## 4 Discussion
For the main P2 phase, both the \(a\) and \(c\) crystallographic axes increased (Table 1). The enlargement of the \(c\)-axis could, in principle, be useful for an easy Na\({}^{+}\) insertion/extraction during cell functioning, but, at the same time, it could favor the instability of the electrode structure [33]. The increase in the \(a\)-axis instead weakens the repulsion in the transition metal layers and, during structure changes upon cycling, particularly at high voltage, the long-range structure can change and the distortion effect will begin to have an impact on the structural reversibility, which can lead to poor capacity retention and rate performance. This scenario seems to suggest that, from the structural point of view, the dopant ions could have an unfavorable influence on the electrochemical properties. The lattice parameter variations can be attributed to the differences in the ionic radii of the stabilized species in octahedral coordination, Mn\({}^{3+}\)/Mn\({}^{4+}\) (suggested by the EPR spectra analysis), and Cu\({}^{2+}\) and Fe\({}^{3+}\) ions (as determined by Mossbauer spectroscopy) [38]. This is particularly true for the quenched samples, where no Mn vacancies should be present. The crystallite sizes of the P2 phase were about twice those of the P'2 phase, for all the synthesized samples, independently of the cooling treatment. This is in line with the higher structural order degree of the P2 polymorph with respect to P'2, as is consistent with the line width of the relative EPR contributions. In particular, the copper ions heavily affected the crystallite size values, which are bigger than those of the pure and iron-doped samples for both the polymorphs (Table 1), as well as the external morphology of the particles, with micron-sized dimensions and without clear platelet particles. The morphology of the samples also changed, passing from the slowly cooled to the quenched samples: the rapid cooling maintained the grains' aggregation, which could be the basis of the worsening of electrochemical performances. It is well known that the downsizing of the grains, decreasing the diffusion paths, favors the Na\({}^{+}\) migration. In particular, the unfavorable morphology could justify the worst performances of the Cu-doped slowly cooled and quenched samples, (Figure 9). The marked influence of the dopant ions was evident in terms of the structural stability as a function of time. The undoped sample had good stability for at least 7-14 days. This is due to the intrinsic characteristics of the P2 polymorph, considered one of the most stable between the P- and O-type layered phases, but this is also valid for \(Cmcm\), the main phase of the NMO-Q sample. The introduction of iron does not markedly change the behavior of NMO: only in the case of Fe02-Q, after 30 days, high degradation was evident, with a concomitant loss of crystallinity of the main P2 phase. The copper substitution, instead, helped to improve the stability because the formed phases remained practically unchanged for up to 30 days. This evidence is in agreement with the literature findings, suggesting copper substitution as a way to enhance the stability of layered electrode materials [14]. We could try to relate the electrochemical performances (Figures 8, 9 and S5) to the structural and morphological peculiarities of the samples. The CV curves were complex, with multiple events, due to the presence of complex mixtures of phases that were electrochemically active. The NMO sample, with the highest amount of P2 phase, had a well-defined CV, in which the redox peak of Mn\({}^{3+}\)/Mn\({}^{4+}\) couple and the peak at about 4.39 V, due to P2-O2 transformation, can be recognized. However, the peak intensities decreased, and the positions shifted to higher potential in the anodic scan, passing from the first to the fifth cycle. These effects could be related to the marked structural changes occurring during cycling. The introduction of dopants limited the intensity decrease during cycling: both the doped samples have better reversibility.