Sites and Mobility of Lithium along the Li1+xTi2-xInx(PO4)3 (0 ≤ x ≤ 2) Series Deduced by XRD, NMR, and Impedance Spectroscopy.

The structure and Li conductivity has been investigated in the Li1+xTi2-xInx(PO4)3 (0 ≤ x ≤ 2) series prepared by the ceramic route at 900 °C. The XRD patterns of 0 ≤ x ≤ 0.2 samples show the presence of rhombohedral (S.G. R3̅c); those of 0.2 ≤ x ≤ 1 samples display both rhombohedral and orthorhombic (S.G. Pbca), and 1 ≤ x ≤ 2 samples exhibit only monoclinic (S.G. P21/n) phases. At intermediate compositions, the secondary LiTiPO5 phase was detected. The Rietveld analysis of XRD patterns was used to deduce unit-cell parameters, chemical composition, and percentage of phases. The amount of In3+, deduced from structural refinements of three phases, was confirmed by 31P MAS NMR spectroscopy. The Li mobility was investigated by 7Li MAS NMR and impedance spectroscopies. The Li conductivity increased with the Li content in rhombohedral but decreased in orthorhombic, increasing again in monoclinic samples. The maximum conductivity was obtained in the rhombohedral x = 0.2 sample (σb = 1.9 × 10-3 S·cm-1), with an activation energy Eb = 0.27 eV. In this composition, the overall Li conductivity was σov = 1.7 × 10-4 S·cm-1 and Eov = 0.32 eV, making this composition a potential solid electrolyte for all-solid-state batteries. Another maximum conductivity was detected in the monoclinic x ∼ 1.25 sample (σov = 1.4 × 10-5 S·cm-1), with an activation energy Eov = 0.39 eV. Structural models deduced with the Rietveld technique were used to analyze the conduction channels and justify the transport properties of different Li1+xTi2-x Inx(PO4)3 phases.

Cation Miscibility and Lithium Mobility in NASICON Li1+xTi2-xScx(PO4)3 (0 ≤ x ≤ 0.5) Series: A Combined NMR and Impedance Study.

Rhombohedral NASICON compounds with general formula Li1+xTi2-xScx(PO4)3 (0 ≤ x ≤ 0.5) have been prepared using a conventional solid-state reaction and characterized by X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and impedance spectroscopy. The partial substitution of Ti4+ by Sc3+ and Li+ in pristine LiTi2(PO4)3 increases unit-cell dimensions and the number of charge carriers. In Sc-rich samples, the analysis of XRD data and 6Li/7Li, 31P, and 45Sc MAS NMR spectra confirms the presence of secondary LiScO2 and LiScP2O7 phases that reduce the amount of lithium incorporated in the NASICON phase. In samples with x < 0.3, electrostatic repulsions between Li ions located at M1 and M3 sites increase Li mobility. For x ≥ 0.3, ionic conductivity decreases because of secondary nonconducting phases formed at grain boundaries of the NASICON particles (core-shell structures). For x = 0.2, high bulk conductivity (2.5 × 10-3 S·cm-1) and low activation energy (Ea = 0.25 eV) measured at room temperature make Li1.2Ti1.8Sc0.2(PO4)3 one of the best lithium ionic conductors reported in the literature. In this material, the vacancy arrangement enhances Li conductivity.