# Structure-Property Relation of Trimethyl Ammonium Ionic Liquids for Battery Applications
## 3 Results and Discussion
### Linear Sweep Voltammetry and Application in Li-ion Based Half-Cells  
The oxidative stability of the ionic liquid-lithium imide salt mixtures were measured in blocking Pt electrode versus a lithium metal electrode configuration. A significant increase in current density implies the upper limit of the electrochemical window (ECW) of the electrolyte. Nevertheless, it should be considered that blocking electrodes may pretend to be too oxidative stable [69]. Table 5 summarizes the results with two different stability limit approaches, namely the current limit (\(j=50\) uA cm\({}^{-2}\)) and the tangential approach. It can be observed that the ammonium based ionic liquids exhibit a wide oxidative stability window of \(E_{\text{ox}}\geq 4.5\) V vs. Li/Li\({}^{+}\) even in case of [FSI]\({}^{-}\) anion salt at \(T=25\)\({}^{\circ}\)C. It is seen that the mixtures containing [TFSI]\({}^{-}\) anions show even greater stability of \(E_{\text{ox}}\geq 5\) V vs. Li/Li\({}^{+}\). Due to the low current response, the current density approach may lead to systematic overestimation of the stability. However, for almost all systems, both approaches  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Ionic Liquid**} & \multicolumn{2}{c}{**Li Salt**} & \multicolumn{2}{c}{\(E_{\text{ox}}\)/\(V\)} & \multicolumn{2}{c}{\(E_{\text{ox}}\)/\(V\)} & \multirow{2}{*}{\(\begin{array}{c}\text{C}_{\text{DC}}\text{z}\text{/}\text{/}\text{/}\text{/}\text{ }\text{/}\text{ }\text  
for determining the oxidative limit (tangential approach/current density limit) result in comparable stability values. Ether containing side chains do not significantly reduce the \(E_{\text{ox}}\) values. In the tangential approach, it can be observed that the addition of lithium salt slightly reduces the \(E_{\text{ox}}\) values in the order of \(0.2-1.5\) V. Overall, the compounds exhibit high oxidative stability, which makes them particularly suitable for use in high-voltage materials. Cycling tests were performed in half-cell configuration (NMC versus Li electrodes) in EL-Cell \"PAT-Cell-Press\" including the pressure detection at \(T=60\)\({}^{\circ}\)C at C/20 current rate. Parameters were determined based on pre-tests of a selection of mixtures in coin cell configuration, cycled at different C-rates at \(40\)\({}^{\circ}\)C. It was found that a current rate lower than 0.05 should be used due to the drop in capacitance at higher C-rates. A more serious issue here is that the relatively high viscosity and ion aggregate formation of the electrolytes can make Li transport into the NMC material difficult, even at high temperatures. Such harsh conditions at low current rates and high temperatures accelerate aging processes. Additionally, cycling against lithium avoids difficulties, which arise against graphite electrodes (e.g., intercalation of electrolyte components without additional additives). Usually the conducting salt concentration should be higher in cell tests; however, to ensure same composition as studied in depth above, we performed cell tests with low concentrated samples at reduced current rates. Cell data are shown in Supporting Information (Figure S2). The results of the cycling tests are shown in Table 5 as an overview and with more detail also in the Supporting Information (Tables S14 and S15). During the cell tests, the internal pressure increases during the cycling (e.g., [N1114][TFSI] +0.25[Li][TFSI]), especially during charging periods. This might be an effect of gas formation and electrolyte decomposition during Li plating on newly created Li surface. A typical observed anode potential shifting around \(\pm 20\) mV can be observed for the mixture of [N1114][TFSI] with lithium salt. However, this charge-discharge jump is superimposed by a drift, which is clearly pronounced in some electrolytes (e.g., [N111(201)][TFSI] mixture or others in SI) up to hundreds of mV range. At the same time, the positive electrode (NMC) drifts correspondingly, which is, however, unexpected for such stable electrode materials.