A study of mixed-anion hydride lithium-ion conductors for application in all-solid-state batteries

Solid state lithium ion conductors are important in the development of electrolytes for all-solid-state lithium ion batteries. Complex hydrides are one such area of interest due to their thermal and chemical stability as well as their light weight and in this thesis the Li-BH_4-NH-NH_2 phase space was investigated. The structures of two lithium borohydride-imides: one orthorhombic with the formula Li_5(BH_4)_3NH, and another cubic with the formula Li_3BH_4NH have been determined through powder diffraction methods. The reaction of lithium imide and lithium borohydride was studied through both in-situ and ex-situ powder diffraction methods and highlighted the formation of a melt upon heating. The studies also showed Li_5(BH_4)_3NH formed rapidly upon heating whilst Li_3BH_4NH did not form until late in the cooling process. Further investigation into the reaction of lithium amide-imide and lithium borohydride also identified another phase, a new hexagonal lithium amide-borohydride-imide, Li_4BH_4NHNH_2. It was subsequently revealed that Li_4BH_4NHBH_2 was apparently the most thermodynamically stable of the three structures, forming upon slow cooling of the Li_2NH + LiBH_4 reaction mixture. Investigations into the lithium ion conductivities of the materials produced promising results, with high conductivities at room temperature. All three materials displayed higher conductivities than lithium borohydride and their mechanical properties made them much more robust when manufacturing half cells as compared to lithium imide