Yazar "Koleva, Violeta" seçeneğine göre listele

Listeleniyor 1 - 3 / 3
  • Küçük Resim Yok
    Öğe
    High-Performance Full Sodium Cells Based on MgO-Treated P2-Type Na0.67(Mn0.5Fe0.5)1-xCoxO2 Cathodes
    (Mdpi, 2023) Taskiran, Nermin; Altundag, Sebahat; Koleva, Violeta; Altin, Emine; Arshad, Muhammad; Avci, Sevda; Ates, Mehmet Nurullah
    Herein, we design a cathode material based on layered Na-2/3(Mn1/2Fe1/2)O-2 for practical application by combining the Co substitution and MgO treatment strategies. The oxides are prepared via solid-state reactions at 900 degrees C. The structure, morphology, and oxidation state of transition metal ions for Co-substituted and MgO-treated oxides are carefully examined via X-ray diffraction, IR and Raman spectroscopies, FESEM with EDX, specific surface area measurement, and XPS spectroscopy. The ability of oxides to store sodium reversibly is analyzed within a temperature range of 10 to 50 degrees C via CV experiments, galvanostatic measurements, and EIS, using half and full sodium ion cells. The changes in the local structure and oxidation state of transition metal ions during Na+ intercalation are monitored via operando XAS experiments. It is found that the Co substituents have a positive impact on the rate capability of layered oxides, while Mg additives lead to a strong increase in the capacity and an enhancement of the cycling stability. Thus, the highest capacity is obtained for 2 at.%-MgO-treated Na-2/3(Mn1/2Fe1/2)(0.9)Co0.1O2 (175 mAh/g, with a capacity fade of 28% after 100 cycles). In comparison with Co substituents, the Mg treatment has a crucial role in the improvement of the lattice stability during the cycling process. The best electrode materials, with a chemical formula of 2 at.%-MgO treated Na-2/3(Mn1/2Fe1/2)(0.9)Co0.1O2, were also used for the full cells design, with hard carbon as an anode. In the voltage window of 2-4 V, the capacity of the cells was obtained as 78 mAh/g and 51 mAh/g for applied current densities of 12 mA/g and 60 mA/g, respectively.
  • Küçük Resim Yok
    Öğe
    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.
  • Küçük Resim Yok
    Öğe
    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.