Identifying Li+ ion transport properties of aluminum doped lithium titanium phosphate solid electrolyte at wide temperature range

• Ac impedance spectroscopy of Li1 + xAlxTi2 -x(PO4)3 (x = 0 ~ 0.4) has been investigated at 123 K-333 K.
• Undoped LiTi2(PO4)3 shows two bulk and one grain boundary region in impedance spectrum.
• Relative density of ceramic has a significant impact on the conductivities of both bulk and grain boundary.

A series of Li1 + xAlxTi2 − x(PO4)3 samples (x = 0–0.4) were prepared by solid state reaction and were characterized. Lattice parameters a and c decrease continuously with increase of Al content. The relative density of the Al-doped samples is about 97%, which is much higher than 75% of the undoped LiTi2(PO4)3 sample. Detailed ac impedance measurements from − 150 °C to 60 °C reveal clearly that three electrical response regions with different relaxation time constants coexist in the undoped LiTi2(PO4)3 sample but only two in all Al-doped samples. The bulk ionic conductivities for the Al-doped samples show no significant variation from x = 0.1 to x = 0.4. Their ionic conductivities are slightly higher than those of the undoped sample. However, this increase is not caused by increasing bulk ionic conductivity through introducing more lithium ions via doping, but it is mainly attributed to a densification effect.