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

Li1.2 + 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 57Fe Mössbauer spectroscopy. The rhombohedral (R 3 ¯ c) 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 57Fe Mössbauer 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 Li1.2 + 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.