Structural and morphological changes of Co(OH)2 and valence modification of cobalt during the first charging by pretreatment, discharging at 57.8 mA g−1 for 7.5 h before the first charging, were investigated by transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray diffractometry. Consequently, metallic Co nanoparticles were formed on the Co(OH)2 particles after pretreatment. A comparison of the results for the pretreated and untreated electrodes shows that for the pretreated electrode, the Co nanoparticles were sequentially oxidized to Co(OH)2, Co3O4 and CoOOH. Moreover, the electric conductivities of the untreated and pretreated Co(OH)2 electrodes were 3.6 × 10−5 and 2.0 × 10−3 S cm−1, respectively. The difference in electric conductivity was ascribed to that in surface morphology of Co(OH)2 by the pretreatment and the following reoxidation which led to the formation of highly conductive mosaic CoOOH. This contributes to the formation of a dense, highly conductive network on the surface, great improving the electrochemical performance.