In Situ Neutron Powder Diffraction of Li6C60 for Hydrogen Storage

Li6C60 is so far the best performing fullerene-based hydrogen-storage material. The mechanism of its reversible hydrogen absorption is not, however, totally known. Here, we report the thermal evolution of Li6C60 upon deuterium absorption up to 330 degrees C and under 60 bar deuterium gas pressure using in situ high-intensity neutron powder diffraction. The temporal resolution of the collected data allowed the hydrogenation of Li6C60 and the segregation of lithium hydride (here LiD) processes to be distinguished from a mechanistic point of view. During absorption, Li6C60 undergoes several phase transitions, involving the partial segregation of Li in the form of hydride and the expansion of the face-centered cubic (fcc) lattice, followed by a body-centered cubic (bcc) rearrangement of the deuterated fullerenes. The amount of absorbed deuterium was determined by the analysis of the variation in the scattered neutron intensity and confirmed by an ex situ desorption measurement. This analysis clarifies the complex physicochemical reactions behind the absorption mechanism of this model material and highlights the importance of intercalated lithium in triggering the hydrogenation process.