(645c) 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.