We recently proposed a new battery system based on the redox of lithium peroxide (Li2O2)/lithium oxide (Li2O) at the cathode (lithium-peroxide battery system). In this system, the use of Li2O with cobalt ions partially substituted for lithium ions (Co-doped Li2O) is key to its realization. In this study, to further improve the cell performance, we prepare various Co-doped Li2O samples by a mechanochemical process using different cobalt source materials (e.g., LiCoO2, Co3O4, and CoO) and comparatively investigate them. Amongst the investigated cathode materials, the Co-doped Li2O sample prepared using LiCoO2 with a Co/(Co + Li) ratio of 0.09 exhibits the best performance. Monitoring of the pressure in the cell reveals that this Co-doped Li2O cathode can be charged to 270 mAh g−1 without O2 evolution involving its decomposition. Charge and discharge at 270 mAh g−1 is repeated more than 50 times. In addition, the rate-capability tests reveals that the redox reaction between peroxide and oxide ions is fast and that the cathode can be discharged at a high current density of 1000 mA g−1.