A Co-doped Li2O (CDL) was synthesized using a planetary ball mill and examined as a cathode material for lithium-ion batteries. When vinylene carbonate, which is often employed as an additive in an electrolyte, was used not only as an additive in the electrolyte but also as a treatment agent for CDL, the specific capacity of CDL was greatly enhanced and the charge-discharge cycles with specific charging and discharging capacities of 400 mAh/g could be repeated for more than 50 times. Co K-edge XANES spectroscopy and quantitative analysis of peroxide species revealed that the redox reactions of cobalt ions, formation/neutralization of O 2p electron holes, and generation/annihilation of peroxide species occurred during the charge-discharge process. (C) 2017 The Electrochemical Society. All rights reserved.