Influence of preparation temperature on ionic conductivity of titanium-defective Li1+4xTi2−x(PO4)3 NASICON-type materials

NASICON-type Li1+4xTi2−x(PO4)3 samples (0 ≤ x ≤ 0.2) have been synthesized by conventional solid-state reaction. Structural factors that affect Li conductivity were investigated with powder X-ray diffraction (XRD), scanning electron microscopy, nuclear magnetic resonance (NMR) and impedance spectroscopy techniques. The increment in lithium enhances Li–Li repulsions and increases Li conductivity of x = 0.1 samples with two orders of magnitude with respect to that of the stoichiometric x = 0 sample. In LiTi2(PO4)3 phase, Li ions mainly occupy sixfold M1 sites (CQ ~ 40 kHz), while in Li1+4xTi2−x(PO4)3 samples, Li ions are also allocated near triangular windows that connect M1 and M2 cavities (CQ ~ 60 kHz). T he Li rearrangement increases long-range motions of lithium. XRD and 31P MAS-NMR patterns showed variable amounts of secondary LiTiPO5, TiP2O7 and Li4P2O7 phases besides NASICON compounds. The formation of non-conducting secondary phases at the surface of NASICON particles decreases overall conductivity of x = 0.2 samples. The Li1.4Ti1.9(PO4)3 (x = 0.1 sample) prepared at 800 °C displays at room temperature high "bulk" conductivity, 1.6 × 10−4 S cm−1, low activation energy, 0.30 eV, and good overall DC conductivity, 2.7 × 10–6 S cm−1. The small amount of secondary phases detected in this sample makes it a good candidate for solid electrolyte in all solid-state batteries.