Ionic conduction in ammonia functionalised closo-dodecaborates MB₁₂H₁₁NH₃ (M = Li and Na)

Metal hydroborates and their derivatives have been receiving attention as potential solid-state ion conductors for battery applications owing to their impressive electrochemical and mechanical characteristics. However, to date only a fraction of these compounds has been investigated as solid-state electrolytes. Here, MB12H11NH3 (M = Li and Na) hydroborates are synthesized and investigated as electrolyte materials for all-solid-state batteries. The room temperature alpha-NaB12H11NH3 was structurally solved in P2(1)2(1)2(1) (a = 7.1972(3) & Aring;, b = 9.9225(4) & Aring;, and c = 14.5556(5) & Aring;). It shows a polymorphic structural transition near 140 degrees C to cubic Fm3(-)m. LiB12H11NH3 and NaB12H11NH3 exhibit cationic conductivities of sigma(Li+) = 3.0 x 10(-4) S cm(-1) and sigma(Na+) = 1.2 x 10(-4) S cm(-1) at 200 degrees C. Hydration is found to improve ionic conductivity of the hydroborates. It is presumed that modest ionic conductivities could be due to a lack of significant re-orientational dynamics in the crystal structure resulting from the presence of the bulky -NH3 group in the anion.