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Good effect of 3d metal coexistence is obtained for a cobalt containing material of NaFe_1/2Co_1/2O₂. O3-NaFe_1/2Co_1/2O₂ delivers a large reversible capacity of 160 mAh g⁻¹ based on Co^3+/4+ redox in a low-voltage region and Fe^3+/4+ in a high-voltage region in the voltage range of 2.5–4.0 V (Figure 1.12) (Yoshida et al. 2013). Furthermore, relatively good cycling stability of the reversible capacities and excellent rate-capability are obtained. O3-NaFe_1/2Co_1/2O₂ undergoes reversible 0.7 Na extraction from the structure, implying that iron migration is suppressed by coexistence with cobalt compared to O3-type Fe, Fe-Ti, and Fe-Ni systems, but further Na extraction leads to large irreversible capacity and capacity degradation (Kubota et al. 2016). Further improvement and compositional modification have been conducted for O3-NaFe_1/2Co_1/2O₂. For examples, O3-Na[Fe_1/3Co_1/3Ni_1/3]O₂ (Vassilaras et al. 2014) and O3-Na[Fe_1/4Co_1/4Ni_1/4Mn_1/4]O₂ (Li et al. 2014) deliver reversible capacities of ca. 165 and 180 mAh g⁻¹, respectively, which are larger than 160 mAh g⁻¹ for O3-NaFe_1/2Co_1/2O₂. Although Ti-substitution reduces the reversible capacity, O3-Na[Fe_1/4Co_1/4Ni_1/4Mn_1/8Ti_1/8]O₂ delivers a reversible capacity of ca. 130 mAh g⁻¹ in the voltage range of 2.0–4.1 V and exhibits excellent cycle stability and rate-capability (Yue et al. 2015). Optimization of the composition including the iron and cobalt contents and particle morphology is expected to further enhance the electrode performances of Na[Fe,Co,M]O₂.