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 O-2 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). (C) 2015 Elsevier B.V. All rights reserved.