## 3 Results
### Electrochemical Results  
The sodium storage properties of layered cathodes were evaluated in a half-cell configuration. Cyclic voltammetry (CV) can provide useful information regarding the thermodynamic and kinetics of sodium intercalation reactions, which indeed determine the shapes of the voltammograms. As an example, the voltammograms of Fe-doped samples are reported in Figure 8, while in Figure S5 in the Supplementary Materials, those of all the other samples are shown. The voltammograms appeared quite different between the samples. In all the cases, many redox peaks can be observed, which were an indication of the multiple reactions taking place during sodium intercalation/deintercalation, due to the complex phase's stabilization and to the multiple reactions of the layered polymorphs. For these materials, the redox processes were expected to be mainly due to Mn ions [9; 12; 31; 32]. The most defined phenomena can be observed for the NMO sample (Figure S5A in the Supplementary Materials). The oxidation peak at about 2.45 V was due to the Mn\({}^{3+}\)/Mn\({}^{4+}\) redox couple [33]. In the reduction step, the lower intensity of the peak suggested the partial irreversibility of the phenomenon because some Mn ions can maintain the 4+ oxidation state. The peak at 4.39 V can be ascribed to the P2-O2 phase transition, which is considered detrimental to capacity retention: by setting the cutoff voltage to 4 V, this phenomenon could be avoided at the expense of some capacity loss [13]. The multiple pairs of the small redox peaks below 3 V were attributed to the structural rearrangement of the layers, while the other small events between 3 and 4 V may be due to Na\({}^{+}\) vacancy ordering [12]. During cycling, the peak intensity decreased, suggesting a certain degree of irreversibility of the involved processes. It is also clearly evident that the oxidation peak attributed to Mn\({}^{3+}\)/Mn\({}^{4+}\) shifted to a higher potential from the first to the fifth cycle, suggesting the need for a higher driving force to extract sodium from the host, probably due to the structural changes of the layered material. The CV of the Cu02 sample (Figure S5C) was different from that of NMO, but in line with other Cu doped layered samples [34]: the main peaks located at 4.04 V/3.81 V could be attributed to the Cu\({}^{2+}\)/Cu\({}^{3+}\) redox couple [14], presenting a small polarization and a good kinetics. The Mn\({}^{3+}\)/Mn\({}^{4+}\) redox peak at about 2.45 V, clearly evident in NMO, is now very low and broad. The other redox events can again be explained by sodium vacancy ordering. Apart from the first cycle, the CV curves were over-imposable, suggesting the good reversibility of the phenomena. The peak due to P2-O2 transition is lacking, as well as in Fe02 (Figure 8), suggesting the possible dopant efficacy to improve the cyclability, due to the decrease in the P2 amount, as demonstrated via XRPD (Table 1). The CV of the Fe02 sample showed many redox events in the oxidation range of 3-4 V and, in the cathodic scan, between 2 and 3 V. These can be attributed to the iron contribution (Fe\({}^{3+}\)/Fe\({}^{4+}\)) and to the layer's rearrangement/gliding. The CV curves of the quenched samples were quite different from their slow-cooled analogs (Figures 8 and S5). The NMO-Q showed flat curves without clear evidence of redox phenomena. This may be due to the absence of the  
Figure 8: CV curves of (**A**) Fe02 and (**B**) Fe02-Q samples. P2 phase and the presence of the main \(Cmcm\) phase, which is more structurally distorted. The CV of the Cu02-Q showed narrower peaks with a more defined Mn\({}^{3+}\)/Mn\({}^{4+}\) oxidation phenomenon, without polarization, showing some similarity with Cu02. The same is true for the Fe02-Q voltammograms, which are broader than those of Fe02 but with redox events again being recognizable. The rate capability tests for slowly cooled and quenched samples are reported in Figure 9A,B, respectively.