Stable Interface Formation between TiS₂ and LiBH₄ in Bulk-Type All-Solid-State Lithium Batteries

In this study, we assembled a bulk-type all-solid-state battery comprised of a TiS₂ positive electrode, LiBH₄ 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 second discharge capacity was as high as 205 mAh g^-1, corresponding to a TiS₂ utilization ratio of 85%. The 300th discharge capacity remained as high as 180 mAh g^-1 with nearly 100% Coulombic efficiency from the second cycle. Negligible impact of the exposure of LiBH₄ 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 TiS₂/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 LiBH₄ occurred at the TiS₂ surface at the battery operating temperature of 393 K and/or during the initial charge. During this oxidation reaction of LiBH₄ with hydrogen (H₂) release just beneath the TiS₂ surface, a third phase, likely including Li₂B₁₂H₁₂, precipitated at the interface between LiBH₄ and TiS₂. Li₂B₁₂H₁₂ has a lithium ionic conductivity of log(σ / S cm^-1) = -4.4, charge transfer reactivity with Li electrodes, and superior oxidative stability to LiBH₄, 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 Li₂B₁₂H₁₂, which are thermodynamically available in the ternary Li-B-H system. (Figure Presented).