Probing Lithium Diffusion Pathways in LiBH4-LiBr by QENS and Impedance Spectroscopy

The key to high-performance electrochemical devices such as batteries, hydrogen and solid-oxide fuel cells, and electrolytic cells is the fast transport of ions through electrolytes. Lithium borohydride (LiBH4) is a potential fast ion conductor, however, its use in solid-state batteries is hampered due to the presence of unfavorable diffusion channels with high activation barriers at RT. The stabilization of highly conductive hexagonal-LiBH4 using halide (e.g., Cl, Br, or I) substitution and nanoconfinement inside porous oxide scaffolds (e.g., silica) have been considered the most effective methods to achieve Li+ conductivity approaching ~1 mS cm-1 at RT. Previous studies on LiBH4-based solid electrolytes considered the poorly conducting orthorhombic-LiBH4 with some structural changes observed in the presence of ammonia, ammonia borane or methylamine, however, the underpinning mechanism is not well understood. For the first time, we have retained the highly conducting hexagonal-LiBH4 in the presence of ammonia. Analyzing motion of BH4‒/NH3 species and the impact of NH3 on the reorientation of BH4‒ in different phases of LiBH4 using simultaneous QENS and impedance measurements will not only help link the molecular diffusion processes with Li+ mobility in our system but also understand hydrogenation-dehydrogenation processes in various hydride materials.