The lithium garnet ceramic has been considered as one of promising solid-state electrolytes for secondary lithium batteries. However, improvement of its ionic conductivity is hindered by the high porosity related to the severe volatilization of lithium components during sintering. In order to find the solution for this problem, Li2O additives in concentration of 2-8 wt.% were introduced into the cubic Li6.75La3Zr1.75Ta0.25O12 (LLZTO) at the solid-state reaction processes. It is found that the Li2O additives lead to formation of glassy-like phases at the grain boundaries related to the liquid-phase sintering behavior, which eliminates the residual pores therein and increases the relative density from 91.5% to 97.3%. Measurement of conductivity indicates that 6 wt.% is the optimum concentration of Li2O which leads to an ionic conductivity of 6.4 x 10(-4) s cm(-1) at room temperature. This value is approximately 3 times larger than that of the ceramic without the additive. (C) 2013 Elsevier B.V. All rights reserved.