Dehydrogenation of LiBH$_4$ nanoclusters: A first-principles study

Recent experimental studies\footnote{A.F. Gross et al. J. Phys. Chem. C 112, 5651, 2008}$^,$\footnote{X. Liu et al. Chem. Mater. 23, 1331, 2011} show faster desorption kinetics, improved reversibility and more favorable thermodynamics for confined LiBH$_4$ nanoparticles than the bulk. Using density functional theory calculations, we first discuss the geometries and energetics of LiBH$_4$, LiH, LiB, Li and B clusters. Secondly, we study the effects of particle size on the decomposition pathway of LiBH$_4$ clusters. Our calculations show that only very small clusters of LiBH$_4$ (up to 12 formula units) are significantly destabilized relative to the bulk. High stability of small clusters of LiBH$_4$ originates from the fact that surface energies are very low for bulk LiBH$_4$. (100), (010), (101) and (011) surfaces are almost degenerate with surface energies of 0.113, 0.102, 0.115 and 0.097~J/m$^2$, respectively. Clusters of LiH, LiB, Li and B are more strongly destabilized than the LiBH$_4$ clusters upon decreasing the cluster size. We show that, in contrast to the bulk, destabilized clusters of LiBH$_4$ decompose to (LiB)$_n$ clusters. Finally, we present some of our preliminary NMR chemical shift results for different LiBH$_4$ surface terminations.