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submitted on 16.06.2020 and posted on 19.06.2020by Joshua Tuffnell, Jędrzej K. Morzy, Rui Tan, Qilei Song, Caterina Ducati, Thomas Bennett, Siân E. Dutton
IL@MOF (IL: ionic liquid; MOF: metal-organic framework) materials have been proposed as a candidate for solid-state
electrolytes, combining the inherent non-flammability and high thermal and chemical stability of the ionic liquid with the
host-guest interactions of the MOF. In this work, we compare the structure and ionic conductivity of a sodium ion containing
IL@MOF composite formed from a microcrystalline powder of the zeolitic imidazolate framework (ZIF), ZIF-8 with a
hierarchically porous sample of ZIF-8 containing both micro- and mesopores from a sol-gel synthesis. Although the
crystallographic structures were shown to be the same by X-ray diffraction, significant differences in particle size, packing
and morphology were identified by electron microscopy techniques which highlight the origins of the hierarchical porosity.
After incorporation of Na0.1EMIM0.9TFSI (abbreviated to NaIL; EMIM = 1-ethyl-3-methylimidazolium; TFSI =
bis(trifluoromethylsulfonyl)imide), the hierarchically porous composite exhibited a 40 % greater filling capacity than the
purely microporous sample which was confirmed by elemental analysis and digestive proton NMR. Finally, the ionic
conductivity properties of the composite materials were probed by electrochemical impedance spectroscopy. The results
showed that despite the 40 % increased loading of NaIL in the NaIL@ZIF-8micro sample, the ionic conductivities at 25 °C were
8.4x10-6 and 1.6x10-5 S cm-1 for NaIL@ZIF-8meso and NaIL@ZIF-8micro respectively. These results exemplify the importance of
the long range, continuous ion pathways contributed by the microcrystalline pores, as well as the detrimental effect of
discontinuous and tortuous mesoporous pathways which show a limited contribution to the overall ionic conductivity.