Effects Of Nonmetal Element Additives In （Mg₂Ni, LiBH₄） Hydrogen Storage Alloy: Afirst-Principles Study

Nonmetal atoms （B, C and Si） are designed to add into Mg₂Nihydrogen storage alloy and its hydride. First-principles density-functionaltheory calculations have been performed to investigate their crystalstructures, electronic and thermodynamic properties. The calculationresults present that nonmetal additions in Mg₂NiH₄show more effectivedestabilization than metal additions. Especially for B and C, the decreasesof formation enthalpies of Mg₂NiH₄reach0.19and0.21eV·atom^-1. TheNiH₄structure near B or C in Mg₂NiH₄hydride becomes the tripod-likeNiH3structure. The results show that the thermodynamic stabilities ofMg₂Ni and Mg₂NiH₄exhibit a nearly linear decrease with the increasingcontent of nonmetal atoms. The calculated dehydrogenation energies are59.39,58.12,55.84and55.30kJ·mol-1H2for Mg₂NiH₄, Mg₂NiB0.5H₄,Mg₂NiC0.5H₄and Mg₂NiSi0.5H₄, respectively. It is found that the additionof nonmetal atoms favors the dehydrogenation reaction for Mg₂Nihydrogen storage material. In addition, the effects of nonmetals to theheat capacities and vibrational entropies of Mg₂Ni and Mg₂NiH₄are also analyzed.The graphite fragments are used to establish the calculated models ofLiBH₄mechanically milled by graphite additive. First-principlesdensity-functional theory （DFT） calculations have been performed tooptimize the crystal structures of LiBH₄inserted different graphitefragments. It is found that the unsaturated sites introduced by graphitefragments are occupied by BH3complexes and H atoms. Thedehydrogenation energies are calculated to investigate thedehydrogenation ability of LiBH₄with graphite fragments. The resultsshow BH3complexes bonded with graphite fragments have the lowerdehydrogenation energy relative to pure LiBH₄system, which is animportant reason of the improvement of thermodynamic property ofLiBH₄system. The hydrogen diffusion behaviors of LiBH₄with graphitefragments are investigated by the calculation of the nudged elastic band（NEB）. It is found that the hydrogen diffusion barriers of β-H are muchlower than the other types of H atoms in LiBH₄/graphite fragmentssystems, and also much lower than those in pure LiBH₄system. Thismeans that the existence of β-H significantly enhances the H-exchangekinetics of LiBH₄system. At the same time, the effects of sizes ofgraphite fragments in LiBH₄system are discussed.