A Novel Ni-rich O3-Na[Ni0.60Fe0.25Mn0.15]O2 Cathode for Na-ion Batteries

O3-type layered oxide materials are being considered as one of the most promising cathodes for Na-ion batteries owing to their higher capacity, however, they usually suffer from structural damage at the highly desodiated state. To achieve the stable/high-capacity O3-type Na-ion cathodes, a series of Ni-rich O3–Na[NixFeyMn1-x-y]O2 (x ​= ​0.6, 0.7 and 0.8) oxide cathodes were successfully prepared and the phase transitions at high voltage were systematically investigated. Combined with the electrochemical measurements and structural characterizations, the structural transitions from O3 to O′3, P3, O3″ phases during the Na+ (de)intercalation process were demonstrated in the voltage range of 2.0–4.2 ​V. Moreover, several reasons for the high-voltage capacity decay are revealed: 1) the thermodynamic instability of high-voltage phase due to less Na+ in the crystal structure; 2) large volume change during the high-voltage phase evolution with inferior Na+ diffusion kinetics; 3) formation of microcracks and cathode-electrolyte interphase on the surface of cathode particles. To address the above issues, a reasonable upper cut-off voltage of 4.0 ​V was set to prevent the formation of O3″ phase and reduce electrolyte decomposition, which leads to a high reversible capacity of ~152 mAh g−1 (~467 ​Wh kg−1) with a superior capacity retention of ~84% after 200 cycles at 0.5C, showing great Na storage performance. This work provides insights on the relationship of the structure-property for the further development of high-performance Ni-rich O3-type Na-ion cathodes.