Role of native defects and the effects of metal additives on the kinetics of magnesium borohydride

Abstract

Magnesium borohydride (Mg(BH₄)₂) has been considered as a potential material for hydrogen storage. In this paper, density functional theory studies have been carried out on native point defects and electrically active impurities (Ni and Ti) in Mg(BH₄)₂. A detailed analysis of the geometrical structures, energetics and migration of the defects reveals that hydrogen related defects are charged and their formation energies are Fermi-level dependent. We propose a specific mechanism for the decomposition of Mg(BH₄)₂: the self-diffusion of Mg_i²⁺ is the rate-limiting process for decomposing Mg(BH₄)₂. Moreover, Ni and Ti impurities can tailor the hydrogen desorption kinetics of Mg(BH₄)₂ by shifting the Fermi level, but the effects of impurities on shifting the Fermi levels are related to how the impurity is incorporated into Mg(BH₄)₂.