Dopant-Free Synergy of Fe Gradient and Na Vacancies for Boosted Na⁺ Transport in O3-Type Cathodes

Abstract

O3-type NaNi1/3Fe1/3Mn1/3O2 (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 Na0.95Ni0.33Fe0.28Mn0.39O2 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.