# Structure-Property Relation of Trimethyl Ammonium Ionic Liquids for Battery Applications
## 3 Results and Discussion
### Self-Diffusion Coefficients
[FSI]- containing samples tend to higher values than those incorporating [TFSI]-. This is consistent with literature findings for cyclic ammonium ionic liquids, where the [FSI]- samples showed higher ionicities than the [TFSI]- samples with the same cation and mild decreasing of \(I_{\text{HR}}\) for higher \(T\)[51; 57]. A similar value (\(I_{\text{HR}}\approx 0.6\)) and minor change in the ionicity upon \(T\)-increase was also found for a pure ammonium [TFSI]- ionic liquid [15]. For phosphonium ionic liquids with hydrocarbon and ether functionalities, the ionicity also does not change significantly and addition of lithium salt leads to decreased \(I_{\text{HR}}\) values. Several literature results showed the existence of [Li(A)x]1-x clusters for the amide-type ionic liquids as the interaction of the anion with the lithium cation is much stronger than with the ionic liquid cation [40; 49; 62; 63]. These complexes would then reduce the amount of charge carriers, which would lead to a lower ionicity. Higher temperature is then expected to reduce the amount of aggregated species leading to higher ionicities. However, the contrary situation is observed here. Possible reasons for this may lie in a gradual change of the contribution of different transporting mechanisms, i.e., vehicular and structure diffusion mechanism [64], as seen in the concentration dependence of the lithium transfer [65]. Further investigations on the temperature-dependence of the lithium coordination and aggregation of ions might give further insight, if the ionicity interpretation can be related to the observed \(T\)-dependence. Other rationalizations of the ionicity, apart from the degree of unpaired ions, are for instance charge transfer form anion to cation [66] or  
Figure 7: Temperature-dependent ionicity \(I_{\text{HR}}\) of the ionic liquids and mixtures with lithium salts for cations with (**a**) alkylated side chains and (**b**) attached ether-groups. different diffusional states [67]. For pure ionic liquids, the deviation between experimental and Nernst-Einstein conductivity is frequently interpreted by velocity anti-correlations between the different ions [51]. Application of the Stokes-Einstein Equation (8) in the commonly used form is restricted since it contains two unknown parameters, the constant for the diffusion conditions \(\gamma\) and the Stokes radius \(r_{\rm H}(i)\). From the experimental data, only the product of the two parameters can be obtained, see Figure 8.  
The values for \(\gamma\times r_{\rm H}(i)\) are always highest for the lithium cations with the exception of the 1.5 M samples at higher temperatures. For the [TFSI]\({}^{-}\) samples, the anions have higher values than the organic cations, whereas for the [FSI]\({}^{-}\) electrolytes the situation is inverted. The values are affected by temperature to a minor extent with a tendency of decreasing for higher temperatures and with higher effect on [Li]\({}^{+}\) ions. The findings of the lower \(\gamma\times r_{\rm H}(i)\) values for organic cations than for anions in samples with \(i\) = [TFSI]\({}^{-}\), and reversed situation in those with \(i\) = [FSI]\({}^{-}\), can be explained by the Van der Waals radius. For samples containing [TFSI]\({}^{-}\), the cations are smaller than the anion or comparable in size, leading to higher values of \(\gamma\times r_{\rm H}(i)\) for the anions [66]. The much higher \(\gamma\times r_{\rm H}(i)\) values for the small lithium cations are a clear indication of the formation of complexes with the anion if the same friction constant \(\gamma\) for all ions are proposed. The ion aggregation around [Li]\({}^{+}\) also explains the stronger decrease of \(\gamma\times r_{\rm H}(i)\) with temperature by the change in anion coordination or in the contributions of different diffusion mechanisms. Using the Van der Waals radii\({}^{62}\) from ab initio calculations as the Stokes radii in the Stokes-Einstein Equation (8) allows for the elimination of one unknown quantity [57].