We previously proposed a lithium-ion battery system based on redox reactions between Li2O2 and Li2O by producing a Co-doped Li2O (CDL) cathode. CDL is synthesized by the mechanochemical method with Li2O and Co-dopant as the source materials in a planetary ball mill. Here, we aim to reveal the relation between the planetary ball milling conditions (the number and size of media balls, rotational speed, and processing time) and the characteristics of CDL. Moreover, we intend to optimize these conditions to improve the performance of CDL. While CDL decomposes into Li2O and LiCoO2 when the impact energy from the collision of balls is too high, we successfully suppress the decomposition reaction of CDL to obtain CDL with higher Co-doping levels than that previously reported by optimizing synthetic conditions. The CDL prepared under optimal conditions delivers a specific capacity of 450 mAh g(-1) and exhibits stable discharge behavior for at least 30 cycles. (C) The Author(s) 2018. Published by ECS.