Tailoring P2/P3 Biphases of Layered NaxMnO2 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 NaxMnO2 nanosheets with tunable P2/P3 biphase structures are synthesized by a novel dealloying–annealing strategy. The optimized P2/P3–Na0.67Mn0.64Co0.30Al0.06O2 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 Na0.67Mn0.64Co0.30Al0.06O2 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. Co‐substituted NaxMnO2 nanosheets with tunable P2/P3 biphase structures are synthesized by a novel dealloying–annealing strategy. The optimized P2/P3–Na0.67Mn0.64Co0.30Al0.06O2 cathode exhibits excellent cycling stability and rate capacity due to the biphase synergistic effect. Operando X‐ray diffraction/Raman techniques are employed to unravel the structural evolution of prepared samples upon Na+ extraction/insertion processes.