## 4 Discussion  
The Na\({}_{0.67}\)MnO\({}_{2}\) cathode material is currently attracting attention for its application in SIBs, mainly due to the high capacity and structural stability of the P2 phase, the main stable polymorph for this sodium composition. The wide range of possible cationic substitutions further allows for improving its functional properties. However, to develop electrochemically performing manganese-based P2 cathode materials, the elemental substitution necessitates the control of the Mn oxidation state. We achieved this goal by the combined use of structural and spectroscopic techniques. Cu- and Fe-doped Na\({}_{0.67}\)MnO\({}_{2}\) samples were successfully synthesized as mixtures of phases [31,32] due to the low temperature of thermal treatment, 800 \({}^{\circ}\)C, that was intentionally chosen to verify if the stabilization of the polymorph mixtures could allow to reach higher capacities or achieve better cycling stability, thanks to the favorable cushioning of structural transitions [7,11,36]. As previously demonstrated by XRPD, P2 and P2 were the main phases for all the samples, apart from NMO-Q. This is in line with the literature findings on Na\({}_{0.67}\)Mn\({}_{1.x}\)Mg\({}_{x}\)O\({}_{2}\)[11], in which the quenching process favors, as in our case, \(Cmcm\) stabilization for undoped and slightly doped samples, apart from x = 0.2, the same doping level used in the present work, for which the P2 phase is instead stabilized. In our case, in fact, the doping always allowed the stabilization of the P2 hexagonal polymorph as the main phase, independently of the cooling treatment, even if the amount of the P'2 phase increased in the quenched samples. This observation, based on the XRPD evidence (Table 1), was also consistent with the EPR results. In fact, the P'2 phase is stabilized when a low number of Mn vacancies is formed,  
Figure 9: Discharge capacity values for slowly cooled (**A**) and quenched (**B**) samples. with a consequent reduction in the amount of Mn\({}^{4+}\) ions, which are, therefore, preferentially surrounded by Mn\({}^{3+}\) ions, thereby giving rise to broad EPR lines with g-values that are generally far from the ideal 1.996 value, which is typical of samples with Mn\({}^{4+}\) as the unique magnetic ion. For slowly cooled samples, the EPR spectra of the Cu-doped samples showed a lower intensity with respect to pure and Fe-doped samples. This evidence suggests a complex behavior that cannot be explained only on the basis of the possible variation of manganese oxidation states as a consequence of the dopant introduction or quenching treatment [11]. O-type polymorphs were also present (Table 1), apart from the Cu02 sample, which only contained P-type polymorphs. The intergrowth of the P- and O-type phases is not new for layered cathodes: similar composites were obtained experimentally but were also simulated by structure modeling to calculate the diffraction patterns, to be compared with the complex experimental patterns [27; 31; 32; 37]. For slowly cooled samples, the O polymorphs' amount is similar for NMO and Fe02, while the presence of Cu ions suppresses the gliding of planes, avoiding the segregation of O-type phases. For the quenched samples, the O-type phases are present in all the samples in a higher amount for NMO-Q, demonstrating that the cooling treatment is a prevailing factor with respect to doping. We demonstrated that iron and copper can easily substitute for manganese on the octahedral sites of the layers, as suggested by the absence of phases only containing the dopant ions and by the changes in lattice parameters with respect to undoped NMO, which was particularly evident in the quenched samples. The Mossbauer data suggested that iron ions were present in both the P-type polymorphs, with a preference for the more regular octahedral sites of the P2 structure. The distribution of the dopants in both the structures can be predicted by the unit cell volume of P2 and P'2: after doping, they are about 2-3% and 4-6% higher with respect to the undoped sample.