The Influence Of Defect On The Structure And Hydrogen Storage Property Of LiBH₄·NH₃

Hydrogen energy is concerned as a clean and green energy because the Hydrogen is themost abundant element in the universe and its combustion product is clean and pollution-free.The storage of hydrogen is the most critical issue for the application of hydrogen energy, andthe improvement of hydrogen storage technology can overcome the problem of hydrogenstorage. Recently, a new type of B-N-H complexes M(BH4)m·nNH3(M=metal) were found.They are considered to be the most potential hydrogen storage materials, due to theirgravimetric hydrogen densities and dehydrogenation properties.In this paper, the structure and hydrogen storage property of pure and doped LiBH4·NH3were studied. Our calculation uses the VASP software package, which is based on the densityfunctional theory using the generalized gradient approximation (GGA). The interactionsbetween the nucleus and the inner electrons and the valence electrons are dealt with theprojector augmented wave (PAW), and PW91generalized gradient function was selected asexchange correlation function. Firstly, we study the crystal structure, the H atom dissociationenergy and the electronic structure of pure LiBH4·NH3. Then we use Mg, Al, Zn atomsreplacing a Li atom in LiBH4·NH3, and discuss the effect of dopant on the hydrogen releaseproperties from three aspects: the substitution, substitution-vacancy defects andsubstitution-frenkel defects in the LiBH4·NH3. The main content of this paper is divided intothree parts (the third chapter to the fifth chapter):The first part, the crystal structure of LiBH4·NH3shows that there are strong interactionsbetween B-H and N-H. The Bader charge indicates that it is mainly ionic bond between Liand [BH4] unit and [NH3] group is almost neutral. By the calculation of the hydrogendissociation energy, QTAIM analysis and the density of state property, we further determinethe type and strength of interactions between B-H and N-H. At the same time, we find thatthere is a weak interaction between Li and N atom, which can effectively inhibit the ammoniarelease in the dehydrogenation process. Therefore, weakening interatomic covalence of B-Hand N-H bonds and enhancing the interaction between metal atoms and N atoms (M-N) arethe key points to improve the property of dehydrogenation of LiBH4·NH3.The second part, we use the light metal atoms Mg and Al replacing a Li atom inLiBH4·NH3, and study the doping effect on the crystal structure and atomic interaction fromthe three situations, including the substitution, substitution-vacancy defects andsubstitution-frenkel defects. For the Mg and Al substituted systems, Mg substitution occurs more easily than Al substitution. Mg substitution makes the interaction between B-H and N-Hweakened obviously, and the interaction of Mg-N is stronger than that of Al-N, so the Mgsubstitution can better improve the dehydrogenation properties of LiBH4·NH3. Whensubstitution and vacancy defects exist simultaneously, there has an collaborative effect on therelease of hydrogen. Substitution-vacancy defects reduce the interaction of B-H and N-H, andincrease the interaction between the metal (Mg, Al) and N atoms effectively.Substitution-frenkel defect is in favor of H atom diffusion in the crystal, because it makes thehydrogen vacancy-interval pair to be formed more easily.The third part, we investigated the doping effect on the property of dehydrogenationfrom three aspects by using transition metal Zn as substitution atom. We found that the spinpolarization has a little effect in the Zn substitution systems. Substitution can weaken theinteractions of B-H and N-H, which is in favor of hydrogen release. Meanwhile, substitutioncan also enhance the interaction between the metal atom and N atom, which is propitious tothe stabilization of NH3group. We also discussed the coexistence situation of Zn substitutionand hydrogen vacancy defects. The results indicated that the coexistence of Zn substitutionand hydrogen vacancy defects have a stronger superimposition effect on the hydrogen releaseproperty. The substitution of Zn increases the possibility of forming hydrogen vacancy, andthe superimposition defects are effective to reduce the interaction of B-H and N-H. In the caseof coexistence of substitution and frenkel pair defects, the substitution can enhance theformation of vacancy-interval pair, and thereby promote the diffusion of H atom in thecrystal.The novel conclusions and ideas of this work are listed as follows:1. Based on the first principle calculations, we studied the crystal structure and electronicstructure of LiBH4·NH3in detail and deeply discussed the bonding characteristics betweenatoms from the view points of Bader charge and electronic state density.2. The topological analysis of electronic density was used to calculate the topologicalproperties of bonds in the clasters which were taken from the optimized crystal cell. Bycomparing the topologiacal properties of bonds B-H, N-H, and M-N before and after Mg, Al,Zn substitution, we analysed quantitatively the strength changes of these bonds which arerelative to the release of H atoms.3. We found that metal substitution is beneficial to the formation of H vacancy. Therehave collaborative and promoted effects between metal substitution and H vacancy for therelease of hydrogen. 4. The study of the frenkel defects indicates that Mg, Al, Zn substitution is propitious tothe hydrogen diffusion in the crystal cell.