Ether-Functionalized Sulfonium Ionic Liquid and Its Binary Mixtures with Acetonitrile as Electrolyte for Electrochemical Double Layer Capacitors: A Molecular Dynamics Study.

Abstract

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-methoxyethyl)ethylmethylsulfonium bis(trifluoromethanesulfonyl)imide [S12G1][NTf2] and its mixtures with acetonitrile (ACN). The viscosity and ionic conductivity of the neat ionic liquid are in quite good agreement with the experimental results in a wide range of temperatures. The addition of ACN decreases viscosity and, consequently, increases ionic conductivity and diffusion coefficients. Typical alternating layers of ions close to the electrodes surfaces are observed in supercapacitors built with [S12G1][NTf2] when a high voltage is applied, ΔΨ = 3.0 V. Sharp layers of the 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 implies the 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 for the charge accumulation are 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 increases the charging time. Charging of the supercapacitors is dependent on ion exchange and counterion adsorption at ΔΨ = 3.0 V.