The garnet-type Nb-doped cubic lithium ion-conducting solid Li6.75-xLa3Zr1.75Nb0.25O12-0.5x (LLZ-Nb) was prepared with 0.46 wt% Al2O3 using a sol-gel precursor. LLZ-Nb sintered at 1,150 degrees C for 36 h exhibited a bulk conductivity of 6.13 x 10(-4) S cm(-1) and a grain boundary conductivity of 7.76 x 10(-3) S cm(-1) at 25 degrees C. The cell resistance of Li/LLZ-Nb/Li was gradually increased with storage time over 5 months. The increase in cell resistance was attributed to both the grain boundary and interfacial-resistance between LLZ-Nb and the lithium electrode. The Li/LLZ-Nb/Li cell showed a short-circuit for 280 s polarization at 0.5 mA cm(-2) and no short-circuit for 41 h polarization at 0.1 mA cm(-2). LLZ-Nb was stable in saturated LiOH and LiCl aqueous solution. (C) 2013 The Electrochemical Society. All rights reserved.