The development of electrochemically stable Li-ion solid electrolytes with high conductivity is crucial for a successful performance of all-solid-state batteries. Li-containing argyrodite sulfides with the formula Li6PS5Z (Z=Cl, Br, I) are considered as promising solid electrolytes due to their high conductivity in the order of 10−4 to 10−3 S cm−1 and their wide stable electrochemical potential (up to 5 V vs Li/Li+). In this work, argyrodites were mixed with LiBH4 at different ratios to promote the partial substitution of halide Z− ions by [BH4] - units. The powder characteristics and the ionic conductivity were systematically studied as function of mechanical and thermal steps applied during the synthesis of the prepared samples. The compositions Li6PS5Cl0.83(BH4)0.17 and Li6PS5I0.83(BH4)0.17 revealed a significant enhancement on the conductivity (4.0·10−4 and 7.6·10−4 S cm−1, respectively) comparing to the pristine argyrodites. In addition, the wide electrochemical stability window from 0 to 5 V makes these compounds attractive for their future integration in high-power all-solid-state batteries.