A high-entropy cathode for sodium-ion batteries: Cu/Zn-doping O3-Type Ni/Fe/Mn layer oxides

The O3 layered oxides have been extensively investigated as cathode materials for sodium-ion batteries due to their remarkable theoretical capacity. However, the large radius of Na+ lead to complex phase transitions and tortuous diffusion channels during extraction/insertion, which ultimately restricts the Na+ diffusion kinetics and cyclic stability. In this study, we report a Zn and Cu co-doping strategy to the high entropy layer oxide Na [Ni0·2Fe0·2Mn0·4Cu0·15Zn0.05]O2 (HEO-CuZn) using the sol-gel method to prepare the oxides. Here, Zn is used to stabilize the structure of layer between the transition metal and O atoms, which slows down the adverse phase transition. In the fabricated sample, the element Cu provides additional capacity and inhibits excessive oxidation. The prepared HEO-CuZn used as cathode for Na+ ion battery exhibits better electrochemical performance, delivering a high specific capacity of 148 mA h g−1, and 84 % capacity retention after 100 cycles, which is attributed to the expanded Na+ transfer channels, reduced activation energy and interface side reactions, thus enhancing the Na+ diffusion kinetics. Meanwhile, the high entropy strategy stabilizes the overall structure by reducing the John-Teller distortion and suppressing the Na+/vacancy order, and effectively avoids adverse phase transitions such as O′3 and P′3. This study provides a new insight for the advance of the next generation high specific energy sodium-ion battery.