In this work, the influence of sintering additive (borosilicate glass, 65Li(2)O center dot 27B(2)O(3)center dot 8SiO(2)) on the density and total (bulk + grain-boundary) conductivity of the tetragonal Li7La3Zr2O12 (LLZ) has been investigated. Optimal temperature of the sintering was chosen as 1150 degrees C since the composite with 1 wt% of the glass has the highest density and total conductivity after annealing at this temperature. Then, series of composites Li7La3Zr2O12 - x 65Li(2)O center dot 27B(2)O(3)center dot 8SiO(2) (x = 1-10) were prepared. Relative density of the composite samples is higher than one of the sintered LLZ. The phase composition of each sample was characterized using X-ray diffraction; all reflexes are indexed as tetragonal LLZ. The conductivity of the composites was studied by complex impedance spectroscopy in the temperature range of 20-250 degrees C. The 65Li(2)O center dot 27B(2)O(3)center dot 8SiO(2) glass addition of 3 wt% leads to an increase in the total conductivity of the samples by two orders of magnitude up from 2.6.10(-7) to 1.5-10(-5) S cm(-1) at 25 degrees C. (C) 2016 Elsevier B.V. All rights reserved.