Electrochemical Properties and Structure of Single Crystal O3‐NaNi _1/3 Fe _1/3 Mn _1/3 O ₂ at Different Cut‐Off Voltages

Understanding the sodium storage mechanism of single‐crystal O3‐NaNi 1/3 Fe 1/3 Mn 1/3 O 2 (SC‐NFM) in different voltage ranges is essential for sodium‐ion batteries. We systematically investigate the impact of different charge voltages on its electrochemical performance and structural evolution. Electrochemical tests reveal a slow capacity decay at 3.8V, with capacity retention reaching 93.5% after 200 cycles (1 C). In contrast, when the charge voltage is at 4.2 V, the retention drops sharply to 20.17%. A similar trend is observed in the SC‐NFM|HC full cell. Morphological characterization indicates that intergranular microcracks in cycled particles initiate and propagate with increasing voltage. Ex situ X‐ray diffraction results indicate a complex phase transition process (O3–P3–OP2–O3) during a 2–4.2 V voltage range. The formation of the detrimental OP2 phase intensifies structural deformation and impedes Na + intercalation, thereby accelerating capacity fade. As a result, more pronounced structural degradation and side reactions are observed at 4.2 V. The d Q /d V analysis further validates the occurrence of enhanced irreversible phase transitions and increased voltage polarization at higher voltages and over extended cycling. This study elucidates how different voltages accelerate structural degradation and performance deterioration of SC‐NFM, providing crucial insights for optimization of cycling and voltage management in sodium‐ion batteries.