The ionic liquids (ILs) doped with metal salts have become a real alternative as electrolytes for batteries, but the right chose of these compounds for reaching the adequate properties and performance is still a challenge and strategies are therefore needed for achieving it.

Thermophysical properties of the IL 1-butyl-1-methylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide ([bmpyr][TFSI]) and its mixtures with the Bis(trifluoromethane)sulfonimide lithium salt (from 0.1 m to saturation level) were determined in this work. These properties are density (ρ), speed of sound (U), and corresponding derived magnitudes, such as the bulk modulus and the thermal coefficient, as well as electric conductivity (s) against temperature.

Density shows a linear decreasing dependence with temperature and a clear increase with the salt addition, whereas thermal expansion coefficient increases with temperature and salt addition.

Speed of sound decreases with both temperature and salt concentration, and the adiabatic compressibility calculated by means of the well-known Laplace equation increases, as expected, with temperature in all the studied cases although a small variation with concentration can be observed. The thermal coefficient increases with temperature and with salt concentration.

Electric conductivity increases with temperature following the Vogel-Fulcher-Tammann (VFT) equation and decreases with the addition of salt.