Dependence of ionic conductivity on composition of fast ionic conductors Li1+xTi2-xAlx(PO4)(3), 0 <= x <= 0.7. A parallel NMR and electric impedance study

Materials with nominal compositions Li1+xTi2-xAlx(PO4)(3), 0 less than or equal to x less than or equal to 0.7, have been prepared and studied with X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and electrical impedance techniques. Substitution of Ti4+ by Al3+ reduces the unit cell dimensions of the NASICON framework but enhances the grain interior conductivity. The overall conductivity is dominated by the grain boundary composition. Detection of new broad peaks in the P-31 and Al-27 MAS NMR spectra supports the formation of an amorphous Li-conducting phase that accounts for the increase observed in the overall conductivity. In samples with x > 0.5, crystallization of Li4P2O7 as a segregated phase is accompanied by the appearance of a new electrical response. In these samples, a decrease in overall conductivity, associated with a modification of the amorphous phase, is also observed.