Ionic Liquids (ILs), solely composed of ions, are a unique class of electrolytes that lack hydration shell. In an electrochemical system, the two fundamental concepts of interest are: (1) bulk phase structure as it effects the diffusivity and conductivity of charge carriers, and (2) interface structure as it influences charge transfer and capacitive behavior. The ionic interactions govern the phase behavior in the bulk such as viscosity and conductivity. Seminal work showed that the bulk phase of ILs is heterogeneous [1] at the nanoscale and that the presence of small charged species such as Li+ yields complex solvation structures. [2, 3] To design a strategy towards improving the diffusivity and conductivity of the charge carrier Li+ in ILs, we investigate the solvation structure of binary IL mixtures that display eutectic phase behavior. Specifically, the Li+ coordination in binary ILs were investigated by Raman Spectroscopy. In a mixture containing dicyanamide and bis(trifluoromethylsulfonyl)imide anions, Li+ shows preferred interactions with dicyanamide and the coordination dependency on lithium salt concentration becomes more complex compared to neat IL. The macroscopic conductivity decreases with salt addition up to 0.5 M and does not show further reduction with increasing the concentration to 1 M, suggesting microscopic changes in the local solvent structure that is responsible for a transport mechanism pertinent to IL mixtures where multi ions are present. In order to understand the interfacial behavior of ILs and how it impacts capacitance, electrochemical Surface Enhanced Raman Spectroscopy (eSERS) is performed. The initial results show possible ion adsorption on electrode surface that is potential dependent. Further investigation into the structure of the IL electrical double layer and its implications for energy storage systems are underway and will be discussed in this presentation.

Directing bulk and interface interactions in ionic liquids to modulate ion solvation, conduction and structure in electrochemical systems

255th National Meeting (2018)

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