The electrochemical and structural properties of cobalt oxide films which are deposited at different sputtering gas-ratios of O2/(Ar+O2) are investigated. In order to examine the electrochemical properties of the as-deposited films, all solid-state thin-film supercapacitors (TFSCs) are fabricated. There consist of Co3O4 electrodes and an amorphous LiPON thin-film electrolyte. It is shown that the capacitance behaviour of the Co3O4/LiPON/Co3O4 TFSCs is similar to bulk-type supercapacitor behaviour. It is further shown that the electrochemical behaviour of the TFSCs is dependent on the sputtering gas-ratios. The gas-ratio dependence of the capacitance of the oxide electrode films is discussed based on X-ray diffraction (XRD) and electrical results for the Co3O4 films.