Correlating the dispersion of Li@Mn6 superstructure units with the oxygen activation in Li-rich layered cathode

Abstract

The oxygen activation, contributing to the high capacity (> 250 mA h g−1) of Li-rich transition metal (TM) layered oxides xLi2MnO3•yLiTMO2 (TM = Mn, Ni, Co, Fe, etc.), is rooted in the unique 180o Li-O-Li configuration due to the ordering arrangement of Li@Mn6 superstructure units in Li2MnO3 component (equivalent to Li[Li1/3Mn2/3]O2), but the relationship between the oxygen activation and the distribution of Li@Mn6 superstructure units has not established. Herein, we comprehensively investigated the dispersion behavior of Li@Mn6 superstructure units during the synthesis of a model compound Li[Li1/6Mn1/3Ni1/3Sb1/6]O2 (0.5 Li[Li2/3Mn1/3]O2•0.5 Li[Ni2/3Sb1/3]O2) combining ex-situ X-ray diffraction (XRD) and in-situ/ex-situ transmission electron microscope (TEM). It revealed the entire process from the formation of Li@Mn6 superstructure units, to the gradual fusion with Sb@Ni6 superstructure units, eventually to the complete dispersion at 1100 °C. The systemic electrochemical tests demonstrated that, the dispersion of Li@Mn6 superstructure units effectively suppressed the irreversible oxygen activation, and the best capacity and voltage retentions were obtained in the solid solution with the complete dispersion of Li@Mn6 superstructure units. This work benefits the design of high performance Li-rich layered oxides with the modest anionic redox activity through the local structural tuning.