Biphasic high-entropy layered oxide as a stable and high-rate cathode for sodium-ion batteries

O3-phase layered oxides are promising cathode materials for sodium-ion batteries. However due to the strong Na+–O2- electrostatic attraction and the large size of Na ion, the O3 type cathodes suffer inferior rate capability and undesired phase transitions induced fast capacity fading. Herein, we propose a synergetic strategy of secondary phase (P2) and high-entropy to alleviate the aforementioned problems. Such a novel biphasic high-entropy (B-HE) layered cathode Na0.85Li0.05Ni0.25Cu0.025Mg0.025Fe0.05Al0.05Mn0.5Ti0.05O2 exhibits a specific capacity of 122 mAh g−1 at 0.2C, outstanding rate capacity of 81.8 mAh g−1 at 10C, and long-term cycling stability of ∼ 90% capacity retention after 1000 cycles at 10C. In situ XRD and electrochemical characterization demonstrate reversible phase transition of O3/P2 ↔ P3/P2 ↔ OP2/OP4, and the long-term existed P phase with fast Na ion migration paths. This biphasic high-entropy design strategy provides a promising road for advanced cathodes of Sodium-ion batteries.