Combustion Resistant Borohydrides and Their Chemical Interactions with Li-Metal Surfaces: An Experimental and Theoretical Study

Borohydrides are important molecular entities for a myriad of applications from organic synthesis to components of functional materials and devices. All borohydrides have been thought to be susceptible to spontaneous ignition when exposed to a flame. Herein we demonstrate that this is not always true by identifying several borohydride rich materials that are resistant to combustion when contacted with a torch. One of these materials is a Li⁺ salt of a carborane anion that depending on its coordination environment exists as a unique ionic liquid that has a nearly naked Li⁺ countercation. This has provided us with the first opportunity to spectroscopically probe the interactions of such carborane anions with Li metal in a solvent free environment. We found that this carborane anion is immune to deleterious reduction at Li-metal surfaces, as evidenced by XPS, EDS and SEM analysis of the Li-Metal surface after exposure to the ionic liquid. Additionally, NMR analysis of the ionic liquid after stirring it with Li powder shows no reaction. Calculations show that the cage skeleton is reduced at the surface monolayer, but as the reduced form is removed from contact with Li-metal, the cage reverts to the closo-form, demonstrating reversibility.

Among a series of combustion resistant borohydrides, we describe the synthesis of a carboranyl ionic liquid whose chemical compatibility against Li metal is investigated through experiment and theory.