O3-NaFe1/2Co1/2O2 shows initial capacity of 160 mAh/g and an average operating voltage of 3.1 V (vs. Na) with good cyclability, and is a promising candidate of the cathode materials for sodium-ion secondary batteries (SIBs). Here, we found that the cyclability of the slowly-cooled sample is much worse than that of quenched one, even though the former sample keeps the O3-type structure. The energy dispersive X-ray spectroscopy (EDX) images suggest that the slow-cooled sample (Nax Fe1-yCoy O2) is inhomogeneous in the Fe concentration (1 - y), perhaps triggered by the Na deficiency (1 - x). We ascribed the poor cyclability in the slowly-cooled sample to the concentration inhomogeneity (Δy). The Δy is further responsible for the fluctuation of the lattice constants (a and c), as revealed by the Williamson-Hall plot.
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