Designing water/air-stable Co-free high-entropy oxide cathodes with suppressed irreversible phase transition for sodium-ion batteries

Layered transitional metal oxides are promising cathode materials for sodium-ion batteries that benefit from their high energy densities. Nevertheless, the complicated irreversible phase transitions and poor air stability limit their energy efficiency and implementation in practical devices. Here, a new Co-free high-entropy oxide (HEO) layered cathode NaFe0.2Cu0.1Ni0.2Mn0.3Ti0.2O2 is proposed to enhance the reversible phase transition, improve the air stability, and further decrease the cost. The HEO cathode displays excellent sodium-ion storage performance with a capacity of 121 mAh g−1, competitive cycling stability (∼83.8% capacity retention after 200 cycles at 2 C), and enhanced water resistance (capacity of 105.2 mAh g−1 after soaking in water for 2 h and capacity retention of 83% after 200 cycles at 1 C). Moreover, the as-obtained NFCNMT‖hard carbon full cells exhibit superior performance with reversible specific capacity 115 mAh g−1 and approximately ∼73% capacity retention after 100 cycles at 1C. Ex-situ X-ray diffraction and transmission electron microscopy reveal the completely reversible O3 → P3 → O3 structure evolution and suppressed violent changes in lattice parameters owing to the high-entropy design. Furthermore, the galvanostatic intermittent titration technique and in-situ electrochemical impedance spectroscopy reveal that the HEO cathode exhibits fast Na+ diffusion kinetics. For sodium-ion batteries, this work provides a new strategy for suppressing irreversible phase transformation and enhancing air stability.