New insights into understanding the exceptional electrochemical performance of P2-type manganese-based layered oxide cathode for sodium ion batteries

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 Na_2/3Ni_1/3Mn_2/3O₂ cathode, and new insights into understanding the structure-performance correlation are gained. Na_2/3Ni_1/3Mn_2/3O₂ 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 Na_2/3Ni_1/3Mn_2/3O₂ 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 Na_2/3Ni_1/3Mn_2/3O₂ during the Na⁺ insertion/extraction process. The present results suggest that P2-type Na_2/3Ni_1/3Mn_2/3O₂ is an extremely promising cathode material for advanced long-life SIBs towards grid storage application.