Understanding on the structural and electrochemical performance of orthorhombic sodium manganese oxides

Abstract

We investigate the orthorhombic Na_0.67[Ni_xMn_1−x]O₂ (x = 0 and 0.05) cathode materials that provide high capacity for prolonged cycles. X-ray absorption studies revealed that the redox activity of the Mn^3+/4+ and Ni^2+/3+ pairs is effective in suppressing the Jahn–Teller effect of Mn³⁺ ions because of the network with Ni²⁺ ions. This effect influenced the smooth voltage variations in the voltage profile for Na_0.67[Ni_0.05Mn_0.95]O₂, whereas several complicated voltage plateaus associated with the first-order phase transition were noticed in Na_0.67MnO₂. Operando synchrotron X-ray diffraction and transmission microscopy studies confirmed the simplicity of the phase transition for Na_0.67[Ni_0.05Mn_0.95]O₂ due to suppression of the Jahn–Teller effect of Mn³⁺ in the oxide lattice. These findings, along with the capacity retention during prolonged cycling and the acceptable thermal properties, make high-capacity sodium-ion batteries feasible, inexpensive, and safe for energy storage application.