# Structure-Property Relation of Trimethyl Ammonium Ionic Liquids for Battery Applications
## 3 Results and Discussion
### Molecular Structure, Thermal Properties and Density
The only exception is the mixture of [N1114][TFSI] with 1.5 M [Li][TFSI], which has a melting point that is 1 \({}^{\circ}\)C higher than the pure ionic liquid. The diverse transition behavior is also reported for a phosphonium ionic liquid with comparable cation structure and [FSI]\({}^{-}\) anion [30]. Upon increasing the concentration of [Li][FSI], the thermal behavior changes from the second type for the pure ionic liquid to the third type for samples with 0.5 and 1 M concentration, and finally results in a first type behavior for high lithium salt concentrations [30]. _Influence of anions type and cation side chains_: For the ionic liquids with the alkylated [N1114]\({}^{+}\) cation and their 0.25 M Li-salt mixtures, the melting points and crystallization temperatures (if present) are slightly lower for the samples with the [TFSI]\({}^{-}\) anion compared to the [FSI]\({}^{-}\) anion. However, the situation is different in case of the samples with [N111(2 OI)]\({}^{+}\) cation. For the ionic liquids with this cation, the samples with the [FSI]\({}^{-}\) anion show slightly lower melting points, but higher crystallization temperatures than the [TFSI]\({}^{-}\) analogues. The values for the melting point and solid-solid transition of the pure [N1114][TFSI] are in good agreement with literature reports [31]. Elongation of the alkyl side chain from butyl to pentyl leads to an increase in the crystallization and melting points of roughly 10 \({}^{\circ}\)C. As a consequence, [N1115][TFSI] is the only sample with purely alkyl side chains, that is not a room temperature ionic liquid. For alkylated samples, the addition of lithium salt leads to a change in thermal behavior from the second to third type, so that only a glass transition is obtained in the cooling trace. The functionalization of the cation side chain with an ether group in \(\gamma\)-position leads to an increase in the melting temperatures compared to the case of the butyl side chain for both the [TFSI]\({}^{-}\) and [FSI]\({}^{-}\) anion. In case of the pentyl group, the substitution of the \(\gamma\)-methylene group by an oxygen leads results in a decrease of the melting point by nearly 30 \({}^{\circ}\)C, giving the lowest melting point among the pure ionic liquids in this set for [N111(2O2)][TFSI]. _Supercooling of the samples_: Supercooling, meaning the occurrence of a liquid state below the melting point of a compound for longer times, is a common finding in the field of ionic liquids [32; 33]. All the samples showing a thermal behavior of the third type have a strong tendency for supercooling with a trend that is more pronounced for samples with the [TFSI]\({}^{-}\) anion and ether functionalized side chains. Furthermore, the samples with dissolved lithium salts showed stronger supercooling than the pure ionic liquids. For some samples, this kinetic hindrance of crystallization even leads to the non-existence of crystallization temperatures under these experimental conditions and the formation of glasses upon cooling, so that these samples fall into the first and third types of DSC traces. The lowest supercooling is found for the sample [N111(2 OI)][FSI], showing only 22 \({}^{\circ}\)C difference between crystallization and melting, while [N111(2 OI)][TFSI] with 0.25 M [Li][TFSI] has a difference of 72 \({}^{\circ}\)C between both transitions. Due to the pronounced supercooling, many of the ionic liquids' physicochemical properties could also be measured in the liquid state below their melting point. _Solid-solid transitions:_ In addition to the above-mentioned thermal properties, many of the samples showed additional solid-solid transition in the DSC traces, whereby the greater variety of transitions is obtained upon heating. The solid-solid transitions can be found for all four types of cations and two types of anions investigated here. A similar variety of solid phases is also found for ionic liquids with small phosphonium cations and various anions [34], as well as other ammonium ionic liquids with the anions used here [35].