Properties\nof Rich-Na<sub>e</sub> Effect and Zero-Phase\nTransition in P2–Na<sub>0.67</sub>(Ni<sub>0.1</sub>Mn<sub>0.8</sub>Fe<sub>0.1</sub>)<sub>1–<i>x</i></sub>Mg&

Mn-rich ternary cathodes are highly regarded as a potential\noption\nfor sodium-ion batteries (SIBs) due to their low cost and high theoretical\ncapacity. Nonetheless, cycling stability was hindered by the occurrence\nof high-voltage phase transitions. In this work, Na<sub>0.67</sub>(Ni<sub>0.1</sub>Mn<sub>0.8</sub>Fe<sub>0.1</sub>)<sub>1–<i>x</i></sub>Mg<sub><i>x</i></sub>O<sub>2</sub> (NaNMF-Mg<i>x</i>) cathode materials with high-voltage zero-phase transitions\nproperty were successfully synthesized. Amazingly, it was found that\nthere was a valuable strengthening in the occupancy of stabler Na<sub>e</sub> sites by employing the Mg-doping strategy; and the calculation\nhighlighted excellent structural stability and conductivity of NaNMF-Mg0.04,\nwhich has the lowest thermodynamic formation energy and a narrow band\ngap. The combination of theory and experiment demonstrated the underlying\nmechanisms of Mg substitution. Especially, Mg doping had the potential\nto regulate Na<sub>e</sub>/Na<sub>f</sub> ratio, and the ratio of\nNaNMF-Mg0.04 reached the maximum, indicating its most remarkable\n“rich-Na<sub>e</sub>” effect. Moreover, ex-situ XRD\nand ADF-STEM certified that NaNMF-Mg0.04 cathode maintained an intact\nP2 phase structure during high-voltage charging process. The “rich-Na<sub>e</sub>” and “zero-phase transitions” effects\nenabled NaNMF-Mg0.04 cathode to express remarkable initial capacitance\n(119.5 mAh g<sup>–1</sup>, 0.1 C), stability (80.0% over 200\ncycles), and energy density (356.5 Wh kg<sup>–1</sup>). This\nunique mechanism provided fresh insights into revisiting the relationship\nbetween structure and performance and might open up a new idea for\ndesigning novel Mn-rich ternary cathodes with zero-phase transitions\nproperty in the future.