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 Li 1+ x Ti 2– x Sc x (PO 4 ) 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 Ti 4+ by Sc 3+ and Li + in pristine LiTi 2 (PO 4 ) 3 increases unit-cell dimensions and the number of charge carriers. In Sc-rich samples, the analysis of XRD data and 6 Li/ 7 Li, 31 P, and 45 Sc MAS NMR spectra confirms the presence of secondary LiScO 2 and LiScP 2 O 7 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 ( E a = 0.25 eV) measured at room temperature make Li 1.2 Ti 1.8 Sc 0.2 (PO 4 ) 3 one of the best lithium ionic conductors reported in the literature. In this material, the vacancy arrangement enhances Li conductivity.