The ion-exchange study by LiMn2O4 for Na-ion cathodes: an investigation of structural and electrochemical performance

The sodium manganese oxide phase was synthesized by an ion-exchange process in the glovebox using LiMn2O4 electrodes. For this process, LiMn2O4 cathodes were discharged at specific voltage values that correspond to the redox reaction values in cycling voltammetry measurements and then the cell was disassembled, and the cathode was used for Na-ion cell by Na metal. The newly assembled cell was discharged to 1.5 V for the ion-exchange process. To understand the mechanism during the ion-exchange process, the cells were disassembled in each redox voltage during the charging and discharging of the cell for structural analysis. The XRD patterns of each electrode were analyzed by Rietveld refinement and the possible reaction mechanism for the ion-exchange process was investigated. It was found that there are lambda-MnO2, Li2MnO3, and NaMn2O4 phases in the electrodes which formed at different cut of voltages. According to Fourier Transform Infrared Spectroscopy measurements, the presence of Na-O bands was confirmed the successful ion-exchange within the materials. Structural properties were further examined using Scanning Electron Microscopy combined with Energy Dispersive X-ray analysis dot mapping and X-ray photoelectron spectroscopy analysis, supported by X-ray diffraction experimental results. The electrochemical performance of the ion-exchanged electrodes was investigated by cyclic voltammetry, electrochemical impedance spectroscopy, galvanostatic cycling, and C-rate measurements. The results showed that there was a significant change in the redox reaction mechanism by the ion-exchange process. According to galvanostatic measurements, the ion-exchanged electrodes showed better performance up to 50 cycles, but a phase change in the electrodes during the cycling caused a sharp decrease in capacity. Ex-situ XRD analysis after 100 cycles revealed the formation of the Na2Mn3O7 phase which is electrochemically inactive, and it has Mn4+ ions in the structure. The results suggest that the ion-exchange mechanism is a successful method, but the crystal structure has a crucial role in the cycling process of the cells.