Study On Microsture And Properties Of Li-N-H Metal Complex Hydrogen Storage Materials

In recent years,Li-N-H metal complex hydrogen storage materials attract much attention due to L₃N reacting with H₂,which have theoretical capacity of 10.5wt.%hydrogen and capability of reversible reaction.It is more advantageous than conventional hydrogen storage alloys.As one of foundation investigations of Li-N-H materials,the crystal structure of Li imide has no definite conclusion yet.Different H atoms locations in Li₂NH crystal affect much on Li₂NH+H₂/LiNH₂+LiH reaction enthalpy.Therefore,it is important for understanding mechanism of hydrogen storage and reaction dynamics to clarify the crystal structure of Li₂NH.In chapter 3,the crystal structure and locations of hydrogen atoms of lithium imide (Li₂NH) are studied by employing first-principle plane wave pseudopotential method based on the density function theory(DFT).Three models are served as investigation the effects of the Li,N and nearest-neighbor N-H bonds to N-H bonds orientation respectively.The calculated results show that Li₂NH crystal can be described to a layered tetragonal crystal (P4₂/m) structure.Four N-H bonds of each conventional cell align two layers.The two imide groups in the same layer prefer to be antiparallel and the imide groups in the nearest-neighbor layers tend to be vertical.The density of state(DOS) and the electron local function(ELF) analyses show strong ionic interaction between the Li and N-H dimmer,while the bonding between the N and H has covalent character.Our P4₂/m structure of Li₂NH crystal yields a hydrogen storage Li₂NH₂/LiNH₂ reaction enthalpy of 69.6 KJ/mol H₂ at T=0K in good agreement with experimental reports of～66 KJ/mol H₂ for this reaction.Alough,geometry of LiNH₂ crystal settled down in laboratory as early as in 1972, different exchange and correlation functions caused specific different results during calculating procedure.Thus,in chapter 4,the local density approximation(CA-PZ) and the generalized gradient approximation(PBE,RPBE and PW91) are applied for structure optimization.The calculated results show that LiNH₂ gets the lowest total energy by using GGA-PW91 function.Also,the crystal constant approaches to experiment result most closely. They could be concluded that GGA-PW91 function is more suitable than others in dealing with structure and properties of Li-N-H metal complex hydrogen materials.