# Structure-Property Relation of Trimethyl Ammonium Ionic Liquids for Battery Applications
## 3 Results and Discussion
### Molecular Structure, Thermal Properties and Density
\begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline \multirow{2}{*}{**Ionic Liquid**} & \multicolumn{2}{c}{**Lithium Salt**} & \multirow{2}{*}{\(T_{\text{c}}\) [a]/\({}^{\circ}\)C**} & \(T_{\text{g}}\)/\({}^{\circ}\)C & \(T_{\text{cc}}\) [b]/\({}^{\circ}\)C & \(T_{\text{ss}}\)/\({}^{\circ}\)C & \(T_{\text{m}}\) [a]/\({}^{\circ}\)C**} & \(\rho^{25\) \({}^{\circ}\)C/g mL\({}^{-1}\)**} \\ \hline
[N1114][TFSI] & - & \(-30\) & - & - & \(-103\) [a]; \(-100\) [b]; \(-38\) [b] & \(9\) [b] & \(18\) & \(1.3919\) \\
[N1114][TFSI] & \(0.25\) & - & \(-78\) & \(-25\) & - & \(9\) & \(1.4173\) \\
[N1114][TFSI] & \(1.5\) & - & \(-48\) & \(-15\) & - & \(19\) & \(1.5268\) \\
[N111(201)][TFSI] & - & \(-17\) & - & - & - & \(38\) & \(-\)[c] \\
[N111(201)][TFSI] & \(0.25\) & \(-36\) & - & - & - & \(36\) & \(-\)[c] \\
[N111(201)][TFSI] & \(1.5\) & - & \(-56\) & - & - & - & \(1.5725\) \\
[N1114][FSSI] & - & \(-18\) & - & - & \(-34\) [a]; \(-35\) [b] \(9\) [b] & \(22\) & \(1.3028\) \\
[N1114][FSSI] & \(0.25\) & \(-30\) & - & - & \(-32\) [a]; \(-28\) [b]; \(8\) [b] & \(15\) & \(1.3186\) \\
[N111(201)][FSSI] & - & \(12\) & - & - & - & \(34\) & \(-\)[c] \\
[N111(201)][FSSI] & \(0.25\) & \(-18\) & - & - & \(-33\) [a]; \(-28\) [b] \(9\) [b] & \(18\) & \(1.3914\) \\
[N1115][TFSI] & - & \(-17\) & - & - & \(-2\) [b]; \(19\) [b] & \(29\) & \(1.3638\) \\
[N1115][TFSI] & \(0.25\) & - & \(-72\) & \(-48\) & \(-8\) [b]; \(8\) [b] & \(17\) & \(1.3854\) \\
[N111(202)][TFSI] & - & - & \(-83\) & \(-66\) & \(-55\) [b] & \(0\) & \(1.4141\) \\
[N111(202)][TFSI] & \(0.25\) & - & \(-79\) & \(-18\) & - & \(-3\) & \(1.4341\) \\ \hline \hline \end{tabular}  
* [a] Observed in the cooling step (\(-1\) \({}^{\circ}\)C min\({}^{-1}\)); [b] observed in the heating step (\(+1\) \({}^{\circ}\)C min\({}^{-1}\)); [c] no sufficient supercooling to obtain a liquid at 25 \({}^{\circ}\)C.  
\end{table} 
Table 1: Thermal transitions of the pure ionic liquids and Li-salt mixtures obtained by DSC at \(\pm 1\) \({}^{\circ}\)C min\({}^{-1}\) scanning rates in closed Al-crucibles and their densities at 25 \({}^{\circ}\)C. Thermal transitions include crystallization temperatures \(T_{\text{c}}\) upon cooling, glass transition points \(T_{\text{g}}\), cold crystallization temperatures \(T_{\text{cc}}\), (transition from glassy to crystalline upon heating), transition temperatures for the change of crystalline phases \(T_{\text{ss}}\) and the melting temperatures \(T_{\text{m}}\). Uncertainty in the temperatures is \(\pm 1\) \({}^{\circ}\)C.  
Figure 1: Molecular structures of the ionic liquids’ ions and their abbreviations. the cooling and heating traces. This results in three different types that are commonly observed [29]. The first type is characterized by the complete absence of ordered crystalline structures leading only to the formation of amorphous glasses at the glass transition temperature \(T_{\rm g}\). Samples of the second type show crystallization \(T_{\rm c}\) upon cooling and melting \(T_{\rm m}\) upon heating similar to the situation for most organic salts. Ionic liquids that belong to the third type of thermal transition show a glass transition upon cooling, followed by cold crystallization \(T_{\rm cc}\) from the supercooled liquid state in the heating cycle and subsequent melting. _Overview of the thermal behaviors_: Most bulk ionic liquids and the mixtures with lower lithium salt content belong to the second type, showing crystallization upon cooling. The mixtures with high amount of lithium salt form glasses upon cooling. The sample with the alkylated [N1114]\({}^{+}\) cation and 1.5 M [Li][TFSI] shows an additional \(T_{\rm cc}\) (third type). The mixture of 1.5 M LiNTf\({}_{2}\) in [N111(2 OI)]\({}^{+}\) is the only sample where ordered solids could not be observed under the conditions explored. Glass transitions are only found for ionic liquids with the [TFSI]\({}^{-}\) anion. For the mixtures of ionic liquids with lithium salts the values for the crystallization and melting temperature are slightly lower than for the bulk liquids.