Insights Into Mg 2+ ‐Driven Lattice Stabilization and Na + Kinetics Regulation in O3‐Type NaMn 0.5 Fe 0.5 O 2 Cathodes for Sodium‐Ion Batteries

Sodium‐ion batteries (SIBs) are emerging as cost‐effective alternatives to lithium‐ion systems owing to the natural abundance of sodium. Among layered cathodes, O3‐type NaFe 0.5 Mn 0.5 O 2 exhibits high theoretical capacity and environmental compatibility but suffers from structural degradation driven by O3→P3 transitions, volume variation, and transition‐metal migration during cycling. Herein, Mg 2+ is incorporated to regulate the lattice stability and electrochemical performance of NaFe 0.5 Mn 0.5 O 2 via grain boundaries/interface difustion at solid state. The sample Mg4‐NFM was synthesized via a solid‐state route and systematically characterized. The x‐ray diffraction combined with Williamson–Hall analysis reveals Mg‐induced lattice modulation, micro‐strain regulation, and crystallite‐size variation, while SEM, FTIR, Raman spectroscopy, and BET measurements provide complementary insights into morphology, bonding, vibrational characteristics, and surface properties. Electrochemical evaluations demonstrate that the 0.48 At% Mg atoms in the hybrid sample deliver the optimal performance, achieving an initial discharge capacity of 83.3 mAh g − 1 with excellent capacity retention over 100 cycles at 0.1C and Coulombic efficiency exceeding 98.5%. Density functional theory calculations reveal that Mg incorporation enhances thermodynamic stability and increases the density of states near the Fermi level, indicating improved electronic conductivity. Furthermore, nudged elastic band calculations identify an ultralow Na + diffusion barrier of 0.113 eV, confirming favorable ion‐transport kinetics.