On the influence of the cation vacancy on lithium conductivity of Li1 + xRxTi2 − x(PO4)3 Nasicon type materials

Structural features responsible for outstanding Li conductivity of Li1 + xRxTi2 − x(PO4)3 (LRTP) Nasicon samples (0 ≤ x ≤ 0.6 and R = Al, Sc, In) prepared by the ceramic route have been analyzed by XRD, ND, MAS-NMR and impedance spectroscopy. The structural analysis showed that all samples display the rhombohedral (S.G. R-3c) symmetry. The structural site occupancy has been investigated by 7Li, 27Al/45Sc and 31P MAS-NMR spectroscopy. The Fourier map differences deduced from high-resolution ND patterns of LAlTP samples revealed that Li ions occupy Li1 sites and to a lower extent Li3/Li3′ sites inside Li2 cavities. The location of Li at 3 sites minimizes electrostatic Li (Li1-Li3) repulsions, enhancing local mobility in LRTP samples. A maximum of conductivity was detected for 0.2 ≤ x ≤ 0.4, when a significant amount of vacant Li1 sites was created at the intersection of conduction pathways. The increment of vacant Li1 sites explains the existence of two Li motion regimes detected by 7Li NMR and impedance spectroscopy. In the low temperature regime, activation energy and migration entropy of Li have been related by the Meyer-Neldel relationship. In the high-temperature regime, further investigation is required to assess the role of vacancy in lithium conductivity.