Dopant-free synergy of Fe gradient and Na vacancies for boosted Na⁺ transport in O3-type cathodes

O3-type NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM) is a promising sodium-ion battery cathode due to its low cost and high theoretical capacity, but its structural instability, Fe migration, and Jahn-Teller distortion lead to severe phase transitions and capacity decay. Here, we report a dopant-free radial composition and Na-deficiency co-regulation strategy, which constructs a non-typical O-phase by controlling the radial Fe gradient and Na vacancy concentration, simultaneously optimizing the electronic structure and Na⁺ transport kinetics. X-ray diffraction (XRD) and ex situ X-ray absorption spectroscopy (XAS) reveal that Na_0.95Ni_0.33Fe_0.28Mn_0.39O₂ possesses wider interlayer spacing and a more robust Ni/Mn-O framework. Density functional theory (DFT) calculations show that Fe migration induces self-adaptive modulation of the transition metal layers, strengthening Ni/Mn-O bonds and downshifting the O 2p band, suppressing lattice oxygen oxidation and achieving Ni oxidation protection and structural stabilization. The material delivers 76.3% capacity retention after 200 cycles at 1 C (2.0-4.0 V), demonstrating excellent cycling and rate performance. In situ XRD and other electrochemical analysis methods indicate enhanced Na⁺ diffusion, reduced charge-transfer resistance, and highly reversible structural evolution, effectively suppressing irreversible phase transitions and microcracks. This study demonstrates that tuning elemental distribution and Na stoichiometry can synergistically improve Na⁺ transport kinetics and structural stability, providing a novel strategy for designing high-performance sodium-ion battery cathodes.