Revealing the role of the amorphous phase in Na_0.74CoO₂/C/N composite cathode

To optimize the Na_0·74CoO₂-based cathode materials for sodium-ion batteries, we fabricated a series of Na_0·74CoO₂/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 Na_0·74CoO₂/C/N composite cathode.