Electrochemical oxygen evolution coupled structure and capacity decay of single-crystal LiNi0.6Co0.2Mn0.2O2 cathode materials

Abstract

Understanding the oxygen evolution of layered oxide cathode materials at high cut-off voltages is an enormous challenge. Herein, the single-crystal LiNi0.6Co0.2Mn0.2O2 (SC622) cathodes have been studied to investigate the relationship between electrochemical oxygen evolution behavior, structural degradation, and electrochemical performance at high cut-off voltages. The oxygen evolution behavior can be detected at around 4.6 V (with 84 % lithium extraction) via in-situ differential electrochemical mass spectrometry and ex-situ synchrotron soft X-ray absorption spectroscopy tests. Density functional theory results show that lattice oxygen atoms are lost easily from the surficial region but stable in the bulk area at high voltages. This provides scientific theoretical fundamentals for using SC622 cathode material at high cut-off voltages. Excessive lithium extraction results in uneven distribution of lithium inside SC622 grains, leading to lattice distortion and uneven deformation, eventually causing slippage and cracks. These bulk damages accelerate oxygen release and severely degrade the electrochemical performance of SC622. Both surface doping and fluorinated electrolyte can suppress the oxygen evolution of SC622 cathode materials at an ultrahigh cut-off voltage of 4.7 V. However, an appropriately high cut-off voltage, such as 4.5 V, maybe a more pragmatic strategy, achieving a “sweet point” of high specific energy and long cycle-life of cathode materials such as SC622.