Correlation between Fe/Mn/Ni Ratio and Structural Properties in P2-Na2/3Mn2/3Fe1/3– x Ni x O2 ( x = 0, 0.02, 0.05, 0.1, and 1/3)

P2-type layered oxides, particularly those based on Mn/Fe/Ni, are promising cathode materials for sodium-ion batteries but have structural instability caused by phase transitions during sodiation/desodiation. We show that the phase behavior of the P2-type material Na0.66Mn2/3Fe1/3–xNix series (x = 0, 0.02, 0.05, and 1/3) is strongly influenced by the Fe/Mn/Ni cationic ratio, which is directly coupled to the Mn3+/Mn4+ ratio. Powder X-ray diffraction reveals a systematic variation in the lattice parameter as a function of cation distribution, reflecting the corresponding changes in cation radii. Furthermore, the two binary systems exhibit different structural behaviors, with the Ni/Mn system forming a superstructure, whereas the Fe/Mn system does not exhibit such a phenomenon. The progressive narrowing of the linewidth determined by electron paramagnetic resonance indicates that the proportion of Mn3+ cations decreases as Ni2+ substitution in the P2 lattice increases. In situ XRD between 1.5 and 4.2 V vs Na/Na+ shows that, in the absence or at low amounts of Ni2+ (x < 0.05), the P2 phase coexists with an undesired, Jahn–Teller-distorted orthorhombic P′2 phase. Notably, no high-voltage structural transformations to O2 or OP4/Z phases are observed above 4.0 V. At x ≥ 0.05, the Mn3+ fraction is sufficiently reduced to suppress the formation of the P′2 phase in the low-voltage regime during discharge. These findings underscore the structural stabilization of Fe/Mn-based P2-type layered oxides achieved through minimal Ni substitution.