# Structure-Property Relation of Trimethyl Ammonium Ionic Liquids for Battery Applications
## 3 Results and Discussion
### Viscosity  
Viscosity is one of the most important characteristics of ionic liquids since this class of materials suffers from the inherent disadvantage of slower dynamics compared to molecular fluids. Due to the presence of the strong, long-ranged Coulomb forces, low temperature molten salts have viscosity values that are at least one order of magnitude higher than conventional solvents and electrolytes. Decreasing the viscosity of ionic liquids is therefore a key aspect in the focused optimization of their properties. Critically, viscosity determines the transfer of the electroactive species in battery electrolytes and therefore limits the electrochemical reactions and charge/discharge rates. Beyond modification of the molecular structure of ionic liquids to achieve higher fluidities, a mixing of ionic and molecular liquids is another practical tool to achieve electrolytes with good transport properties. However, this approach to some extent comes at the cost of the advantages of pure ionic liquid electrolytes, such as negligible vapor pressure, increased safety and high chemical stability. The obtained viscosities of the ionic liquids at 25 \({}^{\circ}\)C, the fitting parameters following the VFT Equation (3), the Angell strength factor \(\delta_{\eta}\) and activation energy \(E_{\alpha,\eta}\) obtained from the Arrhenius Equation (4) in the range of 25 to 60 \({}^{\circ}\)C are given in Table 2. Experimental viscosity values are given in the supporting information. \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Ionic Liquid**} & \multirow{2}{*}{\begin{tabular}{c} **Li Salt** \\ **\(c\)/mol-dm\({}^{-3}\)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} \(\eta^{25\ \text{C}}/\) \\ **mPa s** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} \(\eta_{0}/\) \\ **mPa s** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} \(B_{\eta}/\)K** \\ **mPa s** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} \(T_{0,\eta}/\)K** \\ **mPa s** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} \(\delta_{\eta}/\) \\ **kJ mol\({}^{-1}\)** \\ \end{tabular} } \\ \hline
[N1114][TFSI] & - & 99.2 & 1.772 & 784.9 & 174.1 & 4.51 & 32.9 \\
[N1114][TFSI] & 0.25 & 147.3 & 2.073 & 760.4 & 182.3 & 4.17 & 36.0 \\
[N1114][TFSI] & 1.5 & 1933 & 1.229 & 979.4 & 196.8 & 4.98 & 58.6 \\
[N111(201)][TFSI] & - & 59.6 & 2.805 & 632.7 & 180.1 & 3.51 & 29.0 \\
[N111(201)][TFSI] & 0.25 & 87.7 & 2.138 & 746.5 & 174.1 & 4.29 & 31.3 \\
[N111(201)][TFSI] & 1.5 & 833.9 & 1.675 & 926.5 & 189.3 & 4.89 & 48.9 \\
[N1114][FSI] & - & 55.1 & 2.053 & 835.4 & 148.8 & 5.61 & 25.1 \\
[N1114][FSI] & 0.25 & 62.0 & 2.183 & 819.7 & 153.1 & 5.35 & 25.9 \\
[N111(201)][FSI] & - & 38.9 & 1.996 & 800.9 & 146.3 & 5.47 & 23.3 \\
[N111(201)][FSI] & 0.25 & 45.5 & 4.502 & 550.0 & 178.8 & 3.08 & 24.7 \\
[N1115][TFSI] & - & 136.9 & 1.650 & 806.0 & 178.2 & 4.52 & 35.9 \\ \hline \end{tabular}
\end{table} 
Table 2: Viscosity \(\eta\) of the pure ionic liquids and mixtures with lithium salts at 25 \({}^{\circ}\)C, VFT fitting parameters (\(\eta_{0}\), \(B_{\eta}\), \(T_{0,\eta}\)), Angell strength factor \(\delta_{\eta}\) and activation energy \(E_{\alpha,\eta}\) (in the range of 25 to 60 \({}^{\circ}\)C) for the \(T\)-dependent viscosity. Figure 2: Temperature-dependent densities of the ionic liquids with (**a**) purely alkyl side groups and (**b**) ether functionalization in the side-chain. At 25 \({}^{\circ}\)C, the ionic liquids or their mixtures with lithium salt incorporating the [FSI]\({}^{-}\) have significantly lower viscosity values than the ones with the [TFSI]\({}^{-}\) anion and a common cation. The viscosity of the [TFSI]\({}^{-}\) samples are higher than those of [FSI]\({}^{-}\) ionic liquids by a factor of 1.80 for the [N1114]\({}^{+}\) cation and 1.53 for the [N111(2O1)]\({}^{+}\) cation. Elongation of the side chain from butyl to pentyl leads to an increase in viscosity by 38% for the alkylated sample, while for the ether-substituted samples, even a slight decrease is found (IL without salt).