NMR and electric impedance study of lithium mobility in fast ion conductors LiTi 2 − xZr x(PO 4) 3 (0 ≤ x ≤ 2)

Abstract

Materials of the LiTi 2 − x Zr x (PO 4) 3 series (0 ≤ x ≤ 2) were prepared and characterized by powder X-ray (XRD) and neutron diffraction (ND), 7Li and 31P Nuclear Magnetic Resonance (NMR) and Electric Impedance techniques. In samples with x < 1.8, XRD patterns were indexed with the rhombohedral R3̅ c space group, but in samples with x ≥ 1.8, XRD patterns display the presence of rhombohedral and triclinic phases. The Rietveld analysis of the LiTi 1.4Zr 0.6(PO 4) 3 neutron diffraction (ND) pattern provided structural information about intermediate compositions. For low Zr contents, compositions deduced from 31P MAS-NMR spectra are similar to nominal ones, indicating that Zr 4+ and Ti 4+ cations are randomly distributed in the NASICON structure. At increasing Zr contents, differences between nominal and deduced compositions become significant, indicating some Zr segregation in the triclinic phase. The substitution of Ti 4+ by Zr 4+ stabilizes the rhombohedral phase; however, electrical performances are not improved in expanded networks of Zr-rich samples. Below 300 K, activation energy of all samples is near 0.36 eV; however, above 300 K, activation energy is near 0.23 eV in Ti-rich samples and close to 0.36 eV in Zr-rich samples. The analysis of electrical data suggests that the amount of charge carriers and entropic terms are higher in Zr-rich samples; however, the increment of both parameters does not compensate lower activation energy terms of these samples. As a consequence of different contributions, the bulk conductivity of Zr-rich samples, measured at room temperature, is one order of magnitude lower than that measured in Ti-rich samples.