Superior Reversible Hydrogen Storage Properties and Mechanism of LiBH₄–MgH₂–Al Doped with NbF₅ Additive

LiBH₄ is one of the most potential candidates for hydrogen storage materials among several sorts of complex borohydrides. Utilizing reactive hydride composites on LiBH₄ could destabilize the thermodynamics and improve dehydrogenation behaviors, such as the excellent reversibility of LiBH₄–MgH₂ and the fast dehydrogenation of LiBH₄–Al. The strategy of combining both outstanding effects of MgH₂ and Al to form LiBH₄–MgH₂–Al system has been proposed. However, reduction of hydrogen capacity during cycles has not been solved for the LiBH₄–MgH₂–Al system, which is considered as the principal problem. In this work, we investigated the reversible hydrogen storage performance and reaction mechanism of LiBH₄–MgH₂–Al doped with/without NbF₅ additive. It can be found that the dehydrogenation of 4LiBH₄–MgH₂–Al can release about 9.0 wt % H₂ quickly without incubation period, compared with 2LiBH₄–MgH₂. Moreover, it is the first time to achieve completely reversible hydrogen desorption property of LiBH₄–MgH₂–Al by doping with NbF₅ and dehydrogenating under hydrogen back pressure in experiment. Microstructure analysis shows that the formation of Mg–Al alloys could result in the formation of Li₂B₁₂H₁₂ and subsequently lead to the capacity degradation. With the additive NbF₅, it shows a totally different pathway and a significant inhibition effect on the alloying between Mg and Al, leading to an improved de/rehydrogenation behavior without the byproduct Li₂B₁₂H₁₂. Meanwhile, NbF₅ could be hydrogenated into NbH₂ and react with element B to form NbB₂, promoting the reaction between Mg/Al metals and B element to form MgAlB₄. On the other hand, those niobium compounds could facilitate the products MgAlB₄ and LiH to be fully rehydrogenated into LiBH₄, MgH₂, and Al, which contributes to the complete reversibility of LiBH₄–MgH₂–Al. A better understanding of the capacity fade mechanism of LiBH₄–MgH₂–Al system and the effects of additives might promote further development of high-capacity hydrogen storage materials