MgF₂ surface loading stabilizes the interfacial chemical environment of O3-Type NaNi_1/3Fe_1/3Mn_1/3O₂ cathodes for sodium-ion batteries

Abstract

O3-NaNi_1/3Fe_1/3Mn_1/3O₂ (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.

Keywords

Sodium-ion batteries / NaNi_1/3Fe_1/3Mn_1/3O₂ / MgF₂ surface loading / Interfacial chemical environment

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