Effect of the presence of chlorides on the synthesis and decomposition of Ca(BH_4)_2

摘要

Calcium borohydride is one of the most interesting compounds for solid-state hydrogen storage, in particular because of its high hydrogen capacity. In this paper, the synthesis of Ca(BH_4)_2 by metathesis reaction via ball milling of a mixture of LiBH_4 and CaCl_2 is described. The effectiveness of this synthesis technique and the possible substitution of Cl ions in the borohydride phases is analysed depending on the back-pressure used for milling. When performed by ball milling under Ar, the metathesis reaction is not successful. A large quantity of a solid solution Ii(BH_4)_(1-x)Cl_x remains in the sample and CaHCl is formed rather than Ca(BH_4)_2. In contrast, the use of H_2 back-pressure during milling favours the borohydride phases rather than CaHCl and leads to the formation of a solid solution Ca(BH_4)_2-yCly where [BH_4]~- groups are partially substituted by Cl ions. This compound has a similar structure as (3-Ca(BH_4)_2 but with smaller lattice parameters. It is present in the as-milled sample together with LiCl and Li(BH_4)_(1-x)Cl_x. The decomposition of the mixture occurs at lower temperature than for pure LiBH_4 but higher than for pure Ca(BH_4)_2. The presence of chlorides in the structure of borohydride compounds changes dramatically the thermal properties of the material prepared and should be considered each time a metathesis reaction is used for synthesis.