Solid state electrolytic properties of mixtures of Li(BH₄)_0.75I_0.25 and amorphous 0.75Li₂S·0.25P₂S₅ are reported in this study. The enhanced Li-ion conductivities for the Li(BH₄)_0.75I_0.25 phase after addition of 0.75Li₂S·0.25P₂S₅ were found to be within the range for practical solid-state electrochemical storage near room temperature. The highest ionic conductivities are found for the borohydride/sulfide system at 1:2 wt ratio with ∼10⁻³ S cm^-1 at room temperature and an activation energy of 0.30(2) eV. Combined experimental analyses with powder X-ray diffraction and infrared spectroscopy suggest that the synthesized material still include both [PS₄³⁻] and [BH₄⁻] entities. Density functional theory (DFT) calculations provide more insights about the origin of the observed ionic behavior according to the induced structural modifications in the BH₄–BH₄ interactions. Finally, the electrolyte functionality and its compatibility toward an active material are successfully demonstrated by means of cyclic voltammetry (Au vs. Li⁺/Li) in a large voltage window (up to 5 V) and battery cycling tests with TiS₂ electrode, respectively.