In this study, we assembled a bulk-type all-solid-state battery comprised of a TiS 2 positive electrode, LiBH 4 electrolyte, and Li negative electrode. Our battery retained high capacity over 300 discharge-charge cycles when operated at 393 K and 0.2 C. The 2 nd discharge capacity was as high as 205 mAh g -1 , corresponding to a TiS 2 utilization ratio of 85 %. The 300 th discharge capacity remained as high as 180 mAh g -1 with nearly 100 % coulombic efficiency from the 2 nd cycle. Negligible impact of the exposure of LiBH 4 to atmospheric-pressure oxygen on battery cycle life was also confirmed. To investigate the origin of the cycle durability for this bulk-type all-solid-state TiS 2 /Li battery, electrochemical measurements, thermogravimetry coupled with gas composition analysis, powder X-ray diffraction measurements and first-principles molecular dynamics simulations were carried out. Chemical and/or electrochemical oxidation of LiBH 4 occurred at the TiS 2 surface at the battery operating temperature of 393 K and/or during the initial charge. During this oxidation reaction of LiBH 4 with hydrogen (H 2 ) release just beneath the TiS 2 surface, a third phase, likely including Li 2 B 12 H 12 , precipitated at the interface between LiBH 4 and TiS 2 . Li 2 B 12 H 12 has a lithium ionic conductivity of log(σ / S cm -1 ) = -4.4, charge transfer reactivity with Li electrodes, and superior oxidative stability to LiBH 4 , and thereby can act as a stable interface that enables numerous discharge-charge cycles. Our results strongly suggest that the creation of such a stable interfacial layer is due to the propensity of forming highly stable, hydrogen-deficient polyhydro-closo-polyborates such as Li 2 B 12 H 12 , which are thermodynamically available in the ternary Li-B-H system.

Stable battery operation of a bulk-type all-solid-state lithium-sulfur battery was demonstrated by using a LiBH4 electrolyte. The electrochemical activity of insulating elemental sulfur as the positive electrode was enhanced by the mutual dispersion of elemental sulfur and carbon in the composite powders. Subsequently, a tight interface between the sulfur-carbon composite and the LiBH4 powders was manifested only by cold-pressing owing to the highly deformable nature of the LiBH4 electrolyte. The high reducing ability of LiBH4 allows using the use of a Li negative electrode that enhances the energy density. The results demonstrate the interface modification of insulating sulfur and the architecture of an all-solid-state Li-S battery configuration with high energy density.