The Influence Of Defect On The Surface Structure And Hydrogen Storage Propetry Of LiBH₄

Hydrogen energy has become more and more important in the21st century, andhydrogen storage technology has made great progress. Although hydrogen storage materialshave different shortcomings, their prospects are still very broad. Solid complex hydrides aremainly composed by the light metal (such as: Li, Na, K, Al, etc.) with the B, N, etc. LiBH4isone of the promising materials for hydrogen storage due to its high gravimetric andvolumetric hydrogen densities of18.5wt.%and121kg H2/m3. LiBH4as a hydrogen storagematerial is an ionic compound and its mechanism of hydrogen release is different with alloy.In recent years, scientists have done a lot of modification researches.In this study, the first principles calculations adopted was based on density functionaltheory using the generalized gradient approximation (GGA), the covalent interactions weredealed with PAW potential and PW91generalized gradient function was selected as exchangecorrelation function. First of all, The effects of Mg substitution on the structure and hydrogenstorage property of LiBH4(010) surface were investigated. Then the geometry, electronicstructure, dehydrogenation energy and hydrogen atom migration on the Al, Ti substitutionsurfaces were investigated. Finally, the article analyzed the effect on the hydrogen desorptionof LiBH4(100) surface with Mg, Al substitution and Li vacancy.The first chapter gives a brief review on the experimental and theoretical studies for theLiBH4hydrogen storage materials. The second chapter provides some basic knowledgeinvolved in this thesis. The studies are divided into three main parts (the third chapter to thefifth chapter):In the first part, The geometry, electronic structure, dehydrogenation energy andhydrogen atom migration on the clean and Mg doped LiBH4(010) surfaces have been studiedby first-principles calculations based on density functional theory (DFT). Electronic structurerevealed that The B-H bond lengths increase and the strength of the bond becomes weaker onthe Mg doped surface. The interaction between Mg and H atoms is stronger, which weakensthe strength of B-H bond. By calculating the diffusion pathway of H atom from one [BH4] tothe nearby [BH4] on the clean and Mg doped LiBH4(010) surfaces, we found that themigration barrier energy comes down from4.03to3.09eV. This indicates that the H atoms are easier to diffuse on the Mg doped LiBH4(010) surface.In the second part, The geometry, electronic structure, dehydrogenation energy andhydrogen atom migration on the Al, Ti doped LiBH4(010) surfaces have been studied byfirst-principles calculations based on density functional theory (DFT). Electronic structurerevealed that the B-H distances increase and the strength of the bond becomes weaker on theAl, Ti doped surface. The interactions of Al and H or Ti and H atoms are stronger, whichweaken the strength of B-H bond. By calculating the diffusion pathway of H atom from one[BH4] to the nearby [BH4] on the Al, Ti doped LiBH4(010) surfaces, we found that themigration barrier energies come down to3.20and2.98eV, respectively. This indicates thatthe H atoms are easier to diffuse on the Al, Ti doped LiBH4(010) surface.In the third part, we analyzed the effect on the hydrogen desorption of LiBH4(100)surfaces with Mg, Al substitution and Li vacancy. Clean LiBH4(100) and LiBH4(010)surfaces have similar properties. The interaction between B and H atoms are strong and Hatoms are difficult to dissociatived. Mg, Al replaced and Li vacancy systems improve thecapacity of dehydrogenation; Mg replaced and Li vacancy systems are the best and Alreplaced is better than the clean system. By calculating the diffusion pathway of H atom fromone [BH4] to the nearby [BH4] on the clean and LiBH4(100) with Mg, Al replaced and Livacancy surfaces, we found that all of the migration barrier energies come down. Thisindicates that the H atoms are easier to diffuse on the LiBH4(100) surfaces with Mg, Al,replaced and Li vacancy.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 theLiBH4surfaces and discuss the properties of interatomic bonding. Further more we researchthe influence of defects on the surface structure and hydrogen storage property of LiBH4. Itprovide a theoretical foundation and support for the synthesis of new hydrogen storagematerials.2. By the calculations of the surface electronic structure, we compare the electron densityvalue of BCP between each pair of B and H atoms, and B and metal atoms before and afterMg, Al, Ti substitution, which is quantitative to analyse strength changes of the B-H bond. 3. It found that metal substitution and Li vacancy are beneficial to form H vacancy andmake H more easier to escape. Mg doped LiBH4(010) and Li vacancy on the LiBH4(100)surfaces have the best capability of hydrogen release.4. Hydrogen diffusion process on the nearby LiBH4surface is identified by using theclimbing image nudged elastic band method (CI-NEB). It found that metal substitution and Livacancy can reduce the energy barrier of diffusion and the energy barriers of Ti dopedLiBH4(010) and Al doped LiBH4(100) surfaces are the lowest.