Tailoring the properties of ammine metal borohydrides for solid-state hydrogen storage

A series of halide-free ammine manganese borohydrides, Mn(BH4)2·nNH3, n=1, 2, 3, and 6, a new bimetallic compound Li2Mn(BH4)4·6NH3, and the first ammine metal borohydride solid solution Mg1xMnx(BH4)2·6NH3 are presented. Four new crystal structures have been determined by synchrotron radiation powder X-ray diffraction and the thermal decomposition is systematically investigated for all the new compounds. The solid-gas reaction between Mn(BH4)2 and NH3 provides Mn(BH4)2·6NH3. The number of NH3 per Mn has been varied by mechanochemical treatment of Mn(BH4)2·6NH3-Mn(BH4)2 mixtures giving rise to increased hydrogen purity for n/m1 for M(BH4)m·nNH3. The structures of Mg(BH4)2·3NH3 and Li2Mg(BH4)4·6NH3 have been revisited and new structural models are presented. Finally, we demonstrate that ammonia destabilizes metal borohydrides with low electronegativity of the metal (cp ~1.6) are generally stabilized