In this work, we report that modification of the chemical composition of grain boundaries of La2/3-xLi3xTiO3 double perovskite, one of the most promising Li-ion conducting solid electrolytes, can be a convenient and versatile way of controlling the space charge potential, leading to a mitigated electrical resistance of the grain boundaries. Two groups of additives are investigated: lithium-enriching agents (Li3BO3, LiF) and 3dmetal ions (Co2+, Cu2+), both expected to reduce the Schottky barrier. It is observed that Li-containing additives work effectively at a higher sintering temperature of 1250 degrees C. Regarding copper, it shows a much stronger positive impact at lower temperature, 1150 degrees C, while the addition of cobalt is always detrimental. Despite overall complex behavior, it is documented that the decreased space charge potential plays a more important role in the improvement of lithium conduction than the thickness of the grain boundaries. Among the proposed additives, modification of La2/3-xLi3xTiO3 by 2 mol.% Cu2+ results in the space charge potential reduction by 32 mV in relation to the reference sample, and the grain boundary specific conductivity increase by 80%, as measured at 30 degrees C. Introduced additive allows to obtain a similar effect on the conductivity as elevating the sintering temperature, which can facilitate manufacturing procedure. [GRAPHICS] .