Effects of different charge cut-off voltages on the surface structure and electrochemical properties of LiNi0.6Co0.2Mn0.2O2

ABSTRACT Herein we combine X-ray photoelectron spectroscopy, AC impedance experiments and high resolution transmission electron microscopy to provide new insight into the degradation mechanism of a nickel-rich cathode material under different charge voltages. The degradation is closely related to the charge/discharge cut-off voltage. We use a simple in-situ method to generate a stable Li2WO4 layer with a thickness of ∼10 nm on the LiNi0.6Co0.2Mn0.2O2 surface during the calcination process using (NH4)2WO4. This decorated layer prevents side reactions and reduces formation of the insulators LiF and Li2CO3 on the surface of the particles. Electrochemical impedance spectroscopy shows that the Li2WO4 layer has no effect on the interphase film resistance of the cathode/electrolyte, which remains stable during the cycling process. The charge transfer of LiNi0.6Co0.2Mn0.2O2 is improved a lot by the decorated layer. Transmission electron microscopy shows that increasing amounts of a rock-salt phase substance forms with increasing cut-off voltage, while for the Li2WO4-decorated sample only a small amount of rock-salt forms on the surface of the cathode material. This study illustrates the relationship between changes in the surface structure with different cut-off voltages and the electrochemical performance, and provides an effective approach to improve the cycling performance of nickel-rich cathode materials.