Enhancement of lithium transport by controlling the mesoporosity of silica monoliths filled by ionic liquids

A systematic study of the effect of the mesopore diameter of silica monoliths with hierarchical porosity (meso-/macroporosity) on charge transport of confined ionic liquid with lithium salt is reported. Different measurements have been performed including ionic conductivity, Li+ transference number, and Li-7 MAS NMR. For a concentration of 0.5 M of Li bis-trifluorosulfonylimide an optimum of ion transport is found for N-methyl, N-propylpyrrolidinium bis-trifluorosulfonylimide confined within silica mesopore of 10 nm diameter. For this value of mesopore diameter, the ionogel monolith shows better charge transport behaviour than the same, non confined ionic liquid. Confining ionic liquid within silica monoliths with hierarchical porosity (meso-/macroporosity) allowed also to evidence that macropores has no effect on ionic transport. The optimum of mesoporosity of silica for Li+ diffusivity seems to be related to a good compromise between the amount of Li+ interacting with the surface silanols and the amount of Li+ in the volume of the pores. Smaller mesopores should give rise to too strong interactions of Li+ with surface silanols whereas larger mesopores should lead to Li+ strongly interacting with bis-trifluorosulfonylimide anions in the volume of the pores.