Solid-State Hydriding Mechanism in the LiBH4 + MgH2 System

The LiBH4 + MgH2 system has great potential in reversible hydrogen storage for fuel cell vehicles. However, it has always been dehydrogenated and rehydrogenated in the liquid state until recently. The solid-state hydriding and dehydriding are necessary in order to achieve hydrogen uptake and release near ambient temperature. In this study, the solid-state hydriding mechanism of 2LiH + MgB2 mixtures has been investigated. It is found that the solid-state hydriding proceeds in two elementary steps. The first step is the ion exchange between the Mg2+ and Li+ ions in the MgB2 crystal to form an intermediate compound (Mg1-xLi2x)B-2. The second step is the continuous ion exchange and simultaneous hydrogenation of (Mg1-xLi2x)B-2 to form LiBH4 and MgH2. This finding is consistent with the observed diffusion-controlled hydriding kinetics.