Effect of additives on the synthesis and reversibility of Ca(BH4)2

Metal borohydrides are potential materials for solid-state hydrogen due to their high gravimetric and volumetric hydrogen densities. Among them, Ca(BH4)2 is particularly interesting because of the predicted suitable thermodynamic properties. In this work, we investigate a new synthesis route using high-pressure reactive ball milling. Starting from CaH2 and CaB6 with a TiCl3 or TiF3 as additive, a reaction yield of 19% is obtained after 24 h milling at room temperature and 140 bar H2. The presence of Ca(BH4)2 is confirmed by the presence of the stretching mode of the [BH4]− group in the infrared spectra of the as-milled samples. Using in situ XRD, we observe the recrystallisation of a poorly crystallised Ca(BH4)2 phase present after milling. The reversible decomposition/formation of Ca(BH4)2 is obtained with higher yield (57%) using higher temperature and TiF3 as additive but not with TiCl3 despite its similar electronic structure. The differences observed using different additives and the influence of the anion are discussed.