Investigation of Li and H dynamics in Li6C60 and Li6C60Hy

We report the extensive investigation of Li and H dynamics in Li6C60 and Li6C60Hy, by combining 7Li and 1H solid state NMR measurements with DC/AC conductivity, in order to evaluate the potential application of these systems for energy-storage purposes. 7Li NMR results show a local motion of Li ions above 200 K in both pristine and hydrogenated compounds, with activation energies of 90-150 meV and correlation times of about 30 ps. Evidences of Li interdiffusive dynamics are given by conductivity measurements in Li6C60 already above 120 K, with activation energies of 240 meV, suggesting that ionic conductivity is of the order of 10-5 Scm-1 at room temperature, with correlation times of about 150 ps. On the other hand, the Li6C60Hy behaves like a semiconductor with a high energy gap (ca. 2.5 eV), suggesting that diffusion of intercalated Li ions is prevented. 1H NMR measurements indicate the absence of H motions for the whole temperature range investigated (up to 360 K), neither on macroscopic or local scale. Li6C60 good properties for H2-storage are confirmed in terms of absorption capacity (5 wt% H2), moreover we found that around 35% of lithium segregates in LiH form, leaving Li4C60H40 as the final hydrogenation product.