Increased discharge capacity of a Li-air activated carbon cathode produced by preventing carbon surface passivation

Abstract

A significant discharge capacity increase (larger than 3 times) for the gas-diffusion-electrode (GDE) used in Li-air cells was demonstrated through modification of the carbon surface with long-chain hydrophobic molecules. The capacity loss of the Li-air activated carbon cathode was found to be caused by the formation of undesired surface passivation. The mechanism of such passivation was identified as the formation of dense Li oxide films directly on the surface of the carbon during the oxygen reduction reaction. Such dense layers of Li oxide are here identified as the root cause of the undesired passivation, which blocks electrochemical reactions, increases the impedance and drops the discharge voltage rapidly. This investigation reveals that the capacity for the gas-diffusion-electrode can be substantially increased, if the activated carbon is modified by attaching long-chain hydrophobic molecules onto the surface. The carbon surface modification significantly delays the formation of the dense Li oxide layers. Therefore, the discharge capacity for the GDE is substantially increased.