First Principles Study On Mechanisms Of Improvement Of Mg（BH₄）₂Hydrogen Storage Material By Doping

Borohydride Mg（BH₄）₂has a lot of advantages, such as high storage capacity, lightmass, low price, and reversibility, which makes it as a most promising hydrogen storagematerial. However, its high thermal stability, the high absorption/release hydrogentemperatures, slow charge/recharge hydrogen kinetics obstruct its practical applicationsas a hydrogen storage media. Therefore, modifying the thermal stability of borohydrideMg（BH₄）₂has become a key issue. In this paper, calculations on the electronic structureof Mg（BH₄）₂, stability and influence of dopants on the dehydrogenation properties ofMg（BH₄）₂were performed by means of first principles.It was shown that the low temperature phase of Mg（BH₄）₂is an isolator with a bandgap of6.2eV between the conduction and valence bands. The B-H bond is a covalentbond, while interaction between Mg and B atoms shows an ionic characteristic. Furtherstudies on the mechanisms of improving the thermal stability and dehydrogenationproperties of Mg（BH₄）₂by doping were carried out. Firstly, the effect of elements Al, Ca,N, Li and Na, which was solely doped on the stability and the dehydrogenation ofMg（BH₄）₂was identified. It was shown that the site occupation behaviors of dopants inMg（BH₄）₂are different, and the mechanism that dopant affecting dehydrogenation of thedoped systems is sensitive to the site occupation. Al, Li and N intend to replace the Batom, and Ca shows strongly possibility to replace the Mg atom, the Na, however, willoccupy an interstitial site. The stability of Mg（BH₄）₂will be enhanced if Ca and Noccupy their mostly preference sites, while the stability of other doped systems isreduced. The improvement of the dehydrogenation of doped Mg（BH₄）₂is mainlyoriginated by the destroying the BH₄group with doping. By considering the two factors,i.e. the occupation energy, and the hydrogen dissociation energy, the N is the mosteffective dopant. At low concentration, The N could replace the B atom and reduce inhydrogen dissociation energy about1.0eV, while at high concentration, it will occupythe second stable site（the interstitial site） and dramatically reduce the hydrogendissociation energy.Secondly, the transition metals Ti, Fe, Nb and Ni were selected as dopants to studytheir influence on Mg（BH₄）₂. Results illustrate that the four elements intend to replacethe Mg atom, but the hydrogen dissociation energy is the lowest if they replace the Batom. Mechanisms that each dopant used to improve the dehydrogenation properties ofMg（BH₄）₂are different. In the case that dopant replaces Mg atom, Ti may drive theformation of TiB2, Fe and Ni attract one H atom in the BH₄groups around them anddestroy the BH₄groups, and Nb tends to generate an NbB2phase.Thirdly, the influence of co-doping on the dehydrogenation of Mg（BH₄）₂was in investigated. Three combinations,（Li, N）,（Li, Al） and （Na, Al） were selected. When thedistance between two dopants is small, there is an evident “coupling effect”, however,the coupling effect is weak between Li and N. It also shows that this effect is weakenedwith increase of the distance between the two dopants and will vanish when the distancebetween dopants is large enough, therefore, the system behaviours as a simplecombination of two elements individually doped in. For each co-doping group, the （Li,N） doping mainly weakens the N-H bonds, but less affects the bond interactions amongatoms in the vicinity of Li atoms. Therefore, this doping has weak influence on theimprovement of dehydrogenation of Mg（BH₄）₂. It is significantly different from the （Li,Al） group to the （Li, N） group. The M-I doping, i.e., distance of Li and Al in Mg（BH₄）₂is the smallest, shows the best effect on the dehydrogenation of Mg（BH₄）₂.（Na, Al）group has a similar effect as （Li, Al） effect as （Li, Al） group. However, owing to thelarge energy required to dope in this group, it is hard to achieve practically.