We have designed the air electrode for rechargeable Li–O2 batteries by adapting conventional current collectors-carbon papers (CPs), as freestanding substrates. The MnO2@CP electrodes are prepared by simply floating CPs on KMnO4 solution at room temperature. CPs act as the reducing agents as well as the freestanding substrates. Birnessite-type MnO2 nanosheets are observed to in situ grow vertically from the surface of CPs, thus building 3D porous architecture. The nanosheets are uniformly distributed and interconnected each other, which contributes to an improved electrical connection among the MnO2 catalyst and CPs. The cycling tests using the electrode as the cathode in rechargeable Li–O2 batteries exhibit high reversibility and superior cycling stability, over 90 cycles with a capacity of more than 1000 mA h (g MnO2)−1 and a high coulombic efficiency of around 100% in the voltage rang of 2.2–4.4 V.