First-Principles Study Of The Effect Of Dopants On The Dehydrogenation Characteristics Of LiAlH₄ And LiBH₄

As it is lightweight, safe, highly abundant, efficient in absorption and desorption, the studies of complex metal hydride on hydrogen storage, carrier and transform are very important.Because the complex metal hydride as a hydrogen carrier has a promising future and most studies have focused on experiment presently, therefore, we concentrate on theoretical studies, especially, the effects of dopants on the dehydrogenation characteristics of LiAlH₄ and LiBH₄ in this thesis.In chapter 3, based on the plane-wave pseudo-potential method of First-Principles, the effect and mechanism of dopant Fe on the dehydrogenation characteristics of LiAlH₄ were studied. The dopant formation energy, electronic density of states and dissociation energy of H, were investigated. The bonding state and the structure-stability of the crystal were analyzed. The results indicate that when Fe occupied the interstitial site, the Al site or the Li site, the dehydrogenation properties of LiAlH₄ were all improved. Fe tended to occupy the interstitial site. Analysis of the electronic structure reveals that there is strong interaction between the dopant Fe and its nearest neighbouring Al atoms when Fe occupied the interstitial site. Meanwhile, interaction between the dopant Fe and its nearest neighbouring H atoms is also considerably strong. The result is the stability of the [AlH₄] ligand was distorted, therefore, the dehydrogenation properties of LiAlH₄ were improved. In general, dopant Fe improve the dehydrogenation properties of LiAlH₄, which is consistent with the experimental findings.In chapter 4, the effect and mechanism of dopant Ti on the dehydrogenation properties of LiBH₄ were studied. The dopant formation energy, electronic density of states and overlap population, were investigated. The bonding state and the structure-stability of the crystal were analyzed. Results shows that Ti tended to occupy the Li site in LiBH₄. Analysis of the electronic structure reveals that the stability of the [AlH₄] ligand was distorted by the strong interaction between the dopant Ti and its nearest neighbouring B atom when Ti occupied the Li site. Therefore, the interaction between the B and the H was reduced. As a result, dehydrogenation properties of LiBH₄ were improved. The strong interaction is conducive to the formation of Ti-B compounds.In chapter 5, the effect and mechanism of dopant F on the dehydrogenation characteristics of LiBH₄ were studied. The dopant formation energy, electronic density of states and overlap population, were investigated. The bonding state and the structure-stability of the crystal were analyzed. Results shows that F tended to occupy the H site in LiBH₄. Analysis of the electronic structure reveals that the interaction between F and Li, compared with interaction between Li and H in pure LiBH₄, was stronger. The distance Li-F, compared with distance Li-H in pure LiBH₄, was shorter. The distance B-F, compared with distance B-H in pure LiBH₄, was longer. This shows that F and B repel each other, F and Li attract, it is tend to the formation of LiF compounds. In addition, the distance between B and H was increased, it is conducive to improve dehydrogenation properties of LiBH₄.