In situ X-ray diffraction studies on the mechanism of capacity retention improvement by coating at the surface of LiCoO 2

Abstract

Synchrotron based in situ X-ray diffraction technique has been used to study the mechanism of capacity fading of LiCoO 2 cycled to a higher voltage above the normal 4.2 V limit and to investigate the mechanism of capacity retention improvement by ZrO 2 surface coating on LiCoO 2. It was found that the capacity fading of LiCoO 2 cycled at higher voltage limit is closely related to the increased polarization rather than the bulk crystal structure damage. The capacity of uncoated LiCoO 2 sample dropped to less than 70 mAh g −1 when charged to 4.8 V after high voltage cycling. However, when the voltage limit was further increased to 8.35 V, the capacity was partially restored and the corresponding structural changes were recovered to the similar level as seen in fresh sample. This indicates that the integrity of the bulk crystal structure of LiCoO 2 was not seriously damaged during cycling to 4.8 V. The increased polarization seems to be responsible for the fading capacity and the uncompleted phase transformation of LiCoO 2. The polarization-induced capacity fading can be significantly improved by ZrO 2 surface coating. It was proposed that the effect of ZrO 2-coating layer on the capacity retention during high voltage cycling is through the formation of protection layer on the surface of LiCoO 2 particles, which can reduce the decomposition of the electrolyte at higher voltages.