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    Interfacial Evaluation in ZnO-Coated NaxMn0.5Fe0.5O2 Cathodes and Hard Carbon Anodes Induced by Sodium Azide: Operando EIS and Structural Insights
    (Wiley-V C H Verlag Gmbh, 2025) Whba, Rawdah; Dogan, Ebru; Duygulu, Ozgur; Alanazi, Abdullah K.; Arshad, Muhammad; Stoyanova, Radostina; Koleva, Violeta
    This article explores the synthesis and electrochemical properties of NaxMn0.5Fe0.5O2 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 NaN3 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 NaN3 presodiation collectively improve sodium-ion battery performance.
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    Optimized performance of Na0.67Mn0.5Fe0.5O2@TiO2 and presodiated hard carbon (Pre-SHC) full-cells using direct contact method
    (Elsevier, 2025) Dogan, Ebru; Whba, Rawdah; Altin, Emine; Moeez, Iqra; Chung, Kyung Yoon; Stoyanova, Radostina; Koleva, Violeta
    We report the synthesis and electrochemical performance of an optimized core-shell structure composed of P2type Na0.67Mn0.5Fe0.5O2 coated with TiO2. The structural properties are characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM), which confirm the successful formation of the core-shell structure. Electrochemical performance is evaluated through tests on halfcells and full-cells. Na0.67Mn0.5Fe0.5O2@TiO2 as cathode and sodium metal as anode are used in half-cells, while in full-cells, presodiated hard carbon (Pre-SHC) anodes are prepared via a direct-contact method. Cyclic voltammetry (CV) tests show similar redox behavior for uncoated and TiO2-coated Na0.67Mn0.5Fe0.5O2. Galvanostatic cycling tests are performed using two different voltage windows of 1.5-3.5 V and 1.5-4.3 V and capacity retention values are compared. Performance analysis of the full-cells reveals the best conditions for the presodiation process for the hard carbon (HC) anode. The first charge and discharge capacity values are used to determine the optimized presodiation conditions. Long-term cycling tests for both uncoated and TiO2-coated Na0.67Mn0.5Fe0.5O2 cathodes show significantly improved capacity retention and stability for the Na0.67Mn.0.5Fe0.5O2 @TiO2 cathode over 500 cycles at 0.5 and 1.0C rates. This study highlights the effectiveness of the TiO2 coating in enhancing the electrochemical performance and stability of Na0.67Mn0.5Fe0.5O2 cathode material.