Entropy-Stabilized Layered Oxides with Transition Metal Vacancies for High-Energy-Density Sodium Storage

The growing demand for lithium-ion batteries has intensified concerns over lithium resource sustainability, driving researchers to expore sodium-ion batteries (SIBs) as a viable alternative. Layered transition metal oxides are promising SIB cathodes but suffer from structural instability and sluggish diffusion kinetics. Here, we report a dual-strategy approach integrating high-entropy engineering and transition metal vacancy modulation to develop O3-type layered oxides NaMn0.26□0.04Fe0.3Ni0.2Cu0.1Ti0.05Mg0.05O2 (HEO-□0.04). The intentional introduction of 4% TM vacancies triggers a dual optimization mechanism: (1) elevating Mn valence (Mn3+ → Mn4+) to suppress Jahn–Teller distortions and (2) activating reversible anionic redox via Na-□-O configurations. Compared with HEO, HEO-□0.04 exhibits superior sodium storage performance, delivering a 3.7% higher initial discharge capacity (132.1 vs. 127.4 mAh g–1 at 0.1 C), 12% enhanced initial Coulombic efficiency, and exceptional rate capability (83.6% vs. 68.2% capacity retention at 5 C). In situ XRD analyses reveal that TM vacancies stabilize reversible O3 ↔ P3 phase transitions, enabling a 78.5% capacity retention after 200 cycles at 1 C. DFT calculations reveal that TM vacancies enhance ionic diffusion and stabilize high-entropy oxides via reduced Na+ migration barriers and suppressed metal cation displacement. The HEO-□0.04||hard carbon full cell delivers an energy density of 304.2 Wh kg–1 (based on the cathode active mass) and powers commercial LEDs, demonstrating practical viability. This work establishes a universal strategy of entropy-vacancy synergy for designing stable high-energy-density SIB cathodes.