Preparation and Electrochemical Properties of Li1+xAlxGe2-x(PO4)3 Synthesized by a Sol-Gel Method

NASICON-type Li1+xAlxGe2-x(PO4)3 solid state lithium ionic conductors were synthesized by a sol-gel method using citric acid and ethylene glycol. The obtained precursors were sintered at various temperatures and the NASICON-type single phase was observed in a range of x = 0-0.6. The highest electrical conductivity was obtained for Li1.4Al0.4Ge1.6(PO4)3 sintered at 900degC for 11 h in air. The total conductivity was 1.22x10[?]3 S cm[?]1 at 25degC, and the bulk and grain boundary conductivities were estimated by impedance profile analysis to be 1.70x10[?]3 and 4.30x10[?]3 S cm[?]1, respectively. Sintered pellets of Li1.4Al0.4Ge1.6(PO4)3 were immersed in distilled water, saturated LiCl aqueous solution, and a saturated LiCl and LiOH aqueous solution at 50degC for one week; X-ray diffraction patterns of these samples dried at 220degC under vacuum showed no significant change from that of the pristine sample. The electrical conductivity of Li1.4Al0.4Ge1.6(PO4)3 was decreased to 1.4x10[?]4 S cm[?]1 at 25degC by immersion in distilled water, while immersion in the saturated LiCl aqueous solution increased the conductivity to 4.95x10[?]3 S cm[?]1 at 25degC. Li1.4Al0.4Ge1.6(PO4)3 was stable in the saturated LiCl and LiOH aqueous solution. Li1.4Al0.4Ge1.6(PO4)3 was unstable in contact with Li metal and Li-In alloy, but was stable in contact with Li7Ti5O12.