Improved lithium-ion transport in NASICON-type lithium titanium phosphate by calcium and iron doping

Li12 + xTi1.9 - xFexCa0.1 (PO4)(3) (x = 0.1-0.4) compounds were synthesized by a sol-gel method and studied by X-ray diffraction, SEM and Fe-57 Mossbauer spectroscopy. The rhombohedral (R (3) over barc) phase prevailed for all studied samples, while for x >= 0.3 an orthorhombic Pbca phase appeared. The substitution of Ti4+ by Fe3+ and Ca2+ led to an increase of the unit cell parameters of the rhombohedral phase. The hyperfine Fe-57 MOssbauer parameters showed a single signal ascribable to Fe3+ in an anisotropic distribution of charges surrounding the probe atom. The double substitution of Ti4+ by Fe3+ and Ca2+ in Li12 + xTi1.9 - xFexCa0.1 (PO4)(3) resulted in a net improvement of the ionic conductivity, as compared to pristine LiTi2(PO4)(3). Ionic conductivities of 3.3 . 10(-4) S cm(-1) and 2.4 . 10(-5) S cm(-1) at room temperature, and 5.23 . 10(-4) S cm(-1) and 8.44 . 10(-5) S cm(-1) at 373 K were observed for Li1.3Ti1.8Fe0.1Ca0.1(PO4)(3) and Li1.6Ti1.5Fe0.4Ca0.1(PO4)(3), respectively. (C) 2013 Elsevier B.V. All rights reserved.