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

NASICON-type Li 1+x Al x Ge 2-x (PO 4 ) 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 Li 1.4 Al 0.4 Ge 1.6 (PO 4 ) 3 sintered at 900°C for 11 h in air. The total conductivity was 1.22 × 10 −3 S cm −1 at 25°C, and the bulk and grain boundary conductivities were estimated by impedance profile analysis to be 1.70 × 10 −3 and 4.30 × 10 −3 S cm −1 , respectively. Sintered pellets of Li 1.4 Al 0.4 Ge 1.6 (PO 4 ) 3 were immersed in distilled water, saturated LiCl aqueous solution, and a saturated LiCl and LiOH aqueous solution at 50°C for one week; X-ray diffraction patterns of these samples dried at 220°C under vacuum showed no significant change from that of the pristine sample. The electrical conductivity of Li 1.4 Al 0.4 Ge 1.6 (PO 4 ) 3 was decreased to 1.4 × 10 −4 S cm −1 at 25°C by immersion in distilled water, while immersion in the saturated LiCl aqueous solution increased the conductivity to 4.95 × 10 −3 S cm −1 at 25°C. Li 1.4 Al 0.4 Ge 1.6 (PO 4 ) 3 was stable in the saturated LiCl and LiOH aqueous solution. Li 1.4 Al 0.4 Ge 1.6 (PO 4 ) 3 was unstable in contact with Li metal and Li-In alloy, but was stable in contact with Li 7 Ti 5 O 12 .