471084 Solid State Thermal Reaction of NaOH and Mn3O4 Drives the Formation of Sodium-Manganese Oxide Birnessite for Aqueous Electrochemical Energy Storage
Sodium-ion aqueous electrochemical energy storage devices have been widely studied recently due to their low-cost and environmental-friendliness compared with commonly used non-aqueous Li-ion batteries. Here, sodium-manganese oxide materials were synthesized via a solid state reaction by thermal treatment of Mn3O4 and NaOH at various molar ratios. Energy-dispersive x-ray spectroscopy (EDS) measurements confirmed that the atomic ratio of sodium to manganese varied from 0.02 to 0.29. The atomic structures of sodium-manganese oxide were studied with synchrotron X-ray and neutron scattering using pair distribution function analysis and Rietveld refinement. The results showed the structural evolution from monoclinic Mn5O8 nanoparticles to layered MnO2 birnessite with the increasing sodium content. Cyclic Voltammetry (CV) measurements showed Na0.29MnO2 had a specific capacitance of 211 F g-1 at a scan rate of 5 mV s-1 in a 0.1 M Na2SO4 electrolyte in a three-electrode half-cell, 60% higher than that of pure manganese oxide (132 F g-1). In-situ XRD measurements showed crystalline structure change and the d-spacing of (001) diffraction peak of sodium-manganese oxide varied 4% during charge and discharge processes, demonstrating its ability to achieve high capacity. Electro-kinetics analysis showed that more capacitive contribution was observed in sodium-manganese oxide compared with that of pure manganese oxide, indicating its high-rate power performance. Our synthesis methods as well as structure and electrochemical studies of sodium-manganese oxide materials provided an insight into the development of innovative electrode materials with high capacity and high power performance for aqueous electrochemical energy storage.