Synthesis and Characterization of Distorted Orthorhombic Type Na0.7MnO2 Cathode Material By Spray Pyrolysis

Abstract

Because the lithium-ion batteries have characteristics of high capacity, high energy density, long cycle life and safety, lithium-ion batteries have been widely used as power sources for electric vehicles, portable electric device, and hybrid electric vehicles in the battery market. However, lithium resource is not abundant element and unevenly distributed in earth. Indeed, the price of lithium has dramatically surged due to commercialization of lithium batteries for power sources of vehicles and energy storage systems. In order to resolve the above problem, sodium-ion batteries using 3d - transition metals have been studied in earnest from 1980s. Sodium resource is possible to be compared with lithium resources as counterpart because sodium is one of the most opulent elements on the Earth. In addition, sodium is the second lightest and smallest alkali metal. Above all, intercalation chemistry of sodium is similar to lithium. For this reason, we decided to attempt to prepare distorted orthorhombic - Na0.7MnO2 based on Mn3+/4+ redox reaction synthesized by spray pyrolysis. Na0.7MnO2 powders were prepared by an ultrasonic spray pyrolysis method. Stoichiometric amounts of manganese nitrate tetra-hydrate and sodium nitrate as the starting materials were dissolved in distilled water at a molar ratio of Na:Mn of 0.7:1 at room temperature. The dissolved solution was added to a mixture solution of citric acid and sucrose. The resulting aerosol stream was introduced into a vertical quartz reactor heated to 400oC. The precursor powders were calcined from 1000 to 1300oC for 10h in a furnace under an air atmosphere at a heating rate of 5oC/min and then followed by slowly cooling to room temperature. The synthesized Na0.7MnO2 powders were identified by X-ray diffraction (XRD) with Cu kα radiation and analyzed by Rietveld refinement. The particle morphologies and size of the obtained precursors and powders were observed using scanning electron microscopy (SEM). Galvanostatic charge/discharge test was performed using a R2032-type coin cell. The electrochemical performances of Na0.7MnO2 were measured by using a constant charge-discharge rate of 9mAg-1 at voltages between 1.5 and 4.3V. Sodium layered oxide, Na0.7MnO2, with the distorted orthorhombic (space group:Cmcm) type structure was synthesized by an ultrasonic spray pyrolysis method and characterized as positive electrode for sodium batteries. The Na0.7MnO2 electrode delivered a high specific capacity of 228 mAhg-1 at 0.05C rate in the 1.5–4.3 V range. The capacity retention after 25 cycles is about 90% in the 1.5–4.3 V range. This reason of high capacity retention would be structural stability of the distorted orthorhombic Na0.7MnO2 electrodes during cycling. In order to confirm this reason, we conducted the in situ X-ray diffraction at first charge/discharge process. Furthermore, to understand the relationships between electrochemical properties and structural changes, cycled electrode were subjected to ex situ X-ray diffraction. Figure 1