The demand for new energy storage technologies has been rising in recent years with the increased adoption of electric vehicles and intermittent renewable electricity sources such as wind and solar power. Among the new energy storage technologies under development, the Li-O2 battery chemistry represents an option with a very high energy density potential. One of the key components of Li-O2 batteries is the O2 electrode in which the discharge product (Li2O2) is deposited. Among the possible materials to be used in the manufacture of these electrodes, carbon nanotubes represent one of the best options: besides high conductivity and the possibility to tailor structures with high porosity, carbon nanotubes present catalytic activity and can be used as support for other catalysts focused on specific electrochemical reactions. This work presents a model of a Li-O2 battery developed to evaluate the impact of catalytic activity on the capacity of the cell. The model considers a generic catalytic activity, which is varied in a range, combined with different current densities to evaluate the impact of power density and reaction rate in the distribution of discharge products in the electrode. Results suggest a strong compromise between power density and energy density because high power density leads to low energy density and vice versa. The relationship between catalytic activity and properties of carbon nanotubes is also discussed considering the use of this material in the manufacturing of electrodes.