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Na x Ni 0.5 Mn 0.5 O 2 (0.5 ≤ x ≤ 1.2)-layered oxides have been prepared and studied as cathode materials in sodium metal cells. The influence of sodium content on the structure and electrochemical performance of Na x Ni 0.5 Mn 0.5 O 2 (0.5 ≤ x ≤ 1.2) have been investigated. When x is between 0.5 and 0.8, the materials crystallize in the P2 phase. For x in the range of 0.9−1.2, novel P3-type materials have been obtained. Of great interest is the P3-type material with the specific composition Na 0.9 Ni 0.5 Mn 0.5 O 2 because it can deliver high discharge capacities (141 and 102 mA h g −1 at 10 and 100 mA g −1 , respectively). Compared to P2 Na x Ni 0.5 Mn 0.5 O 2 (0.5 ≤ x ≤ 0.8) materials, it exhibits much better cycling stability (78% capacity retention after 500 cycles in the voltage range of 1.5−4.5 V) and an initial Coulombic efficiency of ∼100%, which is more desirable for practical use. In addition, ex situ X-ray absorption near-edge structure spectra reveal that the redox reaction of nickel ions predominantly contributes to the capacity. Operando X-ray diffraction demonstrates reversible phase changes during the charge/discharge. Density functional theory calculations indicate that P3 NaNi 0.5 Mn 0.5 O 2 shows a low Na + -diffusion barrier of 237 meV. This unexplored class of P3 cathode materials induces new perspectives for the development of layered cathode materials and more energy-dense sodium ion batteries.