In this study, we investigated the effect of excess sodium (Na) in a NaMnO2 structure using one-step heat treatment at 900 °C followed by quenching in liquid nitrogen (N₂). According to the X-ray diffraction (XRD) analysis, there was a competition between the monoclinic and orthorhombic phases, and we found that there were two monoclinic phases with similar structural properties. Therefore, we focused on revealing the formation of two isostructures of the monoclinic phase triggered by Na ions. We found that the lattice parameters and β angle changed from 113° to 105° in the samples with increasing Na content. Structural analysis of the powders using the XRD data was conducted using Rietveld refinement, and the phase ratios for all samples were calculated. The sample with x = 1.3 showed a 95% α-phase. To understand the formation of the two isostructures, we performed Density functional theory (DFT) calculations to examine their band structure, stability, and formation energy. A structural analysis of the excess Na-doped samples was performed using common techniques, and it was found that excess Na caused the formation of a coating on the grains in the form of sodium oxide. To validate this prediction, we conducted inductively coupled plasma mass spectrometry (ICP-MS), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy coupled with energy dispersive X-ray (SEM–EDX) analyses using the basic properties of these techniques and their interactions with materials. In the second part of the study, we produced HC from locally sourced olive leaves and investigated their structural properties. The electrochemical properties of the electrode materials were examined using a half-cell configuration as electrodes with Na metal and a full-cell configuration using x = 1.3 cathode and HC anode. A direct-contact pre-sodiation strategy was used as the anode in the full-cell measurements. It was found that the full cells had initial capacity values of 150 mAh/g for the voltage range 1.5–4.3 V and 120 mAh/g for the voltage range 1.5–3.5 V.

Sodium-induced phase shift in α-NaMnO2 and electrochemical properties of the full cells using hard carbon anodes derived from regional olive leaves
    Abstract
    1 Introduction
    2 Experimental section
    3 Results and discussion
        3.1 Structural properties
        3.2 Electrochemical properties
            3.2.1 Half cells measurements
            3.2.2 Full-cell test results
    4 Conclusion
    Acknowledgements 
    References

• Ebru Dogan(Physics Department, Inonu University, Malatya 44280, Turkey)
• Serdar Altin(Physics Department, Inonu University, Malatya 44280, Turkey) Corresponding author
• Sibel Ozcan(Physics Department, Aksaray University, Aksaray 68100, Turkey)
• Canan Aksu Canbay(Physics Department, Fırat University, Elazig 23200, Turkey)
• Mesut Karta(Department of Metallurgy‑Iskenderun Vocational School of Higher Education, Iskenderun Technical University, Hatay, Turkey)
• Tolga Depci(Department of Petroleum and Natural Gas Engineering, Iskenderun Technical University, Hatay, Turkey)