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.