## 1 Introduction  
Sodium-ion batteries (NIBs) are nowadays considered to be the most promising candidate to replace Lithium-ion batteries (LIBs) in the near future, mainly due to the chemical similarity of lithium and sodium ions and the high natural abundance of sodium [1, 2]. The main limiting issue for NIBs application is related to their cathode counterpart, which underwent uncontrollable phase transitions and volume changes during repeated charge-discharge cycles, preventing short-term battery commercialization. For this reason, the main efforts of researchers have been devoted to developing high-performance cathodes [3, 4]. The most intriguing of the identified materials are the transition layered metal oxides, Na\({}_{x}\)MO\({}_{2}\) (M = Mn, Co, Ni, Fe), due to their high theoretical specific capacity [5]. This is particularly true for Na\({}_{0.67}\)MnO\({}_{2}\) (NMO) with a P2-type structure (following the current Delmas notation for this class of materials [6]) and a capacity of \(>175\) mAh/g [7]. The P2-type phase, with AA-BB octahedral layer stacking, allows the direct migration of sodium between adjacent prismatic sites, thus enabling rapid Na+ ions transport and showing better applicability with respect to the O3-type phase. In addition, NMO has low-cost constituting elements and the environmental friendliness of manganese. Unfortunately, some challenges are related to the irreversible structural distortions caused by Mn3+ (Jahn-Teller) ions, poor cycling stability due to severe volume changes and mechanical stress during intercalation/deintercalation, and an unsatisfactory energy density limited by the low redox potential of the Mn3+/Mn4+ redox couple [5]. Attempts to solve these issues include cation doping or co-doping and nano-structuring, as well as the formation of composites with graphene, strategies used for many other electrode materials [8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18]. As has been well demonstrated in the battery literature, the samples' morphology plays an important role in the electrochemical performances. It was proven that, for layered materials, the performance can be optimized by adjusting the Na/Mn ratio, the sintering temperature, and the cooling rate [13], which at the same time can be useful to control the Mn vacancies that were suggested to form on transition metal layers due to the uptake of oxygen during the cooling process, resulting in an increase in the manganese oxidation state and the formation of manganese-deficient phases [11; 19]. It was suggested that the quenching process from high temperatures could avoid the Mn vacancies, stabilizing an orthorhombic P2 form with the \(Cmcm\) space group [7; 10]. However, a high Mn3+ amount is still present in the sample, making unavoidable the multiphase transitions during cycling. A clear correlation between the cooling treatment and structure stabilization was not identified, particularly for the doped Na0,67 MnO2 samples [11; 15]. The doping was suggested to improve the structural stability during sodiation/desodiation and, indeed, the long-term cyclability. One of the most investigated doped compounds is Na0,67 MnO67 Ni0,333O2, with the substitution of 1/3 of the manganese in the original NMO with nickel: in this case, an increase in the redox potential and the improvement of cycling performance have been demonstrated [8]. Starting from this stoichiometry, other compositions were obtained by substituting manganese with variable amounts of Fe, Co, Ti, Al, Mg, and Zn or a combination of them, achieving very complex stoichiometries [9; 10; 11; 12; 13; 14; 15]. However, Ni and Co are toxic and expensive elements that should be avoided for the necessary transition toward the next generation of sustainable materials for NIBs. Concerning the other dopants, it seems that divalent cations may enhance the structural stability of the main phase by increasing the oxidation state of manganese and limiting the Jahn-Teller effect related to Mn3+ ions.