## 3 Results  
### Structural and Morphological Results  
In Figure 1, the XRPD patterns of undoped and doped samples obtained from slow cooling (A) and quenching (B) are reported. The patterns of slowly cooled samples are similar: for all of them, the main peaks can be assigned to the P2 phase (Figure 1A, stars) as expected, due to their stoichiometry [11; 13]. However, after a more careful observation, the peaks of all the patterns appear asymmetric or broadened: this is compatible with the stabilization of a distorted form of P2 polymorph with monoclinic symmetry, which will be indicated as P'2, together with the main P2 phase. Low peaks, particularly evident in the 30-45\({}^{\circ}\) 20 range, can be detected, probably due to secondary phases, apart from Cu02. The quenching process introduced visible differences in the patterns, particularly for the NMO-Q sample (Figure 1B): a higher number of peaks is evident with respect to NMO, while the quenched doped samples more closely resemble their slowly cooled analogs [11]. The effect of the cooling step on the phase's stabilization has also been studied in the literature. It was reported that slow cooling preserves the P2 phase (with manganese vacancies on the transition metal octahedral layers, due to oxygen uptake), while quenching avoids the vacancy formation and allows the stabilization of the orthorhombic _Cmcm_ phase [7; 11; 26]. The type and amount of the stabilized phases, as well as their main structural parameters, were determined by Rietveld structural refinement of all the patterns, using the TOPAS 3.0 software [22]. The results are reported in Table 1; as an example, in Figure S1 in the Supplementary Materials, the graphical comparisons between the experimental and calculated patterns of Cu02 and Cu02-Q samples are shown. A good agreement was found, as evident from the difference curve, suggesting reliable results; this was also demonstrated by the agreement indices of all the refinements (Table 1). In the case of the main peak at about 16\({}^{\circ}\), suffering the preferred orientation effect, a small residual is detectable (Figure S1). Figure 1: XRPD patterns of the samples obtained from the slow cooling (**A**) and quenching (**B**) processes. The stars mark the main reflections of P2 polymorph (valid for both (**A**,**B**) panels). The P2 and monoclinic P'2 polymorphs [11] represent the main phases for all the samples, apart from NMO-Q, which instead contains the _Cmcm_ polymorph as the main phase. The weight percentage ratio of the main phases P2 (or _Cmcm_) and P'2 (see Table 1) decreases, passing from NMO to the doped samples, suggesting, for these last ones, the presence of a higher amount of P'2 phase. For the quenched samples the ratios further decrease with respect to the slowly cooled samples but maintain the same trend [7; 11]. Apart from the Cu02 sample, constituted by only the P2 and P'2 phases, for all the others, variable amounts of O-type phases were detected, which mainly explains the peaks in the 30-45\({}^{\circ}\) angular range (Figure 1). For the slowly cooled samples, the total percentage of O-type polymorphs was low, while it increased in the Fe02-Q and increased markedly in the NMO-Q samples. Differences in the lattice parameters of the P2 phase and, therefore, in the cell volumes were found between the NMO and doped samples: the volume was slightly higher for the Fe-doped samples, for both slowly cooled and quenched samples. In particular, the \(c\) parameter increased, passing from the NMO to Cu- and Fe-substituted samples, with a marked increase for Fe02-Q. The crystallite sizes of P2 and P'2 were similar, independently of the doping, while those of P'2 were smaller than those of P2. The Cu-doped samples instead had larger crystallite sizes for both polymorphs. One aspect that could affect the storage capacity of electrode materials is the structural stability in air.