Li–O2 battery electrochemistries involved H2O have been reported to show reduced charge overpotential recently. However, water in electrolyte diffusing to Li anode side will inevitably corrode Li metal and degrade battery performance severely. Herein, a “H2O self-trapping” cathode of activated graphene is proposed to reserve certain amount of bound water, which will participant in the electrochemistry reactions of Li–O2 batteries in dry system. Under the participation of trapped water, an ultra-low charge plateau at about 3.3 V obtained, which is mainly attributed to the electrochemical decomposition of LiOH. This the first time that such excellent round-trip efficiency achieved using pure carbon cathodes during fully discharge-recharge process. Moreover, these results provide convictive evidence of the water catalysis in the presence of a “H2O self-trapping” cathode, which are instructive for the design of optimized cathode materials that will be used in more practical Li–O2 batteries.