Inorganic hydrides for energy storage as the hydrogen or as battery materialsAuthor(s): Torben Rene Jensen
A wide variety of complex metal borohydrides, closo-borates and carborates have been discovered and characterized during the past decade, revealing an extremely rich chemistry including fascinating structural flexibility and a wide range of compositions and physical properties. New classes of boron based electrolytes for batteries have recently been discovered along with high hydrogen density materials relevant for hydrogen storage. Here we present new design criteria of functional electrolytes developed by analysis of the underlying phenomena responsible for the high ion conductivity. This is both useful for mono- and divalent cations including Mg2+. Structural dynamics in the solid state, i.e. entropy effects, are of extreme importance for ionic conductivity. Disorder of high temperature polymorphs often lead to increased dynamics. The structure of high temperature polymorphs can be stabilized to lower temperatures by anion substitution. Disorder in the solid state can also be created by stabilization of eutectic molten states by nano-structuring. Neutral molecules and a 3D network of dihydrogen bonds may increase the coordination flexibility and thereby increase cation mobility. We conclude that the chemistry of boronhydrides is very divers, towards rational design of multi-functional materials, including new ion-conductors for batteries and hydrogen storage materials.