## 4 Discussion
This may be due to the coexistence of a higher amount of structurally different phases. In fact, the doping and the quenching caused the increase in P'2 polymorph amount, as well as O-type phases, particularly in the quenched samples. The Fe-doped samples and, partly, the Cu samples (at least at lower potentials) had broader peaks, suggesting that the electrochemical phenomena occurred in a wide potential window, due to the increase in the amount of distorted polymorphs. These observations were in agreement with the line-broadening of EPR spectra. In the slowly cooled samples, the NMO had the highest capacity values (about 162 mAh/g); however, these rapidly decreased by increasing the C-rates, even if the capacity at 2C was again acceptable. A similar trend was verified by [PERSON] et al. [37] on layered Na\({}_{0.66}\)Li\({}_{0.18}\)Mn\({}_{0.71}\)Ni\({}_{0.21}\)Co\({}_{0.08}\)O\({}_{2+\delta}\) P2/O3 composite. The minor presence of the O3 phase, together with the main P2 phase, is responsible for the satisfactory rate capability. The O3 phase can supply more sodium ions, while the enlarged layered spacings of the P2 phase are beneficial for the easy diffusion of sodium ions upon the charging process. A good rate capability is shown by the Fe02 sample, also having some amount of O3 Na+ reservoir phase, but in this case, the enlargement of the lattice parameters of P2 phase and the stabilization of a larger amount of P'2 polymorph seem to negatively influence the electrochemical performance. Cu02 has the worst performance, although with a lower decrease in capacity during cycling at an increasing C-rate. The Cu-doped samples' poor performance may mainly be due to the kinetic limitations related to the higher diffusion paths of sodium, because of the larger particle sizes, and due to the absence of O-type phases. So the NMO, containing a higher amount of P2 with respect to Fe02 and Cu02 and with smaller lattice parameters, could better buffer the structural changes during cycling. In some papers, it has been suggested that both the Cu\({}^{2+}\) and Fe\({}^{3+}\) in the P2 phase are active redox couples providing themselves a contribution to the overall capacity [14; 21]. In fact, from the CV data of the Cu-doped samples (Figure S5), the peak at about 4 V can be attributed to the Cu\({}^{2+}\)/Cu\({}^{3+}\) redox couple [14]. Despite the possible dopant contribution to the capacity, in the slowly cooled samples, the prevailing factor on the capacity values is the P2 phase prevalence, together with a small amount of O-type phases (particularly O3). The effect of dopant, as demonstrated by the EPR measurements, is different from what was expected. In fact, particularly in the case of the Cu-doped samples, the amount of Mn\({}^{4+}\) did not increase, as demonstrated by the decrease in EPR signal intensity with respect to NMO (Figure 4), suggesting a possible decrease in oxygen absorption and, in turn, a lower number of manganese vacancies with respect to NMO. This evidence could also help to explain the poor electrochemical capacities. The performance of the quenched samples was lower with respect to the slowly cooled samples. The reason for this difference could be due to the different morphologies and to the different stabilizations of the phases. Infact, NMO-Q (Figure 2) presented particles that seemed to melt together, forming large aggregates, which was different from NMO. In addition, the main phase of this sample was the _Cmcm_ polymorph, with a more distorted structure, limiting easy Na+ diffusion. In addition, a high percentage of O-type phases was stabilized, which may have worsened the electrochemical performance. In fact, the intervention of intermediate sites in the sodium migration needs to overcome a high-energy barrier for the O3-type structure; the inevitable complex phase transition and weak kinetics performance will directly influence the electrochemical properties.