Ether-Functionalized Sulfonium Ionic Liquid and its Binary Mixtures With Acetonitrile as Electrolyte for Electrochemical Double Layer Capacitors: a Molecular Dynamics Study.

Ether-functionalized sulfonium ionic liquids have been investigated as promising electrolytes in electrochemical storage energy devices due to their wide electrochemical window, high ionic conductivity and low viscosity. In spite of that, the viscosity of neat ionic liquids is still high for supercapacitor applications. Here we have used atomistic molecular dynamics simulations to describe transport properties, structure and supercapacitor performance of (2-methoxy-ethyl)-ethyl-methylsulfonium bis(trifluoromethanesulfonyl)imide [S12G1][NTf2] and its mixtures with acetonitrile (ACN). The viscosity and ionic conductivity of neat ionic liquid are in quite good agreement with the experimental results in a wide range of temperature. The addition of ACN decreases viscosity and, consequently, increases ionic conductivity and diffusion coefficients. Typical alternated layers of ions close to the electrodes surfaces are observed in supercapacitor built with [S12G1][NTf2] when is applied high voltage, ΔΨ = 3.0 V. Sharp layers of solvent adsorbed on the surface of electrodes are observed in the mixtures containing ACN. The charge accumulated on the electrodes is barely affected by the amount of ACN, which imply in similar performance in terms of capacitance. However, the charging times follow the viscosities of the electrolytes, that is, the electrolytes with high content of ACN have higher power performance. At low voltage, the rearrangements of ions close to electrodes responsible to the charge accumulation is very low in neat [S12G1][NTf2]. The sharp layer of ACN makes this rearrangement easier, but it acts as a barrier to ion exchange at higher voltages, which increase the charging time. Charging of the supercapacitors is dependent of ion exchange and counter-ion adsorption at ΔΨ = 3.0 V.