Structural transformation and electrochemical properties of a nanosized flower-like R-MnO2 cathode in a sodium battery

High-energy density and low-cost sodium–ion batteries are being sought to meet increasing energy demand. Here, R-MnO2 is chosen as a cathode material of sodium–ion batteries owing to its low cost and high energy density. The structural transformation from the tunnel R-MnO2 to the layered NaMnO2 and electrochemical properties during the charge/discharge are investigated at the atomic level by combining XRD and related electrochemical experiments. Na≤0.04MnO2 has a tunnel R-MnO2 phase structure, Na≥0.42MnO2 has a layered NaMnO2 phase structure, and Na0.04−0.42MnO2 is their mixed phase. Mn3+ 3d4[t2gβ3dz2(1)3dx2−y2(0)] in NaMnO2 loses one 3dz2 electron and the redox couple Mn3+/Mn4+ delivers 206 mA h g−1 during the initial charge. The case that the Fermi energy level difference between R-MnO2 and NaMnO2 is lower than that between the layered Na(12-x)/12MnO2 and NaMnO2 makes the potential plateau of R-MnO2 turning into NaMnO2 lower than that of the layered Na(12−x)/12MnO2 to NaMnO2. This can be confirmed by our experiment from the 1st–2nd voltage capacity profile of R-MnO2 in EC/PC (ethylene carbonate/propylene carbonate) electrolyte. The study would give a new view of the production of sustainable sodium battery cathode materials.