## 3 Results and Discussion  
All the experimental data are available in the Supplementary Materials. Tables S1-S3 show density, viscosity and electrical conductivity of [EMIm]F at different temperatures, respectively. The density of [EMIm]F was measured for temperatures between 303.15 K and 353.15 K. The temperature dependence of the density of [EMIm]F is shown in Figure 3. Meanwhile, the densities of ILs with the same cationic group and different anionic groups were also compared [43; 44; 45; 46]. Figure 3 shows that the density of [EMIm]F decreases linearly with the increase of temperature for the other ILs. At a certain temperature, the density of [EMIm]F is higher than that of [EMIm][OAc] and [EMIm][DCA], and is lower than that of [EMIm][Tf\({}_{2}\)N], [EMIm][BF\({}_{4}\)] and [EMIm][EtSO\({}_{4}\)]. Correspondingly, the molar volume, calculated by dividing the molar mass by the density, increases linearly with the increase of temperature. The relationships between the density, molar volume, and temperature can be described by the following fitting equations:  
\[\rho=1.516-1.22\times 10^{-3}\;T \tag{1}\]  
\[V_{\rm m}=76.21+0.1296\;T. \tag{2}\]  
To study the volume changes of a pure liquid caused by the temperature at a constant pressure, a parameter, \(\alpha\), known as the thermal expansion coefficient, is defined as  
\[\alpha\ =\ \frac{1}{V}(\frac{\partial V}{\partial T})_{p}\ =\ -\frac{1}{\rho}(\frac{ \partial\rho}{\partial T})_{p}\ =\ \frac{1}{V_{m}}(\frac{\partial V_{m}}{\partial T})_{p}, \tag{3}\]  
where the subscript \(p\) indicates constant pressure. According to the calculus principle, Equation (3) can also be expressed as  
Figure 2: Synthesis process of [EMIm]F.  
\[\alpha\ =\ (\frac{\partial\ln V}{\partial T})_{p}\ =\ -(\frac{\partial\ln\rho}{ \partial T})_{p}\ =\ (\frac{\partial\ln V_{m}}{\partial T})_{p}. \tag{4}\]  
As we discussed above, both the density and molar volume have a linear relation with the temperature. Namely, \((\partial\rho/\partial T)_{p}\) and \((\partial V_{m}/\partial T)_{p}\) are constant. The thermal expansion coefficient \(\alpha\) varies with the density according to Equation (3). However, \(\alpha\) is a constant from Equation (4) because the plot of \(\ln\rho\) versus \(T\) is also linear. Actually, the two perspectives are not so conflicting due to different linear relations supposed in a rational scope (\(R^{2}>0.99\)) for the two equations. In the literature, both alternatives were reported in order to evaluate the volume expansivity for ILs [44; 47; 48]. For simplification, our work considers the thermal expansion coefficient of [EMIm]F a constant from Equation (4): \(1.09\times 10^{-3}\) K\({}^{-1}\). The dynamic viscosity of [EMIm]F was measured via a DV-2T viscometer. The viscosities of ILs with the same cationic group and different anionic groups were compared, as shown in Figure 4. The viscosity of [EMIm]F is obviously larger than that of other ILs below 320 K, and decreases sharply with the temperature increase. The viscosity of [EMIm]F approaches that of other ILs above 320 K. As is known, the viscosity of ILs is mainly dependent on the combination of hydrogen bonds and van der Waals forces. The viscosity variation of [EMIm]F indicates that the interaction forces between the cation and anion are susceptible to temperature. In general, the relationship between temperature and viscosity can be expressed as an exponential equation, commonly known as an Arrhenius-type equation:  
\[\eta\ =\ \eta_{\infty}\exp(-\frac{E_{a}}{RT}), \tag{5}\]  
where \(R\) is the ideal gas constant, \(\eta_{\infty}\) is the viscosity at the infinite temperature, and \(E_{\rm a}\) is the apparent activation energy. According to the fitting results, the characteristic parameters of [EMIm]F can be obtained: \(\eta_{\infty}\) = \(2.07\times 10^{-7}\) mPa\(\cdot\)s, \(E_{\rm a}=-5.39\times 10^{4}\) J\(\cdot\)mol\({}^{-1}\). Figure 4: Viscosity of [EMIm]F and similar ILs as a function of temperature. Figure 3: Temperature dependence of the densities of [EMIm]F and similar ILs.