Colloidal interactions in ionic liquids: Effects of surface area and particle type

The increasing demand for batteries with high energy and power density, yet at the same time reliable and safe, has challenged the researchers to look for new materials. Ionic liquid gels have emerged as a suitable electrolyte candidate given the improved mechanical properties provided by the gels and the high ionic conductivity, non-flammability, low vapor pressure, and wide thermal stability from the ionic liquids. In this work, dispersions of silica (SiO2), alumina (Al2O3), and titania (TiO2) nanoparticles in 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM-BF4) ionic liquid were studied. The silica and alumina nanoparticles form gels at concentrations between 5 and 7.5 wt%, whereas the titania nanoparticles simply sediment. Despite the solid-like behavior (gel), the ionic conductivity and glass transition temperature remains relatively unchanged with respect to that of the pure BMIM-BF4. However, for low nanoparticle concentration a small increase in conductivity is observed for all dispersions, whereas with increasing concentration the conductivity drops below that of the pure BMIM-BF4. Dispersions with silica nanoparticles form stronger gels compared to those of alumina nanoparticles. The stabilization and interaction mechanisms present in the dispersions are discussed in function of the surface area, the nanoparticle hydrodynamic diameter, the surface charge, and the surface functional groups