## 3 Results and Discussion
### Preliminary Test in Na Half Cells  
The EMIFSI and N\({}_{1114}\)FSI electrolytes both show an electrochemical stability window extending from 0.1 to 4.5 V vs. Na\({}^{+}\)/Na\({}^{\circ}\), which is highly appealing for NIB systems. These electrolytes are also able to support the reversible intercalation of sodium ions in HC electrodes. Thanks to these features, both electrolyte formulations were selected for preliminary tests in Na/HC and Na/NMO half cells. The NaTFSI salt was preferable to NaClO\({}_{4}\) in these tests in light of the unrealistic use of perchlorate-based solution in commercial electrolytes. In Figure 5, the CV test results of Na/HC (Figure 5A) and Na/NMO (Figure 5B) cells are shown. A large irreversible feature around 0.6 V vs. Na\({}^{+}\)/Na\({}^{\circ}\) is observed in the first cathodic scan of the hard carbon electrodes (Figure 5A) due to the SEI formation. HC electrodes exhibit a two-step storage mechanism: the first one is related to the sodium-ion intercalation into graphene layers above 200 mV (vs. Na\({}^{+}\)/Na\({}^{\circ}\)) and the second one is associated with the sodium insertion into micropores formed by the pseudo-graphitic domains with an oxidation state close to that of Na metal, which is detected below 200 mV [31; 32; 33; 34]. The reversibility of the Na\({}^{+}\) intercalation process is highlighted by the presence of two well-evidenced features around 0.15 and 0.7 V in the following anodic scan for both electrolytes. The first CV profile of NMO cathodes (displayed in Figure 5B) shows several features in both the anodic and cathodic scans, which is in line with the expected multi-stage de-sodiation/sodiation mechanism for this material [19; 35]. As already described in the literature [35], the monoclinic \(\alpha\)-NaMnO\({}_{2}\) shows eight different oxidation peaks during the de-sodiation process, which are located at 2.59 V, 2.63 V, 2.73 V, 2.80 V, 2.97 V, 3.14 V, 3.48 V and 3.59 V. Whereas, the sedation process involves five reduction peaks centered at 2.47 V, 2.62 V, 2.87 V, 3.08 V and 3.45 V. Nevertheless, the voltammograms of Figure 5B show only five separated peaks. Figure 4: Current-voltage profiles obtained by cathodic CV measurements run on Na/HC cells containing different 0.1 NaClO\({}_{4}\)-0.9 IL electrolytes (panels from (**A**–**G**)). Scan rate: 0.05 mV s\({}^{-1}\). Temperature: 20 \({}^{\circ}\)C. The voltage values are based on the Na\({}^{+}\)/Na\({}^{\circ}\) redox couple. Figure 5: Current-voltage profile related to the 1 st CV cycle of Na/HC (panel (**A**)) and Na/NMO (panel (**B**)) cells containing the 0.1 NaTFSI-0.9 EMIFSI (solid traces) and 0.1 NaTFSI-0.9N\({}_{1114}\)FSI (dotted traces) electrolytes. Scan rate: 0.05 mV s\({}^{-1}\). Temperature: 20 \({}^{\circ}\)C. The voltage values are referred to the Na\({}^{+}\)/Na\({}^{\circ}\) redox couple. The cells containing the N\({}_{1114}\)FSI electrolyte (dotted traces) display a larger area under the CV profiles for both HC and NMO electrodes, supporting the greater reversibility of the electrochemical intercalation processes. No appreciable decomposition process is observed, so it can be concluded that these electrolytes are stable towards the electrode chemistries. A preliminary evaluation of the electrochemical performance of HC (Figure 6A) and NMO (Figure 6B) electrodes in 0.1 NaTFSI-0.9 EMIFSI (solid traces) and 0.1 NaTFSI-0.9N\({}_{1114}\)FSI (dotted traces) is reported in Figure 6, in terms of the initial voltage-capacity profile obtained at a 0.1C current rate. According to the CV results (Figure 5), the hard carbon electrode displays a two-step sodium incorporation process [18; 31; 36]: (_i_) a reduced pseudo-plateau around 0.10-0.15 V related to the sodium adsorption into the porosity of the structurally disordered carbonaceous material and (_ii_) a sloping curve at higher voltages, which accounts for the Na\({}^{+}\) intercalation into pseudo-graphitic regions.