Since electrode surfaces primarily house the sites of oxygen reduction/evolution reactions, surface chemistry is pivotal for both charge and discharge processes of Li-O-2 batteries. Herein, we introduce oxygen-containing functional groups associated with defects on the carbon fiber papers (functionalization) via acid treatment and investigate their effects on the rechargeable nonaqueous Li-O-2 batteries. The discharge platform increases from 2.68 V to 2.79 V, meanwhile the decreased charge overpotential (a drop of 0.84 V) is also observed after functionalization. It is found that the discharge products formed on pristine carbon fiber exhibit a huge toroidal-like structure, in contrast to the porous flake clusters with much smaller size on the functionalized carbon fiber. On the other hand, the smaller product clusters are found to be much easier to be oxidized, leading to remarkably reduced charge overpotential. In this regard, the decomposition of electrolyte and corrosion of carbon cathode are dramatically alleviated, thus leading to greatly promoted cycling performance. All the experimental facts suggest that the oxygen functional groups associated with defects could provide tremendous active sites for the nucleation of discharge products and result in remarkably decreased overpotential for both the oxygen reduction and the oxygen evolution reaction in our cases. (C) 2018 Published by Elsevier Ltd.