[en] The ionic conduction and electrochemical and thermal stabilities of the LiBH₄–LiCl solid-state electrolyte were investigated for use in bulk-type all-solid-state lithium-sulfur batteries. The LiBH₄–LiCl solid-state electrolyte exhibiting a lithium ionic conductivity of $\mathrm{l}\mathrm{o}\mathrm{g}\left(\mathrm{\sigma <!--\; ??\; -->}/\mathrm{S}\mathrm{c}{\mathrm{m}}^{-<!--\; ???\; -->1}\right)=-<!--\; ???\; -->3.3$ at 373 K, forms a reversible interface with a lithium metal electrode and has a wide electrochemical potential window up to 5 V. By means of the high-energy mechanical ball-milling technique, we prepared a composite powder consisting of elemental sulfur and mixed conductive additive, i.e., Ketjen black and Maxsorb. In that composite powder, homogeneous dispersion of the materials is achieved on a nanometer scale, and thereby a high concentration of the interface among them is induced. Such nanometer-scale dispersals of both elemental sulfur and carbon materials play an important role in enhancing the electrochemical reaction of elemental sulfur. The highly deformable LiBH₄–LiCl electrolyte assists in the formation of a high concentration of tight interfaces with the sulfur-carbon composite powder. The LiBH₄–LiCl electrolyte also allows the formation of the interface between the positive electrode and the electrolyte layers, and thus the Li-ion transport paths are established at that interface. As a result, our battery exhibits high discharge capacities of 1377, 856, and 636 mAh g^–1 for the 1st, 2nd, and 5th discharges, respectively, at 373 K. These results imply that complex hydride-based solid-state electrolytes that contain Cl-ions in the crystal would be integrated into rechargeable batteries. (paper)