Sodium-ion batteries are potential alternatives to lithium-ion batteries due to the natural abundance, and relatively low cost, of sodium. When used as a Na-ion battery cathode material, P2-type Na0.67Mn0.65Fe0.35O2 (NMFO) displays high reversible capacity (185 mAhg−1) and undergoes structural transitions between P63/mmc, P63 (OP4) and orthorhombic Cmcm during charge-discharge cycling between 1.5 and 4.3 V. Using Rietveld crystal structure refinement, we report that these structural transitions are completely suppressed in Na0.67Mn0.625Fe0.25Ni0.125O2 (NMFNO) during cycling. Interestingly, during discharge to 1.5 V, a mixture of two separate P63/mmc phases appears. Reversible capacity and specific energy of NMFNO are superior up to 100 cycles in the 1.5–4.0 V range, and to at least 200 cycles for 2.0–4.0 V. NMFNO displays first-cycle specific energy of 335 Whg−1, compared with 275 Whg−1 for NMFO. Scanning Electron Microscopy of the cathode surfaces after 200 cycles reveals performance-eroding cracks, and a solid electrolyte interface (SEI). Electrochemical Impedance Spectroscopy (EIS) shows that the total impedance of NMFNO between 1000 kHz and 0.1 Hz is significantly lower than NMFO after 200 cycles. We conclude that Ni substitution stabilizes the crystal structure by suppressing structural transitions in NMFO during cycling.