Breaking Capacity–Stability Trade‐Off in Sodium Layered Oxide via Entropy‐Mediated Dual‐Site Engineering

Layered high‐entropy oxides represent a promising class of cathode materials for sodium‐ion batteries (SIBs), owing to the sodium's natural abundance and advantageous electrochemistry. Conventional high‐entropy designs, however, typically introduce multiple redox‐active or inert elements, inevitably forcing a compromise between entropy‐stabilized structural integrity and high specific capacity. Here, we demonstrate a dual‐site modification approach for an O3‐type Na 0.91 Ca 0.02 (Ni 0.3 Li 0.05 Fe 0.1 Mn 0.4 Ti 0.1 Mg 0.05 )O 2 cathode, by incorporating Ca 2+ pillars in the Na layers and a high‐entropy configuration within the transition‐metal slab. The optimized cathode material exhibits a high reversible capacity of 145.2 mAh g −1 at 0.1 C, remarkable rate performance (81.3% capacity retention at 2 C), and exceptional cycling stability (92.6% capacity retention after 800 cycles at 5 C) between 2.0 and 4.2 V. In situ X‐ray diffraction and complementary kinetics analyses reveal that this design effectively suppresses the detrimental P3–OP2 phase transition above 4.0 V and promotes rapid Na + transport. Our results establish that the synergistic entropy engineering and cationic substitution can reconcile high capacity with long‐term cyclability, providing a strategic design route to practical high‐energy cathode materials for SIBs.