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 Na_0.29MnO₂ 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.