Synthesis, characterization of Li x CoO2 (x>1.0) and their electrochemical behaviors are examined. From compositional analysis, the solid solution limit of excess lithium is found around Li/Co=1.15. These products contain oxygen defects in order to maintain electronic neutrality. To evaluate the effect of oxygen defects, high pressure oxygen treatment was done for these samples. The electrochemical behavior of the treated samples shows much higher efficiency in the first cycle. The reversible capacity shows dependence on the composition and the highest one as much as 140mAhg−1 was obtained at Li/Co=1.10. The capacity linearly decreased with Li/Co ratio beyond 1.10 due to decreasing amount of cobalt in the products. This lead to better cycling performance with the reversible capacity of ca. 160mAhg−1, when the higher cut off voltage as 4.52V was set for the charge process.