Tailoring P2/P3 Biphases of Layered Na_x MnO₂ by Co Substitution for High-Performance Sodium-Ion Battery

P-type layered oxide is a promising cathode candidate for sodium-ion batteries (SIBs), but faces the challenge of simultaneously realizing high rate capability and long cycle life. Herein, Co-substituted Na_x MnO₂ nanosheets with tunable P2/P3 biphase structures are synthesized by a novel dealloying-annealing strategy. The optimized P2/P3-Na_0.67 Mn_0.64 Co_0.30 Al_0.06 O₂ cathode delivers an excellent rate capability of 83 mA h g^-1 at a high current density of 1700 mA g^-1 (10 C), and an outstanding cycling stability over 500 cycles at 1000 mA g^-1 . This excellent performance is attributed to the unique P2/P3 biphases with stable crystal structures and fast Na⁺ diffusion between open prismatic Na sites. Moreover, operando X-ray diffraction is applied to explore the structural evolution of Na_0.67 Mn_0.64 Co_0.30 Al_0.06 O₂ during the Na⁺ extraction/insertion processes, and the P2-P2' phase transition is effectively suppressed. Operando Raman technique is utilized to explore the structural superiority of P2/P3 biphase cathode compared with pure P2 or P3 phase. This work highlights precisely tailoring the phase composition as an effective strategy to design advanced cathode materials for SIBs.