# Structure-Property Relation of Trimethyl Ammonium Ionic Liquids for Battery Applications
## 3 Results and Discussion
### Viscosity
The largest effect on the viscosity is found for the side chain functionalization, and the ratio of alkyl to ether is more distinct for the [TFSI]\({}^{-}\) samples with values of 1.66 for the [N1114]\({}^{+}\)/[N111(2O1)]\({}^{+}\) and 2.37 for the [N1115]\({}^{+}\)/[N111(2O2)]\({}^{+}\) combination. For the [FSI]\({}^{-}\) samples, ether functionalization also reduces the viscosity significantly, but to a lower extent (1.42 for the [N1114]\({}^{+}\)/[N111(2O1)]\({}^{+}\) pair). For all samples, the viscosity clearly increases upon addition of lithium salt. For the concentrated 1.5 M [Li][TFSI] samples, the raise in viscosity at 25 \({}^{\circ}\)C is distinct compared to the pure ionic liquids being 19.5 times the value for the alkylated [N1114][TFSI] and 14.0 times the value for the ether-ionic liquid [N111(2O1)][TFSI]. However, at 100 \({}^{\circ}\)C, the viscosity quotient between the concentrated solutions and the pure ionic liquid decreased to a factor 3.56 for the alkylated and 3.46 for the ether-substituted cation. Contrary to the sharp decrease in the viscosity ratio of lithium containing solution to pure ionic liquid in case of the 1.5 M samples, the ratio remains approximately constant for the samples with 0.25 M concentration. The \(T\)-dependence of all ionic liquids and their solutions could be well fitted with the VFT-equation (Figure 3). The values for the Angell strength factor are all in a quite narrow interval ranging from 3.05 to 5.61 and do not reveal a general trend. As the viscosities of the samples show no crossover with temperature, it is not surprisingly that the calculated activation energies for the viscous flow \(E_{\alpha,\eta}\) show the same trends as the viscosity values. In particular, a decrease in \(E_{\alpha,\eta}\) is observed upon replacing [TFSI]\({}^{-}\) by [FSI]\({}^{-}\), alkyl by ether side-chain, as well as for reducing the side-chain length and lithium content. Factors governing the viscosity of the cation and anion are not independent of each other as seen by the difference in viscosity for the alkyl and ether substituted cation when exchanging the anion. Similar findings related to a coupling of cation and anion factors affecting the transport properties of ionic liquids are also found in other studies [37]. Lower viscosities for the liquids based on the [FSI]\({}^{-}\) anion than for ones based on [TFSI]\({}^{-}\) are a general finding [40] and might be attributed to the smaller ion size or the stronger interaction of the latter with the cation, as it was shown in the literature for the [C\({}_{2}\)C\({}_{1}\)im]\({}^{+}\) cation [43]. The increase in viscosity upon elongation of the side chain found for the alkylated sample can be understood in the strengthening of a liquid microstructure consisting of aggregated nonpolar hydrocarbon and polar ionic components [42; 44]. In the case of 1-alkyl-3-methyl imidazolium [C\({}_{\rm{C}}\)C\({}_{1}\)im][TFSI] ionic liquids, the clear emergence of this nanostructure is reported to occur when the side chain length is equal to or larger than pentyl (x \(\geq\) 5) [45]. The underlying cause for the significantly lowered viscosity of the ether-substituted ionic liquids is the change in the cation conformation. While the side-chains of alkylated ionic liquids usually obtain linear conformation, the ether-groups are kinked and coordinate the positively charged part of the cation [37; 42; 46]. The origin for this curled structure is thought to be in the formation of intramolecular hydrogen bonding to the hydrogens in \(\alpha\)-position to the cation center [47]. This curling of the ether chains leads to more spherical ions with better shielding of the positive charge and blocking of coordination sites at the cation. This shielding and blocking become more distinct for (slightly) longer chains and reduces the overall interactions with the anion. In addition, the more polar ether-substituents suppress the formation of a nanostructure, which is contrary to less polar hydrocarbon groups.