Water-stable lithium ion conducting solid electrolyte of the Li1.4Al0.4Ti1.6 − xGex(PO4)3 system (x = 0–1.0) with NASICON-type structure

•Electrical conductivity of Li1.4Al0.4Ti1.6 − xGex(PO4)3 as function of x was examined.•The highest conductivity of 1.29 × 10− 3 S cm− 1 at 25 °C was observed for x = 0.2.•Li1.4Al0.4Ti1.4Ge0.2(PO4)3 is stable in the saturated LiOH and LiCl solution.

Solid ionic conductors composed of Li1.4Al0.4Ti1.6 − xGex(PO4)3 (x = 0–1.0) with the NASICON-type structure were synthesized with a precursor prepared using the sol–gel method. The electrical conductivity was examined as a function of x in Li1.4Al0.4Ti1.6 − xGex(PO4)3 at various sintering temperatures and for various sintering periods. The highest electrical conductivity was obtained for Li1.4Al0.4Ti1.4Ge0.2(PO4)3 sintered at 900 °C for 11 h in air. The total and bulk conductivities of the sintered pellet were 1.29 × 10− 3 and 2.35 × 10− 3 S cm− 1 at 25 °C, respectively. The grain boundary resistance of Li1.4Al0.4Ti1.4Ge0.2(PO4)3 was significantly increased to 552 Ω cm2 from 12.2 Ω cm2 by immersion in distilled water at 50 °C for one week, whereas the bulk resistance was not increased. However, no significant increase of the bulk and grain boundary resistance was observed after immersion in a saturated aqueous solution of LiOH and LiCl.