Significant effects of graphite fragments on hydrogen storage performances of LiBH4: A first-principles approach

The graphite fragments are used to establish the calculated models of LiBH4 mechanically milled by graphite additive. Density-functional theory calculations have been performed to optimize the crystal structures of LiBH4 inserted by different graphite fragments. It is found that the unsaturated sites introduced by graphite fragments are occupied by BH3 complexes and H atoms. The dehydrogenation energies are calculated to investigate the dehydrogenation ability of LiBH4 with graphite fragments. The results show that BH3 complexes bonded with graphite fragments have lower dehydrogenation energy than pure LiBH4, which is an important factor of the improvement of thermodynamic property of LiBH4 system. The hydrogen diffusion behaviors of graphite fragments doped LiBH4 are investigated by the nudged elastic band method. It is found that the hydrogen diffusion barriers of β-H (the H atoms of BH3 complexes bonding with C atoms) diffusing to other sites are small in the LiBH4/graphite fragment systems. The results show that the existence of β-H type significantly enhances the hydrogen diffusion kinetics of the graphite fragments doped LiBH4.