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Study on Microsture and Properties of Li-N-H Metal Complex Hydrogen Storage Materials

Author: YuDaLong
Tutor: ChenYuHong
School: Lanzhou University of Technology
Course: Condensed Matter Physics
Keywords: first-principle crystal structure electron structure hydrogen storage material Li2NH LiNH2
CLC: TG139.7
Type: Master's thesis
Year: 2009
Downloads: 137
Quote: 2
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Abstract


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.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > The alloy learn with a variety of properties of alloys > Other special nature of the alloy > Hydrogen storage alloy
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