Boosting electrochemical reaction and suppressing phase transition with a high-entropy O3-type layered oxide for sodium-ion batteries

Complex phase transitions induced by interlayer slides in layered cathode materials lead to poor cycling stability and rate capability for sodium-ion batteries. Herein, we design and prepare a new six-component high-entropy oxide (HEO) layered cathode O3–Na(Fe0.2Co0.2Ni0.2Ti0.2Sn0.1Li0.1)O2 to enable highly reversible electrochemical reaction and phase-transition behavior. The HEO cathode exhibits good cycling performance (capacity retention of ∼81% after 100 cycles at 0.5C) and outstanding rate capability (capacity of ∼81 mA h g−1 at 2.0C) due to the higher sodium diffusion coefficient (above 5.75 × 10−11 cm2 s−1) than most reported O3-type cathodes. Moreover, the high-entropy cathode has superior compatibility with the hard carbon anode and delivers a specific capacity of 90.4 mA h g−1 (energy density of ∼267.5 W h kg−1). Ex situ X-ray diffraction proves that the high-entropy designing effectively suppresses the intermediate phase change to achieve reversible O3–P3 phase evolution, and in turn stabilizes the layered structure. X-ray absorption spectroscopy and Mössbauer spectrum of 57Fe suggest that Ni2+/Ni3.5+, Co3+/Co3.5+, and part of Fe3+/Fe3.5+ redox reaction contribute the charge compensation. The enhanced performance can be attributed to the disordered distribution of multi-component transition metals in HEO suppressing the ordering of electric charges and sodium vacancies, thereby inhibiting the interlayer slide and phase transition.