Microstructural Study of MgB₂ in the LiBH₄-MgH₂ Composite by Using TEM

The hampered kinetics of reactive hydride composites (RHCs) in hydrogen storage and release, which limits their use for extensive applications in hydrogen storage S1and energy conversion, can be improved using additives. However, the mechanism of the kinetic restriction and the additive effect on promoting the kinetics have remained unclear. These uncertainties are addressed by utilizing versatile transmission electron microscopy (TEM) on the LiBH₄-MgH₂ composite under the influence of the 3TiCl₃·AlCl₃ additives. The formation of the MgB₂ phase, as the rate-limiting step, is emphatically studied. According to the observations, the heterogeneous nucleation of MgB₂ relies on different nucleation centers (Mg or TiB₂ and AlB₂). The varied nucleation and growth of MgB₂ are related to the in-plane strain energy density at the interface, resulting from the atomic misfit between MgB₂ and its nucleation centers. This leads to distinct MgB₂ morphologies (bars and platelets) and different performances in the dehydrogenation kinetics of LiBH₄-MgH₂. It was found that the formation of numerous MgB₂ platelets is regarded as the origin of the kinetic improvement. Therefore, to promote dehydrogenation kinetics in comparable RHC systems for hydrogen storage, it is suggested to select additives delivering a small atomic misfit.