To rapidly promote the development of electric vehicles, an efficient cathode catalyst for Li-O2 batteries is urgently needed. In the present study, we prepared a new type of doped carbon catalyst derived from nori biomass for the cathode of Li-O2 batteries, using a hydrothermal carbonization and pyrolysis method. The catalyst presented a graphene-like nanosheet structure, a high surface area, and excellent ORR/OER activity. Li-O2 batteries with this catalyst exhibited superior round-trip efficiency (at current densities of 500mA/g, the corresponding coulombic efficiency was 99.8%) and excellent cycling stability (100 stable cycles at 200mA/g under capacity limitation). Furthermore, the charge–discharge overpotential could be reduced dramatically by adding LiI to the electrolyte, resulting in greatly enhanced battery performance. The battery’s energy efficiency was over 90%, even after 100 cycles at limited capacity. We concluded the following: (i) the high surface area and nanosheet structure of the nori catalyst provided sufficient space not only to accommodate the discharge products but also to guarantee that oxygen, soluble catalyst, and lithium ions could be freely transported; and (ii) these combined with the redox mediator LiI that was added to the electrolyte, which could freely access the interior of the air electrode, easily reacting with the solid discharge products and effectively changing the morphology and chemical nature of the discharge products. We believe these factors were responsible for the significantly enhanced performance of the resulting Li-O2 batteries, suggesting this method may be a facile way to achieve a high-performance Li-O2 battery.