# Structure-Property Relation of Trimethyl Ammonium Ionic Liquids for Battery Applications
## 3 Results and Discussion
### Molecular Structure, Thermal Properties and Density
Potential applications of these ionic liquids and their mixtures are in the field of organic plastic crystals. _Summary and rationalization of the thermal behavior:_ These overall diverse, complex and hard to predict behavior of the thermal transitions for samples with similar structures highlights the need for a fine-tuning of the molecular structure to achieve ionic liquids with low solidification temperatures. This allows for batteries with expanded application ranges. The ionic liquids and mixture show no clear trend in the melting transition with the chosen anion, side chain length and functionalization, which is contrary to the situation found for transport properties (shown below). For instance, a common finding for ionic liquids with multiple ether substituents are significantly reduced melting temperatures and often occurrence of only glass transition when compared to the alkylated analogues [36; 37]. The only overall tendency for the thermal transitions is that the mixtures of IL with 0.25 M lithium salt show lower melting and crystallization temperatures. Accordingly, the formation of binary electrolytes consisting of ionic liquids and lithium salt can be a valuable tool to increase the liquid range of battery electrolytes. The lowering of the melting point for ionic liquid-lithium salt mixtures is a common finding [38] and can be attributed to increasing disorder in the resulting mixed crystal leading to a lower difference in entropy between solid and liquid state, as found for eutectic systems [39]. The entropic contribution of the dissolved lithium salt also serves as an explanation why the supercooling and glass formation is less pronounced for the [FSI]\({}^{-}\) samples, as these have lower [Li]\({}^{+}\) contents compared to the [TFSI]\({}^{-}\) mixture. Furthermore, a trend for the increase in glass transitions temperature upon increasing lithium salt concentration was found in the literature and attributed to the stronger ion-ion interaction of the small, charge localized Li-cation with the anions [40]. In this set, the effect is observed for [N1114][TFSI] where the glass transition is raised from \(-78\)\({}^{\circ}\)C for the 0.25 M sample to \(-48\)\({}^{\circ}\)C for the 1.5 M sample. This increase in the \(T_{\text{g}}\) upon increase of [Li]\({}^{+}\) is directly coupled to the increase of viscosity, that will be discussed below, as these two properties are related to each other [41]. The density values of the ionic liquids and their mixtures, on the contrary, show clear trends, Figure 2. The density increases significantly for the same cation when [FSI]\({}^{-}\) is replaced by [TFSI]\({}^{-}\). In addition, a density increase is also observed upon replacing the methylene unit in the side chain by an oxygen atom. Elongation of the side chain again leads to a decrease of the densities. Thus, the densities of the ionic liquids increase with increasing content of sulfur and fluorine. Increasing densities upon exchange of [FSI]\({}^{-}\) with [TFSI]\({}^{-}\) and addition of lithium salt was found also for ionic liquid electrolytes based on the 1-ethyl-3-methyl imidazolium cation [C\({}_{2}\)C\({}_{1}\)im]\({}^{+}\). The significant change in densities between the ether and alkyl samples can be rationalized by a change in the cation conformation as the difference between the weight of the methylene group and the oxygen is too small to account for these relatively large differences. Change in cation conformations from a linear side chain arrangement of the alkyl groups to a kinked one for ether-cations with curling of the side chain around the cation is well known in the literature [37; 42]. All temperature-dependent densities showed a linear behavior within the investigated \(T\)-range.