Nanoconfined LiBH4 and Enhanced Mobility of Li+ and BH4- Studied by Solid-State NMR

The structural and dynamical properties of LiBH4 confined in porous carbon and ordered porous silica are studied using H-1, Li-7, and B-11 solid-state NMR. The B-11 and Li-7 NMR resonances of LiBH4 confined in porous carbon (broad pore size distribution up to <60 nm) are strongly broadened compared to bulk LiBH4. This line broadening is dominated by anisotropic susceptibility effects induced by the nanostructured carbon host. Because of the lack of resolution caused by the anisotropic susceptibility broadening, we studied confined LiBH4 in ordered porous silica (MCM-41 pore size: 1.9 nm). In the Li-7 and B-11 spectra, a bulk-like LiBH4 resonance is observed together with an additional, more narrow component. Above T = 313 K, this component showed a typical J-coupling pattern in both B-11 and H-1 spectra corresponding to highly mobile BH4- species. Static B-11 solid-state NMR measurements compared with second moment calculations show that these BH4- species not only rotate as in the bulk material but also experience translations through the crystal lattice. Static Li-7 measurements show that Li+ is also highly mobile. Therefore, we conclude that nanoconfinement of LiBH4 strongly enhances diffusional mobility of borohydride anions and lithium in this material.