Sodium ion batteries (SIBs) are emerging as one of the most promising candidates for large-scale energy storage due to the abundance of sodium. Layered manganese-based oxides, owing to relatively high capacity and low cost, exhibit great potential as SIBs cathode materials, but the cycling life remains a big challenge towards practical applications. Herein, unprecedented electrochemical performance is achieved in P2-type layered Na2/3Ni1/3Mn2/3O2 cathode, and new insights into understanding the structure-performance correlation are gained. Na2/3Ni1/3Mn2/3O2 delivers outstanding cycling stability (~ 80% capacity retention for 2000 cycles, 0.01% capacity loss per cycle),excellent rate capability (70.21% capacity retention at 20 C compared to 0.1 C), and a useable reversible capacity of about 84 mAh g-1 through tailoring its operating voltage range of 2.0-4.0 V. Moreover, the crystal structure of Na2/3Ni1/3Mn2/3O2 is investigated in depth at atomic resolution, and sodium atoms located at 2d Wyckoff sites in different layers are clearly observed and directly distinguished for the first time. Both in-situ X-ray diffraction (XRD) and ex-situ high-resolution transmission electron microscopy (HRTEM) results reveal that the exceptional electrochemical performance is mainly attributed to the superior structural stability of Na2/3Ni1/3Mn2/3O2 during the Na+ insertion/extraction process. The present results suggest that P2-type Na2/3Ni1/3Mn2/3O2 is an extremely promising cathode material for advanced long-life SIBs towards grid storage application.