LAUSR

Abstract To optimize the Na0·74CoO2-based cathode materials for sodium-ion batteries, we fabricated a series of Na0·74CoO2/C/N-based composites by sintering the mixtures of ZIF-4(Co) (Co [H2C3N2]2) and Na2CO3, which possess various molar Na/Co ratios (X = 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2 and 2.0), at 800 °C for 6 h. By performing the electrochemical characterizations, we found that the composite with X = 0.7 (NC-0.7) exhibits the optimum sodium storage performance, i.e., excellent Na+ storage capacity (107.9 mA h g−1 at 0.1 C), and high cycling stability (74% of the discharge capacity retention after 500 cycles at 10 C). In addition, for this NC-0.7 composite, we discovered a large difference (48.1 mA h g−1) between the charging (59.8 mA h g−1) and discharging (107.9 mA h g−1) capacities for the first cycle. Using the X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and scanning transmission electron microscope, we clarified the atomic-scale origin of the large capacity difference, and revealed the role of the amorphous phase in the as-prepared Na0·74CoO2/C/N composite cathode.