Understanding on the structural and electrochemical performance of orthorhombic sodium manganese oxidesElectronic supplementary information (ESI) available: XRD, DFT, cell performance, HT-XRD, DSC and TGA results of o-sodium manganese oxide compounds, lattice parameters from DFT and Rietveld refinement results (fresh and after 50 cycles). See DOI: 10.1039/c8ta08796b

We investigate the orthorhombic Na 0.67 [Ni x Mn 1− x ]O 2 ( 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 Mn 3+ ions because of the network with Ni 2+ ions. This effect influenced the smooth voltage variations in the voltage profile for Na 0.67 [Ni 0.05 Mn 0.95 ]O 2 , whereas several complicated voltage plateaus associated with the first-order phase transition were noticed in Na 0.67 MnO 2 . Operando synchrotron X-ray diffraction and transmission microscopy studies confirmed the simplicity of the phase transition for Na 0.67 [Ni 0.05 Mn 0.95 ]O 2 due to suppression of the Jahn-Teller effect of Mn 3+ 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. Sodium manganese oxide with divalent Ni in prismatic sites, represented as o -Na 0.67 [Ni 0.05 Mn 0.95 ]O 2 , shows high capacity with excellent cyclability.