First-principles Study On Metal Doping Effect On Dehydrogenation Of Li-B-H Systems

In this paper,using the first-principles density-functional theory,we have calculated the occupation energy of metal,the dehydrogenation energy,the density of states,the bond population and the band structure for all studied systems,aiming at understanding the metal doping effects on the dehydrogenation of Li-B-H systems.The specific contents of our works include:(1)Al/Ti with larger electronegativity relative to Li are incorporated into mother system LiBH4(space group Pnma)by substitution for Li atom(0.338,0.375,0.613),B atom(0.689,0.375,0.573)and interstitial site(0.5,0.5,0.5),producing LiBH4-Al/Ti systems.We have studied the dehydrogenation properties of LiBH4 modified by Al/Ti,and discussed the catalytic effect of Al/Ti on the decomposition of LiBH4.In this work,systems with transition metal Ti are considered spin polarization calculations.(2)Mg is designed to add into mother systems LiBH4(space group Pnma)and Li2B12H12(an intermediate compound in the decomposition of LiBH4,space group C2/m)by occupation of Li atom sites(0.338,0.375,0.613)for LiBH4 and(0.75,0.75,0.5)for Li2B12H12,introducing LiBH4-Mg and Li2B12H12-Mg systems,respectively.We have investigated the effect of Mg on the dehydrogenation of LiBH4 and Li2B12H12,and analyzed the reaction mechanism between LiBH4 and MgH2.Our results are shown in the following.For LiBH4-Al/Ti systems with A1 and Ti as dopants,the bonding interaction between B and H is covalent nature,and between Li and B/H is ionic nature.Al/Ti addition decreases the bonding interaction of B-H,Li-B and Li-H.Al atom interacts with its neighboring B atom to form Al-B bond within Li-and interstitial site-substituted systems with Al(Li7AlB8H32 and Li8B8AlH32 without H vacancy,Li7AlB8H3,and Li8B8AlH31with H vacancy).This Al-B bond in systems with H vacancy exhibits stronger covalent interaction and shorter distance(?2.1 A,very close to the Al-B bond length of-2.02 A in AlB2 molecule)compared to that in systems without H vacancy.Ti atom makes bond with its neighboring H atom in systems with Ti.Spin polarization effect is not obvious on relaxed LiBH4-Ti systems:the total energy changes slightly before and after spin polarization calculation,and the lowest dehydrogenation energy can be obtained without spin polarization.Pure LiBH4 has a nonmetallic character with band gap of 7.019 eV.This band gap becomes narrower or disappears when Al/Ti is added into LiBH4 bulk,resulting in the metal-like/metallic character shown in some LiBH4-Al/Ti systems(Li8B8AlH32,Li7TiB8H32,Li8B7TiH32 and Li8B8TiH32).Substitution with Al/Ti destabilizes LiBH4,leading to a reduction in the dehydrogenation energy(Ed).Taking LiBH4-Al systems for an example,with Al addition,the dehydrogenation energy decreases in the order of Li8B8H32>Li8B7AIH32>Li7AlB8H32>Li8B8AlH32,and this descending order is also reflected on the scaled bond order of B-H(BOsB-H)and the band gap(Eg).The improvement in the dehydrogenation properties of LiBH4 with Al/Ti addition may be ascribed to four facts:(1)The expansion of cell volume provides more sites for H-diffusion;(2)The decrease in bonding strength of B-H,Li-H and Li-B tailors the structural stability of LiBH4;(3)The metal-like/metallic character is favorable for the decomposition of LiBH4;(4)The Al-B bond in systems with Al or Ti-H bond in systems with Ti may drive the formation AlB2 or Ti hydride with catalytic activity during dehydrogenation reaction,respectively,which have a positive effect on H-desorption.For Al-doped systems,Al atom prefers to substitute for B atom,but this only weakly affects the dehydrogenation energy compared to pure LiBH4.Interstitial site-substituted system Li8B8AlH32 with relatively lower dehydrogenation energy shows good dehydrogenation performance,but the energy cost for Al occupation of interstitial site is rather high with occupation energy(Eocc)of 3.401 eV.In our works,Ti substitution will cost more energy than corresponding A1 substitution,leading to a limited Ti effect relative to active Al effect on LiBH4 decomposition.LiBH4-Al/Ti systems with good dehydrogenation characteristics need to reduce the energy cost for incorporation of Al/Ti into LiBH4 bulk(occupation energy)for practical application.For LiBH4-Mg and Li2812H12-Mg systems with Mg as dopant,the Li-B,Li-H,Mg-B and Mg-H bonds have ionic character,and B-H and B-B(Li2B12H12-Mg systems)bonds exhibit covalent character.The average bonding length of Mg-B(BLMg_B=?2.8 A)for LiBH4-/Li2B12H12-Mg systems is close to that for MgB2 compound(BLMg-B=?2.64 A).The bonding strength of Li-B and Li-H is weaker,and of B-H and B-B is slightly stronger in Mg-doped systems than in Mg-free systems.The energy cost for Mg atom substitution for Li atom in LiBH4 bulk(Eocc=3.168 eV)is higher than in Li2B12H12 bulk(Eocc=3.08 eV),this may help to explain why the reaction temperature for LiBH4+Mg is higher than that for Li2B12H12+MgH2.In our studies,the dehydrogenation properties of LiBH4/Li2B12H12 have been modified by Mg,resulting in favorable H-desorption,which may be attributed to four facts:(1)The larger cell volume provides more sites for H-desorption;(2)The weaker bonding interaction between Li and B/H leads to unstable structure;(3)The interaction between B and H brings the formation of MgB2;(4)Disappearance of band gap results in appearance of metallic character.In the case of Li2B12H12-Mg systems,their higher dehydrogenation energy relative to LiBH4-Mg systems is unfavorable for the dehydrogenation,and their[B12H12]group with strong covalent interaction between B and B is believed to be the main obstacle for the rehydrogenation.We believe that suppression of the formation of Li2B12H12(dehydrogenation intermediate compound of LiBH4)is of great importance for further improvement of hydrogen storage properties of LiBH4-MgH2 system.