Rational Design of High-Na P2-Type Cathodes for Sodium-Ion Batteries: Unveiling Phase Formation Principles and Composition–Structure–Performance Correlations

High-performance cathodes are required for advancing sodium-ion batteries, where both the Na content and transition metal (TM) composition significantly influence electrochemical performance. This work presents a rational design strategy for P2-type layered oxides, integrating increased Na content, reduced Ni/Fe concentration, balanced Mn3+/Mn4+ ratio, and Li incorporation. Guided by this approach, a series of high-Na-content P2-type cathodes was developed, and the key phase formation principles for Fe-containing compositions were revealed. Correlation analysis suggested an optimal composition, Na0.8Li0.07Fe0.12Ni0.11Mn0.7O2, featuring ultralow Ni content, demonstrated enhanced structural stability and Na+ diffusion kinetics. In situ XRD analysis confirmed exceptional structural resilience during cycling, exhibiting minimal lattice strain (1.5% volume variation) attributed to sufficient Na at the Naf site, mitigating TM layer gliding. Electrochemical evaluation revealed outstanding performance: a high reversible capacity (125.8 mAh g-1 at 0.1 C, 2.5-4.5 V), excellent cycling stability (81.6% capacity retention after 500 cycles at 1 C), and superior energy density in full cells (268.1 Wh kg-1). It also exhibited remarkable air stability, retaining structural integrity and 96.9% initial capacity after 10 days of air exposure. This design-oriented strategy not only clarifies the intrinsic phase formation rules but also establishes a paradigm for compositionally guided cathode engineering, bridging fundamental understanding and practical material design.