Atomic Motions in LiBH₄ by NMR

Abstract

1 H, 7 Li, and 11 B NMR measurements were used to understand atomic translational motions in both the lowand high-temperature phases (LT, HT) of LiBH 4 . In the HT phase 7 Li spectra, spin-echo T 2 , and T 1 all indicate very rapid lithium ion diffusion. Just above the phase transition, the hydrogen resonance is broad, about 22 kHz fwhm (full width at half of maximum), showing that H translations remain slow. From 120 to 170°C, a rapidly decreasing T 1D (relaxation time of dipolar spin-order) shows that the hydrogens diffuse increasingly rapidly. This motion eventually results in marked hydrogen line-narrowing centered near 190°C; the hydrogen diffusion is likely relevant to the kinetics of dehydriding. The extent of 11 B line-narrowing demonstrates that the boron atoms also diffuse rapidly at temperatures above 200°C. In the LT phase, the hydrogen T 1D decreases rapidly with increasing temperature, here due to 7 Li diffusion which is too slow for line-narrowing.

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

“…The 11 B and 7 Li NMR resonances of LiBH 4 confined in porous carbon (broad pore size distribution up to <60 nm) are strongly broadened compared to bulk LiBH 4 . This line broadening is dominated by anisotropic susceptibility effects induced by the nanostructured carbon host.…”

“…Above T = 313 K, this component showed a typical J-coupling pattern in both 11 B and 1 H spectra corresponding to highly mobile BH 4 − species. Static 11 B solid-state NMR measurements compared with second moment calculations show that these BH 4 − species not only rotate as in the bulk material but also experience translations through the crystal lattice. Static 7 Li measurements show that Li + is also highly mobile.…”

“…1 The complex metal hydride lithium borohydride, LiBH 4 , is an interesting material because of its relatively high gravimetric hydrogen content of 18.5 wt % and volumetric hydrogen content of 121 kg H 2 /m 3 . 2 LiBH 4 forms an ionic crystal which contains Li + cations and BH 4 − anions. Motion is present throughout the lithium borohydride crystal.…”

“…15−21 The kinetics of the reactions of the desorption and absorption of hydrogen are significantly improved, and the thermodynamics properties have been shown to be modified for NaAlH 4 . 22,23 For LiBH 4 , Gross et al 24 reported for the first time enhanced kinetics after nanoconfinement by melt infiltration in a porous carbon aerogel.…”

“…5,6,25−32 Hwang et al 11 confirmed the formation of B 12 H 12 2− complexes in the decomposition of metal borohydrides by NMR. Conradi et al, 4,6,32,33 Skripov et al, 5,7 and Jimura et al 34 studied the structure and atomic mobilities of LiBH 4 .…”