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_0.91Ca_0.02(Ni_0.3Li_0.05Fe_0.1Mn_0.4Ti_0.1Mg_0.05)O₂ cathode, by incorporating Ca²⁺ 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.