Na-ion batteries have become promising candidates for large-scale energy-storage systems because of the abundant Na resources and they have attracted considerable academic interest because of their unique behavior, such as their electrochemical activity for the Fe3+/Fe4+ redox couple. The high-rate performance derived from the low Lewis-acidity of the Na+ ions is another advantage of Na-ion batteries and has been demonstrated in NaFe1/2Co1/2O2 solutions. Here, a solid solution of NaFeO2- NaCoO2 is synthesized and the mechanisms behind their excellent electrochemical performance are studied in comparison to those of their respective endmembers. The combined analysis of operando X-ray diffraction, ex situ X-ray absorption spectroscopy, and density functional theory (DFT) calculations for Na1-xFe1/2Co1/2O2 reveals that the O3-type phase transforms into a P3-type phase coupled with Na+/vacancy ordering, which has not been observed in O3-type NaFeO2. The substitution of Co for Fe stabilizes the P3-type phase formed by sodium extraction and could suppress the irreversible structural change that is usually observed in O3-type NaFeO2, resulting in a better cycle retention and higher rate performance. Although no ordering of the transition metal ions is seen in the neutron diffraction experiments, as supported by Monte-Carlo simulations, the formation of a superlattice originating from the Na+/vacancy ordering is found by synchrotron X-ray diffraction for Na0.5Fe1/2Co1/2O2, which may involve a potential step in the charge/discharge profiles.