  Iodine added to iodide-based ionic liquids leads to extraordinarily efficient
charge transport, vastly exceeding that expected for such viscous systems.
Using terahertz time-domain spectroscopy, in conjunction with dc conductivity,
diffusivity and viscosity measurements we unravel the conductivity pathways in
1-methyl-3-propylimidazolium iodide melts. This study presents evidence of the
Grotthuss mechanism as a significant contributor to the conductivity, and
provides new insights into ion pairing processes as well as the formation of
polyiodides. The terahertz and transport results are reunited in a model
providing a quantitative description of the conduction by physical diffusion
and the Grotthuss bond-exchange process. These novel results are important for
the fundamental understanding of conduction in molten salts and for
applications where ionic liquids are used as charge-transporting media such as
in batteries and dye-sensitized solar cells.
