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 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 andLi/Li,P, andSc 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.R.K. would like to thank the Tunisian Ministry of Higher Education and Scientific Research for financial support. This work was funded by Spanish MINECO MAT2013-46452-C4-2R and MATERYENER3-CAM (S2013/MIT-2753) projects and the European NANOLICOM (FP7-PEOPLE-2009-IRSES) project