Structural transformation and electrochemical properties of a nanosized flower-like R-MnO 2 cathode in a sodium battery.

High-energy density and low-cost sodium-ion batteries are being sought to meet increasing energy demand. Here, R-MnO 2 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-MnO 2 to the layered NaMnO 2 and electrochemical properties during the charge/discharge are investigated at the atomic level by combining XRD and related electrochemical experiments. Na ≤0.04 MnO 2 has a tunnel R-MnO 2 phase structure, Na ≥0.42 MnO 2 has a layered NaMnO 2 phase structure, and Na 0.04-0.42 MnO 2 is their mixed phase. Mn 3+ 3d 4 [t 2g β3d z 2 (1)3d x 2 - y 2 (0)] in NaMnO 2 loses one 3d z 2 electron and the redox couple Mn 3+ /Mn 4+ delivers 206 mA h g -1 during the initial charge. The case that the Fermi energy level difference between R-MnO 2 and NaMnO 2 is lower than that between the layered Na (12- x )/12 MnO 2 and NaMnO 2 makes the potential plateau of R-MnO 2 turning into NaMnO 2 lower than that of the layered Na (12- x )/12 MnO 2 to NaMnO 2 . This can be confirmed by our experiment from the 1st-2nd voltage capacity profile of R-MnO 2 in EC/PC (ethylene carbonate/propylene carbonate) electrolyte. The study would give a new view of the production of sustainable sodium battery cathode materials.