-5 S/cm. The mode of ionic relaxation in grain boundary region was found to be different in yttrium doped samples compared to nominal composition. Core level x-ray photoelectron spectroscopy (XPS) was performed for Al and P. While Al was found to be in single coordination state, P was found to possess two coordination environment – both of which were in tetrahedral configuration. 27Al MAS NMR studies suggested distortion at tetrahedral and octahedral sites due to partial replacement of Al3+ and Ti4+ cations by Y3+. It was found that yttrium enhanced the overall density of the ceramic material – which however did not help increase the bulk Li+ ion conductivity due to blocking effect by aluminum and yttrium phases at grain boundary.