Effect of Bulk Diffusivities of Co-solvents on the Microscopic Dynamics of an Ionic Liquid

Room temperature ionic liquids (RTILs) are considered superior over organic electrolytes in terms of their thermal stability, volatility, voltage window, and device life expectancy, making them promising electrolyte materials for electrical double layer capacitors. However, pure RTILs often exhibit high viscosity, low conductivity, and poor diffusivity, which can affect the charging and discharging rates, impacting device performances. The addition of co-solvents is found to change the viscosity of RTILs, leading to improved physico-chemical characteristics that affect their bulk and interfacial properties. Even though a direct correlation of co-solvent polarity with cation diffusivity has been established, an impact of co-solvent bulk diffusivity on the cation dynamics of RTILs has never been explored. Here, using different neutron scattering spectrometers and molecular dynamics simulation, we showed a presence of a phase separation into an ionic liquid-rich and a solvent-rich phase of 1-butyl-3-methyl-imidazolium bis(trifluromethylsulfonyl)imide, [Bmim⁺ ][TFSI⁻], mixed with four different co-solvents of nearly the same dipole moment, where we observed a scaling of long-range translational mobility of the [Bmim⁺] cation with the bulk diffusivities of the organic solvents.