Tuning Orbital Hybridization and Suppressing Jahn–Teller Effect in P2–Na _0.67 Fe _<i>x</i> Mn _{1– <i>x</i>} O ₂ via Fe/Mn Ratio for High-Performance Sodium-Ion Batteries

P2-type Fe/Mn-based layered oxides are promising cathode materials for high-capacity sodium-ion batteries (SIBs), yet their performance is plagued by uncontrolled Jahn–Teller distortion and irreversible phase transitions at high voltages. This study focuses on low-cost P2- Na0.67FexMn1–xO2 (x = 0.1–0.5) layered cathodes. By precisely controlling the Fe/Mn ratio, we elucidate synergistic mechanisms among crystal structure, electronic properties, and electrochemical performance using combined in/ex situ techniques including X-ray diffraction, X-ray absorption spectroscopy, and density functional theory calculations. Key revelations include the following: (i) increased Fe content expands the c-axis lattice, widening Na+ diffusion channels and reducing charge-transfer resistance; (ii) higher Fe/Mn ratios elevate Mn valence (Mn3+ → Mn4+), suppressing Jahn–Teller distortion, while enhanced Fe 3d–O 2p orbital hybridization narrows the bandgap to 0.299 eV, boosting electronic conductivity; and (iii) the optimal NFM-55 (x = 0.5) cathode delivers a high reversible capacity of 181.6 mAh g–1 (1.5–4.2 V) with superior cycling stability. This enhancement originates from reversible P2-Z phase transitions and voltage-dependent charge compensation. This work demonstrates that Fe/Mn ratio control synchronously optimizes orbital hybridization and structural stability, offering perspectives for designing high-performance SIB cathodes.