The First-principles Investigation Of The Li-N-H Complex Of Hydrogen-storage Mechanism

In order to study the hydrogen storage material of complex hydride of Li-N-H system,we investigated the macroscopic thermodynamic properties,reaction micro-reaction mechanism,reaction transition state,as well as the influence of hydrogen absorption and desorption properties by element substitution,the purpose of this paper is to reveal the mechanism of hydrogen storage about Li-N-H system,and provide theoretical reference for the experimental research.The main results are summarized as follows:1.The electronic structure and formation enthalpy of compounds maybe in the Li-N-H system have been studied.By using the chemical potential phase diagram of compounds of Li-N-H system to analysis the balance of LiNH2 constraints and the competition between the possible exist of each step reaction.The calculation results shown that the Fermi energy in the top of the valence band,obviously,it has non-metallicity.For LiNH2,the interaction between Li+and[NH2]-anion has an ionic character.For Li2NH,the interaction of the N-H bond is stronger,and Li atom is loss electronic form Li+.The formation enthalpy is agreement well with the others theoretical results,the LiNH2 is-212.27 kJ/mol,LiH is-91.66 kJ/mol,NH3is-102.27 kJ/mol,Li4NH is-309.72 kJ/mol,Li3N is-189.11 kJ/mol.According to the results of study the balance of chemical potential,we found that the larger region of LiNH2 is exist in the whole system at 0 K.With the change of the hydrogen pressure,exist 6 steps reversible reaction in the system,Li4NH participate in the three-step reaction,NH3 participate in part of the reversible reaction.2.The electronic structure and reaction transition state of the each molecular in the reaction of LiH+NH3?LiNH2+H2 have been studied.The calculation results show that the reaction of LiH and NH3 proceeds via an intermediate configurations as molecular LiNH4 complexes.The results show that in the reaction LiH and NH3 of the complex molecules as the reaction intermediate configuration.With the whole reaction proceeding,the charge population of Li-N bond and H1-H2 bond increases gradually,when with bond-breaking of Li-H1,N-H2 and forming of H1-H2,H2 is to provide an H atom by LiH and NH3,respectively.The reaction barrier is 46.28 kJ/mol,reaction energy is 35.68 kJ/mol.The EHOMO-LUMO of transition state is-1.779 eV,indicating the lowest chemical activity.In the whole process of the reaction,the charge transfer occurs mainly between H1 and H2,Li and N.3.The transition metal M(M=Ti,V,Mn,Co)replacing the part Li in the LiNH2 have been discussed the crystal structure,binding energy,electronic structure,and H vacancy formation energy for LiNH2 influence.The results show that the calculated cell parameter of LiNH2 is in agreement with the experimental results.The Ti and Co crystal cell parameters changed little while V and Mn substituted formed Li(M)NH2 the lattice volume greatly change.After substituted,the reduce of binding energy of Li(M)NH2 imply reduced stability.The electronic structure analysis found that the N-H bond is weaker after substituted,indicating that the fracture of N-H bond easier to released H atoms freely,including the role of Ti the most obvious.The value of H vacancy formation energy after substituted are calculated 1.365 eV,0.829 eV,0.486 eV,-0.079 eV,respectively.It means that after transition metal elements substitute Li(M)NH2 can reduce the dissociation energy of H,which improved performance of H sorption and desorption,moreover Ti is more suitable for the LiNH2 as catalyst.