A three-in-one strategy of high-entropy, single-crystal, and biphasic approaches to design O3-type layered cathodes for sodium-ion batteries

O3-type layered oxides are promising cathodes for sodium-ion batteries (SIBs). However, severe volume changes, irreversible phase transitions, and sluggish Na⁺ ion transport kinetics lead to structural collapse and severe capacity loss. Herein, a three-in-one strategy “high entropy, single crystal, and biphase” is proposed to design O3-type layered cathodes for SIBs, which achieves enhanced structural stability and Na⁺ transport kinetics by the combination effect of multimetal high-entropy, the single crystal, and Li substitution. The as-prepared high-entropy oxide (HEO) cathode, Na(Fe_1/6Co_1/6Ni_1/6Mn_1/6Ti_1/6)Li_1/6O₂, exhibits a high reversible capacity of 140.3 mAh g⁻¹, robust cycling stability, exceptional rate capability (86 mAh g⁻¹ at rates of 15C), excellent air-stability, and water-resistance ability. In situ X-ray diffraction reveals that the HEO cathode has highly reversible phase transitions and small volume change (ΔV=3.28 %). Ex situ X-ray absorption spectroscopy reveals that reversible Ni²⁺/Ni⁴⁺, Fe³⁺/Fe^3.6+, and Co³⁺/Co^3.6+ redox couples provide charge compensation for the high-entropy cathode at 2.0∼4.2 V. Notably, the full-cell battery based on the high-entropy cathode and hard carbon anode delivers a specific capacity of 134.3 mAh g⁻¹ and an energy density of 390.8 Wh kg⁻¹. This work provides valuable insights into the design of novel high-performance high-entropy cathodes for SIBs, highlighting a promising avenue for advancing rechargeable battery technology.