Lithium-ion batteries (LIBs) consisting of LiCoO2 and graphite electrodes exhibit a trade-off relationship between their reversible capacity and cycle/calendar life in terms of the charge cut-off voltage. That is to say, a higher charge cut-off voltage leads to a larger reversible capacity and shorter cycle life. In order to develop LIBs that satisfy both performance criteria (i.e., have a high reversible capacity as well as a long cycling life), the degradation mechanism of the LiCoO2 electrode under float charge conditions and high temperatures is investigated while focusing on the relationship between the structural deterioration of the electrode and capacity fading. Durability tests performed on graphite/LiCoO2 cells under float charge conditions (4.4 V at 60 °C) induced a drop in the open-circuit voltage as well as capacity fading in the LiCoO2 electrode along with the dissolution of a large number of cobalt ions. Acoustic emission histometry, X-ray diffraction, and transmission electron microscopy analyses of the LiCoO2 electrode after the float charge tests revealed that the degradation of the LiCoO2 electrode during the float charge tests occurred as per the following steps: (1) the HF generated by the decomposition of LiPF6 reacts with the charged LiCoO2 electrode, (2) the charged LiCoO2 electrode is disproportionated into CoO2 and Co2+ ions, and finally (3) the CoO2 having an O1 structure decomposes into cobalt oxides containing cobalt ions in a lower oxidation state, which is associated with the evolution of oxygen gas.