α-MnO₂/Carbon nanotube/Carbon nanofiber composite catalytic air electrodes for rechargeable lithium-air batteries

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 -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.