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 AI 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 4LiBH(4)-MgH2-Al can release about 9.0 wt % H-2 quickly without incubation period, compared with 2LiBH(4)-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.