## 3 Results and Discussion
### Electrochemical Stability
#### 3.1.4 Cathodic Stability  
The electrochemical stability towards reduction was investigated by cyclic voltammetry (Figure 4) in the cathodic range on HC-working electrodes capable of reversibly intercalating sodium ions [18]. The CV measurements were carried out at a scan rate (0.05 mV s\({}^{-1}\)) considerably lower than the anodic ones (1 mV s\({}^{-1}\)) in order to promote the intercalation (de-intercalation) processes of sodium ions which, in cases of higher rates, may be hindered by kinetics. The FSI-based electrolytes (Figure 4A,E) show a large feature around 1 V that is present in the first cathode scan but absent in the following cycles and, therefore, is attributable to the formation of passive SEI constituted by the products coming from the electrolyte degradation [28; 29; 30] on the working electrode. Once formed, the SEI allows the reversible intercalation/de-intercalation of the Na\({}^{+}\) cations [18]. The reversibility of the sodium intercalation process in the HC-working electrode is further highlighted by composite peaks that appear constant when cycling at about 0.5-0.6 V in the cathode scans, whereas a symmetric feature around 0.6-0.7 V is observed in the anode scans. No feature re  
Figure 3: Current-voltage profiles obtained by anodic CV measurements run on Na/0.9 EMITFSI-0.1 NaClO\({}_{4}\)/C cells in the presence of moisture contaminations at different anodic cut-offs: 4.6 V (panel (**A**)), 4.2 V (panel (**B**)) and 4.0 V (panel (**C**)). Scan rate: 1 mV s\({}^{-1}\). Temperature: 20 \({}^{\circ}\)C. The voltage values are referred to the Na\({}^{+}\)/Na\({}^{\circ}\) redox couple.  
lated to the IL electrolyte reduction is observed up to 0.01 V, clearly showing the possibility of reversibly intercalating sodium cations. This avoids irreversible degradation phenomena of the electrolyte components, indicating how the FSI anion can form optimal interphases even on sodium-intercalating electrodes. For instance, the ability of the FSI anion to form optimal protective films on LIB anodes, such as lithium metal, graphite and silicon, is widely reported in the literature [14]. Conversely, the electrolyte formulations containing the TFSI (Figure 4B-D,F) and IM\({}_{14}\) (Figure 4G) anions do not exhibit any significant profile attributable to the intercalation process of Na\({}^{+}\) ions within the HC electrode. The samples containing the ionic liquids EMITFSI (Figure 4B) and N\({}_{1114}\)TFSI (Figure 4F) show a clear profile around 0.5 V in the first cathode scan due to the formation of a passive film, while the subsequent anodic scan shows an even broader peak (in particular, EMITFSI) at about 1.5 V. However, this is more likely due to the oxidative processes catalysed by degradation products of the electrolyte, rather than the de-intercalation of sodium. Poor electrochemical activity is observed in the subsequent cycles. This behaviour may be due to the poor ability of the TFSI and IM\({}_{14}\) anions to form stable, passive films on anodic active materials, as previously observed in lithium batteries [14]. Overall, almost negligible cathodic currents were measured using these IL electrolytes down to 0.01 V vs. Na\({}^{+}\)/Na, proving the strong electrochemical robustness towards reduction. To summarize, the anodic and cathodic CV measurements indicate a large electrochemical stability window up to 4.5 V and down to 0.01 V vs. Na for IL-based sodium-ion electrolytes.