Li-air cells based on Li foil as an anode electrode, freestanding carbon nanotube/nanofiber mixed buckypaper as an air ͑cathode͒ electrode, and organic electrolyte were assembled. The air electrode was made with single-wall carbon nanotube ͑SWNT͒ and carbon nanofiber ͑CNF͒ without any binder. The discharge capacity was strongly dependent on both the discharge current density and the thickness of the air electrode. A discharge capacity as high as 2500 mAh/g was obtained for an air electrode at a thickness of 20 m with a discharge current density of 0.1 mA/cm 2 ; however, it was reduced to 400 mAh/g when the thickness of the air electrode was increased to 220 m. For a 66 m thick air electrode, the discharge capacity decreased from 1600 to 340 mAh/g when the discharge current density increased from 0.1 to 0.5 mA/cm 2. The scanning electron microscope images on surfaces of the air electrode from a fully discharged cell showed that the voids at the air side were almost fully filled by the solid deposition; however, the voids at the membrane side were still wide open.

Air electrodes, made with a mixture of carbon nanotube (CNT)/carbon nanofiber (CNF) and with/without α-MnO2 nano-rods, were prepared for Li-air cells. The charge capacity and cyclability were found to increase largely for the cells made with the α-MnO2 catalyst; however, the first cycle discharge capacities were no different for the cells made with and without the α-MnO2 catalyst. It was found that the discharge capacity of the Li-air cell was mainly due to oxygen deficiency from the pinch-off of the diffusion channel by the deposition product at the air side of the air electrode. Electrochemical impedance spectra at different cycles demonstrated that the charge transfer resistance was increased and decreased during discharge and charge processes, respectively, due to the change of porosity, oxygen concentration, and rate of coefficient of chemical reaction in the air electrode.