Cation Miscibility and Lithium Mobility in NASICON LiTiSc(PO) (0 ≤ x ≤ 0.5) Series: A Combined NMR and Impedance Study.

Rhombohedral NASICON compounds with general formula LiTiSc(PO) (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 Ti by Sc and Li in pristine LiTi(PO) increases unit-cell dimensions and the number of charge carriers. In Sc-rich samples, the analysis of XRD data and Li/Li, P, and Sc MAS NMR spectra confirms the presence of secondary LiScO and LiScPO 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 S·cm) and low activation energy (E = 0.25 eV) measured at room temperature make LiTiSc(PO) one of the best lithium ionic conductors reported in the literature. In this material, the vacancy arrangement enhances Li conductivity.