Sodium-ion batteries (SIBs) have garnered considerable attention as a viable option for large-scale energy storage,with O3-type layered transition metal oxides identified as one of the most promising cathode materials due to their superior specific capacity.However,the stability of these materials at elevated voltages remains a critical challenge,hindering their broader application.In this study,O3-NaNi_1/3Fe_1/3Mn_1/3O₂ was systematically characterized using scanning electron microscopy(SEM), transmission electron microscopy(TEM),and in-situ X-ray diffraction(XRD) to elucidate the relationship between microstructural evolution and electrochemical stability.The results reveal that phase transitions significantly impair Na? diffusion kinetics. Notably, the irreversible P3-O3' phase transition at high voltages above 4.1 V results in a reduction of the Na⁺ diffusion coefficient by at least five orders of magnitude,which is reflected by a substantial increase in internal resistance.Moreover,the O3' phase emerging during discharge triggers the formation of the P'3 phase,deviating it from the electrochemical pathway established during charging and thereby severely compromising the material’s cycling stability.