Formation of intermediate compound Li₂B₁₂H ₁₂ during the dehydrogenation process of the LiBH₄- MgH₂ system

Intermediate compound comprising [B₁₂H₁₂] ^2- anion formed in the dehydrogenation is regarded to be responsible for the degradation of reversibility of metal borohydrides, whereas little is known about the formation of Li₂B₁₂H₁₂ in the LiBH₄-MgH₂ combined system. In this study, we investigated the dehydrogenation process of the LiBH₄-MgH₂ system in a heating run under different hydrogen pressures. Our experimental results demonstrated, for the first time, that Li₂B₁₂H ₁₂ was formed under hydrogen pressure ≤1.0 MPa, but not formed under 2.0 MPa. Moreover, the formation of Li₂B₁₂H ₁₂ was found to negatively depending on the hydrogen pressure, and also influence subsequent formation of MgB₂: the more Li ₂B₁₂H₁₂ that formed, the less MgB₂ obtained. These findings indicate that suppression of the formation of intermediate compound Li₂B₁₂H₁₂, such as by using adequate hydrogen pressures (e.g., 2.0 MPa), is of great importance for further improvement of hydrogen storage properties of the LiBH ₄-MgH₂ system.