Li2O has been evaluated as a sintering aid for Gd0.1Ce0.9O2-delta(GDC). Using 2.5 mol% ratio of Li2O to GDC (5LiGDC), dense samples with relative density of 993% were achieved at sintering temperature as low as 900 degrees C. A high total conductivity of 0.059 S cm(-1) at 600 degrees C was obtained for the 5LiGDC samples sintered at 1000 degrees C (5LiGDC1000), while 5LiGDC samples sintered at 1400 degrees C showed a lower conductivity of 0.017 S cm(-1) at 600 degrees C. It has been found that Li2O has the tendency to accumulate in the grain boundary region to form Li-Gd-Ce-O phases when the 5LiGDC sintering temperature is at 1000 degrees C or below, leading to an increase in the grain boundary conductivity. Increasing the 5LiGDC sintering temperature above 1000 degrees C will accelerate the vaporization of Li2O, association of the oxygen vacancy and formation of additional pores in the bulk, resulting in a decrease of both the grain boundary and grain interior conductivity. Secondary ion mass spectrometry (SIMS) results have confirmed the existence of Li ions for the 5LiGDC samples sintered at or below 1000 degrees C, while most of Li species has vaporized for the 5LiGDC samples sintered above 1000 degrees C. (c) 2014 Elsevier B.V. All rights reserved.