Abstract
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.
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