Correlation between regulated structure of Li-rich layered oxide and low-potential TM redox

Abstract

The anionic (oxygen) redox is known responsible for the high specific capacity and the transition metal (TM) migration of the Li- and Mn-rich (LMR) layer-structured oxide cathode materials. The reversibility of the TM redox was taken for granted; less attention has been paid to the relationship between structural degradation (TM migration and oxygen loss) and the redox behavior of the TM ions at low potentials. In combination of the crystalline and electronic structural characterizations and the electrochemical evaluation, we revealed in this article that the TM migration (Li-TM mixing) and the intensive anionic oxidation at high potentials suppress the Co2+/3+ redox and trigger the Mn3+/4+ redox at low potentials. The Mn3+/4+ redox accelerates the structural distortion and reduces the average discharge potential though both the Co2+/3+ and Mn3+/4+ redox contribute to charge compensation and the total capacity. Slightly decreasing the initial delithiation (charge cut-off potential decreases to 4.6 V vs. Li+/Li) mitigates the TM migration in Li1.2Ni0.13Co0.13Mn0.54O2 and significantly improves its structural stability and cycling performance after the charge cut-off potential recovers to the “normal” value (4.8 V) in the subsequent cycling. The dependence of the electronic structure of the Co3d and Mn3d orbitals on structural degradation is clarified with the density functional theory (DFT) calculations. It was shown that the TM migration in LMR results in the decrease of the unoccupied states of the Co3d orbitals and the shifting of the unoccupied states of the Mn3d orbital towards the Fermi energy level. These findings reveal the correlation between the structural degradation and the behavior of the low-potential TM redox, and will inspire ways to excavate a higher capacity in LMRs by activating more TM redox couples.