Na0.67MnO2 (NMO) stands out among the layered cathode materials used for sodium batteries due to its high capacity values, low cost, and environmental compatibility. Unfortunately, many issues arise during cycling, but customizing the nanostructure and doping can help mitigate them. Our goal was to synthesize undoped and Cu- or Fe-doped NMO samples via the sol-gel route, with different cooling phases to room temperature, either naturally or by temperature control. The formation of a polymorph mixture and differences in the external morphology of the powder grains were observed.

Using spectroscopic techniques, Mössbauer spectroscopy for Fe-doped samples, and electron paramagnetic resonance, we obtained information on the oxidation states of transition metals and provided insights into magnetic ordering and the possible presence of magnetic impurities. The results from cyclic voltammetry and galvanostatic cycling were interpreted based on the spectroscopic data: the introduction of substituents generally worsens capacity values due to the decrease in the amount of P2 and the introduction of structural distortions. The structural stability of the samples in air over time was also analyzed via X-ray diffraction, demonstrating the positive effect of Cu presence.