Cho et at. reported that coating LiCoO2 with oxides can improve the capacity retention of LiCoO2 cycled to 4.4 V. Since that time, a number of other groups have confirmed that finding. This review article summarizes some of this early work and then focuses on work from our laboratory that helps clarify the role of the coating in cells charged to 4.5 V. We confirm that 30% higher energy density than that accessed by LiCoO2 normally used in a commercial cell (upper cut-off potential of 4.2V) can be obtained with excellent capacity retention. An in situ XRD study proves that the mechanism for the improvement in capacity retention by coating proposed by Cho et al. is incorrect. Further experiments described here identify the suppression of impedance growth in the cell as the key reason for the improvement caused by coating. Other methods that are also able to suppress the impedance growth associated with repeated charging to 4.5 V have been developed to improve the energy density of LiCoO2 without sacrificing capacity retention. Good capacity retention cannot be attained for cycling LiCoO2 above 4.5 V with respect to Li metal, presumably because of the structural changes between the O3 phase and the H1-3 phase that occur near 4.55 V. (C) 2003 Elsevier Ltd. All rights reserved.