Interfacial Evaluation in ZnO‐Coated Na x Mn 0.5 Fe 0.5 O ₂ Cathodes and Hard Carbon Anodes Induced by Sodium Azide: Operando EIS and Structural Insights

This article explores the synthesis and electrochemical properties of Na x Mn 0.5 Fe 0.5 O 2 powders, prepared via a conventional solid‐state reaction. Subsequently, the powders are functionalized with a ZnO protective coating through a wet‐chemical approach employing zinc acetate in ethanol. Structural characterization confirmed that the ZnO layer maintained the P2‐type ( P6 3 /mmc ) structure, while energy‐dispersive X‐ray spectrometry mapping verified the successful coating. Electrochemical analyses, including electrochemical impedance spectroscopy (EIS) and cyclic voltammetry, revealed that although the redox reaction mechanism remained unchanged, the charge–transfer resistance ( R ct ) depended on the coating thickness. ZnO‐coated NMFO electrodes exhibited initial discharge capacities of 159.3, 153.6, and 124.8 mAh g − 1 with respective capacity retentions of 48.9%, 41.9%, and 52.0% after 100 cycles for ZnO contents of 0.2, 0.4, and 0.6 wt.%. The galvanostatic intermittent titration technique results indicated that the diffusion coefficients varied with the coating conditions. Operando EIS from 1.5 to 4.3 V showed stable bulk resistance ( R b ) but voltage‐dependent variations in solid electrolyte interface resistance ( R SEI ) and R ct . Additionally, sodium azide is used to presodiate the hard carbon (HC) anode to enhance the full‐cell performance. The ZnO‐coated cathode paired with NaN 3 presodiated HC delivered a capacity of over 120 mAh g − 1 at C/10. Ex situ analysis after 500 cycles confirmed structural stability, demonstrating that ZnO coating and NaN 3 presodiation collectively improve sodium‐ion battery performance.