Properties of Rich-Na_e Effect and Zero-Phase Transition in P2–Na_0.67(Ni_0.1Mn_0.8Fe_0.1)_1–<i>x</i>Mg_<i>x</i>O₂ Cathodes for Rapid and Stable Sodium Storage

Mn-rich ternary cathodes are highly regarded as a potential option for sodium-ion batteries (SIBs) due to their low cost and high theoretical capacity. Nonetheless, cycling stability was hindered by the occurrence of high-voltage phase transitions. In this work, Na0.67(Ni0.1Mn0.8Fe0.1)1–xMgxO2 (NaNMF-Mgx) cathode materials with high-voltage zero-phase transitions property were successfully synthesized. Amazingly, it was found that there was a valuable strengthening in the occupancy of stabler Nae sites by employing the Mg-doping strategy; and the calculation highlighted excellent structural stability and conductivity of NaNMF-Mg0.04, which has the lowest thermodynamic formation energy and a narrow band gap. The combination of theory and experiment demonstrated the underlying mechanisms of Mg substitution. Especially, Mg doping had the potential to regulate Nae/Naf ratio, and the ratio of NaNMF-Mg0.04 reached the maximum, indicating its most remarkable “rich-Nae” effect. Moreover, ex-situ XRD and ADF-STEM certified that NaNMF-Mg0.04 cathode maintained an intact P2 phase structure during high-voltage charging process. The “rich-Nae” and “zero-phase transitions” effects enabled NaNMF-Mg0.04 cathode to express remarkable initial capacitance (119.5 mAh g–1, 0.1 C), stability (80.0% over 200 cycles), and energy density (356.5 Wh kg–1). This unique mechanism provided fresh insights into revisiting the relationship between structure and performance and might open up a new idea for designing novel Mn-rich ternary cathodes with zero-phase transitions property in the future.