Investigation On The Influence Mechanics Of The Hydrogen Related Defects And Addition Of Metallic On The Dehydrogenation Performance Of LiNH₂

Li-N-H hydrogen storage materials have many advantages, such as, high-capacity hydrogen storage, light mass, low cost, rich raw materials and so on. So many scientists are interested in it. However, its applications are greatly hindered by its high operating temperature and its poor kinetics. In this thesis, a first-principles plane-wave pseudo-potential method based on the density functional theory was used to investigate the interaction of defect and doped atom on the dehydrogenation performance of LiNH2. And, we uncovered the interaction mechanisms of defects in Li-N-H materials, which are quite essential to improve dehydrogenating properties of hydrogen storage materials, and provide a theoretical basis to the development of low temperature dehydrogenation materials. It is significant for design and optimization of Li-N-H hydrogen storage materials. The obtained results are as follows:The result of binding-energy can't reflect the dehydrogenating properties of defect and doped of LiNH2. In the LiNH2 which contain interstitial H defect In equilibrium, there are a quantity of interstitial H defects; the formation energy of interstitial H defect is reduced by doped Mg and Ti, which increases the concentration of interstitial H. Interstitial H introduces the defect energy level in the gap, which reduces the width of the gap, and increases the dehydrogenation performance of LiNH2. The strength of N-H bond in [NH2]-was weakened by interstitial H, so that hydrogen in LiNH2 is relatively easy to release. The covalent bond between interstitial H and N of [NH2]- explains that the escape of NH3 from the dehydrogenation reaction of LiNH2 system. The strength of N-H bonds are not equal in doped LiNH2, a part of N-H bonds are weaker, and other N-H bonds are strong, the hydrogen of weaker N-H bonds are easy to release.In the LiNH2 which contain vacancy defect and Frankel defect, our study shows that formation of Frankel defect is much easier than vacancy defect. Formation energy is greatly reduced by doping with Mg and Ti, which increases the possibilities of defect formation. Formation of Frankel defect by doping with Mg is more difficult, which reduce the concentration of interstitial H. The formation of Frankel defect by doping with Ti is more easier, which increase the concentration of interstitial H. Vacancy would enable the electronic of neighbor N, H atom at a higher energy state, and would prone to disconnect the bond between H and N, thus it make the hydrogen easy to release. The band gap is narrowed by doping Mg and Ti with vacancy defect, so the dehydrogenating properties of LiNH2 are greatly improved. The present of vacancy does not make any difference to LiNH2. Frankel defect has less influence, so it is easy to generate. The band gap is narrowed by doped Mg and Ti in the Frankel defect, so the dehydrogenating properties are improve greatly. Vacancy defect leads to the split of the lowest energy level of LiMgNH2 and LiTiNH2, which will make electrons go into a higher band, hydrogen favorable to the release of and hydrogen. The atom H near the vacancy is escape easily because of its existence. The result of the function of alloying element and vacancy can add up together to improve the hydrogen releasing. Frankel defect has little effect on hydrogen releasing. The atom in gaps of LiMNH2 with Frankel defects has little effect on its hydrogen ability. Mg, Ti and vacancy can add up together to improve the hydrogen releasing.