High‐Entropy Engineering Regulates Na‐Ion Transport and Phase Evolution in O3‐Type Layered Oxide Cathode

Layered oxides have emerged as one of the most promising cathode materials for sodium‐ion batteries owing to their high capacities and energy densities. However, they undergo multiple phase transformations during cycling, leading to structural instability and performance degradation. High‐entropy design has recently been recognized as an effective strategy to mitigate such instability in O3‐type layered oxide cathodes. In this work, an O3‐type NaMn 0.35 Fe 0.15 Ni 0.25 Cu 0.10 Al 0.15 O 2 cathode, with a configurational entropy of ≈1.51 R (at transition metal site), is reported with an expanded Na‐layer spacing of 3.3170 Å, enabling facile Na + transport. Operando Synchrotron X‐ray diffraction confirms suppressed monoclinic distortions, preserving the hexagonal framework during cycling. As a result, the cathode delivers a specific discharge capacity of 168.3 mAh g −1 at 0.1C and 122.6 mAh g −1 at 1C, retaining 83.7% after 250 cycles at 1C. Full cells with hard carbon deliver 108.1 mAh g −1 at 0.1C, highlighting the practical applicability of the material.