ALBERT

Description: Lithium-oxygen (Li-O2) batteries have the highest theoretical energy density of all the Li-based energy storage systems, but many challenges prevent them from practical use. A major obstacle is the sluggish performance of the air cathode, where both oxygen reduction (discharge) and oxygen evolution (charge) reactions occur. Recently there have been significant advances in the development of graphene-based air cathode materials with a large surface area and high catalytic activity for both oxygen reduction and evolution reactions. However, most studies reported so far have examined air cathodes with a limited areal mass loading rarely exceeding 1 mg/cm2. Despite the high gravimetric capacity values achieved, therefore, the actual (areal) capacities of those batteries were far from sufficient for practical applications. Here, we present the fabrication, performance, and mechanistic investigations of high mass loading (up to 10 mg/cm2) graphene-based air electrodes for high-performance Li-O2 batteries. Such air electrodes could be easily prepared within minutes under solvent-free and binder-free conditions by compression molding holey graphene because of the unique dry compressibility of this graphene structural derivative with in-plane holes. High mass loading Li-O2 batteries prepared in this manner exhibited excellent gravimetric capacity and thus ultrahigh areal capacity (as high as ~40 mAh/cm2). The batteries were also cycled at a high curtailing areal capacity (2 mAh/cm2), with ultrathick cathodes showing a better stability during cycling than thinner ones. Detailed postmortem analyses of the electrodes clearly revealed the battery failure mechanisms under both primary and secondary modes, which were the oxygen diffusion blockage and the catalytic site deactivation, respectively. The results strongly suggest that the dry-pressed holey graphene electrodes are a highly viable architectural platform for high capacity, high performance air cathodes in Li-O2 batteries of practical significance.