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

Abstract Li 1.2 + x Ti 1.9 − x Fe x Ca 0.1 (PO 4 ) 3 (x = 0.1–0.4) compounds were synthesized by a sol–gel method and studied by X-ray diffraction, SEM and 57 Fe Mossbauer 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 Ti 4 + by Fe 3 + and Ca 2 + led to an increase of the unit cell parameters of the rhombohedral phase. The hyperfine 57 Fe Mossbauer parameters showed a single signal ascribable to Fe 3 + in an anisotropic distribution of charges surrounding the probe atom. The double substitution of Ti 4 + by Fe 3 + and Ca 2 + in Li 1.2 + x Ti 1.9 − x Fe x Ca 0.1 (PO 4 ) 3 resulted in a net improvement of the ionic conductivity, as compared to pristine LiTi 2 (PO 4 ) 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 Li 1.3 Ti 1.8 Fe 0.1 Ca 0.1 (PO 4 ) 3 and Li 1.6 Ti 1.5 Fe 0.4 Ca 0.1 (PO 4 ) 3 , respectively.