Lithium Hydrazinidoborane: A Polymorphic Material with Potential for Chemical Hydrogen Storage

Herein, we describe the synthesis and characterization (chemical, structural, and thermal) of a new crystal phase of lithium hydrazinidoborane (LiN₂H₄­BH₃, LiHB), which is a new material for solid-state chemical hydrogen storage. We put in evidence that lithium hydrazinidoborane is a polymorphic material, with a stable low-temperature phase and a metastable high-temperature phase. The former is called β-LiHB and the latter α-LiHB. Results from DSC and XRD showed that the transition phase occurs at around 90 °C. On this basis, the crystal structure of the novel β-LiHB phase was solved. The potential of this material for solid-state chemical hydrogen storage was verified by TGA, DSC, and isothermal dehydrogenations. Upon the formation of the α-LiHB phase, the borane dehydrogenates. At 150 °C, it is able to generate 10 wt % of pure H₂ while a solid residue consisting of polymers with linear and cyclic units forms. Reaction mechanisms and formation of bis­(lithium hydrazide) of diborane [(LiN₂H₃)₂­BH₂]⁺­[BH₄]⁻ as a reaction intermediate are tentatively proposed to highlight the decomposition of β-LiHB in our conditions