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

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 Na_0.04−0.42MnO₂ is their mixed phase. Mn³⁺ 3d⁴[t_2gβ3d_z²(1)3d_x²−y²(0)] in NaMnO₂ loses one 3d_z² 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.