Synergistic Rigidity–Flexibility Engineering of O3-Type Sodium Layered Oxide Cathodes Through Site-Specific High-Entropy Regulation

O3-type layered oxides have emerged as promising cathode materials for sodium-ion batteries (SIBs) owing to their high theoretical capacity and elemental abundance. However, complex phase transition and anisotropic lattice strain undermine their structural integrity and cycling stability. Herein, a site-specific high-entropy strategy is proposed that integrates the rigidity of Ca²⁺ in the alkali-metal (AM) layers and the flexibility of high-entropy multi-cation configurations in the transition-metal (TM) layers to synergistically enhance the electrochemical performance of O3-type NaNi_0.5Mn_0.5O₂. The Ca²⁺ rigidity in the AM layers acts as a structural pillar, exerting a pinning effect that suppresses excessive TM slab gliding and stabilizes Na⁺ migration pathways. Simultaneously, the high-entropy flexibility in the TM layers, achieved through the random distribution of multiple cations, introduces adaptive local coordination environments that accommodate anisotropic lattice distortions and mitigate severe Jahn–Teller distortion of Ni³⁺O₆ octahedra. This dual-layer regulation considerably increases the interlayer spacing ratio (d_O-Na-O/d_O-TM-O), which not only promotes a more moderate and reversible structural evolution but also improves Na⁺ diffusion kinetics during cycling. Therefore, the engineered cathode exhibits enhanced specific capacity and cycling stability in both half and full cells. This work offers a scalable strategy toward the development of high-performance SIBs for practical applications.