The effects of ZrO2-coating on the surface of LiCoO2 have been studied using synchrotron-based in situ x-ray diffraction (XRD) technique. Although both ZrO2-coated LiCoO2 and uncoated LiCoO2 showed capacity fading during high voltage (4.8 V) cycling, the ZrO2 coated one showed better cycle performance. After observing capacity fade for both samples through 10 charge-discharge cycles, in situ XRD patterns were collected during the 11th charge with reduced current density. With reduced current density, both samples restored part of their lost capacity. However, in situ XRD data shows that the ZrO2-coated LiCoO2 sample has much better structural change behaviors than the uncoated LiCoO2. The partial restoration of the lost capacity and the recovered structural variations observed at reduced current density clearly indicates that the lost capacity is very closely related to the increased polarization, which is most likely due to the precipitation of the electrolyte decomposition products on the surface of LiCoO2 cathode material. ZrO2 coating layer may provide some protection for the LiCoO2 cathode surface and help to reduce the electrolyte decomposition at higher voltages. (c) 2006 The Electrochemical Society.