The cubic Ca/Ta-substituted Li-6.55(La2.95Ca0.05)(Zr1.5Ta0.5)O-12 (LLCZTO) electrolytes were synthesized at 800 degrees C with Li3BO3 as additives. The optimal amount of Li3BO3 and its influences on the microstructure, crystal structures, Li+ conductivity and the stability of the Li-6.55(La2.95Ca0.05)(Zr1.5Ta0.5)O-12 were studied by SEM, XRD and EIS. Among all the samples, when the molar ratio of Li3BO3 to the Li-6.55(La2.95Ca0.05)(Zr1.5Ta0.5)O-12 is 4:5, the highest Li+ conductivity of 1.33 x 10(-4) S cm(-1) at 30 degrees C is obtained. When the LLCZTO samples are exposed in air, the Li+ conductivity is deteriorated possibly owing to the side reactions between the LLCZTO and the H2O or CO2 in the air. The Li3BO3 addition can alleviate such deterioration of the Li+ conductivity. (C) 2017 Elsevier B.V. All rights reserved.