Theoretical Study On The Effect Of Hydrogen Vacancy And Ti，Nb, Mg Substituted On The Structure And Property Of The LiNH₂

Hydrogen energy is one of the ideal clean energies in the future and has become animportant research field all over the world．The storage of hydrogen is the most critical issuefor the application of hydrogen energy. Metal-nitrogen-hydrogen （metal-N-H） systemscomposed of light elements which have moderate gravimetric and volumetric storage capacity,good reversibility and low cost etc. They have the highest potential for use as hydrogenstorage materials. But it still shows several problems such as high operating temperatures andpoor dynamic performance, which hinder that the large-scale application. In recent years,much work has been done to render the hydrogenation or dehydrogenation process reversibleunder practical conditions.In this paper, the research object was light metal nitrogen hydride. Firstly we studied thestructure of LiNH₂bulk, then discussed the changes of properties about the structure, electrondensity distribution and state density distribution by Ti, Nb, Mg substitution in LiNH₂. Wefurther focused on clarifying the geometrical and electronic structures and interatomicbonding effect of H vacancies in Ti_, Nb_,or Mg-substituted LiNH₂. The first-principlescalculations adopted in the present study are based on density functional theory （DFT） usingthe generalized gradient approximation with the Perdew and Wang （PW91） exchangecorrelation function to describe interaction between electronic and ionic. We used theprojector augmented wave method embodied in the Vienna ab initio simulation program（VASP）. The PW91function of generalized gradient approximation GGA was employed forthe exchange correlation of electrons. The main content of this paper was divided into threeparts:The first part showed the theoretical studies on the properties of LiNH₂regarding thecrystal construction, electron density distribution and the density of states （DOS）. We foundthat N-H bond is mainly covalent bond, meanwhile it also has certain ionic bond property.The Bader charge analysis showed that Li atom has+0.84electricity and it is mainly theionic bond but there is also a small amount of covalent interaction between Li and N atom.Weaking interatomic covalent of N-H bond is the key point to improve the capacity ofdehydrogenation in LiNH₂.The second part, we used Ti, Nb, Mg atoms which have different electronegativity toreplace Li atom in the supercell of LiNH₂, respectively, and discuss the improvement ofcrystal structure and the interatomic bonding with substitution. By calculating the heat of formation, it found that Ti and Nb atoms are more easier to replace Li than that of Mg. Therehave differents results on N-H bond in the crystal with substituted metallic element Ti, Nband Mg. The Ti substitution has a larger effect on N–H bond around the Ti atom and theelectron density ρ（rc） at the BCP of the N–H bond come down. Ti sustitution have differenteffect on the adjacent NH2. Nb and Mg sustitution have the same influence on the adjacentNH2, but Nb atom is stronger. The bonding electrons which come from H s and N s, p orbitaldecrease after metal elements modification, and the interaction between N-Metal （substitutedmetal） atoms strengthen.The third part, on the basis of the introduction of Ti, Nb and Mg atoms, we bring inhydrogen vacancy defects to research the effect on the properties of releasing hydrogen. Metalsubstitution increase the possibility of forming hydrogen vacancy and Ti or Nb replaced ismore propitious to form hydrogen vacancy. The ability of Hydrogen released in system canstack with the both existence of substitution and vacancy defect. Superimposition effect isstronger when Ti or Nb substitution, which is effective to reduce the interaction of N-H bondand beneficial to release hydrogen. By researching the hydrogen atoms diffusion path in thebulk, we found that the substitutions which are more conducive to hydrogen diffusion canreduce the energy barrier of hydrogen diffusion.The novel conclusions and ideas of this work are listed as follows:1. Based on density functional theory of the first principle methods, we study on thecrystal structure and electronic structure of LiNH2and deeply discuss the properties ofinteratomic bonding by analysing the electricity and electronic state density of atoms incrystal.2. The electron density topology analysis method is used to compare the electron densityvalue of BCP between each pair of N and H atoms, and N and metal atoms before and after Ti,Nb, Mg substitution in supercell, which is quantitative to analyse strength changes of the N-Hbond and the capacity of H atom escaped.3. It found that metal substitution is beneficial to form H vacancy. There havecollaborative and promoted effect between metal substitution and H vacancy for the release ofhydrogen.4. Hydrogen diffusion process in the bulk of LiNH2is identified by using the climbingimage nudged elastic band method （CI-NEB）. The study of minimum energy path （MEP） ofhydrogen diffusion provides the result that metal substitution can reduce the energy barrier ofdiffusion, and thus is propitious to the hydrogen diffusion in the bulk.