First-principles study of decomposition mechanisms of Mg(BH4)2·2NH3 and LiMg(BH4)3·2NH3

Abstract

The decomposition mechanisms of Mg(BH₄)₂·2NH₃ and LiMg(BH₄)₃·2NH₃ were studied by using density functional theory calculations. Compared to that of Mg(BH₄)₂·2NH₃, the incorporation of LiBH₄ with the formation of LiMg(BH₄)₃·2NH₃ slightly increased Bader charges of B atoms, meanwhile it decreased Bader charges of N atoms. Mg(BH₄)₂·2NH₃ shows a low ammonia vacancy diffusion barrier, but relatively high ammonia vacancy formation energy, which lead to a low concentration of NH₃ vacancies and limit NH₃ transportation. In contrast to that of Mg(BH₄)₂·2NH₃, LiMg(BH₄)₃·2NH₃ has a relatively high ammonia vacancy formation energy and diffusion barrier, which suppresses ammonia release. The incorporation of LiBH₄ and Mg(BH₄)₂·2NH₃ does not decrease but increases the hydrogen formation barrier of LiMg(BH₄)₃·2NH₃, resulting in a slight increase in the dehydrogenation peak temperature, consistent with experimental results.