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

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 Li 2 WO 4 layer with a thickness of ∼10 nm on the LiNi 0.6 Co 0.2 Mn 0.2 O 2 surface during the calcination process using (NH 4 ) 2 WO 4 . This decorated layer prevents side reactions and reduces formation of the insulators LiF and Li 2 CO 3 on the surface of the particles. Electrochemical impedance spectroscopy shows that the Li 2 WO 4 layer has no effect on the interphase film resistance of the cathode/electrolyte, which remains stable during the cycling process. The charge transfer of LiNi 0.6 Co 0.2 Mn 0.2 O 2 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 Li 2 WO 4 -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.