Sodium Stoichiometry-Driven P2/O3 Biphase Layered Oxides with Enhanced Na+ Kinetics and Structural Stability for Sodium-Ion Batteries

P2/O3-type Ni/Mn-based layered oxides are regarded as promising cathode materials for sodium-ion batteries (SIBs) because of their high energy density. However, their practical application is limited by low initial Coulombic efficiency, sluggish Na+ kinetics, transition-metal dissolution/migration and irreversible phase transitions during cycling. Herein, a controlled P2 phase was achieved through elemental ratio regulation, enabling systematic synthesis of a series of NaxNi0.4Co0.1Mn0.5O2(x-NCMO) materials with tailored P2/O3 ratios. The optimized composition (x = 0.8), containing 16.6% P2 and 83.4% O3 phases, achieves an optimal phase equilibrium, thereby maximizing the synergistic coupling between the two layered polymorphs. This biphasic architecture demonstrates significantly enhanced Na+ transport kinetics and exceptional electrochemical performance, high initial capacity of 168.65 mAh g−1 and excellent rate performance, maintaining 84.88 mAh g−1 at 10 C, outperforming most reported P2/O3 biphasic cathodes. Structural analysis and electrochemical analysis reveal that elemental ratio regulation modulates the TM–O electronic structure, promotes electronic transport, and accelerates Na+ migration. These effects collectively reduce polarization, stabilize the structure, and thereby improve rate capability and long-term cycling capacity retention. This work provides an effective design strategy for designing high-performance layered oxide cathodes with improved structural and interfacial stability.