Tuning the Sodium Content for Highly Stable O3-Type Sodium Layered Oxides

O3-type layered oxides are promising cathodes for sodium-ion batteries (SIBs), but the influence of the sodium (Na) content on phase transitions across different voltage windows remains unclear. In this work, a series of gradient Na-content materials, Na_xLi_0.05Ni_0.45Mn_0.25Mg_0.05Ti_0.25O₂ (x = 0.70-0.95), were synthesized via solid-state reaction. In situ X-ray diffraction (XRD) and full-field transmission X-ray microscopy analyses reveal that increasing the Na content alters the discharge capacity and cycling stability. Despite similar phase evolution above 3 V, significant differences emerge below 3 V, where the persistence of the O3-phase plateau becomes a key indicator of structural integrity. In situ XRD confirms that higher Na content extends this plateau duration. These results highlight a direct correlation between the length of the low-voltage plateau and bulk structural stability, offering new guidance for designing robust cathodes via tuning of the Na content and voltage windows.