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 LiLi 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 (C-Q similar to 40 kHz), while in Li(1+4)xTi(2-x)(PO4)(3) samples, Li ions are also allocated near triangular windows that connect M1 and M2 cavities (C-Q similar to 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 degrees C displays at room temperature high "bulk" conductivity, 1.6x10(-4) S cm(-1), low activation energy, 0.30 eV, and good overall DC conductivity, 2.7x10(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.