Decomposition behaviour of Mn(BH4)2 formed by ball-milling LiBH4 and MnCl2

Article abstract of DOI:10.1016/j.jallcom.2011.10.067

Manganese borohydride-based materials are of interest for hydrogen storage, as the Mn(BH₄)₂ compound has a theoretical hydrogen capacity of 9.53 wt.%. However, more needs to be known about the decomposition behaviour, in order to try to introduce reversibility. In this work, samples were prepared by ball-milling xLiBH₄ and MnCl₂ (where x = 2 or 3). The Mn(BH₄)₂ phase had XRD reflections consistent with the P3₁12 structure proposed by ?erny et al., and Raman spectroscopy indicted that the [BH₄]^- units were in an ionic tetrahedral configuration. The 3LiBH₄ + MnCl₂ sample contained excess LiBH₄ which exhibited an orthorhombic to hexagonal phase change at 93 °C indicating partial substitution of Cl ^- for [BH₄]^-. Thermal decomposition of Mn(BH₄)₂ occurred between 130 and 181 °C for the 2LiBH₄ + MnCl₂ sample and 105-145 °C for 3LiBH ₄ + MnCl₂, with the concurrent evolution of hydrogen and diborane. Analysis of the decomposition products shows the formation of amorphous boron, and the probable formation of manganese metal (deduced from the presence of manganese oxide in a sample subsequently exposed to air).