MgF2 surface loading stabilizes the interfacial chemical environment of O3-Type NaNi1/3Fe1/3Mn1/3O2 cathodes for sodium-ion batteries

O3-NaNi1/3Fe1/3Mn1/3O2 (NFM) has drawn considerable attention as a cathode material for sodium-ion batteries, but its practical application is restricted by interfacial parasitic reactions, continuous impedance growth, and phase-transition-associated structural instability. Here, a MgF2 surface layer is introduced onto NFM through a post-treatment surface-loading strategy to regulate the interfacial chemical environment of the cathode. The MgF2 loading preserves the O3-type bulk framework and the dominant transition-metal valence states of NFM, while generating a non-uniform Mg/F-enriched surface layer. Among the investigated samples, NFM@1%MgF2 exhibits the best overall electrochemical performance, delivering 103.01 mA h g-1 after 100 cycles at 1C with a capacity retention of 85.65%, achieving 82.09 mA h g-1 at 10C, and retaining 75.17% of its capacity after 300 cycles at 3C. Galvanostatic intermittent titration technique, electrochemical impedance spectroscopy, and post-cycling X-ray photoelectron spectroscopy collectively show that MgF2 loading enhances apparent Na+ transport kinetics, suppresses interfacial side reactions, and retards impedance growth during cycling. Operando X-ray diffraction further reveals that MgF2 does not alter the intrinsic O3-P3-O3 transition sequence of NFM but markedly mitigates the associated structural response. In addition, an NFM@1%MgF2||hard carbon full cell exhibits 85.43% capacity retention after 100 cycles at 1C. These results establish MgF2 surface loading as an effective route for interfacial stabilization in O3-type NFM cathodes and suggest a generalizable strategy for regulating interfacial chemistry in sodium-ion battery cathode materials.