First-principles Study Light Metal LiAlH₄,LiBH₄ Hydrogen Storage And Desorption

In view of fossil energy crisis and environmental pollution, seeking security, economic and clean energy is increasingly urgent. Hydrogen Energy, because of its environmentally friendly, renewable and higher energy release per mass, attracts people's attention. The light metal materials are one of the most potential materials for high-density hydrogen storage in the form of metal hydrides. For example, the metal hydride LiAlH4, its theoretical hydrogen storage can reach 10.5%, and it is a good candidate for hydrogen storage. However, one of the shortcomings of the materials for applications is the higher temperatures for hydrogen release. At lower temperatures the hydrogen release process is slow, and there exist high diffusion barrier. In this dissertation, by using ab initio calculations based on density functional theory (DFT) we studied the changes of structures and stability of the metal hydrides in the process of hydrogen release. We also calculated the diffusion barrier of hydrogen in the process. Our aim is to reveal the changes of structures in the atomic scale and give some hints to low the temperature for hydrogen release. By using density functional calculations we explored the hydrogen releaseprocess in the monoclinic LiAlH4, and try to understand the reasons why it is so difficult for the hydride LiAlH4 to release hydrogen. First, the crystal structure and electronic property of the bulk LiAlH4 was determined, and the calculated lattice constants were in excellent agreement with experiment. The calculated band gap is 4.8eV, indicating the bulk LiAlH4 is an insulator which again agrees with experiment. Next, we removed hydrogen one by one from the bulk LiAlH4 and to check the changes of its structures and stability. The results show that the binding energy of hydrogen is mainly in the range of -2.49 eV—-2.94 eV. The binding energy is relatively lower after the first hydrogen removed. With hydrogen removed process, the unit cell volume gradually decreases, and Al atoms prefer to form clusters. We also calculated hydrogen release process for LiBH4 system and compared with the LiAlH4. We found the same phenomenon occurred in the hydrogen release process.