Li conductivity in Li1+xTi2-xAlx(PO4)3 (0.3≤ x≤ 0.7) ceramics prepared from sol-gel precursors

LiTiAl (PO) (LTAP) x = 0.3, 0.4, 0.5 and 0.7) compounds, prepared at 800 °C by sol-gel, have been studied by X-ray diffraction, Nuclear Magnetic Resonance, Scanning Electron Microscopy and Impedance Spectroscopy. The increment of lithium enhances Li–Li repulsions, favoring the creation of vacancies at the conduction paths intersection (M1 sites), increasing Li mobility along (… M1-M2-M1 …) channels in NASICON phases. The partial elimination of segregated AlPO, LiAlPO and LiTiPO phases increased Ti, Al solubility in NASICON, improving overall conductivity in ceramics prepared at 950 °C. The maximum of “bulk” conductivity was produced in x = 0.4 sample, as a consequence of the creation of M1 vacancies and allocation of Li in M2 cavities with optimal Li/vacancy distribution. The presence of secondary phases increased grain boundary impedance, shifting maximum of overall conductivity to x = 0.3 compound. Obtained results are compared with those reported previously in LTPA samples prepared by the ceramic route.